Evolution and characterization of fracture patterns: Insights from multi-scale analysis of the Buxa dolomite in the Siang Valley, Arunachal Lesser Himalayan fold-thrust belt

Journal of Structural Geology 123 (2019) 54–66

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Evolution and characterization of fracture patterns: Insights from multi-scale analysis of the Buxa dolomite in the Siang Valley, Arunachal Lesser Himalayan fold-thrust belt

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Abhisek Basaa,1, Farzan Ahmeda, Kathakali Bhattacharyyaa,∗, Ankur Royb a b

Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, West Bengal, India Petroleum Engineering Centre (ADRACEPE), Indian Institute of Technology Kharagpur, West Bengal, 721302, India

ABSTRACT

Fractures at different scales are an integral component of shallow crustal deformation associated with the progressive evolution of a fold-thrust belt (FTB). We characterize the evolution of fractures from the Buxa dolomite of the Main Boundary thrust (MBT) sheet in the frontal segment of the Arunachal Lesser Himalayan FTB. Regionally, the MBT sheet forms a fault-bend antiform. Based on unfolding of strata, early formed low- and moderate-angle fractures are interpreted to be prefolding layer-parallel shortening structures, while the dominant late-stage high-angle fractures are inferred to have formed synchronous to folding, tracking the progressive deformation of the orogenic wedge. The high-angle fracture set is the dominant one at both outcrop and microscopic scales, forming ∼44% and ∼45% of the total number of fractures, respectively. Microfracture analysis indicates that ∼41% of the high-angle fractures, ∼22% of the moderate-angle fractures and ∼10% of the low-angle fractures form veins, respectively. Cumulative plots of fracture spacing values collected from outcrops and under the microscope reveal that this parameter is best described by power-law distributions that indicate scale-independence of fracture spacing. However, the coefficients of variation (Cv) of spacing of these high-angle fractures reflect scale-dependent clustering.

1. Introduction During progressive deformation in an orogenic wedge fracture systems form at different stages (Nickelsen, 1979; Dunne, 1986; Gray and Mitra, 1993; Hennings et al., 2000; Ismat and Mitra, 2001; Hanks et al., 2004; Tavani et al., 2006; Bellahsen et al., 2006; Mynatt et al., 2009; Storti and Salvini, 2001; Reif et al., 2012; Gomez-Rivas et al., 2014), and may have genetic association with folding (Allmendinger, 1982; Couzens and Dunne, 1994; Fisher and Anastasio, 1994; Ismat and Mitra, 2001; Tavani et al., 2006; Watkins et al., 2015; Ukar et al., 2017). These fractures often get modified from their original mode of formation (Dunne, 1986; Ismat and Mitra, 2001, 2005a; Guiton et al., 2003; Bergbauer and Pollard, 2004; Florez-Nio et al., 2005; Ramsey and Chester, 2004; Sanz et al., 2008; Mynatt et al., 2009), and get reactivated during progressive deformation (Cruikshank et al., 1991; Mitra and Ismat, 2001; Ismat and Mitra, 2005b; Bellahsen et al., 2006). Statistical analysis of opening-mode fracture spacing can be best described by power-law distributions (Genter et al., 1997; Gillespie et al., 2001). Scaling relationships of spacing differing from power-law, however, are not uncommon and fracture spacing datasets may be found to have exponential (Priest and Hudson, 1976; Pineau, 1985),

log-normal (Sen and Kazi, 1984; Narr and Suppe, 1991; Rives and Petit, 1992; Becker and Gross, 1996; Pascal et al., 1997) or gamma distributions (Huang and Angelier, 1989; Gross, 1993; Castaing et al., 1996; Bonnet et al., 2001). These variations in scaling relationships have been attributed to fracture formation under varying deformation conditions (Gillespie et al., 2001) or to propagation interference by mechanical bedding boundaries during fracture growth (Hooker et al., 2009). Additionally, intersection of fractures leads to the formation of connected fracture networks that can act as efficient pathways for fluid migration (Evans and Battles, 1999; Fitz-Diaz et al., 2011; Guerriero et al., 2013). In the rock record these can be manifested as veins. The presence of fluids affects the rheology (Wojtal and Mitra, 1986; Badertscher and Burkhard, 2000; Kennedy and Logan, 1997) and strength of rocks (Hubbert and Rubey, 1959; Etheridge, 1983; Jaeger et al., 2009), thereby impacting the overall deformation mechanisms of the wedge. Additionally, fracture formation alters the porosity and permeability of the host rocks. Therefore, characterizing the evolution of fracture networks is essential to examine the impact of fracture sets on fluid flow. Internal thrust sheets of orogenic belts that deform at deeper crustal conditions and under high deformation temperatures record early layer

Corresponding author. E-mail address: [email protected] (K. Bhattacharyya). 1 Present address: Université de Pau et des Pays de L’Adour, Laboratoire des Fluides Complexes et de Leurs Réservoirs, E2S, UMR CNRS TOTAL 5150, Bâtiment IPRA, BP1115, 64013, Pau cedex, France. ∗

https://doi.org/10.1016/j.jsg.2019.03.004 Received 30 March 2018; Received in revised form 15 March 2019; Accepted 15 March 2019 Available online 22 March 2019 0191-8141/ © 2019 Elsevier Ltd. All rights reserved.

