HR: 12:05h
AN: T22C-08    [Abstracts]
TI: Upper mantle earthquakes in the Himalayan collision: Flexure of the continental lithosphere?
AU: * Monsalve, G
EM: monsalve@cires.colorado.edu
AF: University of Colorado, 2200 Colorado Ave, Boulder, CO 80309, United States
AU: McGovern, P
EM: mcgovern@lpi.usra.edu
AF: Lunar and Planetary Institute, 3600 Bay Area Blvd, Houston, TX 77058, United States
AU: Sheehan, A
EM: afs@cires.colorado.edu
AF: University of Colorado, 2200 Colorado Ave, Boulder, CO 80309, United States
AB: Earthquake locations from the Himalaya Nepal-Tibet seismic experiment (HIMNT) during 2001-2003 suggest that the uppermost mantle in the Indian lithosphere deforms by brittle processes. We found clusters of upper mantle earthquakes beneath the transition from the Ganges Plains to the southernmost Himalaya, and beneath the southern Tibetan Plateau. Previous studies (e.g. Chen and Yang, 2004) have also noted the presence of upper mantle earthquakes beneath the southern Tibetan Plateau. Using a finite element model, we establish a link between upper mantle seismicity and flexural bending of the Indian lithosphere. The goal is to determine the steady-state stress field in the Indian plate in response to loading. Our models assume plane strain and use Cartesian coordinates on a representative arc-normal cross section of the Indian plate at the Himalayan collision zone. We use a time-dependent viscoelastic regime in a vertically layered lithosphere with a non-Newtonian rheology, overlying an asthenosphere with a constant and Newtonian viscosity. The crust is assumed to be composed by quartzite in its upper part and diabase in its lower portion; for the upper mantle we use olivine rheologies. The modeled stress field reveals that the highest differential stresses occur in the uppermost mantle, with magnitudes of around 1 GPa, concentrating roughly at the regions where upper mantle earthquakes originate. The predicted orientations of principal stresses just below the model crust-mantle boundary are consistent with previously determined focal mechanisms of near-Moho earthquakes in southern Tibet, with the maximum compressive stresses oriented in a nearly horizontal Himalayan arc-normal direction, and the maximum extensional stress being nearly horizontal and arc-parallel. The finite element modeling suggests that flexural bending of the Indian plate generates sufficiently large background stresses to put upper mantle rocks in near-brittle failure conditions.
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8138 Lithospheric flexure
DE: 8150 Plate boundary: general (3040)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting