UCLA

Two hypotheses on the origin of the ‘Isabella tomographic anomaly,’ which has been interpreted as either a lithospheric delamination (Ducea and Saleeby, 1998) or a remnant of the Farallon plate attached to the Monterey micro-plate (Wang　et al., 2013), were tested. P-wave receiver functions and tomography based on teleseismic events recorded by 37 permanent stations and 4 newly installed stations were used to construct a simple geometry tomographic model of the Isabella anomaly, and to test whether the Monterey micro-plate can be connected with the location of the Isabella anomaly. A rectangular block model was used to fit the arrival-time residual pattern, and the best-fit model has its top surface located at 50km below the Great Valley. The block model dips 65º toward the Sierra Nevada with 100 km thickness. Receiver functions suggest that the remnant Farallon plate is not stalled at the base of the crust beneath the Coast Ranges as has been suggested from refraction studies (Brocher et al., 1999). Instead, the Monterey micro-plate lies in the mantle. Its top and bottom surfaces can be observed as a negative-positive dipole signal in the receiver functions. Negative-positive dipole signals at 35 km observed in the Coast Ranges' receiver functions were interpreted as the P-to-S converted phases from the Monterey micro-plate. These P-to-S converted phases from the top of the oceanic crust could be traced from the Coast Ranges to the Great Valley. The depth of the top surface of the Monterey micro-plate suggested from the receiver functions is consistent with the location of the rectangular block found from tomography, supporting the hypothesis that the Isabella anomaly is a remnant of the Farallon plate that is attached to the Monterey micro-plate. The combined results of the receiver functions and tomography study suggest that the partially subducted Monterey micro-plate extends east beyond the San Andreas fault and that the initially low angle of subduction in the west increases its dip east below the Great Valley.