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					                                                                                  How close is the link between plate tectonics and magmatism in western North America?
                                                                                                                                                                                                                                                                                               Allen F. Glazner
                                                                                                                                                    Dept. of Geological Sciences, CB#3315, University of North Carolina, Chapel Hill, NC, 27599 USA; afg@unc.edu

                         The Curious and Enigmatic Links Between                                                                                             Space-Time Patterns Seen in the NAVDAT Dataset                                                                                                                    Data from Mexico--Evidence for a Sweep?                                                                                                  What Triggered the Return of Magmatism?
                                Tectonics and Magmatism                                                                                        Several space-time patterns are evident in the NAVDAT animations. These include:                                                                           The dataset for Mexico is far less complete that that for the U.S., but several interesting patterns are apparent. I constructed a plot
                                                                                                                                                                                                                                                                                                          showing distance of magmatism from the continental margin vs. age by projecting data onto a line parallel to the Gulf of California.                                                              When magmatism resumed after MFZ passage it formed east-west
 Most magmatism on Earth is assumed to be driven by plate tectonic processes. Since the widespread acceptance of plate tec-                    1) A sweep from Montana into Nevada from 50 to about 20 Ma.                                                                                                Data from the Trans-Mexican Volcanic Belt were omitted. Evidence for an inland sweep in the early Tertiary is weak, although it will                                                              belts along the trace of the subducted MFZ, nearly perpendicular to
 tonics about 35 years ago, magmatism in western North America has been explained by subduction along the west coast of                                                                                                                                                                                   become stronger when Mesozoic data are added. There is a significant pattern of westward younging that is also evident in the ani-                                                                the trench, and continued for 15-20 m.y. Although noted long ago
 North America and destruction of the subduction system by development of the San Andreas transform fault system. However,                     2) A clockwise sweep around the Colorado Plateau from New Mexico to southern Nevada, from about 30 to 15 Ma.                                               mations. It appears from this analysis of limited data that there are patterns in Mexico consistent with changes in slab dip.                                                                     (Glazner and Supplee, 1982; Glazner and Bartley, 1984), the cause of
 re-analysis of space-time patterns of magmatism in western North America calls many of the classic patterns of magmatism into                                                                                                                                                                                                                                                                                                                                                                              this return is not known. In the early and middle Tertiary, subducted
 question.                                                                                                                                     3) A burst of magmatism at about 16 Ma in northern Nevada, followed by outward sweeps to Yellowstone, southwestern Oregon,                                                                                                                                                                                                                                   oceanic lithosphere north of the MFZ was ~15 m.y. older than that to
                                                                                                                                               and the central Sierra Nevada.                                                                                                                                                                                                                                                                                                                               the south, and after ~28 Ma this older lithosphere abutted the slab
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            window on the south. It is possible that the return was caused by
                                                                                                                                               4) A burst of magmatism in the Sierra Nevada at 3.5 Ma.                                                                                                                                                                                                                                                                                                      establishment of normal subduction south of the MFZ, but this expla-
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            nation does not account for the extensive trench-perpendicular belts
                                                                                                                                               5) Several more local migrations, including from Phoenix north onto the Colorado Plateau and from the San Francisco Bay area                                                                                                                                                                                                                                 of magmatism, nor for continuation of magmatism after the slab win-
                                                                                                                                               north to the Geysers geothermal field.                                                                                                                                                                                                                                                                                                                       dow formed. Magmatism may have resumed owing to formation of the
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            slab window, but this does not account for magmatism that occurred
                                                                                                                                               These patterns are best seen in computer animations (see computer).                                                                                                                                                                                                                                                                                          before the slab window formed (figure at left). The triggering event
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            remains a geodynamic puzzle.




