POST-CRETACEOUS DEXTRAL OFFSET OF 65 KM ACROSS OWENS VALLEY, CALIFORNIA, IMPLIED BY CORRELATION OF THE GOLDEN BEAR AND COSO DIKE SETS

The Golden Bear and Coso Dike sets are distinctive K-feldspar porphyry dikes that crop out on opposite sides of Owens Valley. Petrographic, chemical, isotopic, and geochronologic data suggest that the dike sets are correlative. The correlation requires ~65 km of dextral offset across the valley since the dikes were intruded ca. 83 Ma, which greatly exceeds previous estimates. The Golden Bear and Coso dikes strike roughly perpendicular to Owens Valley, and neither reappears on the opposite side of the valley. New U-Pb zircon data from four samples indicate that both sets were intruded ca. 83 Ma. The dikes cut similar, distinctive wall rocks, including newly dated 102 Ma leucogranites (Independence and Sardine plutons in the Sierra Nevada; Coso leucogranite in the Coso Range). The sets have nearly identical textures and mineralogy, with K-spar phenocrysts up to 8 cm across and euhedral quartz bipyramids ~1 cm across. Major and trace element contents also are indistinguishable; however, interpretation of preliminary Sr isotopic data is more complicated. Isotopic compositions of contiguous portions of each dike set vary and compositional ranges of the two sets do not overlap (Golden Bear: 0.7069-0.7073, Coso: 0.7074-0.7084) Chilled margins are consistently less radiogenic than adjacent dike cores. We interpret the data to suggest that the dike sets are correlative, but that they have been variably contaminated by their wall rocks. Consequently, we interpret the two sets to have once been continuous and later offset from each other by approximately 65 km of dextral shear. We hypothesize that dextral offset of the dike sets was accommodated by a through-going right-slip fault system, extending northward up Owens Valley from the western side of the Coso Range. If faulting began during the Pliocene as some studies suggest, an offset of 65 km requires either slip rates were at least 25% faster than current geodetically measured rates (5 mm/yr) or earlier inception of faulting, perhaps during the late Miocene regional transition from orthogonal extension to dextral transtension. Pliocene onset and a slower slip rate also is possible if there was significant late Cretaceous or early Cenozoic offset.