Abstract Title: CORRESPONDENCE OF REGIONAL STRAIN AND RELATIVE STRENGTH INFERRED FROM SHALLOW MANTLE VELOCITY VARIATIONS Abstract Author(s): Glenn Biasi,, Nevada Seismological Laboratory, University of Nevada, Reno, NV 89557, glenn@seismo.unr.edu Abstract: Scaling relationships that increase compressional velocity in the shallow upper mantle likewise increase strength. Temperature is perhaps the best known such effect. Signs of velocity derivatives for composition and partial melt also agree with their affect on shear strength. This means that maps of shallow mantle velocity structure can be interpreted in terms of relative strength of the mantle contribution to total lithospheric strength. The partitioning of strength of the lithosphere between crust and mantle is harder to pin down. Regional scale tomographic imaging does not uniquely resolve absolute velocity, so one cannot say from the images alone whether velocity, and thus strength, varies around so low a value that the mantle is always weak compared to the crust, or whether high velocity anomalies are in fact strong at relevant strain rates. Significant mantle strength, even in the western Basin and Range, cannot be dismissed out of hand on the basis of high surface heat flow.. At strain rates of To address the mantle contribution to lithospheric strength, two strategies are employed, both beginning with P-wave velocity images from California and Nevada. First, we compare shallow tomographic velocity maps to locations of active faults. Faults should generally avoid strength in the upper mantle where it is kinematically feasible. The clearest example of this is the Sierra Nevada block in California. The core high-velocity structure along the entire length of the Sierra Nevada beneath the western foothills is internally undeformed and moving as a coherent block. Low present and paleo-temperatures in the foothills indicate that the velocity scaling here is due primarily to Laramide and post-Laramide subduction-related suppression of temperature. Similar age and size batholithic complexes southern California without the high velocity "foundation" have been extensively faulted. Faulting in the southern Sierra is concentrated where the high velocities have been lost, perhaps due to delamination fro Low velocities scale asymmetrically. Once mantle strength is small compared to the crust, further decreases are predicted to have little further effect on surface faulting and shallow crustal kinematics define resulting deformation. Fault patterns in the Walker Lane generally north of the White Mountains and south of the Pyramid Lake indicate that the crust is the dominant contributor, at near zero (relative) velocity contrast. Seismicity in the Central Nevada Seismic Belt occurs on the west edge of a high-velocity body, indicating that the mantle anomaly defines the eastern extent of faulting. The minimum correlation length for mantle velocity and faulting is not clear. A ~50x50 high velocity block west of the Pyramid Lake fault appears to deflect deformation around it. This generally granitic block could locally retain the upper mantle qualities of the Sierra nearby to the west. Another small anomaly aligns with the Garlock fault, and is reasonably interpreted as a downgoing mantle lithospheric sheet set in motion by north-south compression perpendicular to the fault. GPS provides a second strategy for assessing mantle contribution to strength. We compare a map of GPS-derived shear strain to the shallow mantle velocities. In general the comparison is very encouraging. Features such as the Walker Lane, which is a zone of slow to neutral mantle velocities, correspond to zones of focused strain in the geodetic inversion. The Sierra Nevada appears as an island of low strain in good agreement with inferences from the velocity structure. Deflection of the strain field in Central Nevada Seismic Belt is mirrored in the velocity structure. The good correspondence between shallow mantle velocity variations, faulting, and regional strain indicates that tomographic images produced under Earthscope experiments will contribute useful strength and mantle viscosity constraints in models of western U.S. dynamics.