Abstract
Two new geologic observations at the Pleistocene Sutter Buttes volcano (SBV) give evidence of the likely topographic setting of the Sacramento Valley prior to the eruption of the SBV 1.6-1.4 Ma. 1) Within the central volcanic dome complex of the SBV there is a granitic outcrop about 650 m in diameter. This is the only exposure of crystalline basement in the Sacramento Valley. The granite is directly overlain by an andesite dome. Biotite from this granite yielded an Ar-Ar step-heating integrated age of 111.19+ or -0.14 Ma, with no obvious step-heating plateau. We are presently attempting U-Pb zircon dating of the granitic rocks. 2) The 16 Ma Lovejoy basaltic flood lava is found in subsurface, probable paleochannels, throughout the Sacramento Valley. However, the Lovejoy is not exposed in the SBV despite excellent exposures of the entire Sacramento Valley sedimentary section from Late Cretaceous to Pleistocene on the periphery of the central volcanic dome complex. The section conspicuously lacks the 16 Ma Lovejoy basalt. These observations suggest that the area that became the SBV was a topographic basement high for at least 16 Ma prior to the eruption of SBV. The granitic outcrop in the SBV is about 250-300 m elevation above sea level, about 220-270 m above the surrounding Sacramento Valley floor elevation. This outcrop is interpreted to be an exposure of the underlying crystalline crust, which elsewhere near SBV lies at a depth about 2 km based on nearby well intercepts. The Lovejoy flood basalt flowed around this preexisting topographic high and was never deposited in the area of the SBV. The SBV lies on top of a narrow magnetic and gravity high extending N-S through the much of the Great Valley. Little is known about the geologic nature of the Great Valley Geophysical Anomaly (GVGA). It has been proposed that this linear geophysical feature is the suture zone between Coast Range ophiolitic crust on the west and Sierran granitic crust to the east. It is proposed that structural deformation, perhaps faulting, may have locally uplifted granitic basement along the GVGA. The GVGA is currently seismically inactive in the area surrounding the SBV. Whatever the nature of this geophysical anomaly it likely provided a structural pathway through the crust for magmas that erupted to form the SBV 1.6-1.4 Ma in this no-slab, post-subduction tectonic setting.