LINKING PRECAMBRIAN TO MESOZOIC DETRITAL ZIRCON SOURCES AND INTRA-ARC DEPOSITS TO EVALUATE THE OROGENIC EVOLUTION OF THE MESOZOIC SIERRA NEVADA CORDILLERAN ARC

The Mesozoic Sierra Nevada arc of eastern CA is a type long-lived Cordilleran arc displaying temporally variable orogenic responses to tectonic boundary conditions. New detrital zircon U-Pb ages from Mesozoic intra-arc sediments exposed in Sierran host rock pendants show systematic variations in provenance that likely reflect this evolving orogenic response.

Triassic terrestrial sediments with minor submarine deposits, part of a large package of altered, intermediate-felsic, calc-alkaline volcanics lying unconformably over pre-Mesozoic strata, show unimodal Triassic detrital ages, few pre-Mesozoic grains, and no provenance changes with depositional environment. These sediments sourced the proximal, somewhat emergent Triassic arc. Jurassic shallow- to deep-marine deposits with interbedded mafic-felsic volcanics unconformably overlie Triassic rocks to the east, and Permo-Triassic oceanic rocks and pre-Mesozoic strata to the west. This widespread overlap sequence shows major Jurassic peaks younging from ~200-150 Ma from E-W across the Sierra with few Triassic grains and a spread of pre-Mesozoic ages. Less widespread lower Cretaceous sediments, deposited as the arc gradually became emergent, are dominated by ~145-170 Ma detrital ages with decreasing abundance of older grains. These Jurassic ages reflect magmatic activity of the largely submarine yet continental arc. Detrital ages of terrestrial and volcanogenic Late Cretaceous intra-arc sediments reflect the Cretaceous magmatic flare up of the then highly emergent arc.

We compare pre-Mesozoic detrital ages from these Mesozoic deposits (~45% of ages) to compiled detrital age data from pre-Mesozoic SW Laurentian rocks. The spread of pre-Mesozoic ages found in Sierran intra-arc sediments suggests recycling of W Laurentian passive margin strata. The dearth of ~1.85 Ga detrital ages indicates limited input from allochthonous Paleozoic rocks. Neoproterozoic-Paleozoic detrital ages are consistent with coeval outboard Cordilleran arc basins but the sources of these grains are unclear. We cannot discriminate whether pre-Mesozoic detrital ages in intra-arc basins are sourced from intra-arc Sierran framework rocks, inboard SW Laurentian passive margin strata, or outboard fringing arcs with the current Sierran detrital zircon data alone.