REGIONAL DECOLLEMENTS AND THEIR INTERACTIONS WITH A SILICIC MAGMATIC SYSTEM CONSTRAINED BY PASSIVE SEISMOLOGICAL OBSERVATIONS, SOUTHWESTERN GREAT BASIN

Pronounced simple-shear extension of the lithosphere will produce substantial deformation underneath areas unfaulted at the surface. Defining the geometry of such systems is difficult because of the absence of a direct connection between the surface and structures underneath. Lack of profound differences of geophysical properties across decollements tends to make them invisible to most geophysical techniques. We have found that P to S conversions from teleseismic earthquakes can image these structures. Additionally, this technique can reveal the direction of seismic anisotropy associated with the shear zone. In the southwestern Great Basin, this approach located a west-dipping shear zone extending under the Sierra Nevada from the Death Valley region. Anisotropy suggests a net ENE lineation to this shear zone under the Sierra, which matches the anisotropy in the upper mantle revealed from SKS shear-wave splitting measurements, but a NW lineation is suggested at shallower depths to the east. We fail to find the same decollement within the Coso geothermal field at the southern edge of this part of the Basin and Range. It appears that in this area strain is localized about the upper crustal magma chamber, with the magma chamber absorbing extensional strain over its depth range (about 5-15 km) and decollements rooting into the top and bottom of the magma chamber. These latter decollements are more local than that seen to the north, and separate non-straining middle crust from more broadly deforming upper and lower crust. These results suggest that magmatic systems can greatly alter the accommodation of strain both vertically and horizontally. Failure to date to identify a seismic conversion connecting the regional decollement to the north with those in the Coso area suggests that near-vertical faults are absorbing the variations in strain. This is likely to produce different overall strain geometries in areas extending with and without magmatic systems.