Thermal structure and rheology of upper mantle beneath Zhejiang by slappypappy116

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									  VOI. 41   NO.   2                       SCIENCE IN CHINA (Series D)                                                April 1998




                  Thermal structure and rheology of upper mantle
                       beneath Zhejiang Province, China *
            LIN C h u a n ~ o n g($$I$%)',         SHI ~anbin(*Z%$)',               HANXiuling          ($$%@)',
                           CHEN Xiaode ( %$@$)I             and ZHANG Xiao' ou             (!jEtt/J\@)'
             (1. Institute of Geology, State Seismological Bureau, Beijing 100029, China; 2. Institute of Geology,
                                      Chinese Academy of Sciences, Beijing 100029, China)

                                                   Received October 8, 1997


    Abstract       The paleogeotherm derived from spinel and garnet lherzolites xenoliths for the upper mantle beneath Zhe-
    jiang Province, China is higher than the oceanic geotherm but is similar to the geotherm for the upper mantle beneath
    eastern China constructed by Xu et al. and the upper mantle geotherm of southeastern Australia. The crust-mantle
    boundary defined by this geotherm is about 34 km, while the lithosphere-asthenosphere boundary is about 75 km. This
    result coincides well with geophysical data. The study of rheological features of the xenoliths has revealed that at least
     two periods of deformation events occurred in the upper mantle beneath this reglon. The first event might be related to
    upper mantle diapir occurring in this region before or during late Tertiary, and the second might be related to the oc-
    currence of small-scale shear zones in the upper mantle.

    Keywords:         mantle xenolith, upper mantle geotherm, rheology of upper mantle, Zhejiang Province.


     Thermal state and structure of the upper mantle are the critical data for understanding the
structure and dynamics of the lithosphere, and even for its 3-dimensional or 4-dimensional map-
ping[l-31 . The detailed study of mantle xenoliths may provide not only the data for constructing
the thermal structure of the upper mantle but also the data for understanding its rheology. In
Zhejiang Province there is a large areal distribution of Cenozoic basalts, most of which contain
mantle xenoliths of both spinel and garnet lherzolite compositions. Therefore, it is an ideal locali-
ty for studying the above-mentioned topic. The main purpose of this paper is to get insight into
the thermal structure and rheology of the upper mantle beneath this region.
       The samples studied were collected from the Cenozoic basalts at Caijiawan, Xinchang City
and Xilong, Quzhou City. The former is Shenxian Group basalts widely outcropped in Shenxian
and Xinchang area. The age of the basalts was used to be assigned to Pliocene epoch[41. Recently,
however, Liu et al. have pointed out that the basalts in this region can be classified into early
Miocene (having K-Ar age of 2 0 . 7 Ma) and late Miocene to early Miocene (having K-Ar age of
                              .
10.38-4.77 ~ a[51 ) The Xilong basalt appears as a small elliptical pipe of nepheline basalt, 200
m in length and 50 m in width. The age of this basalt is considered to be comparable to Shenxian
Group. A detailed study of the petrology and geochemistry of mantle xenoliths from Xilong basalt
has been carried out by several author^'^,^], but the xenoliths from Xinchang basalts were poorly
reported only in some papers, and a preliminary study has been carried out by the present authors
                    In
r e ~ e n t l y ' ~ ] . previous study, however, no complete and reliable upper mantle geotherm for this
region can be accepted, because of the relatively small amount of samples studied and the

    *   Project supported by the National Natural Science Foundation of China (Grant No. 49472105)
     172                             SCIENCE IN CHINA (Series D)                               Vol. 41


unreliability of the geothermobarometers used for the study. Moreover, the rheology of the upper
mantle has not been studied in detail.

