Heat flow results from plate boundary observatory boreholes in California

Graduation date: 2011An integral component to understanding lithospheric rheology, fault mechanics, and geodynamic processes is knowledge of the subsurface thermal regime. This study presents thermal data from 26 new borehole sites drilled during the installation of borehole strain meters as part of the Plate Boundary Observatory (PBO). These data extends existing coverage for heat flow and radiogenic heat production measurements for California, which helps constrain the role of temperature in geologic processes such as the spatial and temporal pattern of deformation within this plate boundary zone. The boreholes are clustered in five areas: Anza (n=5), Parkfield (n=7), San Juan Batista (n=5), San Francisco (n=5), and Northern California (n=4). Temperature measurements are made to the bottom of each borehole, which vary in depth from 97 to 245 m. Measured temperatures were corrected for the 3-dimensional effects of terrain and more than 900 thermal conductivity and 68 radiogenic heat production measurements were determined from drill cuttings and core samples. Terrain corrected thermal gradients are combined with harmonic mean thermal conductivities to calculate heat flow. New heat flow values fall between approximately 52 and 95 mW m⁻² in Anza, 42 and 121 mW m⁻² in Parkfield, 67 and 109 mW m⁻² in San Juan Batista, 82 and 93 mW m⁻² in the San Francisco Bay Area, and 38 and 65 mW m⁻² in Northern California. New heat flow measurements reported for Northern California are low to moderate, consistent with values typically found in continental fore-arc regions along subduction zones. Heat flow values in the other areas show elevated heat flow that is characteristic for the California Coast Ranges