Strath terraces in the Santa Ynez Valley suggest late Quaternary activity on a detachment fault beneath the Western Transverse Ranges, California

Nate W. Onderdonk, Andrew Farris, Edward Tyler, Ani M. Pytlewski, Antonio Garcia, & Shannon A. Mahan

Published August 14, 2017, SCEC Contribution #7638, 2017 SCEC Annual Meeting Poster #124

High rates of shortening across the western Transverse Ranges (WTR) have been interpreted from geodetic, geologic, and geomorphic data. Studies of active deformation of the WTR have been focused on structures along the Ventura-Santa Barbara coast. These structures are typically inferred to sole into a regional detachment fault, or system of detachment faults, that dip north and underlie the WTR. To test geometric models of these detachment faults, and evaluate their potential seismic hazard, we are using Quaternary surfaces and deposits in the interior of the WTR to determine uplift rates and patterns. We present preliminary results from the Santa Ynez Valley, which is a low area within the WTR located between two major reverse faults; the Santa Ynez fault and the Little Pine fault, both of which dip away from the valley and have lifted mountain ranges in their hanging walls. Despite being in the footwall of these faults, the major drainages of the Santa Ynez Valley have abundant strath terraces, indicating Quaternary incision due to uplift. We are using field mapping, GPS surveys, active channel longitudinal profiles, soil characterizations, and luminescence dating to study these terraces.

Luminescence dates from three terrace levels along the upper Santa Ynez River show large differences between BOSL and postIR-IRSL methods. More work is needed to resolve these differences, but current data indicate incision rates between 0.5 and 3 mm/yr. In Santa Cruz Creek, in the eastern Santa Ynez Valley, terrace heights and postIR-IRSL dates from three strath terrace levels indicate incision rates of 1 to 1.5 mm/yr since 82 Ka, increasing to more than 2 mm/yr during the Holocene. In Zaca Creek 20 km to the west, mapping and postIR-IRSL dating of two strath terrace levels suggest incision rates of approximately 1 mm/yr since 85 Ka.

These preliminary data from the Santa Ynez Valley suggest regional Quaternary rock uplift rates of at least 1 mm/yr. The terraces are only slightly deformed by a few minor faults and folds that cross the studied drainages and terrace genesis cannot be attributed to these local structures, or to climate forcing. The fact that rock uplift is occurring in the footwall of the major faults that bound the valley indicates that the uplift is regional and most likely due to slip on a detachment at depth. Continued work will refine and expand these data, and use them to model fault geometries and offsets at depth during the past 100 Ka.

Key Words
uplift, faults, western Transverse Ranges, Santa Barbara, Santa Ynez, detachment, Luminiscence

Onderdonk, N. W., Farris, A., Tyler, E., Pytlewski, A. M., Garcia, A., & Mahan, S. A. (2017, 08). Strath terraces in the Santa Ynez Valley suggest late Quaternary activity on a detachment fault beneath the Western Transverse Ranges, California. Poster Presentation at 2017 SCEC Annual Meeting.

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