Partially Molten Middle Crust Be by pengxiuhui

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 Partially Molten Middle Crust Beneath Southern
 Partially Molten Middle Crust Beneath Southern                                                                      crust (4, 8, 11). Beneath the southern part
                                                                                                                     crust (4, 8, 11). Beneath the southern part
                                                                                                                     of the survey this anomalously thick crust is
                                                                                                                     of the survey this anomalously thick crust is
   Tibet: Synthesis of Project INDEPTH Results
   Tibet: Synthesis of Project INDEPTH Results                                                                       composed largely of the structurally imbri-
                                                                                                                     composed largely of the structurally imbri-
                                                                                                                     cated, pre-Eocene, passive continental mar-
                                                                                                                    cated, pre-Eocene, passive continental mar-
  K. D. Nelson,* Wenjin Zhao, L. D. Brown, J. Kuo, Jinkai Che,
  K. D. Nelson,* Wenjin Zhao, L. D. Brown, J. Kuo, Jinkai Che,                                                       gin of northern India; beneath the northern
                                                                                                                    gin of northern India; beneath the northern
    Xianwen Liu, S. L. Klemperer, Y. Makovsky, R. Meissner,
    Xianwen Liu, S. L. Klemperer, Y. Makovsky, R. Meissner,                                                          part, at least in the upper crust, it is com-
                                                                                                                     part, at least in the upper crust, it is com-
 J. Mechie, R. Kind, F. Wenzel, J. Ni,J. Nabelek, Chen Leshou,
 J. Mechie, R. Kind, F. Wenzel, J. Ni, J. Nabelek, Chen Leshou,
                                                                                                                     posed of the Gangdese batholith and Lhasa
                                                                                                                     posed of the Gangdese batholith and Lhasa
                                                                                                                     block, which formed the southern active
                                                                                                                     block, which formed the southern active
       Handong Tan, Wenbo Wei, A. G. Jones, J. Booker,
       Handong Tan, Wenbo Wei, A. G. Jones, J. Booker,                                                               continental margin of Asia prior to the
                                                                                                                    continental margin of Asia prior to the
   M. Unsworth, W. S. F. Kidd, M. Hauck, D. Alsdorf, A. Ross,
   M. Unsworth, W. S. F. Kidd, M. Hauck, D. Alsdorf, A. Ross,                                                       collision (12). The boundary between the
                                                                                                                    collision (12). The boundary between the
        M. Cogan, Changde Wu, E. Sandvol, M. Edwards
        M. Cogan, Changde Wu, E. Sandvol, M. Edwards                                                                 two is the Zangbo suture zone, which crops
                                                                                                                     two is the Zangbo suture zone, which crops
                                                                                                                    out approximately in the middle of the
                                                                                                                    out approximately in the middle of the
                                                                                                                    survey, coincident with the Yarlung-Zangbo
                                                                                                                    survey, coincident with the Yarlung-Zangbo
INDEPTH geophysical and geological observations imply that partially molten mid-
INDEPTH geophysical and geological observations imply that a partially molten mid-                                  River valley. Crustal thickening south of
                                                                                                                    River valley. Crustal thickening south of
crustal layer exists beneath southern Tibet. This partially molten layer has been produced
crustallayer exists beneath southern Tibet. This partially molten layer has been produced                           the suture is the result of convergence dur-
                                                                                                                    the suture is the result of convergence dur-
by crustal thickening and behaves         fluid the time scale of Himalayan deformation.
by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation.                            ing the collision between India and Asia
                                                                                                                    ing the collision between India and Asia
It is confined      the south by the structurally imbricated Indian crust underlying the
It is confined on the south by the structurally imbricated Indian crust underlying the                               (12). Recent observations suggest that the
                                                                                                                    (12). Recent observations suggest that the
Tethyan and High Himalaya and is underlain, apparently, by stiff Indian mantle lid. The
Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The                           Lhasa block crust may have been substan-
                                                                                                                    Lhasa block crust may have been substan-
results suggest that during Neogene time the underthrusting Indian crust has acted
results suggest that during Neogene time the underthrusting Indian crust has acted as                               tially thickened before the collision while it
                                                                                                                    tially thickened before the collision while it
   plunger, displacing the molten middle crust to the north while at the
a plunger, displacing the molten middle crust to the north while at the same time      time                         was part of the southern active margin of
                                                                                                                    was part of the southern active margin of
                                                                                                                    Asia (13). In the early 1970s, Dewey and




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contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene
contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene                                 Asia (13). In the early 1970s, Dewey and
evolution of the Himalaya is essentially record of the southward extrusion of the partially
evolution of the Himalaya is essentially a record of the southward extrusion of the partially                       Burke hypothesized that a midcrustal zone
                                                                                                                    Burke hypothesized that a midcrustal zone
molten middle crust underlying southern Tibet.
molten middle crust underlying southern Tibet.                                                                      of intrusion exists beneath the Tibetan Pla-
                                                                                                                    of intrusion exists beneath the Tibetan Pla-
                                                                                                                    teau, largely on the basis of analogy with
                                                                                                                    teau, largely on the basis of analogy with
                                                                                                                    older, more deeply eroded, mountain belts
                                                                                                                    older, more deeply eroded, mountain belts
                                                                                                                    (14). Subsequently geophysical investiga-
                                                                                                                    (14). Subsequently geophysical investiga-
The Himalaya and adjacent Tibetan Plateau
The Himalaya and adjacent Tibetan Plateau                          Project INDEPTH (2) recently undertook
                                                                   Project INDEPTH (2) recently undertook           tions undertaken during the early 1980s
                                                                                                                    tions undertaken during the early 1980s
play a central role in the study of collisional
play a central role in the study of collisional                   geophysical and geological investigations in
                                                                  geophysical and geological investigations in      hinted that such a layer might actually exist
                                                                                                                    hinted that such a layer might actually exist
orogens. It is generally assumed that the
orogens. It is generally assumed that the                                                1).
                                                                  southern Tibet (Fig. 1), INDEPTH is a col-
                                                                  southern Tibet (Fig. 1). IN DEPTH is a col-       beneath at least southern Tibet. These sur-
                                                                                                                    beneath at least southern Tibet. These sur-
tectonic processes at work in this active
tectonic processes at work in this active                         laborative geoscience project between the
                                                                  laborative geoscience project between the         veys demonstrated that heat flow is locally
                                                                                                                    veys demonstrated that heat flow is locally
continent-continent collision zone similarly
continent-continent collision zone similarly                      Chinese Academy of Geological Sciences
                                                                  Chinese Academy of Geological Sciences            extreme, consistent with the existence of
                                                                                                                    extreme, consistent with the existence of
shaped older collisional orogens and, to a
shaped older collisional orogens and, to a                        and investigators from U.S., German, and
                                                                  and investigators from U.S., German, and          molten granite at a depth of 10 to 20 km
                                                                                                                    molten granite at a depth of 10 to 20 km
significant degree, the continents as a
significant degree, the continents as a                           Canadian geoscience institutions. The
                                                                  Canadian geoscience institutions. The             beneath Yamdrok Tso (Fig. 1) (15), and
                                                                                                                    beneath Yamdrok Tso (Fig. 1) (15), and
whole. Despite the attention given to the
whole. Despite the attention given to the                         project builds on the efforts of previous
                                                                  project builds on the efforts of previous         identified an electrically conductive layer
                                                                                                                    identified an electrically conductive layer
region in recent years, little consensus has
region in recent years, little consensus has                      Sino-French, Sino-British, and Sino-U.s.
                                                                                                        Sino-U.S.
                                                                  Sino-French, Sino-British, and Sino-U.S.          within the crust north of the Zangbo suture
                                                                                                                    within the crust north of the Zangbo suture
developed with regard to the mechanical
developed with regard to the mechanical                           collaborative efforts in Tibet (3). IN DEPTH
                                                                                                       INDEPTH
                                                                  collaborative efforts in Tibet (3). INDEPTH       (16).
                                                                                                                    (16).
evolution of the Plateau lithosphere since
evolution of the Plateau lithosphere since                        collected a pilot near-vertical incidence
                                                                  collected a pilot near-vertical incidence             These observations are supported and
                                                                                                                        These observations are supported and
the onset of collision about 50 million years
the onset of collision about 50 million years                     common-midpoint (CMP) reflection profile
                                                                  common-midpoint (CMP) reflection profile          extended by the INDEPTH results, The
                                                                                                                    extended by the INDEPTH results. The
                                                                                                                                         IN DEPTH results.
ago (1). In the hope of advancing knowl-
ago (1). In the hope of advancing knowl-                          and companion wide-angle reflection data
                                                                  and companion wide-angle reflection data          new CMP reflection profiles reveal a set of
                                                                                                                    new CMP reflection profiles reveal a set of
edge of the geologic structure of this region,
edge of the geologic structure of this region,                    in southern Tibet during the summer of
                                                                  in southern Tibet during the summer of            reflections constituting a prominent, undu-
                                                                                                                    reflections constituting a prominent, undu-
                                                                   1992 (4, 5). During the summers of 1994
                                                                   1992 (4, 5). During the summers of 1994          latory, reflection horizon at depths of 15 to
                                                                                                                    latory, reflection horizon at depths of 15 to
 K. D. Nelson, M. Cogan, C. Wu, Department of Earth
K. D. Nelson, M. Cogan, C. Wu, Department of Earth
Sciences, Syracuse University, Syracuse, NY 13244,
                                                                  and 1995 a larger program involving the
                                                                  and 1995 a larger program involving the           20 km. These reflections extend from just
                                                                                                                    20 km. These reflections extend from just
Sciences, Syracuse University, Syracuse, NY 13244,
USA.
USA.                                                              acquisition of CMP reflection, wide-angle
                                                                  acquisition of CMP reflection, wide-angle         north of the Tsangpo suture to the north
                                                                                                                    north of the Tsangpo suture to the north
W. Zhao, J. Che, X. Liu, Chinese Academy of Geological
W. Zhao, J. Che, X. Liu, Chinese Academy of Geological            reflection, broadband earthquake, magneto-
                                                                  reflection, broad band earthquake, magneto-
                                                                              broadband                             end.ofthe IN DEPTH survey ("1" in Fig. 2)
                                                                                                                    end of the INDEPTH
                                                                                                                    endof the INDEPTH survey ("1" in Fig. 2)
Sciences, Beijing 100037, China.
Sciences, Beijing 100037, China.
L. D. Brown, M. Hauck, D. Alsdorf, A. Ross, Institute for
                                                                  telluric and surface geological data was un-
                                                                  telluric and surface geological data was un-      (6). This reflection band locally exhibits
                                                                                                                    (6). This reflection band locally exhibits
L. D. Brown, M. Hauck, D. Alsdorf, A. Ross, Institute for
the Study of the Continents, Cornell University, Ithaca,
the Study of the Continents, Comell University, Ithaca,
                                Cornell                           dertaken (INDEPTH II) [see companion re-
                                                                  dertaken (IN DEPTH II) [see companion re-         anomalously high amplitude and negative
                                                                                                                    anomalously high amplitude and negative
NY 14853, USA.
NY 14853, USA.                                                    ports in this issue (6-9)]. The INDEPTH
                                                                  ports in this issue (6-9)]. The INDEPTH           polarity (6), and at wide-angle, a strong
                                                                                                                    polarity (6), and at wide-angle, a strong
J. Kuo, Lamont Doherty Geological Observatory, Pali-
J. Kuo, Lamont Doherty Geological Observatory, Pali-              transect is - 300 km long and extends from
                                                                              -300
                                                                  transect is -300 km long and extends from         P-to-S conversion (7)-properties suggest-
                                                                                                                    P-to-S conversion (7)-properties suggest-
sades, NY, 10964, USA.
sades, NY, 10964, USA.
                                                                                                     (-27043'N,
                                                                  the crest of the Himalaya (-27°43'N,
                                                                  the crest of the Himalaya (-2r43'N,               ing the local occurrence of fluids along the
                                                                                                                    ing the local occurrence of fluids along the
S. L. Klemperer and Y. Makovsky, Department of Geo-
S. L. Klemperer and Y. Makovsky, Department of Geo-
physics, Stanford University, Stanford, CA 94305, USA.
physics, Stanford University, Stanford, CA 94305, USA.            89°09'E) to roughly the center of the Lhasa
                                                                  89°09'E) to roughly the center of the Lhasa       reflecting horizon. Receiver function anal-
                                                                                                                    reflecting horizon. Receiver function anal-
R, Meissner, Institut fOr Geophysik, Christian-Albrechts-
R.
R. Meissner, Institut fur Geophysik, Christian-Albrechts-
                      fUr                                         block (-30°35'N, 91°25'E). Most of the
                                                                  block (-30 00 35'N, 91°25'E). Most of the
                                                                                        91°25'E),                   yses of INDEPTH teleseismic data show
                                                                                                                              INDEPTH
                                                                                                                    yses of IN DEPTH teleseismic data show
Universitaet zu Kiel, 24098 Kiel, Germany.
Universitaet zu Kiel, 24098 Kiel, Germany.
                                                                  data were acquired in the Yadong-Gulu rift,
                                                                  data were acquired in the Yadong-Gulu rift,       that a marked low-velocity zone lies within
                                                                                                                    that a marked low-velocity zone lies within
J. Mechie and R. Kind, GeoForschungsZentrum Pots-
J. Mechie and R. Kind, GeoForschungsZentrum Pots-
dam (GFZ), 14473 Potsdam, Germany.
dam (GFZ), 14473 Potsdam, Germany.                                which is the largest of a set of roughly north-
                                                                  which is the largest of a set of roughly north-   the crust beneath the northern half of the
                                                                                                                    the crust beneath the northern half of the
F. Wenzel, Geophysikalisches Institut, Universitaet
F. Wenzel, Geophysikalisches Institut, Universitaet               south trending Neogene to Quaternary rifts
                                                                  south trending Neogene to Quaternary rifts        IN DEPTH survey, the top of which broadly
                                                                                                                    INDEPTH survey, the top of which broadly
Karlsruhe,
Karisruhe, 76187 Karlsruhe, Germany.
Kansruhe, 76187 Karlsruhe, Germany.                               that extend across the Himalaya and south-
                                                                  that extend across the Himalaya and south-        coincides with the reflection band at depths
                                                                                                                    coincides with the reflection band at depths
J. Ni and E. Sandvol, Department of Physics, New Mex-
J. Ni and E. Sandvol, Department of Physics, New Mex-
ico State University, Las Cruces, NM 88003, USA.
ico State University, Las Cruces, NM 88003, USA.                  ern Tibetan Plateau. The flat floors of the
                                                                  ern Tibetan Plateau. The flat floors of the       of 15 to 20 km (Fig. 2, top) (8). The
                                                                                                                    of 15 to 20 km (Fig. 2, top) (8). The
J. Nabelek, College of Oceanography, Oregon State Uni-
J. Nabelek, College of Oceanography, Oregon State Uni-            half-graben and graben valleys that form the
                                                                  half-graben and graben valleys that form the      low-velocity zone extends southward in the
                                                                                                                    low-velocity zone extends southward in the
versity, Corvallis, OR 97331, USA.
versity, Corvallis, OR 97331, USA.
L. Chen, H. Tan, W. Wei, China University of Geo-
                                                                  rift provided a logistically feasible- though
                                                                  rift provided a logistically feasible though      subsurface at least to the outcrop position of
                                                                                                                    subsurface at least to the outcrop position of
L. Chen, H. Tan, W. Wei, China University of Geo-
sciences, Beijing, China.
sciences, Beijing, China.                                         discontinuous route for geophysical work in
                                                                  discontinuous route for geophysical work in       the Zangbo suture, south of which it appears
                                                                                                                    the Zangbo suture, south of which it appears
A. G. Jones, Geological Survey of Canada, 1 Observatory
A. G. Jones, Geological Survey of Canada, 1 Observatory           the otherwise mountainous terrain of south-
                                                                  the otherwise mountainous terrain of south-       to die out. The new MT results indicate
                                                                                                                    to die out. The new MT results indicate
Crescent, Ottawa, Ontario, Canada.
Crescent, Ottawa, Ontario, Canada.
J. Booker and M. Unsworth, Geophysics Program, Uni-
                                                                  ern Tibet. The Yadong-Gulu and its sister
                                                                  ern Tibet. The Yadong-Gulu and its sister         that anomalously electrically conductive
                                                                                                                    that anomalously electrically conductive
J. Booker and M. Unsworth, Geophysics Program, Uni-
versity of Washington, Seattle, WA 98195, USA.
versity of Washington, Seattle, WA 98195, USA.
                                                                  rifts are accommodating ongoing east-west
                                                                  rifts are accommodating ongoing east-west         crust exists at depth beneath the entire
                                                                                                                    crust exists at depth beneath the entire
W. S. F. Kidd and M, Edwards, Department of Geo-
                      M.
W. S. F. Kidd and M. Edwards, Department of Geo-                  extension of the Tibet Plateau (10).
                                                                  extension of the Tibet Plateau (10).              northern two-thirds of the INDEPTH sur-
                                                                                                                    northern two-thirds of the INDEPTH sur-
                                                                                                                                                  IN DEPTH
sciences, SUNY-Albany, Albany, NY 12222, USA
sciences, SUNY-Albany, Albany, NY 12222, USA                          The thickness of the crust beneath Tibet
                                                                      The thickness of the crust beneath Tibet      vey (north of the Kangmar dome), begin-
                                                                                                                    vey (north of the Kangmar dome), begin-
• To whom correspondence should be addressed.
* To whom correspondence should be addressed.                     is roughly twice that of normal continental
                                                                  is roughly twice that of normal continental       ning at a depth of a few tens of kilometers,
                                                                                                                    ning at a depth of a few tens of kilometers,

