On day 3, we travel south through the Western Transverse Ranges, to the coastal area. Where we were on day 2, the San Andreas fault is oriented parallel to the tectonic plate motion, so it accommodates entirely strike-slip motion; in this portion of California, the San Andreas is striking more NW, meaning that in order for the relative plate motion to be accommodated here, there needs to not only be strike-slip motion, but also a significant component of shortening, which creates shortening structures like thrust faults, reverse faults, fault-related folds, and which all creates uplift and significant topography. We first see evidence for this in the Western Transverse Ranges, then see excellent evidence for it “in action” along the coast. At Ventura, we inspect several aspects of the Ventura Avenue Anticline, one of the fastest uplifting structures drive over the active scarp of the Ventura Avenue Anticline Thrust fault (6-7 mm/yr of uplift). We see this uplift history recorded by uplifted fluvial terraces of the Ventura River, as well as in the uplift history of marine terraces at Pitas Point.
The transpressional deformation that is required at the plate-scale is partitioned between strike-slip and dip-slip shortening structures at many scales–from the large (San Andreas and Western Transverse Ranges) to the intermediate (Santa Ynez Fault and Ventura Avenue Anticline) to the very small (outcrop-scale). We then visit Arroyo Burro Beach, where, due to the different orientations of the cliff face, we are able to appreciate strike-slip and dip-slip components of the transpressional deformation required to uplift the coastal area in the southwest part of Santa Barbara.
Entering the Western Transverse Ranges from the northFluvial terraces recording the uplift of the Ventura Avenue Anticline; these terraces have range in age from 80 ka to 30 ka; given their current elevations, they record the uplift history of the underlying structure to be 5-7 mm/yr over this time period.Marine terraces recording the holocene (<10k yr) uplift history of the Ventura Avenue anticline at Pitas Point; distinct terraces with age dates constrained by marine shells of 950, 2090, 4400 and 6700 years before present observed at this location (area with the palm trees in this image). These discrete terraces record 4 large-magnitude uplift events on the underlying structures; each of these events requires an earthquake in the magnitude range of 7.5-8.0 in order to explain the observed magnitude of uplift (Rockwell et al., 2016).Shortening structures on the southeast-facing cliff face at Arroyo Burro Beach, Santa BarbaraShortening structures at Arroyo Burro Beach (photo by C. Hanagan)A fine evening for a beach stroll to look at the transpressional structures responsible for the uplifted topography along the cliff face at Arroyo Burro BeachFelipe inspecting the distributed strike-slip deformation best expressed on the southwest-facing cliff face at Arroyo Burro BeachDiscussing the partitioning of strike-slip and shortening required to accommodate the relative plate motion and geometry at this locationArroyo Burro Beach–evidently some people visit this field site not just for the excellent structure outcrops, but also for its virtues as a “beach” for “fun?” And we’ve been looking the wrong direction, at the cliff face, the whole time? Will look into this in the future.Carpinteria State Beach, our campsite for the night.
