GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 164-2
Presentation Time: 8:15 AM


ATTIA, Snir1, PATERSON, Scott R.1 and SALEEBY, Jason2, (1)Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, CA 90089-0740, (2)Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd, MC 100-23, Pasadena, CA 91125

The Sierra Nevada of eastern California represents a Cordilleran arc displaying episodic magmatic and orogenic activity from the Permian to Cretaceous. Tectonostratigraphic relationships and zircon geochronology from intra-arc deposits preserved in pendants reveal systematic shifts in surface evolution across the Mesozoic Sierran arc.

Isolated exposures of Permo-Triassic volcanics and sediments suggest that an earliest arc assemblage may have originally been more extensive. Triassic terrestrial and marine strata unconformably overlying pre-Mesozoic strata show unimodal Triassic detrital ages and few pre-Mesozoic grains. Jurassic marine deposits para-/un-conformably overlie prebatholithic rocks and Upper Triassic marine strata with minor carbonates. Lower Jurassic, deep to marginal marine sediments exhibit two distinct detrital zircon facies: coarser sediments show only Jura-Triassic ages whereas fine-grained strata show Jurassic major age peaks, few Triassic ages, and a spread of pre-Mesozoic detrital ages. By ca 180 Ma, the latter varied distribution is dominant. This trend continues with progressively younger major age peaks as marine Jurassic strata gave way to terrestrial Cretaceous sediments.

Pre-Mesozoic detrital ages in Mesozoic intra-arc strata match either prebatholithic framework and SW Laurentian margin detrital ages, indicating sediment recycling from older sources except in the Triassic when the locally emergent arc dominated retro- and intra-arc depositional systems. No detrital zircon age shifts are observed in response to proposed latest Jurassic arc/terrane collisions. Structural relationships are instead consistent with prolonged Middle to Late Jurassic contraction across the Sierra. The Early Jurassic is characterized by exhumation of the Snow Lake terrane, transition to deeper marine deposition across the Sierra, reorganization of sediment dispersal networks, and a pronounced magmatic lull. These shifts are accompanied by a decrease in volcanic and detrital zircon U/Th, and a ~175-185 Ma transient spike towards more evolved isotopic compositions of arc rocks. Thus, a dramatic shift in lithospheric architecture, magma sources, and arc surface expression occurred between ~180-195 Ma, possibly indicating extension across the arc and SW Cordilleran margin.