SEISMIC INTERPRETATION AND STRUCTURAL ANALYSIS ALONG A TRANSECT FROM THE BABOQUIVARI MOUNTAINS TO THE TRANSITION ZONE OF THE COLORADO PLATEAU: IMPLICATIONS FOR MODELS OF CORE-COMPLEX DEVELOPMENT

The Catalina and Pinaleño Mountains core complexes in southeast Arizona are established as prime examples of kinematic crustal extension models in which initial extension occurs along a breakaway fault, which subsequently is deformed into a synform and abandoned in response to isostatic rebound, with new faults breaking forward in the dominant transport direction. However, these core complexes can more broadly be seen as elements of a linked mid-Tertiary regional extension system that spans ~180 km along a SW-NE transect from the Baboquivari Mountains to the transition zone of the Colorado Plateau. Analysis of geological data covering these highly extended terranes, in conjunction with reprocessing and interpretation of a suite of industry 2-D seismic reflection profiles across the region, illuminate subsurface structural features related to Cenozoic crustal extension and provide new constraints on evolution of core complexes in southeast Arizona. U-Pb zircon dating of a sill that was emplaced within and parallel to the San Pedro fault zone provides a minimum age of faulting on this part of the presumed detachment system, and other geochronological studies further suggest that the Galiuro detachment (presumed breakaway) and uplifted Catalina detachment were coeval. These timing constraints seem to preclude early abandonment of the up-dip Galiuro detachment fault while the down-dip Catalina segment continued to move. Geological and seismological data indicate that viable alternative models explain observations perhaps at least as well as previous core-complex models. In contrast to the “traditional” model often employed for these structures, our models suggest that the southwest- and northeast-dipping normal-fault systems on the flanks of the Galiuro Mountains extend to mid-crustal depths beneath the San Pedro trough and Sulphur-Springs Valley, respectively, and are not simple breakaway faults for the Catalina and the Pinaleño Mountains core complexes; neither are these range-bounding faults directly related to the San Pedro fault and the Eagle Pass detachment fault.