LARGE-MAGNITUDE EXTENSIONAL DEFORMATION IN THE SOUTH MOUNTAINS METAMORPHIC CORE COMPLEX, ARIZONA - EVALUATION WITH PALEOMAGNETISM

Paleomagnetic data are used to test controversial aspects of Cordiller an metamorphic core complexes, including the original dip of extension al structures, origin of the mylonitic front, and applicability of rol ling-hinge models. We obtained paleomagnetic data (115 sites, 82 accep ted for analysis) 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. These rocks yiel d dual polarity, high unblocking temperature, and high to moderate coe rcivity magnetizations. Positive baked contact tests indicate that foo twall rocks possess primary thermoremanent magnetizations (TRMs) or hi gh-temperature thermochemical remanent magnetizations (TCRMs) acquired early in their cooling history and during ductile and brittle extensi onal deformation of structurally higher rocks. This is consistent with thermochronologic data indicating rapid synkinematic cooling from cry stallization through the range of laboratory unblocking temperatures f or the magnetic mineralogy of these rocks (between about 22 and 17.5 M a). Paleomagnetic data are considered as two populations based on the structural asymmetry of the South Mountains metamorphic core complex: (1) a front side characterized by northeast-dipping (similar to 10 deg rees) mylonitic fabrics and brittle extensional structures, and (2) a back-dipping side characterized by rollover of the mylonitic zone to f orm a southwest- or back-dipping (similar to 15 degrees) mylonitic fro nt. Comparison of paleomagnetic data from these two sides suggests tha t the back-dipping mylonitic front was synkine-matically tilted about 10 degrees down-to-the-southwest. The data support a folded shear zone hypothesis for origin of the mylonitic front and the interpretation t hat footwall rocks possess primary, Miocene-age TRMs or TCRMs. A secon d regional fold test involved data from sites on both flanks of the to pographically prominent northeast-trending mountain range-scale antifo rm. The negative result from this fold test demonstrates that this str ucture formed early in the extensional history and prior to magnetizat ion acquisition by the plutons. We obtained a well-grouped footwall gr and mean from 62 front-side and 20 back-dipping site means (N = 82, D = 1.0 degrees, I = 51.7 degrees, k = 41.8, alpha(95) = 2.5 degrees). W e calculated this grand mean with the assumption that front-side sites have remained structurally untilted, whereas back-dipping side sites require removal of 10 degrees of southwest dip. This grand mean is sta tistically indistinguishable (95% confidence level) from time-averaged Miocene expected directions. We thus conclude that the current gentle dip of front-side mylonites and detachment faults is original, Theref ore, both ductile and brittle extensional deformations of the South Mo untains metamorphic core complex were accommodated along low-angle str uctures (dip of less than or equal to 15 degrees). Our interpretation refutes the widespread applicability of models that predict metamorphi c core complexes to represent tilted crustal blocks originally bounded by moderate-angle normal faults and does not support rolling-hinge mo dels of metamorphic core complex evolution that require a moderate-ang le ramp.