Two hypotheses were tested for the origin of the Isabella tomographic anomaly, which has been interpreted as either a lithospheric drip (Jones et al., 1994; Ducea and Saleeby, 1998) or a remnant of the Farallon plate, possibly attached to the Monterey microplate (Wang et al., 2013). P‐wave receiver functions and travel‐time residuals based on teleseismic events recorded by 41 stations were used to construct simple geometric tomography models of the Isabella anomaly and to test whether the Monterey microplate, or a similar fragment of the Farallon slab, can be connected with the location of the anomaly. The travel‐time residual pattern was modeled with a slab geometry composed of a high‐velocity rectangular block, and a lithospheric drip was modeled as a dipping cylindrical anomaly. Both gave statistically similar fits, but the cylindrical block appears less well constrained than the rectangular block. The top surface of the best‐fit rectangular block was located at 50 km below the Great Valley and dips 65° northeast toward the Sierra Nevada, with 100 km thickness. West of the Isabella anomaly, receiver‐function P‐to‐S converted phases from the top and bottom surfaces of the oceanic crust of the Monterey microplate were modeled as late‐arriving negative–positive dipole signals. Such dipoles are observed at times that place their origin at 10 km below the Moho at the Coast Ranges. These converted phases from the oceanic crust could also be traced from the Coast Ranges to the Great Valley, where the top surface of the slab model is located. The combined result of the receiver functions and a geometric tomography model is consistent with the fossil slab hypothesis, which initiated as a flat subduction system that later went through a delamination process.

Online Material: Figures of receiver functions, sorted by back azimuth and ray parameter at 31 stations.

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