Cenozoic Crustal Extension in Southeastern Arizona and Implications for Models of Core Complex Development

In popular models of Cordilleran-style metamorphic core-complex development, initial extension occurs along a breakaway fault, which subsequently is abandoned and deformed into a synform in response to isostatic rebound and new faults breaking forward in the dominant transport direction. The Catalina core complex and associated geology in southeastern Arizona have been pointed to as a type example of this model. From southwest to northeast, the region is characterized by the NW-SE trending Tucson basin, the Catalina core complex, the San Pedro trough and the Galiuro Mountains. The Catalina core complex is bounded by the top-to-the-southwest Catalina detachment fault along its southwestern flank and the low-angle, northeast-dipping San Pedro fault along its northeastern flank. The Galiuro Mountains expose non-mylonitic rocks and are separated from the San Pedro trough to the southwest by a system of low- to moderate-angle southwest-dipping normal faults. This Galiuro fault system (referred to as Galiuro detachment fault in this study) is widely interpreted to be the breakaway zone for the Catalina core complex. It is inferred to be folded into a synform beneath the San Pedro trough, to resurface to the southwest as the San Pedro fault, and to have been abandoned during slip along the younger Catalina detachment. This study aimed to test this model through analysis of field relations and geochronological age constraints, and reprocessing and interpretation of 2-D seismic reflection data from the Catalina core complex and San Pedro trough. In contrast to predictions of the traditional breakaway zone model, we raise the possibility of a moderate-angle, southwest-dipping detachment fault beneath the San Pedro trough, with gently to moderately northeast-dipping rocks in the hanging wall, that could extend to mid-crustal depths beneath the eastern flank of the Catalina Mountains. We present an alternative kinematic model in which extension was accommodated by a pair of top-to-the-southwest normal fault systems (the Catalina and Galiuro detachment faults), with the only major difference between the two being the magnitude of displacement, which was greater for the Catalina detachment.