Large-magnitude extensional deformation in the South Mountains metamorphic core complex, Arizona

Abstract

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-hinge models. We obtained paleomagnetic data (115 sites, 82 accepted for analysis) from the weakly deformed interior of a synkinematic, footwall intrusive suite and Proterozoic footwall rocks of the South Mountains metamorphic core complex, central Arizona. These rocks yield dual polarity, high unblocking temperature, and high to moderate coercivity magnetizations. Positive baked contact tests indicate that footwall rocks possess primary thermoremanent magnetizations (TRMs) or high-temperature thermochemical remanent magnetizations (TCRMs) acquired early in their cooling history and during ductile and brittle extensional deformation of structurally higher rocks. This is consistent with thermochronologic data indicating rapid synkinematic cooling from crystallization through the range of laboratory unblocking temperatures for the magnetic mineralogy of these rocks (between about 22 and 17.5 Ma). 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 (∼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. Comparison of paleomagnetic data from these two sides suggests that the back-dipping mylonitic front was synkinematically tilted about 10 down-to-the-southwest. The data support a folded shear zone hypothesis for origin of the mylonitic front and the interpretation that footwall rocks possess primary, Miocene-age TRMs or TCRMs. A second regional fold test involved data from sites on both flanks of the topographically prominent northeast-trending mountain range–scale antiform. The negative result from this fold test demonstrates that this structure formed early in the extensional history and prior to magnetization acquisition by the plutons. We obtained a well-grouped footwall grand mean from 62 front-side and 20 back-dipping site means ( N = 82, D = 1.0, I = 51.7, k = 41.8, α 95 = 2.5). We calculated this grand mean with the assumption that front-side sites have remained structurally untilted, whereas back-dipping side sites require removal of 10 of southwest dip. This grand mean is statistically 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. Therefore, both ductile and brittle extensional deformations of the South Mountains metamorphic core complex were accommodated along low-angle structures (dip of ≤15). Our interpretation refutes the widespread applicability of models that predict metamorphic core complexes to represent tilted crustal blocks originally bounded by moderate-angle normal faults and does not support rolling-hinge models of metamorphic core complex evolution that require a moderate-angle ramp.