Sediment Underplating During Shallow Subduction

During the Late Cretaceous to early Cenozoic, southern California was underplated by subducted sediments as the western margin of the arc and inner part of the forearc basin were simultaneously removed from the margin. Now partly exhumed to the surface, the Pelona-Orocopia-Rand schists crop out along a belt extending from the southern Sierra Nevada to southwestern Arizona and are inferred to record an evolution from normal subduction prior to the early Late Cretaceous to flat subduction extending into the early Cenozoic. Protolith and emplacement ages decrease from >90 Ma in the northwest to <60 Ma in the southeast as revealed by 40Ar/39Ar thermochronology and detrital zircon U-Pb ages.  These results define a cycling interval during which sediment was eroded, deposited in the trench, subducted, and accreted.  Metasandstones in the older schists originated primarily from the western belt of the Sierran–Peninsular Ranges arc. Younger units were apparently derived by erosion of progressively more inboard regions, including the southwestern edge of the North American craton.  The transition from outboard to inboard sediment sources appears to have coincided with removal of arc and forearc terranes along the Nacimiento fault, which most likely involved either thrusting or strike slip faulting.