Deformation history of the NW salient of the Eastern Ghats Mobile Belt, India

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Abstract

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.

Introduction

Fold-thrust belts (FTB) are characteristic features of convergent mobile belts. Though they are commonly reported from Phanerozoic mobile belts like the Himalayas (Valdiya, 1984), Alps (Dewey et al., 1973), Zagros (Bird, 1978) and Taiwan fold belts (Davis et al., 1983), they have also been observed in Proterozoic mobile belts like Grenville (Davidson, 2001), Broken Hill (White et al., 1995) and Churchill province (Gibb, 1978). The FTBs consist of stack of enechelonly arranged nappes showing vergence towards the foreland. The frontal thrust marks the sole thrust or decollement that forms the leading imbricate structure with the splay thrusts behind (Ramsay and Huber, 1987). The thrusts are normally listric in nature and adopt a ramp-flat geometry while cutting across different rock types in the stratigraphic section. The hanging wall rocks produce fault-bend folds over such ramp-flat parts while gliding over the thrust planes. This results in thickening and a large amount of shortening of the thrust wedge leading to the development of parallel folds and axial planar cleavage. Hence the thrust structure guides the structural style of the overriding block (Fig. 1(a)) (McClay and Coward, 1981, Suppe, 1983). However, the basement remains undeformed. Contrary to this, another model postulates that thrusting takes place much after folding and metamorphism of the cover rocks and the thrusts cut across the preexisting structures of the nappes. Moreover, the basement is deformed along with the cover (Fig. 1(b)) (Fischer and Coward, 1982, Ramsay, 1997). The FTBs on the map display a curvilinear geometry because of the salient and recess structures (Marshak and Tabor, 1989, Macedo and Marshak, 1999, Spraggins and Dunne, 2002).

In this paper we describe the FTB structure from the northwestern margin of the Proterozoic Eastern Ghats Mobile Belt of India (Fig. 2). The sole thrust is exposed at the surface as a well defined ductile shear zone that demarcates the terrane margin between a low grade cratonic foreland and a highly deformed granulitic mobile belt. A prominent salient structure is observed on the northwestern part of the mobile belt, which has been investigated in this paper with regard to the structural geology, and a model has been proposed to explain its origin.

Section snippets

Regional geological setting

The Eastern Ghats Mobile Belt (EGMB, Fig. 2 inset) of the east coast of India represents a regional granulite belt belonging to a wide spread of ages from Archaean to Upper Proterozoic (Sarkar and Paul, 1998 for review; Rickers et al., 2001). Continental collision between India and Antarctica during Mesoproterozoic time, however, marks the most significant event in the evolution of the belt. The belt comprises a host of granulitic supracrustals such as khondalites (garnet–sillimanite–graphite

Geological setting of the craton in the foreland

Fig. 5(a) and (b), respectively, show the False Colour Composite and generalized geological map of the NW salient of the FTB (Fig. 5(b) is to be compared with Fig. 3 for detailed geology). The Bastar and Singhbhum cratons lie to the west and north, respectively. The salient has a flat convex outline with broad apex and nearly orthogonal end points (EP in Fig. 5(b)). As many authors have dealt with the geology of the cratons extensively (Ramakrishnan, 1990 and references therein), the

Structure of the TBSZ, the decollement

The deformation structures of the TBSZ have been described in reference to apex and end points of the salient (Fig. 3, Fig. 6).

The TBSZ in the apex part shows a thrust character. It has a width of nearly 2 km and shows linear geometry with NNE–SSW strike. The dip varies from 45 to 60° towards SE (Fig. 6(b)). In an overall listric geometry of the decollement, the above high dip corresponds to the frontal ramp of the decollement. The thrust is marked by quartzofeldspathic mylonites, showing a

Structure of the Lathore nappe

Immediately overlying the decollement, the Lathore nappe occurs as a huge tectonic slab occupying 80% of the area of the salient (Fig. 4). The charnockitic gneiss is the main rock type in the nappe (Fig. 3) followed by basic granulites, khondalites (near Gandhmardan Hill) and calc granulites which carry the signature of multiple generations of folding. The folds include coaxial F1 and F2 that are developed along NNE–SSW axes. These folds are classified as parallel to highly flattened parallel

Structure of the Turekela thrust

The Turekela thrust underlies the Turekela nappe (Fig. 4). The thrust in the western margin dips gently to SE and shows NW vergence. The Turekela nappe in the study area is extremely eroded and has been left out as a klippe. Similar to Turekela klippe there could be few other klippes in the area tectonically resting over the Lathore nappe. These are yet to be mapped. The Turekela thrust is very conspicuous at Dholmandal where the khondalites of the Turekela klippe are extremely sheared and show

Structure of the Turekela nappe

The Turekela nappe represents the top most nappe in the NW salient. It occurs as a klippe surrounded by the rock of the Lathore nappe (Fig. 3). The root zone of the klippe lies further towards east. The klippe is dominated by khondalites followed by calc granulites. Mineralogically khondalites consist of sillimanite, graphite, feldspar, garnet and quartz. Leptynite veins derived from the melting of the above assemblage occur concordantly in the khondalites. Turekela nappe exhibits multiple

Mechanism of formation of the salient

The formation of the salient is to be discussed in light of the FTB structure of the EGMB. It has been mentioned earlier that the thrusting took place subsequent to folding and granulite metamorphism which happened during Eastern Ghats orogeny in Mesoproterozoic period (Ramakrishnan et al., 1998). Thrusting juxtaposed the mobile belt against the craton forming FTB on the NW front, subsequent to the orogeny. The TBSZ acted as the decollement for the Eastern Ghats rocks which have been split up

Conclusion

The NW salient of the EGMB is characterized by episodic deformational history largely belonging to two distinct events. While the earlier phase is marked by polyphase folding (F1, F2 and F3) and synkinematic granulite metamorphism the later includes thrusting, thrust related folding and retrogression of granulites into amphibolites. The folds noticed in both the nappes, as part of the earlier phase of deformation, are attributed to a buckling origin which is obviously the result of

Acknowledgements

The authors are thankful to DST and CSIR for sponsoring the research work. Review by the anonymous referees helped in improving the early version of the manuscript. Personal communication with Profs J.G. Ramsay, B. Tikoff, J. Suppe, A. Kronenberg and C. Merguerian while revising the manuscript is gratefully acknowledged. Discussion with Profs B.P. Kashyap and K. Narasimhan of Dep. Metallurgy, IIT Bombay helped greatly in revision. Miss Arpita Mandal helped during preparation of the manuscript.

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