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parallel shortening (LPS) structures generally by dislocation-controlled deformation mechanisms (Groshong, 1975; Sibson, 1977; Mitra and Elliott, 1980; Engelder and Geiser, 1980; Knipe, 1989). These structures are modified and often overprinted by later fractures as thrusting initiates and fault-related folds form (Mitra and Elliott, 1980; Suppe, 1983; Gray and Mitra, 1993; Mitra and Bhattacharyya, 2011), progressively bringing rocks to shallower crustal conditions and lower temperatures due to thrusts cutting up-section and concomitant erosion (Wojtal and Mitra, 1988; Bhattacharyya and Mitra, 2011, 2014). In contrast, external thrust sheets deforming under lower temperature conditions may accommodate the early LPS as fractures that are reoriented, reactivated, along with growth of newer fracture sets, associated with movement along the thrust planes and subsequent fault-related folding (Couzens and Dunne, 1994; Ismat and Mitra, 2005b; GomezRivas et al., 2014). Additionally, fault-related folds, as they are cored by ramps, have components of both buckling and bending that result in varying shortening directions during their evolution (Groshong, 1975; Ramsay and Huber, 1987; Cooke et al., 1999; Cosgrove and Ameen, 1999; Ismat and Mitra, 2005a; Ismat, 2008). Therefore, deciphering the evolution of fracture systems during progressive deformation of a faultrelated fold, having both structural and lithological control (Nelson, 1985), can be quite challenging in natural settings (Price, 1966; Nickelsen, 1979; Ismat and Mitra, 2001; Watkins et al., 2015). In addition, if the fold is not completely preserved or exposed, working out the fracture evolution can become potentially even more challenging. In this study, we address the evolution of a fracture system that is spatially associated with a partially exposed fault-bend fold from an external thrust sheet, the Main Boundary thrust (MBT), of the eastern Arunachal Himalayan fold thrust belt (FTB; Figs. 1 and 2). This is the first such study from the Himalayan FTB. At the outcrop scale, fractures are the dominant structure in the MBT sheet. The Lesser Himalayan Sequence (LHS) lies in the hanging wall of the MBT. The LHS is repeated by a series of thrust faults in the Himalayan FTB and is bounded by the Main Central thrust (MCT) in the north and the MBT in the south. Compilation of existing published data from different transects of the Himalayan FTB reveals that the LHS accommodates the highest convergence-related shortening of the entire Himalayan FTB (∼114–458 km; Bhattacharyya and Ahmed, 2016 and references therein). The exposed LHS in the frontal portion of the Siang window of the study area is dominated by the Buxa dolomite (Fig. 1; Acharyya, 1994). The dominant deformation structure in the Buxa dolomite of the MBT sheet is a fracture network that lies in spatial association with a fault-bend fold of ∼17.4 km wavelength (Fig. 2; Ahmed et al., 2017). To examine the spatial distribution of these fractures within the Buxa dolomite, we first address the fracture density variation from the frontal segment of the MBT sheet, at both outcrop and microscopic scales of observations (Fig. 2). Then, we establish the relative temporal evolution of the different fracture sets, using both outcrop and microstructural observations, and their temporal relationship with respect to the fault-bend fold of the MBT sheet. We investigate scale-effects on fracture clustering by comparing the coefficients of variation (Cv) of fracture spacing, calculated at two different resolutions of observation, i.e., outcrop and microscopic scales. Additionally, we quantify the number of fractures, considering only the presence or absence of vein fillings, to evaluate a first-order fracture permeability among the different fracture sets, in their finite deformed state.