1 684
1684                                                         SCIENCE
                                                             SCIENCE        *
                                                                            •   VOL. 274
                                                                                VOL. 274   *
                                                                                           •   6 DECEMBER 1996
                                                                                               6 DECEMBER 1996
 and that a highly electrically conductive
 and that a highly electrically conductive                from northward-decreasing crustal Q asso-
                                                          fwm northward-decreasing crustal Q asso-
                                                          from northward-de<:reasing              Q              steepening of the MHT at this location (a
                                                                                                                 steepening of the MHT at this location (a
 body coincides with the horizon at depths
 body coincides with the horizon at depths                ciated with the development of the mid-
                                                          ciated with the development of the mid-                ramp), a northward decrease in reflectivity
                                                                                                                 ramp), a northward decrease in reflectivity
 of 15 to 20 km imaged on the CMP profiles
 of 15 to 20 km imaged on the CMP profiles                crustal partial melt layer. The observation
                                                          crusta 1 partial melt layer. The observation
                                                          crusta!                                                of the MHT lOne (due to widening of the
                                                                                                                 of the MHT zone (due to widening of the
                                                                                                                                 zone
 beneath the northern Yadong-Gulu rift
 beneath the northern Yadong-Gulu rift                          Sg
                                                          that SI is a clear arrival in the INDEPTH
                                                          that Sg is a clear arrival in the INDEPTH
                                                                                              IN DEPTH           ductile shear zone with depth), or north-
                                                                                                                 ductile shear zone with depth), or north-
 (Damxung grJben) (9). Additionally, the
                   graben)
 (Damxung graben) (9). Additionally, the                  wide-angle data for travel paths in the
                                                          wide-angle data for travel paths in the                ward decreasing signal penetration. The
                                                                                                                 ward decreasing signal penetration. The
 MT data indicate that the highly electrical-
 MT data indicate that the highly electrical-             southern part of the survey, whereas it is
                                                          southern part of the survey, whereas it is             midcrustal zone of high electrical conduc-
                                                                                                                 midcrustal zone of high electrical conduc-
 ly conductive crust extends outside the
 ly conductive crust extends outside the                  absent beyond about 40 km for travel paths
                                                          absent beyond about 40 km for travel paths
                                                          ab5ent           about                                 tivity observed beneath the northern two-
                                                                                                                 tivity observed beneath the northern two-  tWO-
 northern Yadong-Gulu rift, both to the
 northern Yadong-Gulu rift, both to the                   in the northem part of the survey (7) is
                                                          in the northern part of the survey (7) is              thirds of the INDEPTH survey has a north-
                                                                                                                 thirds of the IN DEPTH survey has a north-
                                                                                                                                INDEPTH
 northwest and southeast. Taken together,
 northwest and southeast. Taken together,
                                     T aken               broadly supportive of this interpretation.
                                                          broadly supportive of this interpretation.             dipping tongue that projects upward be-
                                                                                                                 dipping tongue that projects upward be-
 these observations imply that the reflection
 these observations imply that the reflection
          ob5ervations                                    The reflectivity in the south provides infor-
                                                          The reflectivity in the south provides infor-          neath the north flank of the Kangmar dome
                                                                                                                 neath the north flank of the Kangmar dome
 horizon at depths of 15 to 20 km imaged on
 horizon at depths of 15 to 20 km imaged on               mation on the geometry of crustal structures
                                                          mation on the geometry of crusta I structures
                                                                                       crustal                   (9). The observation suggestS that partially
                                                                                                                 (9). The observation suggests that partially
                                                                                                                                          suggests
 the CMP profiles marks the top of a mid-
 the CMP profiles marks the top of a mid-                 underlying the T ethyan Himalaya.
                                                          underlying the Tethyan Himalaya.
                                                                            Tethyan                              molten midcrustal material approaches the
                                                                                                                 molten midcrustal material approaches the
 crustal partial melt layer underlying the
 crustal partial melt layer underlying the                    The most prominent feature visible on
                                                              The most prominent feature visible on              surface beneath the north limb of the dome.
                                                                                                                 surface beneath the north limb of the dome.
                                                                                                                                                          dome.
 northern part of the INDEPTH survey. The
 northern part of the lNDEPTH survey. The
                              INDEPTH                     the southern part of the CMP traverse is the
                                                          the southern part of the CMP traverse is the           Taken together, the observations suggest
                                                                                                                 Taken together, the observations suggest
 local occurrence of bright spots exhibiting
 local occurrence of bright spots exhibiting              Main Himalayan thrust (MHT)-the
                                                          Main Himalayan thrust (MHT)-the                        that the midcrustal partial melt layer- dies
                                                                                                                 that the midcrustal partial melt layer dies
 negative reflection polarity and strong P-
 negative reflection polarity and strong P-
                reflect ion                               thrust fault along which India is currently
                                                          thrust fault along which India is currently            out southward in the subsurface between
                                                                                                                 out southward in the subsurface between
                                                                                                                                            sub5urface
 to-S conversion along this horizon suggests
 to-S conversion along this horizon suggests              underthrusting southern Tibet ("4" in Fig.
                                                          underthrusting southern Tibet ("4" in Fig. Fig.        the suture and Kangmar dome.
                                                                                                                 the suture and Kangmar dome.
 that locally it also marks the top of actual
 that locally it also marks the top of actual             2) (6). This fault is traceable as a discrete
                                                          2) (6). This fault is traceable as a discrete              The Moho is only locally observed on
                                                                                                                    The Moho is only locally ob5erved on
                                                                                                                                                    observed
 magma accumulations.
 magma accumulations.                                     gently north-dipping reflection to
                                                          gently      north-dipping     refle<:tion
                                                                                        reflection     to        the INDEPTH CMP profiles near the
                                                                                                                 the INDEPTH CMP profiles near the
                                                                                                                       IN DEPTH
      Currently it is is possible to trace the
       Currently it not possible to trace the
                           not                            -28°45'N, where it disappears beneath the
                                                          -28°45'N , where it disappears beneath the
                                                           -28°45'N,                                             southern end of the transect, where it oc-
                                                                                                                 southern end of the transect, where it oc-
 top of this inferred partial melt layer un-
 top of this inferred partial melt layer un-              Kangmar dome. South-dipping reflections
                                                          Kangmar dome. South-dipping reflections                curs at a depth of -75 km (4, 6). To the
                                                                                                                                       --75
                                                                                                                 curs at a depth of - 75 km (4, 6). To the




                                                                                                                                                                            Downloaded from www.sciencemag.org on June 7, 2010
 equivocally south of the Zangbo suture, be-
 equivocally south of the Zangbo suture, be-              observed beneath the south limb of the
                                                          observed beneath the south limb of the.                north, its position beneath the survey is
                                                                                                                 north, its position beneath the survey is
 cause of a gap in the CMP coverage across
 cause of a gap in the CMP coverage across                Kangmar dome indicate that it is the sur-
                                                          Kangmar dome indicate that it is the sur-              constrained by receiver function determina-
                                                                                                                 constrained by receiver function determina-
 the Zangbo River gorge and the sparseness
 the Zangbo River gorge and the sparseness                face expression of a basement-cored anti-
                                                          face expression of a basement-cored anti-              tions from the broadband earthquake data
                                                                                                                 tions from the broadband earthquake data
                                                                                                                                   broad band
 of the broadband coverage. However, sev-
 of the broadband coverage. However, sev-                 form in the hanging wall of the MHT. The
                                                          form in the hanging wall of the MHT. The               (8). These indicate that the Moho lies be-
                                                                                                                 (8). These indicate that the Moho lies be-
 eral observations suggest that the layer does
 eral observations suggest that the layer does            observed across-strike extent and reflection
                                                          observed across-strike extent and reflection
                                                          ob5erved                                               tween depths of about 70 and 80 km along
                                                                                                                 tween depths of about 70 and 80 km along
 extend somewhat to the south, roughly to
 extend somewhat to the south, roughly to                 character of the MHT are strikingly similar
                                                          character of the MHT are strikingly similar            the length of the survey, implying that the
                                                                                                                 the length of the survey, implying that the
 the north limb of the Kangmar Dome (Fig.
 the north limb of the Kangmar Dome (Fig.                 to those of basal thrust decollement ob-
                                                          w those of basal thrust decollement ob-
                                                          to                                                     crust-mantle boundary is essentially flat be-
                                                                                                                 crust-mantle boundary is essentially flat be-
 1): (i) Heat flow measured south of the
 1): (i) Heat flow measured south of the
  I):                                                     served on reflection profiles in fossil colli-
                                                          served on reflection profiles in fossil colli-         neath the region. The spacing of the broad-
                                                                                                                 neath the region. The spacing of the broad-
 suture in Yamdrok Tso (latitude 29°N) is
 suture in Yamdrok Tso (latitude 29"N) is
                             T so             29°N)       sional orogens such as the southern Appa-
                                                          sional orogens such as the southern Appa-              band stations, however, is such that one or
                                                                                                                 band stations, however, is such that one or
 high (average 146 mW/mi2)) (15). (H) At
 high (average 146 mW/m 2 (15). (ii) At
                                     2         (ii)       lachians, with the obvious exception that
                                                          lachians, with the obvious exception that              more Moho imbrications, of the scale sug-
                                                                                                                 more Moho imbrications, of the scale sug-
 this location the CMP data exhibit a prom-
 this location the CMP data exhibit a prom-               the depth to the decollement at equivalent
                                                          the depth to the decollement at equivalent             gested to occur beneath southern Tibet on
                                                                                                                 gested to occur beneath southern Tibet on
 inent subhorizontal reflection at a depth of
          subhorizontal
 inent subhorirontal reflection at a depth of                                               considerably
                                                          positions in these olderbelts is conSiderably
                                                          positions in these older belts is considera bly        the basis of earlier fan profiling experiments
                                                                                                                 the basis of earlier fan profiling experiments
 -18 km, similar in appearance and depth to
 --18 km, simi lar in appearance and depth to
    18         similar                                    less.
                                                          less.                                                  (-10 to 15 km throw) (11), cannot be
                                                                                                                 (-10 to 15 km throw) (11), cannot be
                                                                                                                            IS                 (J I),
 the bright reflections imaged north of the
 the bright reflections imaged north of the
                  refle ctions                                It is unknown whether the disappear-
                                                              It is unknown whether the disappear-               ruled out. Indeed, some kind of north-dip-
                                                                                                                 ruled out. Indeed, some kind of north-dip-
suture (6). (iii) The MT data indicate that
suture (6). (iii) The MT data indicate that
                  (Hi)                                    ance of the MHT reflection beneath the
                                                          ance of the MHT reflection beneath the                 ping structure or structures do appear to be
                                                                                                                 ping structure or structures do appear to be
highly electrically conductive middle crust,
 highly electrically conductive middle crust,
 highl ~'                                                 Kangmar dome results from northward
                                                          Kangmar dome resu lts from northward
                                                                              results                            developed in the lower crust or uppermost
                                                                                                                 developed in the lower crust or uppermost
beginning at a depth of a few tens of kilo-
 beginning at a depth of a few tens of kilo-
 meters, extends without apparent break be-
 meters, extends without apparent break be-                Fig. 1. Generalized geologic
                                                                                    geologic
                                                           Fig. 1. Generalized gedogic               TarE
 neath the suture, as far south as the Kang-
neath the suture, as far south as the Kang-                map of the INDEPTH transect
                                                           map of the INDEPTH transect
 mar dome (9); and (iv) the wide-angle data
 mar dome (9); and (iv) the wide-angle data                regoo showing locations of
                                                           region
                                                           region showing locations of                      T                  200 MT linc
                                                                                                            Pi
show that reflections exhibiting strong P-
show [hilt reflections exhibiting strong P-
           that renL'Ctions                                INDEPTH controlled-source
                                                           INDEPTH controi!ed-SOIXCe
                                                           IN DEPTH controlled-source
to-S conversion ;n the upper crust extend
 to-S conversion in the upper crust extend
                         in                                seismic, earthquake. and
                                                           seismic. earthquake,
                                                           seismic, earthquake, and                     <
sporadically -50 km south of the suture
sporadically -50 km south of the suture                    magnetotelluric
                                                           magnetoteHuric
                                                           magnetotelluric        recording
                                                                                  recording     India
 (-28°50'N) (7). We therefore infer that
 (-28°50'N) (7). We therefore infer that
 (- 28"50'N)                                               programs. "CMP line," near-
                                                           programs. "CMP line: near-
                                                                                line,"
the partial melt layer extends southward in
the partial melt layer extends southward in
        pilrtial                                     [n    vertical incidence common
                                                           vertical incidence common
the subsurface, albeit probably dying away,
the subsurface, albeit probably dying away,
        subsurfilce.
                                                           midpoint reflection profile.
                                                           midpoint reflection profile.
                                                           "MT profile,· approximate line
                                                           "MT profile," approximate m
                                                                 profile," appmximate line       -
to roughly the north limb of the Kangmar
to roughly the north limb of the Kangmar                   of magnetotelluric recording
                                                          of magnetotelluric recording
dome.
dome.                                                      stations; "Seismic stations,"
                                                                                   stations,"
                                                          stations; "Seismic stations:
                                                           statoos;
      The IN DEPTH CMP profiles exhibit
      The INDEPTH CMP profiles exhibit
      Tht' INDEPTH                                        temporary seismic stations
                                                          temporary seismic stations
relatively few reflections from the crust be-
relatively few reflections from the crust be-              (REFTEKs) established for
                                                          (REFTEKs) established for
neath the prominent 15- to 20-km-deep
neath the prominent 15- to 20-km-deep
                                 l5-      ZO-km-deep      wide-angle recording of CMP
                                                          wide-angle recording of CMP
horizon described above. with the notable
horizon described above, with the notable
                           above,                                    ("Wide-Angle"
                                                          shots ("Wide-Angle" and
                                                          shots (WIde·Angle" and
exception of a steeply north-dipping reflec-
exception of a steeply north-dipping reflec-               "Broadside")
                                                          "Broadside") and broadband
                                                                                broadband
                                                          "BroadsKlel and Ixoadband
tion originating in the deep crust beneath
         originating
tion (lriginilting in the deep crust beneath              earthquake recording ("Pas-
                                                          earthquake recording ("Pas-
the Gangdese batholith ("2" in Fig. 2) (6).
the Gangdese batholith ("2" in Fig. 2) (6).
        G,iOgdese                                         sive"). The geologic base map
                                                          sive1. The geologic base map
                                                          sive").
                                                          is generalized from published
                                                          is generalized from published
      contrast,
In contrast, to the south, where the 15- to
In con trast, to the south, where the 15- to              sources (20, 43) and local
                                                          sources (20, 43) and local
20-km-deep horizon is not observed, the
ZO-km-deep
20-km-deep horizon is not observed, the                   geological obserVatiOns
                                                          geological observations made
                                                          gOOIogical observations made
crust is richly reflective down to, at least
crust is richly reflective down w, at least
                                          to,             by INDEPTH personnel. HHC,
                                                          by INDEPTH personnel. HHC,
locally, the Moho at the base of the --75-
locally, the Moho at the base of the -75-           75-   High Himalayan crystallines;
                                                          High Himalayan crystaJ!ines;
                                                                               crystallines;    _
                                                                                                                                                      CMI'linc     /'

km-thick crust underlying the T ethyan Hi-
km-thick crust underlying the Tethyan Hi_
                       undedying         Tethyan Hi-      STD, South Tibetan detach-
                                                          STD, South Tibetan detach·detach-
malaya ("3" in Fig. 2). As there is no obvi-
malaya ("3" in Fig. 2). As there is no obvi-              ment system; RZT, Aef1bu-
                                                          ment system; RZT, Renbu-   Renbu-                                          HIfC
ous change in the near surface conditions or              Zedong thrust; GT, Gangclese                                                                Seismic Stations
ous change in the near surface conditions or
                          nt'ar                                                  Gangdese
                                                          Zedong thrust; GT, Gangdese
                                                          thrust: NTSZ, Nyainqentan-
                                                                                                                                                      Wi< .... ~n~~     0
data
dilta acquisition parameters coincident with
data acquisition parameters coincident with               thrust; NTSZ, Nyainqentan-
                                                          thrust;                                                                                     Wide-Angle
                                                                                                                                                      ....w'·... II'A
                                                                                                                                                      Passive+WA
                                                                                                                                                                        C
                                                                                                                                                                        C
                                                                                                                                                                        0
this north-south variation, it likely results
this north-south variation, it likely results
th is                                 It                  glha shear zone.
                                                          glha shear                                                                         t)   I   ft.."......w.
                                                                                                                                                      Broadside         6


