Eastern Indian Shield Research Articles

Uncovering the hidden architecture of the Rengali Province, eastern Indian shield using high-resolution aero-geophysics, ground-gravity, structure: Insights towards geology and tectonics

Uncovering the hidden architecture of the Rengali Province, eastern Indian shield using high-resolution aero-geophysics, ground-gravity, structure: Insights towards geology and tectonics

Southeastern extension of Singhbhum Shear Zone, Eastern Indian Shield: A detailed appraisal

Trans-crustal, northerly dipping, arcuate Singhbhum Shear Zone (SSZ) constituting the interface of Proterozoic North Singhbhum Mobile Belt (NSMB) and Singhbhum Craton (SC) is established over a strike length of about ∼200 km from Baharagorato Chakradharpur in the west beyond which it bifurcates around the wedge shaped Chakradharpur granite. Based on geological mapping and meso to micro scale structural analysis, present work examines the SSZ’s southeastern extension beyond Baharagora, indicating its southward continuation for about 50 km to Dubukidihi within the NSMB lithopackage. Bangriposi Shear Zone (BSZ) which defines the terrane boundary between the SC and NSMB in this sector, splays towards north as Bangriposi West Shear Zone (BWSZ) and both transects the Dhanjori Basin longitudinally. BSZ and BWSZ are interpreted to be cogenetic with SSZ. NSMB in this area comprises of pelitic and psamopelitic rocks intruded by mafic–ultramafic magmatism and granitoids. Considering a north to south compression regime during accretion of NSMB with SC, the study area, which forms the interface between the craton and mobile belt, is at an acute to obtuse angle with the direction of overall stress field. It is marked by several high strain zones characterized by N-S shear band cleavages with moderate easterly dips, and northerly pitching stretching/mineral lineation. Consistent westerly vergent folds on mylonitic plane and orientation of stretching lineation indicates top to WSW movement in the shear zones. Tectonically interleaved mafic–ultramafic schist within the Romapahari granite are sites of copper mineralization with enhanced nickel concentration pointing towards future exploration sites for epigenetic copper, uranium mineralization which is so characteristic of SSZ.

Petrogenesis of magmatic charnockite-biotite granite suite from parts of the Chotanagpur Granite Gneissic Complex (CGGC), eastern Indian shield: Implication for the break down of the Columbia Supercontinent

A rare porphyritic charnockite that is girdled by and mineralogically grades to biotite granite occurs as a part of the Chotanagpur Granite Gneissic Complex (CGGC) in and around Massanjore, Jharkhand, India. Preservation of certain textural features, including (a) euhedral to subhedral grains of orthopyroxene, (b) low dihedral angle subtended by orthopyroxene and plagioclase grains, and (c) relict intergranular and porphyritic textures, are consistent with the view that orthopyroxene in the studied rocks has a magmatic origin. Preserved magmatic features and other petrological attributes of these rocks do not support any significant mass change beyond a few tens of microns during the overprinting high-grade metamorphism. The geochemical variation of the felsic rock suite (porphyritic charnockite and biotite granite) indicates that they are cogenetic and are derived from a ferroan A-type granitoid magma by crystal fractionation. The observed geochemical trend of the studied felsic rock suites has been simulated by phase equilibria modelling in the open system using the system components Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2. The observed mineralogical and geochemical attributes and the results of the modelling study are consistent with a petrogenetic process in which high magma temperature (> 900 °C), low pressure, and low water activity in the parental melt favoured the separation of an orthopyroxene bearing cumulate assemblage (orthopyroxene + plagioclase + quartz + K-feldspar + ilmenite + magnetite) in the initial part of the magmatic differentiation. Removal of this anhydrous cumulitic assemblage raised the bulk H2O content in the residual melt. Orthopyroxene became unstable with respect to biotite in the evolved melt that eventually crystallised minerals that formed the biotite granite. An increase in magma fO2 also restricts the orthopyroxene stability in felsic magma. Taken together all the petrological and geochemical attributes, we demonstrate that fractionation of an orthopyroxene-bearing crystal cumulate from the melt is essential to form the charnockites, and that the biotite granite forms from the evolved melt after the fractionation. The charnockite-biotite granite association of the studied area was formed in an extensional tectonic setting, presumably during the breakdown of the Columbia Supercontinent.

