Quantifying Late Quaternary Deformation along the Santa Ynez River, Santa Maria Basin, California

The fault bounded Santa Maria Basin, located on the Central Coast of California, is positioned in an area of convergence between the rotating Western Transverse Ranges and the non-rotated Southern Coast Ranges. The Santa Ynez River Fault (SYRF) is an east-west trending fault that parallels the Santa Ynez River west of Lake Cachuma, California and defines the southern structural boundary of the Santa Maria Basin. However, the rate and style of Late Quaternary deformation and uplift in this region and the potential for seismic hazard along the fault is lacking. Fluvial terraces are key geomorphological components of fluvial systems that can be used to provide insights into regional and local uplift and deformation. The Santa Ynez River delineates the northern edge of the Santa Ynez Mountains and flows west through the Santa Ynez Valley to its mouth at the Pacific Ocean. The Santa Ynez River Field Area is a 10 km stretch of the Santa Ynez River just west of Lake Cachuma where terraces are well developed and the SYRF cuts through terraces and the active river (Figure 1). If there has been Quaternary movement of the SYRF we expect to find deformation in these areas. An initial survey of the area identified five terrace levels ranging from 8 m to 135 m above modern river level. The fluvial terraces are being mapped as separate units, surveyed for deformation with GPS based transects, and sampled for optically stimulated luminescence (OSL) dating. These combined methods will allow us to document the geomorphic characteristics and landform evolution of the lower Santa Ynez River, evaluate the possibility of Late Quaternary activity of the SYRF, and determine the rate of Late Quaternary regional uplift along the western Santa Ynez River in the Santa Maria Basin providing a possible basis for augmentation of the seismic hazards for Santa Barbara County.