Timing, Rates, Episodicity, and Sediment Provenance of Subduction Accretion: Establishing a Geochronologic Framework for Long-term Accretion in the Franciscan Subduction Complex

Stanford University, Stanford CA

Investigators

Abstract

The Franciscan subduction complex of California is the world's best studied ancient accretionary prism. The extensive, diverse data sets from the Franciscan are critical inputs for models for a very broad range of subduction-related processes, complimenting geophysical and geologic studies of modern subduction zones (which are almost entirely marine) and studies of other ancient on-land subduction complexes. Critically, however, the Franciscan and most other subduction complexes have proven very difficult to date and this severely impedes interpretation of many aspects of their development. The last ten years have seen major advances in several powerful geochronologic techniques that are highly applicable to subduction complex rocks, including U-Pb dating of detrital zircon grains from sandstones, U-Th-He dating of detrital zircons, 40Ar/39Ar dating of micas, and Lu-Hf dating of garnet and lawsonite. This project involves an integrated campaign to apply these methods to already well characterized target units across the full width of the Franciscan complex, to units accreted progressively from Jurassic through Miocene time. This new geochronologic framework will effectively complement a very large body of existing Franciscan work that is compromised by poor age control. The new zircon data will also be highly complementary with other large detrital zircon data set being generated from the Great Valley forearc basin, Klamath Province, Cordilleran foreland basin, etc. Synthesized, these data sets will address such issues as: (1) What was the chronology of accretion in this classic subduction system? What were rates of accretion and how did they vary over time? (2) How did the tectonics and paleogeography of the greater Franciscan-Great Valley-Sierra Nevada subduction system evolve over 165 million years of geologic time? (30 How do rates and episodes of accretion correlate with events in the Sierra Nevada magmatic arc, Great Valley forearc basin, Cordilleran foreland, as well as motions of Pacific basin paleoplates? (4) What were the sediment sources for Franciscan metasedimentary rocks and how did they evolve over time? Subduction is the geologic process by which one of the Earth's major tectonic plates moves beneath an adjacent plate and eventually descends into the Earth's mantle, over time periods of tens of millions of years. It is one of the most important global processes creating and modifying the Earth's surface. It generates chains of volcanoes, sedimentary basins, mineral deposits, and modifies the structure and composition of the crust and mantle. Subduction-generated volcanoes, earthquakes, and tsunamis are among the most severe natural hazards on Earth. Investigations of subduction processes use two broadly complementary approaches. Modern, active subduction zones are targets for geophysical, drilling, geologic, geochemical, and modeling studies, but lay almost entirely beneath the ocean surface, which severely limits direct observation. Ancient, extinct subduction zones are targets for extensive surface mapping, sampling, and geophysical study. Some ancient subduction zones expose a rock record of time-varying subduction over protracted periods sometimes exceeding 100 million years. The goal of this project is to provide a greatly improved framework of the ages of various rock subunits within the well-studied ancient Franciscan subduction zone in California. This improved geochronologic framework will allow much more robust interpretations of many other existing data sets from this subduction zone.