Upper crustal deformation above foundering lower lithosphere, southern Sierra Nevada Microplate, California

Abstract

Quaternary faulting and background seismicity in the southern Sierra Nevada are concentrated east and southeast of the Isabella anomaly, a high velocity body in the upper mantle interpreted to be lower Sierran lithosphere that is foundering or convectively descending into the asthenosphere. We analyzed seismicity in this region to evaluate patterns of upper crustal deformation above and adjacent to the Isabella anomaly. Earthquakes in the southern Sierra and San Joaquin Valley were relocated using joint hypocentral inversion and double-difference techniques, and groups of focal mechanisms were inverted for the components of a reduced deformation rate tensor. From east to west, the regional seismogenic deformation field derived from this analysis exhibits the following characteristics: (1) horizontal plane strain in the southern Walker Lane belt and southwestern Sierra Nevada consistent with distributed NNW-directed dextral shear; (2) heterogeneous extension and crustal thinning in the high Sierra and western foothills east of the Isabella anomaly; (3) pronounced CCW rotation of the principal strains from regional trends in the SW Sierra Nevada; (4) horizontal plane strain in the southwestern San Joaquin Valley; and (5) a 20- to 40-km-wide zone of transpression directly east of the San Andreas fault. The heterogeneous extension in the southern Sierra occurs above a zone of anomalous low upper mantle P-wave velocities, which suggests that the crust is extending above delaminated lithosphere (and possibly upwelling asthenosphere). The CCW rotation of strain trajectories in the SW Sierra occurs directly southeast of the Isabella anomaly. Speculatively, thickened lithosphere above and possibly connected to the anomaly may form a strong backstop that resists through-going NW-directed shear propagating westward into the Sierran microplate from the southern Walker Lane belt, and consequently shearing is deflected to the WNW around the southwestern margin of the anomaly. Alternatively, the Isabella anomaly may generate gravitational buoyancy forces that modify the local stress regime and perturb the deformation field, thus rotating the strain trajectories.

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