Is the Isabella anomaly a fossil slab or the foundered lithospheric root of the Sierra Nevada batholith?

Abstract:

The Isabella Anomaly is a volume of relatively high seismic velocity upper mantle beneath the southern Great Valley in California. We deployed ~45 broadband seismometers in central California to test two main hypotheses for the origin of the Isabella Anomaly. One suggests that the Isabella Anomaly is the foundered lithospheric root of the southern Sierra Nevada batholith, which delaminated on account of eclogite-rich composition and translated westward as it began to sink into the asthenosphere. The other hypothesis suggests that the Isabella Anomaly is a fossil slab fragment attached to the Monterey microplate that lies offshore of central California and thus it is mechanically coupled to the Pacific plate. Prior seismic imaging with ~70 km station spacing cannot resolve the landward termination of Monterey microplate lithosphere beneath coastal California or where/if the Isabella Anomaly is attached to North America lithosphere beneath the Great Valley. The new temporary broadband array consists of 40 broadband seismometers with ~7 km spacing extending from the central California coast to the western Sierra Nevada batholith, plus some outliers to fill gaps in the regional network coverage. The temporary array was initially deployed in early 2014 and will continue to record until October 2015 so the complete data are not yet available. Preliminary Ps scattered wave images show an abrupt ~6 km increase in Moho depth eastward across the San Andreas fault, a strong positive impedance contrast that dips westward from ~7-25 km beneath Great Valley, and a sharp Moho with a slight westward dip beneath the western edge of the Sierra Nevada batholith. Apparently low impedance contrast characterizes the Moho beneath the eastern Great Valley and foothills, consistent with near mantle velocities in the lower crust. Processing of the cumulative data that will be available in October 2015 and incorporation of new tomography models into scattered wave imaging are needed before assessing the significance of potential uppermost mantle interfaces that may represent edges of the Isabella Anomaly. Results from Ps and Sp scattered wave imaging, ambient noise surface wave tomography, teleseismic body-wave tomography, and teleseismic shear wave splitting will be presented.