LATE CRETACEOUS TRANSITION FROM CONTRACTION TO EXTENSION IN THE GRANITE WASH MOUNTAINS, WESTERN ARIZONA

The metamorphic core complex belt of western Arizona lies along the eastern boundary of the Mojave region. Large-magnitude Miocene extension has exhumed mid-crustal rocks that record the Cretaceous-Paleogene history of this area, including Maria Fold & Thrust Belt (MFTB) structures such as the Hercules Thrust. Within the Granite Wash Mountains, the Hercules Thrust is present as a ductile shear zone that puts Proterozoic basement on top of McCoy Mountains Formation. The thrust is cut by the Tank Pass and Granite Wash plutons. Recent work has suggested a phase of Late Cretaceous extension along the Harcuvars, indicating the transition from contraction to extension occurred at the end of the Mesozoic. We present preliminary field, microstructural, and geochronologic data aimed at addressing when the MFTB was active and what drove the transition from contraction to extension. Mylonitic fabrics in the uppermost McCoy Formation indicate thrusting post-dated deposition of this unit. New detrital zircon U-Pb dates constrain the maximum depositional age of deformed McCoy sandstone to the Late Cretaceous. Mylonitic fabrics strike NW-SE and dip NE with an average lineation of 037/48. Fabrics show oblique grain shape fabrics and asymmetric porphyroclasts with mixed shear sense indicators. Kyanite occurs parallel to mylonitic fabrics and shows kink banding. Mylonitic fabrics are cut by the Tank Pass pluton, which yields a zircon U-Pb crystallization age of ~77 Ma. Discrete shear zones oriented NE-SW up to 15 cm wide occur locally in Tank Pass. These shear zones dip SE and record sinistral/top-NE kinematic indicators, whic are more consistent with a transitional strain field than uniquely MFTB shortening or Laramide or Miocene extensional strain. Quartz microstructures record a wide range of temperature–stress conditions, suggesting deformation occurred during post-magmatic cooling. The unstrained Granite Wash pluton, which has a zircon U-Pb crystallization age of ~74 Ma, is similarly post-kinematic with respect to the Hercules Thrust but do not feature discrete shear zones.