Cratons and Fold Belts of India

Orogenic (Gr. Oros means mountain and genic means birth) belts or orogens are some of the most prominent tectonic features of continents. These terms are, however, not synonymous to Mountain belt which is a geographic term referring to areas of high and rugged topography. Surely, mountain belts are also orogenic belts but not all orogenic belts are mountains. In this book, the orogenic belts, also called mobile belts, are termed fold belts because they are made up of rocks that show large-scale folds, and faults/thrusts and metamorphism with evidence of melting or high mobility in the core region during orogenesis. These belts are characteristically formed of (a) thick sequences of shallow water sandstones, limestones and shales deposited on continental crust and (b) deep-water trubidites and pelagic sediments, commonly with volcanoclastic sediments and volcanic rocks. Typical mobile belts, rather fold belts as titled here, have rocks that were deformed and metamorphosed to varying degrees and intruded by plutonic bodies of granitic compositions. Some fold belts are also characterized by extensive thrust faulting and by movements along large transcurrent fault zones. Even extensional deformation may be found in such belts. Most belts show a linear central region of thick multiply deformed and metamorphosed rocks bordered by continental margins, but some belts are also having oceanic margin on one side.

With the basic concept of plate tectonics and mountain building processes discussed in Chap. 1, we now attempt to understand the evolution of the fold belts or mobile belts or orogenic belts in light of the plate tectonics theory. We believe that orogenic belts, being fold-thrust belts, formed at convergent margins where the adjacent plates had moved toward each other and collided. These margins are called subduction zones and are classified as either oceanic or sub-continental, depending upon whether the crust of the overriding plate above the subduction zone is oceanic or continental. Oceanic subduction zones are marked by a trench and by an arcuate chain of volcanoes on the overriding plate, called island arcs or ocean island arcs. Subcontinental convergent margins are also marked by an oceanic trench and by an arcuate chain of volcanoes that are built on the continental crust. These margins are called continental arcs or Andean-type continental margins. The Andes is the simple orogenic belt that formed when oceanic lithosphere (Nazca plate) subducted eastward under a continental margin of South America.

The Aravalli mountain of Rajasthan, western India, has NE-SW orographic trend and runs for over 700 km through the States of Gujarat, Rajasthan, Haryana and Delhi. This belt is made up of Proterozoic supracrustal rocks of the Aravalli and Delhi Supergroups, which are distinct from each other in their deformational and metamorphic history. These Proterozoic supracrustals were deposited on the Archaean basement gneiss complex, called Banded Gneissic Complex (BGC) which is predominantly a polymetamorphosed, multideformed rock-suite of tonalite-trondhjemite gneiss, amphibolite, migmatite and granitoid, most of which are Archaean (3.4–2.6 Ga old) (see Chap. 3, Sect. 3.5).

The fold belts of the central Indian region are sandwiched between the Rajasthan-Bundelkhand craton in the north and the Bastar craton in the south. The age of 3.3 Ga of the tonalite-trondhjemite-granodiorite (TTG) rocks in these adjacent cratons of Bastar and Rajasthan-Bundelkhand (Bandyopadhyay et al., 1990) suggests that these continental nuclei once united to form a mosaic in northern Indian peninsula, as stated in Chap. 2. In the present geological set up the location of the central Indian fold belts is shown in Fig. 5.1 (overview in Fig. 5.1b). Of these four fold belts, namely the Mahakoshal, Satpura, Sakoli and Dongargarh, the former two are located in the ENE-WSW trending Central Indian Tectonic Zone (CITZ) lying between the Bundelkhand and Bastar cratons, whereas the latter two belts are located within Bastar craton and hence south of the Central Indian Shear zone (CIS) regarded as the southern limit of the CITZ.

The Singhbhum fold belt (SFB) or Singhbhum Mobile belt, also called North Singhbhum belt (Saha, 1994) in eastern India is sandwiched between the Singhbhum craton in the south and Chhotanagpur Granite-Gneiss terrain (CGGC) in the north (Fig. 6.1). Although slightly curvilinear, the fold belt has nearly E-W strike like that of the Satpura fold belt (Chap. 5) and is believed to have evolved in the Proterozoic Satpura orogeny (Sarkar and Saha, 1962). The Singhbhum fold belt is made up of metasedimentary and metavolcanic rocks categorized in three domains. Gupta and Basu (2000) of Geological Survey of India gave the following lithostratigraphy of the Singhbhum fold belt.

The Eastern Ghats Mobile Belt (EGMB) derives its name from the mountain of Eastern Ghats in the east coast of India. Apart from reconnaissance survey by individual geologists (C.S. Middlemiss, T.L. Walker, L.L. Fermor, V. Ball), the Geological Survey of India provided a geological map of the EGMB on 1:50,000 scale in later part of the 20th century and also gave periodical reviews in its Reports (see Ramakrishnan and Vaidyanadhan, 2008). Recently, Ramakrishnan et al. (1998) gave a new geological map (1: 1000,000) with a summary of regional geology of the EGMB.

Pandyan Mobile Belt (PMB) is the name given by Ramakrishnan (1993, 1988) to the Southern Granulite Terrain (SGT) situated to the south of the E-W trending Palghat-Cauvery Shear Zone (PCSZ) (Fig. 8.1). The name Pandyan is adopted after the legendary dynasty that ruled this part of South India in the historical past. Interestingly, the SGT has been defined variously by different workers. According to Fermor (1936), this terrain is a part of the large “Charnockite Province” located to the south of the orthopyroxene-in (Opx-in) isograd, delineated along a line straddling the join Mangalore-Mysore-Bangalore-Chennai (Pichamuthu, 1965).