HEAT FLOW AND SEISMOTECTONICS OF THE WESTERN TRANSVERSE RANGES

As part of an ongoing investigation of thermal controls on the rheology of the Earth’s crust and the seismogenic behavior of active faults, we have measured heat flow in 11 wells in the Bardsdale, Torrey Canyon, Broadoaks, Big Mountain, Del Valle, Aliso Canyon, Placerita and Pacoima oil fields of the eastern Ventura basin. These measurements extend to depths as great as 2.9 km and provide unique constraints on the thermal state of the crust in a region of active compressional deformation that includes the 1971 Mw 6.7 San Fernando and 1994 Mw 6.7 Northridge earthquakes. Heat flow in the eastern Ventura basin ranges from a low of 42 mW/m² at the boundary with the central Ventura basin near Fillmore to a high of 63 mW/m² in the thin sediments of the Placerita oil field bordering the San Gabriel fault on the eastern margin of the basin. Simple thermal models indicate that almost all of the observed variation in heat flow is due to the thermal effects of varying Neogene sedimentation rates superimposed on a background heat flow of approximately 65 mW/m² . The new heat flow measurements have been combined with heat flow measurements from the central Ventura basin (De Rito et al., JGR, v. 94, 1989) to estimate temperatures at seismogenic depths. The results yield a comprehensive framework for investigating the relationship between temperature and seismicity in the region. Our analysis indicates that the deepening of seismicity from a maximum depth of approximately 15 km along the San Fernando rupture to approximately 18 km along the Northridge rupture and then to depths as great as 27 km in the west Ventura basin (Bryant and Jones, JGR, v. 97, 1992) corresponds to a seismic-aseismic transition temperature of 350 to 400 ^oC. These thermal constraints on the maximum depth of seismogenic faulting could be incorporated in future seismic hazard assessments of the western Transverse Ranges.