Abstract Refraction of markers, viz., cleavages and faults, across rock layers is a well-documented structural geological feature that happens as the evolving structures cut through layers of different 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 disjunctive foliation planes. Although the rock looks 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 been already known 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 (e1) 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.

中文翻译：

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低品位变质沉积岩中剪切诱发机械地层引起的微裂缝折射

摘要 标记物（即解理和断层）在岩层上的折射是一种有据可查的构造地质特征，它发生在不断演化的构造穿过不同能力的层时。我们研究了印度北阿坎德邦 Garhwal 小喜马拉雅山 Rautgara 组的古元古代低品位变灰岩。重点是切割片状矿物丰富解理 (c-) 和分离叶理平面的富含斑岩碎屑的微石 (m-) 域的微裂缝。尽管岩石在细观上看起来是未剪切的，但在微观尺度 S-C 组构中，少数石英斑岩的阴影区和尾部表现出从顶部到 SW 的韧性剪切。从这块岩石中已知的平均运动涡度数 (Wm) 约为 0.73。我们对薄片的重新研究揭示了与各种已知模拟和分析模型的发现相匹配的裂缝折射模式，即，（i）c 域和 m 域之间更高的能力对比有利于延伸裂缝而不是剪切裂缝，后者发展在 m 域中更多。剪切断裂在 c 域中占主导地位。(ii) 在能力较低的层内，裂缝和“层法线”之间的角度 (ϴ) 更高 (>70°)。(iii) 脆性区域中主要的简单剪切产生与主剪切 Y 平面成角度的 P 平面。在一种这样的情况下，从 15 个连续的子平行 c 域和 m 域的薄截面测量的 ϴ 表明，最粘稠的 m 域比最低黏性 c 域的粘性高约 24 倍。此外，在八个 c 层中，最粘稠的 c 域是 3。比最不粘稠的 c 域粘稠 4 倍。类似地，在七个 m 域中，粘性最大的 m 域的粘度是粘性最低的 m 域的四倍。了解岩石中不同层的粘度比将有助于建立更好的模拟和分析构造模型。然而，我们对类似于所研究岩石类型的线弹性材料的一般剪切数值模型表明，流变对比不影响富含石英的砂岩和富含云母的区域之间边界处的剪切断裂的曲率. 间隔很近的杂质/凹口可能会使裂缝域跨层边界弯曲，从而产生裂缝折射的“假”印象。此外，第一主应变轴 (e1) 不会重新定向跨层，除非靠近凹口。尽管如此，