GEOLOGIC MAPPING OF THE GOLDEN BEAR, COSO AND COTTONWOOD PASS DIKES: FURTHER TESTING OF 65 KM OF DEXTRAL OFFSET IN THE CRETACEOUS EASTERN SIERRA

Past research suggests that the tectonically active region of Owens Valley, California, has experienced 65 km of dextral slip over the past 83 million years. That conclusion was based on a possible correlation of the Golden Bear in the eastern Sierra Nevada to the Coso dikes of the Coso range using geochronology, geochemistry, and dike and wallrock lithology. If correlated, these dikes were tectonically offset 65 km from one another after emplacement. While these previously collected data are sound, alternate interpretations were not ruled out. Our and others’ follow-up studies showed that these and another set of similar dikes at Horseshoe Meadow, CA were emplaced over a period of 1.5 million years and paleomagnetic data constrained the possible correlation geometry. Here we document further mapping, focusing on the extent of the Cottonwood Pass dikes at Horseshoe Meadows which constitute multiple parallel dikes as wide as ~10 meters. Thus far, our data suggests that it is unlikely that the emplacement of the Golden Bear and Coso dikes are one unique event, instead indicating they may be separate intrusive bodies. Future zircon/apatite (U-Th)/He thermochronologic data will allow us to demonstrate the age of acquisition of the paleomagnetic signal as well as describe near-surface exhumation at temperatures ranging 25-175° C. Reconstructing the cooling history of these dikes helps infer the nature of their inter-relationship while also revealing their near-surface exhumation history. The new data is relevant to understanding the late Mesozoic and early Cenozoic tectonic development of the region, specifically deformation during the Laramide orogeny.