21 Structural Geology and Tectonics of the Western United States

Wednesday, 24 May 2017
8:30 AM - 11:50 AM
Room 325A

PRESERVATION OF A SUBDUCTION-RELATED GEOTHERMAL GRADIENT IN THE SCHIST OF SIERRA DE SALINAS, CENTRAL CALIFORNIA COAST RANGES

Paper number: 21-9
Program: Theme Sessions
Time: 11:05 AM – 11:25 AM

Authors

Abstract

The schist of Sierra de Salinas, located in the Coast Ranges of central California, is widely regarded to represent a product of Laramide shallow subduction. The schist preserves an inverted metamorphic gradient, recording peak temperatures that diminish with increasing structural depth from >700 ˚C to ~575 ˚C. This study seeks to clarify the processes that led to the construction of the exposed metamorphic gradient and the timescale over which it formed. We analyzed four samples collected across a range of structural depths using U-Pb detrital zircon geochronology to assess the timing of schist emplacement. All analyzed samples contain abundant Cretaceous detrital zircon grains, with diminishing percentages of Jurassic, Triassic, and Proterozoic grains, consistent with a Sierra Nevada arc source. Approximately 5% of the total grains yield ca. 1.38 Ga ages, pointing to an ultimate source of either Mojave Desert or central Idaho for these grains. Samples collected more than ~1 km from the upper plate-bounding shear zone yield a U-Pb maximum depositional age of ca. 84 Ma. The youngest zircon grains within this sample show limited textural and geochemical evidence for metamorphic recrystallization. In contrast, samples collected within ~1 km of the shear zone show up to 40 µm thick cathodoluminescence-bright rims that truncate interior oscillatory-zoned domains. These samples yield weighted mean ages from the youngest grain populations of ca. 78 Ma. The apparent upsection younging may be explained in three ways: 1) the schist may represent a subducted but otherwise intact stratigraphic section, 2) recrystallization of metamorphic zircon occurred within the Sierran arc, and/or 3) recrystallization of metamorphic zircon occurred in the subduction zone. We favor scenario 3 because recrystallized zircon is abundant at the highest structural levels where the highest peak temperatures are reported and partial melting has taken place, suggesting a possible causative relationship. We conclude that the structurally deepest levels of the schist, and perhaps the schist as a whole, were deposited after ca. 84 Ma and emplaced beneath a hot upper plate at ca. 78 Ma, forming an inverted metamorphic record of the geothermal gradient that existed during subduction.

Geological Society of America Abstracts with Programs. Vol. 49, No. 4

doi: 10.1130/abs/2017CD-292922

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