WILD AND CRAZY GEOLOGY OF THE TUCSON REGION, ARIZONA

Tucson, Arizona is located in a broadly west-northwest trending southwestern boundary of North America, where the transcontinental arch intersects the continental margin. The Paleozoic sedimentary cover is relatively thin, and Mesozoic basin formation and arc magmatism produced a structurally complex crust that was overprinted by Laramide contraction and subsequent Tertiary extension. Basin and Range physiography is reflected in a series of disconnected mountain ranges, each with wildly varying geologic records. Progress has been made in assembling the evidence from these postage stamps into a coherent geologic history; many questions are unanswered, but the emerging picture makes for quite a story. Salient features include: formation of Jurassic Arc, high-angle faulting and basin formation in the Late Jurassic-Early Cretaceous; subsequent Laramide deformation included transpressional inversion of these basins, development of potentially major northwest trending strike slip faults, large-scale caldera eruptions, thrust faulting to separate calderas from their plutonic roots, and intrusion of large bodies of peraluminous granite. The details of sequencing of these various structural features between mountain ranges is still problematic, and Laramide contractional overprinting of older Mesozoic structures makes determination of their kinematics problematic. In Tertiary time, a late Oligocene period of north-south extension was followed by top-to the southwest extension that formed the well known Catalina Core Complex. The final period of deformation has produced the current basin and range physiography. Existing geologic mapping provides a foundation for identifying key areas to target detailed analysis to unravel this complex history and place it in the larger context of Cordilleran tectonics.