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Strain rates assessed from brittle fracture, associated with earthquakes, and total brittle-ductile deformation measured from geodetic data have been compared to paleostrain from Quaternary geology for the intraplate Great Basin of the western United States. These data provide an assessment of the kinematics and mode of lithospheric extension that the western U.S. Cordillera has experienced in the last 5-10 million years. Strain and deformation rates were determined by the seismic moment tensor method using historic seismicity and fault plane solutions. By subdividing the Great Basin into areas of homogeneous strain it was possible to examine regional variations in the strain field. Contemporary deformation of the Great Basin occurs principally along the active seismic zones: the southern Intermountain Seismic Belt - 4.7 mm/ a maximum deformation rate, along most of the western boundary, the Sierra Nevada front - 28.0 mm/ a maximum deformation rate, and along the west central Nevada seismic belt - 7.5 mm/ a maximum deformation rate. The earthquake related strain shows that the Great Basin is characterized by regional E-W extension at 8.4 mm/ a in the north that diminishes to NW-SE extension of 3.5mm/ a in the south. These results show -8-10 mm/ a deformation associated with earthquakes that compares to -9mm/ a determined from satellite geodesy and tectonic plate models, implying that modern strain is generally reliant on earthquakes. Zones of maximum extension correspond to belts of shallow crust, high heat flow, and Quaternary basaltic volcanism, suggesting that these parameters are related through an effect such as a stress relaxation allowing bouys, uplift and ascension of magmas.

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