Structure, intrusion, and tectonic origin of the Independence dike swarm, eastern California

The Jurassic (∼148 Ma) Independence dike swarm is exposed over a distance of >600 km and has served as a valuable temporal and structural marker across eastern California. This study investigates new structural, field, and compositional data associated with the swarm's tectonic origin, deformation and emplacement, and proposes models to explain these data. In addition, this study explores an enigmatic relationship between deformed mafic dikes and their undeformed granitic host.; In the eastern Sierra, Independence dikes within Jurassic plutons cut and are themselves deformed by NNE-striking reverse-sense mylonitic shear zones, providing strong evidence for syndeformational dike intrusion. Many dikes contain a penetrative, solid-state ductile fabric which formed during sinistral shear across the dikes. Mylonitization and sinistral shear along Sierran dikes may have occurred along a Late Jurassic transpressional Cordilleran arc. Swarm intrusion likely occurred in response to a rapid change in North American plate motion rather than during rifting of the arc or in response to a collisional event.; Composite Independence dikes may indicate the mode of dike emplacement and magma interaction at depth. Most Sierran composite dikes contain subparallel mafic intrusions but a few contain both mafic and felsic phases. Along more southerly portions of the swarm, in the Spangler Hills and in the Granite and Fry mountains, composite dikes contain abundant enclaves. These features may correlate with dike compositions. In the Sierra, composite dikes are dominantly either mafic or felsic, whereas more southerly composite dikes include intermediate phases. If dikes tapped plutons emplaced at similar paleodepths, then dike features and compositional variability may reflect different degrees of mixing along dike conduits.; Rock mechanics experiments indicate that mafic rocks should be more competent than quartz-rich granite. Dikes which are strongly mylonitized along their margins adjacent to nearly undeformed granite present a paradoxical relationship that defies experimental flow laws for these rock types. Several mechanisms may be responsible for this phenomenon including the presence of a weak mineral and/or finer grain size in the dike, reaction softening, or hydrolytic weakening.