TECTONIC MODEL FOR THE COSO RANGE, EASTERN CALIFORNIA

A study of the Coso Range, located along the western margin of the Basin and Range province in eastern California, shows fault trends and motions that fit well with the Pliocene to Recent transtensional setting. Features of the pre-Cenozoic basement, including ductile shear zones, dike swarms, and cataclastic fault zones, can be used to infer motions on younger structures. These offset can be combined with new radiometric dates to understand the timing of Cenozoic displacements better. Active faults are mostly oriented west-northwest and about north-south. The relative ages with mutually-crosscutting relationships are evident between these sets. This is especially true in the southwestern portion of the Coso Range, where these two fault systems' interaction coincides with the Coso Geothermal Field. A major structure is the Haiwee Spring fault zone (trending north-northwest), which may accommodate right-lateral slip within the Coso Range. Strike-slip displacement is recorded by offset Late Cretaceous dikes and by the offset of another regional fault. In contrast, the Coso Wash fault zone has more of a dip-slip component. Overall, these two fault systems form an oblique-extensional system. Geothermal activity is focused in the area west of the Haiwee Spring and Coso Wash fault zones, where Quaternary volcanism coincides with the complex interaction between the fault sets. Correlation of structures within and adjacent to the geothermal area suggests that Quaternary volcanism is localized within a pre-mid Pleistocene graben segmented by left-lateral slip along several west-northwest-trending faults before, or perhaps, during the volcanic activity. Extensional deformation fits well with the interpretation of the area as an early stage metamorphic core complex.