                                                                                                                                                                                                                                                   Montana
                                                                                                                                                                                                      Washington
                                                    Coney and Reynolds (1977)
                                                    compiled age data on igne-
                                                    ous rocks in the southwest-               Dickinson and Snyder (1979) extended
                                                    ern U.S. and proposed an                  Atwater’s analysis and proposed that develop-
                                                    inland sweep during the
                                                    Laramide orogeny, followed
                                                                                              ment of a slab window should have affected
                                                                                              magmatism, causing an expanding area of
                                                                                                                                                                                                        Oregon
                                                                                                                                                                                                                             Idaho
                                                                                                                                                                                                                                                             Wyoming
                                                                                                                                                                                                                                                                                                                                                                                                                                                                      Does This Trend Continue Into Mexico? (No)
                                                    by a rapid westward sweep                 basaltic volcanism and progressive northward
                                                    during the middle Cenozoic.               shutoff of the ancestral Cascades arc.
                                                    They related these sweeps                                                                                                                                         Nev.
                                                                                                                                                                                                                                           Utah
Plate tectonic analysis of Atwater (1970). This     to changes in dip of the
                                                                                                                                                                                                                                                                       Colorado
paper laid the groundwork for understanding how     subducted slab.
magmatism should have been controlled by plate-
tectonic setting and predicted widespread subduc-
tion-related magmatism before development of the                                                                                                                                                                                                                                                                                                                                                                                                                                                                      The figure at left shows a latitude-age plot for Mexico
San Andreas system.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      data in NAVDAT, with the U.S. plot joined at the same
                                                                                                                                                                                                    California
                                                                                                                                                                                                                                                  Arizona
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      scale. The MFZ pattern that seems so clear in the U.S. is
                                                                                                                                                                                                                                                                        New
                                                                                                                                                                                                                                                                        Mexico
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      not apparent in Sonora and surrounding states. Full anal-
   Analysis of the developing NAVDAT dataset, using computer animation to illuminate space-time patterns, demonstrates that:                                                                                                                                                                                            Where was the Subduction-Related Magmatism?                                                                                                                                                   ysis awaits a fuller Mexican dataset, but it seems unlikely
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      that the pattern will develop there.

   • Subduction-type (e.g., intermediate) volcanism is poorly linked to the subduction system; there was little magmatism in the
      southwestern U.S. during the predicted time of subduction.
                                                                                                                                                                                                                                                                                                          In order to study how magmatism changed during the transition from
   • There is little evidence that slab windows controlled magmatism.                                                                                                                                                                                                                                     subduction to transform motion, I constructed a subset of the NAVDAT
   • Magmatism was clearly migratory in the Cenozoic, but not in ways that can be explained by plate-tectonic processes.                                                                                                                                                                                  dataset using only samples from within the region below, which is
   • Magmatism was migratory at length scales ranging from 1000s of km (continental) to 10s of km (county).                                                                                                                                                                                               essentially the Basin and Range of the United States. This subset
                                                                                                                                                                                                                                                                                                          avoids samples from west of the San Andreas fault, because these
   • In the southwestern U.S. there is strong evidence that magmatism began when the subducted Mendocino fracture zone
                                                                                                                                                                                                                                                                                                          have been significantly displaced since they were emplaced, and sam-
      passed under an area; however, this trend is not seen in northern Mexico.
                                                                                                                                                                                                                                                                                                          ples from the Rio Grande Rift region, which has had a long and com-
   • There is support for a “Coney-Reynolds” pattern in northern Mexico.                                                                                                              Changes in Subduction Angle?                                                                                        plicated history that seems to be different from that of the Basin and
                                                                                                                                                                                                                                                                                                          Range.
                                                                                                                                              The figure belows shows the NAVDAT dataset for the U.S. plotted as age vs. longitude, equivalent to the Coney and Reynolds
                                                                                                                                              (1977) plot*. Several features deserve mention. First, although there is a general trend from older (Mesozoic) rocks in the west to
                                                                                                                                              younger (Cenozoic) rocks in the east, the trend is quite scattered. Second, the correspondence of the much more extensive
                                                        What is NAVDAT?                                                                       dataset with the Coney-Reynolds swath is not very strong. Third, there is a younging trend from east to west, largely reflecting the

