Upper Paleocene to middle Miocene fluvial-deltaic rocks in the Los Angeles and Ventura basins were deposited by a Colorado paleoriver prior to 300 km of dextral displacement on the San Andreas fault. During the late Miocene, movement on the fault and associated rifting in the Salton trough rerouted the paleoriver into the proto–Gulf of California. Since the Pliocene, translation of a moving Colorado deltaic depocenter along the fault has filled the Salton trough progressively southeastward. The postulated Colorado paleoriver extended north approximately along the lower course of the present river where it joined with Gila and Amargosa paleotributaries. Late Cenozoic tectonism disrupted the Amargosa paleoriver, and the modern course of the Colorado River through the lower Grand Canyon was developed.

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