The Pelona, Orocopia, and Rand Schists (PORS) of southern California and southwestern Arizona (USA), compose a subduction complex emplaced beneath continental crust during low-angle subduction of the Farallon plate. Orocopia Schist has recently been discovered at Cemetery Ridge, Arizona, even farther inland from the continental margin than previously known PORS outcrops. Zircon U-Pb ages from Orocopia Schist metasandstone at Cemetery Ridge are dominantly Proterozoic, Jurassic, Cretaceous, and Paleogene. We interpret Cretaceous and older ages as detrital; they exhibit a probability distribution similar to that of other bodies of Orocopia Schist. Paleogene ages, which are rare elsewhere in the PORS, likely record metamorphic growth. The Orocopia Schist at Cemetery Ridge is intruded by early Miocene igneous rocks, which range from strongly foliated (older) to unfoliated (younger). This pattern is similar to that exhibited by metamorphic core complexes in southern Arizona. Orocopia Schist in a far inland setting has also recently been described within the footwall of the Plomosa Mountains detachment fault, 70 km northwest of Cemetery Ridge. The widespread distribution of underplated trench sediments in southern California–southwestern Arizona implies that similar rocks may be more common in convergent plate margins than previously recognized. Underplating of trench sediment can affect both composition and strength of lower crust.

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