On Day 2, we travel along nearly the full trace of the San Andreas fault through segments that ruptured during the 1857 Fort Tejon earthquake. Along the way, we note several geomorphic features of the fault zone, including sag ponds, pressure ridges, linear valleys, and offset geomorphic markers. We will stop at Wrightwood, the site of important paleoseismic studies constraining the earthquake history on the San Andreas Fault; we will discuss the specifics of this site and how its particular features make it particularly well-suited for paleoseismic study. At Appletree Flats, Palmdale, and near Lake Hughes, we will note the variability in the width and expression of the San Andreas fault. We also make one last stop to look at the Punchbowl Fault and associated deformation at Devil’s Punchbowl County park, where progressive transpressive deformation adjacent to the fault resulted in uplift, folding, erosion, and the development of unconformities that record the slip history on this fault. As we head north, we make a quick stop at Wheeler Ridge to look at shortening and uplift features before continuing on to Carrizo Plain National Monument, where we will see classic outcrops of offset drainages and other geographic indicators like pressure ridges that constrain the geometry and slip rates on the San Andreas fault near the northern termination of the 1857 Fort Tejon earthquake rupture.
Photos from Day 2:
Students inspecting the wide damage zone in the San Andreas Fault at Appletree Flats.A wide zone of deformation on the San Andreas Fault at Appletree Flats. Panorama of uplift in a restraining bend at the outcrop of the San Andreas Fault near Palmdale along Highway 14. Group photo in front of the Palmdale San Andreas fault outcrop (photo from D. Lamb).Panoramic view of folded Pliocene shallow oceanic San Francisquito Formation and arkosic sandstones of the Miocene-Pliocene Punchbowl Formation, in a transpressional series of faults adjacent to the strike-slip Punchbowl Fault, Devil’s Punchbowl County Park. Angular unconformities in this stratigraphic section record the history of distinct periods of uplift, folding, and erosion associated with past activity of this fault system.Folded sedimentary beds adjacent to the Punchbowl Fault at Devil’s Punchbowl (Photo from D. Lamb)The group exploring the folded sediments at Devil’s Punchbowl (photo from D. Lamb)Panorama of the group marking the intersection of Wallace Creek and the San Andreas Fault, Carrizo Plain National MonumentOffset drainages along linear ridge marking the location of the San Andreas Fault, Carrizo Plain National MonumentThe group lined up along the San Andreas Fault, marking the offset of Wallace Creek. At this location, the abandoned drainage of paleo-Wallace Creek on the west has been age dated at 3800 years; because it has been offset 420 feet (130 m) since then, the slip rate on the San Andreas at this location can be constrained to be 34 mm/yr over this time interval (Sieh and Wallace, 1987).Dragon’s Back Pressure Ridge, a location where rocks are deformed and uplifted due to strike-slip motion on a portion of the San Andreas Fault that is slightly non-planar, resulting in local transpression and uplift, Carrizo Plain National Monument. Group photo in front of the Dragon’s Back pressure ridge in Carrizo Plain National Monument (photo from D. Lamb)