                                                          SCIENCE
                                                          SC:IENCE
                                                          SCIENCE       *
                                                                        •   VOl. 274 * 6 DECEMBER 1996
                                                                            VOL. IH • 6 DECEMBER 1996
                                                                            VOL. 274                                                                                1685
                                                                                                                                                                    1685
            IV
                 " Y" , .il 1- 1. . -,-", .; ,.                                                                                                                  4M.-           i=X,11W           i.AV.h..M
                 .-.,    11
                              w'.".       -
                                              -j.    -
                                                            .
                                                                  c,                                                     1.
                                                                                                                           !. , 7-
                                                                                                                                     "., i----A
                                                                                                                                          ',          I!r
                                                                                                                                                      J*            !,
                                                                                                                         ..,w
                                                                                                                                                  q
       1.           -I                    ..1       .. "Z
                                                                                                                         1,
                                                                                                                                     ,.,                             -




mantle beneath the region. This inference
mantle beneath the region. This inference                                    observations, it appears that, as the Indian
                                                                             observations, it appears that, as the Indian                                   of the partial-melt layer, which presumably
                                                                                                                                                            of the partial-melt layer, which presumably
is based on the region ally coincident obser-
                  regionally
is based on the regionally coincident obser-                                 lithosphere underthrusts southem Tibet, its
                                                                             lithosphere underthrusts southern Tibet, its                                   is a dynamically maintained melt front, ap-
                                                                                                                                                            is a dynamically maintained melt front, ap-
vations of dipping reflections on fan profiles
vations of dipping reflections on fan profiles                               attached crust warms and partially melts,
                                                                             attached crust warms and partially melts,                                      parently
                                                                                                                                                            parently intersects the downgoing Indian
                                                                                                                                                            parendy intersects the downgoing Indian
 ( 11),
(11), the north-dipping lower crusta reflec-
                                  crustal
(//), the north-dipping lower crustalI reflec-                               thereby
                                                                             thereby contributing to a partially molten
                                                                             thereb~' contributing to a partially molten                                    crust between the Kangmar dome and
                                                                                                                                                            crust between the Kangmar dome and
tion observed beneath the Gangdese batho-
tion observed beneath the Gangdese batho-                                    midcrustal layer
                                                                             midcrustallayer that underlies the northem
                                                                             midcrusrallayer that underlies the northern                                    Zangbo
                                                                                                                                                            Zangbo suture, some 220 to 270 km north of
                                                                                                                                                                                                   north
                                                                                                                                                            langbo suture, some 220 to 270 km nonh of
lith on the INDEPTH CMP profile ("2" in
lith          INDEPTH
lieh on the IN DEPTH CMP profile ("2" in                                     part of the IN DEPTH survey. The thickness
                                                                                         INDEPTH
                                                                             part of the INDEPTH survey. The thickness                                      the Himalayan thrust front (MFT tip). The
                                                                                                                                                                                           (MFT
                                                                                                                                                            the Himalayan thrust front (M FT tip). The
Fig. 2), and the observation that teleseismic
Fig. 2), and the observation that teleseismic                                of the partial melt layer is not yet known.
                                                                             of the partial melt layer is not yet known.                                    top of the layer probably coincides region-
                                                                                                                                                            top of the layer probably coincides region-
waves emerging from the north beneath the
waves emerging from the north beneath the                                    The fast upper mantle, and lack of Neogene
                                                                             The fast upper mantle, and lack of Neogene                                     ally with the wet granite solidus,
                                                                                                                                                            ally with the wet granite solidus, consistent
INDEPTH survey exhibit clear mode con-
INDEPTH survey exhibit clear mode con-                                       to recent mafic volcanism in the region
                                                                             to recent mafic volcanism in the region                                        with the elevated heat flow in the region.
                                                                                                                                                                                                    region.
                                                                                                                                                            with the elevated heat flow in the region.
version near the Moho, whereas those
version near the Moho, whereas those                                         imply that the warming results from crustal
                                                                             imply that the warming results from crustal                                    In detail this boundary may be a complex,
                                                                                                                                                            In detail this boundary may be a complex,
emerging from the south do not (17) Al-(17).
emerging from the south do not (J 7).. Al-                                   thickening, not intrusion of mantle-derived
                                                                             thickening, not intrusion of mantle-derived                                                   evolving
                                                                                                                                                            continuously evolving zone of granite intru-
                                                                                                                                                            continuously evolVing zone of granite intru-
though the interpretation of these seismo-
though the interpretation of these seismo-                                   melts into the crust. Kinematically, the par-
                                                                             melts into the crust. Kinematically, the par-                                  sions at the top of the layer.
                                                                                                                                                            sions at the top of the layer.
logically defined fabrics is problematic, they
                             problematic,
logically defined fabrics is problemadc, they                                tially molten middle crust is apparently be-
                                                                             tially molten middle crust is apparently be-                                       In accord with Zhao and Morgan's hy-
                                                                                                                                                                                  Zhao
                                                                                                                                                                In accord with lhao and Morgan's hy-
apparently do not represent a break (or
                       represent
apparently do not rep resent a break (or                                     ing added to the trailing (down-dip) end of
                                                                             ing added to the trailing (down-dip) end of                                    pothesis (20), it also appears that the partial
                                                                                                                                                            pothesis (20), it also appears that the partial
breaks) in the modem Indian lithosphere,
breaks) in the modern Indian lithosphere,                                    the High Himalayan Crystalline thrust
                                                                             the High Himalayan Crystalline thrust                                          melt layer acts as a fluid decoupling layer on
                                                                                                                                                            melt layer acts as a fluid decoupling layer on
as flexural analysis of topographic and grav-
as flexural analysis of topographic and grav-                                       (HHC),
                                                                             sheet (HHC), which surface geological ob-
                                                                             sheet (H HC), which surface geological ob-                                     the time scale of Himalayan deformation
                                                                                                                                                            the time scale of Himalayan deformation
 ity data imply that the Indian lithosphere
ity data imply that the Indian lithosphere                                   servations indicate is being displaced up-
                                                                             servations indicate is being displaced up-                                     (strain rates of to - IS per second). Thermo-
                                                                                                                                                                             10-15                Thermo-
                                                                                                                                                            (strain rates of 10- 15 per second). T hermo-
extends as a mechanically continuous unit
extends as a mechanically continuous unit                                    ward and southward relative to the under-
                                                                             ward and southward relative to the under-                                      chronologic data (22), together with
                                                                                                                                                            chronologic data (22), together with
 (supports a bending stress) to a point north
(supports a bending stress) to a point north                                 thrusting Indian plate (21). This is essen-
                                                                                                      (21).
                                                                             thrusting Indian plate (2/). This is essen-                                    INDEPTH field geological and CM P cross-
                                                                                                                                                                                               CMP
                                                                                                                                                            INDEPTH field geological and CMP cross-
of the INDEPTH survey (18). This infer-
         INDEPTH
of the IN DEPTH survey (18). This infer-                                     tially the mechanical behavior hypothe-
                                                                                          mechanical
                                                                             tially the me<:hanical behavior hypothe-                                       line data, indicate that the footwall of the
                                                                                                                                                            line data, indicate that the footwall of the
ence is also consistent with seismological
ence is also consistent with seismological                                   sized by Zhao and Morgan in 1987 (20),
                                                                                       Zhao
                                                                             sized by lhao and Morgan in 1987 (20),                                         northem Yadong-Gulu rift (Nyainqen-
                                                                                                                                                            northern Yadong-Gulu rift (Nyainqen-




                                                                                                                                                                                                                    Downloaded from www.sciencemag.org on June 7, 2010
results that indicate that relatively fast
results that indicate that relatively fast                                   with the caveat that the protolith for the
                                                                             with the caveat that the protolith for the                                     tanglha range) has undergone pronounced
                                                                                                                                                            tanglha range) has undergone pronounced
(cool) upper mantle extends from northem
                mantle
(cool) upper mande extends from northern                                     partial melt layer may well become Asian
                                                                             partial melt layer may well become Asian                                       uplift (-15 km) and rotation during the
                                                                                                                                                            uplift (-15 km) and rotation during the
India to roughly the center of the Tibetan
India to roughly the center of the Tibetan                                   (Lhasa block crust) toward the north. From
                                                                                                              north.
                                                                             (Lhasa block crust) toward the north. From                                     Late Miocene to Recent opening of the rift,
                                                                                                                                                            Late Miocene to Recent opening of the rift,
          (19).
Plateau (19)
Plateau (19)..                                                               a mechanical perspective, this distinction is
                                                                             a mechanical perspective, this distinction is                                  without the development of a correspond-
                                                                                                                                                            without
                                                                                                                                                            without the development of a correspond-
     In view of both the crust and mantle
     In view of both the cruSt and mantle
                             crust                                           probably unimportant. The southem limit
                                                                             probably unimportant. The southern limit                                       ingly deep graben. As with the extensional
                                                                                                                                                            ingly deep graben. As with the extensional


STD
STD                                                     Kangmar
                                                        Kangmar            Dome
                                                                           Dome           Zangbo
                                                                                          Zangbo      Suture
                                                                                                      Suture

   t                                                                                        t
             28°00'
             28'00'              28°15'                28°30'

                                                                       •
                                                                       +     29°00'
                                                                             29"00'                  29°30'


                                                                                                         ,
                                                                                                               29°45'            30°00'
                                                                                                                                 3O~OO'                           30°15'
                                                                                                                                                                  30"15'


                                                                                                                                                                                        -
                                                                                                                                                                                                30~'
                                                                                                                                                                                                30°30'

                                                                                                                                                                                                  --          0



                                                                                                                        -
                               '-                                                                                                                                                           ~




                 --0-:.'-r
                         r
                                                                                                                        -..
                                                                                                                                                                         -     -LVZ
                                                                                                                                                                                        ,
                                                                                                                                                                                            -.
                                                                                                                                                                                                         .r
                                                                                                                                                                                                         ~
                                                                                                                                                                                                              10
                                                                                                                                                                                                              10
                                                                                                                                                                                                          IWt {sI
                                                                                                                                                                                                          twt (s)

                                                       t
                                                                                                         "        ,                                                                                           20
                                                                                                                                                                                                               0

                              0
                              ®                       E-
                                                       ~
                                                                                                                                                                     70 km
                                                                                                                                                                                    -4--- Moho

                                                                , •    • •        •   •      , , •                       •                                   •                 • ,

                                                                                                                                                                                                     ~
                                                                  4    4          4          S   3   4                   4
                                                                                                                                 "                4                             4   5


                                                                                            Kangmar           Dome
 s.
 S.    MFT~~                          ~       MeT                             STD
                                                                                                         +               .¥'Zangbo                                  Suture




           100 km
Fig. 2. (Top) Composite of selected INDEPTH geophysical observations
Fig. 2. (Top) Composite of selected INDEPTH geopl1ysical observations
          (Top) CompOSite                         geophysical                                            suture; LVZ, midcrustal low-velocity zone evident in shear-wave velocity
                                                                                                         suture; LVZ, midcrustal low-velocity         evident in shear-wave velocity
along the Yadong-Gulu rift in southern Tibet; red,, composite north-south
along the Yadong-Gulu rift in southern Tibet; red, composjte north-south
             Yaoong·Gulu                      red composite                                              profiles north of the Zangbo suture. (Bottom) Interpretive lithosphere-scale
                                                                                                                                                                     lithosphere-scale
                                                                                                         profiles north of the Zangbo suture. (Bottom) Interpretive lithosphere·scale
CMP profile described in (6); blue, one-dimensional shear-wave velocity
CMP profile described in (6); blue, one-dimensional shear-wave velocity                                  cross-section of the Himalayan collision zone (see text).. MFT, Main Frontal
                                                                                                         cross-section                    collision
                                                                                                         cross·section of the Himalayan COllision       (see text) MFT. Main Frontal
                                                                                                                                                             text). MFT,
profiles derived from broadband earthquake data (7); blue stipple, wide-angle
profiles derived from broadband earthquake data (7); bluestipple. wide·angle
                      broad band                     blue stipple, wide-angle                            thrust; MCT, Main Central thrust; STD, South Tibetan detachment system.
                                                                                                         thrust; MGT. Main Central thrust; STD. South Tibetan detachment system.
                                                                                                         thrust; MCT,                       STD,
reflection observed in CMP data gap beneath and just north of the Zangbo
reflection obserVed in CMP data
            observed                 beneath and just north of the Zangbo                                Numbers refer to features discussed in text.
                                                                                                         Numbers refer to features discussed in text.

