THE WESTERN MARGIN OF THE SIERRA NEVADA BATHOLITH (SNB) AS RECORDED IN THE CALIFORNIA GREAT VALLEY SUBSURFACE: STRUCTURE, COMPOSITION AND MAGMATIC SOURCE REGIME

The western margin of the Cretaceous SNB constitutes a significant portion of the eastern two-thirds of the California Great Valley basement, lying nonconformably beneath Upper Cretaceous to Eocene forearc basin strata. Out of ~300 basement cores studied, ~60 per cent are Early Cretaceous SNB plutons. Of these ~20 per cent are gabbroic, noritic and dioritic cumulates, with the remainder being tonalites and low-K granodiorites. The mafic cumulates occur in probable chains of ring dike intrusive complexes, as locally exposed along the adjacent Sierran Foothills, which render linear gravity-magnetic anomalies. These anomalies are traditionally misinterpreted as being derived from Middle Jurassic ophiolite, which is not supported by the basement core suite. Metamorphic wallrocks for the subsurface SNB are typical of the Sierran Foothills belt, consisting of andalusite spotted slate (Jurassic turbidites), Paleozoic meta-ophiolite, and subordinate Jurassic arc/forearc mainly mafic metavolcanics. These all commonly possess static contact metamorphic overprints. Rare low-grade metabasite cores lacking contact metamorphic overprints occur in the extreme west (deepest) cores, and likely represent samples akin to the Middle Jurassic Coast Range ophiolite. U/Pb zircon ages on noritic through tonalitic cores range from ca. 140-116 Ma, and Sr-Nd isotopic data reveal a depleted mantle source. The host rock regime for the Great Valley subsurface and adjacent Sierran Foothills SNB consists of extremely depleted ophiolite accreted from Paleozoic Panthalassa abyssal lithosphere as primitive forearc during the Permo-Triassic initiation of the SW Cordilleran convergent margin. Geochemical and structural maturation of this primitive forearc entailed multiple cycles of arc/forearc mafic magmatism, and cycles of compressional and extensional tectonism in Triassic through Jurassic time. Widely superposed on the subsurface SNB and its wallrocks are retrograde mylonitic and chloritized cataclasite breccia fabrics (~35 per cent of all core types). Structural and thermochronological data indicate that these fabrics developed during a distinct phase of rapid tectonic exhumation in the Late Cretaceous, prior to and during the early stages of depositional overlap by Upper Cretaceous forearc basin strata.