A DENSE SEISMIC TRANSECT ACROSS CENTRAL CALIFORNIA AND THE ISABELLA ANOMALY

The Isabella anomaly is a high velocity seismic body located in the uppermost mantle beneath the Great Valley of Central California (~36°N). This anomaly has long been recognized in seismic tomography studies, however, the geologic interpretation of this feature remains contentious. The conventional interpretation is that the Isabella anomaly is related to the on-going delamination of the dense mafic root associated with the distillation of the Sierra Nevada batholith which lies directly to the east. Alternatively, the anomaly has been interpreted as a fossil slab fragment associated with the stalled subduction of the Monterey microplate which lies just offshore. One of the challenges in interpreting the Isabella anomaly is the relatively poor resolution west of the Sierras which is caused by a lack of seismic data in and around the Great Valley. To help further constrain the subsurface structure and the Isabella anomaly, a dense line of 40 broadband seismometers was deployed in Central California stretching from the Sierra foothills to the coastline and recorded continuous data for approximately two years. Here we report on our on-going research efforts which include body and surface wave tomography as well as scattered wave imaging derived from P and S wave receiver functions. The most prominent feature observed in the receiver function imaging is a high amplitude crustal velocity discontinuity which is observed over ~80 km distance. This westward dipping feature begins near the surface beneath the Sierra foothills and reaches ~25 km depth beneath the Great Valley. No additional boundaries are observed below this feature, including the Moho, and we speculate that this feature represents the top of a high velocity ophiolite body that underlies most of the Great Valley. Body wave travel-time measurements indicate that the Isabella anomaly extends further west to beneath the coastal range. Additionally, receiver function data suggest the presence of eastward dipping velocity discontinuities in the upper-most mantle below the western Great Valley. These seismic observations support the hypothesis that the Isabella anomaly is a slab fragment rather than a piece of delaminating lithosphere.