P-wave tomography of potential convective downwellings and their source regions, Sierra Nevada, California

Teleseismic P-wave tomography using the Sierra Nevada Earthscope Project (SNEP) deployment, older temporary deployments in the Sierra, and broadband stations from permanent and USArray Transportable Array (TA) stations was derived from starting models either lacking lateral variation (one dimensional [1-D]) or created from three-dimensional (3-D) surface-wave models. The use of multiple starting models permits examination of the robustness of different features while limiting the inherent ambiguities of teleseismic body-wave tomography. Our results confirm that mafic residuum of the Mesozoic Sierran batholith has been removed from the eastern Sierra north to at least 39 degrees N. Low-wavespeed material near the Moho under the eastern Sierra is probably silicic lower crust and warm and possible melt-laden upper mantle. If the residuum remains in the upper mantle, there are three possible locations for it: a highwavespeed (+similar to 5%) body extending down to similar to 250 km near 36 degrees N, 119.3 degrees W termed the Isabella anomaly, an unusually high-wave-speed (to + 10%) and shallow (top40-50 km) anomaly at the south end of the Gorda slab near 40.5 N, 122.25 W termed the Redding anomaly, and a more ill-defined region of high wavespeeds in the crust to uppermost mantle (<70 km) along the western foothills of the Sierra termed the Foothills anomaly. The Foothills anomaly is most pronounced from 36.5 to 38 N and is distinguished from high-wavespeed lithosphere to the west by an unusually deep Moho. We infer that this body could either reflect in situ lower lithospheric material from the Mesozoic arc or could be material displaced from the east. The Isabella anomaly is nearly equant in plan view with a diameter similar to 100 km and a plunge to the east of 60-70. This anomaly contains more than enough material to account for the lower lithosphere thought to have been under the eastern Sierra south of similar to 38 N until ca. 10 Ma. The Redding anomaly cannot be trivially separated from the Gorda slab and could either represent imbrication of the downgoing oceanic lithosphere or entrained continental lithosphere, possibly in part from the Sierra. Both the Redding and Isabella anomalies appear to have connections to the Foothills anomaly, but these connections are at the limit of resolution of this data set and could be artifacts from adjacent but unrelated high-wavespeed bodies. Our preferred inference that the Isabella anomaly has been derived from the eastern Sierra suggests that foundering of lithospheric material is not simply described as a two-dimensional (2-D) delamination and/ or Rayleigh-Taylor instability as modeled in the literature to date.