A multicomponent Isabella anomaly: Resolving the physical state of the Sierra Nevada upper mantle from Vp/Vs anisotropy tomography

Abstract

The Isabella anomaly, a prominent upper-mantle high-speed P-wave anomaly located within the southern Great Valley and southwestern foothills of the Sierra Nevada, has been interpreted either as foundering sub-Sierran lithosphere or as remnant oceanic lithosphere. We used Vp/Vs anisotropy tomography to distinguish among the probable origins of the Isabella anomaly. S waveforms were rotated into the Sierran SKSFast and SKSSlow directions determined from SKS-splitting studies. Teleseismic P-, SFast-, SSlow-, SKSFast-, and SKSSlow-wave arrival times were then inverted to obtain three-dimensional (3-D) perturbations in Vp, Vp/VsMean, and percent azimuthal anisotropy using three surface wave 3-D starting models and one one-dimensional (1-D) model. We observed the highest Vp/Vs anomalies associated with slower velocities in regions marked by young volcanism, with the largest of these anomalies being the Mono anomaly under the Long Valley region, which extends to depths of at least 75 km. Peak Vp/Vs perturbations of +4% were found at 40 km depth. The low velocities and high Vp/Vs values of this anomaly could be related to partial melt. The high wave speeds of the Isabella anomaly coincide with low Vp/Vs values with peak perturbations of −2%, yet they do not covary spatially. The P-wave inversion imaged the Isabella anomaly as a unimodal eastward-plunging body. However, the volume of that Isabella anomaly contains three separate bodies as defined by varying Vp/Vs values. High speeds, regionally average Vp/Vs values (higher than the other two anomalies), and lower anisotropy characterize the core of the Isabella anomaly. The western and shallowest part has high wave speeds and a lower Vp/Vs values than the surrounding mantle. The eastern and deepest part of the anomaly also contains high speeds and lower Vp/Vs values but exhibits higher anisotropy. We considered combinations of varying temperature, Mg content (melt depletion), or modal garnet to reproduce our observations. Our results suggest that the displaced garnet-rich mafic root of the Mesozoic Sierra Nevada batholith is found in the core of the Isabella anomaly. If remnant oceanic lithosphere exists within the Isabella anomaly, it most likely resides in the shallow, westernmost feature. Within the Sierra Nevada, the highest upper-mantle anisotropy is largely contained within the central portion of the range and the adjacent Great Valley. Anisotropy along the Sierra crest is shallow and confined to the lithosphere between 20 and 40 km depth. Directly below, there is a zone of low anisotropy (from 170 to 220 km depth), low velocities, and high Vp/Vs values. These features suggest the presence of vertically upwelling asthenosphere and consequent horizontal flow at shallower depths. High anisotropy beneath the adjacent western foothills and Great Valley is found at ~120 km depth and could represent localized mantle deformation produced as asthenosphere filled in a slab gap.