MAKING THE PLATE BOUNDARY: LITHOSPHERIC EVOLUTION IN THE WAKE OF THE MENDOCINO TRIPLE JUNCTION

The tectonic evolution of the Coast Ranges in northern California reflects the suite of thermal and deformational processes that are modifying the North American crust and driving the formation of the primary plate boundary structure – the San Andreas – Hayward – Rodgers Creek - Maacama Fault system in central and northern California. As a consequence of the migration of the Mendocino triple junction (MTJ) there is a fundamental change in plate interactions leading to the formation of the San Andreas transform system, The systematic migration of the MTJ allows a space-for-time substitution to be made. With this we can isolate the stages in the evolution of the plate boundary. In particular we see the following: 1. In association with the southern edge of the subducting slab (i.e. at the latitude of Covelo) there is substantial (ephemeral) crustal thickening, which in concert with the thermal pulse associated with the opening of the slab window, drives metamorphism of the middle and lower crust. 2. The subsequent crustal thinning through a swath in the Coast Ranges drives the observed enhanced surface heat flow, and appears to help localize the development of the plate boundary fault systems. This crustal thinning appears to be accentuated in the Willits to Ukiah corridor and may help drive the rapid development of the Maacama fault. 3. South of Ukiah, additional crustal thinning appears to be minimal and the tectonics are dominated by the maturation of the fault systems, such that by the San Francisco Bay region, the Hayward fault as become a primary component of the plate boundary. Associated with the substantial changes in crustal structure is a complex pattern of uplift and subsidence and associated exhumation and basin development that migrates with the MTJ.