Deformed Latest Pleistocene Fluvial Terraces Reveal Complex Active Faulting within Tectonic Transition Zone, Mendocino Triple Junction, Northern California

The transition between the San Andreas fault (SAF) system and the southern Cascadia subduction zone (CSZ) at the Mendocino Triple Junction (MTJ) encompasses a broad zone of complex deformation, the Mendocino deformation zone. Here, there are discrepancies between types of geological structures (transform or thrust faults) and recorded geodetic velocity vectors of plate motion. Though SAF-type stress is recorded north of the MTJ, there has been little geological evidence for resultant strain at these latitudes on the coast. We focus on the Van Duzen fault (VDF)—a possible subsidiary fault of the Little Salmon fault system, one of the southernmost active thrust faults within the onshore fold and thrust belt associated with CSZ. The VDF deforms young river terraces of varying age, which we use to develop a relative-age framework to contextualize activity along the VDF. Geomorphic analysis and a paleoseismic excavation across the VDF display deformation attributed to compressional stresses, which postdates young (3–11 ka) terrace deposition, at roughly 0.06–0.38 mm/yr. We hypothesize that the transition between CSZ and SAF tectonic regimes is geologically manifest through the orientation of compressional structures (i.e., VDF), which may illuminate dynamics associated with the migrating triple junction, past and present.