Seafloor scarps, stepover geometry, and kinematics of the Newport-Inglewood fault zone offshore Oceanside, California

James Conrad, Daniel S. Brothers, Maureen L. Walton, Ray W. Sliter, & Peter Dartnell

Published August 15, 2018, SCEC Contribution #8732, 2018 SCEC Annual Meeting Poster #160

High-resolution Chirp sub-bottom data were collected in 2018 off the southern California coast between San Mateo Point and Carlsbad to image the right-lateral Newport-Inglewood (NI) fault zone and related faults on the outer shelf and upper slope. The data show two main strands of the NI fault (the Dana Point and Oceanside strands) separated by a ~3-km left step (the San Onofre stepover) northwest of Oceanside. Seafloor scarps showing apparent vertical offsets of 1-2 m have been imaged along both main strands of the NI fault, providing clear evidence of post-transgressive (i.e., Holocene) offset. Sediment wedges deposited along both of these scarps have been cored and sampled for future radiometric dating.

Our new Chirp profiles, along with existing deeper-penetrating seismic reflection data, help to constrain deformation along the left-step in the NI fault offshore of the San Mateo Point-Oceanside coastline. Between the main NI fault strands, the stepover zone emplaces folded Miocene strata against younger flat-lying shelf sediments. This transpressional pop-up structure is bounded to the NE by an arcuate, SW-dipping reverse fault (imaged in deeper-penetrating industry seismic data) that up-warps adjacent young shelf sediments, linking the two main strands of the NI fault.

About 5-8 km NW of the NI stepover zone, the San Onofre (SO) fault, which trends obliquely up the slope from the south, appears to meet the NI fault at an angle of about 20°. Seafloor deformation apparent in bathymetric data reveals right-lateral offset of the slope along the SO fault. New Chirp profiles collected over this fault indicates offset of the Holocene drape layer. These results suggest that right-lateral slip is actively transferring onto the NI fault from the SO fault offshore San Mateo Point.

Key Words
Newport-Inglewood, Oceanside, San Onofre, strike-slip, hazard, seismic, chirp, transpression, offshore

Citation
Conrad, J., Brothers, D. S., Walton, M. L., Sliter, R. W., & Dartnell, P. (2018, 08). Seafloor scarps, stepover geometry, and kinematics of the Newport-Inglewood fault zone offshore Oceanside, California. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)