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INVESTIGATION OF CENOZOIC CRUSTAL EXTENSION INFERRED FROM SEISMIC REFLECTION PROFILES AND FIELD RELATIONS, SE ARIZONAMid-Tertiary metamorphic core complexes in the Basin and Range province of the western North American Cordillera are characterized by large-magnitude extensional deformation. Numerous models have been proposed for the kinematic evolution of these metamorphic core complexes. Such models generally invoke footwall isotatic rebound due to tectonic denudation, and the presence of a weak middle crust capable of flow at mid-crustal levels. In popular models of Cordilleran-style metamorphic core-complex development, 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. In southeast Arizona, the Catalina and Pinaleño Mountains core complexes have been pointed to as type examples of this model. In this study, the “traditional” core-complex model is tested through analysis of field relations and geochronological age constraints, and by interpretation of seismic reflection profiles along a transect incorporating these core complexes. Elements of these linked core-complex systems, from southwest to northeast, include the Tucson Basin, the Santa Catalina-Rincon Mountains, the San Pedro trough, the Galiuro Mountains, the Sulphur Springs Valley, the Pinaleño Mountains, and the Safford Basin. A new digital compilation of geological data, across highly extended terranes, in conjunction with reprocessing and interpretation of a suite of industry 2-D seismic reflection profiles spanning nearly sub-parallel to regional extension, illuminate subsurface structural features related to Cenozoic crustal extension and provide new constraints on evolution of core complexes in southeast Arizona. The main objective is to develop a new kinematic model for mid-Tertiary extension and core complex evolution in southeast Arizona that incorporates new geological and geophysical observations. Geological and seismological data indicate that viable alternative models explain observations 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. In our interpretations and models, these oppositely vergent fault systems are not the breakaway faults for the Catalina and Pinaleño detachment systems.