D.C. Mishra

Crustal structure based on gravity–magnetic modelling constrained from seismic studies under Lambert Rift, Antarctica and Godavari and Mahanadi rifts, India and their interrelationship

Modelling of gravity and magnetic anomalies along selected profiles across the Lambert Glacier, Antarctica constrained from the results of deep seismic sounding (DSS) along a profile provide: (i) 6–7 km thick sediments of low density (2.35 g/cm3) which represent Permian–Triassic sediments as they are exposed along the margins of this basin; (ii) high density (2.75 g/cm3) and high susceptibility rocks with a stable natural magnetization of 0.025–0.035 gauss as the basement; (iii) a high density body (2.80 g/cm3) along the western shoulder which may represent mid crustal granulite and charnockite rocks exposed in the Prince Charles mountains; (iv) a thin crust of 25 km with a high density (3.05 g/cm3) and high velocity underplated lower crust. These signatures suggest that the Lambert Rift and the Amery ice shelf represent an active continental rift valley with signatures of magmatism. The adjoining Prydz Bay basin along the east coast of Antarctica is also characterized by 5–6 km thick Permian–Triassic–Cretaceous sediments with seaward dipping reflectors of basic volcanic rocks. The Godavari and the Mahanadi Gondwana rift valleys of almost same period and length (500–600 km) across the east coast of India are also characterized by Permian–Triassic–Cretaceous sediments of almost same density and thickness. The Mahanadi Basin depicts a thin crust (33–34 km) with a high density (3.00 g/cm3) underplated lower crust similar to the Lambert Rift and a low density (2.65 g/cm3) layer in the upper crust. It indicates substantial rift magmatism in the coastal part of the Mahanadi Basin and adjoining continental shelf which forms a part of the Early Cretaceous volcanic province of East India. As the east coast of India and Antarctica were juxtaposed together at the time of evolution of these rift valleys, the above similarities between them might be due to their common heritage. The signatures of magmatism in the Lambert Rift of Antarctica and the Mahanadi Basin of India indicate that the thermal source at the time of their evolution was located under the eastern part of East Antarctica which might be responsible for its uplift supplying sediments of same provenance and fossil record in these basins.

Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties

Abstract The Bouguer anomaly and the total intensity magnetic maps of Saurashtra have delineated six circular gravity highs and magnetic anomalies of 40–60 mGal (10 −5 m/s 2 ) and 800–1000 nT, respectively. Three of them in western Saurashtra coincide with known volcanic plugs associated with Deccan Volcanic Province (DVP), while the other three in SE Saurashtra coincide with rather concealed plugs exposed partially. The DVP represents different phases of eruption during 65.5±2.5 Ma from the Reunion plume. The geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition, which varies from ultramafic/mafic to felsic composition of volcanic plugs in western Saurashtra and an alkaline composition for those in SE Saurashtra. Detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc-alkaline differentiation trend and a continental tectonic setting of emplacement. The alkaline plugs of SE Saurashtra are associated with NE–SW oriented structural trends, related to the Gulf of Cambay and the Cambay rift basin along the track of the Reunion plume. This indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma. The Junagadh plug with well differentiated ring complexes in western Saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude. This implies that other plugs are also associated with mafic/ultramafic components, which may not be differentiated and may be present at subsurface levels. Paleomagnetic measurements on surface rock samples from DVP in Saurashtra suggest a susceptibility of 5.5×10 −2 SI units with an average Koenigsberger ratio ( Q n ) of almost one and average direction of remanent magnetization of D =147.4° and I =+56.1°. The virtual geomagnetic pole (VGP) position computed from the mean direction of magnetization for the volcanic plugs and Deccan basalt of Saurashtra is 30°N and 74°W, which is close to the VGP position corresponding to the early phases of Deccan eruption. Modeling of gravity and magnetic anomalies along two representative profiles across Junagadh and Barda volcanic plugs suggest a bulk density of 2900 and 2880 kg/m 3 , respectively and susceptibility of 3.14×10 −2 SI units with a Q n ratio of 0.56 which are within the range of their values obtained from laboratory measurements on exposed rock samples. The same order of gravity and magnetic anomalies observed over the volcanic plugs of Saurashtra indicates almost similar bulk physical properties for them. The inferred directions of magnetization from magnetic anomalies, however, are D =337° and 340° and I =−38° and −50° which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs. Some of these plugs are associated with seismic activities of magnitude ≤4 at their contacts. Based on this analysis, other circular/semi-circular gravity highs of NW India can be qualitatively attributed to similar subsurface volcanic plugs.

