Western Continental Margin Of India Research Articles

Cretaceous–Paleocene extension at the southwestern continental margin of India and opening of the Laccadive basin: constraints from geophysical data

Abstract. Previous geophysical investigations of the western continental margin of India (WCMI) confirm the two-phase breakup history of the margin with the first breakup taking place between India and Madagascar that created the Mascarene Basin in the Late Cretaceous and the second breakup event in Early Paleocene with Seychelles separating from India. Despite numerous geoscientific studies along the WCMI, the opening of the Laccadive basin, situated along the southern part of the margin, remains poorly constrained. In this study, we evaluate the multi-channel seismic reflection and gravity anomalies at the margin to identify the early rift signatures in conjunction with the magnetic anomaly identifications in the Mascarene Basin. The analysis led to the identification of two trends of extensional structures, a NNW–SSE-oriented structure over the Laccadive Ridge north of Tellicherry Arch, interpreted to result from ENE–WSW extension, and a SSW–NNE-oriented structure in the Laccadive basin region towards the south, interpreted to result from NW–SE extension. Previous plate reconstruction models of the Mascarene Basin using marine magnetic lineations suggest that the ENE–WSW extension observed over the Laccadive Ridge could be related to the India–Madagascar separation. We associate the pattern of sediment deposition and the presence of a Paleocene trap volcanics, linked with the NW–SE grabens observed in the Laccadive basin region, to the extension between the Laccadive Ridge and the western coast of India after the separation of Madagascar from India. We further propose that the anticlockwise rotation of India and the passage of the Réunion plume have facilitated the opening of the Laccadive basin.

Interpretation and origin determination of oil seepages using marine seismic data: a case study of the Western Continental Margin of India

This article discusses the experience of a joint using remote sensing data, marine seismic data and open geoscientific data to search for natural mineral oil seepages. The study was carried out as part of an assessment of oil prospects in the offshore sedimentary basins of the Arabian Sea on the west coast of the Indian Peninsula. The methodology is based on visual interpretation of natural and anthropogenic oil seepages using Sentinel-2 multispectral imageries. Modern offshore seismic data is used to establish the mineral origin of oil seepages by the presence of reservoirs and fault systems as possible sources of hydrocarbons and channels for their migration to the surface, respectively. The final map of detected oil seepages location is the result of research. Areas of natural mineral oil seepages accumulation confined to the western continental margin of India have been identified. Recommendations for exploration activities in the Kutch, Saurashtra and Kerala-Konkan offshore sedimentary basins were given. The acquired experience of visual interpretation of multispectral satellite data confirms efficiency of marine seismic data application for delineation of natural oil seepages of mineral origin in the areas of intensive navigation. The proposed methodology of research can be used as an indirect assessment of oil prospects in the Western Continental Margin of India and other areas of the world ocean.

Mineralogy, geochemistry and genesis of the polymetallic crusts from the Chagos-Laccadive Ridge, Arabian Sea, India

Recent offshore exploration by the Geological Survey of India (GSI) located ferromanganese crusts (FeMn) along the Chagos-Laccadive Ridge (CLR) offshore western continental margin of India. CLR FeMn crusts grew on phosphorite substrate rock (phosphorite cemented sand/silt with moderate carbonate fluorapatite (CFA) cement and goethite). Phosphorite precipitation is the product of intense upwelling and phosphorus regeneration due to organic matter degradation and fish debris dissolution. CLR FeMn crusts are of hydrogenetic origin precipitated in an oxic environment. As a result of hydrogenetic processes under oxic conditions, Fe-vernadite dominated the mineral phase of FeMn crusts. CLR FeMn crusts formed when the North Indian deep-water layer mixed with oxygen-depleted waters from the Arabian Sea, Persian Gulf, and Red Sea. Three genetic layers are identified in the FeMn crusts; Type-I, Type-II, and Type-III developed in response to fluctuation in dissolved oxygen during periods of intensified suboxic bottom waters and monsoon maxima associated with past climatic events in the Arabian Sea. Type-I layers are columnar showing low Mn/Fe ratio (∼1), reflectivity, Mg (average 1.24%), and Ni (average 0.29%). Dendritic Type-II layers show high Mn/Fe ratios (average 12.29), reflectivity, Mg (average 3.51%) and Ni (average 1.69%). Type-III layers show very low Mn/Fe ratios (average 0.21), reflectivity, Mg (average 1.4%) and Ni (average 0.01%) and contain a substantial amount of terrigenous debris of aeolian origin. Elements of economic interest such as Mn, Co, Ni, Cu, Mo, Li, Te, and REY in CLR FeMn crusts are depleted owing to terrigenous contribution.