1     Rock type and basic features of xenoliths

      Mantle xenoliths in Xilong basalt have been classified in previous papers into three types as
spinel lherzolite, garnet lherzolite and olivine pyroxenite. It was found that most of the garnet
lherzolite xenoliths here contain a small amount of spinel. Most of our samples are garnet lherzo-
lite except one (ZXL-2) with a small amount of spinel. Mantle xenoliths from Xinchang basalt
comprise mainly spinel lherzolite, garnet lherzolite and a small amount of spinel pyroxenite. The
basic features of the xenoliths are comparable to those of the Xilong xenoliths, and especially the
garnet in garnet lherzolite from both localities has been altered and exhibits pink color, but the
chemical compositions of the core of the grain can still be used to estimate the equilibrated temper-
ature and pressure[6-83 .
      The textures of the xenoliths can be classified into protogranular (coarse granular), porphy-
roclastic, tabular equigranular and the transition between these textures. The porphyroclastic tex-
ture is most common in xenoliths from the studied region, while tabular equigranular texture is
relatively rare, indicating that the xenoliths were subjected to deformation process to different ex-
tent under upper mantle conditions.
       The dislocation substructures of olivine from the xenoliths have been revealed by using the
 oxidation decorating technique. T h e observation has shown that the dislocation substructures of o-
 livine here are similar to those of xenoliths elsewhere in eastern China. They include dislocation
 tilt wall, dislocation loops and subgrain structures typical of high-temperature dislocation creep.
 However, some dislocation substructures representative of high strain rate event such as slip band
  and those representative of lower temperature plastic deformation such as dislocation tangle have
  also been found. All these may indicate that the xenoliths from this region experienced relatively
  complicated deformation history.

 2     Estimation of equilibrium temperature and pressure of xenoliths

 2.1    Selection of geotherrnometers and geobarometers
       More than 20 geothermometers and geobarometers have been proposed for calculating the e-
 quilibrium temperature and pressure of xenoliths. Recently, Brey and ~ o h l e r ' ~ ] assessed ex-
                                                                                         have
 isting geotherrnometers, while XU['O] has tested 17 existing geotherrnometers by using experimen-
 tal data of natural system. Their results have shown that the two pyroxene geothermometers pro-
 posed by Bertrand and Mercier['" are the most reasonable geothermometer at present, while the
 calibrated geothermometer of Nickel et al. [ I 2 ] is also suitable for temperature estimation of natural
 mantle xenolith. In this paper we used these two geothermometers for temperature estimation of
 the xenoliths. In addition, a comparison has been carried out by using geotherrnometers common-
 ly used in previous papers.
       Pressure estimation of garnet lherzolite has been carried out by using Wood' s[l3] and Nickel
 and Green's[14' geobarometers, which were commonly used in current references. As for pressure
 estimation of spinel lherzolite, the single pyroxene geobarometer proposed by ~ e r c i e r " ~ ]which
                                                                                                    ,
  was widely used in previous papers, has recently been considered unreliable. Recently a geo-
 barometer suitable for spinel lherzolite has been proposed by Kohler and re^''^] based on calcium
  No. 2                          THERMAL STRUCTURE & RHEOLOGY O F UPPER MANTLE                                                   173


                                                     e
exchange between olivine and ~ l i n o p ~ r o x e n It. is an only geobarometer that can be used for pres-
sure estimation of spinel lherzolite so far, although it has not been widely accepted. We used this
geobarometer in the present paper for pressure estimation of spinel lherzolite. The results of esti-
mation are listed in table 1.
                Table 1 Physical parameters for the upper mantle beneath Zhejiang Province derived from spinel
                                               and garnet lherzolites xenoliths
   --


   No.        T/C      P/GPa      Z/km      oo/GPa ol/GPa            io/s-'           r)olPa.s-   '       E1/s-I          r)l/Pa.s-l

 ZXC37
 ZXC-38
 ZXC-36
 ZXC-55
 ZXL-22
 ZXC-35
 ZXL-25
 ZXG 13
 ZXL-11
 ZXL- 15
  ZXC-8
 ZXC-52
 ZXG12
  ZXL-5
  ZXC-7
 ZXL-16


