Analysis of Holocene Marine Terraces, Cape Mendocino to Mattole River, Northern California: Interpretations and Implications to Mendocino Triple Junction Tectonics

Abstract:

The southern terminus of the Cascadia subduction zone (CSZ) of the Pacific Northwest terminates at the Mendocino Triple Junction, a region of elevated seismicity. Here, episodic, tectonically driven uplift is likely responsible for the formation of a suite of Holocene-aged marine terraces. In 1992, a M 7.1 thrust mainshock and two ~M 6.5 aftershocks occurred ~ 4 km offshore of Cape Mendocino resulting in ~1.4 m of uplift, measured from intertidal species stranded upon uplifted wave-cut platforms (Carver et al., 1994; Merritts, 1996). Using high resolution LiDAR data (NOAA, 2012), we constructed a detailed map of the uplifted 1992 wavecut platform and 9 adjacent higher terrace surfaces between Cape Mendocino and the mouth of the Mattole River. The risers associated with these surfaces are between 1 and 3 m high, similar to that of the 1992 event. This work offers insight into the upper plate crustal response to seismic events and inter-seismic periods in the tectonically complex southern end of the CSZ. By relating the elevations of the shoreline angles of these terraces to an established sea level curve (Gibbs, 1986) and a single absolute age on one terrace (Merritts, 1996) we estimate their ages of formation as all less than 6 ka. Eustatic sea level in the last 6 ka has been fairly stable indicating that these terraces formed during co-seismic uplift events. Marine terrace formation requires relative sea level stability thus there is no indication of vertical interseismic deformation along this portion of the subduction zone. This late Holocene behavior differs from other portions of the CSZ to the north where interseismic recovery has been documented. The presence of multiple, relatively small terrace risers in the last 6 ka suggests this portion of the subduction zone may rupture in smaller events on the megathrust or subsidiary faults.