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

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.