CONSTRAINING TRANSITIONS IN STRENGTH CONTROL, FLUID FLOW, AND MASS MOVEMENT ACROSS THE BRITTLE-DUCTILE TRANSITION ZONE IN THE CORDILLERA BLANCA, PERU

Crystalline-cored uplifts overlain by low-angle, brittle-ductile, detachment faults are ubiquitous in Cordilleran-style orogens undergoing extension. Deciphering the rates and mediators of deformation, exhumation, and integration of these structures from depth to surface occurs is key to understanding the time-dependency of syn- to post-contractional extension of the mid to upper crust. Polyphase deformation histories, limited surface exposure, and equivocal age interpretation of associated geological structures makes it challenging to adequately delimit these processes on kyr to myr timescales, inhibiting our ability to interpret rates with the degree of precision needed to decipher processes on relevant timescales. The 70-km, well-exposed, Miocene-to-Pliocene Cordillera Blanca shear zone in central Peru deforms the ca. 6 – 5 Ma granodiorite that overlaps with the onset of extension, limiting the potential for secondary alteration, overprinting, or mixing of intracrystalline cooling and recrystallization ages.

Here, we pair high- precision, multi-mineral, ⁴⁰Ar/³⁹Ar geochronology with hydrogen stable isotope analysis of micas from 12 transects through the Cordillera Blanca shear zone to provide a 4-dimensional view of exhumation accompanying post-magmatic cooling and establishment of a pervasive, fault-hosted meteoric-hydrothermal system. These processes facilitate strain softening via variable mechanisms which both assist in rapid capture and exhumation of mylonites through the brittle-ductile transition zone during a period of <2.5 Ma from the onset of extension across the length of the fault and <1.5 Ma at single localities along strike. These data integrate with existing low temperature thermochronology, zircon geochronology, and geochemical views from the modern meteoric hydrothermal network to constrain the rates of ductile fabric development, exhumation through the brittle-ductile transition zone, and integration of fluid pathways from the surface to mid-crust during synconvergent extension.