EVIDENCE FOR REVERSE SENSE DUCTILE SHEAR DURING THE LARAMIDE OROGENY IN SOUTHWESTERN ARIZONA: QUITOBAQUITO HILLS

New thermochronologic and microstructural data from the Quitobaquito Hills provide insight into conditions of deformation in the middle crust of southwestern Arizona during Late Cretaceous to early Paleogene crustal shortening associated with low angle subduction. The mylonitic Quitobaquito shear zone juxtaposes Paleoproterozoic (~1.6 Ga) gneiss and schist structurally above Jurassic (175–170 Ma) meta-volcanic and meta-volcaniclastic rocks. The shear zone dips ~50° SSE and exhibits top-to-the-north, reverse-sense kinematics based on mineral (quartz) stretching lineations in outcrop and σ-type and domino-type fragmented porphyroclasts, mica fish, and S-C fabrics in thin section. The main shear zone is ~30 m thick and underlain by at least one footwall imbricate in the Jurassic sequence. Argon-argon and zircon (U-Th)/He thermochronologic data from the footwall and hanging wall of the shear zone reveal distinct time-temperature paths from approximately 70 to 55 Ma, interpreted to represent the timing of displacement on the shear zone. From ~55 Ma to present, both blocks of the shear zone experienced similar exhumation histories, including a period of moderate cooling during Eocene time. Quartz and feldspar recrystallization fabrics and deformation mechanisms bracket deformation temperature between 450–600°C, consistent with thermochronologic data and greenschist- to lower-amphibolite-facies mineral assemblages in the Jurassic metamorphic rocks. Local metamorphic foliations are parallel to the Quitobaquito shear zone and metamorphism of footwall rocks is interpreted as synkinematic with the shear zone. The timing and kinematics of the Quitobaquito shear zone indicate that it is an expression of Laramide contractional tectonism in southwestern Arizona. We did not find structural or thermochronological evidence to suggest that the shear zone is a reactivated strand of the postulated Jurassic Mojave-Sonora megashear.