STRUCTURAL MAPPING OF TRANSTENSIONAL ELEMENTS AND FABRICS IN THE COSO/CHINA LAKE REGION, SOUTHEASTERN CALIFORNIA: AN EARLY PROGRESS REPORT

The Coso/China Lake area (CCL) in southeastern California takes up ~12 of the 14 mm/yr relative transtensional motion between the Sierran microplate and the Colorado Plateau. The CCL is part of the seismically and volcanically active dextral transtensional corridor known as the Eastern California shear zone/Walker Lane and results from the eastward-step of PA-NA plate boundary deformation in the wake of the NW-moving Sierran microplate. Intense seismicity, Quaternary bimodal volcanism, shallow magmatism (~4 km) and high heatflow are concentrated in the CCL between the Sierra Nevada and the Argus Range. Extremely well exposed coeval normal and conjugate wrench fault arrays accommodate oblique extension in a bulk constrictional regime. Vertical partitioning of strike-slip and normal faulting in the brittle crust is indicative of the youth of this transtensional system (~2-3 Ma).

Geologic field studies detailing the structural elements produced in zones of active transtension are limited. The CCL is a natural laboratory in which to document the ways that real rocks behave and resolve complex 3-D compatibility issues in response to ongoing transtensional deformation. Previous workers have performed geologic and fault mapping, deformation rates and transport direction across the CCL are documented by an extensive GPS network, and instantaneous stretching and shortening axes have been derived from seismicity. Our current work, recently initiated in the CCL, will attempt to document and characterize structures and fabrics present in the Mesozoic basement rocks and the overlying cover that record active transtensional deformation at all scales. A progress report summarizing early reconnaissance field mapping and preliminary structural observations and indications will be presented. Map transects from fault block margins inward will assess the impingement of deformation into blocks. Patterns of deformation in the Coso basement will be compared to that of the overlying volcanics and to Sierran basement rocks outside of the transtensional zone, to determine the role of basement forcing of the volcanics/cover by preexisting or younger structures in the basement, and whether the volcanics/cover present a snapshot of instantaneous strain that is similar to or different from the strain pattern of the basement.