Magnetotelluric imaging reveals fragmented crustal blocks beneath the Konkan plains adjoining the Western Ghats, Deccan Volcanic Province, India

The Deccan plateau and the Konkan plain of the western continental margin of India are significant morphotectonic features. Seismic activity along with regional-scale geophysical and geological studies suggests that the plateau is tectonically disturbed and the traps together with the sub-basaltic terrain have been experiencing structural deformation along pre-existing linear zones. The recent Palghar swarm activity has drawn significant attention on the crustal architecture of the Konkan plain and the adjoining Western Ghats region. Here, we present the results of a magnetotelluric study carried out along a 100 km long E-W profile traversing major tectonics features between the Konkan plain and the Bhatsa swarm activity region. Two-dimensional modelling of the distortion corrected impedance tensors reveals high resistivity crustal blocks down to 35–40 km dissected by faults/fractures. These crustal blocks are underlain by a moderately conductive mantle lithosphere which is also characterized as a seismic low velocity and low density layer. The persistent elevated topography of the Western Ghats could be explained by a buoyant, low-velocity/low resistivity upper mantle coupled with reactivation of deep crustal features during the Quaternary period. The model yields some westward dipping conductive and resistive zones coinciding with the exhumed topography highs. The west coast fault in the western and the Ghod fault in the eastern part of the profile are imaged as crustal-scale steeply dipping fault zones segmenting the Eastern Dharwar craton (EDC) crust into the EDC1 and the EDC2. The model in conjunction with earlier results from the Palghar swarm activity region suggests southward continuation of the major conductivity anomalies obtained at Palghar. This study proposes that NW extension of the Kurudwadi rift and its interaction with local structural features is possibly affecting the stress state of the region leading to local seismicity.

Phosphorite occurrences on the western continental margin of India: An update

Marine phosphorites are well known from the tropical upwelling zones along the west coast of North America, South America, and African continents. Despite having similar oceanographic and topographic set-ups, comparable scales of phosphorite occurrences have not been reported from the western margin of the Indian subcontinent. However, Seabed surveys carried out by the Geological Survey of India during the last decade unearthed phosphatic crusts and nodules from several elevated topographic domains off the west coast of India. These new findings necessitate an update to the phosphorite zone map of the present-day Ocean. On the seamounts and spurs off the west coast of India, ferruginous and non-ferruginous phosphorites occur in the form of crusts and nodules within the water depth range of 300 to 800 m. Geochemical evidence and concurrent occurrences of varied types of phosphorites within the same topographic domain point to the prevalence of multiple episodes of short-duration phosphogenesis along the western margin of India.

Surface trace of the active Katrol Hill Fault and estimation of paleo-earthquake magnitude for seismic hazard, Western India

Estimation of seismogenic potential through geological studies of faults is an important step in seismic hazard evaluation and mitigation. We investigated the Katrol Hill Fault (KHF), an E-W trending and south dipping high angle reverse fault in the seismically active Kachchh Basin at the western continental margin of India. While several faults in the eastern part of the basin are presently seismically active, the KHF is the only fault in the entire basin that is shown to have produced three Late Quaternary surface faulting events. We illustrate here an integrated geological approach using field mapping, microscopic analysis (petrography and quartz surface textures using SEM) and shallow geophysical surveys using GPR for determining the precise surface trace of the active KHF. We also determined the displacement and slip-rate for each surface faulting event. Based on the Late Quaternary surface faulting parameters deduced, we calculated the moment magnitude (Mw) of the events using well established empirical relationships. These calculations yielded Mw values consistently ≥ 6.5. Our study shows that the KHF is capable of producing high magnitude earthquakes. We infer that these surface faulting events may be of relatively shallow focus as higher magnitude earthquakes in recent times along other faults in the basin, except the 1819 Allahbund earthquake, did not produce surface rupture. We recommend a revision of seismic hazard risk and mitigation strategies of the region as a consequence of our findings.

