Deformation conditions during syn-convergent extension along the Cordillera Blanca shear zone, Peru

Strain localization across the brittle-ductile transition is a fundamental process in accommodating tectonic movement in the mid-crust. The tectonically active Cordillera Blanca shear zone (CBSZ), a similar to 200-km-long normal-sense shear zone situated within the footwall of a discrete syn-convergent extensional fault in the Peruvian Andes, is an excellent field laboratory to explore this transition. Field and microscopic observations indicate consistent topdown-to-the-southwest sense of shear and a sequence of tectonites ranging from undeformed granodiorite through mylonite and ultimately fault breccia along the detachment. Using microstructural analysis, two-feldspar and Ti-in-quartz (TitaniQ) thermometry, recrystallized quartz paleopiezometry, and analysis of quartz crystallographic preferred orientations, we evaluate the deformation conditions and mechanisms in quartz and feldspar across the CBSZ. Deformation temperatures derived from asymmetric strain-induced myrmekite in a subset of tectonite samples are 410 +/- 30 to 470 +/- 36 degrees C, consistent with TitaniQ temperatures of 450 +/- 60 to 490 +/- 33 degrees C and temperatures >400 degrees C estimated from microstructural criteria. Brittle fabrics overprint ductile fabrics within similar to 150 m of the detachment that indicate that deformation continued to lower-temperature (similar to 280-400 degrees C) and/or higher-strain-rate conditions prior to the onset of pervasive brittle deformation. Initial deformation occurred via high-temperature fracturing and dissolution-precipitation in feldspar. Continued subsolidus deformation resulted in either layering of mylonites into monophase quartz and fine-grained polyphase domains oriented subparallel to macroscopic foliation or the interconnection of recrystallized quartz networks oriented obliquely to macroscopic foliation. The transition to quartz-controlled rheology occurred at temperatures near similar to 500 degrees C and at a differential stress of similar to 16.5 MPa. Deformation within the CBSZ occurred predominantly above similar to 400 degrees C and at stresses up to similar to 71.4 MPa prior to the onset of brittle deformation.