Crustal structure and domain tectonics of the Dharwar Craton (India): insight from new gravity data

Abstract The Precambrian Dharwar Craton of Peninsular India comprises three distinct crustal domains, namely the Western Dharwar Craton (WDC), the Eastern Dharwar Craton (EDC) and the Eastern Ghats Mobile Belt (EGMB). The domain boundaries are demarcated by bipolar (positive and negative) gravity anomalies, measured along a 600 km long Udipi–Kavali seismic line. The two and half dimensional modelling of the gravity anomalies, well constrained by available seismic depth-sections, identified the two contact zones. One occurs between the WDC (relatively low density) and the EDC (intermediate density), to the east of the Chitradurga Schist Belt and the second occurs between the EDC and the EGMB (high density), to the east of Cuddapah Basin as west verging thrust faults. The three-layer crustal structure is up to 41 km thick beneath the WDC, thinning to 37 km beneath the west coast. Under the EDC, the depth of the Moho varies from 38 km towards the west to 40 km beneath the Cuddapah Basin, and decreases to a depth of 35 km beneath the EGMB. A high-density steeply inclined ridge-like body is modelled between 5 and 20 km depth beneath the Closepet Granite, and a similar body is modelled under the EGMB. It is suggested that the three domains of the Dharwar Craton were brought and welded together at the suture zones in the period from Late Archaean to Late Proterozoic through collisional tectonics. The apparent offsets in the Moho depth may be the imprint of the relative domain movement that probably was initiated by the convergent plate motion. The steeply inclined ridge-like high-density bodies are interpreted as stacked lower crustal blocks from the convergent tectonic setting. The Cuddapah Basin, with a 10 km thick shelf-marine sedimentary sequence, is interpreted as a peripheral foreland basin abutting the EGMB. A lopolith of likely mafic composition is modelled at 10 km depth, and is interpreted as a product of juvenile magmatism beneath the EDC.

Geodynamics of NW India: Subduction, lithospheric flexure, ridges and seismicity

Spectral analysis of digital data of the Bouguer anomaly map of NW India suggests maximum depth of causative sources as 134 km that represents the regional field and coincides with the upwarped lithosphere — asthenosphere boundary as inferred from seismic tomography. This upwarping of the Indian plate in this section is related to the lithospheric flexure due to its down thrusting along the Himalayan front. The other causative layers are located at depths of 33, 17, and 6 km indicating depth to the sources along the Moho, lower crust and the basement under Ganga foredeep, the former two also appear to be upwarped as crustal bulge with respect to their depths in adjoining sections.The gravity and the geoid anomaly maps of the NW India provide two specific trends, NW-SE and NE-SW oriented highs due to the lithospheric flexure along the NW Himalayan fold belt in the north and the Western fold belt (Kirthar -Sulaiman ranges, Pakistan) and the Aravalli Delhi Fold Belt (ADFB) in the west, respectively. The lithospheric flexures also manifest them self as crustal bulge and shallow basement ridges such as Delhi — Lahore — Sagodha ridge and Jaisalmer — Ganganagar ridge. There are other NE-SW oriented gravity and geoid highs that may be related to thermal events such as plumes that affected this region. The ADFB and its margin faults extend through Ganga basin and intersect the NW Himalayan front in the Nahan salient and the Dehradun reentrant that are more seismogenic. Similarly, the extension of NE-SW oriented gravity highs associated with Jaisalmer — Ganganagar flexure and ridge towards the Himalayan front meets the gravity highs of the Kangra reentrant that is also seismogenic and experienced a 7.8 magnitude earthquake in 1905. Even parts of the lithospheric flexure and related basement ridge of Delhi — Lahore — Sargodha show more seismic activity in its western part and around Delhi as compared to other parts. The geoid highs over the Jaisalmer — Ganganagar ridge passes through Kachchh rift and connects it to plate boundaries towards the SW (Murray ridge) and NW (Kirthar range) that makes the Kachchh as a part of a diffused plate boundary, which, is one of the most seismogenic regions with large scale mafic intrusive that is supported from 3-D seismic tomography.The modeling of regional gravity field along a profile, Ganganagar — Chandigarh extended beyond the Main Central Thrust (MCT) constrained from the various seismic studies across different parts of the Himalaya suggests crustal thickening from 35-36 km under plains up to ∼56 km under the MCT for a density of 3.1 g/cm3 and 3.25 g/cm3 of the lower most crust and the upper mantle, respectively. An upwarping of ∼3 km in the Moho, crust and basement south of the Himalayan frontal thrusts is noticed due to the lithospheric flexure. High density for the lower most crust indicates partial eclogitization that releases copious fluid that may cause reduction of density in the upper mantle due to sepentinization (3.25 g/cm3). It has also been reported from some other sections of Himalaya. Modeling of the residual gravity and magnetic fields along the same profile suggest gravity highs and lows of NW India to be caused by basement ridges and depressions, respectively. Basement also shows high susceptibility indicating their association with mafic rocks. High density and high magnetization rocks in the basement north of Chandigarh may represent part of the ADFB extending to the Himalayan front primarily in the Nahan salient. The Nahan salient shows a basement uplift of ∼ 2 km that appears to have diverted courses of major rivers on either sides of it. The shallow crustal model has also delineated major Himalayan thrusts that merge subsurface into the Main Himalayan Thrust (MHT), which, is a decollment plane.

