Rapid upward transport of mid-crustal mylonitic gneisses in the footwall of a Miocene detachment fault, Whipple Mountains, southeastern California

摘要: Mylonitic gneisses of upper greenschist to middle amphibolite facies grade are exposed below the Whipple detachment fault in Mountains metamorphic core complex. Fabric and microstructural analyses thick (>3.5 km) mylonitic sequence indicate that it represents an intracrustal zone non-coaxial laminar flow with a predominant sense northeastward shear. The top this shear is front, abruptly gradational (locally within several meters) limit pervasive ductile strain between distinctive non-mylonitized crystalline rocks their lower, mylonitized equivalents. Mylonitization Oligo-Miocene age (26±5 Ma) estimated have occurred at depths 16±4 km (4.4±1.1 kb) temperatures 460–535 °C. Fission track and40Ar/39Ar determinations from collectively document rapid cooling above 450 °C 200 20 18 Ma ago. Rapid attributed post-20 uplift footwall evolving low-angle system extensional origin. NE-rooting mylonites kinematically coordinated (same direction shear), but faults appear cut across million years after formation.