Evidence of lithosphere erosion in the Eastern Indian shield from multi-scale potential field modelling: geodynamic implications

Evidence of lithosphere erosion in the Eastern Indian shield from multi-scale potential field modelling: geodynamic implications

Pillow Morphology of Bathani Volcano-sedimentary Sequence of Eastern Indian Shield: Inferences on the Geotectonic and Geo-Environments

Abstract The Palaeo-proterozoic Bathani volcano-sedimentary sequence (BVSs) at the northeastern fringe of the Central Indian Tectonic Zone (CITZ), comprises three distinct litho-domains: (i) intrusive granitoid, having xenoliths of basalt, pyroxenite-gabbro-anorthosite suits (Northern Domain), (ii) low-grade volcano-sedimentary sequence along with concordant mafic-ultramafic intrusion (Central Domain) and (iii) folded clastic metasedimentary rocks (Southern Domain). The granitoid is intruded into the Central Domain, where pillow basalt with primary structure is well exposed. The volcanological and morphometric study of the pillows of Central Domain is attempted to understand the physicochemical characteristics and tectonic setup of the magma emplacement. The pillows are small to medium normal-sized with spheroidal, lozenge, and tubular shapes and preserve the primary features like radial and concentric cracks, selvage, pipe vesicles, voids, drain away cavities, way-up features, etc. A low-viscosity magma outpouring over a steep to moderate depositional slope is being deciphered from the morphological study of pillows. The stretching and fast-spreading growth mechanisms with low effusion under a rapid cooling rate have been inferred for these pillows. The morphological features of the rock types and pillows of the BVSs suggest a shallow water (<1000m) divergent rift basin of the back-arc tectonic setting. The trace elemental discrimination plots support the basaltic composition of pillows with E-MORB and back-arc basin environment. The lithological setup, tectonic setting, and regional correlation of BVSs advocate it as an eastern continuation of the Palaeo-proterozoic Mahakoshal belt within the Son Narmada Lineament (SNL) zone of CITZ.

3D shear-wave velocity structure of the Eastern Indian Shield from ambient noise tomography: Ultramafic lower crust underneath the Singhbhum Craton

Singhbhum Craton (SC) in the Eastern Indian Shield (EIS) is known to have preserved Archean crust which might provide insights into the Precambrian tectonics. In this study, ambient noise cross-correlation of about 1 year of continuous data recorded in 15 broadband seismic stations across EIS is performed. Rayleigh wave group velocities at 4–22 s periods obtained by frequency-time analysis of the cross-correlograms were subjected to tomographic inversion to produce group velocity maps. The spatial resolution of these maps was found to be ∼100 km. Inverted 1D shear-wave velocity (VS) models at regular grid points of these tomographic maps were combined to construct the first 3D VSmodel of the SC and adjoining regions within the EIS. The 3D model highlights a high VS(3.6–3.8 km/s) structure in the 4–20 km depth range and an ultra-high VS(∼4.0–4.2 km/s) structure at depths ≥ 20 km within the SC only. The VS of the upper layer and its regionalisation match with the well-studied granite batholith within the SC. The ultra-high velocitylayer in the lower crust, spatially spanning ∼100 km × 150 km, is distinctly imaged for the first time and its VS indicates probable ultramafic composition. The existence of ultramafic composition at ∼800 MPa lithostatic pressure requires ultra-high temperature metamorphism, possible through mantle upwelling/melting. The previous studies of upwarped crust-lithosphere beneath the SC and the lower-crustal ultramafic composition inferred in this study probably confirm the delamination of the lithosphere beneath the SC at some point during the Precambrian eon.