Geodynamic events leading to formation of passive western continental margin of India

The geodynamic events of continental breakup and origin of northwest Indian Ocean led to the development of passive continental margin, off western India. However, causal mechanisms and relative chronology of these geodynamic events are not clearly known because of complex regional-scale ridges-basin physiography, multi-stage rifting in a short-time span and thick sediment cover. The Laxmi and adjacent Gop basins constitute key tectonic elements and geophysical investigations on them have come up with sharply divergent explanations of continental rifting and ocean spreading. We present geochemical results of the Laxmi Basin (LB) basement, recovered by the International Ocean Discovery Program Expedition-355 and interpreted in light of existing geophysical results. The basement is identified as continental rift basalt, different from the Deccan/Madagascan basalts. We suggest the basement eruption at ~75 Ma causing igneous underplating which triggered the extension/rifting in Laxmi and Gop basins. The rifting translated into ocean spreading only in the Gop Basin and not in the Laxmi Basin. The geodynamic events echoed soon with similar relative chronology in western India with Reunion plume impact and the Deccan eruption followed by second extension/rifting that culminated in India-Seychelles breakup.

Holocene paleoenvironmental changes in the marginal marine basin of Great Rann of Kachchh, western India: Insights from sedimentological and mineral magnetic studies on a ∼60 m long core

We carried out sedimentological and mineral magnetic studies on a ∼60 m long core recovered from the central part of the Great Rann of Kachchh (GRK), a marginal marine basin, located on the western continental margin of India to understand the Holocene paleoenvironmental changes. We critically analysed the sedimentation pattern, sediment characteristics along with paleoclimatic signatures of global and regional relevance. In GRK basin, the sedimentation started in early Holocene (∼10.6–9.3 kyr BP) under rapidly rising post glacial sea level with very high sedimentation rate (8.71 cm/yr to 2.37 cm/yr) which is also seen in several marginal marine basins across the globe. The abundant laminations, coarse grained sediment flux and environmental magnetic parameters (χlf, S-ratio) suggests that the sedimentation occurred in developing monsoonal conditions under wetter yet fluctuating climate during this period. After ∼9 kyr BP, the environmental magnetic proxies show transition toward relatively wetter condition with peak at ∼6.5 kyr BP. This wetter climatic phase on the other hand corroborates with reduction of sedimentation rate to 0.46 cm/yr. This reduction in sedimentation rate under strengthening monsoonal conditions probably occurred due to rapid filling up of the basin on account of earlier high sedimentation in the basin. The sedimentation rate is reduced significantly after the onset of aridity at ∼4 kyr BP except a minor wet spell recorded during ∼1.5 to 1 kyr BP before drying of the GRK basin.

Phylogenetic diversity of culturable marine bacteria from sediments underlying the oxygen minimum zone of the Arabian Sea and their role in nitrate reduction

AbstractThe eastern Arabian Sea has a unique and permanent oxygen minimum zone (OMZ) extending along the western continental margin of India. The Arabian Sea OMZ is a site of intense denitrification, contributing to 20% of the global oceanic denitrification, playing a significant role in the nitrogen cycle. This study reveals the members of the culturable marine bacterial communities thriving and functioning in the sediment underlying the Arabian Sea OMZ, through 16S rDNA phylogenetic analysis and elucidating their nitrate reduction potential. Phylogenetic analyses revealed that these cultured bacteria distributed into five phyla, viz. Firmicutes (45.7%) Gammaproteobacteria (36.84%), Actinobacteria (8.7%), Bacteroidetes (5.26%), and Alphaproteobacteria (3.5%). The presence of denitrification genes such as narG, nirS, and nosZ in the cultured bacteria (39%) explains their importance in this process. Present study on the diversity and distribution of bacterial communities in the OMZ surface sediment and their probable role in the nitrogen cycle will provide insight into the microbial biogeochemical processes occurring in this region.