  ZXC-46      1 124       1.95       63      0.014     0.063 8.633 X lo-''           5.405 x loZo 1.063 x lo-''               1.976 x 1019
  ZXG48       1128        1.89       61      0.010 0.049 3.607X10-''                                    5.
                                                                                     9 . 2 4 1 ~ 1 0 ~ ~8 3 2 ~ 1 0 - l 3 2 . 8 0 1 ~ 1 0 ' ~
  ZXC-22      1134        1.95       63      0.014     0.075 1 . 2 0 9 X 1 0 ~ ~ ' 3.861X10Z0 2.599X10-l2 9 . 6 1 8 X 1 0 ' ~
  ZXC20       1 135       1.95       63      0.013 0.085 9.858 x lo-''               4.396 x loz0 4.012 x lo-''               7.063 x 1018
   ZXC-1       1 141      2.11       68      0.010     0.063 4.331 x lo-''           7.696~    loz0 1 . 5 6 5 1 0 1 2 1 . 3 4 2 1019
                                                                                                                   ~                    ~
  ZXC-16       1141       2.11       68                                                                  .
                                             0.013 0.080 1 . 0 0 3 ~ 1 0 ' ~4 . 3 2 1 ~ 1 0 ~3~ 3 6 1 x 1 0 - ' ~ 7 . 9 3 3 ~ 1 0 ~ '
  ZXC-43       1143       2.20       71      0.010     0.051 4 . 1 7 8 ~   lo-''     7.978~    loz0 7 . 6 7 7 ~               2.214~    1019
  ZXC-50       1146       2.28       73                                                                                       1. ~
                                             0.012 0.064 7 . 5 5 2 ~ 1 0 - " 5 . 2 9 6 ~ 1 0 ~1~. ~ 6 0 1 x 1 0 ~ ~ 3 3 2 ~ 1 0 ' ~
  ZXC-33       1 148      2.13       62      0 , 0 1 2 0.061 9.563 x l o i 4 4 . 1 8 2 ~       lo2'     1.739~                1 . 1 6 9 1019
                                                                                                                                        ~
  ZXC-41       1150       1.96       64      0.014     0.061 2 . 0 2 7 ~ 1 0 - ' ~ 2 . 3 0 2 ~ 1 0 ~2~. 2 5 1 ~ 1 0 ~ '9~. 0 3 3 ~ 1 0 ' ~
   ZXL-2       1 151      1.88       62      0.009 0.042 5 . 5 7 6 ~ 0 "   1         5.380~    loz0 7 . 7 1 2 ~ 0 - l 3
                                                                                                                   1          1 . 8 1 5 1019
                                                                                                                                        ~
   ZXC-9       1151       1.96       64      0.012 0.075 1 . 2 7 9 ~ 1 0 - ' ~ 3 . 1 2 8 ~ 1 0 ~4~ 5 0 5 ~ 1 0 - ' ~ 5 . 5 5 0 ~ 1 0 ' ~
                                                                                                          .
   ZXC-42      1 153      2.17       70      0.012     0.057 1.077 x 1 0 1 4 3 . 7 1 4 ~       lo2'     1 . 5 7 6 lo-''
                                                                                                                   ~          1.205 x 1019
   ZXN-8       1177       2.30       74      0.010                                                                            ' .
                                                       0.048 1 . 1 2 5 X 1 0 - ' ~ 2 . 9 6 2 ~ 1 0 ~1~. 7 0 3 ~ 1 0 ~ 9 ~ 3 9 4 ~ 1 0 ' ~
  ZXN-20       1197       2.40       77      0.012 0.044 3 . 3 8 0 ~ 1 0 - ' ~ 1 . 1 8 3 x 1 0 ~ ~2 . 1 6 0 x 1 0 - ~ ~ 6 . 7 8 7 x 1 0 ' ~
   ZXL-10      1235       2.38       76      0.010 0.040 6 . 0 6 9 ~ 1 0 - ' ~ 5 . 4 9 9 ~ 1 0 ' ~ 5 . 1 1 9 ~ 1 0 - ' ~ 2 . 6 0 4 ~ 1 0 ' ~
   ZXL-19      1238       2.40       77       0.011 0.037 8.791X10-'4                4.171~10'~4.264~10-'~ 2.892~10'~
   ZXC21       1259       2.25       72       0.014    0.075 4 . 0 8 4 ~ 1 0 - ' ~ 1 . 1 4 3 ~ 1 0 ' ~ 8 . 7 8 4 ~ 1 0 - " 2 . 8 4 6 ~ 1 0 "
   ZXG17       1295       2.42        78      0.011    0.086 4.303X10-l3 8.521X 10"                     3 . 1 0 2 X 1 0 - ' ~ 9.241x1016
       T, Temperature estimated using Bertrand and Mercier's geothermometer; P, pressure estimated using Kohler and ~ r e y ' s ' ~ '
 (above solid line)[14' and (below solid line) geobarometers; 2 , depth inferred from equation: Z = 4 . 2 + 3 0 . 3 P , where P is pres-
 sure in GPa; oO. differential stress estimated from paleoblast grain size; 01, differential stress estimated from neoblast grain size; s o
 and 70, strain rate and equivalent viscosity estimated using oo stress value; i l and 71, strain rate and equivalent viscosity estimated
 using ol stress value.

