Structural Geometry of Mobile Belts of the Indian Subcontinent

The South Delhi orogeny is constrained by a correlative study between the deformational fabric and geochronology of the metarhyolite and granite of the South Delhi terrane (SDT) around Beawar- Rupnagar-Babra, Rajasthan, NW India. The SDT contains metaconglomerate (Bar conglomerate), calcareous schist, mica schist, amphibolite, metarhyolite (G₁) and three phases of granites (G_2-4), which underwent three stages of deformations (D_1-3). The D₁ produced F₁ reclined/recumbent folds and S₁ fabric in a greenschist facies metamorphic conditions. The D₂ produced F₂ folds which were coaxial with F₁ along NE-SW axis, S₂ crenulations and ductile shear zones. The D₃ produced NW-SE trending F₃ folds. The S₁ fabric in the metarhyolite (G₁) and, G₂ and G₃ granites differs from that of the G₄ granite. It shows low temperature of formation in the former (G_1-3), characterized by recrystallization of quartz by bulging and development of biotite, muscovite, epidote. In the later (G₄), it is characterized by high temperature of formation as indicated by dynamic recrystallization of the plagioclase by subgrain rotation and grain boundary migration. Based on these differences, we interpreted that the intrusion of the G₄ is syntectonic with the D₁ deformation while the metaryolite (G₁), G₂ and G₃ granites were pretectonic to the D₁. The G₄ (Sewariya granite) was dated previously at 860 Ma that constrains the age D₁ deformation and South Delhi orogeny. The G₁, G₂ and G₃ (Sendra granite equivalent) were earlier dated at ca. 970 Ma which probably indicates age of rifting.

Sediments of the Aravalli Supergroup have acquired a complex superposed fold pattern during Neoproterozoic amalgamation (Grenvillian Orogeny) of north Indian blocks (Marwar block, Bundelkhand craton, North Delhi crustal block) and collision with the Central Indian Craton. The Upper Aravalli metasediments reveal a prograde mono-metamorphic history with continuous increase in metamorphic conditions towards the Delhi Supergroup contact. The transition from phyllites to garnet-bearing mica schist is accompanied by strain increase seen in tightening of F₂ folds. Microstructures and deformation mechanism in quartz document an increasing temperature gradient from ~280 to 500 °C towards the Aravalli-Delhi contact. Garnets, grown prior to the contact-parallel, pervasive second cleavage indicate crystallisation along a prograde P-T-path from 400 °C/4 kbar to 500 °C/7 kbar. Older metamorphic records in the northern Aravalli-Delhi mobile belt sector (e.g. Sandmata Complex, Mangalwar Complex, and North Delhi Terrane) cannot be related with deformation and metamorphism in the Aravalli Supergroup. Considering the pattern of bivergent thrusts with the Neoproterozoic active continental margin along the western side of the Delhi Fold Belt, the lack of any magmatic signature and the inferred contractional fold and thrust deformation along the eastern proximity can be interpreted as a retro-wedge setting.

Detailed structural mapping carried out in different parts of the Central Indian Tectonic Zone (CITZ) combined with an analysis of spatio-temporal evolution of different domains have shown evolution in three tectono-thermal events. The felsic gneisses, referred as Tirodi Gneiss, belong to four generations. The oldest gneissic unit, Tirodi Gneiss—I (>2450 Ma), has development of a paleosol horizon at the top, which was involved in anoxic weathering, and forms the basement of the Sausar Group. The Sausar Group was deposited during the period ca. 2400–2250 Ma and contains glaciogenic sediments of Paleoproterozoic “snowball earth” event and ore-grade manganese deposits. The first deformation marked by the development of isoclinal folds with axial planar cleavage and coeval granulite facies metamorphism of the Sausar Group took place at ~2100 Ma, when grey gneisses and migmatites, Tirodi Gneiss—II, were developed. The upright to steeply inclined isoclinal folds in the Sausar Group with regional ~EW axial traces were developed during the second deformation (~1600 Ma), which was also associated with granulite facies metamorphism and development of grey gneisses, Tirodi Gneiss—III, during the second cycle of the Satpura orogeny. The Tirodi Gneiss—IV (~1450 Ma) were developed during the waning phase of the Satpura orogeny. Low intensity deformation in CITZ marked by open folds with ~NS axial traces and thermal event (~950 Ma) were possibly related to the amalgamation of India and Australia to form East Gondwana along ~NS trending Pinjarra Orogen (~1100–900 Ma) on the western margin of Western Australia.

Tectonic development of the Bengal Basin is related to the complex interplay among the Himalaya orogen to the north, the Indo-Burma orogen to the east, and Stable Indian Craton to the west. To the north, convergence tectonic loading presumed to transfer from the deformation front, along the Oldham and Dauki faults to the south and shaped not only the Shillong Plateau and Assam Basin, but also the northern part of the Bengal Basin. To the east, oblique subduction related transpressional tectonics splits along different morphotectonic units separated by large-scale transpressive dextral strike-slip faults and produces fold thrust belt, thrust front, and emerging fold belt, whose western limit is marked by the deformation front to the east. The Stable Shelf part of the basin to the west is characterized by the presence of passive margin rift faults with numerous graben and half-graben structures, which show sign of tectonic reactivation in response to the ongoing N-S collision and E-W subduction of the Indian Plate.

In Andaman subduction complex Indian plate is subducting towards east below Burma plate. In this subduction complex an accretionary prism comprising of Upper Cretaceous ophiolite and Eocene sediments is exposed in Andaman Islands. Map pattern and the field distribution show dismembered ophiolites occurring in different N-S trending thrust slices where the western slices have low dip (8°–10°) and the eastern slices have steep dip (65°–70°) towards east. Besides, few N-S trending back thrust and E-W trending out of sequence thrust have affected the disposition of the litho units. Oligocene-Miocene forearc sediments are exposed on both side of the accretionary prism. Eocene sediments deposited in trench slope basin have very irregular fold geometry which is due to changes to the basin floor topography during its upliftment along with ophiolite. The forearc sediments showing proximal to distal fan facies have regular fold geometry with N-S striking axial plane and very low plunge (18°–25°) either north or south. Based on the field disposition it is suggested that on land emplacement of Andaman ophiolite took place after the deposition of Oligocene-Miocene sediments. The forearc sediment was deformed during on-land emplacement of the accretionary prism. Similarity in petrographic character, age of the ophiolite, occurrence in an accretionary prism and field disposition suggest that both the ophiolite bodies of Andaman and Naga Hills of Indo Burma Ranges (IBR) represent Neotethyan crust. The ophiolites with Neotethyan crust situated in western belt of IBR have also similarity with the ophiolites of Eastern belt of IBR and on-land emplacement of IBR ophiolites took place during India-Asia/Burma plate collision in Late Miocene time.

The Kohat Fold and Thrust Belt (KFTB) lies in the north western margin of Himalayan arc in the Pakistan. It covers a part of outer Himalayan Foreland Basin where 800 m thick Eocene package is predominantly exposed at the surface forming different structures. Current research describes the structural geometry and kinematics of the central KFTB and identifies the role of mechanical heterogeneity in defining the surface structuration as well as its impacts on the deeper horizons. The east west trending, open to tight and overturned folds having curvilinear axes associated with thrust fault system dominates the exposed structural geometry of the area. Three mechanically different stratigraphic packages have been identified in the research area. The thick middle (Late Paleocene to Middle Eocene) clayey and mechanically soft package is sandwiched between upper and lower mechanically competent packages. The upper package comprises of Middle Eocene to Late Miocene rocks while the lower package consists of Paleozoic up to Paleocene rocks. Our structural modelling suggests that the deformation within the exposed upper competent stratigraphic package is disharmonic in structural relation to the lower stratigraphic package, which is comprised of south verging, ramp-related fore-folds. The middle, soft and plastic stratigraphic package is acting as surface of fault-fold decoupling between the lower and upper competent packages. The structural balancing reveals that the area has undergone 42.35% shortening. The mode of deformation is thin-skinned, while the facing of structures show southward propagation of deformation as a result of the Indo-Eurasian plates collision.

The Eastern Ghats Mobile Belt (EGMB) lies to the east of the Archaean Bastar and Dharwar Cratons, and to the southeast of the Singbhum Craton in the Indian shield. Along its western boundary, the EGMB granulites have been thrust westward as a nappe over the Bastar Craton along a mylonitic contact zone. Earlier studies considered the northern boundary of the EGMB with the Singhbhum Craton to be a thrust, although this interface is geometrically parallel to the west-directed transport direction of the granulitic nappe. Detailed geological studies along this northern margin reveal that the c. 1.0 Ga granulites of the EGMB do not share a direct contact with the Archaean granite-greenstone terrane of Singhbhum, but are actually juxtaposed against a Late Archaean (2.8–2.5 Ga) high grade terrane referred to as the Rengali Province. Structural studies reveal that the EGMB-Rengali Province contact has a WNW-ESE strike with sub-vertical dip, with prominent asymmetric markers indicating dextral strike-slip shearing along a horizontal transport vector. Microstructural studies indicate that fabric formation during strike-slip deformation is controlled by plastic deformation of quartz, while other minerals remained passive or deformed in a brittle manner. Electron Back-Scatter Diffraction (EBSD) studies on selected samples from the contact zone indicate that quartz deformed mostly by prism <a>, rhomb <a> and basal <a> slip, with asymmetry indicating dextral simple shearing. The Rengali Province samples show that earlier shortening (pure shear) microstructures and quartz CPO patterns were also sheared dextrally by this later deformation. These results confirm that the northern boundary of the EGMB is not a thrust, but a strike-slip shear zone that operated well after granulite metamorphism, and at lower temperatures characteristic of the greenschist facies. Shortening structures in the Rengali Province and the Singhbhum Craton are related to an older deformation event unrelated to emplacement of the EGMB. Gravity studies across the contact confirm that the shear zone continues vertically to a depth of at least 25 km. Thus, integrated geological and geophysical studies confirm the strike-slip nature of the craton-mobile belt boundary.

Voluminous pseudotachylites occur along NE-SW trending Gangavalli sinistral strike-slip fault in the Southern Granulite Terrane of South India. We made a field, microscope and geochemistry study of the pseudotachylites, and determined the parent rock composition, frictional shear resistance (τ_f) and melt pressure (P_m). Based on the result, we made a preliminary interpretation of source property of the earthquake. The pseudotachylite veins belong to two types, (i) the fault veins were produced by in-situ melting, these were used to compute the τ_f during coseismic slip, and (ii) the injected veins formed due to dilation of the pre-existing weak planes, these have been used to calculate the P_m. The pseudotachylites bear chemical similarity with charnockites in that they show andesite to granite composition in TAS diagram, calc-alkaline trend in AFM plot, possess REE fractionation with LREE enrichment and lack Eu anomaly. Hence, pseudotachylites were derived from melting of the charnockite. Further, the pseudotachylites are dominated by hexagonal β-quartz clasts that suggest the maximum temperature of melting was at 1550 ℃. The fault veins exhibit thickness/displacement ratio varying between 0.03 and 0.1. Assuming that there is no loss of melt from fault veins, maximum shear resistance is estimated at 48.95 MPa, characteristic of large magnitude-earthquake. The injected veins are thicker, show varied geometry and dominantly are aligned in NE-SW direction. The melt pressure P_m > σ₂, stress ratio Φ = 0.87 and driving pressure R′ = 0.9 were calculated from stereoplot and 3D Mohr circle. Higher stress ratio indicates σ₂ ≈ σ₁ that leads to flip-flop of σ₁ from horizontal to vertical. This was probably the result of stress drop during stick-slip mechanism.

Here we present, Anisotropy of Magnetic Susceptibility (AMS) studies on quartzites of Champaner Group, Gujarat, Western India. As quartzites are dominantly present within each formation of the Champaner Group, they have been selected for the study. Our study on AMS signifies two prominent striking planes of magnetic foliation within the rocks. The rocks have (i) ENE-WSW to E-W and (ii) N-S to NE-SW trends. The former trend matches with the regional magnetic foliation of Godhra Granites (GG) and neighboring Banded Gneisses (BG), while the later one does not match with any of the trends resulted due to last phase of deformation within Southern Aravalli Mountain Belt (SAMB). Such heterogeneity among the later trends signifies further continuation of emplacement of GG after syn-tectonic pulse and regional deformation. This latter phase of granite led to the development of broad open N-S trending folds within the supracrustals along with its basement and doming up of sequences towards the eastern periphery of the Champaner Group.

In the Garhwal Himalaya, the Main Central Thrust Zone (MCTZ) is bounded by the Main Central Thrust (MCT) in the south and Vaikrita Thrust (VT) in the north. The MCTZ along the Alakhnanda, Bhagirathi and Yamuna valleys is marked by different small-scale structures and are classified as early structures i.e. pre-thrusting, structures due to progressive ductile shearing (syn-thrusting) and late structures (post-thrusting). The detailed study of small-scale structures/kinematic indicators such as sigmoidal foliations, shear band (S-C) structures, asymmetric porphyroclast tails, intergranular faults, folded layering and sheath folds suggest top to SSW sense of movement in the rocks of MCTZ of Garhwal Himalaya. The strain studies and crystallographic preferred orientation of quartz reveal that mesoscopic ductile shear zones grow in a narrow zone in response to very high strain, which deformed the internal crystallographic fabric of rock. The study of compressive stress from several mesoscopic sigmoidal foliations suggest that, in MCTZ the maximum compressive stress direction acted in the NNE-SSW direction horizontally synchronous to the northward movement of Indian Plate.

The Himalayan Range is generally classified into a number of broad longitudinal tectonic belts. Despite a long history of investigation, some fundamental issues of their stratigraphy and structure are still unresolved. Especially, there has been considerable controversy over delineating the Greater Himalayan and Lesser Himalayan belts of Nepal. The Greater Himalayan thrust sheet represents the hanging wall of the Main Central Thrust. In Nepal, the thrust sheet forms two large open folds: the Great Midland Antiform in the inner zone and the Great Mahabharat Synform in the outer part. The Main Himalayan Thrust and Main Central Thrust constitute respectively the floor and roof of a mega duplex where some detached Lesser Himalayan horses are exposed in various tectonic windows. The Main Himalayan Thrust plays a role of sole thrust in the imbricate stack developed within the foreland fold-and-thrust belt. The key structural and stratigraphic aspects of thrust sheets, tectonic windows, klippen, and intermontane basins are discussed together with the neotectonic activity in the Nepal Himalaya.