1686
1686
'686                                                                         SCIENCE * VOL. 274 *• 6 DECEMBER 1996
                                                                             SCIENCE • VOL. 274    6 DECEMBER 1996
core complexes of western North America, quence of it. Geochronological data indi-
core complexes of western North America, quence of it. Geochronological data indi-            that partial melt is widely developed with-
                                                                                              that partial melt is widely developed with-
this geometry essentially requires that the cating that the High Himalayan granites
this geometry essentially requires that the cating that the High Himalayan granites           in the middle crust of the Tibetan Plateau
                                                                                              in the middle crust of the Tibetan Plateau
middle crust has flowed laterally to accom- intruded at ~20 to 22 Ma (33), contempo-
                                                            -20
middle crust has flowed laterally to accom- intruded at -20 to 22 Ma (33), contempo-          and that where distinct low-velocity zones
                                                                                              and that where distinct low-velocity zones
modate the opening of the rift [as described raneous with ductile movement on the
modate the opening of the rift [as described raneous with ductile movement on the             occur the proportion of partial melt is
                                                                                              occur the proportion of partial melt is
 in (23, 24)]. Presumably, it is also flowing MCT (34), would then suggest that melting
in (23, 24)]. Presumably, it is also flowing MCT (34), would then suggest that melting        locally high or the upper crustal rocks
                                                                                              locally high or the upper crustal rocks
to accommodate the northern impingement within the HHC, and its consequent de-
                                                within
to accommodate the northern impingement wii:hin the HHC, and its consequent de-               locally have somewhat higher velocity
                                                                                              locally have somewhat higher velocity
of the Indian lithosphere.
of the Indian lithosphere.                      tachment from the underthrusting Indian
                                                tachment from the underthrusting Indian       than in adjacent regions.
                                                                                              than in adjacent regions.
    The existence of a midcrustal partial plate, initiated somewhat before ~ Ma. -20
    The existence of a midcrustal partial plate, initiated somewhat before - 20 Ma.               A regionally developed, weak, mid-
                                                                                                 A regionally developed, weak, mid-
melt layer beneath southern Tibet, while Essentially synchronous initiation of the
melt layer beneath southern Tibet, while Essentially synchronous initiation of the            crustal partial melt layer would also provide
                                                                                              crustal partial melt layer would also provide
not demonstrable a priori by thermal mod- South Tibetan detachment system, result-
not demonstrable a priori by thermal mod- South Tibetan detachment system, result-            a simple explanation for the essential flat-
                                                                                              a simple explanation for the essential flat-
eling, is plausible in light of such modeling. ing from ramping of the HHC (develop-
eling, is plausible in light of such modeling. ing from ramping of the HHC (develop-          ness of the Tibetan Plateau (39), spatial
                                                                                              ness of the Tibetan Plateau (39), spatial
Doubling the thickness of a generic granit- ment of a topographic front) combined
Doubling the thickness of a generic granit- ment of a topographic front) combined             relationship between gravity and topogra-
                                                                                              relationship between gravity and topogra-
oid continental crust, having global average with weakening of its top due to granite
oid continental crust, having global average with weakening of its top due to granite         phy on the Plateau (40), and results of
                                                                                              phy on the Plateau (40), and results of
continental heat production, overlying intrusion, follows from this scenario.
continental heat production, overlying intrusion, follows from this scenario.                 analog buckling studies (41), all of which
                                                                                              analog buckling studies (41), all of which
mantle with global average mantle heat
mantle with global average mantle heat             While the geologic evolution outlined
                                                   While the geologic evolution outlined      imply that the upper crust of the Tibetan
                                                                                              imply that the upper crust of the Tibetan
flux, will result in the interior of the thick- above implies that a midcrustal partial
flux, will result in the interior of the thick- above implies that a midcrustal partial       Plateau is regionally decoupled from the
                                                                                              Plateau is regionally decoupled from the
ened crust warming to temperature suffi- melt zone is regionally developed beneath
ened crust warming to a atemperature suffi- melt zone is regionally developed beneath         lower crust and mantle lithosphere below.
                                                                                              lower crust and mantle lithosphere below.
cient to produce partial melting in the pres- southern Tibet, IN DEPTH data only con-
                                                                 INDEPTH
cient to produce partial melting in the pres- southern Tibet, INDEPTH data only con-          Given that the crust underlying the entire
                                                                                              Given that the crust underlying the entire
ence of water (~600°C) after a few tens of strain such a layer to exist in the vicinity
                (-600°C)
ence of water (-600°C) after a few tens of strain such a layer to exist in the vicinity       Plateau is anomalously thick, such a layer
                                                                                              Plateau is anomalously thick, such a layer
millions of years (25). If the crust has higher of the northern Yadong-Gulu rift. Deter-
millions of years (25). If the crust has higher of the northern Yadong-Gulu rift. Deter-      conceivably could have developed through
                                                                                              conceivably could have developed through
than normal concentration of heat produc- mining the regional extent of the layer is          crustal thickening alone (as outlined
                                                                                              crustal




