TECTONIC AND HYDROLOGIC IMPLICATIONS OF NEW HEAT FLOW MEASUREMENTS IN THE SANTA CLARA VALLEY, CALIFORNIA

The U.S. Geological Survey has acquired precision temperature logs from 7 monitoring wells in the Santa Clara Valley as part of a multidisciplinary ground-water investigation. The combination of the temperature logs with thermal conductivity measurements on cores and cuttings yields vertical profiles of heat flow through the Quaternary sediments that contain the primary aquifers of the basin. These data, together with lithologic, physical properties and water chemistry data from the wells, reveal a complex pattern of advective and conductive heat transfer within the valley. Subsurface temperatures and temperature gradients are depressed to depths up 200 meters, particularly in the holes along the southwestern side of the valley, where the flow of cool water dominates the shallow ground-water regime. There is evidence for a gradual increase in temperature along the flow path out to the southern San Francisco Bay, an observation consistent with heating of ground water by the vertical conductive heat flux from the underlying, relatively impermeable sedimentary units. In all 7 holes temperature profiles below 200 meters depth are relatively linear and consistent with conductive heat transfer. Heat flow measurements at these depths range from 73 to 96 mW m-2 with a mean of 86 mW m-2. This high heat flow is consistent with the measurements elsewhere in the region and characteristic of the Coast Ranges in central and northern California. The measurement of 73 mW m-2 is 8 mW m-2 lower than the next lowest value and comes from the Evergreen well drilled at the edge of the Evergreen basin on the northeast side of the valley. This anomalously low value may reflect the thermal effects of shallow crustal thickening due to compressional deformation across the boundary zone between the Silver Creek and southern Hayward faults.