Andy Freed

Purdue Univ.

19 September, 2003

Evidence of Powerlaw Flow in the Mojave Desert Mantle

Laboratory experiments suggest that, under certain temperature and stress conditions, viscous flow in deforming lithospheric rocks should be controlled by dislocation creep, characterized by a strain rate proportional to stress raised to a power, n. This powerlaw relationship has not been geodetically observed. Here we successfully explain the spatial and temporal evolution of transient surface deformation measured with GPS following the 1992 Landers and 1999 Hector Mine earthquakes in southern California by powerlaw flow (n = 3.5), predominantly in a warm and wet upper mantle. These results suggest that recovery-controlled dislocation creep is the dominant mechanism of viscous flow following earthquakes. We can rule out Newtonian flow (strain rate linearly proportional to stress) as a reasonable explanation of both the spatial and temporal patterns of postseismic transient motions, implying that the common assumption of Newtonian flow in numerical models of ductile deformation within the crust and upper mantle may be invalid. The model results also preclude significant flow in the lower crust, supporting the contention that, at least beneath the Mojave Desert, the mantle is the weaker region.