Provenance of Cenozoic Indus Fan Sediments (IODP Sites U1456 and U1457)

ABSTRACTProvenance analysis of IODP Expedition 355 cores in the Laxmi Basin sheds new light on the erosional evolution of the Himalayan belt and its western syntaxis during the Neogene and on large-scale mass-wasting and magmatic events that affected the western continental margin of India in the mid-Miocene and early Paleocene. In the cored Laxmi Basin succession, heavy minerals are far less affected by selective diagenetic dissolution than in foreland-basin sandstones exposed along the Himalayan front. Occurrence of euhedral aegirine and apatite in lower Paleocene mudrocks can be tied to alkaline volcanism affecting the adjacent western Indian margin during the late stage of Deccan activity. In the mid-Miocene Nataraja Slide (the second-largest mass-transport deposit reported from passive margins worldwide), dominant carbonate detritus and depleted heavy-mineral suites (including apatite, garnet, and locally augite or rare aegirine) reveal gravitational failure and sliding of the entire succession of carbonate and siliciclastic Paleogene to lower Neogene strata originally accumulated offshore of the Saurashtra margin of western India. Contrary to previous inferences, reworking of Indus-derived detritus by the slide was negligible. The overlying upper Miocene to lower Pleistocene turbidite package has the same feldspatho-litho-quartzose to litho-feldspatho-quartzose signature of modern Indus fluvio-deltaic sand, indicating that amphibolite-facies metamorphic rocks have been widely exposed in the Himalaya–Karakorum orogen since at least the mid-Miocene. Pleistocene nannofossil oozes with planktonic foraminifera at the top of the fan contain a very subordinate litho-feldspatho-quartzose terrigenous fraction including augitic clinopyroxene, suggesting mixing of dominant biogenic debris with minor detritus contributed both by the Indus River and by a river draining western peninsular India, possibly the paleo-Narmada or the paleo-Tapti.

Structural fabric over the seismically active Kachchh rift basin, India: insight from world gravity model 2012

The Kachchh rift basin (KRB), located in the western continental margin of India, is very important due to repeated large magnitude occurrence of intraplate earthquakes. The present study elucidates the subsurface structural features using wavelength filtering, total horizontal derivative (THD) and 2.5 dimensional (2.5D) density modelling of the gravity data. Power spectrum analysis of the gravity data gives the cut-off wavelength of 100 km that has been used for wavelength filtering for regional-residual separation. The residual gravity anomaly significantly enhances the shallow features (like Allah-Bund fault, Kachchh mainland uplift, Wagad uplift, and Pachham uplift) and gives an NW–SE subsurface structural trend. The predominant direction for major lineaments derived by the THD of gravity data shows the ENE–WSW as well as NE–SW and NW–SE directions. The 2.5D density model of the regional gravity anomaly reveals the presence of high-density magmatic underplating layer and intrusive mafic body in the lower crust that is in accordance with the previous studies. The regional slope of the Moho in the KRB is towards the SW direction. An intrusive mafic body in the lower crust and volcanic plugs in the upper crust act as stress concentrator which is responsible for seismicity in the KRB. Several new lineaments have been identified based on the present study, which may be accountable for seismicity over the different part of KRB.

Tracing multiple sources of sediments using trace element and Nd isotope geochemistry: provenance of the Mesozoic succession in the Kutch Basin, western India

