A TOP-TO-BOTTOM TIMELINE FOR PERMIAN TO EARLY CRETACEOUS EXTENSION WITHIN THE SOUTHERN SIERRA NEVADA ARC: ZIRCON U-PB-HF STUDIES OF CRYSTALLINE ROCKS FROM THE KERN PLATEAU AND GREENHORN MOUNTAINS

Zircon U-Pb-Hf isotope data from 19 Jurassic volcano-plutonic rocks from the Kern Plateau and 4 metasediments from the Greenhorn Mountains provide new insight to Mesozoic Sierran intra-arc extension. We redefine the hornblende-rich Summit Gabbro as the Summit gabbro-granite complex, a ca. 148 Ma (n=9) bimodal unit representing deeper equivalents of the Independence dike swarm. This complex includes the 9-km-diameter, gabbro-granite Osa Creek ring complex, a likely sub-volcanic structure located on the Kern Plateau shear zone (KPsz). Distinct from previously recognized Miocene-Pliocene volcanic structures on the Plateau, we assign hypabyssal-to-volcanic dikes and flows found across the central Kern Plateau to the new Tubatulabal volcanic unit. Regional context and U-Pb data justify its division into a ca. 148 Ma bimodal group (n=3; Tubatulabal B), and a ca. 160-170 Ma basalt-andesite-rhyolite group (n=4; Tubatulabal A). We interpret Tubatulabal B as the near-surface expression of the Summit and Independence units, with Tubatulabal A being the extrusive equivalent of Middle Jurassic granitoids that dominate the Kern Plateau, most notably the Sacatar quartz diorite. New U-Pb detrital zircon data from the SW Sierra document construction of the entire southern Sierra arc largely on parautothonous ribbons of Laurentian crust. Complete crustal rifting along the KPsz is documented by a ~40 eHf-unit pull-up at 148±1 Ma. We propose these latest Jurassic zircons crystallized from upwelling aliquots of depleted-mantle-derived basalt comingled with differentiated deep-crustal mafic magmas. By integrating published studies, we construct a top-down timeline for crustal extension across the breadth of the southern Sierra Nevada. Published works include: eHf data from the El Paso terrane that support extension from Permian subduction initiation into the early Triassic; Triassic-Jurassic volcano-sedimentary strata that document an extensive arc graben system; highly discordant detrital zircon that indicate intense Middle Jurassic hydrothermal circulation; detrital zircon trace element data that record Middle-to-Late Jurassic evacuation of deep-crust magma chambers beneath the southern arc; and basin formation in the western arc that documents earliest Cretaceous extension of the upper crust.