ABSTRACT The Carrizo Plain, the only closed basin in California’s Southern Coast Ranges, preserves landforms and deposits that record both climate change and tectonic activity. An extensive system of clay dunes documents the elevations of late Pleistocene and Holocene pans. Clay dune elevations, drowned shorelines, eroded anticlinal ridges, and zones of perturbed soil chemistry provide evidence of two lake levels higher than today’s (currently 581 m above sea level [masl]), one at ~591 masl at ca. 20 ka and another at ~585 masl that existed at ca. 10 ka, based on optically stimulated luminescence (OSL) dates on clay dune sediment. Two cores from the abandoned floor of the lake provide additional evidence of a long-lived lake in the Carrizo Plain during the late Pleistocene. The longer of the two cores (~42 m) was sampled for palynology, environmental magnetism, and scanning electron microscope–petrography. The magnetic susceptibility signal contains two notable features corresponding to sedimentary materials consistent with reducing conditions. The higher of these features occurs near the surface, and the lower occurs at ~18 m depth. A 14 C date on charcoal from the upper reduced zone places the top of this zone at no older than 22.6–20.9 cal ka. This date is consistent with the OSL date on geomorphic features associated with a highstand above ~591 masl. Assuming that reducing conditions correspond to at least a few meters’ depth of relatively fresh water, the new 14 C date suggests that the upper reduced zone represents a marine isotope stage (MIS) 2 pluvial maximum lake in the Carrizo Plain. Pollen and ostracodes from the reduced sediments indicate a wetter and cooler climate than today. These conditions would have been capable of sustaining a lake with water much less saline than that of the modern lake. The timing of the oldest documented highstand (no later than 20 ka) is consistent with a modified jet stream migration model and is not consistent with a tropical incursion model. Northeast-to-southwest asymmetry across the lake floor may be consistent with southwestward tilting driven by Coast Range shortening normal to the San Andreas fault, as is seen throughout the region.

Abstract The 1911 Chon–Kemin (Kebin) earthquake culminated c. 30 years of remarkable earthquakes in the northern Tien Shan (Kyrgyzstan and Kazakhstan). Building on prior mapping of the event, we traced its rupture in the field and measured more than 50 offset landforms. Cumulative fault rupture length is >155–195 km along 13 fault patches comprising six sections. The patches are separated by changes of dip magnitude or dip direction, or by 4–10 km-wide stepovers. One <40 km section overlaps and is parallel to the main north-dipping rupture but is 7 km north and dips opposite (south). Both ends of the rupture are along mountain front thrust faults demonstrating late Quaternary activity. We computed the moment from each fault patch using the surface fault traces, dip inferred from the traces, 20 km seismogenic thickness, rigidity of 3.3×10 10 N m −2 and dip slip converted from our observations of the largely reverse sense of motion vertical offsets. The discontinuous patches with c. 3–4 m average slip and peak slip of <14 m yield a seismic moment of 4.6×10 20 Nm ( M w 7.78) to 7.4×10 20 Nm ( M w 7.91). The majority of moment was released along the inner eastern rupture segments. This geological moment is lower by a factor of 1.5 from that determined from teleseismic data.

Abstract The San Andreas fault is arguably the best-studied strike-slip fault in the world, and our understanding of its evolution continues to improve as new data become available. This trip will take the participants to a selection of some classical as well as some new sites that will highlight a revised understanding of the San Andreas fault's recent history based on our geomorphological, paleoseismologic, and paleoclimate investigations.