Quantification of competency contrast from refraction of shear-induced micro-fractures in meta-sedimentary rock, Gangori Shear Zone, Bhagirathi river section, NW Indian Lesser Himalaya

We study the meta-greywacke of Rautgara Formation, Garhwal Lesser Himalaya, India. The focus is on the micro-fractures which cut the flaky-mineral rich cleavage (c-) and porphyroclast rich microlithon (m-) domains of a disjunctive foliation. Although the rock does not show shear in meso-scale, shadow zones and tails of the quartz porphyroclasts under an optical microscope exhibit a top-to-SW ductile shear. Kinematic vorticity number (Wm) of 0.8 was calculated by porphyroclast aspect ratio method, on 80 semi-elliptical quartz porphyroclasts, assuming a Newtonian viscous rheology. Our observations match the results of previous analogue- and analytical models for various prototype rocks. For example: 1. higher competency contrast between c- and m-domains favors extension fractures over shear fractures (extension fractures tend to develop more in m-domains whereas the shear fractures in c-domains); 2. angle (ϴ) between fracture and `layer normal' is higher in less-competent layers; 3. dominant simple shear gives rise to P-brittle planes at an acute angle to the shear direction (Y-plane); and 4. stress drop during fracturing may inhibit slip along shear-induced fractures. Our calculations indicate that the rheological contrast (derived from the variation of ϴ) refracts the shear-induced fracture at the domain boundaries. In one such case, ϴ measured for 15 successive c- and m-domains show that the most viscous m-domain is 24 times more viscous than the lowest viscous c-domain. Additionally, out of eight c-layers, the most viscous c-domain has a viscosity 3.4 times more than the least viscous c-domain. Similarly, out of seven m-layers, the most viscous m-domain has a viscosity 4 times more than the least viscous m-domain. Hence, it appears that ductile shear and low grade metamorphism have caused prominent inhomogeneity in the rheological configuration of the greywacke.