TECTONICS OF THE COSO RANGE, CALIFORNIA

Study of the Coso Range, located along the western margin of the Basin and Range province in eastern California, supports well the transtensional setting for the area. Features of the pre-Cenozoic basement, including ductile shear zones, dike swarms, and cataclastic fault zones, provide a basis for interpreting younger structural offsets. New radiometric dates provide limits on the ages of Cenozoic displacement. The two dominant fault trends in the area are west-northwest and about north-south; however, relative age relationships are not everywhere consistent, and mutually-crosscutting relationships are evident between these sets. This is especially true in the southwestern portion of the Coso Range, where the interaction of these two fault systems coincides with the significant geothermal resource. A major feature of the tectonic model 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 (described by others) appears to accommodate more of the dip-slip component. Overall, these two fault systems form an oblique-extensional system. Both fault zones appear to be integral parts of the structural framework bisecting the Coso Range. Geothermal activity, however, appears to have been focused in the area west of the Haiwee Spring and Coso Wash fault zones, where Quaternary volcanism coincides with complex interaction among the two dominant fault trends. Correlation of structures within and adjacent to the geothermal area supports a tectonic reconstruction in which the Quaternary volcanism was localized within a pre-mid Pleistocene graben that was 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.