FOUNDERING OF HETEROGENEOUS MATERIAL, SIERRA NEVADA, CALIFORNIA

Xenoliths erupted with Neogene volcanics in the Sierra Nevada have indicated that the mafic complement to the felsic Sierra Nevada batholith remained under the range until ~8 Ma, after which it disappeared. The seismically fast Isabella anomaly (4-5% fast in P wavespeed) in the upper mantle under the southeastern San Joaquin Valley has been proposed to be comprised of this material, but an alternative hypothesis is that this body is a piece of the Farallon Plate that joined the Pacific plate shortly after subduction. Our addition of shear-wave tomography to the published P-wave tomography for the region reveals that the Isabella anomaly is heterogeneous, with a core between about 100 and 200 km depth having the highest vp/vs ratio and lowest transverse anisotropy sandwiched between material with low vp/vs ratios above and below, with the deeper part exhibiting higher anisotropy. By comparing these variations with plausible petrologies, we find it likely that the core anomaly is more garnet-rich than the rest of the body and most plausibly represents the "arclogite" that previously underlay the Sierran batholith. The deeper material is more plausible cold peridotite. The magnitude and variation of properties in the Isabella anomaly is inconsistent with an east-dipping oceanic slab. We thus infer that the Isabella anomaly does indeed represent foundering of the lowest crust and mantle lithosphere that previously underlay the Sierra. The geometry we infer resembles the vertical distribution of material inferred from a numerical 2-D model by Saleeby et al. (Geosphere 2012 p. 1286), where the deeper parts of the downwelling are comprised of mantle lithosphere and the middle and upper parts contain a significant fraction of arclogite. When combined with earlier work showing that the volume of material in the Isabella anomaly agrees within uncertainties with that removed and that the mass surplus in the anomaly is consistent with subsidence inferred in the San Joaquin Valley, the inference of the Isabella anomaly as a subcrustal, intracontinental foundering body seems well substantiated.