The Geomorphic Transition between the Santa Barbara and Ventura Fold Belts near Rincon Point, California

ABSTRACT The Santa Barbara and Ventura fold belts have very different uplift rates based on marine terrace data: ~1-2 m/ky and ~6-8 m/ky, respectively. The onshore transition between these fold belts occurs at Rincon Creek, near Carpinteria, where the first emergent marine terrace rises from below sea level to nearly 200 m elevation over less than 10 km. A statistical comparison of normalized stream steepness values in weak rocks supports higher uplift rates east of Rincon Creek within the field area. Optically stimulated luminescence dating of the first emergent marine terrace between Carpinteria and Rincon Creek yields ages ranging from ~18-40 ka including error. The most reliable of these ages yield an average age of 32.81 ± 6.06 ka. This age is younger than previous dates of ~45 ka on the Punta Gorda terrace east of Rincon Creek but within the range of ages collected on MIS 3 terraces in Santa Barbara. These ages suggest the Carpinteria Bluffs marine terrace is continuous with the Punta Gorda terrace. Based on the age of 32.81 ± 6.06 ka and a local sea level curve, uplift rates on the Carpinteria Bluffs marine terrace are 1.8 ± 0.6 m/ky near Tar Pit Park and 3.2 ± 1.1 m/ky near the Carpinteria Bluffs Nature Preserve. Because the terrace is tilted toward the west, there is an east-west gradient in uplift rates from ~0 m/ky where the wave-cut platform emerges from below sea level at Carpinteria Beach to ~4-5 m/ky near La Conchita. The age and vertical offset of marine terrace deposits indicate minimum vertical slip rates of 0.06 m/ky and 0.20 m/ky on the Railroad fault and Carpinteria fault, respectively. Digital topographic analysis indicates Rincon Point is a geomorphic boundary between the subsiding Carpinteria basin and the uplifting Rincon Mountain. This geomorphic boundary may coincide with a proposed tear fault, but it is most likely influenced by the position of the Red Mountain fault, which takes a left turn offshore of Rincon Point. The profile of the Carpinteria Bluffs marine terrace can be interpreted as either vertically offset or kinked across Rincon Creek. The former supports the presence of a tear fault with an east-side-up sense of slip, but the latter discounts the tear fault in favor of uplift by the Red Mountain fault. Alluvial cover, brush, and private property have hindered discovery of convincing tear fault outcrops. A subaqueous delta at Rincon Point may conceal a bedrock ridge that coincides with the trend of the proposed tear fault, but no direct evidence for a structural origin of Rincon Point is available