Plume and Plate Tectonics Model for Formation of some Proterozoic Basins of India along Contemporary Mobile Belts: Mahakoshal — Bijawar, Vindhyan and Cuddapah Basins

Proterozoic basins in India mostly belong to two periods, viz. Paleo-Mesoproterozoic (~1.9–1.6 Ga) and Meso-Neoproterozoic (1.1–0.7 Ga) periods that show a long hiatus of ~0.5 Ga between the upper and the lower groups. We have considered Mahakoshal — Bijawar, lower Vindhyan and lower Cuddapah (Cuddapah Supergroup) basins in the former group while the latter consists of upper Vindhyan, and upper Cuddapah (Kurnool) basins. They mostly occur along the contemporary Proterozoic collision zones and are sub parallel to them. The Mahakoshal and Bijawar Supergroups occur along the Satpura Mobile Belt (SMB) and Bundelkhand craton, respectively with Bundelkhand craton as basement and are overlain by the Vindhyan Supergroup of rocks along the SMB. They are almost sub-parallel to the mobile bet, SMB. The Cuddapah Supergroup of Paleoproterozoic period and the Kurnool Group of Neoproterozoic period of Cuddapah basin occur along the Eastern Ghat Mobile Belt (EGMB). Based on the exposed contemporary dyke swarms and sills of mafic and ultramafic rocks that are exposed far apart and their extent sub-surface based on geophysical data, it is suggested that a large plume/superplume existed during Paleoproterozoic period (~1.9 Ga) under the Indian continent. It was responsible for the breakup (rifting) of the then cratons and provided margins for deposition of the former older group of rocks with shelf type of sediments and large scale mafic/ultramafic intrusives. The latter younger groups formed during subsequent convergence in Meso-Neoproterozoic period (~1.1–0.7 Ga) as foreland basins on the stable platform of the rifted cratons. That largely explains their undisturbed nature and absence of magmatic rocks. This convergence also caused large scale deformation as folds and faults in the former group of rocks as they collided with adjoining cratons and formed orogenic belts of that time. In the above examples, the Mahakoshal, Bijawar, lower Vindhyan and lower Cuddapah basins of Paleoproterozoic period formed during the rifting phase due to the plume/ superplume while the upper Vindhyan and upper Cuddapah (Kurnool) basins of Neoproterozoic period formed during subsequent convergence as foreland basins. As they formed during different stages of plate tectonics viz. rifting and convergence, they show long hiatus of 0.5–0.6 Ga between the older and the younger groups. The same plume/superplume at 1.9 Ga might also be responsible for the breakup of the contemporary Columbia supercontinent as rocks of similar ages have been reported from other parts of this supercontinent.