MARINE TERRACE FORMATION ASSOCIATED WITH NORTHERN MIGRATION OF THE MENDOCINO TRIPLE JUNCTION AT CAPE MENDOCINO, CALIFORNIA

Significant increases in uplift rates and changes in terrace morphology occurs along the southernmost Cascadia subduction zone between Bear River and Singley Flat in northwestern California near Cape Mendocino. The 1992 M 7.1 Petrolia earthquake caused ~1.4 m of episodic uplift, resulting in vertical displacement recorded on 16 km of the coast; uplift did not occur north of Singley Flat at either Bear River or Cape Ranch. In order to determine whether episodic uplift events similar to the 1992 M 7.1 earthquake are not occurring at these northern sites, high resolution LiDAR data and field mapping were used to create a geologic map and to estimate elevations of multiple marine terraces. A surface classification model was used to quantify slope and surface roughness, evaluating whether surfaces meet the distinct topographic characteristics of marine terraces. A Pleistocene sea-level curve (Waelbroeck et al., 2002) was used to correlate interpreted terraces with sea-level highstands and for estimation of uplift rates. Modern uplift rate estimates are 2.7 m/ky for Bear River, 2.7-3.8 m/ky for Cape Ranch, and 2.8 m/ky for Singley Flat. Bear River and Cape Ranch each have two Holocene marine terraces, the youngest with 10 m tall risers. Singley Flat, within the uplift zone of the 1992 earthquake, has six Holocene marine terraces separated by 1-1.5 m tall risers. These differences suggest the Bear River fault, several splays of which separate these two zones of marine terraces, is active and responsible for increased uplift rates north of Singley Flat. The mechanism responsible for uplift north of Cape Mendocino may be different from the episodic coseismic uplift events occurring south of Cape Mendocino. The ages, elevations, and morphologies of Holocene marine terraces near Cape Mendocino suggest that there are temporal changes in uplift rates potentially associated with the northward migration of the Mendocino Triple Junction and possibly with complexities of the southern end of the Cascadia subduction zone.