FLUID INCLUSION AND MICROSTRUCTURE INSIGHTS INTO DEFORMATION CONDITIONS OF THE CORDILLERA BLANCA DETACHMENT SHEAR ZONE, PERU

The Cordillera Blanca Detachment (CBD) shear zone in central Peru represents an important class of mid-crustal extensional structures in convergent orogenic settings. The CBD is 200 km long and dips WSW at less than 40°. At the Quebrada Honda transect, the average S₁ foliation strikes 110° and dips 34° SW. Miocene basin-fill sediments and ignimbrites form the hanging wall and granodiorites of the 12-5 Ma Cordillera Blanca Batholith form the footwall. Previous workers have estimated the onset of detachment faulting at 5.4 Ma. Our research uses fluid inclusions and their microstructural settings to constrain pressure, temperature, and fluid composition during deformation. The CBD, as perhaps the only active example of orogen-normal, syn-convergent extension, has important implications for understanding the kinematics of earlier detachment faulting episodes in convergent tectonic regimes.

Microstructure features of mylonite samples from Quebrada Honda include 2-9 mm diameter feldspar porphyroclasts embedded in a matrix of fine grain recrystallized quartz. Sigma-type shear sense indicators of feldspar porphyroclasts with quartz tails are prevalent. Recrystallized quartz exhibits subgrain rotation and grain boundary migration, qualitatively indicating moderate-high temperature deformation along the CBD. Fluid inclusions occur in three settings, all interpreted to be synkinematic to CBD deformation: 1) recrystallized quartz veins, 2) quartz tails of sheared porphyroclasts, 3) quartz-filled Mode I fractures in feldspar porphyroclasts. Microstructure measurements indicate Mode I fractures average 68° from the up-dip S₁ foliation. ~90% of observed fluid inclusions occur in a scattered distribution, ~10% occur in linear arrays interpreted as annealed microcracks. Linear arrays average 78° from the up-dip S₁foliation. Inclusions are 0.5-6.0 μm in diameter, and ~75% have liquid and vapor phases at ~25°C.

Continued work on this project will focus on heating/freezing stage fluid inclusion analysis to determine melting point, critical point, and homogenization temperatures, which will allow us to constrain fluid composition, molar volume, and P/T deformation conditions along the CBD.