Arc-parallel compression in the NW Himalaya: Evidence from structural and palaeostress studies of brittle deformation from the clasts of the Upper Siwalik, Uttarakhand, India

Abstract

The sub-Himalayan Upper Siwalik rocks, between the Main Boundary Thrust (MBT) to the north and the Main Frontal Thrust (MFT) to the south, are intensely brittle sheared and jointed. Our field studies around Dehradun (India) furnished at least eight small-scale brittle slip directions, viz., ∼top-to-SW/SSW (up), top-to-SW/SSW (down), top-to-NE/NNE (up), top-to-NE/ENE (down), topto-NW (down), top-to-SE/SSE (up), top-to-SE/SSE (down) and top-to-NW/NNW (up). Additionally, we report near-vertical faults, four sets of joints (inclined: J 1 and J 2 ; near-vertical: J 1V and J 2V). Palaeostress analyses using T-TECTO Studio X5 with all joint sets reveal two compression directions ∼ENE-WSW and ∼NNW-SSE. We propose two possible temporal relations between the joint sets: (i) J 1 , J 2 , J 1V and J 2V are coeval (∼ENE-WSW compression) and (ii) J 1V and J 2V developed coevally (∼ENE-WSW compression) followed by J 1 and J 2 (∼NNW-SSE compression), because arc-parallel compression (if any) occurs later than arc-perpendicular compression. The presence of already well-known strike-slip faults, viz., the Yamuna tear fault and the Ganga tear fault, at high angles, ∼55 • and ∼85 • to the orogenic trend, implies a possible arc-parallel compression in the Siwalik Himalaya in the study area. This ∼NNW-SSE compression could also indicate a localised stress reorientation due to the curvature of the Thrust planes, viz., the MFT and the Asan Thrust (as observed in plan view) close to the study area. This study further shows that arc-parallel compression need not be restricted to the inner arc of an orogen, and/or, as in the case of the Himalaya, near the syntaxes.

Brittle tectonics in the western Arunachal Himalayan frontal fold belt, northeast India: Change in stress regime from pre‐collisional extension to collisional compression

“…The stress ratio cannot be determined from the T‐TECTO Studio X5 solely from fracture data. The present algorithm of the software can determine the stress ratio only when the fault slip data are utilized (Dutta et al, 2019).…”

“…Brittle deformation is most commonly manifested by the development of mesoscale joints. The dynamic palaeostress analysis of such brittle structures is critical for determining the regional palaeostress conditions that exist in the study area (e.g., Dutta et al, 2019; Maurya, Shaikh, & Mukherjee, 2021; Shaikh et al, 2020; Vanik, Shaikh, Mukherjee, Maurya, & Chamyal, 2018). We investigate palaeostress conditions using structural data consisting of 141 joint planes collected in the field (Figure 2).…”

“…Two other slip senses: top‐to‐SE down (Figure 8c) and top‐to‐SW down (Figure 8d) noted in the Bhareli Formation are difficult to interpret and may be related to the orogen‐parallel ~E‐W extension phase of the Siwalik sequence in the phase 3 extension (Section 6). Orogen‐parallel shear has recently been discussed from sub‐Himalaya in Dutta et al (2019) and from the Lesser Himalaya in Mahato, Mukherjee, and Bose (2019) and Biswas, Bose, Dutta, and Mukherjee (2022), all from the western Himalayan sector. It is significant to note only a few places fault striae are documented.…”

“…The Siwalik rocks are folded into anticlines and synclines (Thakur & Pandey, 2004; Wesnousky, Kumar, Mahindra, & Thakur, 1999). Eight strike‐slip faults are reported from the area and the trend of these faults are N‐S, north‐northeast (NNE)‐south‐southwest (SSW), and NE‐SW (Dutta, Biswas, & Mukherjee, 2019; Jayangondaperumal, Dubey, & Sen, 2010). Based on small‐scale brittle slip directions, an attitude of the near‐vertical and inclined joint sets, an arc‐parallel compression in the Siwalik Himalaya is proposed based on palaeostress analysis (Dutta et al, 2019).…”

Evidence of Atlantic Extension in South-East Togo: Case of Fracturing in the Sandstone Relics of Tohoun

“…This software requires as inputs, the geometrical parameters of planes (Dip direction/dip) and striae on planes, as well as fault type and the precision for each plane measurement. Three methods are proposed for the calculation of tensors namely: 1) Direct Inversion [37] [38] [39] [40]; 2) Numerical Dynamical Analysis (NDA; [41], review in [42]); and the 3) Dihedra calculation [43] [44] [45]. The Direct Inversion methods are used for inherited faults systems [44] [46], while the NDA allows determining the parameters of deformation axes rather than stressing characteristics.…”