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.

A well-defined axis of maximum dilation within the ca. 148 Ma Independence dike swarm is significantly offset across Owens Valley. Dilation by diking within the axis of maximum dilation is greater than 5%, commonly exceeds 10%, and locally ranges over 40%. Elsewhere in the swarm, dilation rarely ranges above 2%. The axis of maximum dilation steps ∼75–130 km rightward across Owens Valley, although the offset is difficult to measure precisely because the dike swarm is diffuse and intersects the valley at a relatively low angle. Comparison with other recently investigated geologic markers favors 65 ± 5 km of dextral offset since 83.5 Ma and perhaps an additional 10–65 km of offset prior to 83.5 Ma. Although Owens Valley is a locus of modern dextral slip, regional relations suggest that most of the 65 km of dextral displacement accumulated in Latest Cretaceous–early Paleogene (Laramide) time, when Cordilleran subduction was strongly right-oblique. Thermobarometric, structural, stratigraphic, and geochronologic evidence from the southern Sierra Nevada have previously been interpreted to reflect south-directed tectonic unroofing of deep-crustal rocks to form a metamorphic core complex during Laramide time. Large-magnitude Laramide right slip across Owens Valley thus may have been transferred southward into extension in the southern Sierra Nevada. Linked systems of late-Laramide to post-Laramide strike-slip faults and metamorphic core complexes have long been recognized in the plutonic-metamorphic core of the northern Cordillera. Recognition of this tectonic style in California suggests that it may have characterized most of the western Cordilleran orogen at this time.