 2.2   Chemical compositions of minerals in xenoliths
     Microprobe analyses of chemical compositions of minerals from 18 spinel lherzolite and 20
 garnet lherzolite samples have been carried out. Multi-points detection has been carried out within
    174                                 SCIENCE IN CHINA (Series D)                            Vol. 41


a single grain in order to determine the homogeneity of the compositions of the mineral. The anal-
ysis has shown that no composition zoning has occurred in these grains, and the data can be used
to estimate the equilibrium temperature and pressure of the xenoliths studied. A special pro-
gram'11 has been used in the detection of Ca content in olivine, in order to use the geobarometer of
Kohler and Brey (1990) for pressure estimation of spinel Iherzolite. All analyses were carried out
using a Cameca SX50 electron microprobe at Institute of Geology, Chinese Academy of Sciences.
Several hundreds data have been obtained, but have not been listed in this because of the limit of
space.

3    Estimation of rheological parameters of the upper mantle

      As has been noted above, mantle xenoliths in this region experienced a relatively complicated
deformation history. In order to get better understanding of the deformation conditions and histo-
ry, the relevant deformation parameters including differential stress, strain rate and viscosity
should be determined.

3.1       Estimqtion of differential stress
     The differential stress during the deformation of xenoliths can be estimated by using mi-
crostructural piezometers, such as free dislocation density, recrystallized grain size, subgrain size,
spacing of dislocation walls. We used here recrystallized grain size and spacing of dislocation walls
as piezometers . As was noted above, the xenoliths here exhibit predominantly the transition from
protogranular to porphyroclastic texture. The grain size of olivine in these xenoliths shows a bi-
modal distribution, representative of coarser porphyroclast (paleoblast) and smaller recrystallized
grain (neoblast) . The measurement has shown that the two grain sizes are relatively homogeneous
(with standard deviation of less than 20% ) . Ave Lallemant et al. [ I 7 ' have pointed out that the
homogranular textures of paleoblast are representative of steady-state conditions in the upper man-
tle, and the grain size yield stress compatible with large scale, steady state flow in the upper man-
tle. The superimposed microstructures, including neoblasts, would appear to be related to later
deformation process. We have measured the two kinds of grains separately, and used them to esti-
mate the differential stress of two different stages of deformation. The paleopiezometer we used is
proposed by Ross et al. [ l a ' . Moreover, the spacing of dislocation walls is also commonly regarded
as a useful paleopiezometer, which yields stress representative of later stage of deformation, just
like the neoblasts. We used here paleopiezometer proposed by ~ o r i u m i ' l ~ ' .
          The results show that the paleoblasts yield stress much lower than neoblasts and spacing of
dislocation walls. The value is within a range of 6-16        MPa, and is relatively stable. As was not-
ed by Ave Lallemant et al. , this relatively stable low stress might be representative of large-scale
steady-state flow of the upper mantle. The neoblast and the spacing of dislocation walls yield
much higher stress with greater variation. T h e neoblasts yield stress within the range of 30-100
MPa, while the spacings of dislocation walls yield stress value of 10-80        MPa for spinel lherzolite
and 50-290 MPa for garnet lherzolite. We believe that the stress obtained from the two piezome-
ters might represent the stress values of two different deformation events, or at least the different
stages of one event. This conclusion, however, still cannot be confirmed, as significant differ-
ences may arise when using different piezometers. It is commonly accepted that the recrystallized
grain size of the various paleopiezometers is the most reliable. We will use and discuss here only
    No. 2                THERMAL STRUCTURE & RHEOLOGY O F UPPER MANTLE                                                175


the stress estimated by recrystallized grain size (table 1 ) .

3.2    Estimation of strain rate and equivalent viscosity
      Once the P, T and differential stress were determined, the strain rate can be calculated
through the high temperature flow law of peridotites. We used the equation proposed by Cabanes
and ~ r i ~ u e " ~ ]
                :
                        E = 2 . 6 9 x 1 0 ~ ~ e- (63000 + 1 6 1 0 P ) / T ] a3.',
                                                   x~[
where E is strain rate (in s - I ) , P is pressure in GPa, T is temperature in K, and a is differential
stress in GPa. The equivalent viscosity can be calculated based on the equation 7 = a / 3 E . The re-
sults of the calculation are listed in table 1 . In table 1 we used two different stress to calculate the
strain rate and equivalent viscosity. They should represent the strain rate and viscosity of the
large-scale steady state flow and the later deformation event, respectively. As has been noted
above, no chemical composition zoning in the same grain has been found, and even the chemical
compositions of paleoblasts are consistent with those of the neoblasts. It seems, therefore, that
the two deformation events occurred under the same temperature condition, i. e. after the diapir
when a new equilibration was reached.