New Insights into the Gold Mineralization in the Babaikundi–Birgaon Axis, North Singhbhum Mobile Belt, Eastern Indian Shield Using Magnetic, Very Low-Frequency Electromagnetic (VLF-EM), and Self-Potential Data

The North Singhbhum Mobile Belt (NSMB) in the eastern Indian shield is an excellent example of a diverse lithological domain with multiphase shear/fracture zones. It is a well-acknowledged prospective region known for various mineralizations, e.g., Cu, Au, U, and Fe. The present study was conducted in the Babaikundi–Birgaon Axis (a shear and fracture zone within NSMB) within Babaikundi village. The study suggests that anomalous zones are possibly related to gold-associated sulfide mineralization using photomicrography and geophysical methods such as magnetic, self-potential (SP), and very low-frequency (VLF) electromagnetic (EM) methods. Photomicrography on quartz samples and Scanning Electron Microscope (SEM) Backscattered-Electron (BSE) images show that gold particles are present in the quartz reef vein in Babaikundi. Magnetic data are presented in the total magnetic intensity (TMI), upward continuation, and analytical signal maps. VLF-EM data were analyzed using different processing techniques to delineate anomalous auriferous zones. Furthermore, the SP data were inverted using the Particle Swarm Optimization (PSO), Very Fast Simulated Annealing (VFSA), and Genetic Algorithm (GA) approaches for comparative study of the resultant model parameters and their suitability considering a 2D inverted resistivity section derived using VLF-EM. The photomicrography study indicates the existence of gold/ pyrite/pyrrhotite within the Babaikundi–Birgaon axis, which corresponds to a low magnetic anomaly based on a magnetic study; a high current density, a high Fraser value, and a low resistivity based on the VLF-EM study; and a negative SP value was found based on the SP study. The combined study generally supports mapping auriferous zones and demarcates the Babaikundi–Birgaon axis within the Babaikundi–Birgaon area.

3D Inversion of Gravity and Aeromagnetic Data over the Western North Singhbhum Mobile Belt, Eastern Indian Shield, for Delineating Prospective Sulfide (Au) Mineralization Zones

3D Inversion of Gravity and Aeromagnetic Data over the Western North Singhbhum Mobile Belt, Eastern Indian Shield, for Delineating Prospective Sulfide (Au) Mineralization Zones

Three-dimensional Moho depth model of the eastern Indian shield and its isostatic implications

Three-dimensional Moho depth model of the eastern Indian shield and its isostatic implications

New insights into the crustal magnetization of the eastern Indian shield inferred from the aeromagnetic data-based depth to the bottom of the magnetic sources (DBMS)

New insights into the crustal magnetization of the eastern Indian shield inferred from the aeromagnetic data-based depth to the bottom of the magnetic sources (DBMS)

Origin of clinopyroxene-ilmenite symplectites in mafic granulites from eastern parts of the Chotanagpur granite gneissic complex, East Indian shield

Abstract In this manuscript, we report a rare occurrence of rod-like intergrowths of clinopyroxene-ilmenite that variably replace titanite in a suite of high-pressure mafic granulites from the Chotanagpur Granite Gneissic Complex, Eastern Indian Shield. Garnet proximal to the clinopyroxene-ilmenite intergrowth is invariably replaced with symplectic clinopyroxene-plagioclase or a rind of plagioclase. Textural modeling of the reaction textures and mineral compositions suggests that the garnet-titanite became unstable and the ilmenite-clinopyroxene preferentially develop after titanite. The presence and texture of halogen rich apatites within the ilmenite-clinopyroxene symplectite are consistent with a fluid mediated process. Thermodynamic modeling in the NCFMAST (+H2O) system, demonstrates that the clinopyroxene-ilmenite symplectite was formed along a steeply decompressive retrograde P-T path (from ~13 to ~7 kbar, at ~800 °C), in the presence of partial melts. The study demonstrates that relative to Fe, Mg, and Ca, Ti was less mobile and that the mobility of Ti was restricted within the confines of titanite being replaced by the clinopyroxene-ilmenite symplectite. This study implies that besides the ligand activity, the fluid/rock ratio exerts a strong control on the length scale of Ti transport in natural rocks.

