The Laxmi Basin, which is a deep ocean basin situating adjoining to the northwestern continental margin of India, contains a linear seamount chain consisting of the Raman Seamount, Panikkar Seamount and the Wadia Guyot, located over the axial basement high in the Laxmi Basin representing the Panikkar Ridge. The geomorphology of this seamount chain was studied by earlier researchers; however, its characteristic geophysical signatures, possible genesis and age of formation remains to be clearly established. We attempted for a detailed understanding of the morphological characteristics and the geophysical signatures over the spatial extent of these seamounts using a fresh set of multibeam bathymetry data and sea-surface gravity and magnetic anomalies, complemented by the existing seismic reflection sections. Based on the seafloor and basement topography signatures observed together from the multibeam bathymetry data and the seismic reflection sections, we infer that all the three seamounts are associated with the presence of surface/subsurface secondary peaks on their central parts. The free-air gravity anomaly maps show that the seamounts are associated with a very short wavelength gravity high, superimposed on the short wavelength gravity low representing the Panikkar Ridge. The magnetic anomaly maps suggest that all these seamounts are associated with complex magnetic signatures. Based on the presence of morphological features that are generally associated with the volcanic activity and by considering the above complex geophysical signatures and the updated magnetic isochron map of the Laxmi Basin, we support multiphased volcanic origin for the Raman-Panikkar-Wadia seamount chain, emplaced at an age younger to 63.28 Ma.

Analysis of high-resolution multibeam bathymetry, sub-bottom profiles, and multichannel seismic reflection data provides a novel understanding of a large-scale submarine landslide in the Cochin offshore region, southwestern continental margin of India. This massive submarine landslide is referred to name as the “Cochin slide”. The Cochin slide exhibits a U-shaped slide scar with an average gradient of ~ 2°, which opens in the NNE-SSW trend in the lower slope and Laccadive Basin. The slide scar has a width of ~ 46 km and a total perimeter length of ~ 60 km, estimated with a volume of ~ 240 km 3 excavated from this region. A non-sinuous channel with a length of ~ 36 km and having incisions varying from ~ 140 to 40 m from head to toe has developed within the slide scar. The submarine channel might have formed after the slide event facilitating channel incisions in the upper slope. The mass transport deposits associated with the slide have distributed over a wide area (~ 5700 sq. km.) in the lower slope and Laccadive Basin, with a maximum run-out length of ~ 138 km. The preconditioning factors or trigger mechanisms for the Cochin slide could not be precisely established yet. However, sedimentation during different geological times and the occurrence of weak layers might have contributed to the slope instability. The morphology of the Cochin slide, its correlation with the extensive faulting in the continental shelf-slope province, the presence of structural elements, and records of previous earthquakes suggest that the tectonic/geological processes strongly influence the triggering of the slide.

•The crustal models derived reveal Moho depth at ∼40 km near MBSASZ.•The increased crustal thickness near MBSASZ can be related to collision.•MBSASZ and PCSZ have different tectono-magmatic history.•Eastward marine prolongation of MBSASZ and PCSZ up to the COB.•The tectonic events in SGT are vital to understand the Precambrian shields.

Submarine landslides in the continental slopes of passive margins are recognized as coastal hazards capable of generating tsunami waves. Potential submarine landslide-prone zones in the slope regions can be identified from preliminary geophysical investigations. In the present study, we used multibeam bathymetry and sub-bottom profiling data to study the slope morphology of Palar Basin, off Chennai, covering an area of 3500 km 2 between 300 m and 3300 m water depths. The continental slope of Palar Basin is very steep (mean slope gradient, 38°) and are marked by a series of cracks parallel to the outer shelf. The identified cracks were up to 130 m deep, 10 km long and resembled "crown cracks" that often act as precursors for submarine landslides. These cracks were either a consequence of fault slips or gas hydrate dissociation or a combination of both. One of the wide crack observed was accompanied by a slump with a run-out distance of 1.6 km. Generally, these cracks are suspected of generating slope failures along a glide plane, if the preconditioning factors are favourable. In addition to these surface expressions, five distinct submarine landslides, which are 'disintegrative' in nature were also found. The disintegrative landslides have curved head scarps and were on average 2.70 km wide and 1.25 km long. Multiple side-walls indicated that failures might have occurred as multiple events and probably caused by over steepening of the continental slope. Based on the crack system and several failures identified along the margin, we marked probable 'landslide susceptible zones' in the region.

