Neotectonics of the Big Sur Bend, San Gregorio-Hosgri Fault System, Central California

The right-lateral San Gregorio-Hosgri fault system (SGHF) extends mainly offshore for about 400km along the central California coast and is a major structure in the distributed transform margin of western North America. We mapped a poorly known 64-km-long section of the SGHF offshore Big Sur between Piedras Blancas and Point Sur using high-resolution bathymetry, seismic reflection, and marine magnetic data. In this region, the SGHF is characterized by multiple strands, step overs, scarps and lineaments, shutter ridges, deflected drainages, and other geomorphic features consistent with strike-slip faulting. Analysis of offset shelfbreak gullies suggest a lateral slip rate of about 3.35mm/year. Vertical slip rates range as high as 0.8 to 1.2mm/year. Lateral slip combined with high vertical slip rates result in a northwest decrease in shelf width, a northward increase in shelf and upper slope gradient, and progressive incision of submarine canyon heads. The SGHF between Point Sur and Piedras Blancas trends 321 degrees and forms a 105-km-long transpressive bend (the Big Sur Bend) between more north trending transtensional fault sections to the south (120km long, 336 degrees trend) and north (180km long, 337 degrees trend). This transpressional bend and SGHF splay faults have had a significant role in shaping the modern geomorphology of the central California coast, controlling or influencing the locations of mountainous uplifts, large coastal headlands, embayments, and rivers. Plain Language Summary The northwest trending Hosgri-San Gregorio fault zone extends along the central California coast for 400km. We mapped a poorly known 64-km-long section of this important fault zone offshore of Big Sur between Piedras Blancas and Point Sur using high-resolution geophysical tools and methods. In this offshore region, surface and subsurface features indicate that the fault zone is active and is characterized by both right-lateral and vertical displacement. The northeast side of the fault is moving about 3.3mm/year to the southeast as it is being uplifted about 0.8mm/year. The Big Sur part of the fault zone forms a large bend between more northerly trending parts of the fault zone to the north and south. Fault movement through this Big Sur bend controls or influences locations of large coastal headlands, mountainous uplifts, rivers, and embayments.