Early Cretaceous mafic dykes from the Chhota Nagpur Gneissic Terrane, eastern India: Evidence of multiple magma pulses for the main stage of the Greater Kerguelen mantle plume

Early Cretaceous NNW- to WNW-trending dolerite dykes of the eastern Indian Shield, collectively termed the Raniganj-Koderma swarm, are studied for their emplacement ages and petrogenetic history, and assessed for a possible linkage with the Greater Kerguelen plume. 40Ar/39Ar dates of four dolerite dykes from different locations in the Chhota Nagpur Gneissic Terrane, indicate three pulses of emplacement ca. 118–116 Ma, ca. 112–111 Ma, and ca. 109 Ma. Geochemistry of 19 samples analysed herein (and an additional 12 samples from literature) shows sub-alkaline high-Mg tholeiitic basaltic andesite compositions. All the dyke sets belonging to the Raniganj-Koderma swarm with sub-sets WNW-trending ca. 118 Ma, NNW-trending ca. 116 Ma Salma, and a N-S trending ca. 109 Ma dyke showing similar chemistry. However, NW-trending dykes emplaced ca. 112–111 Ma have slightly different geochemical characteristics. Key trace element geochemistry data, particularly Nb/Y, Zr/Y, Nb/Yb, Ti/Yb, Th/Nb and Th/Yb ratios, indicate derivation from mantle melts generated from interaction of a plume with a spreading ridge (producing OIB – E-MORB melts) with a minor role for interaction with an enriched lithospheric mantle that was metasomatized during an earlier (pre-Mesozoic time) subduction event. Spatiotemporal distribution of the studied dolerite dykes connects them with the second plume head phase of the Greater Kerguelen mantle plume.

India-Antarctica collisional ridge structure below Nellore Schist and Eastern Ghats Belts as revealed by seismic studies

In Columbia, Rodinia and Gondwana supercontinental assembly periods, eastern margin of India lay quite close to the east Antarctica. During these periods, this region underwent prolonged sedimentation, magmatism, oceanic subduction, multiple accretion and continent-continent collision which have left their imprints in the geological and geophysical observations. In the present study, we have analyzed crustal seismic velocity structure along the DSS profiles located in the eastern part of the Indian shield that cuts across the Proterozoic Cuddapah Basin, Godavari and Mahanadi Gondwana grabens. The study revealed presence of a prominent high velocity (6.2-6.5 km/s) hidden ridge structure, which is 30 to 50 km wide and located at a shallow subsurface depth beneath the Nellore Schist Belt and Eastern Ghats Belt. It underlies below Kaligiri-Duttaluru along the Kavali-Parnapalle profile, Darsi-Addanki along the Alampur-Koniki-Ganapeswaram profile and Sattupalli-Ashwaraopet in Kallur-Polavaram profile. This ridge structure is conspicuously characterized by high gravity anomaly gradients from -90 to -15 mGal. Gravity patterns over these regions further indicate that this ridge which appears to continue from the north of Chennai to Bhavanipatna region in Bastar Craton, may have been formed due to the collision of East Antarctic terrain with eastern Indian Shield after the cessation of supra-subduction at the end of Mesoproterozoic. Further, this ridge structure is flanked by Proterozoic sediments on either side; in Cuddapah Basin on its western side and East Coast Sedimentary Basin on its eastern side, which together with the possible occurrence of Proterozoic sediments in other basins, would indicate that the extent of Proterozoic sedimentation was much larger than hitherto known. We suggest that this region was rifting during Paleo-Mesoproterozoic period and possibly a shallow marine basin existed, when it was an integral part of the Columbia supercontinent assembly. It underwent another rifting phase and denudation during the Gondwana period as indicated by thin patches of Gondwana sediments which are lined up all along the east coast. Quite likely, the rifting between India and Antarctica during the Gondwana super assembly period initiated in the Permo-Triassic Gondwana period itself, with final separation taking place much later during early Cretaceous period.

