Tridib Kumar Mondal

Fractures are the manifestation of brittle deformation and act as vital conduits for fluid transport in upper crustal rocks. To measure rock strength and stability, infer deformation mechanisms, and reconstruct the stress condition under which they developed, a systematic examination of their geometrical characteristics is essential which also provide insights on how upper crustal rocks respond to stress. Since fractured rock bodies frequently consist of interconnected networks of different fracture sets, topological characterization aids in quantitative assessment of their connectivity, which directly affects comprehension of their permeability and, consequently, the history of fluid migration through the host rock body. Additionally, characterization of fracture networks has direct implications in recent applications like nuclear waste disposal and carbon sequestration which contribute significantly to environmental sustainability.The present study examines the origin and characterizes subsequent networking of fractures developed within younger granites (~ 2.61 Ga) of the Chitradurga Schist Belt, an Archean age granite-greenstone belt from the Western Dharwar Craton of peninsular India integrating field-based observations with network topology and fractal analysis. We systematically document the geometrical attributes of fracture patterns developed within the granites across varying outcrop scales to understand their formation and characterize them topologically to assess their connectivity and record if fracturing patterns, intensity, density and connectivity vary across scales and also spatially along the areal extent of the granitic plutons. It is found, that although indicative of being formed by the activation of a Riedel shear system under the same tectonic stress regime, the networking patterns which the fractures have developed through their mutual interaction vary spatially in their geometrical, topological and fractal characters. Our study ventures upon the possible causes of this variation and highlights the role of ambient stress state, rheology, pre-existing mechanical anisotropy, orientation of pluton margin and its proximity to adjacent shear zone and superimposition of fractures behind the development of these spatially varying fracture network patterns across the areal extent of the granitic plutons.

Granite emplacement is often assisted by shear zones. These shear zones may be synmagmatic and can lead to the development of various structural features, ranging from complex fabric to faults and their interactive networks. The Neoarchean Closepet Granite (2.56-2.51 Ga) of Eastern Dharwar Craton, India is replete with faults and fractures of various orientations, which are used to decipher the regional brittle tectonics. Detailed analysis of strike-slip and high-angle oblique-slip faults using the Right Dihedron and Rotational Optimization methods reveals an overall E–W oriented horizontal regional compressive stress. The area is further subdivided into smaller domains to observe variations in the local stress field. Results indicate a distinct deformation pattern across the region. The northern portion is influenced by ENE-WSW directed regional compression, while the southern portion is shaped by ESE-WNW directed regional compression. Fault orientations and their kinematics across the pluton indicate that the faults are associated with a large-scale Riedel shear along the pluton boundary. In the north, deformation is compatible with an NW-SE sinistral shear zone, while in the south, fault patterns are consistent with an NNE-SSW dextral shear zone. This variation in shear is attributed to the overall geometry of the pluton boundary. We also interpret that, during the emplacement and the post-emplacement period, the rheological boundary between the granite and the surrounding host rock, evolved into a shear zone, facilitating the development of faults across the Closepet pluton.

Dykes are essentially magma filled fractures within the earth’s crust often formed by the pressure imparted by the intruding magma. Magnitude of the magma overpressure has been traditionally determined utilizing elastic properties of the host rock and the complete dimension i.e., full length and maximum width of the fractures. Full exposures of dykes (from tip to tip) are rare, however, as most of the dyke bodies encountered in the field are subject to erosion or disruption along its length as a result of geological time, making estimation of aspect ratios challenging.We propose a new method of estimating total length and maximum width of dykes from their partial outcrops featuring at least one exposed tip. Taking into account the fact that majority of dykes form as dominantly opening mode fractures with an elliptical shape of opening, the method involves solving the equation of this ellipse using every conceivable combination of a pair of ground points recorded on the dyke margin considering the visible tip as the origin. Validity of the method has been checked using published data obtained from incomplete dyke outcrops exposed in the caldera walls of Miyake-jima volcano in Japan. The calculated estimates are in line with the results acquired through a previous published method. The present method has been effectively utilized to calculate the aspect ratios of partially exposed mafic dykes emplaced within the younger granite of the Chitradurga Schist Belt in the western Dharwar craton of peninsular India. We discuss the ranges of their magma overpressure and depths of origin as well as the stress intensity factors associated with the host granite.