south are the MCT, the Pelling-Munsiari-Bomdila thrust (PT), the Ramgarh-Shumar thrust (RT), the Lesser Himalayan duplex, the Main Boundary thrust (MBT), and the Main Frontal thrust (MFT) (Yin and Harrison, 2000; Acharyya, 2007; DeCelles et al., 2016; Yin et al., 2010; Saha et al., 2011; Basa et al., 2018; Ahmed et al., 2017; Goswami et al., 2016). The LHS rocks are bounded by the MCT and the MBT and are repeated by several thrusts (Fig. 1). Erosion through the MCT and the PT have exposed the footwall LHS rocks forming the Siang window (Fig. 1). The MBT sheet carries the Buxa, the Yambung, the Abor and the Yinkiong Formations in its hanging wall, and is folded in a regional fault-bend antiform that trends along 5°,314° (Ahmed et al., 2017) within the Siang window. The MBT fault zone is not well exposed in the studied area. In this study, we focus on the Buxa dolomite (Fig. 1b) of the forelandmost segment of the MBT sheet, as exposed within the frontal part of the Siang window (Figs. 1b and 2). Fractures are the dominant deformation structures preserved at both outcrop and microscopic scales in these rocks. We mapped the MBT sheet along two ∼ N-S-trending transects. The studied locations lie close to the eastern transect and are projected along a N-S trending cross-section. The fault-bend fold depicted in the cross-section (Fig. 2b) is constrained by projecting the apparent dips of the outcrops that lie closest to the cross-section line. The southerly dip from the Yambung Formation (42°, 220°) has been used to arrive at the geometry of forelimb of the fault-bend fold. The broad hinge zone has a sub-horizontal (2°N) regional dip in the cross-section. We conducted outcrop scale fracture attribute studies from the Buxa dolomite at six locations from the better preserved hinge zone of the fault-bend antiform and at one location from the north-easterly dipping backlimb (57°, 045°) of the fold (Fig. 2). For microfracture analysis, we studied eight samples from the sub-horizontal hinge zone and one from the backlimb of the fold. The Buxa dolomite and the fracture sets are not preserved for fracture attribute study in the backlimb of the fault-bend fold except along a single location in the backlimb. The backlimb of the folded MBT sheet comprises slate, limestone and intrusive rocks, and hence was not considered. Thus, the majority of the locations for this study are from the hinge zone of the fold. 3. Methodology 3.1. Fracture data acquisition The outcrops in the study area are restricted to cross-sections along the road cuts. Two mutually perpendicular or high-angle sections are not exposed in the field. The study region is densely vegetated, and the outcrops are not generally well preserved. We restricted this analysis to the Buxa dolomite of the MBT sheet and, therefore, conducted the study on seven outcrops where the different fracture sets are preserved within the Buxa dolomite (Figs. 2 and 3). The average outcrop length is ∼60 m. These outcrops lie within ∼255–608 m from the inferred MBT trajectory (Fig. 2). Six of these outcrops are from the sub-horizontal hinge zone of the fault-bend fold of the MBT antiform and one is from the backlimb. The Buxa dolomite is not preserved in the backlimb of the MBT that comprises mostly slate, limestone and intrusive rocks and, therefore, most of the study was conducted on the hinge zone of the fold. Based on the abundance of fractures and their cross-cutting relationships, we identified three consistent fracture sets at each of these outcrops that are at low-, moderate- and high-angle to the bedding plane traces in the cross-sectional plane. Due to unavailability of two perpendicular sections in the field, we carried out quantitative analyses of fracture density and spacing using three 1-D scan-lines with varying orientations from each location. The scan-lines were oriented perpendicular to the mean orientation of each of the fracture sets. We selected the scan-line position and its length to capture the highest number of fracture data, resolvable at the scale of observation, from a particular fracture set (Fig. 3). The scan-line lengths varied accordingly within a

2. Geological setting The Arunachal Himalayan FTB forms the easternmost part of the Himalayan FTB, and is located between longitudes 91°30′E and 96°E (Fig. 1). In this segment of the FTB, the major orogen-scale structures that accommodate the convergence-related shortening generally continue from the west, and are manifested by a system of folded thrust faults (Fig. 1). The study area lies in the far-eastern part of the Arunachal Himalaya, where the prominent thrust faults from north to 55

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Fig. 1. (a) Regional map of the Himalayan FTB showing major thrusts and the litho-tectonic units (modified after DeCelles et al., 2016). The Siang window is marked within the black box. (b) Geological map of the Siang window (modified after Acharyya, 1994) showing traces of the major faults from the hinterland to the foreland. The study area is shown in the rectangular black box. A’A is the transect of transport-parallel cross-section. PTBomdila-Pelling-Munsiari thrust; RT- RamgarhShumar thrust, BT- Bomdo thrust GT- Geku thrust, MBT- Main Boundary thrust. AL - Along, B - Bomdo, G - Geku, P - Pasighat, T – Tuting, Y – Yingkiong, LHS – Lesser Himalayan Sequence, SHS – Sub Himalayan Sequence.

3.2. Variability of fracture spacing

range of ∼0.23 m–∼29 m. The microfracture attribute studies were conducted from nine locations that lie within ∼900 m–∼255 m from the inferred MBT trajectory (Fig. 4). The above distance is measured between the fault trajectory, as constrained on the regional cross-section, and the projected locations on the cross-section line. We selected these locations due to their similar lithology and preservation of both highest density and maximum variation in the orientation of fracture sets at the outcrop scale. Microfracture analysis was conducted on three perpendicular thin-sections from each location that were cut along the transportparallel, transport-perpendicular, and bedding-parallel sections. The microfracture analysis was conducted along three scan-lines from each thin-section, similar to the outcrop analysis. The first-order relative timing of various fracture sets, in both outcrop and microscopic scales, were determined using cross-cutting, offset and abutting relationships (Fig. 5). Additionally, based on the presence or absence of vein fillings, we classified fractures into veins and unfilled fractures.

Spacing of microfractures in thin-sections was measured along multiple scan-lines oriented perpendicular to the mean orientation of a fracture set. We used three different scan-lines for high-angle, moderate-angle and low-angle fracture documentation. Here, spacing is defined as the distance between the two nearest fractures along the scan-line (Priest and Hudson, 1976; La Pointe and Hudson, 1985). The coefficient of variation (Cv), i.e., the ratio of standard deviation to the mean, was used to quantify the clustering of fractures (Gillespie et al., 1999, 2001). Cv > 1 denotes a clustered distribution while Cv < 1 refers to an anti-clustered distribution. Cv = 1 signifies a random distribution and a perfectly periodic fracture spacing has Cv = 0. For scanlines with less than 25 fracture spacing data, we modified Cv, following Gillespie (2003).

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Fig. 2. (a) Strip-map showing the spatial extent of dolomite outcrops considered for outcrop scale fracture analysis. (b) A transport-parallel cross-section showing the geometry of the folded MBT sheet from the frontal portion of the Siang window. The Buxa dolomite is marked in blue. The studied locations for fracture attribute analysis at the outcrop and microscopic scales are also shown. MHT- Main Himalayan thrust MBT – Main Boundary thrust MFT – Main Frontal thrust. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

3.3. Effect of folding on fractures To understand the temporal evolution of fracture sets with respect to the fault-bend antiform of the MBT sheet, we measured the orientations of various fracture sets and the bedding of the Buxa carbonate host rocks from 14 different outcrops of the exposed frontal segment of the MBT sheet. The dolomite of the MBT sheet are restricted to these 14 outcrops. Thus, we ensured that the lithology remained similar across the different studied outcrops. For a detailed fracture attribute study from these 14 outcrops, we selected only those sections where the structural features are best preserved. The bedding orientations remained generally uniform within each of these locations. The rest of the locations of the frontal MBT sheet varied in lithology comprising slate, limestone and intrusive rock, and hence were not considered for this study. To arrive at the orientation of the fault-bend antiform of the MBT sheet, we mapped and plotted all the bedding data from the MBT sheet, irrespective of its lithology, along two separate N-S trending transects. The hinge line is estimated at 5°,314° (Fig. 6a). We unfolded the fold first by unplunging it, followed by removal of the dip of the beds (Fig. 6). Next, we examined the relative timing of the fractures with respect to folding by analyzing the clustering pattern of the poles to the fractures as observed using 1% area contouring before unfolding versus after unfolding (Fig. 6). The line of cross-section (Fig. 2) lies close to the eastern transect. The fault-bend fold of the MBT sheet is depicted in the cross-section by projecting the apparent dips of the closest outcrops of the MBT sheet along the line of cross-section.

Fig. 3. Buxa dolomite outcrop located ∼255 m away from the MBT zone showing fractures of different orientations. A bedding-parallel scan-line is marked in dark brown. Fractures at low, moderate and high angles to bedding are marked with pink, blue and black lines, respectively. Scan-lines used for measuring spacing of low, moderate and high-angle fractures are coloured as green, white and dark brown, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 4. Fracture analysis of the Buxa dolomite from seven locations at the outcrop scale and nine locations at the microscopic scale in the MBT sheet. (a) Fracture population distribution at the outcrop and microscopic scales. (b) Variation of fracture density with increasing distance from the MBT zone at the outcrop and microscopic scales. The black dotted-line represents a linear fit showing an increase in fracture density toward the MBT zone. Variation of (c) high-angle fracture density, (d) low- and moderate-angle fracture density with increasing distance from the MBT zone at the outcrop and microscopic scales.

4. Results

cutting relationships observed at outcrop and microscopic scales, we divided the fracture sets into three groups (a) low-angle (0°–30°), (b) moderate-angle (30°–60°), and (c) high-angle (60°–90°) fractures. Here, we include both unfilled fractures and veins. Results from outcrop fracture analysis conducted over seven preserved locations with average outcrop lengths of ∼60 m reveal that the low-, moderate- and high-angle fractures form ∼31%, ∼24% and

4.1. Relative temporal relationships of fracture sets and their population distribution Based on the abundance of fracture sets with respect to the bedding traces on the exposed cross-sectional planes, and also on their cross58

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Fig. 5. (a) Outcrop of the Buxa dolomite showing offset relationships. The fracture set marked in red has offset the pre-existing fracture marked in black. Bedding is represented in dark brown colour. (b) Offset relationship of fracture sets and their temporal evolution. Yellow and red coloured dashed lines represent fractures at low-angle and high-angle to the bedding trace, respectively. The late-stage high-angle vein offsets the early-formed low-angle angle vein. The green coloured line represents scan-line parallel to bedding-trace along which fracture spacing is measured. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

∼45% of the total fractures, respectively (Fig. 4a). Similarly, microfracture analyses conducted along three perpendicular sections indicate that the low- and moderate-angle fractures form ∼28% and ∼29%, respectively, while the high-angle fractures constitute ∼43% of the total fractures (Fig. 4a). Therefore, the high-angle fractures form the dominant fracture set, at both outcrop and microscopic scales. All the studied fractures are exposed along steep cross-sectional planes that very rarely expose sufficient surface areas for identification of slickenlines or plumose structures along them. Therefore, we could not conduct a systematic, comprehensive analysis of shear vs. openingmode fractures based on fracture surface morphology. However, based on 3D orientations of fractures from various locations, their angular relationships with respect to the hinge line and estimated maximum shortening directions, their pre-folding orientations (Fig. 6) and consistent offset patterns, the lower angle fractures are interpreted predominantly as shear-fractures. Similarly, we interpret the high-angle ones as opening-mode fractures (sections 4.2, 5). At both scales of observations, the high-angle fractures cross-cut the low-angle and moderate-angle fractures (Fig. 5). Thus, at a first order, the high-angle fractures are the late-stage fractures. We did not observe a consistent relative timing relationship, based on cross-cutting and offset relationships, between the low- and the moderate-angle fractures across scales. Therefore, we resist from commenting on the relative temporal relationship between the low- and moderate-angle fractures in this study. Microfracture analysis indicates that ∼41% of the high-angle fractures, ∼22% of the moderate-angle fractures and ∼10% of the low-

angle fractures constitute veins (Fig. 7). Thus, at a first order, amongst the studied fracture sets, the high-angle fractures had the highest susceptibility to be filled with cement. Calcite is the most common veinfilling mineral in the studied rocks, with a few veins showing rare quartz grains. All the studied veins, irrespective of their orientations, present blocky textures (Fig. 8a). In some of the veins, there is evidence of systematic coarsening direction from the margin of the vein toward the center (Fig. 8a). Additionally, there is a rare evidence of elongated blocky texture in a high-angle quartz vein (Fig. 8b) with different quartz grains outgrowing each other, due to varying growth rates, thereby recording growth competition (Bons, 2001). This particular vein also has evidence of at least three crack-seal events (Ramsay, 1980) that are recognized by inclusion bands (Fig. 8b). Therefore, based on these microstructures, most of the veins are identified as syntaxial veins (Bons et al., 2012). 4.2. Effect of folding on fracture sets from the MBT sheet Regional structural analysis reveals that the MBT sheet is folded in a fault-bend fold that trends 5°, 314° (Figs. 2 and 6a; Ahmed and Bhattacharyya, 2018). The mean orientations of the north-easterly and the south-westerly limbs are 57°, 037° and 54°, 243°, respectively. To examine the relative timing of folding with respect to the fracture population, we restored the orientation of the beds. On successive stages of removal of plunge followed by dips from the beds, all the poles to the fractures show a variation in their orientations, suggesting that all the plotted fractures of the different fracture sets formed prior or 59

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Fig. 6. (a) Stereoplots showing the orientations of the hinge of the fault-bend fold. Stereoplots showing (b) high-angle fractures (c) low- and moderate-angle fractures at finite deformation stage plunge removal and unfolded stages.

synchronously to the fault-bend fold (Fig. 6). Additionally, on complete unfolding of the fold, the poles to the low- and moderate-angle fractures show a tighter clustering than that of the high-angle fractures (Fig. 6). Although based on limited fracture data, these observations suggest that the low- and moderate-angle fractures formed earlier than the high-angle ones, as the effect of unfolding is higher on the former fracture sets. Therefore, this interpretation agrees with the inferred temporal relationships among the various fracture sets, based on crosscutting and offset relationships (section 4.1). In addition, on complete restoration of the fold, the high-angle fractures restore to a higher angle with respect to bedding (> 75°), while the low-angle fractures cluster toward a lower angle (∼15°). Microstructural studies of the veins indicate that all the veins are syntaxial with blocky textures (Fig. 8). Hence, opening trajectories (Bons et al., 2012) from these veins could not be established for commenting on the type of fractures from the

preserved veins. Therefore, we attempted to examine these fractures with respect to a first-order estimation of maximum compressive direction from conjugate fractures (Blès and Feuga, 1986; Twiss and Moores, 1992), and orientations with respect to the hinge line (Price, 1966; Watkins et al., 2015). Out of the fourteen exposed sites where fracture orientations could be measured directly in the field, ten lie near the hinge zone of the fault-bend fold (Fig. 2). These outcrops also locally preserve conjugate fracture sets, as established from their angular and consistent cross-cutting relationships (Fig. 9). From these conjugate fractures, we estimated an average maximum compressive stress direction (σ1) from the hinge zone of the fold to be oriented at 4°, 142° (Fig. 9a). As we establish the low-angle fractures to be dominantly prefolding, we rotated the finite-state σ1 to the pre-folded stage where bedding is restored to the horizontal, and obtained an orientation of 15°, 149° (Fig. 9b). On comparing the number of analyzed fractures 60

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fractures observed across different scan-lines divided by the total lengths of the scan-lines for each outcrop, varies from 0.16 per cm at ∼608 m from the MBT zone to 0.97 per cm at ∼255 m from MBT zone (Fig. 4b). The fracture density ranges from 0.01 per cm to 0.16 per cm within distances of ∼608 m–∼260 m from the MBT zone, but increases sharply to 0.97 per cm at the studied location closest to the MBT zone. This increase in fracture density with closest proximity to the MBT zone is also observed in the separately analyzed high-angle, moderate-angle and low-angle fracture sets (Fig. 4c and d). Microfracture analysis of nine dolomite samples that lie within ∼900 m and ∼255 m from the MBT zone shows a variation in fracture density from 4.52 per cm at ∼900 m to 11.35 per cm at ∼255 m from MBT zone (Fig. 4b). Although microfracture density does not show a sharp increase toward the MBT zone, it generally increases closer to the fault zone. Additionally, the late-stage, high-angle fractures form the dominant fracture set in both microscopic and outcrop scales (Fig. 4a).

Fig. 7. Plot showing percentage of veins or unfilled fractures relative to the angle between fractures and bedding.

with respect to the rotated σ1 direction, we find that ∼85% of the lowangle fractures and only ∼18% of the high-angle fractures lie within the active shear-fracture ranges that are defined by 30° ± 20° with respect to σ1 (Fig. 9; Blès and Feuga, 1986; Twiss and Moores, 1992; Ismat and Mitra, 2001). Therefore, the low-angle fractures predominantly initiated as shear-fractures. A fault-bend antiform generally develops two sets of high-angle opening-mode fractures, with strikes sub-parallel and sub-perpendicular to the hinge line orientations, respectively (Price, 1966; Rives and Petit, 1992; Bai et al., 2002; Watkins et al., 2015). On comparing the hinge line orientation of the fault-bend fold and the strikes of the measured high-angle fractures, we find ∼77% of the high-angle fractures to be opening-mode fractures with a dominance of hinge-perpendicular fracture sets (∼63%; Fig. 9). Therefore, the low-angle fractures are dominantly shear-fractures while the highangle fractures are predominantly opening-mode fractures. We would also like to point out that Fig. 4 illustrates the number of fractures measured along a particular scan-line, while Fig. 6 depicts the number of fractures whose orientations were measured directly in the field from each representative fracture set. Hence, Fig. 6 illustrates a lower number of fractures than Fig. 4.

4.4. Effect of scale on spacing and density of high-angle fractures Based on fracture population and fracture density data, we interpret the high-angle fractures to be the dominant fracture set in the studied rocks (Fig. 4). Therefore, we examine the effect of scale on the spatial arrangement of these fractures. We measured the fracture attributes from seven outcrops for outcrop scale analysis and compared them with the corresponding seven samples collected from the outcrops for microfracture analysis. At the outcrop scale, the density of the high-angle fractures varies from 0.18 per cm at ∼608 m from the MBT zone to 1.23 per cm at ∼255 m from the MBT zone (Fig. 4c). The high-angle fracture density ranges between 0.01 per cm to 0.18 per cm within distances of ∼608 m–∼260 m from the MBT zone. However, the high-angle fracture density increases to 1.23 per cm at the studied location closest to the MBT zone (Fig. 4c). For microfracture analysis, the high-angle fracture density ranges from 7.14 per cm to 12.87 per cm within distances of ∼608 m–∼255 m from the MBT zone (Fig. 4c). No consistent relationship is observed between high-angle fracture density and the distance from the MBT zone at the microscopic scale. However, the high-angle fracture density measured at every studied location at the microscopic scale is significantly higher relative to the outcrop scale (Fig. 4c). At the outcrop scale, Cv (spacing) of high-angle fractures ranges from 0.30 to 0.67 within distances of ∼608 m–∼255 m from the MBT zone showing anti-clustered arrangement or regular spacing of highangle fractures at all the studied locations. The Cv distribution at the microscopic scale, however, is not as systematic as that at the outcrop scale. The Cv distribution ranges from 0.64 to 1.33 within distances of ∼608 m–∼255 m from the MBT zone (Table 1). However, Cv values at the microscopic scale are higher relative to the outcrop scale in six out of seven studied locations. The location closest to the MBT zone shows

4.3. Fracture density variation within the frontal MBT sheet Although the MBT zone is not exposed in the study area, based on ongoing work on construction of a regional balanced cross-section (Ahmed et al., 2017), we have estimated the sub-surface position of the MBT near the frontal segment of the wedge (Fig. 2b). We analyzed seven outcrops that are well preserved, have the same Buxa dolomite host rocks, and lie within ∼255 m–∼608 m from the sub-surface MBT position (Fig. 4). Fracture density is defined as the number of fractures per unit length of the scan-line (Rouleau and Gale, 1985; Gillespie et al., 1993). The fracture density, measured by adding the total number of

Fig. 8. Photomicrographs of veins. (a) Prominent blocky texture in calcite veins showing growth competition with smaller grains at the boundary. (b) Rare elongateblocky texture in a quartz vein with the growth direction marked, revealing at least three crack-seal events. Calcite vein on the left shows a prominent blocky texture. 61

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Fig. 9. Stereo-plots showing (a) all the conjugate fracture sets. Great circles represent the mean orientation of the conjugate fractures; yellow solid circle marks the mean σ1 orientation (b) Poles to the low-angle fractures along with the rotated σ1 direction marked by the solid green circle. (c) Poles to the high-angle fractures along with the hinge orientation of the regional fold marked by a blue solid circle. (d) Rotated σ1 direction from different parts of the fold as estimated from conjugate fracture sets. (e) Orientation of fractures at different locations of the fault-bend fold along with the bedding plane marked by the red great circle. The finite deformed state and the rotated σ1 directions are marked by black solid circle and red solid circle, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

cutting relationships and their dominance, unfolding of the fractures also suggest that the high-angle fractures possibly form the youngest fracture sets in the studied segment of the MBT sheet. Although the MBT fault zone is not exposed along the studied transect, we observe a general increase in fracture population and density with proximity to the estimated MBT zone across the outcrop and microscopic scales (Fig. 4). Kinematic reconstructions in orogenic belts have demonstrated that the earliest deformation in a thrust sheet is accommodated by layer parallel shortening (LPS) structures (Geiser and Engelder, 1983; Geiser, 1988; Gray and Mitra, 1993; Mitra, 1994; Gray and Stamatakos, 1997; Hogan and Dunne, 2001; Ismat and Mitra, 2005b; Ong et al., 2007; Das et al., 2016). With progressive deformation within an evolving wedge, localization of deformation leads to the formation of large-scale thrust faults, and associated fault-related folds (Boyer and Elliott, 1982; Srivastava and Mitra, 1994; Salvini and Storti, 2001; Bhattacharyya and Mitra, 2009; Parui and Bhattacharyya, 2018). Additionally, the frontal thrust sheets of an orogenic wedge, due to its tapered geometry and the faults cutting upsection along the transport direction (Wojtal and Mitra, 1988; Bhattacharyya and Mitra, 2014), generally record a dominance of shallow-crustal, frictional deformation mechanisms (Sibson, 1977; Mitra et al., 1988; Tavani et al., 2015). The MBT sheet is part of the frontal segment of the Himalayan FTB, and lies ∼4 km north of the mountain front along the studied transect (Fig. 1; Acharyya, 1994; Ahmed et al., 2017). In the study area, the frontal part of the MBT sheet is dominated by carbonate rocks, which accommodate in many cases the early LPS by fractures (Storti and Salvini, 2001; Ong et al., 2007; Yonkee and Weil, 2010; Lacombe et al., 2011). Apart from the carbonate-dominated lithology, the structural position of the studied rocks within the frontal segment of the Himalayan wedge can explain the dominance of shallow-crustal, frictional deformation mechanisms, as manifested by formation of fractures of different orientations. Similar results have been reported from outcrop studies where fracture density increases closer to the faults in the foreland thrust sheets of the orogenic wedge (Wojtal and Mitra, 1986; Tavarnelli, 1997; Antonellini, 2000; Bhattacharyya and Mitra, 2014). An ongoing regional study has revealed that the MBT sheet forms a fault-bend fold (Ahmed et al., 2017). Stereoplot analysis carried out to decipher the effect of folding on the observed fracture sets shows that all the poles to the fractures were affected by folding, and therefore, developed prior to or during folding (Fig. 6). Additionally, by restoring the beds to their original sub-

Table 1 Table showing Cv (spacing) of high-angle fractures at outcrop and microscopic scales. Distance from MBT zone (∼m)

Microfracture analysis

Outcrop-scale analysis

Cv (Spacing)

No. of fractures

Cv (Spacing)

No. of fractures

255 260 304 522 590 600 608 750 900

0.64 1.33 0.99 0.78 0.93 1.27 1.03 0.87 0.56

13 29 23 35 16 19 20 27 18

0.67 0.42 0.7 0.35 0.44 0.3 0.48

65 26 27 26 18 26 57

similar Cv values of 0.67 and 0.64 at outcrop and microscopic scales, respectively. To examine the effect of resolution on fracture spacing, we analyzed log-log cumulative frequency plots of spacing values at outcrop and microscopic scales at the seven studied locations within distances of ∼608 m–∼250 m from the MBT zone. We normalized these cumulative numbers by the length of the scan-line to aid the comparison of fracture spacing attribute across two different scales of observation (Marrett et al., 1999; Ortega et al., 2006). The log-log cumulative frequency plots of all the seven locations are best described by power-laws (Fig. 10). However, there is a slight deviation from the power-law trend at the smallest values of fracture spacing at the microscopic scale. 5. Discussion Based on microscopic and outcrop analyses, we observe a temporal sequence in fracturing within the studied frontal segment of the folded MBT sheet. Based on measured 3D orientations of the exposed fracture planes, the high-angle fractures consistently cross-cut the low- and moderate-angle fractures. Although based on limited data, the effect of unfolding is strongest on the clustering of the poles to the lower-angle fractures relative to the high-angle fractures (Fig. 6). The high-angle fracture set is the dominant one at both outcrop and microscopic scales. Additionally, most prominent fractures can be assumed to be the youngest fracture sets (Ismat and Mitra, 2001). Thus, along with cross62

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Fig. 10. Plot showing the cumulative frequency per cm vs the spacing of high-angle fractures from spacing measurements independently carried out at the outcrop and microscopic scales in seven locations. The trend-line is generated using a power-law.

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horizontal state, the high-angle fractures become steeper (> 75°), while the lower-angle fractures become gentler with respect to the sub-horizontal bedding (∼15°, Fig. 6c), irrespective of their structural position. All the studied veins, irrespective of their orientations, are syntaxial with blocky texture (Fig. 8a). Hence, opening trajectories of the fractures could not be established from these veins (Bons et al., 2012). However, based on their orientations with respect to the estimated average σ1 from conjugate fractures, ∼85% of the low-angle fractures are interpreted to be shear-fractures (Fig. 9; section 4.2). The finite state σ1 (4°, 142°) was rotated to the pre-folded state (15°, 149°) to compare the orientations of the pre-folding low-angle fractures. Therefore, σ1 is at low angle to bedding. Also, we have estimated σ1 from individual locations by restoring the beds to the horizontal position (Fig. 9). Interestingly, σ1 generally lies sub-parallel to the hinge line of the regional fold (5°, 314°) indicating a stress-field that is not compatible with the finite deformed state of the fold. As most of the exposed outcrops of the fold are from the hinge zone, detailed kinematics of the fold could not be worked out. For example, passive rotation and reactivation of conjugate fractures have been reported where the acute bisector can track the extension direction (Ismat, 2015). In the absence of such data, together with the subhorizontal σ1 that forms a very low angle to the bedding, we propose that the shear-fractures are dominantly tracking the early LPS, which is consistent with other results from this study. In contrast, based on the angular relationships between the strikes of the high-angle fractures and the hinge line of the fold (Price, 1966; Rives and Petit, 1992; Watkins et al., 2015), ∼77% of the high-angle fractures are interpreted to be opening-mode fractures (Fig. 9). Therefore, the high-angle opening-mode fractures are related to folding (Fig. 9). Therefore, we attribute the early-stage, low-angle fractures possibly to be the manifestation of the early LPS. We interpret the overprinting of the early fractures by late, high-angle fractures as a result of progressive deformation of the orogenic wedge and successive folding. Additionally, apart from being the dominant fracture set, ∼41% of the high-angle microfractures are veins compared to ∼23% of the lowerangle microfractures. At a first-order, this observation can be speculated as another line of evidence for the high-angle fractures to be later, opening-mode, which were more susceptible to vein formation than the early formed, often healed, lower-angle shear-fractures. As the high-angle fractures are the dominant fracture sets amongst the studied fractures, we analyzed this fracture set at the outcrop and microscopic scales to evaluate the effect of resolution on the spatial arrangement of these fractures. Data-sets obtained along scan-lines, oriented perpendicular to the high-angle fractures, show higher Cv (spacing) values at microscopic scale relative to outcrop scale at six out of seven studied locations (Table 1). Additionally, the high-angle fracture density is significantly higher at the microscopic scale relative to the outcrop scale. Thus, we see a difference in the clustering behavior with the scale of observation and significantly more fractures are detected at higher resolution (Ortega et al., 2006), which organize themselves into clusters (Roy et al., 2016). However, the location studied closest to the MBT zone shows similar Cv (spacing) values across the outcrop and microscopic scales (Table 1). This points out to the possibility that the spatial arrangement of high-angle fractures is fractal in nature. This is further bolstered by the power-law nature of log-log cumulative frequency plots (Fig. 10). A deviation from power-law at the microscopic scale with fracture spacing less than ∼600 μm is observed in all the studied locations from the MBT zone (Fig. 8). A similar deviation from the power-law for naturally deformed rocks has been described as truncation bias due to incomplete sampling of the smallest fractures at the highest resolution (Bacher and Lanney, 1978; Einstein and Baecher, 1983; Odling, 1997; Bonnet et al., 2001). Thus, Cv (spacing) reflects anticlustered or regular spacing of highangle fractures at low-resolution outcrop scale irrespective of the structural position in the MBT sheet. However, there is no systematic variation of Cv (spacing) of these high-angle fractures at high-resolution microscopic scale within the MBT sheet. Although high-angle fracture

population shows power-law spacing distributions (Fig. 10) implying scale-invariance in spacing, Cv (spacing) reflects scale-dependent fracture clustering in most of the studied locations in the MBT sheet (Table 1). 6. Conclusions By integrating detailed fieldwork and multi-scale fracture analysis, we attempted to decipher the fracture evolution of a partially exposed fault-bend antiform from the external thrust sheet of an orogenic wedge during its progressive deformation. In the absence of exposed fracture planes, multiple lines of evidence were combined to arrive at an internally consistent temporal evolution of the fracture sets from a limited outcrop. The MBT sheet forms the frontal segment of the Arunachal Himalayan FTB, and dominantly exposes the Buxa dolomite of the LHS in its frontal segment along the Siang window. Regionally, the MBT forms a fault-bend antiform trending 5°, 314°. At the outcrop and microscopic scales, the dominant deformation structures in these rocks are three sets of fractures that are oriented at 0°–30°, 30°–60° and 60°–90° to bedding. Based on cross-cutting and offset relationships, and the predominance of the fracture set in both outcrop and microscopic scales, the high-angle fractures are interpreted to be the youngest. Additionally, unfolding of the fracture sets affects the poles to all the fracture sets, suggesting that the fractures formed prior to or synchronous to folding. The effect of unfolding is strongest on the clustering of the poles to the lower-angle fractures, suggesting that they are older than the high-angle fractures. Based on their orientations with respect to the estimated average σ1, ∼85% of the low-angle fractures are interpreted to be the early LPS shear-fractures. Based on the angular relationships between the strikes of the high-angle fractures and the hinge line of the fault-bend fold, ∼77% of the high-angle fractures are interpreted to be opening-mode fractures that are related to the fold. Therefore, the overprinting of the early LPS shear-fractures by the later opening-mode fractures track the progressive deformation of the orogenic wedge. Microfracture analysis indicates that ∼41% of the highangle fractures, ∼22% of the moderate-angle fractures and ∼10% of the low-angle fractures constitute veins. Thus, at a first order, amongst the studied fracture sets, the high-angle fractures had the highest susceptibility to form veins. Study of fracture population and density variation within the MBT sheet reveals an overall increase in fracture population and density toward the fault zone across the outcrop and microscopic scales. These high-angle fractures show power-law spacing distributions across outcrop and microscopic scales implying scale-invariance in their spatial arrangement. However, the scale-invariance is not reflected in Cv (spacing) values as they exhibit scale-dependent clustering. Acknowledgements This work was supported by Academic Research Fund of IISER Kolkata to K. Bhattacharyya, and IISER Kolkata IPhD Fellowship to A. Basa and F. Ahmed. The data used for this work are all represented in diagrams. We thank the two reviewers, Gautam Mitra and Hannah Watkins, and the Editors, Enrique Gomez-Rivas and Bill Dunne, for their detailed and critical comments that greatly improved the quality of the Manuscript. We thank Jonti Gogoi and Jyoti Prasad Das for their able assistance in the field. Mr. Rupam Rakshit is acknowledged for preparing thin-sections for microfracture analysis. References Acharyya, S.K., 1994. The Cenozoic foreland basin and tectonics of the eastern subHimalaya: problems and prospects. Himal. Geol. 15, 3–21. Acharyya, S.K., 2007. Evolution of the Himalayan Paleogene foreland basin, influence of its litho-packet on the formation of thrust-related domes and windows in the Eastern Himalayas–A review. J. Asian Earth Sci. 31 (1), 1–17. Ahmed, F., Gogoi, J., Bhattacharyya, K., 2017. Kinematic evolution of the far-eastern

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