Synchronous Development of Mylonite and Pseudotachylyte in Ductile Shear Zone: An Example from the Chitradurga Eastern Margin Shear Zone, Karnataka
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The eastern margin of the Chitradurga schist belt is marked by a NNW-SSE trending sub vertical crustal scale ductile shear zone. The kinematic indicators indicate a predominant sinistral sense of strike-lip movement along the shear zone. Syntectonically emplaced granitic rocks are converted to mylonites and ultramylonites as a result of crystalplastic deformation in the shear zone. In contrast, there are localized zones of brittle failure with attendant functional heat generation exemplified by the development of thin but conspicuous bands and veins of pseudotachylytes, which are emplaced either subparallel with or transgressing the C-planes of the mylonites. From our field and petrographic studies it is interpreted that these two coexisting rock types, namely the mylonite and pseudotachylyte, which are the results of contrasting deformational mechanisms, have generated near synchronously in a progressively developed ductile shear zone. The pseudotachylytes represent the brief interlude of sudden increase in strain rate in an overall ductile regime.Keywords:
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Abstract Field and microstructural observations from the Proterozoic Grizzly Creek Shear Zone suggest that crustal-scale fabric anisotropy exerted a significant control on earthquake rupture propagation during deformation at mid-crustal depths. The shear zone developed in amphibolite-facies supracrustal gneisses and granitoids, and consists of a 0.4–0.7 km-wide zone of high-strain rocks with foliation transposed to 256°/51°NW and top-to-the-south kinematics. The shear zone is overprinted by hundreds of veins of pseudotachylyte, mylonitic pseudotachylyte and ultramylonite. Field observations and whole-rock geochemical data suggest that pseudotachylyte fault veins formed as a result of first-generation rupture through intact rock. Pseudotachylytes are preferentially localized in as many as nine decametre-scale rupture zones dispersed across the width of the shear zone, concordant to foliation. We present a conceptual model for the asymmetric development of anisotropic fabric in a thrust-related fault zone in crystalline metamorphic rocks. Progressive tectonic exhumation of hanging wall rocks during thrusting results in the development of a crustal-scale anisotropic fabric that provides a preferentially weakened zone that could accommodate the propagation of earthquake ruptures from the seismogenic zone into the middle crust.
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The North Tea Lake Mylonite Zone is a late extensional ductile fault that is concordant with and has reworked fabrics of the North Tea Lake Shear Zone, the frontal thrust shear zone of the upper amphibolite–granulite facies Kiosk domain within the interior of the Central Gneiss Belt. North Tea Lake Mylonite Zone fabric is an anomalously fine-grained mylonite compared to Central Gneiss Belt gneisses, and consists of three microstructural domains that display progressive recrystallization and grain size refinement of the protolith granitoid. On the basis of petrography and electron backscatter diffraction, these microdomains are inferred to represent a transition from dominantly dislocation creep to diffusion creep and diffusion-accommodated grain boundary sliding at elevated stress (>100 MPa), low fluid activity, and temperatures ∼500 °C. The North Tea Lake Mylonite Zone is interpreted to mark a step in the progressive transition in deformation mode during late- to post-Ottawan extension and cooling of the Grenville orogen from weak, wide, wet, and warm shear zones to Rigolet-phase cooler, narrow, ultrafine, high-stress shear zones reworking dry protoliths.
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Abstract The ductile shearing along the Shangdan suture zone during the Paleozoic time is a key to understand the collisional deformation and tectonic regime of amalgamation between the North China Block and the South China Blocks. The Wushan ductile shear zone, a branch of the Shangdan suture, records mylonitic deformation that affected granitic and felsic rocks outcropping in an over 1 km wide belt in the western Qinling Orogenic belt. Shear sense indicators and kinematic vorticity number (0.79–0.99) of the mylonites reveal a dextral shear deformation. The quartz c-axis fabrics indicate activation of combined basal 〈a〉 and rhomb 〈a〉 slip, prism 〈a〉 slip and prism 〈c〉 slip. The dynamic recrystallization of quartz is accommodated by combined subgrain rotation and grain boundary migration. These characteristics suggest that the mylonites experienced ductile shear deformation under amphibolite facies conditions at temperatures of ∼500–650 °C. Zircons from granitic mylonite yield a U-Pb age of 910 ± 4.8 Ma, which represents the formation age of the protolith of the mylonite. The ductile shear zone was intruded by a granitic dyke, which yields a zircon U-Pb age of 403 ± 3.5 Ma constraining the minimum age of the ductile shear deformation. Together with regional geology and available geochronological data, these structural characteristics and ages indicate that the Wushan ductile shear zone was formed by dextral shearing following the N-S shortening as a result of collision between the North China and South China blocks along the Shangdan suture.
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SUMMARY regional-scale ductile-brittle shear zone along the southern margin of the Lynn Lake greenstone belt. Preliminary structural analysis has identified six generations of structural fabrics produced by discrete deformational events. The Johnson Shear Zone is a D2 ductile shear zone characterized by the development of dextral transcurrent shear fabrics and generally steep stretching lineations. The development of narrow zones of shallowly plunging stretching lineations in the core of the shear zone reflects kinematics consistent with shear zone development in response to dextral transpression. The overprinting of dextral D2 fabrics by dextral D3 fabrics results in the limited reactivation of the Johnson Shear Zone. Brittle D5 sinistral shear zone parallel deformation represents the final movement event. Gold mineralization associated with the Johnson Shear Zone is characterized by the intense silicification of Wasekwan Group metabasalt and metasedimentary rocks and the introduction of quartz-carbonate-pyrite veinlets during the main D2 shearing event. Subsequent deformation has not resulted in the significant remobilization of gold mineralization. The intrusion of felsic dykes during D2 shearing may represent an important factor in the silicification process.
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