AbstractAn integrated approach involving Sr–Nd isotope, trace and rare earth element analyses tracks multiple sources of the Mesozoic sediments of the Kutch Basin at the western continental margin of India. High (87Sr/86Sr)t (ratio at time of deposition), negative εNd and high concentrations of large-ion lithophile elements (LILEs) indicate the upper continental source. Ratios of Nb/Ta and Zr/Hf suggest sedimentary and felsic igneous sources of sediments. The moderate to high concentration of La, Th and Sc, light rare earth elements (LREE-) enrichment, weak negative Eu anomalies and the relationship between εNd(0) and Th/Sc indicate the dominantly felsic composition of source rocks. However, low contents of Th, low values of (87Sr/86Sr)t and depleted mantle model age TDM < 1600 Ma indicate input from a younger mafic source. Increasing concentrations of Zr, Hf and Nd isotopes and a gradual increase in mean TDM from the older to the younger formations indicate erosional unroofing at the source terrain. The increasing (87Sr/86Sr)t through time relates to increased weathering of the source rock. The overwhelmingly southwesterly palaeocurrent direction of current-generated sedimentary structures, and the mean TDM ages trace suggest source areas of the Kutch Basin to Precambrian rocks in the north and NE of this basin. The TDM ages highlight the dominance of late Palaeoproterozoic source rocks. Nd isotope composition indicates that Proterozoic rocks of Marwar Supergroup and Erinpura Granite, in particular, served as main sediment contributors for the Mesozoic sediments in Kutch. We therefore conclude that the Mesozoic sediments in the Kutch Basin are predominantly of late Palaeoproterozoic age with lesser inputs from rocks of early Mesoproterozoic and early Palaeoproterozoic age.

A local magnitude scale ML for the Saurashtra horst: An active intraplate region, Gujarat, India

The calibration of local magnitude scale to local tectonics is essential for seismic hazard assessment and quantifying the seismicity in active regions. In the present study, we have developed a local magnitude scale ML, for the Saurashtra region, which is a horst, located in the western continental margin of India. The local magnitude scale is developed using 1968 amplitude measurements from horizontal component recordings of 319 earthquakes, obtained from sites in the Saurashtra region, with hypocentral distances ranging from 3 to 298 km. All the 1968 amplitude measurements were inverted simultaneously to determine attenuation curve, magnitudes and station corrections for the studied region. The resultant distance correction term for the Saurashtra is −log(A0) = 1.31 log(r/100) + 0.0002 (r − 100) + 3 for 100 km normalization, where A0 is the distance correction and r is the hypocentral distance. The distance correction term (−log A0) suggests that the attenuation in the Saurashtra region is lower as compared to neighbouring Kachchh region. The station corrections obtained in the present study varies from −0.31 to +0.24. Overall, standard deviation of the magnitude residuals without station correction is 0.28, while with station correction, it is 0.23, which indicates that applying station correction reduces the variance by 31% and brings the average residual closer to zero.

Implications for the lithospheric structure of Cambay rift zone, western India: Inferences from a magnetotelluric study

Broad-band and long-period magnetotelluric (MT) data were acquired along an east-west trending traverse of nearly 200 ​km across the Kachchh, Cambay rift basins, and Aravalli-Delhi fold belt (ADFB), western India. The regional strike analysis of MT data indicated an approximate N59°E geoelectric strike direction under the traverse and it is in fair agreement with the predominant geological strike in the study area. The decomposed transverse electric (TE)- and transverse magnetic (TM)-data modes were inverted using a nonlinear conjugate gradient algorithm to image the electrical lithospheric structure across the Cambay rift basin and its surrounding regions. These studies show a thick (~1–5 ​km) layer of conductive Tertiary–Mesozoic sediments beneath the Kachchh and Cambay rift basins. The resistive blocks indicate presence of basic/ultrabasic volcanic intrusives, depleted mantle lithosphere, and different Precambrian structural units. The crustal conductor delineated within the ADFB indicates the presence of fluids within the fault zones, sulfide mineralization within polyphase metamorphic rocks, and/or Aravalli-Delhi sediments/metasediments. The observed conductive anomalies beneath the Cambay rift basin indicate the presence of basaltic underplating, volatile (CO2, H2O) enriched melts and channelization of melt fractions/fluids into crustal depths that occurred due to plume–lithosphere interactions. The variations in electrical resistivity observed across the profile indicate that the impact of Reunion plume on lithospheric structures of the Cambay rift basin is more dominant at western continental margin of India (WCMI) and thus support the hypothesis proposed by Campbell & Griffiths about the plume–lithosphere interactions.

