Fold-Thrust-Belt Structure of the Proterozoic Eastern Ghats Mobile Belt: A Proposed Correlation Between India and Antarctica in Gondwana (original) (raw)
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Gondwana Research, 2002
Thrusts interspersed with lateral ramp and wrench structures mark the join between Proterozoic Eastern Ghats Mobile Belt and Archaean cratons of the eastern India. The join, referred to as Terrane Boundary Shear Zone, exhibits an arcuate geometry at the NW margin of the Eastern Ghats where the mobile belt shows the presence of northwesterly verging nappes. Based on this, the area has been described to represent the salient part of a fold thrust belt. Juxtaposition of the granulites over the low grade craton is largely attributed to thrusting along the terrane boundary shear zone. As the folds are disoriented and the granulites are retrograded along the thrusts, the terrane boundary shear zone is interpreted to be a retrograde shear zone. The shear zone hosts a number of synkinematic alkali intrusive rocks that constrain the age of thrusting to 1.4 Ga. In the correlative framework of Gondwanaland the terrane boundary shear zone is very likely to be contiguous with the Rayner-Napier boundary of Antarctica.
Deep crustal Shear Zones in the Eastern Ghats Mobile Belt, India: Gondwana correlations
Shear zones are significant in understanding the physicochemical processes, such as mineralization, magmatism, metamorphism, and deformational patterns and bear remarkable influence on many aspects of earth system science. We present here the structural framework of the Eastern Ghats Mobile Belt (EGMB) that constitutes a net work of deep crustal shear zones. It was possible to identify and recognize these shear zones by involving different spatial data sets, field observations along regional traverses and detailed structural analysis in selected critical areas and outcrop mapping and in conjunction with the available published geological maps. We review and present here field description, distribution, geometry and the kinematics of different shear zones of the EGMB and emphasize their significance in terms of tectonic models and Gondwana correlations. The shear zones at the cratonic margins such as the Sileru shear zone, Koraput-Rairakhol shear zone and the Northern boundary shear zone represent a long and continuous suture zone extending for over a few hundreds of kilometers and upto lithospheric depths. All the shear zones witnessed multiple events of magmatism, metamorphism and deformation. The stretching lineations show different orientations in different segments and divide the EGMB into several distinct structural domains. All the shear zones are found to be transpressive in character and subjected to repeated reactivation in space and time. Oblique collision and long lived transpressional tectonic regime during Gondwana amalgamation seem to be responsible for the present structural architecture of the EGMB. The shear zones are the controlling factors for any mineralization, igneous activity, migmatization and retrogression and that they represent 'key laboratories' for understanding the geological processes and ultimately the earth's history. There is a strong need for detailed field based studies involving modern mapping techniques with current tectonic perspectives before we attempt any advanced laboratory based measurements for useful and meaningful interpretations.
Deformation history of the NW salient of the Eastern Ghats Mobile Belt, India
Journal of Asian Earth Sciences, 2003
The Eastern Ghats Mobile Belt of India displays fold-thrust belt structure with a prominent salient on its NW margin. The salient consists of Lathore nappe and Turekela klippe that have overthrust the craton to NW. The rocks of the nappes have undergone granulite facies metamorphism and multiple phases of folding corresponding to the Eastern Ghats orogeny, prior to thrusting. As a result the granulites along the thrust are retrograded to amphibolites and the axial traces of the folds have been truncated against the thrust plane. The basal decollement of the fold-thrust belt is represented by the Terrane Boundary Shear Zone that defines the tectonic margin between the craton and the mobile belt. It occurs as a ductile thrust affecting the cratonic basement as well as the mobile belt suggesting that the basement did not behave as a rigid body during thrusting. Thus the study area is comparable with the Caledonide fold-thrust belt. Further, two large lateral ramps namely Khariar and Paikamal lateral ramps have been developed on the decollement at the lithological contact between tonalite–trondhjemite gneisses/granite gneisses and the latetectonic potassic granites of the craton. Fault-bend folds associate with these ramps too. It is suggested that the salient structure of the fold-thrust belt is the combined result of (1) lateral ramps on the decollement and (2) differential displacement along the sole thrust due to lateral variation in detachment strength.
The granulite terrain of South India consists of three tectonically distinct terranes separated by ductile thrusts. The central terrane namely the Salem-Namakkal Fold Thrust Belt which forms the topics of the present paper is sandwiched between the Northern Archaean Granulite Terrane and southern Neoproterozoic Pandyan Mobile Belt. The Salem-Namakkal Fold Thrust Belt consists of a stack of imbricate thrust sheets showing N to NE vergence. The charnockitic mylonites produce an age of 2530±39 Ma. The imprint of Pan-African event is missing in the charnockitic mylonites. However, the shonkinite dyke emplaced in the ultramafic plutons has produced 801 Ma age. The southernmost Namakkal thrust sheet show isothermal decompression while the northernmost Salem thrust sheet show isobaric cooling history. The granulites are retrograded during shearing. In the Gondwanaland assembly, the Salem-Namakkal Fold Thrust Belt forms a contiguous terrane with the Antananarivo -Tsaratanana Belt of Madagas...
Along its northwestern margin, the Meso-/Neo-proterozoic granulites of the Eastern Ghats Belt are juxtaposed against the Palaeoproterozoic/ Archaean lithologies of the Bastar craton along a crustal ductile scale shear zone. In the shear zone hosted garnet-bearing mafic granulites of the Eastern Ghats Belt, garnet porphyroblasts pre-tectonic with respect to the shearing are mantled by coronal orthopyroxene + An-rich plagioclase vermicular intergrowths. The intergrowths typically overgrow the shear zone fabric defined by the shape preferred aggregates of hornblende, orthopyroxeneclinopyroxene in a recrystallized mosaic of plagioclase. Schreinmakers analysis of reaction equilibria in the NCMASH system suggests that the post-shearing orthopyroxene + plagioclase (An-richer) symplectites formed at the expense of hornblende–garnet–An-poorer matrix plagioclase assemblage. The model reaction has high positive dP/dT slope with orthopyroxene + plagioclase stable in the high temperature side of the reaction. The textural relations are inferred to indicate prograde heating Tmax ∼750 ◦C, Pmax ∼7 kbar post-dating shearing. The gamut of available age data (K–Ar, Rb–Sr, Pb–Pb in zircon, and U–Pb–Th CHIME date in monazites) indicate that the youngest thermal event in and neighboring the shear zone cluster at ∼500–550 Ma, regardless of the vestigal Precambrian dates in rocks from across the shear zone. Since the closure temperature of thermally-driven Pb diffusion in minerals like monazite and zircon is ≥650–700 ◦C, the Early Palaeozoic age is likely to correspond with the ductile shearing and granulite facies prograde heating at T value higher than the blocking temperature of Pb. Since the shearing is correlated with top-to-the-west thrusting of the Eastern Ghats Belt over the cratonic foreland, the Early Palaeozoic prograde granulite facies metamorphism may relate to suturing of the two disparately evolved crustal blocks. The prograde heating may have been caused by burial of the Eastern Ghats granulite facies gneisses neighboring the shear zone due to emplacement of granulites from the hinterland along west-vergent thrust imbrications.