Seismic Profiling of a Breakaway Zone in Southeast Arizona and Implications for Models of Core-Complex Development

The Basin and Range Province of western North America represents a broad zone of Cenozoic crustal extension characterized by various styles of extensional deformation and development of zones of extreme extension manifested as metamorphic core complexes. The Models concerning core complex development define a breakaway zone, along which extension is initially accommodated in the upper crust in which normal fault systems slip in the direction of transport. Slip is initiated as a listric or low-angle normal fault system, which then feeds slip into a subhorizontal or low-angle shear zone at depth. The Models suggest that the breakaway zone normal fault system to have slipped at low-angle during the earlier stages of core complex development. It's been suggested that folding/back-tilting of the original breakaway zone result from the isostatic rebound of the footwall due to unloading of upper plate rocks and exhumation of a portion of the ductile shear zone. The San Pedro trough in Southeast Arizona is an elongate structural depression flanked on the southwest by the Catalina core complex and bounded by the relatively undeformed Galiuro Mountains in the northeast. The detachment system is believed to have initiated from the breakaway zone located on southwestern flank of the Galiuro Mountains. Cenozoic crustal extension revealed through seismic reflection investigations helps us understand the fault geometries and upper crustal structure of the study area. 2-D seismic reflection data in Southeast Arizona shows that the San Pedro Trough contains thin sedimentary cover above Paleozoic and older rocks. Though The detachment fault in the breakaway zone dips SW toward the San Pedro Trough and can reasonably be modeled as a moderate-angle fault that cuts through mylonitic fabric to mid-crustal depths; if this fault forms low-angle shear zone that crosses the San Pedro Trough at shallow depths and re-emerges due to folding/back-tilting during the emplacement of the metamorphic core complex, the fault surface must apparently form the bedrock floor of the trough. Field relations, outcrop data and borehole data also help constrain the structure of the breakaway zone and development of the core complex. Keywords: Metamorphic Core Complex, breakaway zone, crustal extension, San Pedro Trough, detachment fault