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Active megadetachment beneath the western United States

Wernicke, Brian and Davis, James L. and Niemi, Nathan A. and Luffi, Peter (2008) Active megadetachment beneath the western United States. Journal of Geophysical Research B, 113 (B11). B11409. ISSN 0148-0227. http://resolver.caltech.edu/CaltechAUTHORS:WERjgrb08

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Abstract

Geodetic data, interpreted in light of seismic imaging, seismicity, xenolith studies, and the late Quaternary geologic history of the northern Great Basin, suggest that a subcontinental-scale extensional detachment is localized near the Moho. To first order, seismic yielding in the upper crust at any given latitude in this region occurs via an M7 earthquake every 100 years. Here we develop the hypothesis that since 1996, the region has undergone a cycle of strain accumulation and release similar to “slow slip events” observed on subduction megathrusts, but yielding occurred on a subhorizontal surface 5–10 times larger in the slip direction, and at temperatures >800°C. Net slip was variable, ranging from 5 to 10 mm over most of the region. Strain energy with moment magnitude equivalent to an M7 earthquake was released along this “megadetachment,” primarily between 2000.0 and 2005.5. Slip initiated in late 1998 to mid-1999 in northeastern Nevada and is best expressed in late 2003 during a magma injection event at Moho depth beneath the Sierra Nevada, accompanied by more rapid eastward relative displacement across the entire region. The event ended in the east at 2004.0 and in the remainder of the network at about 2005.5. Strain energy thus appears to have been transmitted from the Cordilleran interior toward the plate boundary, from high gravitational potential to low, via yielding on the megadetachment. The size and kinematic function of the proposed structure, in light of various proxies for lithospheric thickness, imply that the subcrustal lithosphere beneath Nevada is a strong, thin plate, even though it resides in a high heat flow tectonic regime. A strong lowermost crust and upper mantle is consistent with patterns of postseismic relaxation in the southern Great Basin, deformation microstructures and low water content in dunite xenoliths in young lavas in central Nevada, and high-temperature microstructures in analog surface exposures of deformed lower crust. Large-scale decoupling between crust and upper mantle is consistent with the broad distribution of strain in the upper crust versus the more localized distribution in the subcrustal lithosphere, as inferred by such proxies as low P wave velocity and mafic magmatism.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1029/2007JB005375DOIArticle
ORCID:
AuthorORCID
Wernicke, Brian0000-0002-7659-8358
Additional Information:© 2008. American Geophysical Union. Received 6 September 2007; revised 21 March 2008; accepted 14 August 2008; published 21 November 2008. This research was supported by National Science Foundation grant EAR-04-53975, DOE cooperative agreement DE-FC28-03RW12232 (Task 3), and the Caltech Tectonics Observatory. Geodetic sites were constructed with technical assistance from the University NAVSTAR Consortium (UNAVCO) facility. The time series for site KTBW were obtained from the PANGA Data Analysis Facility Web site (http://www.panga.cwu.edu/). The time series for site ACAP were obtained from K. Larson. We are grateful to S. Klemperer for prepublication copies of seismic imaging studies in the northern Great Basin and G. Dumond and M. Williams for permission to discuss unpublished data from the Snowbird tectonic zone. We thank Associate Editor W. Schellart and formal reviewers W. Hammomd, P. England, and S. Wesnousky for comments that substantially improved the presentation. We also thank S. Kidder and D. Anderson for comments on an early draft of the manuscript and M. Simons, M. Gurnis, J. Saleeby, and D. Helmberger for useful discussions.
Group:Caltech Tectonics Observatory
Funders:
Funding AgencyGrant Number
NSFEAR-04-53975
Department of Energy (DOE)DE-FC28-03RW12232
Caltech Tectonics ObservatoryUNSPECIFIED
Subject Keywords:tectonics, geodesy, western United States
Issue or Number:B11
Record Number:CaltechAUTHORS:WERjgrb08
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:WERjgrb08
Official Citation:Wernicke, B., J. L. Davis, N. A. Niemi, P. Luffi, and S. Bisnath (2008), Active megadetachment beneath the western United States, J. Geophys. Res., 113, B11409, doi:10.1029/2007JB005375.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:12766
Collection:CaltechAUTHORS
Deposited By: Archive Administrator
Deposited On:22 Dec 2008 23:49
Last Modified:01 Feb 2017 21:07

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