NEW HEAT FLOW MEASUREMENTS FROM PLATE BOUNDARY OBSERVATORY BOREHOLES IN CALIFORNIA

An integral component to understanding lithospheric rheology, fault mechanics, and geodynamic processes is knowledge of the subsurface thermal regime. We present 26 new heat flow values from boreholes drilled during the installation of borehole strain meters as part of the Plate Boundary Observatory (PBO) along the San Andreas Fault. These new values extend existing heat flow coverage in California, and help constrain the role of temperature in determining the spatial and temporal pattern of deformation within this plate boundary zone. The boreholes are clustered by area: north of the Mendocino triple junction (n=4), San Francisco Bay Area (n=5), San Juan Batista (n=5), Parkfield (n=7), and Anza (n=5). The boreholes vary in depth from 97 to 245 m. Temperature profiles were measured in each borehole and more than 899 thermal conductivity measurements were determined from drill cuttings and core samples. Heat production measurements are currently in progress. Temperature gradients have been corrected for the perturbing effects of terrain and this data was combined with thermal conductivity to form thermal resistance plots and calculate heat flow. From north to south, heat flow values range between approximately 42 and 72 mW m^-2 north of the Mendocino triple junction, 78 and 92 mW m^-2 in the San Francisco Bay Area, 68 and 112 mW m^-2 in San Juan Batista, 52 and 116 mW m^-2 in Parkfield, and 53 and 99 mW m^-2 in Anza. With the exception of values north of the Mendocino triple junction, heat flow values are consistent with elevated heat flow that characterizes much of the California Coast Ranges.