Tectonic Evolution of the Early Mesozoic Arc in California and Arizona

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The PI's will test a speculative model for the tectonic setting of the late Triassic to early middle Jurassic arc in Arizona and California. This continental arc has occupied a deep, fault- bounded depression, similar to the present-day arc of Central America. While the modern analogue provides a "snapshot" of surficial processes in an arc graben-depression, the early Mesozoic arc of Arizona and California provides a three- dimensional view of what was an extensional arc, as it evolved over 50 million years. The work will either document the speculative model or modify it, but the relationships to be documented include: (1) determination of patterns and rates of subsidence along the arc, using U-Pb zircon geochronology to date sections of continental strata; (2) characterization of the abundance of high-level rhyolitic intrusions and associated calderas, relative to mesozonal plutons; (3) documentation of patterns of alkalic volcanism within the arc, using major and trace elements that are relatively immobile during greenschist grade metamorphism, and (4) recognition of syndepositional normal faults through facies analysis of sedimentary and volcanic rocks, as well as by zircon dating of cross-cutting relations between faults and intrusions. Implications of this speculative model, which is based in part on our new U-Pb zircon age data, include the possibility that craton-derived quartz sands were funneled along the length of the arc graben-depression and distributed across its width, and beyond to forearc regions, throughout lower and middle Jurassic time. These quartz sands would thus be used as "tracers" to recognize exotic terranes that have been displaced from their site of origin in the Jurassic arc or forearc region of the southwest Cordilleran U.S., particularly if the sands can be fingerprinted through U-Pb dating of detrital zircon. Continuous belts composed of thick continental arc successions may largely originate in extensional settings. The PI's will integrate facies analysis and petrologic study of metavolcanic and metasedimentary rocks with U-Pb zircon dating to document patterns of faulting, subsidence and volcanism in a long- lived arc. These results can then be applied to interpretation of more disrupted continental arc successions of all ages and localities.