4     Construction of upper mantle geotherm for Zhejiang Province and its geophysical implication

4.1     Upper mantle geotherm for Zhejiang Province
      The upper mantle geotherm for Zhejiang Province can be constructed on the basis of the P
and T estimates of mantle xenoliths listed
in table 1( fig. 1) . From table 1 and fig.
1, we can see that the constructed
geotherm is within the range of 900-
1 295°C . The temperature range of spinel
lherzolite is 900-1          130"C, while that of
garnet lherzolite is 1 130-1 295°C . Fig. 1
also shows the transition curve from spinel
lherzolite to garnet lherzolite according to
0' ~ e i l l ' ~ " .I t can be seen that almost all
the data points of spinel lherzolite fall into
the stable field of spinel lherzolite, except
one point which falls just on the transition
curve. All the data points of garnet lherzo-
lite fall into the stable field of garnet lherzo-
lite. All these indicate that the data used     Fig. 1 . A xenolith-derived geotherm ( a ) and the structures ( b ) of
here are reasonable and reliable. The upper     the upper mantle of Zhejiang Province. The spinel lherzolite to garnet
                                                lherzolite transition is taken from the data of 0' ~ e i l l ' ~ ' ]The solid
                                                                                                                     .
         geothem for eastern China 'On-         line is the obtained geotherm for Zhejiang Province, while the dash
structed by XU et al. ['I is also shown in      line is the geotherm for eastern China proposed by Xu et al. ['I. The
fig. 1 for              ~h~ geotherm in the     histogram in the figure can be used to infer crust-mantle boundary,
                                                where the blank column represents spinel lherzolite while the hatched
present paper is slightly lower than that of    column represents garnet Iherzolite. 0 , Data points of spinel Iherzo-
xu   et al. , and there is no inflection at     Iite xenoliths; A , data points of garnet lherzolite xenoliths. CMB:
                                                crust-mantle boundary; the lowest dash line in fig. l ( b ) is believed to
about 60 km depth. The first difference         be the top of asthenosphere.
    176                                 SCIENCE IN CHINA (Series D)                             Vol. 41


can easily be explained by the fact that the geotherm proposed by Xu et al. has taken eastern Chi-
na as a whole. In fact, there are significant differences among different parts of eastern China.
The second difference is possibly caused by the lack of data of Xu et al. at the transition from
spinel lherzolite and garnet lherzolite, and the inflection based mainly on extrapolation. According
to present geotherm no significant inflection has occurred.

4.2       Geophysical implication
      A detailed discussion was given by Xu et al. in their paper about the geophysical implications
of the constructed geotherm for eastern China. As was noted above, the present geotherm is close
to that of Xu et al., except the inflection in their geotherm. Obviously, the present geotherm is
also higher than that of oceanic geotherm and is much higher than that of the Kaapval craton, but
is similar to that for southeastern ~ u s t r a l i a ' ~ 'It is commonly accepted that this high thermal
                                                           .
state may be caused by the deep-seated magmatic process, mantle upwelling (crustal thinning)
and increase of heat flow in the lithosphere. In Zhejiang Province, we believe that mantle up-
welling (diapir) was the main cause for the high geotherm in this region.
     The crust-mantle boundary can roughly be inferred from the constructed geotherm. As the
top of the upper mantle consists of spinel lherzolite, the smallest depth ( t h e lowest temperature)
for spinel lherzolite must represent the crust-mantle boundary['321. From the histogram in fig. 1
we may infer that the crust-mantle boundary in this region is at about 34 km. This result is con-
sistent basically with the recent results of geophysical prospecting[221, in which the crustal thick-
ness for this region determined by gravity data is about 33. 2 km. Fig. 1 also shows that the
boundary between spinel lherzolite and garnet lherzolite is at about 60 km depth (the correspond-
ing temperature is about 1 130°C ), which is in good agreement with the result of Xu et al. ['I. It
is suggested that within the depth range of 34-60         km, the upper mantle comprises mainly spinel
lherzolite, while below 60 km it consists mainly of garnet lherzolite. I t is usually considered that
the temperature of the asthenosphere should be higher than 1 200°C , at which T >O. 7 T,(melt-
ing temperature of peridotite) and peridotite will be softened by plastic flow. If we take this tem-
perature as the boundary between the lithosphere and the asthenosphere, then the top of astheno-
sphere in this region is at about 75 km, and the main composition is still garnet lherzolite. This
may indicate that asthenospheric diapir had occurred in this region before late Tertiary (mantle
xenoliths were entrained by basaltic magma during late Tertiary).

