TRANSTENSION IN THE COSO-CHINA LAKE REGION, NORTHERN EASTERN CALIFORNIA SHEAR ZONE, SOUTHEASTERN CALIFORNIA

The Coso-China Lake region borders the southeastern end of the Sierra Nevada Mountains in the transtensional corridor bounding the eastern margin of the Sierra Nevada Microplate known as the Eastern California Shear Zone/Walker Lane. This zone of transtensional deformation accommodates the NW-directed oblique separation of the Sierran Microplate from western North America and represents an eastward-stepping of the principal Pacific-North American plate boundary zone, which is producing a zone of oblique rifting that may ultimately become the new plate boundary. Within this zone, the Coso-China Lake region is an area of moderate geodetic strain rate (est. 10-15/s), nearly continuous seismicity, high heatflow, active bimodal volcanism, and geothermal activity.

At the latitude of the northern Coso Mountains, the angle of oblique divergence between the Sierra Nevada microplate and the transtensional zone boundary (approximately the eastern Sierra rangefront) is 28°, separating at a rate of ~6 mm/yr between the Argus Range and the southern Sierra Nevada. This combination of simultaneous coaxial and noncoaxial strain components results in a coaxially dominated 3-dimensional non-plane (triaxial) bulk constrictional strain, and produces a complex system of normal, strike-slip, and oblique slip faults and associated constrictional folding and crustal thinning. Faults cut all lithologies including Mesozoic Sierra Nevada batholithic basement, Plio-Pleistocene volcanics, and Quaternary sediments, and are seen to reactivate both preexisting Mesozoic Sierra Nevada basement joints and shear zones, and late Miocene-Pliocene faults which accommodated the uplift and exhumation of the northern Coso range and associated Coso Formation. Younger off-fault brittle fracture and joint geometries kinematically resolve transtension, and are most common around outer block regions but occur internally throughout fault bounded blocks. An examination of the geographic and temporal distributions of faults and brittle features in the northern Coso range will begin to characterize partitioning of strain and kinematic reactivation of existing structures during regional transtension.