Detrital zircon and monazite track the source of Mesozoic sediments in Kutch to rocks of Late Neoproterozoic and Early Palaeozoic orogenies in northern India

Detrital zircon and monazite dating of clastic rocks in the Mesozoic Kutch Basin at the western continental margin of India reveals predominant sediment derivation from rocks of Neoproterozoic Pan-African orogeny, followed by those of Cambro-Ordovician Bhimphedian (or Kurgiakh) orogeny and 850–1000 Ma rocks, with subordinate input from rocks of 700–800 Ma, 1500–1600 Ma, 2400–2500 Ma and 2800–3300 Ma. This finding refutes the existing idea regarding the predominant Mesoproterozoic source inferred for this basin. The dominance of southwesterly palaeocurrent data of Mesozoic rocks in Kutch Basin rules out sediment supply from south or west. Th/U ratios of detrital zircon grains indicate predominantly magmatic and subordinately metamorphic source rock. Petrographic data, particularly the QFR plot supports this interpretation of source rock. Rocks belonging to the Pan-African orogeny are poorly exposed in northwestern India while isolated outcrops of peralkaline granites in the Himalayan region bear testimony of the Bhimphedian orogeny. While the paucity of records of the Pan-African orogeny in western India possibly relates to either burial under the Deccan Flood Basalts or extensive erosion during Mesozoic greenhouse climate, the dearth of rocks of Bhimphedian orogeny results from its occurrence along the present-day Himalayan thrust belt. The absence of detrital zircon grains younger than 458 Ma indicates that post-Ordovician tectono-thermal events skipped the source area. The large gap between youngest detrital zircon and the depositional age of the Mesozoic sediments, suggests long-distance sediment transport as well as sediment recycling. This study, therefore, indicates the existence of widespread younger magmatic rocks to the north during the deposition of Mesozoic of Kutch.

Uplift induced structurally controlled landscape development: example from fault bounded Jumara and Jara domes in Northern Hill Range, Kachchh, Western India

The Kachchh palaeo-rift graben at the western continental margin of India is currently undergoing active coseismic deformation in response to the periodic release of NE-SW oriented compressive stresses along the E-W trending intra-basinal faults. The seismically active Kachchh Mainland Fault (KMF) is the largest intra-basinal fault that is characterized by a narrow zone comprising a chain of domes and anticlines forming the rugged hilly topography of the Northern Hill Range (NHR) in the upthrown block. The present paper deals with the Jumara and Jara domes which have received considerable attention with respect to the Mesozoic biostratigraphy, however, no study has been carried out on geomorphological and neotectonic aspects. The major geomorphological units in the area include a low relief but rugged structurally controlled topography over the domes, the KMF scarp, the precipitous Jaramara scarp, the back slope over the Jaramara scarp and the rugged hilly topography over the large Ukra intrusive body that shows a complicated structural contact relation with the Mesozoic rocks ranging from concordant to discordant. Our detailed field studies supplemented by geomorphological analysis using DEM, topographical profiles, drainage characteristics and longitudinal profiles reveal the dominant control of sustained uplift along the KMF under compression in the development of the present youthful and structurally controlled landscape. Our study also suggests that the imposing E-W trending Jaramara scarp is formed due to the long-term retreat of the KMF scarp in response to uplift, while the present low relief scarp along the KMF is largely the result of post-Miocene uplift. The incision in Quaternary miliolite deposits in the Jara river gorge, deeply incising nature of the rivers, several large and small knickpoints and scanty occurrence of Quaternary miliolite deposits indicate an uplift of ~25 m during the Late Quaternary time.

Extension of flood basalt on the northwestern continental margin of India

The Deccan Continental Flood Basalt (DCFB) in central western India is a large igneous province that covers almost one-sixth area of India. It is erupted in less than 1 million years during the magnetic chron 29 r ( $$\sim $$ 65.6–64.8 Ma) through the Cretaceous–Tertiary boundary. It is believed that nearly an equivalent area of flood basalt, adjoining the DCFB, is submerged on the northwestern continental margin of India (NWCMI). Onshore information on the DCFB is widely reported in geological and geophysical studies. However, knowledge on the offshore extent of flood basalt is poorly known because of paucity of marine geophysical data and lack of an appropriate approach to determine the flood basalt. To fill this gap in knowledge, P-wave velocity in flood basalt as a proxy, drilled wells results, and published seaward dipping reflectors are used to delineate flood basalt extent on the NWCMI. The results of the study reveal that P-wave velocity in the flood basalt varies from 4.1 to 5.2 km/s. Apart from some isolated basement-high features, it is found that flood basalt lies below sediment and carpets the entire NWCMI extending up to the Laxmi–Laccadive ridges. The depth of occurrence of flood basalt ranges from 800 to 7400 m, with a maximum thickness of $$\sim $$ 3900 m in the eastern part of the Laxmi basin. The article presents preliminary results about the extension of flood basalt on the western continental margin of India which may be useful for researchers and Indian oil industries planning exploration activities in Mesozoic sediment of the margin for hydrocarbon prospects.

