ONSET AND CESSATION OF DEFORMATION ALONG A BLIND THRUST IN THE PAST 100 KA: THE SANTA YNEZ RIVER FAULT IN THE WESTERN TRANSVERSE RANGES OF CALIFORNIA

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. Since this reverse fault is not exposed at the surface, little is known about its genesis and deformation history. To evaluate the fault’s Quaternary movement, we use fluvial and marine terraces that intersect the western portion of the Santa Ynez River fault as geomorphic markers of deformation. Field mapping, GIS analyses, elevation surveys, and radiocarbon and luminescence dating indicate that there is significant differential uplift of late Pleistocene terraces across the fault. The deformation of these terraces decreases with decreasing terrace age, and the youngest Holocene terrace is undeformed.

Uplift in the hanging wall of the fault started shortly after the formation of the highest terrace (85-100 ka) but ended before the formation of the lowest terrace (6.6 ka). The uplift rate in the hanging wall decreased from ~2 mm/yr to ~1 mm/yr, whereas in the footwall, fluvial and marine terraces have been uplifting steadily at a rate of ~1 mm/yr over the past 100 ka. Therefore, we interpret that the ~2 mm/yr rate in the hanging wall was localized uplift due to late Pleistocene movement on the Santa Ynez River fault, while there was ~1 mm/yr of background uplift due to movement on deeper, regional structures. The Santa Ynez River fault’s episodic style of deformation is uncommon over timescales less than 100 ka, but it may be a model for how other nearby local structures developed in the late Quaternary.