The 148 Ma Independence dike swarm is a prominent feature of the Jurassic Cordilleran arc, extending >600 km from the eastern Sierra Nevada to the Mojave Desert, California. The swarm is fundamentally mafic in composition (<55 wt% SiO 2 ), although dikes range in composition from basalt to rhyolite. Many dikes in the swarm are composite and contain multiple subparallel sheets or abundant enclaves. Whereas most Sierran composite dikes contain only mafic intrusions, some contain both mafic and felsic sheets. In more southerly portions of the swarm (the Spangler Hills and Granite and Fry Mountains), composite dikes rarely contain subparallel intrusions but instead contain abundant enclaves that locally comprise >50 vol% of a dike. Compositional variability in the Independence swarm as a whole may be correlated with physical characteristics of composite dikes. In the Sierra, where composite dikes show little evidence for interaction between mafic and felsic magmas, compositions are bimodally distributed. In contrast, in the south, where composite dikes are characteristically enclave-rich, intermediate-composition dikes are more common. Elemental and isotopic data for the Independence dikes are consistent with chemical controls on mixing processes. The source for the mafic dikes has a consistent ε Nd (t) value of ~–2, independent of location. This probably reflects derivation from a widespread, isotopically homogeneous source rather than lateral intrusion of the dikes over a great distance from a single source. The isotopic data for the dike swarm as a whole are part of a long-term trend of decreasing isotopic variability over a broad range of bulk composition in the Jurassic through Cretaceous Sierran batholith. Mylonitic shear zones and limited geobarometric data suggest that Sierran dikes represent deeper levels of exposure than dikes in the Mojave Desert, where host rocks are not mylonitized. If dikes along the swarm tapped magmas emplaced at similar paleodepths, then variations in composite dike features and dike compositions along the swarm may reflect different degrees of mixing vertically within dike conduits.

Owens Lake has existed for most of the past 800,000 yr, but the sequence of interconnected lakes and streams of which it was often part, the Owens River cascade, last flourished during late Pleistocene time. A fluctuating, increasingly saline, terminal lake survived into the late Holocene until upstream water diversions to the Los Angeles Aqueduct began in 1913. Shoreline fragments and beach stratigraphy indicate that the lake reached its highest late Pleistocene level around 23.5 ka, during the Last Glacial Maximum, when it was fed by meltwaters from Sierra Nevada glaciers and spilled southward to Searles Lake and beyond. The lake then fell to relatively low levels after 16.5 ka before experiencing terminal Pleistocene oscillations related to hydroclimatic forcing, which involved changing regional precipitation regimes rather than major inputs from Sierra Nevada glaciers. Two major transgressions occurred. The first culminated around 14.3 ka and was probably related to a cooler, wetter regional climate. The second culminated around 12.8 ka and was linked to the earlier wetter phase of the Younger Dryas cold event. However, the high late Pleistocene shoreline is deformed, and the highest beach ranges in elevation from 1140 m to 1167 m above sea level. If the terminal Pleistocene lake overflowed, as suggested here, then its outlet has also been raised since 12.8 ka. This deformation appears to have involved uplift of the Coso Range magmatic complex relative to subsidence and faulting within the Owens Lake graben between the Sierra Nevada and Inyo Mountains frontal faults. Such deformation confounds simple hydroclimatic explanations of lake behavior and must be incorporated into models that seek to interpret the changing form and geochemistry of Owens Lake and the frequency of its spillage southward to Searles Lake.

We have examined the deformation associated with a right-releasing stepover along the dextral Walker Lane belt where it traverses Wild Horse Mesa in eastern California. We use a micropolar inversion of both seismic focal mechanism and fault-slickenline data and compare the results to the micropolar deformation parameters inferred from paleomagnetically determined block rotations and GPS velocities. The focal mechanisms, fault-slickenlines, and GPS velocities all show horizontal shear with a consistent ENE–WSW to E–W maximum extension-rate axis ( d 1 ). A subset of data shows crustal thinning with a similarly oriented d 1 . We interpret these results as a reflection of divergent strike-slip (i.e., transtensional) boundary conditions in a negative flower structure developed in the right-releasing stepover. The fault-slickenline data also show a crustal thickening solution that we attribute to the local accommodation of block rotations. Paleomagnetic data demonstrate clockwise-looking-down rotations of 12.0° ± 2.6° (68% confidence limits) in ca. 3 Ma volcanic rocks, relative to the same rocks outside the stepover. Assuming rotations took 2–3 m.y. gives average microspins (block rotation rates) of 4.0° ± 0.9°/m.y. to 6.0° ± 1.3°/m.y. GPS velocities define a current macrospin (half the continuum rotation rate) of 3.9° ± 0.6°/m.y. to 6.1° ± 1.5°/m.y. These spin components are consistent with expectations for transtension. Our calculations of relative vorticity W from the GPS and paleomagnetic data are generally consistent with values obtained from the inversion of the fault-slickenline data, but the uncertainties in the data do not permit a definitive test of these results.