The marine geophysical investigations carried out in the Eastern Arabian Sea revealed the presence of a bathymetric high feature, referred to as the Sagar Kanya Seamount. The preliminary morphology and geophysical characteristics of this feature were studied along a single transect by earlier researchers, however, a detailed geophysical mapping over the entire extent of this feature is still awaited. The present study aims for such a detailed investigation on the Sagar Kanya Seamount and its adjacent regions using newly acquired high-resolution multibeam bathymetry data, complemented with sea surface magnetic and gravity data. Bathymetric map reveals the presence of well-defined bathymetric high features constituting three seamounts connected with structural high/ridge-like features, together representing a large nearly elliptical bathymetric high complex surrounding a region of nearly flat seafloor, referred to as the Sagar Kanya Bathymetric High Complex (SKBHC). The overall morphology of this feature closely resembles with the shape of a submarine volcanic caldera. The gravity anomalies over this feature are mostly correlatable with topography; elliptical-shaped gravity highs are associated with bathymetric highs and gravity lows are associated with the enclosed flat seafloor. Some magnetic anomalies observed over the SKBHC are correlatable with the topographic highs, while others are observed over the flat seafloor. The magnetic anomalies are interpreted as features of post-caldera volcanism. In view of the proximity of this feature to the Réunion hotspot track, and the tectonic framework of the region, the genesis of the SKBHC is attributed to the Réunion hotspot volcanism.

Palar Basin in India’s southeastern margin is an intracratonic rift basin characterized by a very steep continental slope (gradient, 3°–78°). Recently acquired high-resolution swath bathymetry data from the region documented various geomorphic features on the slope of the Palar Basin, and canyons are most prominent among them. Twenty submarine canyons, grouped as the Palar Canyon System (PCS), have been mapped and identified for the first time in this study. The geological element that controls PCS’s development presents a morphological framework for canyons developed in extremely steep continental slopes, which are rare on continental margins elsewhere. In contrast to many other submarine canyons, the short and low sinuosity canyons in the Palar Basin (1) traverse a large fault (escarpment) exposed up to 1300 m high, (2) erode into a relatively steep margin, and (3) create downstream features akin to the subaerial origin. Because of these unusual characteristics, the environment presents a wide range of possibilities to discuss the processes involved in their origin, evolution, the controlling factors, and sedimentary activity. The present-day PCS architecture implies that the early stages of canyon formation were shaped by bottom–up retrogressive landslides initiated from the disintegrative failures in the oversteepened slope. The evolution of PCS corresponds to the orientation and relative displacement of the fault escarpment, where the movement of the fault generates canyon activity in response to maintaining slope equilibrium. In addition to the fault movement, downward eroding gravity flows and failures along the flanks aid in the canyon shaping processes. For canyons truncated by a large fault, we propose an upslope/bottom–up model of canyon formation that initiates near the steep fault escarpment and incises downslope and propagates upslope to breach the shelf edge. PCS also comprises ellipsoidal depressions and asymmetric bedforms at the downstream reaches of the canyons, classified as “plunge pools” and “mega dunes,” respectively. Seven plunge pools (> 65 m deep) formed at the mouth of some of the canyons are interpreted to reflect the impact of confined high momentum gravity flows, whereas the wave train of mega dunes extending for 10 km opposite to the mouth of a canyon points to the activity of repeated hydraulic jumps in the region.

The Precambrian basement shear zones played a major role in developing offshore structures that are zones of weakness along the eastern continental margin of India. A compilation and analysis of various geophysical datasets, particularly magnetic, gravity, seismicity and seismic reflection data support a clear geometric connection between the shear zones and offshore structures. The presence of shear zones within the crystalline basement in the continental domain and oceanic domain is demarcated primarily by prominent magnetic lineation. We identified three major magnetic lineaments in the offshore Cauvery basin, which correspond to the Plaghat Cauvery shear system (PCSS) of Southern Granulite terrain (SGT). The faults interpreted in the seismic sections are correlated to the extension of PCSS and indicate the location of reactivation of the shear zones which give rise to considerable intraplate seismicity. The geophysical crustal model shows deep crustal-scale faults associated with the marine prolongation of PCSS. The shear zone extended structures in the offshore might have formed at the breakup stage at the onset of rifting and might be extending up to the continent-ocean transition zone (COTZ). The PCSS partly seems to accommodate the stress created by the northward push of Indian plate against the Asian plate. It is also possible that the shear zone might have extended to the nearby land mass that marks it as one of the principal component in Gondwana reconstruction.

