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_Journal of Structural Geology 32 (2010) 1417-1429_ _Patterns of microfracturing are investigated in plagioclase and K-feldspar porphyroclasts formed within granitic mylonites along the boundary of the Eastern Ghats Mobile Belt, India. The mineral assemblage comprising quartz, feldspar, biotite and hornblende suggests lower greenschist facies conditions during mylonitisation, with the contrasting ductile behaviour of quartz and brittle fracturing of feldspars restricting the temperature range during deformation to 300-350°C. Microfracturing of feldspars takes place by concentration of pure shear within the feldspar-rich layers. This may reflect strain partitioning into dominantly pure and simple shear due to the competency contrasts between the two major constituent minerals (quartz and feldspar). The microfractures occur in conjugates (here designated T1 and T2) with T1 inclined in the same direction as the S-fabric and showing an antithetic sense to the NW verging shear, while T2 is inclined in an opposite sense to the S-fabric and displays synthetic shear. The direction of maximum compression occurs at high angles to the C-fabric, and the T1 and T2 fractures are the result of pure shear localized into brittle layers within the mylonite. With progressive shear, the fractures along with their host feldspar grains are rotated. Theoretical graphs are plotted between bulk shear (g) and the angle of initiation (a) of T1 and T2 with respect to C-planes, for fractures hosted in a circular or elliptical object. The kinematics of these fractures are also analyzed with regard to variations in shear strain and sense of shear along them. The sense of shear may vary or remain stable within fractures, depending on their initial angle of inclination with respect to the C-fabric. As T1 is inclined at low angles to the XY plane and in the same direction as the S-fabric, it undergoes maximum shear strain compared to T2 and may even exceed the bulk shear. This facilitates breakdown of feldspar porphyroclasts during progressive deformation. The T1 set maintains an antithetic sense of shear even at low angles with the C-fabric, while T2 displays a synthetic sense in spite of being at high angles to C. The T1 and T2 fractures therefore differ significantly from the classic Riedel, P and Y shears where low angle fractures display synthetic shear and high angle ones (R0) show antithetic displacement with respect to the bulk shear sense._ _1. Introduction_ _It is well established that layered anisotropic rocks may provide strong rheological contrasts which produce coeval brittle and ductile structures during shearing (e.g., Druguet et al., 2009 and references therein). Within granitic rocks, localized high strain zones comprising quartz-rich and feldspar-rich layers may develop even at an early stage of deformation. Park et al. (2006) suggest a dramatic weakening of the bulk rock strength due to the development of such layers, as well as by the breakdown of feldspars into mica. Constituent minerals within shear zones also behave differently during deformation depending on their rheological properties. While some minerals may deform in a ductile manner, others deform as rigid bodies, thus developing a variable microfabric. Quartz and feldspar (plagioclase and K-feldspar) that constitute essential minerals in granite differ significantly in their rheological properties. At low temperature (about 300°C) and at low strain quartz undergoes ductile deformation, whilst feldspar, even at very high strains, can persist as a brittle mineral. As a consequence, they produce markedly different types of fabric (Goodwin and Tikoff, 2002). Quartz is flattened into lensoidal shapes followed by complete recrystallization producing a grain shape fabric (GSF) (Hirth and Tullis, 1992). Conversely, feldspar exhibits microfracturing and mechanical disintegration with only limited dynamic recrystallization. The quartz GSF is considered so unique that it is utilized as a shear sense indicator (Lister and Snoke, 1984)._ _Various types and scales of fractures associated with displacement zones have been described in the literature including R and R0 Riedel shears, and P and Y shears etc. (see Passchier and Trouw, 2005 for a review). In this paper we report a system of shear fractures (designated T1 and T2) that vary in both orientation as well as in sense of shear from those noted above. These are also different from the T-fractures of Petit (1987) that are extensional but lack shear displacement (see Passchier and Trouw, 2005, p. 157). The T1 and T2 shear fractures that we describe play an important role in breaking down feldspar porphyroclasts within mylonites. As feldspars rotate with increasing strain, the fractures also rotate passively within them. Slip along these fractures results in strain concentration leading to mechanical disintegration and dynamic recrystallization. They further control the shape of the porphyroclasts and overall fabric of the mylonite._ _Although an extensive literature exists on quartz shape and crystallographic fabrics in a simple shear environment, there is much less published material on fabrics produced by the microfracturing of feldspar grains. It is generally believed