Holes In The Hole-in-the-plate Model: The Late Cenozoic Thermal Regime Of Coastal California

In California, the heat flow shows a distinct pattern that can give insight into understanding of the nature of tectonism in the late Cenozoic after the cessation of subduction. In Coastal Ranges heat flow is 50% above the continental average whereas in the Great Valley and western Sierra Nevada region it is 50% below the average. The two thermal anomaly zones extend over 500 km south of the Mendocino triple point along the transform regime. The thermal transition zone is sharp in the north and becomes broader toward the south. Low heat flow is still preserved at areas where the subduction ceased 20 My ago. Using the linear symmetry of the structure and heat flow along the transform margin, we present 2D heat conduction models for the extinct California forearc for various times after the cessation of subduction. We tested two proposed tectonic scenarios, namely the slab window scenario and the stalled slab scenario, in terms of their consequences on the surface heat flow. The first scenario infers emplacement of asthenospheric material to shallow depth whereas the latter implies the thermal re-equilibration of the oceanic slab under the continent after subduction stops. The modeling shows that the slab window scenario cannot match the present low heat flow regime in the Great Valley and Sierra Nevada region whereas the stalled slab scenario cannot match the high heat flow in the Coastal Ranges. Low heat flow in the Great Valley cannot be due to sediment blanketing since the sedimentation rate is slow. In the southern Sierra Nevada region low heat flow occurs 20 My after the cessation of subduction. These facts preclude the possibility of formation of a slab window in these regions. The steep lateral decrease in the heat flow from the Coastal Ranges to the Great Valley shows that the source of the Coastal Ranges anomaly is in the crust. The slab window model is unable to predict this sharp change in heat flow. In the absence of a slab window, the present broad Coastal Ranges high heat flow can best be explained by dissipative heating in the ductile zone of the transform boundary. We conclude that heat flow distribution in California represents the superposition of the thermal regime of the stalled slab and shear heating in the ductile zone of San Andreas Fault system.