Abstract

Field relationships and new U-Pb geochronology reveal that a significant number of dikes associated with the Independence dike swarm, both within the type locality and elsewhere, are of Cretaceous age. A new U-Pb zircon age for a diorite dike in the type locality of the Independence dike swarm in California confirms that some mafic dikes in the swarm are coeval with previously dated Late Jurassic felsic dikes. Cretaceous dikes in the eastern Sierra Nevada batholith are located near—and may be genetically related to—coeval mafic and felsic plutons. However, the presence of Cretaceous dikes throughout the swarm demonstrates that this is not a local phenomenon, and caution therefore should be exercised when using Independence-like dikes as temporal markers. We have not identified any consistent petrographic or chemical criteria that distinguish Jurassic Independence dikes from Cretaceous dikes. However, Jurassic dikes in the Sierra Nevada commonly contain a sinistral shear fabric that is absent in both dated Cretaceous dikes and undated dikes that intrude dated Cretaceous plutons. Despite intruding at least 60 m.y. apart, the orientations and petrology of the Jurassic and Cretaceous dikes are remarkably similar. Because dikes were intruded over a time period characterized by large changes in the orientation of plate convergence west of the Sierran arc, it is unlikely that the uniform structural orientation of Jurassic and Cretaceous dikes resulted from regional stresses imposed by the subducting plate. Instead, the orientation of Jurassic and Cretaceous dikes (and of many Sierran Cretaceous plutons) may have been controlled by preexisting faults or by the orientation of the continental margin.

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