New insights into the tectonic history and Quaternary fault activity of the Santa Lucia Bank region, offshore south-central California

Abstract:

Santa Lucia Bank (SLB), located ~60 km offshore Morro Bay, California, is a ~90-km-long and ~20-km-wide uplift at ~500 m water depth. The bank features a ~160-m-relief escarpment along its eastern flank that has been interpreted as a potentially active and seismogenic fault system, perhaps accommodating part of the right-lateral slip between the Pacific and North American plates. We present a 2018 geophysical dataset (multi-channel sparker seismic reflection, multibeam bathymetry, Chirp sub-bottom profiles, high-resolution autonomous underwater vehicle surveys) and complementary piston cores and vibracores that provide comprehensive high-resolution imaging of SLB for the first time. The data reveal limited evidence for Quaternary tectonic activity along the SLB escarpment.

Subsurface imaging of SLB reveals shallow, largely acoustically transparent basement with weakly reflective tilted strata. Just upsection, folded and faulted stratigraphy onlap the acoustic basement. Flat-lying stratigraphy within the Santa Maria Basin east of SLB buries and onlaps deformation along the flank of SLB. Some small-offset faults and shallow folds appear to deform the seafloor atop SLB, though a thin drape of undeformed sediments often caps the deformed strata. Shallow bank-top sediments are predominantly Pliocene age or older based on correlations to an exploration well, and preliminary analysis of piston core samples shows glauconitic sands at the seafloor, implying long exposure. Tilted strata in the acoustic basement support a non-plutonic tectonic origin for the basement formation which is consistent with previous studies suggesting early Tertiary generation in a subduction complex, comparable or equivalent to the Franciscan Complex. Shallow deformation likely originated during Miocene extensional basin growth and/or post-Miocene convergence and transpression. We suggest that SLB is unlikely to pose significant seismic hazard; the old seafloor, undeformed drape, and flat-lying, onlapping stratigraphy support little-to-no Quaternary tectonic deformation. However, piston cores adjacent to the bank show thick sand layers that likely derive from SLB. These sands have the potential to better constrain the earthquake history of the neighboring San Gregorio-Hosgri and/or San Andreas fault systems.