Large-magnitude extensional deformation in the South Mountains metamorphic core complex, Arizona: evaluation with paleomagnetism

Paleomagnetic data are used to test controversial aspects of Cordilleran metamorphic core complexes, including the original dip of extensional structures, origin of the mylonitic front, and applicability of rolling-binge models. Paleomagnetic data were obtained from the weakly deformed interior of a syn-kinematic, footwall intrusive suite and Proterozoic footwall rocks of the South Mountains metamorphic core complex, central Arizona. Paleomagnetic data are considered as two populations based on the structural asymetry of the South Mountains metamorphic core complex: 1) a front side characterized by northeast-dipping (~10°) mylonitic fabrics and brittle extensional structures, and 2) a back-dipping side characterized by rollover of the mylonitic zone to form a southwest- or back-dipping (~15°) mylonitic front. It is concluded that the current gentle dip of front-side mylonites and detachment faults is original. Therefore, both ductile and brittle extensional deformations of the South Mountains metamorphic core complex were accommodated along low-angle structures. -from Authors