Control of pre-existing fabric in fracture formation, reactivation and vein emplacement under variable fluid pressure conditions: An example from Archean Greenstone belt, India

Most of the upper crustal fluid flows are strongly influenced by the pre-existing fractures/foliations in the 10 rocks under a certain state of tectonic stress and fluid pressure condition. In the present study, we analyze a wide range of crosscutting fractures that are filled with quartz veins of variable orientations and thicknesses, from the gold bearing massive metabasalts (supracrustal) of the Chitradurga Schist Belt adjacent to the Chitradurga Shear Zone (CSZ), western Dharwar craton, south India. The study involves the following steps: 1) analyzing the internal magnetic fabric using anisotropy of magnetic susceptibility 15 (AMS) studies, and strength of the host metabasalts, 2) quantifying the fluid pressure condition through lower hemisphere equal area projection of pole to veins by determining the driving pressure ratio (R ́), stress ratio (φ), and susceptibility to fracturing, and 3) deciphering the paleostress condition using fault slip analysis. We interpret that the NNW-SSE to NW-SE (mean 337 o/69o NE) oriented magnetic fabric in the rocks of the region developed during regional D1/D2 deformation on account of NE-SW shortening. 20 However, D3 deformation manifested by NW-SE to E-W shortening led to the sinistral movement along https://doi.org/10.5194/se-2020-30 Preprint. Discussion started: 9 March 2020 c © Author(s) 2020. CC BY 4.0 License.


Introduction
The upper crust is replete with fractures/faults, which act as pathways for fluid flow and vein emplacement. Fracture formation and vein emplacement mechanisms are closely interrelated, and require a detailed study for finding out potential hydrothermal deposits. Fracture formation and reactivation 35 involves a combination of regional stress field (far field stress), stress ratio () and driving pressure ratio (R´) that helps to determine the prevailing fluid pressure condition (Delaney et al., 1986;Jolly and Sanderson, 1997;McKeagney et al., 2004;Mazzarini, 2007;Martinez-Poza et al., 2016;Cucci et al., 2017). However, previous studies suggest that pre-existing anisotropy in host rocks play a significant role in formation and propagation of fractures provided the anisotropy is favorably oriented to the far field 40 stresses (Ikari et al., 2015;Donath, 1961;Hoek, 1964;Attwell and Sandford, 1974). Presence of such https://doi.org/10.5194/se-2020-30 Preprint. Discussion started: 9 March 2020 c Author(s) 2020. CC BY 4.0 License.
favorably oriented anisotropy lowers the shear strength of the host rocks, enabling failure/slip along them at minimum compressive stress, prior to/during vein emplacement. Such conduits are reactivated at both high and low fluid pressures forming pathways for fluid flow (Mondal andMamtani 2013, Lahiri and. Thus, vein emplacement mechanism requires fracture orientations that can be 45 reactivated, when fluid pressure gradually builds and exceeds the normal stresses acting on the fracture wall (Gudmundsson, 2011). This enables dilation of the fracture planes, a mechanism known as faultvalve action (Sibson et al., 1988;Sibson, 1992Sibson, , 1996Sibson, , 2000Boullier and Robert, 1992;Sibson and Scott, 1998;Petit et al., 1999;Cox et al., 2001 among others). Subsequently, fluid flows into the fractures, a phenomenon analogous to burping, triggering an immediate drop in fluid pressure. This sudden drop in 50 fluid pressure is responsible for mineral deposition and vein formation (Cox et al., 1991, Cox, 1995. Vein materials thus deposited seals the fracture/fault planes, preparing the system for the next cycle of fluid pressure build up, rupture, fluid flow and vein formation (Mondal and Mamtani, 2013;Lahiri and Mamtani, 2016;Marchesini et al., 2019). Thus, repeated cycles of elevated and depleted fluid pressure generate criss-cross pattern in veins. Such intricate studies regarding the mechanism of fabric 55 development and fracture formation vis-à-vis vein emplacement helps to provide a detailed insight about the development of brittle structures and their role in understanding the tectonic evolution of Archean cratons.
The Chitradurga Schist belt (western Dharwar craton, south India), is a NW-SE trending Archean greenstone belt, known to harbor a widespread network of veins with potential epigenetic gold bearing 60 lodes (Gupta et al., 2014;Gopalakrishna et al., 2018). We have conducted this study in the meta-volcanic (metabasalt), hosting quartz veins of variable orientations and thicknesses, in and around Chitradurga https://doi.org/10.5194/se-2020-30 Preprint. Discussion started: 9 March 2020 c Author(s) 2020. CC BY 4.0 License. region. We emphasize on understanding the mechanism of vein emplacement under a tectonic environment where propensity of fracture reactivation for vein emplacement is mutually dependent on both fluid pressure condition and the regional far field stresses. The present paper is a comprehensive work which quantifies the fabric in the visually isotropic metabasalts of the Chitradurga Greenstone belt and its role in fracture instigation and in channelizing upper crustal fluids. We also found the reactivation potential of the fractures/faults at variable fluid pressure conditions to investigate the role of fractures/faults which are not favorably oriented to the pre-existing anisotropy and regional stress field, and their contribution towards vein emplacement in the study area.

Geology of the study area
The study has been conducted in the Chitradurga Greenstone Belt of Dharwar craton (southern India; Fig.   1a), which represents a complex geological history. Dharwar craton exposes >3.0 Ga, Archean continental crusts, represented by the TTG (trondjemite-tonalite-granodiorite gneiss) also known as the peninsular 75 gneiss (Jayananda et al., 2006). The craton stabilized during the accretion of Eastern Dharwar Craton (EDC) and Western Dharwar Craton (WDC) at 2.75-2.51 Ga. The zone of accretion of the two tectonic blocks is marked by a shear zone, referred to as Chitradurga Shear Zone (e.g., Naqvi and Rogers, 1987;Chadwick et al., 2003;Jayananda et al. 2006). The eastern part of WDC marked by Chitradurga Schist Belt (CSB) (Fig. 1b), comprises of peninsular gneiss (3.4-3.0 Ga), and younger supracrustal rocks 80 (Beckinsale et al., 1980;Sarma et al., 2011;Taylor et al., 1984). The latter comprises metavolcanics/metabasalts (greenstone belt; greenschist/lower amphibolite facies metamorphism), metamorphosed greywacke-argillite (interbanded with ferruginous chert and banded iron formation), polymict conglomerate, and ferruginous chert. It is believed that the basaltic rocks of the schist belt have an island arc affinity (Chakrabarti et al., 2006). Regionally, the belt is surrounded by older peninsular 85 gneisses and younger granitoids (Ramakrishna and Vaidyanadhan, 2010). Previous geological investigations, revealed the presence of various younger granites ~2.6 Ga; (Jayananda et al., 2006;Chardon et al., 2002) which are associated with the schist belt, such as Chitradurga granite, J. N. Kote granite. The structural investigations of the adjacent meta-sedimentary rocks of the region show that the area has undergone three phases of deformation: D1, D2 and D3 respectively (Chadwick et al., 1989;90 Jayananda et al., 2006;Mondal and Mamtani, 2014).The D1/D2 deformations are coaxial with NW-SE striking axial plane. D1 folds are tight to isoclinal and asymmetric while the D2 folds are open to tight and upright. Earlier studies revealed that the folds (regional open; NE-SW striking vertical axial plane), related to early D3 deformation superposed the D1/D2structures thus resulting in dome-basin geometry in the meta-sedimentary rocks of that region (Chakrabarti et al., 2006;Mondal and Mamtani, 2014). 95 However, later phase of D3 deformation led to the formation of brittle structures in the younger granites of CSB (Mondal and Mamtani, 2016). The NE-SW shortening during D1/D2 deformation was responsible for NW-SE oriented structural elements in the CSB (Chadwick et al., 2003), while NW-SE to E-W shortening direction prevailed during D3 deformation (Jayananda et al., 2006;Mondal, 2018;Mondal and Acharyya, 2018).A number of petrological and geochemical investigations have been carried out on the 100 area in the past. These studies suggest the presence of actinolite, albite, chlorite, epidote, quartz and calcite in the basaltic rocks (Chakrabarti et al., 2006). The CSB holds evidence of progressive metamorphism with a gradual increase in P-T conditions (approximately 3-4 Kb of pressure), from greenschist facies in north to amphibolite facies in south suggesting the depth of deformation to be 10-12 Km (during D1/D2). https://doi.org/10.5194/se-2020-30 Preprint. Discussion started: 9 March 2020 c Author(s) 2020. CC BY 4.0 License. However, recent studies by Acharyya and Mondal, (2019) shows that the brittle deformation during late 105 D3 took place at a shallow depth of ~2-4 km. The dashed rectangular area in figure 1b demarcates the study area. The present study focuses on the dark greyish to blackish, massive to fine grained, altered metabasalt hosting quartz veins, of the region surrounded by meta-sedimentary sequences (Fig. 2).The metabasalts of the study area are devoid of any well-defined field foliation. The foliation planes in the adjacent meta-sedimentary rocks of the region are found to be NW-SE oriented (mean strike/dip is 110 323º/71º NE). Since, the metabasalts are devoid of mesoscopic field fabric, the anisotropy of magnetic susceptibility (AMS) study has been conducted in order to quantify the internal magnetic fabric in them.

Overview of brittle structures
Metabasalts of the study area are replete with fractures and faults of multiple orientations. Some of these 115 fractures and faults are found to host quartz veins forming criss-cross pattern ( Fig. 3a and c) and some of them are devoid of any vein material. Growth of vein materials (quartz crystals) are often found to be perpendicular to the vein wall suggesting that dilation was significant in these veins (Fig.3b). The maximum width and length of the quartz veins are recorded to be ~1 meter and ~130 meters respectively. Some of the quartz veins are found to be displaced by other ones forming crisscross network of veins in 120 the study area (Fig. 3c). Wing cracks filled up with quartz veins are commonly observed (Fig. 3d). At few places, the thicker quartz veins are replete with series of successive fault planes with slickenside lineations on them ( to high. Both Left Lateral Faults (LLF's) and Right Lateral Faults (RLF's) are recorded based on the movement of the hanging wall with respect to the footwall. Presence of congruous steps helps in the discrete identification of the fault planes (Fig. 3g). Although, quartz veins of variable orientations and thicknesses are found, however, most of the veins are predominantly NNW-SSE trending (Fig. 4a). Most of the fractures and faults show NNW-SSE trend (maxima) whereas some others form a WNW-ESE to 130 NE-SW sub maxima respectively ( Fig. 4b & 4c). Some WNW-ESE trending Mode-I (tensional) cracks with prominent tips are also recorded from the study area which are often filled up with quartz veins. It may be noted that the veins with maximum thicknesses are oriented along NNW-SSE direction (Fig. 4d).
In this study, 992 fracture data (strike/dip), 378 vein data (strike, dip and thickness) and 73 fault data (strike, dip and slip) have been measured from the metabasalt exposures of about 13 locations in the entire 135 study area. All 73 shallow to moderately plunging fault planes are used to decipher the paleostress condition. Thus, integrating field observations and data obtained from brittle structures enables to reconstruct the tectonic stress conditions of the craton and helps to quantify the fluid pressure conditions that prevailed during fracture reactivation and vein emplacement.

Methods of analysis and results
The quartz veins have been emplaced in the massive metabasalts of the region that are devoid of any prominent mesoscopic foliation as mentioned above. At places, veins of one orientation are dissected and sometimes displaced by others that led to the formation of mesh like structures (Fig. 3a, c). Sibson (1992) has mentioned that such a mesh is formed when a rock contains fractures of varying orientations 145 that may get reactivated due to rise in fluid pressure. It is mentioned that in the Chitradurga region, veins https://doi.org/10.5194/se-2020-30 Preprint. Discussion started: 9 March 2020 c Author(s) 2020. CC BY 4.0 License. of various orientations show mutually cross-cutting relationships, which implies repeated cycles of vein emplacement (see sect.-3 and Sibson, 1992). Although veins have various orientations, NNW-SSE striking veins are the most common (Fig. 4a). It may be noted that NW-SE to NNW-SSE direction, which defines the maximum strike orientation of quartz veins, fractures and faults, is also the orientation of the 150 adjacent Chitradurga shear zone, the overall trend of the schist belt. This implies that there is a strong structural control on the formation of these quartz veins. It has been shown earlier that the pre-existing anisotropy plays a critical role in propagating fractures and channelizing fluid in rocks (Sanderson and Zhang, 1999, Cox et al., 2001and Ikari et al., 2015. It is also known from rock mechanics investigations that the rock strength variation controls the strain partitioning and influences fluid flow (e.g., Tsidzi, 1990;155 Vishnu et al 2018). Therefore, it is crucial to determine the rock fabric and state of stresses in order to understand the upper crustal fluid flow vis-à-vis vein formation.
In the past, 3-D Mohr circle analysis has been performed by Jolly and Sanderson (1997) using dyke orientation data to examine the magma pressure condition that was responsible for the opening of preexisting fractures during dyke emplacement. Further the work has been extended to understand the fluid 160 pressure condition and vein emplacement mechanism by McKeagney et al., 2004(also see Yamaji et al., 2010.We present vein orientation data from the Chitradurga region (southern India), which is a province of epigenetic gold deposit (Gupta et al., 2014;Gopalakrishna et al., 2018).
We conduct anisotropy of magnetic susceptibility (AMS) study, followed by Brazilian Tensile strength (BTS) determination in order to quantify the internal magnetic fabric and tensile strength of the 165 rocks within the study area. The 3-D Mohr circle construction using quartz vein orientation data helps to recognise the fluid pressure conditions under which they were emplaced. Further, these fluid pressure https://doi.org/10.5194/se-2020-30 Preprint. Discussion started: 9 March 2020 c Author(s) 2020. CC BY 4.0 License.
conditions were integrated with dilation tendency, slip tendency and fracture susceptibility in order to understand the mechanism of vein emplacement in the Chitradurga region. A combination of data obtained from these methods along with paleostress analysis using fault-slip data recorded from the study 170 area provides a comprehensive evaluation of vein forming conditions in Chitradurga greenstone belt.

4.1Anisotropy of Magnetic Susceptibility (AMS)
The quartz veins occur in massive metabasalt, which does not show any visible field foliation.
However, such visibly massive rocks may preserve an internal fabric, which can be recognized on the 175 basis of anisotropy of magnetic susceptibility (AMS) studies (e.g., Tarling and Hrouda, 1993;Maffione et al., 2015;Mamtani and Greiling, 2005;Raposo et al., 2007;Loock et al., 2008;Mondal and Mamtani, 2014;Mondal, 2018).The study involves preparation of cylindrical core samples (25.4 mm diameter × 22 mm height) from oriented metabasalt blocks. These metabasalt block samples have been collected from 13 different locations (Fig. 5) in the study area. The prepared core samples are subjected to an external 180 magnetic field and the induced magnetization for each core sample is measured in different directions.
AMS is considered to be a symmetric second-rank tensor, represented by an ellipsoid with three mutually perpendicular stress axes, K1, K2 and K3 respectively where (K1 ≥ K2 ≥ K3). The orientation and magnitude of each of these principle axes are determined in this analysis, where K1 represents the magnetic lineation, K3 is the pole to the magnetic foliation (K1K2). Using the magnitudes of K1, K2 and K3, several AMS 185 parameters are calculated such as magnetic susceptibility (Km), magnitude of the magnetic foliation (F) and magnetic lineation (L), degree of magnetic anisotropy (Pj or P´) and shape parameter (T). The formulae for the parameters are given below (after Tarling and Hrouda, 1993;Jelίnek, 1981): https://doi.org/10.5194/se-2020-30 Preprint. Discussion started: 9 March 2020 c Author(s) 2020. CC BY 4.0 License.
Here, 1 = ln K1, 2 = ln K2,3 = ln K3 and m = (1.2.3) 1/3 . In the above equations, Pj and T give the measure of the eccentricity and shape of the AMS ellipsoid respectively. The value of T ranges between 195 -1 to +1. Positive and negative values represent oblate and prolate shapes of the AMS ellipsoid (Tarling and Hrouda, 1993).

Tensile strength determination
It is known that the dilation occurs in a direction parallel to the minimum compressive principal stress (σ3), when the fluid pressure (Pf) exceeds the normal stress acting on the fracture wall.
Therefore the fractures may occur at any depth when the effective stress (σ3 -Pf) is sufficient to 215 counteract the tensile strength (T) of the rocks (see Fig.10c; Gudmundsson, 2011). We measure the tensile strength of the host metabasalts to quantify the Pf that prevailed during vein emplacement in the region. The measurement of direct tensile strength requires machined specimens and also involves difficulty in applying tensile load on the cylindrical specimen during analysis. Therefore, tensile strength measurement of rocks using Brazilian test has become imperative in rock mechanics. 220 Compression-induced extensional fractures are generated in the test which essentially involves lineloading on a circular disk placed between two platens (Aydin and Basu, 2006;Basu et al., 2013;see Fig.6).This tensile strength (T) is estimated from the elastic theory (ISRM, 1978;ASTM D3967, 2001): 225 Here, P is peak/failure load, and L and D, are the length and diameter of the disk respectively. For this analysis, 18 core samples were drilled from metabasalt blocks which were later resized to obtain the desirable cores for the analysis (length: diameter = 1: 2).The maximum tensile strength of each specimen at the instance of failure is recorded. The maximum tensile strength for18 samples are averaged out to obtain the approximate tensile strength of metabasalts, which is ~12 MPa. This tensile strength value is 230 further used for quantifying the Pf in 3D Mohr circle using vein orientation data.

Fluid pressure determination
Here, we have used the method proposed by Jolly and Sanderson (1997) to quantify the Pf conditions that led to the vein emplacement in metabasalts of Chitradurga region (southern India).We have used the lower 235 hemisphere equal area projection of the poles to quartz vein data. According to Jolly and Sanderson (1997), girdle distribution of vein pole data implies Pf> σ2, described as a condition where large number of fracture orientations are susceptible to reactivate, while Pf< σ2, represents clustered distribution of vein pole data, where only limited range of fracture orientations reactivate. Depending on the type of distribution (girdle/cluster), parameters such as stress ratio (ϕ) and driving pressure ratio (R´) are calculated using ranges of fracture orientations (θ1, θ2 and θ3) from the following equations provided by Sanderson, 1997 andBaer et al., 1994.
For Pf> σ2, For Pf< σ2, In figure 7a, the lower hemisphere equal area projection of pole to vein data shows girdle distribution, implying high fluid pressure condition(Pf> σ2). From this distribution the orientations of the principle stress axes (σ1, σ2 and σ3) are determined using the Bingham statistics of the Stereonet 9 software 250 (http://www.geo.cornell.edu/geology/faculty/RWA/programs/stereonet.html). σ1 is sub-vertical lying in the empty space devoid of any vein pole data. Subsequently, following Jolly and Sanderson (1997), the planes σ1σ2, σ1σ3 and σ2σ3 are constructed and the range of fracture orientations, θ2 and θ3 are determined along the σ1σ3 and σ1σ2 planes respectively. For this high Pf condition, θ2= 27º, θ3= 59º from which ϕ = 0.72 and R´ = 0.8 are calculated. Thus, such a Pf condition enhances the chances of vein emplacement 255 along various orientations. Although pole to vein data represents a girdle distribution pattern, however the highest density cluster is found around σ3axis indicating a number of veins with similar orientations.
This suggests that the vein forming fluid along these orientations must have been channelized through a pre-existing anisotropy (along a preferred orientation). These data are segregated and plotted separately in the lower hemisphere equal area projection and thus the obtained contour defines the SW cluster (see.

Dilation tendency, slip tendency and fracture susceptibility
Dilation tendency (Td) and slip tendency (Ts) determine the propensity of any fracture orientation to reactivate through dilation or shearing, under a certain state of stress condition (Mazzarini, 2019). While, high dilation tendency ensures reactivation through dilation, high slip tendency 285 elevates chances of opening through shearing (Ferrill et al., 1999). According to, Stephens et al., 2017, fracture planes suffer dilation when the difference between σ1 and the normal stress acting on the plane is close enough to the magnitude of differential stress(σD= σ1 -σ3) and Td= (σ1-σn)/ σD. Slip tendency is denoted by the ratio of shear stress (σs) to normal stress (σn); (Ts= σs/σn) and also depends on the frictional characteristics of the rock (Morris et al., 1996), along with the fracture plane orientation. Under a 290 particular state of stress condition if the ratio of shear stress to normal stress is significantly large, then that particular fracture orientation is susceptible to reactivate. Fracture susceptibility (Sf), is defined as the variation of fluid pressure (ΔPf) within a fracture plane that can lead to fluid induced shear reactivation (Mildren et al., 2002;Stephen et al., 2017). Such reactivations depend on the shear and normal stresses acting on the fracture plane, along with the cohesion (= 0 in this case) and the static coefficient of friction 295 (µs); Sf = σn-(σs/µs).
Lower hemisphere equal area projections (see Fig. 8) of the poles to fracture (vein-filled) data help us to understand the variation in dilation tendency, slip tendency and susceptibility of fractures with respect to their orientations, under both high and low Pf conditions. The diagrams are prepared using Fractend code . It is evident from figure 8a, that the dilation tendency is high for the fracture orientations 300 which are at a high angle to the 3 axis, i.e., pole to these fractures form a well-defined cluster around 3.
These fractures show greater tendency towards dilational opening for both high and low Pf ( Fig. 8a and   b). For fracture orientations having higher slip tendency, i.e., susceptible to shear opening, pole to the fractures are at a low angle to the 3 axis. The fracture planes are therefore oriented at an angle to the maximum compressive stress axis 1, condition favorable for shear reactivation for both, high and low Pf 305 ( Fig. 8c and d). Fracture susceptibility, which involves variation in fluid pressure (Pf) is low for the fracture orientations having high dilation and slip tendencies, which indicates fluid-induced fracture reactivation in metabasalts ( Fig. 8e and f) in the study area. 310 The shallow to moderaly plunging normal faults of the study area with prominent slip directions are used to determine the stress regime under which these fractures and faults were formed and reactivated. Faultslip data (orientations of fault planes and slip directions) recorded from the field were used for paleostress determination. Several methods are proposed for paleostress analyses using fault-slip data (e.g. Angelier, 1994;Dupin et al., 1993;Etchecopar et al., 1981;Gapais et al., 2000;Marrett and Allmendinger, 1990;315 Ramsay and Lisle, 2000; Twiss and Unruh, 1998;Yamaji, 2000;Žalohar and Vrabec, 2007  (3) faults are homogeneous and a part of the same tectonic event (Angelier, 1994;Gapais et al., 2000;Gephart & Forsyth, 1984;Twiss & Unruh, 1998). In this study, we have determined the paleostress direction, using fault-slip data measured from 73 shallow to moderately plunging normal faults (spatially 325 distributed)in the metabasalt by Right Dihedron method. Since, some of the fault planes show variation in their strike orientations, the small amount of inhomogeneity in the data set is reduced in this process.

4.6Paleostress analysis
Thus, the data sets are segregated methodically into homogeneous data subsets using the 'Win_Tensor' software program (version 5.8.6; Delvaux and Sperner, 2003;Delvaux, 2011). In the present analysis all the collected data are represented in a single set and separation is done by using the Right Dihedron 330 method without any sort of manual intervention. Following Delvaux and Sperner, 2003, the data is filtered on the basis of stress ratio (R), orientation of the stress axes and symmetry of the measured sets. Out of 73 data, 30 data are accepted with a low value of counting deviation and nominal counting values of 0 and 100 for σ1 and σ3 respectively. Thus, the best fitted reduced stress tensor is obtained for the accepted data subset (30 out of 73 fault data; see Fig. 9) at a "C" quality ranking. It also provides the relative 335 orientations of the principal stress axes, stress ratio (R= 0.72) and stress regime index (R´=1.25).The NNE-SSW directed extension direction obtained from this paleostress analysis (see Fig. 9) coincides well with the regional D3 extension direction. Data rejected in this process to obtain the best fit stress tensor when treated separately yields a NNE-SSW oriented extension direction with small variations in the R https://doi.org/10.5194/se-2020-30 Preprint. Discussion started: 9 March 2020 c Author(s) 2020. CC BY 4.0 License.
and R´ values. According to Delvaux and Sperner (2003), the obtained stress regime index indicates a 340 pure-strike slip domain which is in a good agreement with sinistral shearing along CSZ.

Fabric development vis-à-vis regional tectonics
It has been mentioned earlier that the metabasalts of the study area lack any distinct visible foliation. 345 However, the AMS analysis suggests, a prominent NNW-SSE to NW-SE oriented magnetic fabric in metabasalts. This magnetic fabric also matches well with the field foliation of the meta-sedimentary sequences, surrounding the metabasalts and is also parallel to the regional trend of CSZ (see Fig. 2).This implies that the fabric in metabasalts of the study area must have been controlled by the regional deformation. Structural investigations suggest that the Chitradurga region was subjected to three 350 deformational events-D1, D2 and D3 respectively. D1 and D2 were found to be coaxial and controlled by NE-SW shortening that led to the development of folds (NW-SE striking vertical axial plane), while D3 was controlled by NW-SE to E-W shortening, resulting in the development of NE-SW striking planar structures (Chakrabarti et al., 2006;Ramadass et al., 2003). The superposition of D3 over D1/D2 formed culmination and depression in the region (dome-basin structures; Type-I interference pattern) 355 (Chakrabarti et al., 2006).Therefore, it is logical to infer that the magnetic fabric in metabasalts are also related to the regional D1/D2 deformation under NE-SW directed shortening that generated the field foliation in the meta-sedimentary sequences. The magnetic lineations in the metabasalts are plunging due NNW through sub-vertical to SSE (Fig. 5b).Earlier, these variations in the plunge of the magnetic lineations had been interpreted as a consequence of superposed deformation (e.g., Mamtani and Sengupta, 2010;Mondal and Mamtani, 2013). Therefore, in the light of regional structural information and above discussions, it is inferred that these variations in magnetic lineations of the metabasalts are the manifestation of dome-basin geometry that were produced due to the superposition of D3 over D1/D2 regional deformation. Recently, Mondal (2018)

Control of regional far-field stress on developing the brittle structures in Chitradurga region.
The above discussions suggest that (a) NNW-SSE oriented magnetic foliation developed during D1/D2 370 deformation under NE-SW directed shortening, (b) variation in plunge of magnetic lineation is a manifestation of dome-basin geometry on account of D3 deformation. It is argued that during D3 deformation under NW-SE to E-W directed shortening the CSZ evolved as a sinistral shear zone (Mondal and Acharyya, 2018). It may be noted that the angle between the mean orientation of the schist belt and the compression direction for D3 deformation are found to be ~45º which also supports the sinistral 375 movement along Chitradurga shear boundary.
It is mentioned in sect. 2 and 3 that, the study area is replete with a number of brittle structures such as fractures, faults. At places, these fractures are filled-up with quartz veins and the present paper is aimed to understand the mechanism behind the formation of these veins. Therefore, it is now essential to evaluate whether and how this brittle structures and their kinematics can be fitted to the regional far-field 380 stresses responsible for deformation in the Chitradurga region. The quartz vein orientation data from https://doi.org/10.5194/se-2020-30 Preprint. Discussion started: 9 March 2020 c Author(s) 2020. CC BY 4.0 License.
northern part of the Chitradurga schist belt reveals that the vein emplacement took place during regional D3 deformation (Mondal and Mamtani, 2014 MPa, which is greater than 4T, suggesting that the fractures in the metabasalts are not purely tensile except for the cracks parallel to D3 shortening direction. However, the calculated value of Δσ is less than 5.7T, indicating that the normal stress on the fractures are not purely compressive also. Therefore the value of Δσ satisfies 4T <σ1 -σ3 < 5.7 T (Sibson, 2000), indicating that these fractures in the study area are 405 extensional shear mode fractures.

5.3Regional tectonics and the mechanism of fracturing, faulting
We discussed earlier in sect. -3, that the fractures and faults recorded in the metabasalts show a wide range of orientations with a NNW-SSE maxima and a WNW-ESE to NE-SW sub-maxima respectively 410 (Fig. 4b).Among which, fractures trending along WNW-ESE to E-W are sub-parallel to the D3 (late phase) shortening direction. As previously mentioned in sect. -3, these WNW-ESE trending fractures have been regarded as tensional fractures. Similar orientations have also been recorded and interpreted as tensile fractures from the micro-granitoid enclaves in Chitradurga granite as evident from the studies of Mondal and Acharyya (2018). However, it is essential to explain the predominance of fractures and faults 415 along the NNW-SSE orientation (forming the maxima). In order to explain this, we refer to the preexisting fabric in the metabasalts of the study area, i.e., the NNW-SSE to NW-SE oriented magnetic fabric developed during D1/D2 regional deformation (Fig. 5a). Earlier studies suggest, that fractures are more likely to propagate along a pre-existing anisotropy, if and when the anisotropy is favorably oriented with respect to the regional stress field (Ikari et al., 2015). The CSZ being a sinistral shear zone exhibits a pure 420 strike slip stress regime, coeval with D3 deformation (NW-SE to E-W directed shortening).
In the later phase of D3 deformation, the favorably oriented NNW-SSE fabrics were reactivated under a congenial stress field, thereby, causing reactivation of the pre-existing fabrics in the metabasalts. This however, fails to justify the occurrence of the ~NW-SE and ~NE-SW oriented fractures within the metabasalts. From field investigations, the NE-SW oriented fractures show dextral movements, while the 425 NNW-SSE and NW-SE oriented fractures are recorded with sinistral movements respectively (see Fig.   3c and d). However, the fracture disposition and consistency in their respective orientations indicate that all of these fractures are related to the same deformational event and have been reactivated under similar stress conditions. Moreover, any other brittle deformational event post D3, have not been recorded from the study area as mentioned earlier. Therefore, we need to explain the occurrence of such variably oriented 430 fractures/faults within a single kinematic framework. The NNW-SSE and NW-SE orientations are most likely to be sinistral, whereas NE-SW orientations form the dextral shear components respectively, considering CSZ to be the sinistral shear boundary. (See Fig. 10b and 4b, c). Thus, the NNW-SSE to NW-SE (P, Y and R) and the NE-SW to ENE-WSW (X and R) fractures coincide with the shear components of a riedel shear system considering the angle of internal friction (Φ) in metabaslts of the study area to be 435 ~30 . It may be noted that the value of Φ is approximated from the Uniaxial Compressive Strength (UCS) studies of the metabasalts core samples following Sivakugan et al., 2014. These fracture planes acted as pathways for fluid flow and vein emplacement during the late D3 deformation. 440 In order to explain the vein emplacement mechanism along these weak zones we need to consider the fluid pressure conditions that prevailed in the study area. In sect. 4.4, we mentioned earlier that the lower hemisphere equal area projection of pole to vein data shows girdle distribution, indicating high Pf condition (~53.2 MPa; >σ2) in the study area. Under such high Pf conditions, veins were emplaced along all possible orientations, including NNW-SSE, NW-SE, WNW-ESE, NE-SW and ENE-WSW trending 445 fractures. The building fluid pressure surpassed the normal stresses acting on the fracture wall, and hence, fluid burped into the weak planes leading to fluid-induced reactivation of the fractures, promoting vein emplacement along them (Fig. 10c).However, the NNW-SSE trending veins show greater thickness and abundance with respect to other orientations (see Fig. 4d and 3e). We found that pole to these veins (NNW-SSE trending orientations) lie within the warm zones of the stereoplots obtained from the Fractend 450 code , indicating higher dilation tendencies (see Fig. 8a and 8b). In Fig. 8a and 8b, it is perceived that pole to these orientations form a cluster around the σ3 stress axis. Similarly, pole to the orientations trending NW-SE and NE-SW respectively, have higher slip tendencies, indicating shear reactivation along them ( Fig. 8c and d).  (Cox et al., 1991(Cox et al., , 2001.

Conclusions
In the present study, we commented on the vein emplacement mechanism of the Chitradurga greenstone belt (Dharwar craton, south India). We analysed the magnetic fabric data recorded from AMS analysis of the metabasalt that hosts the quartz veins. 3D Mohr circle and paleostress analysis have been used to evaluate the vein emplacement vis-à-vis regional deformation. Following are the main findings and conclusions from the study: 1. The NW-SE oriented magnetic fabric recorded in the metabasalts (as evident from the AMS analysis) is a product of the D1/D2 regional deformation on account of NE-SW directed shortening. This fabric was also favourably oriented and therefore, suitable for fracture 490 propagation in relation to the prevailing stress field.    zones stand for vein attitudes that are more susceptible to reactivate under low Pf variation. 'Thermal' color scheme from Thyng et al., 2016. White square (σ1), white diamond (σ2) and white triangle (σ3).  MPa), obtained from the dashed lines representing the mean stress in each case. A conceptual graph shows multiple cycles (n-times) of high and low Pf conditions in the study area justifying fault-valve action that led to the emplacement of vein in the Chitradurga region.