that microfractures oriented at low angles to the C-foliation will display shear which is synthetic to the bulk sense of shear (Simpson and Schmid, 1983; Ghosh, 1993). However, the present study, both theoretically and from thin section observations, shows that alternative fracture orientations and kinematics are possible during bulk simple shear. This work also emphasizes how these kinematically controlled microfractures help determine the overall fabric asymmetry of mylonites._ _2. Regional geological setting_ _The investigated mylonites formed in the Terrane Boundary Shear Zone (TBSZ) that marks the tectonic contact between the Proterozoic Eastern Ghats Mobile Belt (EGMB) and Archean Peninsular Craton (Biswal and Sinha, 2003, 2004) (Fig. 1). The EGMB comprises supracrustals such as khondalites, charnockitic gneisses, calc granulites, quartzites and banded iron formations, together with some mantle derived intrusive suites. It is characterized by polyphase folding with early generations of folds displaying coaxial folding along an NEeSW axis leading to the overall northwesterly trend of the mobile belt (Biswal et al., 1998). In contrast, the Peninsular Craton consists of tonalite trondhjemite gneisses that have been extensively intruded by late tectonic granite. This late granite lacks any deformational fabric and was emplaced at 2.5 Ga (Sarkar et al., 1993). The TBSZ forms a 2 km wide arcuate shear zone separating these two terranes with a distinct syntaxial bend separating EeW strikes in the north from NNEeSSW trends further west (Fig. 1, inset). In addition, the sense of shear also varies along strike from top-to-the NW thrusting in the west to dextral strike slip in the north._ _The EGMB initiated in the Archean and continued to develop into the Upper Proterozoic, although the most important event in its evolution is the Mesoproterozoic Eastern Ghats Orogeny at 1.6 Ga (Ramakrishnan et al., 1998; Mezger and Cosca, 1999). Alkaline plutons that are emplaced near the terrane margin at 517 Ma (Biswal et al., 2007), display a magmatic fabric consistent with the shear fabric present in the TBSZ, and are therefore considered coeval with thrusting and related mylonitization._ _The study area lies in the NW part of the TBSZ, where the moderately inclined shear zone thrusts the EGMB over the Bastar craton by 4 km. The craton in the study area is dominated by late tectonic granite composed of quartz, potassium feldspar, plagioclase and biotite. The rock is coarse and equigranular with occasional graphic intergrowths. There is no sign of deformation in the granite except for mylonitic foliation and stretching lineation._ Ключевые слова: quartz grain, feldspar grain, fracture plotted, parallel fracturing, biermeier, shear strain, sinha, ghats, twin plane, temperature range, biswal, eastern ghats, angle fabric, lowest strain, c-fabric, elliptical grain, shear, gondwana, ha, microfractures, aspect, bulk sense, subsequent evolution, study area, respect, journal structural, granite, volume, dynamic recrystallization, brittle layer, circular object, feldspar, path, elliptical object, differ signicantly, curved dashed, grain size, initial, journal, initial angle, petit, shear zone, inclined, mica, quartz, high angle, mylonites, antithetic sense, simple shear, orientation, shear plane, simple, graph, tbsz, wall, change, rigid, fabric, individual clast, rigid body, analysis, grain, extreme diminution, geological, slope, study, ductile deformation, feldspar porphyroclasts, ghosh, separate, rotation, synthetic sense, mobile, angle cleavage, parallel, india, strain partitioning, fracture wall, antithetic, bastar craton, ductile, brittle, mylonite zone, mylonitic foliation, bulk, ultramylonite zone, circular, fracture hosted, calculated, set maintains, microfractures occur, rheological property, angle, protomylonite mylonite, quartz ribbon, development, mobile belt, recrystallized, protomylonite displaying, display, matrix ratio, progressive shear, external source, structural, pure shear, sinha journal, park, clast, shear fracture, deformation, mylonite, brittle fracturing, ultramylonite, geology, plagioclase, feldspar clast, bulk shear, mineral, antithetic displacement, elliptical, strain analysis, sense shear, ghats mobile, progressive deformation, rigid object, flinn, elsevier, angle measured, riedel shear, aspect ratio, rotation path, ramsay, conjugate set, structural geology, inclination, zone, twin lamella, gf, fractures, progressive, sample, shear sense, shear wall, form, synthetic, society, object, belt, classic riedel, craton, statistical best-t, protomylonite zone, volume gain, davis, graph figs, traverse, nadai, strain, sense, angle fracture, nite strain, acute angle, ductile structure, fracture originating, protomylonite, equation, modal percentage, circular grain, microfractures cut, layer, direction, alsop, recrystallization, photomicrographs, matrix, geological society, undeformed granite, ratio, twin, shear direction, plane, porphyroclasts, mylonite sample, angle c-fabric, set, high, jeffrey, journal structural geology, fracture, feldspar-rich layer, lisle biermeier, observed, longer dimension, showing, ductile matrix, s-fabric, rock, eastern, elliptical geometry, increasing strain, press, sarkar, passive marker