                                                                                                                                                                     Downloaded from www.sciencemag.org on June 7, 2010
than normal concentration of heat produc- mining the regional extent of the layer is          crusta I thickening alone (as outlined
ing radioactive elements, such as has been of considerable interest. Existing data are
ing radioactive elements, such as has been of considerable interest. Existing data are        above). Within the northern Plateau,
                                                                                              above). Within the northern Plateau,
found in the exposed HHC (26), then high- permissive of and arguably imply that par-
found in the exposed HHC (26), then high- permissive of and arguably imply that par-          where Neogene alkalic volcanic rocks are
                                                                                              where Neogene alkalic volcanic rocks are
er temperatures can be obtained. Denuda- tial melt is widely developed within the
er temperatures can be obtained. Denuda- tial melt is widely developed within the             widespread (42) and seismological evidence
                                                                                              widespread (42) and seismological evidence
tion resulting from erosion or normal fault- Tibetan Plateau crust. As noted above,
tion resulting from erosion or normal fault- Tibetan Plateau crust. As noted above,           implies that the upper mantle is hot (19),
                                                                                              implies that the upper mantle is hot (19),
ing, both of which have occurred during the the INDEPTH MT results suggest that the
ing, both of which have occurred during the the IN DEPTH MT results suggest that the          advection of heat into the crust by intrusion
                                                                                              advection of heat into the crust by intrusion
Neogene to Recent evolution of the Hima- partial melt zone extends at least a few
Neogene to Recent evolution of the Hima- partial melt zone extends at least a few             of mantle-derived magmas is likely to have
                                                                                              of mantle-derived magmas is likely to have
laya, similarly act to raise temperatures tens of kilometers in the subsurface out-
laya, similarly act to raise temperatures tens of kilometers in the subsurface out-           contributed to the formation and mainte-
                                                                                              contributed to the formation and mainte-
within the crust further (27). Thus in side the northern Yadong-Gulu rift. Hot
within the crust further (27). Thus in side the northern Yadong-Gulu rift. Hot                nance of such a layer.
                                                                                              nance of such a layer.
southern Tibet, where collision has been in springs, suggestive of elevated heat flow,
southern Tibet, where collision has been in springs, suggestive of elevated heat flow,
progress for 40 to 50 million years (12) and are a ubiquitous feature of the southern
progress for 40 to 50 million years (12) and are a ubiquitous feature of the southern                    REFERENCES AND NOTES
                                                                                                         REFERENCES AND NOTES
where the middle and lower crust of the Tibetan Plateau (35). It has been recog-
where the middle and lower crust of the Tibetan Plateau (35). It has been recog-                1. See. for example. models summarized in: J. F.
                                                                                                   See,        example,
                                                                                               1. See, for example, models summarized in: J. F.
Gangdese magmatic arc was presumably nized for some time that seismic wave
Gangdese magmatic arc was presumably nized for some time that seismic wave                         Dewey, M.Shackleton, Chang,
                                                                                                   Dewey, R. M.Shackleton, C. Chang, Y. Sun Philos.
                                                                                                   Dewey. R. M.Shackleton. C. Chang. Y. Sun Phi/os.
molten at the onset of collision, the exis- propagation velocities are generally low
molten at the onset of collision, the exis- propagation velocities are generally low               Trans. R. Soc. London Ser. A 327, 379 (1988); S. D.
                                                                                                   Trans. R. Soc. London Ser. A 327. 379 (1988); S. D.
                                                                                                                                           327,
tence of a midcrustal partial melt zone is within the Plateau crust compared to con-
tence of a midcrustal partial melt zone is within the Plateau crust compared to con-                                Beaumont,                  369,
                                                                                                   Willett and C. Beaumont, Nature 369. 642 (1994).
                                                                                                   Willett and C. Beaumont. Nature 369, 642 (1994).
                                                                                               2. "INDEPTH"-International Deep Profiling of Tibet
                                                                                               2. "INDEPTH"-Intemational Deep Profiling of Tibet
likely (28).
likely (28).                                    tinental crust worldwide (36-38). Receiv-
                                                tinental crust worldwide (36-38). Receiv-          and the Himalaya.
                                                                                                   and the Himalaya.
    This picture has a number of implica- er function analysis of broadband earth-
    This picture has a number of implica- er function analysis of broad band earth-
                                                                         broadband             3. C. Allegre et a/., Nature 307. 17 (1984); R. M. Shack-
                                                                                               3. C. Allegre et al.• Nature 307, 17 (1984); R. M. Shack-
                                                                                                                 al.,             307,
tions for the structural evolution of the quake data acquired in a transect across
tions for the structural evolution of the quake data acquired in a transect across                 leton, Philos. Trans. R. Soc. London Ser. A 327. 3
                                                                                                   leton, Phi/os. Trans. R. Soc. London Ser. A 327, 3
                                                                                                   leton.                                                  327,
Himalaya. Geometrically, the HHC can be the Plateau, indicate that the middle and
Himalaya. Geometrically, the HHC can be . the Plateau, indicate that the middle and                (1988); T. J. Owens, J. E. Randall, R. T. Wu. R. S.
                                                                                                                   Owens.              Randall,
                                                                                                   (1988); T. J. Owens, J. E. Randall. R. T. Wu, R. S.  Wu,
                                                                                                   Zeng, Bull. Seismol. Soc. A. 83,1959 (1993).
                                                                                                   Zeng. Bull. Seismol. Soc. A. 83. 1959 (1993).
                                                                                                   Zeng,                                 83,
viewed as an ongoing extrusion of the fluid in some places upper crust exhibits low
viewed as an ongoing extrusion of the fluid in some places upper crust exhibits low            4. W. Zhao et al., Nature 366, 557 (1993).
                                                                                               4. W. Zhao et al., Nature 366, 557 (1993).
                                                                                                                al.•              366.
middle crust. In this view the exposed crys- shear wave velocity (<3.5 km/s) across
middle crust. In this view the exposed crys- shear wave velocity «3.5 km/s) across             5. Y. Makovsky et al.• Tectonics 15, 997 (1996).
                                                                                               5. Y. Makovsky et a/., Tectonics 15. 997 (1996).
                                                                                                                       al.,                15,
talline rocks of the High Himalaya and the Plateau, and that the mean crustalI
talline rocks of the High Himalaya and the Plateau, and that the mean crusta        crustal    6. L. D. Brown et al., Science 274, 1688 (1996).
                                                                                               6. L. D. Brown et al., Science 274. 1688 (1996).
                                                                                                                      al..
                                                                                               7. Y. Makovsky et al., ibid., p. 1690.
                                                                                               7. Y. Makovsky et al., ibid., p. 1690.
                                                                                                                                        274,
North Himalayan domes represent progres- velocity in general is low (38). In some
North Himalayan domes represent progres- velocity in general is low (38). In some
                                                                                                                       al.. ibid..
                                                                                               8. R. Kind et al., ibid., p. 1692.
                                                                                               8. R. Kind et al.• ibid.• p. 1692.
                                                                                                               al., ibid.,
sively younger, frozen, snapshots of the par- areas midcrustal low velocity zones are
sively younger, frozen, snapshots of the par- areas midcrustal low velocity zones are          9. L. Chen et al., ibid., p. 1694.
                                                                                               9. L. Chen et al.• ibid.• p. 1694.
                                                                                                                al., ibid.,
tially molten midcrustallayer lying at depth apparent from mode ling these data (sta-
tially molten midcrustal layer lying at depth apparent from modeling these data (sta-
                                                                modeling                      10. P. Molnar and P. Tapponnier. J. Geophys. Res. 83,
                                                                                              10. P. Molnar and P. Tapponnier, J. Geophys. Res. 83.
                                                                                                                          Tapponnier,                          83,
                                                                                                   5361 (1978); R. P. Armijo. P. Tapponnier. J. L. Mer-
                                                                                                   5361 (1978); R. P. Armijo, P. Tapponnier, J. L. Mer-
to the north. The HHC, which is up to tions AMDO, BUDO, TUNL), whereas in
to the north. The HHC, which is up to           tions AMDO, BUDO, TUNL), whereas in                cier, H.Tong-Lin,
                                                                                                                            Armijo,       Tapponnier,
                                                                                                   cier, H.Tong-Lin, ibid. 91, 13803 (1986).
                                                                                                   cier. H.Tong-Lin. ibid. 91. 13803 (1986).
                                                                                                                                 91,
several tens of kilometers thick and con- others they are not. Notably, where low-
several tens of kilometers thick and con- others they are not. Notably, where low-            11. A. Hirn et al., Nature 307, 23 (1984); P. Molnar,
                                                                                              11. A. Him et al., Nature 307.23 (1984); P. Molnar,
                                                                                                                al.•               307, 23                Molnar.
tains widespread poly-deformed migmatitic velocity zones have been recognized, they
tains widespread poly-deformed migmatitic velocity zones have been recognized, they                Philos. Trans. R. Soc. London Ser. A 327, 33 (1988).
                                                                                                                                                  327,33
                                                                                                   Philos. Trans. R. Soc. London Ser. A 327. 33 (1988).
                                                                                                   Phi/os.
gneisses that exhibit evidence for large- are manifest not by an obvious decrease in
gneisses that exhibit evidence for large- are manifest not by an obvious decrease in          12. J. P. Burg and G. Chen, Nature 311, 219 (1984); M.
                                                                                                                           Chen,             311,219
                                                                                              12. J. P. Burg and G. Chen. Nature 311.219 (1984); M.
                                                                                                   P. Searle et al., Geol. Soc. Am. Bull. 98, 678 (1987).
magnitude strain in the presence of melt midcrustal velocity relative to Plateau sta-
magnitude strain in the presence of melt midcrustal velocity relative to Plateau sta-
                                                                                                                                                  98,678
                                                                                                   P. Searle et al.• Geol. Soc. Am. Bull. 98. 678 (1987).
                                                                                                                 al.,
                                                                                              13. A. Yin, M. A. Murphy, T. M. Harrison, Geol. Soc. Am.
                                                                                              13. A. Vino M. A. Murphy, T. M. Harrison, Geol. Soc. Am.
                                                                                                       Yin,       Murphy.              Harrison.
(30) and high-level cross-cutting plutons tions that do not exhibit a low-velocity
(30) and high-level cross-cutting plutons tions that do not exhibit a low-velocity                 Abstr. Progr. 27, A335 (1995).
                                                                                                   Abstr. Progr. 27, A335 (1995).
                                                                                                                    27.
(31), is likely a frozen facsimile of the par- zone, but rather by the local occurrence of
(31), is likely a frozen facsimile of the par- zone, but rather by the local occurrence of    14. J. F. Dewey and K. C. A. Burke, J. Geol. 81, 683
                                                                                              14. J. F. Dewey and K. C. A. Burke. J. Geol. 81. 683
                                                                                                           Deweyand                    Burke,             81,683
tially molten middle crust currently existing somewhat higher velocity upper crustal
tially molten middle crust currently existing somewhat higher velocity upper crustal               (1973).
                                                                                                   (1973).
                                                                                              15. J. Francheteau et al., Nature 307, 32 (1984).
                                                                                              15. J. Francheteau et al., Nature 307,32 (1984).
                                                                                                                           al..           307. 32
at depth north of the Kangmar dome. This material above. Stations SANG and LSA,
at depth north of the Kangmar dome. This material above. Stations SANG and LSA,               16. X. Yuan et al., in Geology of the Himalayas, vol. 1,
                                                                                                                al.,                   ofthe Himalayas, vo!.
                                                                                              16. X. Yuan et al.• in Geology of the Himalayas. vol. 1.
interpretation accords with the now widely which are in the general vicinity of the
interpretation accords with the now widely which are in the general vicinity of the                Papers on Geophysics, L. Guangcen, X. Yuan, A.
                                                                                                                 Geophysics,            Guangcen,
                                                                                                   Papers on Geophysics. L. Guan9cen. X. Yuan. A.        Yuan,
held view of LeFort (31, 32) that genera- INDEPTH survey but outside the Yadong-
held view of Le Fort (31, 32) that genera- INDEPTH survey but outside the Yadong-
                                                IN DEPTH                                           Hirn, Eds. (Geological Publishing House, Beijing,
                                                                                                                                                  House. Beijing,
                                                                                                   Him, Eds. (Geological Publishing House, Beijing.
                                                                                                   Him.
                                                                                                   1990), pp.105-113; V. N. Pham, D. Boyer, P.
tion of the Late Oligocene-Early Miocene Gulu rift, are examples. These stations do
tion of the Late Oligocene-Early Miocene Gulu rift, are examples. These stations do
                                                                                                    1990), pp.105-113; V. N. Pham. D. Boyer. P.
                                                                                                    1990).                                 Pham,
                                                                                                   Therme, X. Yuan, G. Jin, ibid., pp. 115-124.
                                                                                                   Therme,       Yuan,          Jin, ibid.•
                                                                                                   Therme. X. Yuan. G. Jin. ibid., pp. 115-124.
                                                                                                                                                        Boyer,

High Himalayan granites was genetically not exhibit distinct low-velocity zones but
High Himalayan granites was genetically not exhibit distinct low-velocity zones but           17. J. Nabelek et al., Eos (Fall Suppl.) 75, 628 (1994).
                                                                                              17. J. Nabelek et al., Eos (Fall Suppl.) 75, 628 (1994).
                                                                                                                    al.•                        75.
linked to movement on the MCT. Howev- do show that the crust from the near
linked to movement on the MCT. Howev- do show that the crust from the near                    18. Y. Jin, M. K. McNutt, Y. Zhu, J. Geophys. Res. 101,
                                                                                              18. Y. Jin. M. K. McNutt. Y. Zhu, J. Geophys. Res. 101.
                                                                                                       Jin,        McNutt,          Zhu.                     101,
er, in contrast, the geometric evolution de- surface to depths of 40 and 50 km has a
er, in contrast, the geometric evolution de- surface to depths of 40 and 50 km has a               11275 (1995).
                                                                                                    11275 (1995).
                                                                                                    11275(1995).
                                                                                              19. J. Ni, and M. Barazangi, Geophys. J. R. Astronom.
                                                                                              19. J. Ni. and M. Barazangi. Geophys. J. R. Astronom.
                                                                                                       Ni,          Barazangi,
scribed above suggests that initial detach- markedly low shear velocity (3.2 and 3.2
scribed above suggests that initial detach- markedly low shear velocity (3.2 and 3.2               Soc. 72, 665 (1983); N. Beghoul, M. Barazangi, B.
                                                                                                                                     Beghoul,
                                                                                                   Soc. 72, 665 (1983); N. Beghoul. M. Barazangi. B.
                                                                                                           72.                                      Barazangi,
ment of the HHC from the underthrusting to 3.5 km/s, respectively, for the two sta-
ment of the HHC from the underthrusting to 3.5 km/s, respectively, for the two sta-                Isacks, J. Geophys. Res. 98, 1997 (1993); D. E.
                                                                                                                                        98,
                                                                                                   Isacks, J. Geophys. Res. 98. 1997 (1993); D. E.
                                                                                                    lsacks.
Indian plate (initiation of MCT) was prob- tions) in both cases with no discernible
Indian plate (initiation of MCT) was prob- tions)-in both cases with no discernible                McNamara, T. J. Owens, W. R. Walter, ibid. 100,
                                                                                                   McNamara, T. J. Owens. W. R. Waiter. ibid. 100.
                                                                                                    McNamara.
                                                                                                   22215 (1995).
                                                                                                   22215 (1995).
                                                                                                                           Owens,               Waiter,      100,

ably caused by melting in the downgoing positive velocity gradient (38). A reason-
ably caused by melting in the downgoing positive velocity gradient (38). A reason-            20. B. C. Burchfiel et al., Geol. Soc. Am. Spec. Pap. 269
                                                                                              20. B. C. Burchfiel et al .• Geol. Soc. Am. Spec. Pap. 269
                                                                                                                         al.,
Indian crust, rather than being the conse- able interpretation of these observations is
Indian crust, rather than being the conse- able interpretation of these observations is            (1992), p. 41.
                                                                                                   (1992), p. 41.
                                                                                                   (1992).


