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Published 2009 | public
Book Section - Chapter

Seismotectonics of an evolving intracontinental plate boundary, southeastern California

Abstract

An analysis of seismicity in the northern Mojave block, southern Walker Lane belt, and southern Sierra Nevada provides a detailed snapshot of the kinematics of active deformation within a young and probably evolving intracontinental plate boundary in eastern California. Earthquakes in this region were relocated using joint hypocentral inversion, double-difference, and waveform cross-correlation techniques. Groups of focal mechanisms were inverted for the components of a reduced deformation rate tensor, and the inversion results were synthesized in maps of the regional seismogenic deformation field. In general, seismogenic deformation in the Eastern California shear zone and Walker Lane belt is characterized by distributed simple shear and primarily accommodates northwest translation of the Sierra Nevada–Central Valley (Sierran) microplate relative to stable North America. Crustal thinning in the southern Walker Lane belt is subordinate to northwest-directed dextral shear and typically is associated with releasing geometries among dextral faults. There is no significant variation in strain geometry in the transition from the Eastern California shear zone to the Walker Lane belt across the Garlock fault; we interpret this to indicate that this structure is being sheared and rotated clockwise by distributed northwest dextral shear. In contrast, seismogenic deformation in the southeastern Sierra Nevada near latitude 36°N is characterized by horizontal elongation and locally by flattening, i.e., extension in two perpendicular horizontal directions. The transition from wrench tectonics in the southern Walker Lane to flattening in the High Sierra occurs across a 25- to 35-km-wide transtensional zone west of the Sierran range front. The elongation and flattening occur directly east of the "Isabella anomaly," a high-velocity anomaly in the upper mantle interpreted to be lower Sierran lithosphere that is foundering or convectively descending into the asthenosphere. The seismogenic thinning of the southern Sierra may be related to thickening above the descending lithosphere to the west and/or may be driven by tensile buoyancy forces. The seismotectonics of this region may provide insights into the process by which the Walker Lane belt has progressively expanded westward into the Sierran microplate during late Cenozoic time.

Additional Information

© 2009 The Geological Society of America. Accepted 21 July 2008. We performed this study under contract N68936-02-C-0207 from the U.S. Navy. We thank Frank Monastero, U.S. Navy Geothermal Program Office, for supporting this work and engaging us in many stimulating discussions about the seismotectonics of the study region. Hauksson also was supported by NEHRP/U.S. Geological Survey grants 04HQGR0052 and 05HQGR0040. Steve Thompson provided constructive criticism of an early draft of this paper. Comments by volume editor John Oldow and formal reviews by Basil Tikoff and an anonymous reviewer significantly improved the final version. This is contribution number 10,001 of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena.

Additional details

Created:
August 20, 2023
Modified:
January 13, 2024