High-Angle Oblique-Slip Faults in the Southern Eel River Basin: Forearc Deformation at Migrating Triple Junction

Yun, Janet¹, J. Casey Moore², N. Driscoll³, Danny O;Shea⁴, Leah Hogarth³ (1) Chevron Energy Technology Company, Houston, TX (2) UC Santa Cruz, Santa Cruz, CA (3) Scripps Institute of Oceanography, UCSD, La Jolla, CA (4) Humboldt State University, Arcarta, CA

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wo young, east-west trending, south-dipping, high-angle deformation zones are observed in nearshore seismic reflection profiles north of Cape Mendocino in the southern offshore Eel River basin. These deformation zones exhibit possible left-lateral up-to-the-south oblique-slip and are the youngest structures on the continental shelf. Deformation along these high-angle faults likely initiated in the late Pleistocene, with some deformation continuing into the present. Projected onshore, these deformation zones appear to influence onshore topography. The high-angle deformation represents one of at least two types of structures in the southern Eel River basin. On the continental shelf, deformation appears to have changed from northwest-southeast trending broad anticline/syncline pairs that were active in the mid-Pleistocene, to east-west trending high-angle faults that appear active in the late Pleistocene to Holocene. The south-dipping high-angle faults are in a location where north-dipping reverse and thrust faults are mapped onshore. Known gas venting along the trend of one of the high-angle structures suggests that slip may be facilitated by overpressured fluids within the fault zone. The change in deformation type and structural trend may be related to the northward migration of the Pacific plate and the Mendocino triple junction.