The western continental margin of India and the adjacent deep ocean basins were formed by break-up and separation among India, Seychelles and Madagascar since the Late Cretaceous. The initial India-Madagascar separation and the subsequent India-Seychelles separation are believed to have been caused by the Marion hotspot at similar to 90 Ma and the Reunion hotspot at similar to 68.5 Ma, respectively. These geodynamic events resulted in the formation of several bathymetric highs that probably represent imprints of these volcanic events. In the present study these bathymetric high features were mapped comprehensively to understand their morphotectonic characteristics, using a fresh set of multibeam bathymetry, sea-surface gravity and magnetic anomalies, complemented by the available multichannel seismic reflection sections. A high-resolution bathymetric map of the southwestern continental margin of India and the adjoining deep offshore regions has been generated to decipher detailed morphological configuration and distribution of prominent undersea bathymetric features. We also carried out detailed morphometric analysis of these features to deduce the morphological parameters. A total of 33 individual bathymetric high features were identified and classified as seamounts, hills, knolls, guyots and plateaus based on the standardization of undersea feature names published by Intergovernmental Oceanographic Commission (IOC) and International Hydrographic Organization (IHO) in 2013. Multichannel seismic reflection, sea-surface gravity and magnetic data were used to describe the sub-seafloor configuration and qualitative interpretation of the geophysical signatures associated with the bathymetric highs. Interpretation of the multichannel seismic reflection sections suggest that some of these identified features are extrusive in nature, while others are intrusive. These features are associated with characteristic gravity highs superimposed over regional negative anomalies and complex negative and positive magnetic anomalies. The study results suggest that the genesis of the bathymetric highs mapped in the study area could be attributed to the hotspot volcanism, caused by the Marion or Reunion hotspots. We infer that the features in the southwestern continental margin of India closer to the Alleppey-Trivandrum Terrace Complex might have been created by the Marion hotspot volcanism, while those in the Laccadive Basin and eastern sector of the Laccadive Plateau might have been formed by Reunion hotspot volcanism. (C) 2017 Elsevier B.V. All rights reserved.

The Cauvery–Palar basin is a major peri-cratonic rift basin located along the Eastern Continental Margin of India (ECMI) that had formed during the rift-drift events associated with the breakup of eastern Gondwanaland (mainly India–Sri Lanka–East Antarctica). In the present study, we carry out an integrated analysis of the potential field data across the basin to understand the crustal structure and the associated rift tectonics. The composite-magnetic anomaly map of the basin clearly shows the onshore-to-offshore structural continuity, and presence of several high-low trends related to either intrusive rocks or the faults. The Curie depth estimated from the spectral analysis of offshore magnetic anomaly data gave rise to 23 km in the offshore Cauvery–Palar basin. The 2D gravity and magnetic crustal models indicate several crustal blocks separated by major structures or faults, and the rift-related volcanic intrusive rocks that characterize the basin. The crustal models further reveal that the crust below southeast Indian shield margin is ∼36 km thick and thins down to as much as 13–16 km in the Ocean Continent Transition (OCT) region and increases to around 19–21 km towards deep oceanic areas of the basin. The faulted Moho geometry with maximum stretching in the Cauvery basin indicates shearing or low angle rifting at the time of breakup between India–Sri Lanka and the East Antarctica. However, the additional stretching observed in the Cauvery basin region could be ascribed to the subsequent rifting of Sri Lanka from India. The abnormal thinning of crust at the OCT is interpreted as the probable zone of emplaced Proto-Oceanic Crust (POC) rocks during the breakup. The derived crustal structure along with other geophysical data further reiterates sheared nature of the southern part of the ECMI.

