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

October 18, 2017

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 wave-cut 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.

Citation Information

Publication Year 2017
Title Shelf evolution along a transpressive transform margin, Santa Barbara Channel, California
DOI 10.1130/GES01387.1
Authors Samuel Y. Johnson, Stephen Hartwell, Christopher C. Sorlien, Peter Dartnell, Andrew C. Ritchie
Publication Type Article
Publication Subtype Journal Article
Series Title Geosphere
Index ID 70191858
Record Source USGS Publications Warehouse
USGS Organization Pacific Coastal and Marine Science Center

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