Integrated geophysical appraisal of crustal structure and tectonic evolution of the Angria Bank, western continental margin of India

Angria Bank, a submerged plateau with coral reefs, is located off the central-west coast of India. Biologically, the bank attracts scientific community for its coral formations. But geologically, origin and tectonic setting of this feature are not established yet due to scarcity of geological and geophysical data. Newly acquired bathymetry and magnetic data along with existing geophysical data have allowed us to determine crustal structure and tectonic evolution of the Angria Bank. The entire geological setup of the bank consists of 34 km long water plateau (average water depth ~ 20 m), and two prominent spurs extending to its western side. The area of water plateau and entire geological setup are 365 km2 and 1460 km2, respectively. The bank is characterized by two prominent magnetic low with a relative high in the middle. Integrated gravity and magnetic models revealed that flood basalt has carpeted the entire geological structure which later acted as a foundation for coral growth. The crust below the bank is continental in nature and underplated by high-density magmatic material. Moho is almost flat and lies at a depth of ~ 27 km. The results of 3D Euler deconvolution suggest that the study area is characterized by two types of linear trend viz. NE–SW to NNE–SSW and NW–SE. These trends are interpreted mostly as basement faults but at few places, they might be associated with sills/dykes. The geological setup of the bank is fault-bounded and comprised of two horst structure (interpreted as spurs) trending NE–SW to NNE–SSW. Integrated interpretation of the geophysical data revealed that the Angria Bank is an isolated feature, evolved during rifting between India and Seychelles-Laxmi Ridge in the Late Cretaceous.

A Hybrid Multistage Model of Evolution of the Western Ghats at the Passive Western Continental Margin of India

The Western Ghats (WG) is one of the Great Escarpments of the world that developed and persisted for the last ~60 Ma at the passive Western Continental Margin of India (WCMI). Like many such escarpments, the origin and persistence of the WG cannot be explained by a single mechanism, either lithosphere-asthenosphere or surficial processes, and hence, these are debated. Here I suggest a unified multistage model for the evolution of WG based on the available geophysical and geological data, and link its origin and persistence through possible underplating and detachmentcontrolled mid-crustal ductile flow with the characteristics of the Deccan plateau and the offshore region. The implications of the model for lithosphere-asthenosphere dynamics of the Deccan plateau and Stable Continental Region (SCR) seismicity are discussed.

Chromium speciation in the sediments across the oxygen minimum zone, western continental margin of India

This study suggests that varying concentration of dissolved oxygen of overlying bottom water influences geochemical fractionation, speciation, and oxidation state of sedimentary Cr in a marine system. The nature of sedimentary organic matter (labile or nonlabile) and Fe speciation also controls the geochemical fractionation and oxidation state of Cr in the sediment from the continental margin across the oxygen minimum zone of the Arabian Sea. Increasing concentration of sedimentary organic matter (under hypoxic conditions) increased association of Cr with sedimentary organic binding phases. The association of Cr with Fe‐Mn‐oxyhydroxide phase gradually decreased with decreasing dissolved oxygen concentration due to the reduction and dissolution of Fe (III). The sedimentary organic matter was found to be the major hosting phase for Cr (VI) in the shelf sediments. The decrease in sedimentary Fe (III) concentration under hypoxic conditions also prevented Cr (VI) reduction. This study suggests that the type of sedimentary organic matter and Fe (III) concentration may influence benthic exchange fluxes of Cr under variable redox conditions in many coastal/margin systems.