Characterization of Deformation During Syn-Convergent Extension: Insights from the Brittle-Ductile-Transition in the Cordillera Blanca, Peru

The Andes Mountains are a culmination of smaller mountain ranges that form the longest mountain chain in the world, accommodating convergence and the subduction of the Nazca Plate under the South American Plate since the Late Jurassic. Subduction along the Andean margin varies from normal-dipping to flat-slab subduction, with flat-slab segments commonly characterized by crustal thicknesses upwards of ~70 km. The Peruvian segment is of particular interest, as magmatic arcs in this region show an eastward younging trend and active extension along the western margin of the youngest plutonic feature, the Cordillera Blanca batholith. Extension along this margin is limited to high elevations and occurs along an active low-angle normal detachment fault striking parallel to the trend of the ~200-km-long Cordillera Blanca mountain range, which hosts numerous peaks > 6000 m. The detachment exhumes a mylonitic shear zone of deformed batholith rocks. Tectonic fabrics suggest that 1) extension along the fault is parallel to the direction of plate convergence and regional crustal shortening, and 2) the shear zone crossed the brittle-ductile transition while it was actively deforming during exhumation. These conditions make for a unique setting in which to study the mid-crustal brittle-ductile-transition, and strain localization in a syn-convergent extensional setting where the regional principal stresses are known. This project characterizes deformation of the exhumed shear zone using detailed kinematic analyses, deformation temperature analyses, paleopiezometry, and geochemistry. From this work, a crustal strength profiles for the Cordillera Blanca has been constructed, providing a model for the rheologic evolution of this particular region and contributing to the structural community’s understanding of the rheologic conditions that allow for convergence-parallel extension in the mid-crust.