ABSTRACT The current tectonic framework for understanding Cenozoic magmatism in western North America was laid out in a series of influential papers in the early days of the plate-tectonics revolution. These ideas, largely developed through deductive analysis, were so revolutionary yet seemingly self-evident that they quickly passed from hypothesis to axiom. These include the following. (1) Inboard and outboard sweeps of magmatism resulted from shallowing and then rapid steepening of a subducted slab. (2) The Oligocene–Miocene ignimbrite flareup resulted from sinking and rollback of a shallow slab. (3) Late Cenozoic basaltic magmatism resulted from opening of a slab window. (4) The current Cascade arc is the remnant of a much more continuous ancestral arc that ran the length of western North America. When tested against current databases of igneous rock ages and chemical analyses, these conjectures largely fail; some are clearly contradicted, whereas others are possible but ad hoc and unfalsifiable. Ironically, the plate-tectonics revolution nicely explains plate-boundary magmatism in much of the world but is less successful in western North America, where many of these links were first developed. It is time for a second revolution.

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.