Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 20-9
Presentation Time: 9:00 AM-6:00 PM


CRAWFORD, Brandon1, HARTSHORN, Evan J.2, HEMPHILL-HALEY, Mark A.3 and MICHALAK, Melanie J.3, (1)United States Department of Energy, Los Alamos National Laboratory, EES-14, PO Box 1663 D452 Pajarito Road, Los Alamos, NM 87544, (2)Desert Research Institute, Division of Earth and Ecosystem Sciences, 2215 Raggion Pkwy, Reno, NV 89512, (3)Geology, Humboldt State University, 1 Harpst St., Arcata, CA 95521

Cape Mendocino, in northern California, is located within the current mapped transition between the Cascadia subduction zone to the north and the San Andreas fault system, referred to as the Mendocino triple junction (MTJ). The 1992 M 7.1 Cape Mendocino earthquake caused up to ~1.4 m of coseismic uplift, along 16 km of the coast. This uplift was not recorded immediately north of Cape Mendocino at two locations where uplift would have been noticeable (Bear River and Cape Ranch). South of Cape Mendocino, a flight of six Holocene marine terraces at Singley Flat with repeating ~1.5 m tall risers are tentatively correlated to a Holocene sea-level curve in order to estimate ages. We interpret these terraces formed from similar events to the 1992 earthquake: this suggests a recurring rupture pattern along this part of the CSZ. In contrast, to the north of Cape Mendocino, at Bear River and Cape Ranch, a single ca. Holocene terrace with a ~10 m tall riser exists and a flight of prominent Pleistocene terraces are mapped. Based on radiocarbon ages from previous studies and comparison to established sea level curves, we estimate poorly constrained late Pleistocence uplift rates of ca. 2.7m/ky north of Cape Mendocino, and ca. 2.8 m/ky to the south of Cape Mendocino. However, a higher Holocene uplift rate to the south is evidenced by the multiple Holocene terraces not observed to the north. We suggest that the distribution of dissimilar terrace ages and discordant ages may be recording structural and temporal complexity in the southern end of the CSZ. At least six, 1.5 m-high Holocene terraces located along the rupture length of the 1992 M 7.1 event may be typical for this part of the subduction zone, or, may indicate that this is not part of the larger CSZ and is a separate fault. The mapped Bear River fault, immediately north of Cape Mendocino, may represent a structural boundary where terrace formation and preservation to the north and south are significantly different.