UC Santa Barbara

The ~60 km Gaviota Coast of the Western Transverse Ranges, southern California, is a tectonically active region with emergent marine terraces. Rock uplift rates have been questioned near Gaviota Canyon, where the South Branch of the Santa Ynez fault (SBSYF) strikes offshore, oblique to the east-west trending coastline. No absolute dates have been reported within 35 km east of the fault, and unconventional ages reported west of the fault have not been verified. Establishing an accurate terrace chronology is crucial because large (~40 m) differences in paleo-sea level between marine isotope stages (MIS) 3 and 5 significantly change uplift rate estimates, which can impact tectonic models and assessments of seismic hazard. I provide new chronologic constraints for the Gaviota Coast, yielding rock uplift rates up to five times higher than those previously reported west of the SBSYF.

Twelve radiocarbon ages (40-50 ka) and eight optical ages (31-47 ka) from raised beach deposits indicate the first emergent terrace formed during MIS 3. These ages yield time-averaged rock uplift rates between 0.9-2.0 m/ky. In general, higher terrace elevations and rates of uplift (~1.5 m/ky) occur on the eastern Gaviota Coast, though local deviations occur and are controlled by small structures. West of the SBSYF, there is a 16 m drop in the elevation of the first emergent terrace, and rock uplift rates decrease to ~1.1 m/ky. These rock uplift rates are reaffirmed by the elevation spacing of a flight of five marine terraces preserved on the western Gaviota Coast. The terraces are much younger than previously believed, with the paleo-shorelines correlating to intermediate highstands from MIS 3 to late MIS 5 (45-84 ka), rather than from MIS 5-9. The terrace flight is offset by the SBSYF, with up to 0.44 m/ky of differential uplift accommodated across the SBSYF based on offset of the same-age terrace. A knickpoint in the longitudinal profile of Gaviota Creek, as well as a transition from alluvial to bedrock channel conditions across the fault, indicate the SBSYF is active and has a south-side up component of slip.

These results indicate that relatively high rock uplift rates persist for 110 km from Point Conception to Ventura, where even higher rates (up to 6-7 m/ky) have been reported. Because rock uplift rates on the western Gaviota Coast are higher than previously believed, seismic hazard models for the area should be re-evaluated, as recurrence intervals may be shorter and there may be potential for long ruptures along regional faults (i.e. Pitas Point fault system).