TIMING AND IMPLICATIONS OF LATE CRETACEOUS SHALLOW SUBDUCTION REFRIGERATION OF SOUTHERN CALIFORNIA AND SOUTHWESTERN ARIZONA

Geochronologic (U-Pb detrital zircon and metamorphic overgrowths) and thermochronologic (⁴⁰Ar/³⁹Ar cooling age) constraints indicate that subducted trench-derived sediments were accreted beneath Jurassic-Cretaceous arc and cratonal crustal rocks along a ca. 500 km segment of the SW North American margin over a ca. 25 m.y. interval from 90 Ma at the latitude of the southernmost Sierra Nevada to 65 Ma in SW Arizona. In all cases, the accreted rocks – referred to as the Pelona-Orocopia-Rand Schist – structurally underlie continental crust across extensional faults with scarce evidence of intervening lithospheric mantle preserved. Current plate kinematic and mantle tomographic reconstructions of Laramide shallow subduction beneath the southwestern U.S. indicate that initiation of shallow subduction coincided with the arrival of a large oceanic plateau (Shatsky conjugate) at the margin by ca. 88 Ma but indicate that the topographic high of the plateau was rapidly subducted at rates approaching 100 km/m.y. and reached a position 800 km inboard of the trench by ca. 76 Ma. While the younger timing of accretion of schist in SW Arizona may relate to later subduction of the Hess Rise conjugate, this appears to have occurred too late (< 65 Ma) and too far south beneath mainland Mexico. Resolution of these different types of evidence for the timing and nature of shallow subduction requires understanding of the extent to which lithospheric mantle was removed and how much time was required for its removal. The impact of a subducting slab proximity upon mid-crustal rocks can be assessed from the magnitude and timing of subduction refrigeration. We have produced over 200 new, regionally distributed ⁴⁰Ar/³⁹Ar K-feldspar thermal history results from SW North American basement. When combined with prior data, the time of rapid cooling (350-150°C) of crust overlying the shallow subduction corridor was found to vary systematically from 90-85 Ma (southern Sierra Nevada) to 70-65 Ma (SW Arizona) with the youngest cooling ages obtained adjacent to exposures of POR schist. Our results demonstrate regional variability in the depth of the subducting slab beneath the sampled basement and seem to require a protracted and spatially variable process for the removal of subcontinental mantle lithosphere to accommodate shallow subduction.