LAUSR

The Santa Ynez River fault is a blind thrust in the western Transverse Ranges fold and thrust belt of southern California. To evaluate the late Quaternary fault kinematics and landscape development, we used fluvial and marine terraces that intersect the fault as geomorphic markers of deformation. Field mapping, GIS analysis, elevation surveys, and radiocarbon and postIR‐IRSL luminescence dating of these terraces indicate an approximately 50 kyr episode of vertical displacement and folding across the fault during the late Pleistocene. Deformation of terraces began at approximately 85 ka following an extended period of tectonic quiescence and fluvial planation in the area. Multiple terrace levels ranging in age from 95.5 ± 11 to 43 ± 5 ka are lifted and folded across the fault with the amount of deformation decreasing with decreasing age. Terraces younger than approximately 35 ka show no evidence of folding or vertical displacement across the fault, indicating that the fault has not slipped in the past 35 kyr. Deformation of the terraces shows that the Santa Ynez River fault exhibited alternating episodes of activity and quiescence on a timescale of tens of thousands of years. This episodic nature cannot be attributed to directional growth of the fold and thrust belt, fault linkage, or long recurrence intervals and evidence of fault slip since Miocene time suggests the episodic activity is not restricted to late‐Quaternary time. This unusual episodic behavior may be due to surface deformation switching back and forth between different shallow structures in the mountain belt through time.