The cessation of passive margin sedimentation in Paleozoic time and the beginning of arc magmatism in Mesozoic time in southern Arizona are clearly established, and the deposits associated with both Paleozoic sedimentation and Mesozoic volcanism have been or are being studied. The nature of the transition itself remains unknown, however, although coherent and plausible models have been provided for a similar time span in the Mojave Desert and elsewhere along the Cordilleran margin. This project will investigate epiclastic, volcaniclastic, and volcanic strata associated with the Paleozoic/Mesozoic boundary in southern Arizona. An integrative geologic investigation will involve: 1) Detailed field mapping of sections where depositional sequences from Paleozoic to early Mesozoic strata are known or inferred; 2) Facies analysis of strata along the contact, in order to determine a) the tectonic setting of earliest Mesozoic magmatism; 3) U-Pb zircon b) the tectonic setting of earliest Mesozoic magmatism; 3) U-Pb zircon geochronology on volcanic rocks at the contact to establish the initial timing of magmatic activity, or where appropriate as a tool in correlating sequences. This research will fill an important gap in our understanding of late Paleozoic to early Mesozoic tectonic evolution of the southwestern Cordillera by shedding light on the timing and style of the transition from passive-margin sedimentation to subduction and magmatism in southern Arizona.
