Abstract Since the days of John Muir, the striking granitic topography of Yosemite Valley, California, has been understood to have been sculpted by glaciers and presently modified by rockfall. Glacial erosion has provided remarkably clean and extensive exposures of granitic rocks on the vertical walls that provide insights into intrusive relations and rockfall susceptibility. However, it is only with recent remote sensing methods that these exposures have been studied in detail. El Capitan presents an unparalleled exposure of the interior of a granitic plutonic system at the point of interaction between multiple intrusive suites and two sets of mafic dike swarms. The distribution and orientation of these units affected El Capitan's extensive rockfall history, including a huge postglacial rock avalanche at 3.6 ka. This two-day field trip will explore these ideas and apply them to some of the other classic cliffs of Yosemite Valley such as Glacier Point and Half Dome. We will present a new map of El Capitan and discuss the intrusive relationships exposed on the face while visiting several rockfall deposits and some of the classic vistas of Yosemite Valley, including El Capitan Meadow, Glacier Point, Taft Point, and Mirror Lake.

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.

Abstract This study investigates the internal anatomy and petrogenesis of the Tuolumne Intrusive Suite (TIS), which comprises metaluminous, high-potassium, calc-alkaline granitoids typical of the Sierra Nevada batholith. Although the TIS has often been cited as an example of a large magma chamber that cooled and fractionated from the margins inward, its geochemistry is inconsistent with closed-system fractionation. Most major elements are highly correlated with SiO 2 , but the scattered nature of trace elements and variations of initial Sr and Nd isotopic ratios indicate that fractional crystallization is not the predominant process responsible for its chemical evolution. Isotopic data suggest mixing between melts of mantle-like rocks and a granitic melt similar in composition to the highest-silica TIS unit. Monte Carlo models of magma mixing confirm that such processes can reproduce the observed variations in major elements, trace elements and isotopic ratios. Thermobarometry suggests emplacement at depths near 6 km and crystallization temperatures ranging from 660 to 750 °C. Feldspars, hornblende, biotite and magnetite exhibit evidence of extensive low-temperature subsolidus exsolution. The TIS as a whole trends toward more evolved isotopic compositions and younger U–Pb zircon ages passing inward. This pattern indicates a general increase in the proportion of felsic, crustally derived melt in the mixing process, which may have resulted from net accumulation of heat added to the lower crust by intrusion of mantle-derived mafic magma. However, the bulk geochemical and isotopic compositions of the equigranular Half Dome Granodiorite, the porphyritic Half Dome Granodiorite and the Cathedral Peak Granodiorite overlap one another and the contacts between them are commonly gradational. We interpret these map units to represent a single petrological continuum rather than distinct intrusive phases. The textural differences that define the units probably reflect thermal evolution of the system rather than distinct intrusive events.

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.