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Internal deformation of the southern Sierra Nevada microplate associated with foundering lower lithosphere, California

Unruh, Jeffrey and Hauksson, Egill and Jones, Craig H. (2014) Internal deformation of the southern Sierra Nevada microplate associated with foundering lower lithosphere, California. Geosphere, 10 (1). pp. 107-128. ISSN 1553-040X. doi:10.1130/GES00936.1.

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Quaternary faulting and background seismicity in the southern Sierra Nevada microplate are concentrated east and south of the Isabella anomaly, a high-velocity body in the upper mantle interpreted to be lower Sierra lithosphere that is foundering 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. The deformation field derived from this analysis reveals two distinct departures from horizontal plane strain associated with distributed northwest-directed dextral shear east of the Pacific plate: (1) heterogeneous extension and crustal thinning in the high Sierra and western foothills east of the Isabella anomaly; and (2) pronounced counterclockwise rotation of the principal strains from regional trends in the southwestern Sierra Nevada and across the Kern Arch. Based on comparison with a three-dimensional tomographic model, the extension in the southern Sierra is spatially associated with relatively thinner crust and anomalous low P-wave speeds in the upper mantle (40–90 km depth range) directly east of the Isabella anomaly. These relations suggest that seismogenic crustal thinning is localized above upwelling asthenosphere that is replacing foundering lithosphere. Counterclockwise rotation of strain trajectories in the southwest Sierra occurs southeast of the Isabella anomaly, and is associated with seismogenic west-northwest–striking dextral faults. We suggest that the deformation here represents westward encroachment of dextral shear into the microplate from the eastern California shear zone and southern Walker Lane belt. The strain rotation may reflect the presence of local stresses associated with relaxation of subsidence in the vicinity of the Isabella anomaly. Westward propagation of foundering lithosphere, with spatially associated patterns of upper crustal deformation similar to those documented herein, can account for observed late Cenozoic time- and space-transgressive deformation in the southern Walker Lane belt east of the Isabella anomaly, and is a potentially observable consequence of the foundering process in other orogens.

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Hauksson, Egill0000-0002-6834-5051
Additional Information:© 2014 Geological Society of America. Received 7 April 2013; Revision received 7 November 2013; Accepted 10 December 2013; Published online 14 January 2014. We acknowledge support of this research by the National Science Foundation (grant EAR-0607625 to Unruh, grants EAR-0454535 and EAR-0607831 to Jones), and the U.S. Army Corps of Engineers. The ideas and interpretations presented herein were developed in conversations with Jason Saleeby, Zorka Saleeby, Peter Molnar, Greg Houseman, Hersh Gilbert, Anthony Frassetto, Allen Glazner, Frank Monastero, John Dewey, Eugene Humphreys, and Colin Amos. We thank Frank Monastero and Cooper Brossy for comments on an early version of the manuscript, and Jason Saleeby, Jeffrey Lee, and an anonymous reviewer for Geosphere for constructive reviews, all of which significantly improved the final paper.
Group:Seismological Laboratory
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U.S. Army Corps of EngineersUNSPECIFIED
Issue or Number:1
Record Number:CaltechAUTHORS:20140807-085423064
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Official Citation:Internal deformation of the southern Sierra Nevada microplate associated with foundering lower lithosphere, California Geosphere, February 2014, v. 10, p. 107-128, First published on January 14, 2014, doi:10.1130/GES00936.1
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48166
Deposited By: Tony Diaz
Deposited On:07 Aug 2014 16:04
Last Modified:10 Nov 2021 18:04

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