HR: 16:00h
AN: T34A-01 INVITED     [Abstracts]
TI: Constraining the Extent of Crust-Mantle Coupling in Central Asia Using GPS, Geologic, and Shear-Wave Splitting Data
AU: * Flesch, L M
EM: flesch@dtm.ciw.edu
AF: Carnegie Institution of Washington, Dept. of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, DC 20015 United States
AU: Holt, W E
EM: wholt@mantle.geo.sunysb.edu
AF: Dept. of Geosciences, SUNY-Stony Brook, Stony Brook, NY 11794-2100 United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Carnegie Institution of Washington, Dept. of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, DC 20015 United States
AU: Stephenson, M
EM: mas285@cornell.edu
AF: Dept. of Earth & Atmospheric Sciences, Snee Hall, Cornell University, Ithaca, NY 14853 United States
AU: Wang, C
EM: wangcy@cdsn.org.cn
AF: Institute of Geophysics, China Earthquakes Administration, Beijing, 100081 China
AU: Chan, W W
EM: winston@mulitmax.com
AF: Mulitmax Corporation, 1441 McCormick Drive, Largo, MD 20774 United States
AB: We have obtained constraints on mechanical crust-mantle coupling for Tibet and Yunnan/Indo China, by comparing the observed surface deformation field inferred from GPS and Quaternary fault slip rate data, with the mantle deformation field inferred from several SKS shear wave splitting data sets. It was first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction, which is assumed to be parallel to the direction of maximum shear. For asthenospheric flow, we solved for a best-fitting uniform sub-asthenospheric velocity model for Eurasia. The fit, however, was not satisfactory. Solving for separate uniform flow fields in each region improves the fit, although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely. We then considered lithospheric models. For Tibet, vertically coherent deformation (i.e., maximum shear direction from surface deformation is parallel to the fast polarization direction) yields an improved match for left-lateral shear. Both the goodness of fit and the dominance of left-lateral surface faulting in Tibet, argue for a lithospheric source of anisotropy. The misfit for Yunnan is large for either right- or left-lateral shear, which requires a complete crust-mantle de-coupling. Dynamic modeling of Eurasia deformation shows that boundary conditions and topographically induced body forces contribute roughly equally. Because these body forces are applied to the crust, they will only contribute to mantle deformation if crust-mantle coupling is strong. The observed vertical coherence of lithospheric deformation in Tibet thus argues for strong crust-mantle coupling there. Conversely, crust-mantle de-coupling within Yunnan lithosphere makes the specific prediction that mantle deformation would be controlled by boundary conditions alone. To test this, we determined the mantle deformation field generated only by boundary conditions, and found that the fit for Yunnan is much improved for left-lateral shear. The fit is not good for Tibet, further illustrating the need for crust-mantle coupling there. Finally we constructed a "hybrid" lithospheric mantle model, where crustal body-forces were applied in Tibet, but not in Yunnan. The resulting model provides an excellent fit to the entire data set, and is our preferred model. These results have the following implications. First, they imply coherence and strong mechanical coupling for Tibet. This coherence is driven by the dynamics of the upper crust, not the dynamics of the mantle. An upper crust that is substantially weaker than the mantle is not compatible with our results. In addition, they are incompatible with the popular "jelly-sandwich" rheology, and preclude behavior such as large-scale lower crustal flow or mantle delamination. Second, the decoupling in Yunnan implies that the crust is moving south with respect to the mantle at rates as high as ~30mm/yr. Third, there is a fundamental rheological lithospheric transition between Tibet and Yunnan that may provide a key to understanding this significant orogen.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8159 Rheology--crust and lithosphere
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting