Abstract The eastern California shear zone is an important component of the Pacific–North America plate boundary. This region of active, predominantly strike-slip, deformation east of the San Andreas fault extends from the southern Mojave Desert along the east side of the Sierra Nevada and into western Nevada. The eastern California shear zone is thought to accommodate nearly a quarter of relative plate motion between the Pacific and North America plates. Recent studies in the region, utilizing innovative methods ranging from cosmogenic nuclide geochronology, airborne laser swath mapping, and ground penetrating radar to geologic mapping, geochemistry, and U-Pb, 40 Ar/ 39 Ar, and (U-Th)/He geochronology, are helping elucidate slip rate and displacement histories for many of the major structures that comprise the eastern California shear zone. This field trip includes twelve stops along the Lenwood, Garlock, Owens Valley, and Fish Lake Valley faults, which are some of the primary focus areas for new research. Trip participants will explore a rich record of the spatial and temporal evolution of the eastern California shear zone from 83 Ma to the late Holocene through observations of offset alluvial deposits, lava flows, key stratigraphic markers, and igneous intrusions, all of which are deformed as a result of recurring seismic activity. Discussion will focus on the constancy (or non-constancy) of strain accumulation and release, the function of the Garlock fault in accommodating deformation in the region, total cumulative displacement and timing of offset on faults, the various techniques used to determine fault displacements and slip rates, and the role of the eastern California shear zone as a nascent segment of the Pacific–North America plate boundary.

High-resolution single-channel seismic reflection profiles, bathymetry and sides-can sonar imagery from the Puerto Rico trench document the present-day and post-collisional effects of the obliquely subducting southeastern extension of the Bahama Province and the Main Ridge fracture zone on the northern Puerto Rico–Virgin Islands margin. In contrast to an orthogonal system, where it is unlikely that two high-standing ridges will impact the same section of margin, along the Puerto Rico trench convergence is highly oblique and the deformational effects of the two ridges are superimposed and often difficult to isolate. A middle–upper Miocene margin-wide unconformity in the Oligocene–lower Pliocene shallow-water carbonate platform of the Virgin Islands, Puerto Rico, and eastern Hispaniola provides an excellent horizontal reference frame for the timing and impact of the high-standing ridges on the margin. During the past 10 m.y., trenchward tilting (4–6°) of the carbonate platform including the margin-wide erosional surface to >5000 m water depths provides evidence that the margin has experienced significant subsidence. In this paper we present a model of accelerated subduction erosion and diachronous margin subsidence triggered by the ridges sweeping from east to west beneath the Puerto Rico forearc. Evidence for ongoing and past subduction erosion include zones of enhanced seismicity, oversteepening and mass wasting of the forearc slope, and landward migration of the inner trench-slope break. We document over 3 km of Neogene subsidence presumed to be the result of rapid crustal thinning associated with the tunneling of the buoyant, thick (∼20-km-thick crustal/sediment section) southeastern Bahama Province beneath the forearc. During the past 3.5–5 m.y., the volume of material eroded from the overriding plate is estimated to be ∼210 km 3 /km of margin, equivalent to an erosion rate of 42–60 km 3 /m.y./km of margin. A significant reduction in the rate of margin subsidence in the eastern part of the survey area suggests that the Main Ridge fracture zone has had a relatively small erosional impact on the margin, and that under normal conditions, this highly oblique convergent boundary is characterized by relatively slow rates of subduction erosion. Three strike-slip fault zones are imaged in the forearc: the East Septentrional fault zone, the Bunce fault zone, and the Bowin fault zone. The Bunce and Bowin fault zones trend N80°–90°E, subparallel to the predicted North America–Caribbean relative plate motion vector (N70°E) determined by global positioning system (GPS) studies. Seismic reflection profiles across the Puerto Rico trench in the western section of the survey area, where the Bunce fault zone is <10 km from the deformation front, reveal thick (0.75–1.5 km) accumulations of relatively undeformed trench sediment fill. This lack of shortening deformation suggests that convergence approaches pure strike-slip and that strain within the forearc is largely accommodated by the strike-slip fault zones. Strike-slip faulting appears to be progressively partitioned to the east where deformation in the trench is more clearly contractional. Although not a typical plow mark, the Bowin fault zone appears to represent the long-term (∼10 m.y.) track of the underthrusting and colliding southeastern Bahama Province across the forearc area that is consistent with the convergence direction exhibited by GPS results.