HR: 09:00h
AN: U51B-04    [Abstracts]
TI: Present-Day Microplate Tectonics of Tibet and its Relation to Rheological Stratification and Flow in the Lithosphere
AU: * Thatcher, W
EM: thatcher@usgs.gov
AF: U. S. Geological Survey, MS/977 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: Site velocities from 288 Global Positioning System (GPS) stations are used to construct a 10-element quasi-rigid block model of the Tibetan Plateau and its surroundings. Rigid rotations of 5 major blocks are well determined and average translation velocities of 5 smaller blocks can be constrained. Where data are well distributed the velocity field can be explained well by rigid block motion and fault slip across block boundaries. Residual misfits average 1.6 mm/yr compared to typical one standard deviation velocity uncertainties of 1.1 mm/yr. Any residual internal straining of the blocks is small and heterogeneous. Residual substructure might represent currently unresolved motions of smaller blocks. However if so, such blocks must move at nearly the same rate as the larger block within which they lie. Predicted relative motions between blocks agree with the observed sense of slip and along-strike partitioning of motion across major faults. However, predicted slip rates across Tibet's major strike-slip faults are low, only 5-12 mm/yr, a factor of 2-3 smaller than most rates estimated from fault offset features dated by radiometric methods as ~2000 to ~100,000 year old. Previous work has suggested that both GPS data and low fault slip rates are incompatible with rigid block motions of Tibet. The results reported here overcome these objections and provide strong support for the block model. Space geodetic data alone provide only very limited constraints on the depth to which inferred block structure extends and how deformation beneath the blocks is accommodated. With several well-known caveats, seismic shear-wave (SKS) splitting observations may provide better constraints on flow at depth related to present-day surface deformation. Previous studies have suggested correlations between orientation of fast S-wave speed (`fast S orientations') in central and northern Tibet and strain axis orientations determined from present-day surface deformation or integrated deformation calculated from dynamic models for Tibet. These studies, based on continuum model assumptions, have concluded that fast S and strain axis orientations agree, suggesting coherent, coupled deformation through most of the thickness of the lithosphere. Comparison of fast S orientations and relative motions derived from the GPS microplate model suggest another interpretation. First, fast S orientations within identified microplates are nearly parallel to predicted block motions relative to Eurasia (rms misfit=14 deg.). Furthermore, fast S orientations across major strike-slip faults (Jiali, Kunlun) are parallel to predicted relative motions across block boundaries. Together these correlations suggest present-day microplate motions drive or resist more continuous ductile flow at depth. Because of the limited resolution of the S split observations, the depth at which block-like motion yields to ~continuum flow is poorly constrained. However, the source of splitting is unlikely to be in the asthenosphere, because correcting Tibet-Eurasia block motions to a hotspot reference frame (HS3-NUVEL1A) significantly degrades the match of `absolute' block motion to fast S orientations (rms misfit=40 deg.). The 1-2 sec magnitude of SKS split times seems to require significant flow in the mantle lithosphere, so ~rigid block motion of the crust could be directly coupled to upper mantle flow. However, crustal anisotropy may also be important, and low seismic velocity zones in the crust may mimic lower viscosity, so coupling between upper crustal blocks and ductile deformation of mantle lithosphere could occur via lower crustal flow.
DE: 1209 Tectonic deformation (6924)
DE: 1236 Rheology of the lithosphere and mantle (7218, 8160)
DE: 7205 Continental crust (1219)
DE: 8110 Continental tectonics: general (0905)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Union [U]
MN: Fall Meeting 2005