Evidence for paleo-continental margin and folding of relic fragment of oceanic lithosphere, northeast Indian subcontinent

The processes of continental breakup, reassembling of landmasses, and huge deltaic sediment depositions contribute to reconfiguring the Earth's surface. Here, we show a classic example of contiguous structures of paleo-continental margin on the eastern Indian Shield and an ancient fragment of the oceanic lithosphere beneath Bangladesh. In the present work, we collated gravity data from Bangladesh and adjoined parts of the Indian shield and the northern Bay of Bengal to decipher rocks' nature and long-term behavior. The gravity map illustrates N-S oriented alternate bands of ∼150 km wide positive and negative anomalies in the Bangladesh region, besides showing signatures of prominent geomorphic features such as the Shillong Plateau, Sylhet Trough, Burmese Arc, etc. The crustal models reveal the presence of paleo-continental margin segments on the eastern edge of the Singhbhum craton and south of the Shillong Plateau, ∼150 km wide folded oceanic crust beneath Bangladesh, and a low slab dip of the subducting Indian plate beneath the Burma platelet. The inferred continental margin evolved after the breakup occurred between the Rajmahal-Sylhet Line and continental fragments (Elan Bank and southern parts of the Kerguelen Plateau) at about 120 Ma. Subsequently, the sediments carried by two great Himalayan river systems were excessively deposited on the margin and over an older fragment of the oceanic lithosphere, resulting in the progradation of the Bengal delta towards the ocean. The oceanic crust beneath Bangladesh was folded with approximately 150 km wavelength as the compressive stress regime is locally operative due to the eastward moment of the Indian plate and westward propagation of the Indo-Burman Wedge.

Evidence of secular variation in Archean crust formation in the Eastern Indian Shield

Understanding the dominant crustal accretion model in any Archean craton is the key to understanding the dominant geodynamic process responsible for early crust formation during the Hadean (> 4.0 Ga) and Archaean (4.0–2.5 Ga). The continental crust has been proposed to have formed through either horizontal/vertical accretion related to subduction or mantle plume tectonic processes. Here, the Moho depths and average crustal Vp/Vs ratios are modelled at 16 broadband stations in the Eastern Indian Shield (EIS) through HK stacking of radial P-receiver functions (PRFs). These modelled parameters are used to test both plume and subduction models, which might have played a key role in the crustal accretion of the EIS throughout the Archean. We observe a correlation between crustal age and composition within the ellipsoidal Paleoarchean cratonic domain in the Singhbhum-Odisha-Craton (SOC), which reveals an increase in age from the younger granitoid core of the SOC (with thinning of felsic crust) to the surrounding older greenstone belts (with thickening of felsic crust). A thinner mafic crust resulting from multiple magmatic events characterizes the neighbouring Meso-Proterozoic Chotanagpur Granitic Gneissic terrain (CGGT). The Common Conversion Point (CCP) image of radial PRFs reveals northward subduction of the Paleoarchean SOC below the Meso-Proterozoic CGGT.

Crustal architecture of North Singhbhum Mobile Belt, Eastern Indian Shield: Constraints from two‐dimensional and three‐dimensional modelling of Bouguer gravity and aeromagnetic data

The North Singhbhum Mobile Belt (NSMB) that lies in the Eastern Indian Shield between the Chotanagpur Granite‐Gneiss Complex (CGGC) and the Singhbhum Craton (SC) provides a unique opportunity to understand the Proterozoic crustal evolution of the Indian shield. In the present study, an integrated interpretation of petrophysical, Bouguer gravity, and aeromagnetic data was carried out to examine the crustal architecture and structural framework of the region. Three‐dimensional (3‐D) inversion of the aeromagnetic anomalies reveals that the magnetic basement depths range from 0.2 to 9.0 km in the region. The crustal models obtained from two‐dimensional (2‐D) and 3‐D forward modelling of potential field data reveal the Moho at 37–38 km depth beneath the NSMB and at a relatively deeper depth of 39–40 km and 40–44 km beneath SC and CGGC, respectively. Further, the distinct gravity high and the coinciding bipolar magnetic anomalies associated with the Dalma Formation can be attributed to the mafic/ultramafic intrusions. Gravity models across the Singhbhum Shear Zone (SSZ) and South Purulia Shear Zone (SPSZ) exhibit the crustal density inhomogeneity and Moho offset, typical of ancient suture zones of Precambrian terrains. In comparison, the absence of such crustal‐scale signatures across the North Purulia Shear Zone represents its intracratonic nature. Further, the Bouguer gravity and aeromagnetic signatures across the SPSZ, together with the crustal architecture and metamorphic ages, suggest that SPSZ marks the eastward continuity of the Gavilgarh‐Tan Shear Zone along which the amalgamation of northern and southern Indian cratonic blocks possibly took place at ca. 1.0–0.9 Ga.

