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Description
Approximately 5 to 9 Ma, Baja California was transferred from the North American to the Pacific plate. This transfer was accomplished through either strain partitioning into strikeslip motion along the Tosco-Abrejos fault and extensional opening in the Gulf Extensional province or through distributed dextral shear. PSRA is located in the southwest comer of the Colorado River corridor, roughly 25 miles north of Yuma, Arizona and 225 miles northeast of San Diego, California, and is therefore ideally located to assess which of the above proposals is most relevant to the geology of SE California. The stratigraphy of PSRC can be subdivided into three broad lithostratigraphic units. The oldest unit includes a basement complex comprised of Jurassic mylonitic gneisses, the latest Cretaceous-early Tertiary Orocopia Schist and the Jurassic Winterhaven Formation. An over 1 km thick section of mostly Oligocene volcanic rocks intervenes between the older units and capping middle to late Miocene gravels and basalt. A probable early Miocene detachment faults separate the Jurassic mylonitic gneiss from the Orocopia Schist while another probable detachment fault of the same vintage separates the Winterhaven Formation from the mylonitic gneiss and the schist. Though most previous workers have focused on the origin and characteristics of the detachment systems my work suggests that a younger series of structures overprinted the early detachment system and controls the current structural fabric of the park. For example, a major NW trending fault, herein referred to as the Taylor Lake fault, with as much 1 km of right-lateral displacement transects the central portion of PSRA. This fault on aerial photographs can be traced for over 25 miles southward to Yuma, Arizona. In addition, it appears to have developed contemporaneously with a set ofEW trending folds, a set ofEW trending reverse faults, and a set of NS trending normal faults. These structures fold or transect rocks as young as ~9 Ma, but do not appear to affect old gravels associated with the Colorado River (~4.7 Ma). Hence, the Taylor Lake fault and associated structures formed between about 9 and 5 Ma. I therefore speculate that the Taylor Lake fault and associated structures are part of a larger system and likely reflect the transfer of Baja California to the Pacific plate through a net work of distributed dextral shear.
