Episodic Deformation Along a Blind Thrust in the Past 125 Kyr: The Santa Ynez River fault in the western Transverse Ranges of California

Clay Kelty, & Nate W. Onderdonk

Published August 12, 2020, SCEC Contribution #10440, 2020 SCEC Annual Meeting Poster #005

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. Because the fault is not exposed at the surface, little is known about its kinematics and recency of faulting. To evaluate the fault activity in late Quaternary time, we use fluvial and marine terraces that intersect the western part of the fault as geomorphic markers of deformation. Field mapping, GIS analyses, elevation surveys, and radiocarbon and luminescence dating of these terraces indicate that there was significant vertical displacement and folding across the fault in late Pleistocene time. The magnitude of this terrace deformation decreases with decreasing terrace age and the lowest two terraces show no detectable evidence of folding or vertical displacement. Consequently, the average rock uplift rate in the hanging wall of the fault decreased from approximately 2.0 m/ka to 1.0 m/ka, whereas the footwall of the fault has also been rising, but at slower rates that likely increase as a function of distance away from the fault. This terrace deformation pattern indicates: 1) that the Santa Ynez River fault is a blind thrust that elevated the south side of the fault, 2) that the most recent displacement along the fault occurred between approximately 80 to 35 ka, and 3) that the fault is probably not a significant seismogenic hazard. The growth of this blind thrust into a pre-existing river valley also significantly influenced the local fluvial geomorphology. In particular, we infer that the diversion of the Santa Ynez River into the rising south side of the fault and that the contrasting terrace morphology and valley width on both sides of the fault are likely a result of the complex interplay of tectonics, eustasy, and climate over the past 125 kyr. The episodic style of fault displacement and the subsequent landscape response is uncommon over timescales less than 125 kyr, but it may provide insights for how other nearby structures developed in late Quaternary time.

Key Words
Santa Ynez River fault, western Transverse Ranges, Santa Maria Basin, tectonic geomorphology

Citation
Kelty, C., & Onderdonk, N. W. (2020, 08). Episodic Deformation Along a Blind Thrust in the Past 125 Kyr: The Santa Ynez River fault in the western Transverse Ranges of California. Poster Presentation at 2020 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology