HR: 16:15h
AN: V14A-02 INVITED     [Abstracts]
TI: Geophysical Constraints on Present-Day Surface Deformation and Flow at Depth Beneath the Continents
AU: * Thatcher, W
EM: thatcher@usgs.gov
AF: U. S. Geological Survey, MS/977 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: Space geodetic measurements are quantifying the surface deformation of the continental lithosphere worldwide but cannot constrain its downward extension into the lower crust and lithospheric mantle. Seismic, geothermal, and geochemical data can ameliorate this shortcoming and provide bounds on the rheology and deformation of the deeper lithosphere. Large GPS datasets from Tibet, Aegean Greece and Turkey, the western U. S. and elsewhere are rapidly mapping continental deformation on scales from 10s to 1000s of km at high precision. Using the locations of major faults to define block boundaries, the GPS data are fit well to first order using the same rules of rigid plate kinematics that have been so successfully applied on a global scale. However, small departures from perfect rigidity are evident in the data and indicate isolated regions of internal deformation or smaller blocks unresolved by current data. Geodetic measurements provide some further constraints on the depth to which inferred block structure extends and how deformation beneath the blocks is accommodated. Depth estimates for earthquake slip on block-bounding faults indicate block structure persists through the 10-20 km thickness of the seismogenic upper crust. Interpretation of transient deformation occurring up to decades following major earthquakes suggests the crust is stronger than the upper mantle in many regions, though lower crustal flow on longer timescales at higher viscosities is not precluded. Seismological data complement space geodetic observations by providing images of deep crust and mantle velocity structure that may mimic rheological layering and flow of the lithosphere beneath crustal blocks. For example, in regions of over-thickened crust beneath the Tibetan Plateau and Bolivian Altiplano, crustal low velocity zones and anisotropic S-wave velocities may define regions of weak, ductile lower crust where flow is localized. S-wave splitting observed from SKS body phases map anisotropy related to current or fossil flow fabric in the crust, mantle lithosphere, and asthenosphere. On the Tibetan Plateau, agreement between S-anisotropy orientations and predicted block motions suggests that the crustal blocks drive or drag ductile flow in the underlying lithosphere. Identification of a seismic low velocity zone and a high thermal gradient in the crust beneath Tibet suggests flow occurs in both the lower crust and lithospheric mantle. In the North Aegean a change in block motions during the past ~4 Ma seems to be reflected in the orientation of S-wave anisotropy, which is aligned more closely with earlier extension than with present-day strike-slip motions [Kreemer et al., 2004 EPSL]. Patterns of S-wave anisotropy in the western U. S. are complex and have been variously attributed to `absolute' motion of the lithosphere over asthenospheric mantle, shearing across major active faults, and driving flow in the asthenosphere.
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: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005