Stephen R. Hartwell

Shelf evolution along a transpressive transform margin, Santa Barbara Channel, California

High-resolution bathymetric and seismic reflection data provide new insights for understanding the post–Last Glacial Maximum (LGM, ca. 21 ka) evolution of the ~120-km-long Santa Barbara shelf, located within a transpressive segment of the transform continental margin of western North America. The goal is to determine how rising sea level, sediment supply, and tectonics combine to control shelf geomorphology and history. Morphologic, stratigraphic, and structural data highlight regional variability and support division of the shelf into three domains. (1) The eastern Santa Barbara shelf is south of and in the hanging wall of the blind south-dipping Oak Ridge fault. The broad gently dipping shelf has a convex-upward shape resulting from thick post-LGM sediment (mean = 24.7 m) derived from the Santa Clara River. (2) The ~5–8-km-wide Ventura Basin obliquely crosses the shelf and forms an asymmetric trough with thick post-LGM sediment fill (mean = 30.4 m) derived from the Santa Clara and Ventura Rivers. The basin is between and in the footwalls of the Oak Ridge fault to the south and the blind north-dipping Pitas Point fault to the north. (3) The central and western Santa Barbara shelf is located north of and in the hanging wall of the North Channel–Pitas Point fault system. The concave-up shape of the shelf results from folding, marine erosion, and the relative lack of post-LGM sediment cover (mean = 3.8 m). Sediment is derived from small steep coastal watersheds and largely stored in the Gaviota bar and other nearshore mouth bars. Three distinct upper slope morphologies result from a mix of progradation and submarine landsliding. Ages and rates of deformation are derived from a local sea-level-rise model that incorporates an inferred LGM shoreline angle and the LGM wavecut platform. Post-LGM slip rates on the offshore Oak Ridge fault are a minimum of 0.7 ± 0.1 mm/yr. Slip rates on the Pitas Point fault system are a minimum of 2.3 ± 0.3 mm/yr near Pitas Point, and decrease to the west across the Santa Barbara Channel. Documentation of fault lengths, slip rates, and rupture modes, as well as potential zones of submarine landsliding, provide essential information for enhanced regional earthquake and tsunami hazard assessment. INTRODUCTION Continental shelves develop and evolve primarily in response to tectonics, sediment supply, and eustatic fluctuations (e.g., Pratson et al., 2007). In passive and convergent continental margins, tectonics typically exert a regional control through processes such as thermal subsidence or growth of accretionary prisms. In contrast, the role of tectonics in transform continental margins can be both more local and complex, resulting from deformation on crustal faults that may have variable locations, trends, and geometries relative to plate motions and geologic framework. Few investigations have explicitly explored shelf evolution in such heterogeneous settings. Here we investigate a transpressional, 120-km-long section of continental shelf in the Santa Barbara Channel, within the distributed Pacific–North America transform plate boundary in California. The Santa Barbara shelf (Figs. 1 and 2) extends for ~120 km from Point Conception to the Hueneme submarine canyon, ranges significantly in both width (5–19 km), gradient (0.2°–0.8°), and sediment cover, and is cut and deformed by active faults considered capable of generating large earthquakes and tsunamis. The goal of this paper is to document latest Quaternary shelf evolution and geomorphology in this dynamic region in order to both enhance understanding of shelf evolution and to inform regional geologic hazard assessment. Recent high-resolution geophysical data collected for the California Seafloor Mapping Program (CSMP; Johnson et al., 2017a) provide the foundation for this study. Each of eight U.S. Geological Survey (USGS) CSMP publications (Johnson et al., 2012, 2013a, 2013b, 2013c, 2014, 2015a, 2017a, 2017b) include 10 or more thematic map sheets and a digital data catalog. These publications focus on data presentation, and are intended to inform a host of coastal management needs. This report significantly expands the minimalist and local (1:24,000 map scale) geologic analysis in the CSMP publications with a regional integrated focus on latest Pleistocene to Holocene shelf morphology and evolution. Mapping results are herein summarized in sections on seismic stratigraphy, sediment distribution and thickness, active faults and folds, and shelf and upper slope morphology. These results provide the basis for a discussion of local sea-level history and deformation rates, GEOSPHERE GEOSPHERE; v. 13, no. 6 doi:10.1130/GES01387.1 25 figures; 2 tables CORRESPONDENCE: sjohnson@usgs .gov CITATION: Johnson, S.Y., Hartwell, S.R., Sorlien, C.C., Dartnell, P., and Ritchie, A.C., 2017, Shelf evo‐ lution along a transpressive transform margin, Santa Barbara Channel, California: Geosphere, v. 13, no. 6, p. 2041–2077, doi:10.1130/GES01387.1. Received 22 June 2016 Revision received 27 June 2017 Accepted 9 August 2017 Published online 2 October 2017