TERTIARY STRUCTURAL DEVELOPMENT OF OROCOPIA SCHIST AND RELATED LITHOLOGIES IN THE PETER KANE MOUNTAIN AREA OF SE CALIFORNIA

Detailed structural analysis of metamorphic tectonites, low- and high-angle normal faults, and planar dilational dikes in the Peter Kane Mountain Area (PKMA) of southeasternmost California, supported by geochronologic data from this and adjacent areas, reveals a complex extensional history from 48 Ma through 23 Ma. The Late Cretaceous-early Tertiary Orocopia Schist (hereafter “schist”) underlies and is structurally deepest in a series of highly disrupted tectonic units. Above the schist, along the low-angle Chocolate Mountains detachment fault (CMF), are Proterozoic(?) and Mesozoic quartzofeldspathic gneisses, Jurassic(?) Winterhaven Formation and related lithologies, and Oligocene and Miocene volcanic rocks. Sub-parallel, low-angle normal faults separate these upper-plate units. Regionally, ⁴⁰Ar/³⁹Ar studies of muscovite from the schist indicate the onset of extension along the CMF by at least 48 Ma. Nine ion microprobe analyses of zircons from monzogranite that intrudes the schist give a range of ²⁰⁶Pb/²³⁸U ages from 22-26 Ma, with a modal age of 23 Ma. Small-scale normal faults and fractures, asymmetries in protoclasic dikes, and the coeval intrusion of a porphyry related to the monzogranite indicate NE-SW directed extension at this time. The porphyry intrudes all faults, but is deformed only within those structurally higher than the CMF, indicating that late stages of exhumation of the schist were accommodated by these faults. Unlike Miocene core complexes further north and east, ductile and brittle structures associated with the schist are not kinematically related, evinced by the E-W trending retrograde lineation along the CMF. Intrusion of porphyry into all units suggests that the schist and all higher units were at similar structural levels by 23 Ma. Schist in the PKMA is petrologically distinct from that in adjacent areas, reflecting thermal metamorphism and pervasive cataclasis. Particularly notable are the paucity of white mica and relatively low amounts of total mica. These findings suggest that recent estimates for the total amount of Tertiary extension in the lower Colorado River extensional corridor may be too low.