5     Rheological features of the upper mantle beneath Zhejiang Province

          Based on the rheological parameters listed in table 1, a profile of rheological parameters ver-
sus the depth can be constructed. As has been pointed out above, at least two deformation events
can be recognized in the upper mantle of this region. The two events, therefore, should be con-
sidered separately. Fig. 2 shows the results. ( a ) , ( b ) and ( c ) represent the earlier deformation
process, while ( d ) , ( e ) and ( f ) represent the later event. Fig. 2 ( a ) shows that the differential
stress for the earlier event for garnet lherzolite is slightly greater than that for spinel lherzolite.
The stress was relatively low and stable, within the range of 5-15 MPa, decreasing with depth.
Fig. 2 ( b ) and (c) show that the strain rate increases with depth, while the equivalent viscosity
decreases with depth, showing a good linear correlation. Fig. 2 ( d ) , ( e ) and ( f ) show that
during the later deformation process, the differential stress for garnet lherzolite shows a clear
  No. 2                      THERMAL STRUCTURE & RHEOLOGY O F UPPER MANTLE                                                177


tendency to decrease with increasing depth, but this is less clear in spinel lherzolite xenoliths. The
strain and viscosity profiles show slight inflection between garnet lherzolite and spinel lherzolite.
It seems that there were some differences in the later deformation process between spinel lherzolite
and garnet lherzolite.




                             u, - u,/MPa                  -log t / s - '               log tl/Pa. s




                             01 -   as/MPa                 -log i / s - '               log ?/Pa.s
   Fig. 2. The differential stress, strain rate and equivalent viscosity profiles for the upper mantle of Zhejiang Province.
   (a), (b) and ( c ) are for the early deformation stage; while ( d ) , (e) and (f) for the later deformation stage.
     The lithosphere-asthenosphere boundary can also be inferred from these figures. It is com-
monly accepted, that the strain rate value for the asthenosphere is higher than 10-14s-1, while
the viscosity is lower than 1 0 ~ ~ .~ For - the earlier deformation event, these values will com-
                                         s    a
pletely be obtained at 1 200"C, corresponding to a depth of about 75 km, which is in good agree-
ment with the conclusion obtained from geotherm. When considering the high stress level for the
late deformation process, however, we found that the temperature and depth at which the strain
rate and viscosity reach the value for asthenosphere will be greatly decreased, i. e. at about
1050°C or 55 km depth (fig. 2 ( e ) , ( f ) ) . This may indicate that after the diapir of the upper
mantle in this region, a high strain rate and low viscosity zone occurred within the range of 20 km
above the top of the asthenosphere due to local high stress level. Obviously, this low viscosity
zone differs from asthenosphere, so that it is reasonable to regard it as "anomalous mantle". In
consideration of the significant variation of the differential stress even at the same depth, we be-
lieve that this was a small scale and locally existed zone. Nicolas et al. [231 in their study on Massif
Central volcanic region have refined the original hypothesis of one large-scale diapir and hhve sug-
gested the existence of several smaller diapirs to explain this deformed mantle. Recently,
             ' ~ pointed out that it is simpler and more realistic to think of the lithospheric mantle
~ o w n e s has ~ ~
as containing numerous shear zones resulting from lithospheric thinning and rifting. Our result
has shown that the mantle diapir in this region should be a small-scale one, while the low viscosity
zone might be a series of shear zone as proposed by Downes. I n addition, recent geophysical study
in this region also supported this conclusion. For example, the comprehensive geophysical
prospecting in ~ u n x i - ~ e n z h o u [ ~ ~ ] revealed that a high conductivity layer (asthenosphere)
                                        area has
     178                                        SCIENCE IN CHINA (Series D)                                                Vol. 41


is pervasively observed in Zhejiang Province. It has an average buried depth of 100 km, decreas-
ing from west to east, and the depth has become 65 km to the sea shore. Asthenospheric up-
welling has been recognized in 4 low resistivity zones. Obviously, the 4 upwellings of astheno-
sphere should correspond to the small-scale diapirs.

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