QUATERNARY CHRONOLOGY AND UPLIFT OF GAVIOTA COAST MARINE TERRACES, SOUTHERN CALIFORNIA

The ~50 km Gaviota Coast of the Western Transverse Ranges, southern California, is a tectonically active region with emergent marine terraces. Coastal uplift rates have been debated near Gaviota Canyon, where the South Branch of the Santa Ynez fault (SBSYF) strikes southwest offshore, oblique to the east-west trending coastline. No previous numerical dates have been reported from the east side of the fault to Ellwood, 35 km away. Establishing an accurate terrace chronology is crucial because large (~40 m) differences in paleo-sea level between MIS 3 and MIS 5 significantly change estimates of the uplift rate, and thus the perceived seismic hazard. We provide new chronologic constraints for the Gaviota Coast, yielding uplift rates six times higher than those previously proposed.

We identified several sites with marine sands and/or marine mollusk fossils directly overlying the paleo-wave cut platform in the first emergent terrace. Age control at each site was obtained through radiocarbon dating and/or optically stimulated luminescence (OSL). Two calibrated ¹⁴C dates, 41.1 ka and 44.2 ka (median probabilities, with 2σ error <±1.3 ka), indicate the terrace platform was cut during MIS 3. This agrees with a published OSL date of 36±3 ka at Bell Canyon, on the east margin of the Gaviota Coast. These dates support the hypothesis that the coastline east of the SBSYF is younger and experiencing more rapid uplift than to the west of the fault, where the first emergent terrace was previously dated to ~80 ka (0.3 m/ky). Future work will explore how this apparent shift in uplift rate affects the geomorphology of the Santa Ynez Mountains.

The complete chronologic history (additional analyses pending) will allow us to verify the terrace age west of the SBSYF, and assess the lateral extent of the MIS 3 terrace. We tentatively assign an age of 37 ka to the MIS 3 terrace, based on the closest regional highstand to the minimum ¹⁴C age. This yields uplift rates ranging from 1.7-1.9 ±0.5 m/ky, east of the SBSYF. Our results suggest that relatively high uplift rates persist for 85 km from Gaviota to Ventura, where even higher rates (up to 6-7 m/ky) have been reported. If high uplift continues uninterrupted for such a distance, there may be potential for long ruptures along offshore faults (i.e. Pitas Point fault), posing a serious seismic hazard.