Geosphere | 2019
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 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. ■ INTRODUCTION The seismic velocity structure beneath the Sierra Nevada (Fig. 1) has been an important constraint in decoding the uplift history of this range and the geodynamic consequences of uplift on surrounding areas. One widely discussed hypothesis for uplift of the Sierra Nevada is through removal of mafic mantle lithosphere and/or dense lower crust. While the exact mechanisms are still debated, removal of dense, garnet-rich material beneath the Sierra Nevada would result in convective upwelling of asthenosphere directly beneath and east of the range (e.g., Ducea and Saleeby, 1998). Upward migration of hot, buoyant asthenosphere would lead to crustal thinning, range uplift, and volcanism—all of which have been inferred from xenolith, geologic, seismic, and magnetotelluric studies (e.g., Ducea and Saleeby, 1996; Jones et al., 1994; Park, 2004). The distribution of upper-mantle seismic velocities is somewhat consistent with this hypothesis. The western foothills of the Sierra Nevada exhibit fast lower-crust/uppermost-mantle seismic velocities relative to surrounding material (e.g., Biasi and Humphreys, 1992; Fliedner et al., 1996; Jones et al., 1994) and thick crust, up to 55 km (Frassetto et al., 2011). The crust below the Sierra crest has slower velocities and thins to 35 km, which is comparable to crustal thicknesses in the Basin and Range Province. West of the southern Sierra Nevada, there lies an isolated, high-velocity, upper-mantle anomaly, termed the Isabella anomaly (Fig. 1). The magnitude of the Isabella anomaly is large, up to ~10% faster in P-wave speed than surrounding material (e.g., Jones et al., 2014). The Isabella anomaly is an important feature in unraveling the evolutionary history of the Sierra Nevada. GEOSPHERE GEOSPHERE, v. 15, no. 6 https://doi.org/10.1130/GES02093.1 23 figures; 4 tables; 1 set of supplemental files CORRESPONDENCE: [email protected] CITATION: Bernardino, M.V., Jones, C.H., Levandow\xad ski, W., Bastow, I., Owens, T.J., and Gilbert, H., 2019, A multicomponent Isabella anomaly: Resolving the physical state of the Sierra Nevada upper mantle from Vp/Vs anisotropy tomography: Geosphere, v. 15, no. 6, p. 2018–2042, https://doi.org/10.1130/GES02093.1. Science Editor: Shanaka de Silva Published online 8 November 2019 Received 27 November 2018 Revision received 17 June 2019 Accepted 9 August 2019 © 2019 The Authors This paper is published under the terms of the CC\xadBY\xadNC license. *Present address: TetraTech, Inc., Superior, Colorado 80027, USA †Present address: Department of Earth Science and Engineering, Imperial College London, London SW7 2BP, UK Research Paper Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/6/2018/4881695/2018.pdf by University of Colorado Boulder user on 07 January 202