MODERN DEFORMATION AND VOLCANISM IN OWENS VALLEY REACTIVATES A LARAMIDE-AGE SHEAR ZONE

Stratigraphic, geomorphic, and seismic reflection data indicate Pliocene initiation of right-lateral transtension across Owens Valley that localizes the Coso geothermal system. Four independent markers indicate ~65 km of right lateral offset across Owens Valley, most of it between 83.5 and 15 Ma. Active displacement across Owens Valley thus represents reactivation of a pre-existing major shear zone.

Correlations of Jurassic rocks across Owens Valley made in the 1960’s implied ~5 km of dextral offset, consistent with recent initiation, but the correlations are nonunique. Stevens and Stone (2002) inferred 65 km of offset based on correlation of Devonian strata in the Mt. Morrison pendant with similar rocks near Tinemaha Reservoir. We identified three other offset markers—a distinctive zone within the ~150 Ma Independence dike swarm, the 102 Ma Bullfrog and Cactus Flat leucogranites, and the 83.5 Ma Golden Bear and Coso granite dikes—that are all consistent with ~65 km of offset. The Golden Bear-Coso dike correlation has been challenged and an alternative correlation of the Coso dikes proposed that requires only ~25 km of dextral offset. However, the Golden Bear-Coso dike correlation remains the most probable for several reasons, most conclusively because three independent geologic markers indicate the larger offset. Continuity of the Garlock fault across the southern projection of Owens Valley requires most of this offset to be older than 15 Ma. Structures that accommodated the offset are largely concealed by valley fill. However, greenschist-facies phyllonite zones at the western edge of the Coso Range crosscut Independence dikes and contain right-lateral transpressive shear fabrics, and right-lateral transpressive shear zones dated at ~75 Ma crop out at the western edges of the Inyo (Vines, 1999) and White Mountains (Sullivan, 2003). Thus, most of the offset probably is of Laramide age. The Owens Valley shear system lies outside of the main Sierran arc and movement across it postdates the end of major Sierran magmatism at ~84 Ma. Accommodation of the lateral component of relative plate motion may have shifted from the Sierra Crest shear zone within the arc to the eastern margin of the batholith when lithospheric strength within the batholith increased after igneous activity ceased.