Geochemistry of detrital zinc-rich chromite in conglomerates from eastern India

AbstractOn the western part of the eastern Indian shield, Archaean basement (Bonai granite) is overlain by radioactive conglomerate. The conglomerate contains well-rounded, fractured Mg-poor (<0.18 wt.% MgO), Al-rich (up to 21.14 wt.% Al2O3) Zn-rich chromite (>35.0 wt.% Cr2O3) containing up to 15.5 wt.% ZnO. This is the first reported occurrence of detrital zinc-rich chromite having such unusually high ZnO from India, and to our knowledge, the third reported occurrence in radioactive quartz-pebble conglomerate after Witwatersrand, South Africa and Tarkwa, Ghana. Zinc-rich chromite grains are either clean or contain exsolution blebs/lamella of rutile. The conglomerates show evidence for post-depositional hydrothermal fluid influx and fluid-induced mineral alteration. The strong negative correlation between Zn and Fe2+, Al and Cr, and Al and Fe3+, and strong positive correlation between Zn and Al suggest secondary incorporation of Zn and Al by substitution of Fe2+ and Cr (and Fe3+), respectively, leading to partial transformation of (Fe)(Cr,Al,Fe3+)2O4 towards ZnAl2O4 composition. The chromite grains were possibly derived from komatiite. The timing of Zn enrichment, either at the provenance prior to sedimentation, or at the depositional site post-dating sedimentation, remains unresolved.

Early Cretaceous Greater Kerguelen Large Igneous Province and its plumbing systems: A contemplation on concurrent magmatic records of the eastern Indian Shield and adjoining regions

A detailed contemplation is attempted to comprehend the likely connection of a large number of Early Cretaceous mafic, felsic, alkaline (kimberlite, lamproites, ultramafic lamprophyre, etc.), and carbonatite magmatic records of the eastern and north‐eastern region of the Indian Shield and adjoining regions. All these Early Cretaceous magmatic records occupied a large area, covering adjoining regions of south‐eastern Tibet, eastern and north‐eastern India, western Australia, Eastern Indian Ocean, southern Kerguelen Plateau, and north‐eastern Antarctica, are supposed to be part of the Greater Kerguelen Large Igneous Province (LIP) and likely to be related to the break‐up of East Gondwana. Available age results, chiefly ranging from ca. 145 to 100 Ma, support a genetic connection with the Kerguelen mantle plume. However, there are other lines of evidences, particularly radiogenic isotope data that preclude direct involvement of any mantle plume component; likely to be derived from decompression melting related to passive rifting. Two major magmatic phases, ca. 145–130 Ma and ca. 124–100 Ma, have been distinctly identified, which could be related to distinct rifting events. Break‐up of East Gondwana had happened at ca. 137 Ma with separation of Western Australia from Greater India. Later, ca. 124–122 Ma, separation of Antarctica from Eastern India occurred. The later magmatic phases (after ca. 120 Ma) are likely to have an indirect connection with the Greater Kerguelen LIP probably with the mantle plume providing extra heat for melting.