The Krishna-Godavari basin is a rifted passive margin basin that developed orthogonally to the NW-SE trending Pranhita-Godavari graben along the central Eastern Continental Margin of India in response to the continental rifting process and the subsequent seafloor spreading between India and eastern Antarctica during the early Cretaceous period. The 3-D gravity interpretation of both the onshore and offshore sections of the basin integrated with detailed seismic reflection and refraction data provided significant new information about the crustal architecture and the early breakup history of the basin. The gravity-derived crustal models indicate that the crust at the eastern Indian shield margin is 39-41 km thick and thins to as much as 20-23 km at the Ocean Continent Transition (OCT) in the offshore. There is a significant variation in the nature of crust and the configuration along the margin between the three crustal domains encompassing the basin: the Cuddapah basin, Pranhita-Godavari Graben, and the Eastern Ghat Mobile Belt that indicates the lateral segmentation of the margin. A zone of high density (3.0 g/cm(3)) crustal material at the OCT separates the pure continental and oceanic crusts on either side. The Moho is shallowest along this zone and brought the upper mantle notably close to the surface, which indicates that it could be comprised of Proto-Oceanic Crustal rocks. Furthermore, the present study highlights a major basement structural high in the deep offshore area of the margin. The geophysical signatures over this structure suggest that it is a crustal scale feature that is characterized by intrusive volcanic rocks, comprised of low density crust, and lies close to the OCT towards offshore. Based on the inferred tectonic reconstruction, we believe that this structural high could be a continental fragment that was left behind during the process of breakup between India and the Elan Bank. (c) 2012 Elsevier Ltd. All rights reserved.

Due to the lack of clearly discernible magnetic anomaly identifications and fracture zones in the Bay of Bengal (BOB), the early Cretaceous plate reconstruction history of eastern Gondwanaland remains an unresolved issue with regard to the separation of the East Coast of India from the conjugate Enderby Basin of Antarctica. Based on the correlation of oceanic basement information obtained from a vast amount of deep reflection seismic data, we identified 16 NW–SE-trending fracture zones (FZ1–FZ16) in the BOB region. The trends of these fracture zones at the ECMI indicate two distinctly different sets, one set in the direction ~N25°W to N35°W in the southern part (FZ1–FZ7) and the other set in ~N50°W (FZ8–FZ15) direction along the northern part. These two sets of fracture zones further support the earlier inferences about the segmented nature of the ECMI into southern sheared/oblique rifted margin and the northern normal rifted margin. While, the southern part of the ECMI had formed under slow/asymmetric spreading and mixed transform setting with its conjugate western Enderby Basin, the northern part of the ECMI formed after the breakup of the Elan Bank from India at M2. Considering this scenario, we infer the presence of Late Mesozoic (younger to M4) anomalies along the ECMI and the subdued nature of magnetic anomalies in the western BOB can be explained by the obliquity of fracture zones with respect to the coast, as observed in many marginal smooth zones of the world oceans. The disposition of the 85°E ridge with respect to the pre-evolved BOB crust and early Cretaceous volcanic episodes in the region together indicate an emplacement of the ridge from ~105Ma onwards as a result of a hotspot source related to the Kerguelen super plume during its intense eruption activity.

The Bay of Bengal (BOB) lithosphere evolved due to rifting between India and Antarctica and subsequent seafloor spreading process since the early Cretaceous is known for the presence of thick Bengal Fan sediments covering the entire BOB with maximum sediment accumulation of 22km in the offshore Bangladesh. Three-dimensional crustal structure of the BOB is investigated by integrating the sediment isopach map, seismic reflection, refraction and wide-angle reflection data with the 3-D gravity modelling. Large amount of sediment seismic velocities have been used to construct depth varying density contrast with respect to the underlying crustal layer due to deeper burial effect by a quadratic density function. 3-D gravity effect of the sedimentary layer computed for the derived density distribution shows large variations ranging from −5mGal over the Ninetyeast Ridge near the equator to as low as −210mGal at the apex of Bengal Fan in the Bangladesh shelf region. The residual crustal Bouguer anomaly map obtained after subtracting the effect of water and sediment layers and normalised for 6km uniform crust display crustal thickness variations with depth to Moho in the BOB region varying from 11km in the south to as much as 35km in the Bangladesh shelf with Moho depressed locally below both 85° E and Ninetyeast ridges. The total crust map obtained after subtracting the water and sediment layers from the Moho depth map shows an unusually thin oceanic crust of 2–4km in the deep basinal areas of BOB and 10–12km of thick crust below 85° E and Ninetyeast ridges. Due to the widespread nature of observed thin crust as well as the presence of plume traces that have emplaced 85° E and Ninetyeast ridges, we infer its formation during plume–spreading interactions in the eastern Indian Ocean. Further, higher flexural rigidity of lithosphere and cold mantle in the BOB region indicates that huge amount of Bengal Fan sediments were emplaced over the unusually thin oceanic crust and a strong mantle similar to that observed in the Amazon Fan.