 NAVDAT is a comprehensive on-line database of geochemical and geochronological data for igneous rocks in western North
                                                                                                                                              clockwise sweep around the Colorado Plateau. Finally, note that Coney and Reynolds ignored ages younger than 15 m.y.; includ-
                                                                                                                                              ing these younger ages greatly smears out the pattern.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        Conclusions
 America. The dataset is currently being served from GEON (navdat.geongrid.org). The Earthchem site (earthchem.org) provides
 a common portal for NAVDAT, GEOROC, and PetDB.                                                                                               I conclude that evidence for a Coney-Reynolds sweep in the U.S. is not strong.                                                                                                       Effect of the Mendocino Fracture Zone                                                                            • Many of the expected plate-tectonic patterns of magmatism are not apparent in maps and animations drawn using the NAVDAT
                                                                                                                                                                                                                                                                                                                                                                                                                                                    dataset.
                                                                                                                                              *Pre-Cenozoic points on this figure are mostly from USGS Digital Dataset 14; the Cenozoic record is far more complete.                                      Latitude-age plots of for this region (below) show a curious feature. In the southern U.S. Basin and Range, the region inland of the      • A “Coney-Reynolds” magmatic sweep is only weakly shown in the U.S. by the expanded dataset, but such a sweep is stronger in
 NAVDAT is expanding rapidly. It currently contains data for about 10,000 Cenozoic rocks from the western U.S. and Mexico;
                                                                                                                                                                                                                                                                                                          subduction boundary showed little magmatism (“subduction hole” region below and in following figure), and the onset of                    the relatively limited Mexican dataset.
 about half of those entries include geochemical data. In addition, the NAVDAT portal allows searching of the PETROS geochem-
                                                                                                                                                                                                                                                                                                                                                                                                                                                    • Much of the U.S. Basin and Range was devoid of magmatism in the early Cenozoic, supporting widespread shallow subduction

                                                                                                                                                                                                        Los Angeles
                                                                                                                                                                                                                                                                                                          magmatism at any given latitude coincided in time with passage of the subducted Mendocino fracture zone (MFZ) past that latitude.




                                                                                                                                                                                                                                                                                  Las Cruces
 ical database, USGS geochronological dataset DDS-14, and New Mexico geochronological dataset DDS-DB1.




                                                                                                                                                                                                                               Las Vegas
                                                                                                                                                                                                                                                                                                          The MFZ was oriented approximately east-west (see Dickinson and Snyder figure, above left, and Atwater inset in next column).             then.




                                                                                                                                                                                                                                                             Tucson
                                                                                                                                                                                                                                                                                                          Lack of magmatism in the subduction hole is consistent with widespread shallow subduction; if true, then the event must have been         • Magmatism returned to the southern U.S. Basin and Range as the Mendocino fracture zone swept northward; the triggering
                                                                                                                                                                                                                                                                                                          much more extensive than the limited area of shallow subduction proposed by Saleeby (2003).                                               mechanism remains an unsolved geodynamic problem.




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   References Cited
                                                                                                                                                                                                                                                                                                                                                                                                                                                    Atwater, T., 1970, Implications of plate tectonics for the Cenozoic tectonic evolution of western North America: Geological Society
                                                                                                                                                                                                                                                                                                                                                                                                                                                    of America Bulletin, v. 81, p. 3513-3536.
                                                                                                                                                                                                                                                                                                                                                                                                                                                    Coney, P. J., and Reynolds, S. J., 1977, Cordilleran Benioff zones: Nature, v. 270, p. 403-406.
                                                                                                                                                                                                                                                                                                                                                                                                                                                    Dickinson, W. R., and Snyder, W. S., 1979, Geometry of subducted slabs related to San Andreas transform: Journal of Geology, v.
                                                                                                                                                                                                                                                                                                                                                                                                                                                    87, p. 609-627.
                                                                                                                                                                                                                                                                                                                                                                                                                                                    Glazner, A. F., and Bartley, J. M., 1984, Timing and tectonic setting of Tertiary low-angle normal faulting and associated magmatism
                                                                                                                                                                                                                                                                                                                                                                                                                                                    in the southwestern United States: Tectonics, v. 3, p. 385-396.
                                                                                                                                                                                                                                                                                                                                                                                                                                                    Glazner, A. F., and Supplee, J. A., 1982, Migration of Tertiary volcanism in the southwestern United States and subduction of the
                                                                                                                                                                                                                                                                                                                                                                                                                                                    Mendocino fracture zone: Earth and Planetary Science Letters, v. 60, p. 429-436.
                                                                                                                                                                                                                                                                                                                                                                                                                                                    Saleeby, J., 2003, Segmentation of the Laramide slab; evidence from the southern Sierra Nevada region: Geological Society of
                                                                                                                                                                                                                                                                                                                                                                                                                                                    America Bulletin, v. 115, p. 655-668.

				
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