USING QUARTZ PETROFABRIC INTENSITY PARAMETERS TO DELINEATE SHEAR ZONES: A CASE STUDY FROM THE MAIN CENTRAL THRUST IN WESTERN BHUTAN

Ongoing debates over how to best define shear zones within packages of rock that have been pervasively recrystallized impede our ability to accurately map and correlate shear zones. For example, the Main Central thrust (MCT), a top-to-south shear zone mapped across the length of the Himalayan orogen, has been defined using multiple criteria, including changes in lithology, isotopic signatures, deformation temperature, and the conditions and timing of peak metamorphism, which has led to significant variation in its mapped position. Here, we explore a novel approach for delineating the MCT by using statistical intensity parameters of quartz petrofabrics as a proxy for finite strain magnitude. We utilize the cylindricity value, which quantifies quartz c-axis fabric intensity on a scale from 0 (random) to 1 (non-random). We calculated cylindricity values for quartz-rich samples collected along the Puna Tsang Chu and Sarpang transects in southwestern Bhutan, both of which cross exposures of the MCT as mapped by previous studies. Several criteria proposed for delineating the MCT, including the structurally-lowest occurrence of kyanite, leucosomes, and gneissic banding, all occur at approximately the same structural position on these transects, making these transects ideal for testing this new approach. On the Puna Tsang Chu transect, samples between 150-500 m below the MCT (n=5) yielded values between 0.59-0.85. Samples between 12 m above and 36 m below the MCT (n=5) yielded values between 0.86-0.93, indicating highly non-random fabrics. Samples between 2.4-11.7 km above the MCT (n=3) yielded cylindricity values between 0.37-0.68. On the Sarpang transect, cylindricity values are overall lower, but illustrate similar trends. Samples collected between 0.5-2.5 km below the MCT (n=8) yielded values between 0.05-0.55, while samples between 200 m above and 75 m below the MCT (n=3) yielded values between 0.51-0.70. On both transects, our data illustrate increasingly non-random fabrics with proximity to the mapped position of the MCT. These results highlight the potential utility of using petrofabric intensity parameters as a proxy for finite strain magnitude, which can aid in the delineation of shear zones within packages of highly recrystallized rock.