HR: 1330h
AN: S22A-0407    [PDF]
TI: Constraining the Vertical Coherence of Deformation 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, Department 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: Department of Geosciences, SUNY at Stony Brook, Stony Brook, NY 11794-2100 United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, DC 20015 United States
AU: Stephenson, M
EM: stephens@mantle.geo.sunysb.edu
AF: Department of Geosciences, SUNY at Stony Brook, Stony Brook, NY 11794-2100 United States
AB: First-order constraints on the depth dependence of lithospheric strength are provided by the degree of vertical coherence between crustal and mantle deformation. We evaluate the level of vertical coherence for the Tibetan Plateau, and off-plateau region of Yunan, by comparing the strain-rate field that has been calculated for the surface, and the mantle field inferred from mantle anisotropy. A continuous surface strain-rate field is determined from GPS observations and Quaternary fault-slip rates that are interpolated using continuous bi-cubic spline polynomials. Point estimates of the mantle finite-strain field are inferred from measurements of the shear wave splitting fast polarization directions, $\phi$, of core phases, which are assumed to denote the orientation of shear in the lithospheric mantle under transpressional deformation. We evaluate the surface field at the locations of splitting measurements, and calculate a surface-derived prediction of $\phi$, $\phi_{s}$, assuming that the finite-strain shear orientation corresponds to the no-length change orientation. We then use the difference angle $\Delta\phi_{s}$ =$\phi$-$\phi_{s}$ as a measure of vertical coherence. On the Tibetan plateau, vertical coherence is remarkably high under left-lateral shear; the RMS value of $\Delta\phi_{s}$ is less than $10\deg$, which approximately corresponds to the expected uncertainty in the splitting measurements. In contrast, $\Delta\phi_{s}$ is very large for the off-plateau region for either right ($52\deg$) or left lateral ($43\deg$) shear, signifying the absence of vertical coherence. We calculate the mantle strain-rate field assuming that the Indian plate, Tarim Basin, south China Block, Ordos block, and Sunda block, represent rigid lithospheric blocks. GPS measurements are used to define the rotations of these bounding blocks, and we solve for a continuous strain rate field in the deforming interior region. We calculate predicted values of $\phi$, $\phi_{m}$, where splitting measurements are available. The most dramatic change, compared to surface field, is that in Yunan, $\Delta\phi_{m}$ is less than $10\deg$, suggesting that boundary conditions alone are enough to predict mantle deformation field off the plateau. For Tibet, the fit is degraded, compared to predictions from the surface field, with $\Delta\phi_{m}$ = $22\deg$, suggesting a need for a second mantle deformational driving force, namely a contribution from body forces. Indeed previous dynamic modeling of Tibet shows that the surface deformation field (and hence the mantle field) requires a significant body-force contribution. Given that the crust and mantle have distinct strain-rate and velocity fields, at least in certain regions, we next seek to quantify the horizontal shear in the decoupling zone between the two. For this purpose, we determine a mantle velocity field consistent with the splitting observations by again applying the GPS-inferred rotations of the rigid blocks, and also impose the shear wave splitting measurements as directions of no length change in the inversion. With continuous surface and mantle velocity fields we can then calculate a differential velocity field between the crustal and mantle layers. By assuming different thicknesses and viscosity values for the decoupling zone, we calculate the shear strains and resulting shear tractions. These stresses are then compared to the observed surface deformation to place bounds on viscosity values within the decoupling zone.
DE: 8110 Continental tectonics--general (0905)
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Seismology [S]
MN: 2003 Fall Meeting