Hidden Extreme Rate Burial/Exhumation in a Migrating Basin-Orogen System at the Mendocino Triple Junction, California, USA

The rates and kinematics of tectonic processes are generally thought to be reflected in the resulting landscape - with sites of rapid burial and exhumation typically being more rugged or high-relief. Although the plate boundary transition that occurs at the Mendocino triple junction (MTJ; northern California) represents a fundamental plate boundary change from subduction to translation, the landscape of the northern California coast ranges is relatively subdued or low-relief. Additionally, the MTJ region is marked by high levels of seismicity  indicating significant active deformation, but at the surface the effects are relatively minimal.  At present, the MTJ region is characterized by an abrupt change in crustal structure from a small, but deep, sedimentary basin - the Eel River Basin (ERB), north of the triple junction, to the exhumed Franciscan subduction complex (basement) to the south.  

New crustal seismic tomography for the region coupled with new low-T thermochronologic data and existing geophysical data (heat flow, seismicity, gravity) allow us to understand the cause of this basin-basement juxtaposition. Based on integrative modeling of the thermochronologic data with heat flow and other thermal indicators (vitrinite reflectance) we conclude that the ERB - Franciscan crust system migrates with the MTJ and represents the sequential occurrence of two extreme tectonic events. The ERB forms in advance of the MTJ, filling rapidly over a few million years to a maximum thickness of ~8-10 km. This basin is then rapidly exhumed and eroded in ~ 1 million years as the MTJ migrates, with exhumation rates on the order of order 8-10 mm/yr. In spite of these extreme exhumation rates, the resulting landscape is quite subdued, as a result of the migrating locus of tectonic activity, which leads to extreme but short-lived tectonic activity at any single location as the plate boundary system migrates. The Franciscan basement rocks record this burial/exhumation thermal history, but the lack of significant relief means that such tectonics could be easily missed in investigations of plate boundary evolution.