Differential uplift of fluvial and marine terraces across the Santa Ynez River fault, Santa Barbara County, California

Clay Kelty, Nate Onderdonk, Richard Behl, & Antonio Garcia

Published August 13, 2019, SCEC Contribution #9493, 2019 SCEC Annual Meeting Poster #097

The Santa Ynez River fault is a major regional structure along the central California coast that separates the Western Transverse Ranges and the Santa Maria Basin. The fault is interpreted as a steep, south-dipping, oblique left-lateral reverse fault (Sylvester and Darrow, 1979), but its kinematics, recency of faulting, and influence on local topographic development is unknown. Quaternary displacement is suspected along the Santa Ynez River fault because of relatively fast shortening and uplift rates in the region. However, there are no modern tectonic geomorphology or fault assessment studies across this basin-bounding fault. To evaluate the fault’s Quaternary kinematics, we are using fluvial and marine terraces as markers to measure displacement across the western portion of the Santa Ynez River fault. Field mapping, GIS analyses, GPS surveys, soil descriptions, and radiocarbon and luminescence dating allow us to correlate and compare terrace levels in the hanging and footwall of the fault.

Preliminary mapping and surveying suggest that the Santa Ynez River fault has not created detectable surface deformation in the Holocene. This preliminary interpretation is based on the correlation of undeformed, relatively young fluvial terraces across the fault. There is, however, significant differential uplift of late Pleistocene deposits across the fault. In the Western Transverse Ranges (hanging wall), extensive late Pleistocene fluvial deposits are lifted ~200 m above the active river channel. These deposits are likely equivalent or younger than the Orcutt Formation, which has recently been dated to be 85-125 ka. Correlative late Pleistocene terraces in the Santa Maria Basin (footwall) are only lifted 70-100 m above the active river channel. Therefore, tentative uplift rates in the hanging wall of the fault are ~2 mm/yr, whereas uplift rates in the footwall of the fault are ~1 mm/yr. The differential uplift rates are not definitive because numeric terrace ages are still being acquired. However, these preliminary results indicate that the Santa Ynez River fault uplifted the local topography relatively quickly in the late Pleistocene but has not moved in the Holocene.

Key Words
Santa Ynez River fault, Quaternary landscape evolution, tectonic geomorphology, terrace chronology

Citation
Kelty, C., Onderdonk, N., Behl, R., & Garcia, A. (2019, 08). Differential uplift of fluvial and marine terraces across the Santa Ynez River fault, Santa Barbara County, California . Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology