Effects of tectonically induced fabrics on geomechanical properties of rocks, NW Pakistan

Geomechanical properties play important role to predict rocks strength and failure under an applied load for safe engineering purposes. Millions of years deformations, metamorphism, tectonic stresses, and thermal processes associated with crust thickening induce anisotropies in rocks in the form of foliations, lineations, fractures, and shears that affect strength behavior of rocks (Hudson and Harrison 1997; Milnes et al. 2006). Determining geomechanical properties without understanding pre-existing anisotropies at all scale of observations may lead to unrealistic rock strength calculation (Ramamurthy et al. 1993; Hudson and Cosgrove 1997). Tectonically induced penetrative foliations, mineral lineations, mica fish, S-C fabrics, mantle porphyroclast, and shear zones can be used to derive movement of planes along which a rock can potentially collapse during or after construction (Lister and Snoke 1984; Priest 1993; Hudson and Cosgrove 1997; Passchier and Trouw 2005; Chamine et al. 2013; Barton and Quadros 2015; Watkins et al. 2015). Similarly, the planes, which truncate asymmetric crenulated cleavages (S-fabric) in the microlithon against the main crenulation cleavage (C-fabric), can provide additional shear planes for applied loaded stresses. These tectonically induced anisotropies in deformed rocks can potentially affect the engineering characteristics of rocks (Duan and Kwok 2015). Therefore, it is critical to take into account the pattern of metamorphic fabrics, which distinctly exhibit coordinated geometric planes of weakness in tectonically deformed rocks (cf. Miskovsky et al. 2004; Punturo et al. 2014). Most of the times less attention is paid to study tectonically induced fabrics, mineral lineations, and intersection lineations in the rocks that eventually are used in construction and as bedrocks. These structures that are produced as a result of multiple deformations could affect mechanical strength of rocks, which make construction designs more complex and elaborative (Milnes 2006). Indian plate and KIA collision induced multiple penetrative foliations and lineations at all scale of observations in the rocks exposed south and north of the MMT (DiPietro et al. 2008; Ali et al. 2016). This research insight into the affects of tectonically induced geometric structures on rocks compressive and tensile strengths and thus provides a better understanding of the effect of tectonic fabrics on the engineering properties of a rock.

The MMT formed as a result of the Indian plate collision with the KIA ~ 60 to 40 Ma (Fig. 1; Yin 2006). The KIA is formed as a result of intra-oceanic subduction of the Neo-Tethys in the Cretaceous. The MMT mélange zone is the western continuation of the Indus Tsangpo Suture Zone, which is comprised of deformed and metamorphosed northernmost and southernmost marginal rocks of the Indian plate and KIA, respectively (Fig. 1; DiPietro et al. 2008 and references therein). Four penetrative tectonic fabrics have been established in the northernmost rocks of the Indian plate (DiPietro et al. 2008; Ali et al. 2016). NW-SE trending D₁ structures are mostly preserved in the microlithons as crenulated cleavages or in the garnet porphyroblasts. At outcrop scale D₁ are transposed by NE-SW to ENE-WSW trending D₂ structures, which is the most dominant fabric of the northernmost Indian plate. The D₂ structures are later overprinted by D₃ structures that also resulted into regional-scale domal and syntaxial structures. D₃ associated fabric is counterclockwise rotated by D₄ tectonic event (Ali et al. 2016). Chlorite replacement by biotite in the main S₂ crenulation cleavage domain indicates prograde metamorphic conditions associated with the D₂. It has been interpreted that the garnet growth predated D₂, D₃, and D₄ tectonic events because the main S₂ fabric is wrapped around the garnet porphyroblasts (Ali et al. 2016).

The study area comprises MMT hanging and foot walls (Fig. 1). The hanging wall comprises the Indus suture zone and Kamila Amphibolite, whereas the foot wall consists of the Cambrian to Ordovician CGG (Kazmi and Jan 1997) and Permian to Late Mesozoic Alpurai Group metasediments of the Indian plate (DiPietro et al. 2008).

Penetrative planar and linear fabrics were measured from the CGG, melange zone, and Kamila Amphibolite exposed north and south of the MMT respectively (Fig. 1). Petrographic and micro-structural analyses were made from 19 oriented samples that were cut parallel to the main crenulation cleavage. Respectively four and one bulk samples were collected from the CGG and Kamila Amphibolite to study the effect of the metamorphic fabric on the mechanical properties of rocks. A total of two core samples perpendicular and parallel to the main tectonic fabric were obtained according to ASTM specification (ASTM D-3976 1986) from each bulk sample to calculate the UTS and UCS. These tests were conducted at the Department of Mining Engineering, University of Engineering and Technology, Peshawar.

Mesoscopically ENE-WSW trending S₂ pervasive foliation and L²₂ developed parallel to D₂ fold axis (Fig. 6c). The D₂ tectonic event is overprinted by ENE plunging L³₂ intersection lineations (Fig. 6b). The sigmoidal S₁ crenulated cleavage was not observed mesoscopically. To look into the impact of penetrative fabrics on the strength of rocks the selected bulk samples are cored perpendicular and parallel to the main S₂ foliations. Microscopically, the S₂ crenulation cleavage/C-fabric microlithon nicely preserved S₁ crenulated cleavage and is defined by sigmoidal muscovite, biotite, and stretched quartz (Fig. 2a–c). Microscopic differential stresses anastomose the well-developed matrix S₂ around coarse-grained plagioclase and feldspar grains (Fig. 3b, c). S₂ intensification around rigid plagioclase and feldspar grains at strain caps led to precipitation of quartz and fine mica in the strain shadows (Fig. 3c, d). The S₂ crenulation cleavage also deflects around sigmoid to lenticular mica fish trending 22.21° with top-to-SSE shear sense (Fig. 7; Table 2).

The UCS average value of four core samples that were taken parallel to the main S₂ tectonic fabric from the CGG is 51.8 MPa as compare to UCS average value of 45.35 MPa obtained from the core samples taken perpendicular to the S₂ main foliation. The UCS values of amphibolite rock from the KIA are 91.31 MPa and 59.23 MPa parallel and perpendicular to the S₂ main foliations, respectively (Table 3). The S2 sample obtained from comparatively massive part of the CGG shows less variation in strength in core samples cut parallel and perpendicular to the S₂ foliations and lineations. The UCS, c, and τ values are higher in the core samples that were cut parallel to the pervasive S₂ and L²₂ because the parallel shear stress on the sigmoidal crenulated cleavages in the microlithon of the S₂ and layer parallel top-to-SSE shear sense on S₂ mica fish with a mean value of 22.21° counterbalances the parallel applied load (Fig. 9).The UCS differs from sample to sample due to difference in tectonically induced penetrative planar and linear fabrics. The strength of selected core samples varies with orientation of the tectonically induced fabric. The S₁ crenulated cleavage, ENE-WSW trending S₂ main foliation, D₂ mesoscopic folds, L²₂ lineations, petrography, preferred oriented sigmoidal grains, and mica fish have implication for the overall in-situ engineering constructions and tunnels that will be built on the northernmost Indian plate rocks, Indus Suture Zone, and southernmost rocks of the KIA.

Non-coaxial dextral shear sense on strongly preferred oriented sigmoidal grains (plagioclase and feldspar) that formed by crystal-plastic deformation and mica fish enhances foliation parallel strength of the CGG rocks (Figs. 3a–d and 7a–f). However, the UCS values decrease in the core samples that were cut perpendicular to the pervasive S₂ and L²₂ because the perpendicular shear on the sigmoidal crenulated cleavages in the microlithon of the S₂ and S₂ mica fish enhances the applied load, which lead to the failure of core samples (Fig. 9). The lower UTS values were observed in the core samples, which were cut parallel to the main S₂ foliation and L²₂ lineation compared with the core samples cut perpendicular to the tectonic fabric (Table 3; Fig. 9). Core samples obtained parallel and perpendicular to D₂ associated ENE-WSW axial plane S₂ foliations show variable UCS and UTS values, respectively. The S₂ parallel core samples have higher UCS, c, τ, and UTS values than those cut perpendicular to the S₂ main fabric (Table 3).

Tectonically induced structures including foliations, lineations, mineral alignments, mineral alteration, metamorphic textures, and veins at microscopic to mesoscopic scales significantly affect geomechanical properties of tectonized rocks (Ramamurthy et al. 1993; Singh et al. 2012; Ozbek et al. 2018). Genetic complexity in the form of penetrative foliations and lineations makes geomechanical behavior of metamorphic rocks unpredictable (Behrestaghi et al. 1996). According to Everall and Sanislav (2018) tectonically deformed rocks dominantly fail by reactivation of pre-existing planes of weakness that include foliations, fractures, and veins. The above arguments imply that each rock should be analyzed independently for anisotropies because the UCS is significantly reduced in the presence of pre-existing weaknesses developed during multiple tectonic events. In general, affects of penetrative fabrics on geomechanical properties of anisotropic rocks are mostly looked at microscopic to mesoscopic observations without assessing the overprinting fabrics. However, in the current study multiple penetrative foliations that have significantly affected geomechanical properties of rocks collected south of the MMT would have most likely been studied without looking at the affect of strongly preferred oriented sigmoidal porphyroblasts, mica fish, and crenulated cleavages in the microlithon of the main S₂. The UCS and UTS values differ from sample to sample due to variably induced penetrative fabrics (Table 3). The orientation of mica fish, sigmoidal porphyroblasts, and crenulated cleavages to the applied load directly affected the strength of rocks. The UCS significantly increased in the S₂ parallel core samples containing mica fish and S₁ crenulated cleavages in the microlithon by opposing the applied load as result of load parallel shear at the main matrix crenulation cleavage (Fig. 9). Mica fish and S₁ in the microlithon of the S₂ that respectively counterbalanced and facilitated the applied load in foliation parallel and perpendicular core samples would not be detectable without a detailed petrographic analyses. Therefore, the main differential stresses that are preserved in rocks in the form of simple or complex foliations and lineations must be determined before the use of tectonites in any engineering constructions or as aggregates.