Refraction of micro-fractures due to shear-induced mechanical stratigraphy in a low-grade meta-sedimentary rock

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Refracted markers, viz., cleavages and faults, across rock layers are well-documented structural features that develop as the structures propagate through layers of varying competency. We study the Palaeoproterozoic low grade meta-greywacke of the Rautgara Formation, Garhwal Lesser Himalaya, Uttarakhand, India. The focus is on the micro-fractures that cut the flaky-mineral rich cleavage (c-) and the porphyroclast-rich microlithon (m-) domains of the disjunctive foliation planes. Although the rock appears unsheared mesoscopically, in the micro-scale S–C fabric, shadow zones and tails of few quartz porphyroclasts exhibit a top-to-SW ductile shear. A mean kinematic vorticity number (Wm) of ~0.73 has already been determined from this rock. Our renewed study of thin-sections reveal fracture refraction patterns that match with the findings of various known analogue- and analytical models, viz., (i) higher competency contrast between c- and m-domains favours extension fractures over shear fractures, which develop more in the m-domains. Shear fractures dominate in the c-domains, (ii) the angle (ϴ) between fracture and the ‘layer normal’ is higher (>70°) inside the less-competent layers, (iii) a dominant simple shear in the brittle regime produces the P-planes at an angle to the primary shear Y-plane. In one such case, ϴ measured from thin-section for 15 successive sub-parallel 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 the eight c-layers, the most viscous c-domain is 3.4 times more viscous than the least viscous c-domain. Similarly, out of the seven m-domains, the most viscous m-domain has a viscosity four times more than the least viscous m-domain. Knowing viscosity ratio of different layers in rocks will enable better analogue and analytical tectonic models. Our numerical models of general shear on linear elastic materials similar to the studied rock type, however, show that the rheological contrast does not influence the curvature of the shear-induced fractures at the boundaries between the quartz-rich sandstone and the mica-rich domains. Close-spaced impurities/notches may curve fracture domains across the layer boundaries producing a ‘false’ impression of fracture refraction. Moreover, the first principal strain axis (ε1) does not reorient across the layers except close to the notches. Nevertheless, the current study shows micro-scale development of mechanical stratigraphy under the influence of the ongoing tectonic deformation and quantifies the domain-wise competence contrasts with the help of refracted fractures.