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Extension in the Basin and Range Province and East Pacific Margin |
Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 USA
Casper Renggli Institute for Geophysics, Swiss Federal Institute of Technology, ETH-Honggerberg, CH-8093, Zurich, Switzerland
Strain rates assessed from brittle fracture (associated with historic earthquakes) and total brittle-ductile deformation measured from geodetic data have been compared to estimates of palaeostrain, from Quaternary geology for the intraplate Great Basin part of the Basin and Range Province, western United States. These data provide an assessment of the kinematics and mode of lithospheric extension that the western U S Cordillera has experienced from the past few million years to the present. Strain and deformation rates were determined by the seismic-moment-tensor method using historic seismicity and fault-plane solutions for sub-regions of homogeneous strain. Contemporary deformation (with maximum-deformation rates) in the Great Basin occurs principally along the active seismic zones: (i) the southern Intermountain Seismic Belt (ISB), 4.7 mm a–1; (ii) along the western boundary, the Sierra Nevada front, 1.6 mm a1 (28.0 mm a–1 if the M 8.3 1872 Owen Valley, California earthquake is included); and (iii) along the W-central Nevada seismic belt, 7.5 mm a–1. The integrated opening rate across the entire Great Basin is accommodated by E-W extension at 8–10 mm a–1 to the N which diminishes to 3.5 mm a–1 to the S. These results show 8–10 mm a–1 contemporary extension across the entire Great Basin associated with earthquakes that compares to the
Contemporary strain and deformation rates have been determined from geodetic measurements yielding maximum deformation of 11.2 mm a–1 in the Hebgen Lake, Montana portion of the Intermountain Seismic Belt; 3.6 mm a–1 in the Walker Lane, Nevada area; and 2.5 mm a–1 in the Owens Valley, California area adjacent to the Sierra Nevada front. Palaeostrain and deformation rates principally from Quaternary fault-displacement rates gave deformation rates of 7.4 mm a–1 along the southern ISB. Geodetically determined deformation rates compare well with rates determined from seismic moments, whereas poorly constrained palaeo-strain rates from Quaternary geology are
9 mm a–1 value determined from intraplate tectonic models (constrained by satellite geodesy) implying that contemporary strain is generally released by earthquakes. Zones of maximum lithospheric extension correspond to belts of thin crust, high heat flow, and Quaternary basaltic volcanism, suggesting that these parameters are related through the mechanism of extension such as a stress relaxation, allowing buoyant uplift and ascension of magmas. 
10 times smaller than contemporary rates except in parts of central and southern California, Wyoming, parts of Utah, and along the Idaho-Wyoming border.
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