                                              SCIENCE * VOL. 274 *• 6 DECEMBER 1996
                                              SCIENCE • VOL. 274    6 DECEMBER 1996                                                                       1687
                                                                                                                                                          1687
                                                                                 R..Me
                                                                                     :,ti .-:j::,:.,;,   -.,   ,'.        .'r     W
                                                                                                                                .-MORNME.   ::   -Opffiam   0            ...
                                                                                                                                                                               i.,              ,-   BF&IM 0,
21. W. Zhao and J. Morgan, Tectonics 6, 489 (1987).
21. W. Zhao and J. Morgan, Tectonics 6, 489 (1987).
22. T. M. Harrison, P. Copeland, W. S. F. Kidd, 0. M.
22. T. M. Harrison, P. Copeland, W. S. F. Kidd, O. M.                  Bright Spots, Structure, and Magmatism in
                                                                       Bright Spots, Structure, and Magmatism in
    Lovera, Tectonics 14, 658 (1995).
     Lovera, Tectonics 14,658 (1995).
                        14, 658
23. B. Wernicke, in Exposed Cross-Sections of the Con-
23. B. Wernicke, in Exposed Cross-Sections of the Con-                   Southern Tibet from INDEPTH Seismic
                                                                         Southern Tibet from INDEPTH Seismic
    tinental Crust, M. H. Salisbury and D. M. Fountain,
    tinental Crust, M. H. Salisbury and D. M. Fountain,
    Eds. (Kluwer, Dordrecht, 1990), pp. 509-544.
     Eds. (Kluwer, DordrechC 1990), pp. 509-544.
                    Dordrecht,                                                     Reflection Profiling
                                                                                   Reflection Profiling
24. S. Kruse, M. McNutt, J. Phipps-Morgan, L. Royden,
24. S. Kruse, M. McNutt, J. Phipps-Morgan, L. Royden,
    J. Geophy. Res. 96, 4435 (1991).
    J. Geophy. Res. 96,4435 (1991).
                       96, 4435
25. P. England, and A. Thompson, J. Petrol. 25, 894
25. P. England, and A. Thompson. J. Petrol. 25, 894
                           Thornpson,
                                                                  L. D. Brown, Wenjin Zhao, K. D. Nelson, M. Hauck, D. Alsdorf,
                                                                  L. D. Brown, Wenjin Zhao, K. D. Nelson, M. Hauck, D. Alsdorf,
    (1984); _ _ , in Collision Tectonics (Spec. Publ.
    (1984):
    (1984);           in Collision Tectonics (Spec. Publ.              A. Ross, M. Cogan, M. Clark, Xianwen Liu, Jinkai Che
                                                                       A. Ross, M. Cogan, M. Clark, Xianwen Liu, Jinkai Che
     19), M. P. Coward and A. C. Ries, Eds. (Geological
     19), M. P. Coward and A. C. Ries, Eds. (Geological
    Society, London, 1986), pp. 83-94.
    Society, London, 1986), pp. 83-94.
26. C. Pinet and C. Jaupart, Earth Planet. Sci. Lett. 84,
26. C. Pinet and C. Jaupart, Earth Planet. Sci. Lett. 84,       INDEPTH seismic reflection profiling shows that the decollement beneath which Indian
                                                                INDEPTH seismic reflection profiling shows that the decollement beneath which Indian
    87 (1987).
     87 (1987).
                                                                lithosphere underthrusts the Himalaya extends at least 225 kilometers north of the
                                                                lithosphere underthrusts the Himalaya extends at least 225 kilometers north of the
27. L. H. Royden, J. Geophys. Res. 98, 4487 (1993).
27. L. H. Royden, J. Geophys. Res. 98,4487 (1993).
28. Although only loosely constrained, restorable cross
28. Although only loosely constrained, restorable cross         Himalayan deformation front to a depth of -50 kilometers. Prominent reflections appear
                                                                                                            ~50
                                                                Himalayan deformation front to a depth of -50 kilometers. Prominent reflections
    sections of the the Himalaya (29) suggest that rough-
    sections of the the Himalaya (29) suggest that rough-       at depths of 15 to 18 kilometers near where the decollement reflector apparently ter-
                                                                at depths of 15 to 18 kilometers         where the decollement reflector apparently ter-
    ly 500 km of shortening has occurred south of the
    ly 500 km of shortening has occurred south of the           minates. These reflections extend north of the Zangbo suture to the Damxung graben
                                                                minates. These reflections extend north of the Zangbo suture to the Damxung graben
    Tsangpo suture since the initiation of collision at -50
    Tsangpo suture since the initiation of collision at -50
    million years ago (Ma), implying a mean underthrust
    million years ago (Ma), implying a mean underthrust         of the Tibet Plateau. Some of these reflections have locally anomalous amplitudes (bright
                                                                of the Tibet Plateau. Some of these reflections have locally anomalous amplitudes (bright
    rate relative to the Lhasa block of about 11 cm/yr.
    rate relative to the Lhasa block of about         cm/yr.    spots) and coincident negative polarities implying that they are produced by fluids in the
                                                                spots) and coincident negative polarities implying that they     produced by fluids in the
    Thus, lower plate Indian crust that now lies at depth
    Thus, lower plate Indian crust that now lies at depth       crust. The presence of geothermal activity and high heat flow in the regions of these
                                                                crust. The             of geothermal activity and high heat flow in the regions of these
     -220 km north of the of the thrust front, beneath the
                                                                reflections and the tectonic setting suggest that the bright spots mark granitic magmas
     -220 km north of the of the thrust front, beneath the
    Kangmar Dome, has likely been descending and
    Kangmar Dome, has likely been descending and                reflections and the tectonic setting suggest that the bright spots mark granitic
    warming, for >20 million years-a time span comen-
                                                    co men-
    warming, for >20 million years-a time span comen-           derived by partial melting of the tectonically thickened crust.
                                                                derived by partial melting of the tectonically thickened crust.
    surate with that necessary to reach granite minimum
    surate with that necessary to reach granite minimum
    melt temperature (25).




