DETRITAL ZIRCON RECORD OF THE EARLY MESOZOIC SOUTHWESTERN SIERRA NEVADA ARC PRESERVED IN EARLY CRETACEOUS INTRA-ARC AND FOREARC DEPOSITS

Late Triassic through Early Cretaceous detrital zircon grains separated from Early Cretaceous sediments provide a record of arc magmatism that is not obscured by products of the mid- to Late Cretaceous surge, which dominate the exposed Sierra Nevada batholith. Matching U-Pb age probability maxima to the U-Pb dates of exposed arc plutonic rocks provides confirmation that the detrital zircon grains were sourced in the erupting and eroding Sierra Nevada arc. These data suggest that magmatic productivity in the southwestern Sierra Nevada arc increased steadily through the Middle Jurassic, from an Early Jurassic lull through the Late Jurassic. Though apparent magmatic flux drops slightly in the Early Cretaceous, this time was not a lull in arc activity. Instead, the detrital zircon record implies an original footprint of the Early Cretaceous arc extending from its current exposure in the western Sierra Nevada foothills northwestward into the eastern Sacramento Valley, where its relatively mafic roots are presumably buried beneath younger sediments infilling the Great Valley. The sparse record of Late Triassic magmatism preserved in the analyzed intra-arc and forearc deposits likely reflects greater separations in both time and space between the Early Cretaceous basins and the footprint of the Triassic arc. The need for a greater density and geographic distribution of detrital zircon samples to reconstruct the record of arc magmatism is explored through examination of an atypically dense sampling of the Goldstein Peak Formation intra-arc basin deposits and by comparison of new data from the Early Cretaceous Gravelly Flat formation with published data from other Early Cretaceous forearc sediments of the Great Valley Group.