Variations in crustal and lithospheric structure across the Eastern Indian Shield from passive seismic source imaging: Implications to changes in the tectonic regimes and crustal accretion through the Precambrian

Changes in the dominant tectonic regimes and the modes of formation and maturation of the continental crust during the Precambrian are a subject of active debate. Here, we discuss these issues in terms of new information on the crustal and lithospheric architecture of the different terranes in the Eastern part of the Indian Shield (EIS). We present well-constrained estimates of the crustal and lithospheric thickness characterizing different terranes of the EIS through the joint inversion of P-receiver functions (PRF) and fundamental mode group velocity dispersion data of Rayleigh waves (10–100 s). The lithospheric thicknesses are also computed independently using the S-receiver function (SRF) modeling. The study involved 2167 PRFs and 100 SRFs from 15 broadband stations. Our modeling detects a marked crustal (~4–8 km) and lithospheric (~15–20 km) thinning beneath the Singhbhum-Odisha-Craton (SOC), spatially correlating with the central part of the craton comprising the gneissic terrain contrasted by a crustal thickening (~2–6 km) beneath the horseshoe-shaped Iron Ore Group. We propose that such a crustal configuration beneath the Paleoarchean craton may suggest the dominance of vertical tectonic processes such as thickening of an oceanic mafic plateau and recurrent melting producing the TTGs and the Singbhum Granite suite. Infra-crustal reworking involving Rayleigh-Taylor instabilities and gravitational processes could explain the crustal architecture of the SOC much similar to a few other Paleoarchean cratons such as Pilbara (Australia), Barberton and Kappvaal (South Africa). Our modeling has also revealed a relatively smaller degree of crustal (2–4 km) and lithospheric thinning (4–10 km) beneath the Eastern Ghat Mobile Belt, south of the SOC. We observe a relatively thicker crust (~42 km) characterized by a flat Moho over a large area (~40,000 sq. km) encompassing the Chota Nagpur Granite Gneiss Terrane (CGGT). Such thickening is consistent with the convergence and collision tectonics involving the SOC and CGGT. However, we note a 15–20 km lithospheric thinning associated with the CGGT. Such thinning may mainly relate to Phanerozoic rifting along the Damodar graben and Rajmahal magmatism. Broadly, our model of the crustal and lithospheric architecture of the EIS reflects the imprints of northward subduction of SOC beneath the CGGT.

Crustal Configuration and Seismic Stability of the Eastern Indian Shield and Adjoining Regions: Insights for Incidents of Great Earthquakes in the Nepal-Bihar-Sikkim Himalaya

The Eastern Indian Shield (EIS) is comprised of the intracratonic (coal-bearing) Damodar Gondwana basin, rift-controlled extensional Lower Gangetic basin (LGB), and the downward flexed Indo-Gangetic basin (IGB). The present study involves the computations and mapping of the basement configuration, sediment thickness, Moho depth, and the residual isostatic gravity anomaly, based on 2-D gravity modeling. The sediment thickness in the area ranges between 0.0 and 6.5 km, and the Conrad discontinuity occurs at ∼17.0–20 km depth. The depth of the Moho varies between 36.0 and 41.5 km, with the maximum value beneath the Upper Gangetic basin (UGB), and the minimum of ∼36 km (uplifted Moho) in the southeastern part beneath the LGB. The maximum residual isostatic anomaly of +44 mGal in the southern part indicates the Singhbhum shear zone, LGB, and Rajmahal trap to be under-compensated, whereas the northern part recording the minimum residual isostatic anomaly of –87.0 mGal is over-compensated. Although the region experienced a few moderate-magnitude earthquakes in the past, small-magnitude earthquakes are sparsely distributed. The basement reactivation was possibly associated with a few events of magnitudes more than 4.0. Toward the south, in the Bay of Bengal (BOB), seismic activities of moderate size and shallow origin are confined between the aseismic 85 and 90°E ridges. The regions on the extreme north and south [along the Himalaya and the equatorial Indian Ocean (EIO)] are experienced moderate-to-great earthquakes over different times in the historical past, but the intervening EIS and the BOB have seismic stability. We propose that the two aseismic ridges are guiding the lithospheric stress fields, which are being further focused by the basement of the EIS, the BOB, and the N-S extended regional fault systems into the bending zone of the penetrating Indian lithosphere beneath the Himalaya. The minimum obliquity of the Indian plate and the transecting fault systems in the Foothills of the Himalaya channelize and enhance the stress field into the bending zone. The enhanced stress generates great earthquakes in the Nepal-Bihar-Sikkim Himalaya, and on being reflected back through the apparently stable EIS and BOB, the stress field creates deformation and great earthquakes in the EIO.