                                                                                                                                                                                                           Downloaded from www.sciencemag.org on June 7, 2010
    melt temperature (25).
29. D. Schelling and K. Arita, Tectonics 10,851 (1991);
29. D. Schelling and K. Arita, Tectonics 10, 851 (1991);
                                           10, 851 (1991):
    L. Ratschbacher, W. Frisch, G. Liu, C. Ghen, J. Geo-
    L. Ratschbacher, W. Frisch, G. Uu, C. Ghen, J. Geo-
                                     Liu,
    phys. Res. 99, 19917 (1994).
    phys. Res. 99, 19917 (1994).                                Multichannel seismic reflection data were
                                                                Multichannel seismic reflection data were                                               neath the south flank of the Himalaya; (ii)
                                                                                                                                                        neath the south flank of the Himalaya; (ii)
                                                                                                                                                                                         Himalayaj
30. L. Hollister, Geology 108, 31 (1993).
30. L. Hollister, Geology 108,31 (1993).
                           108, 31
31. P. LeFort, Philos. Trans. R. Soc. London Ser. A 326,
31. P. LeFort, Philos. Trans. R. Soc. London Ser. A 326,
                                                                acquired by Project INDEPTH in 1992 and
                                                                acquired by Project IN DEPTH in 1992 and                                                 its southern end lies at the same depth as
                                                                                                                                                         its southern end lies at the same depth as
    281 (1988).
    281 (1988).                                                  1994 by the Fifth and Fourth Geophysical
                                                                 1994 by the Fifth and Fourth Geophysical                                               the basal decollement beneath the High
                                                                                                                                                        the basal decollement beneath the High
32. __, Am. J. Sci. Ser. A. 275, 1 (1975).
                                  275, 1
32. _ _ , Am. J. Sci. Ser. A. 275,1 (1975).                     Prospecting Brigades, respectively, of the
                                                                Prospecting Brigades, respectively, of the                                              Himalaya inferred from surface geology in
                                                                                                                                                        Himalaya inferred from surface geology in
33. R. R. Parrish and K. V. Hodges, Geol. Soc. Am.
33. R. R. Parrish and K. V. Hodges, Geol. Soc. Am.
    Abstr. Progr. 25, A1 74 (1993).
    Abstr. Progr. 25, A174 (1993).
                                                                Ministry of Geology and Mineral Resources
                                                                Ministry of Geology and Mineral Resources                                               eastern Nepal (3); and (iii) it is discordant
                                                                                                                                                        eastern Nepal (3)j and (iii) it is discordant
                                                                                                                                                                        (3);
                                                                                                                                                        with overlying dipping reflections attribut-
                                                                                                                                                        with overlying dipping reflections attribut-
                       A 17 4
34. M. S. Hubbard and T. M. Harrison, Tectonics 8, 865
34. M. S. Hubbard and T. M. Harrison, Tectonics 8,865 8, 865    of China (MGMR) under the direction of
                                                                of China (MGMR) under the direction of
    (1989): K. V. Hodges et al., Science 258, 1466
    (1989); K. V. Hodges et al., Science 258, 1466
    (1989);                                                     INDEPTH scientists. Restricted to the Ya-
                                                                IN DEPTH scientists. Restricted to the Ya-
                                                                INDEPTH                                                                                 ed to thrust imbrication in the Tethyan
                                                                                                                                                        ed to thrust imbrication in the T ethyan
     (1992).
     (1992).
35. J. W. Tong and M. Zhang, in Geological and Ecolog-
35. J. W. Tong and M. Zhang, in Geological and Ecolog-
                                                                                                         (1)],
                                                                dong-Gulu rift by logistics [figure 1 of (1)],
                                                                dong-Gulu rift by logistics [figure 1 of (0],                                           Himalaya. The MHT reflection, which dips
                                                                                                                                                        Himalaya. The MHT reflection, which dips
    ical Studies of the Qinghai-Xizang Plateau, Liu Dong-
                                                   Liu
    ical Studies of the Qinghai-Xizang Plateau, Uu Dong-        the discontinuous survey consists of seven
                                                                the discontinuous survey consists of seven                                              northward at an average of about 120, dis-
                                                                                                                                                        northward at an average of about 12°, dis-
    sheng Ed. (Gordon & Breach, New York, 1981). pp.
                                                  1981),
    sheng Ed. (Gordon & Breach, New York, 1981), pp.            main profiles and four short cross lines, all
                                                                main profiles and four short cross lines, all                                           appears at a depth of about 50 km beneath
                                                                                                                                                        appears at a depth of about 50 km beneath
    841-846.
    841-846.
36. B. Romanowicz, J. Geophys. Res. 87, 6865
                                                                recorded to 50 s using explosive sources.
                                                                recorded to 50 s using explosive sources.                                               the north end of TIB 3 (adjacent to the
                                                                                                                                                        the north end of TIB 3 (adjacent to the
36. B. Romanowicz, J. Geophys. Res. 87, 6865                    Lines TIB 1 and 2 were collected in 1992
                                                                Lines TIB 1 and 2 were collected in 1992                                                Kangmar dome), some 225 km north of the
                                                                                                                                                        Kangmar dome), some 225 km north of the
    (1982); C. Bandon and B. Romanowicz, ibid. 91,
    (1982);
    (1982): C. Bandon and B. Romanowicz, ibid. 91,
    6547 (1986); K. Y. Chun and T. V. McEvilly, ibid., p.
                                         McEvilly,
    6547 (1986); K. Y. Chun and T. V. Mc Evilly, ibid., p.      with a 120-channel DFS V using a geo-
                                                                with a 120-channel DFS V using a geo-                                                   Himalayan thrust front (Main Frontal
                                                                                                                                                        Himalayan thrust front (Main Frontal
    10405.
    10405.                                                      phone group interval of 50 m and analog
                                                                phone group interval of 50 m and analog                                                 Thrust) but still more than 50 km south of
                                                                                                                                                        Thrust) but still more than 50 km south of
37. R. J. Owens, G. E. Randall, F. T. Wu, R. S. Zeng, Bull.
37. R. J. Owens, G. E. Randall, F. T. Wu, R. S. Zeng, Bull.
                                                                cables. Lines TIB 3-11 were collected in
                                                                cables. Lines TIB 3-11 were collected in                                                the Zangbo suture.
                                                                                                                                                        the Zangbo suture.
    Seismol. Soc. Am. 6, 305 (1993).
                         6, 305
    Seismol. Soc. Am. 6,305 (1993).
38. L. Zhu, R. Zeng, F. Wu, T. J. Owens, G. R. Randall,
38. L. Zhu, R. Zeng, F. Wu, T. J. Owens, G. R. Randall,         1994 with a 240-channel digital telemetry
                                                                1994 with a 240-channel digital telemetry                                                    North of the point where the MHT re-
                                                                                                                                                             North of the point where the MHT re-
    Acta Seismol. Sinica 6, 305 (1993): L. Zhao, M. K.
                                   (1993);
    Acta Seismol. Sinica 6, 305 (1993); L. Zhao, M. K.          system (Wave 3) and had a 25-m group
                                                                system (Wave 3) and had a 25-m group                                                    flection disappears, the seismic sections are
                                                                                                                                                        flection disappears, the seismic sections are
    Sen, P. Stoffa, C. Frohlich, Geophys. J. Int. 125, 355
     (1996).
                                           J. Int.
    Sen, P. Stoffa, C. Frohlich, Geophys. J.lnt. 125,355
                                                                interval resulting in       common spread
                                                                interval resulting in a acommon spread                                                  dominated by a subhorizontal band of
                                                                                                                                                        dominated by a subhorizontal band of
                                                                                                                                                        strong reflections at 5 to 6 s, or at a depth of
    (1996).
39. E. Fielding, B. Isacks, M. Barazangi, C. Duncan, Ge-
39. E. Fielding, B. Isacks, M. Barazangi, C. Duncan, Ge-        length of 6 km. In both cases the nominal
                                                                length of 6 km. In both cases the nominal                                               strong reflections at 5 to 6 s, or at a depth of
    ology 22,163 (1994).
    ology 22, 163 (1994).                                       seismic source consisted of 50-kg seismic
                                                                seismic source consisted of 50-kg seismic                                               about 15 to 18 km (Fig. 1). Hereafter refer-
                                                                                                                                                                                   1).
                                                                                                                                                        about 15 to 18 km (Fig. O. Hereafter refer-
40. Y. Jin, M. McNutt, Y. Zhu, Nature 371, 669 (1994).
40. Y. Jin, M. McNutt, Y. Zhu, Nature 371, 669 (1994).
41. J. P. Burg, P. Davy, J. Martinod, Tectonics 13, 474
                                                                charges in boreholes ~ m deep and at
                                                                                       -50
                                                                charges in boreholes - 50 m deep and at                                                 enced as the "Yamdrok-Damxung reflector"
                                                                                                                                                        enced as the "Yamdrok-Damxung reflector"
41. J. P. Burg, P. Davy, J. Martinod, Tectonics 13, 474
     (1994).
    (1994).                                                     intervals of 200 m; additional 200-kg charg-
                                                                                 m;
                                                                intervals of 200 mj additional 200-kg charg-                                            (YDR), it extends from beneath Yamdrok
                                                                                                                                                        (YDR), it extends from beneath Yamdrok
42. S. Turner et al., Nature 364,50 (1993).
                              364, 50
42. S. Turner et al., Nature 364, 50 (1993).                    es were set off every 3000 m.
                                                                es were set off every 3000 m.                                                           Tso on TIB 5 to the Damxung graben on
                                                                                                                                                        Tso on TIB 5 to the Damxung graben on
43. A. Yin et al., J. Geophys. Res. 99, 18,175 (1994): Y.
                                                 (1994);
43. A. Yin et al., J. Geophys. Res. 99, 18,175 (1994); Y.           The most prominent feature of the seis-
                                                                    The most prominent feature of the seis-                                             TIB 11, a distance of almost 200 km.
                                                                                                                                                        TIB 11, a distance of almost 200 km.
     Pan and W. S. F. Kidd, Geology 20,775 (1992): T. M.
                                      20, 775 (1992);
    Pan and W. S. F. Kidd, Geology 20, 775(1992); T. M.
     Harrison, P. Copeland, W. S. F. Kidd, 0. M. Lovera,
    Harrison, P. Copeland, W. S. F. Kidd, O. M. Lovera,         mic profiles in the Himalaya is a gently
                                                                mic profiles in the Himalaya is a gently                                                     The most striking aspect of the YDR is
                                                                                                                                                             The most striking aspect of the YDR is
                           (1995);
     Tectonics 14, 658 (1995); J. P. Burg, Carte Ge-
    Tectonics 14, 658 (1995): J. P. Burg, Carte Ge-             north-dipping band of reflections extending
                                                                north-dipping band of reflections extending                                             the local occurrence of unusually large am-
                                                                                                                                                        the local occurrence of unusually large am-
    ologique du Sud du Tibet, Institut National
    ologique du Sud du Tibet, Institut National
    d'Astronomie et de Geophysique, C.N.R.S, Paris,
                                                                from ~8 to 12 s across TIB 1, continuing as
                                                                      -8
                                                                from -8 to 12 s across TIB 1, continuing as                                             plitudes, or bright spots (Figs. 1 and 2).
                                                                                                                                                        plitudes, or bright spots (Figs. 1 and 2).
    d'Astronomie et de Geophysique, C.N.R.S, Paris,
                                                      et a/.,
     France, scale 1:: 500,000 (1983); W. S. F. Kidd eta/.,
                       500,000 (1983);
    France, scale 1 :500,000(1983): W. S. F. Kidd et al.,       a more subtle feature on TIB 3 to about 15 s
                                                                a more subtle feature on TIB 3 to about 15 s                                            Amplitudes for the bright spots range from
                                                                                                                                                        Amplitudes for the bright spots range from
    Philos. Trans. R. Soc. London Ser. A 327, 287
    Phi/os. Trans. R. Soc. London Ser. A 327, 287               (Fig. 1). Termed the Main Himalayan
                                                                       1).
                                                                (Fig. O. Termed the Main Himalayan                                                       13 to 22 db above background (Fig. 2)
                                                                                                                                                        13 to 22 db above background (Fig. 2)
    (1988); A. Gansser, Geology of the Bhutan Himalaya
     (1988);
    (1988): A. Gansser, Geology of the Bhutan Hima/aya          thrust (MHT), it is interpreted (2) to be the
                                                                thrust (MHT), it is interpreted (2) to be the                                           whereas amplitudes for the rest of the YDR
                                                                                                                                                        whereas amplitudes for the rest of the YDR
     (Birkhauser, Basel, 1983), p. 181: A. Yin et al., J.
                                     181;
    (Birkhauser, Basel, 1983), p. 181; A. Yin et al., J.
                                                                active decollement along which India is
                                                                active decollement along which India is                                                 generally average about 6 db higher than
                                                                                                                                                        generally average about 6 db higher than
    Geophys. Res., 99 18175 (1994).
    Geophys. Res., 99 18175 (1994).
44. Project INDEPTH was supported by the Ministry of
             INDEPTH
44. Project IN DEPTH was supported by the Ministry of           underthrusting southern Tibet because: (i)
                                                                underthrusting southern Tibet because: 0)  (i)                                          background.
                                                                                                                                                        background.
    Geology and Mineral Resources of China, U.S. Na-
    Geology and Mineral Resources of China, U.S. Na-            it can be extrapolated updip to coincide
                                                                it can be extrapolated updip to coincide                                                     Although several bright spots are con-
                                                                                                                                                             Although several bright spots are con-
    tional Science Foundation Continental Dynamics
    tional Science Foundation Continental Dynamics
     Program, National Natural Science Foundation of
    Program, National Natural Science Foundation of
                                                                with the plane of thrust-type seismicity be-
                                                                with the plane of thrust-type seismicity be-                                            cave in shape (Fig. 1), neither tuning nor
                                                                                                                                                        cave in shape (Fig. 1), neither tuning nor
    China, and the Deutsche Forschungsgemeinschaft
    China, and the Deutsche Forschungsgemeinschaft                                                                                                      fOCUSing are adequate to explain the high
                                                                                                                                                        focusing
                                                                                                                                                        focusing are adequate to explain the high
    and GeoForschungsZentrum Potsdam (GFZ), Ger-
    and GeoForschungsZentrum Potsdam (GFZ), Ger-                 L. D. Brown, M. Hauck, D. A[sdorf, A. Ross, M. Clark,
                                                                                                Alsdorf,
                                                                L. D. Brown, M. Hauck, D. Alsdorf, A. Ross, M. Clark,
                                                                 Institute for the Study of the Continents, Cornell Univer-
                                                                                                                                                        amplitudes. Moreover, each of the bright
                                                                                                                                                        amplitudes. Moreover, each of the bright
    many. We thank the personnel of the 4th and 5th
    many. We thank the personnel of the 4th and 5th
    Geophysical Exploration companies of the Ministry
                                                                Institute for the Study of the Continents, Cornell Univer-
                                                                sity, [thaca, NY 14853, USA.
                                                                       Ithaca,
                                                                sity, Ithaca, NY 14853, USA.                                                            spots corresponds to a negative reflection
                                                                                                                                                        spots corresponds to a negative reflection
    Geophysical Exploration companies of the Ministry
    of Geology and Mineral Resources for their efforts
    of Geology and Mineral Resources for their efforts          Wenjin Zhao and Xianwen Liu, Chinese Academy of Geo-
                                                                                             Liu,
                                                                Wenjin Zhao and Xianwen Uu, Chinese Academy of Geo-                                     polarity (Fig. 3), implying that they repre-
                                                                                                                                                        polarity (Fig. 3), implying that they repre-
    on the behalf of Project INDEPTH. Institute for the
    on the behalf of Project INDEPTH. Institute for the          logical Sciences, Beijing 100037, China.
                                                                logical Sciences, Beijing 100037, China.                                                sent a decrease in seismic velocity or den-
                                                                                                                                                        sent a decrease in seismic velocity or den-
                                                                 K. D. Nelson and M. Cogan, Department of Earth Scienc-
                                                                        Nelson
    Study of the Continents Contribution No. 235.
    Study of the Continents Contribution No. 235.
    Geo[ogica[ Survey of Canada Contribution No.
    Geological
                                                                K. D. Ne[son and M. Cogan, Department of Earth Scienc-
                                                                es, Syracuse University, Syracuse, NY 13244, USA.
                                                                es, Syracuse University, Syracuse, NY 13244, USA.
                                                                                                                                                        sity, or both, at the reflecting interface. In
                                                                                                                                                        sity, or both, at the reflecting interface. In
    Geological Survey of Canada Contribution No.
     1996195.
    1996195.                                                                                                        Geolo-
                                                                Jinkai Che, Bejing Computing Center, Ministry of Geo[o-
                                                                Jinkai Che, Bejing Computing Center, Ministry of Geolo-                                 contrast, parts of the YDR that are not near
                                                                                                                                                        contrast, parts of the YDR that are not near
                                                                gy and Minera[ Resources, Beijing, 100083, China.
                                                                gy and Mineral
                                                                gyand Mineral Resources, Beijing, 100083, China.                                        the bright spots exhibit a positive reflection
                                                                                                                                                        the bright spots exhibit a positive reflection
    2 August 1996; accepted 5 November 1996
             1996;
    2 August 1996: accepted 5 November 1996                     * To whom correspondence should be addressed
                                                                • To whom correspondence should be addressed                                            polarity. The most plausible juxtaposition
                                                                                                                                                        polarity. The most plausible juxtaposition

1688
 1 688                                                          SCIENCE
                                                                SCIENCE      *
                                                                             •   VOL. 274
                                                                                 VOL. 274                            •*   6 DECEMBER 1996
                                                                                                                          6 DECEMBER 1996

								
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