Eastern Continental Margin Of India Research Articles

Geochemistry of volcanic glass from Mahanadi offshore region, eastern continental margin of India: Constraints on the contribution of latest Toba super-eruption

Geochemistry of volcanic glass from Mahanadi offshore region, eastern continental margin of India: Constraints on the contribution of latest Toba super-eruption

Gravity-magnetic appraisal of the southern part of the Cauvery Basin, Eastern Continental Margin of India (ECMI): evidence of a volcanic rifted margin

The Cauvery sedimentary basin of India, a part of the Eastern Continental Margin of India (ECMI), has evolved as a consequence of its breakup from East Antarctica in the early Cretaceous. This study covers approximately 10,000 km2 in the southern part of the Cauvery basin. A total of 5,161 gravity-magnetic observations were made in an area bounded by 78°–79°E longitude and 9°–10°N latitude. The gravity anomaly map shows a dominant NE-SW trend with a total relief of 83 mGal from −77 mGal to 6 mGal. The map reveals very prominent NE-SW trending linear gravity high bands and two circular/elliptical gravity lows around Madurai and Ramanathapuram, respectively. The residual gravity map reveals NE-SW trending alternative bands of gravity highs and lows, revealing a ridge-depression structure. The gravity modeling shows the presence of underplated material, which may be related to magmatic activity during the Cretaceous. The magnetic map reveals two prominent E-W trending linear structures, which probably represent the disposition of a shear zone under the Cauvery Basin. Based on the filtered magnetic and gravity map, we have prepared an interpreted basement geology map. The magnetic map and residual gravity map suggest that the basement rock of the basin is crystalline (granite/gneiss) and may not comprise a high-grade metamorphic rock Eastern Ghat Orogeny.

Ecological preferences of living benthic foraminifera from the Mahanadi river-dominated north-western Bay of Bengal: A potential environmental impact assessment tool

The ecological preferences of living benthic foraminifera from the riverine influx dominated eastern continental margin of India have been studied. The living benthic foraminifera were abundant on the upper slope (~100–700 m). Three distinct species assemblages were identified. Assemblage 1 (Ammonia beccari, Pseudononion costiferum, Hanzawaia nipponica, Bolivina frondalis, Bolivina dilatata, Bolivina striatula, Asterorotalia milletti) representing warmer, well-oxygenated water and coarse substrate with low organic carbon is restricted to the inner shelf. Assemblage 2 includes species (Verneuilinulla propinqua, Ammodiscus incertus, Buliminella dubia, Rotaliatinopsis semiinvoluta, Bolivina lowmani, Fursenkoina spinosa) thriving in the regions with abundant food availability and low oxygen concentration on the continental slope. The species in Assemblage 3 (Globocassidulina subglobosa, Epistominella exigua, Gyroidinoides subzelandica, Reophax longicollis, Adercotryma glomeratum, Cystammina pauciloculata, Spiroplectammina biformis) prefer deeper waters with moderate organic carbon and dissolved oxygen. The information of the ecological niches of benthic foraminifera will help in environmental impact assessment and paleoecological studies.

India‐Elan Bank‐East Antarctica Breakup, Crustal Architecture, and Margin Evolution: Results From Constrained Potential Field and Process‐Oriented Gravity Modeling of Conjugate Margin Segments

AbstractWe present here the results of a seismically well‐constrained potential field modeling along with process‐oriented gravity modeling across the Eastern Continental Margin of India (ECMI) and its conjugate East Antarctica Continental Margin (EACM). The study revealed both along‐strike variations in crustal architecture and effective elastic thickness (Te), showing a locally asymmetric pattern. Crustal stretching factors (β) estimated at these margins range between 1.5 and 4.0 with slightly lower values varying over the narrow zone in the Cauvery basin‐western Enderby basin and relatively higher values in other margin segments. The models reveal typical magma‐poor margin characteristics in the Krishna‐Godavari (K‐G) and Cauvery basin segments of ECMI. The ocean‐continent transition (OCT) in the K‐G basin and its conjugate (central Enderby basin) indicates asymmetric rifting marked by the development of a detachment fault separating the combined India‐Elan Bank margin and the EACM. The higher β values and magmatic underplating at OCT in the Mahanadi basin segment of ECMI may be related to the post‐breakup magmatic emplacement associated with the proximal activity of the Kerguelen plume after the separation of Elan Bank from India by a ridge jump during M2/M0 (124–120 Ma). The pre‐rift reconstruction model through the palinspastic restoration of margins suggests that the central part of ECMI (16–20°) is the ideal pre‐rift location for the Elan bank.

An ANN-based identification of geological features using multi-attributes: a case study from Krishna-Godavari basin, India

Artificial neural network (ANN)–based workflow has been applied to the multi-channel seismic reflection data from the Krishna-Godavari (K-G) basin on the Eastern continental margin of India for delineation of prominent structural features such as chimneys and faults. To eliminate the noise and enhance the data quality, the seismic data is conditioned and filtered by data-conditioning techniques such as dip-steered and structural filters. A single attribute is not sufficient for the delineation of structures such as faults and chimneys; hence, an amalgamation of multiple attributes is necessary for the enhancement of laterally continuous seismic events. Hence, multi-attributes such as dip variance, curvature, coherency, energy, similarity, instantaneous frequency, and frequency washout ratio are extracted from the conditioned data which were combined and fed as inputs for the ANN for the chimney and fault detection. The neural network is trained at the identified chimney/non-chimney and fault/non-fault locations, which generates a final output, namely chimney probability attribute and a fault probability attribute, revealing an improved visibility of faults and chimneys in the seismic data. This multi-attribute approach shows more reliability in comparison with individual attribute responses, which helps in better structural interpretation. The study presents a workflow for better visualization of the subsurface structural features which thereby helps in detailed structural interpretation of the seismic data.

Widespread occurrence of methane seeps in deep-water regions of Krishna-Godavari basin, Bay of Bengal

Drilling/coring activities during NGHP-Expeditions 01 and 02 discovered significant gas hydrate deposits in the Krishna-Godavari (KG) basin, a proven petroliferous basin located along the eastern continental margin of India. Active and paleo-cold seeps are also reported from deep waters (>1000 m), indicating methane venting episodes in the KG basin. In the present study, we analyzed geophysical data acquired onboard RV Sindhu Sadhana to study the spatial distribution of gas flares in the KG offshore basin. Twenty-two gas flares were observed in water-column images at seafloor depths ranging from 638 m to 1960 m. All flares except one are detected within the gas hydrate stability zone (GHSZ), which occurs beyond 700–720 m water depths. Most gas flares seem to terminate at these depths, indicating that the bubbles might be coated with a thin methane hydrate skin. Seabed sampling of flare regions shows the presence of chemosynthetic communities and shallow gas hydrates within 2–4 mbsf.The spatial distribution of gas flares is not random and tends to align with the toe-thrust zones and shale diapiric mounds. These compressive structures formed due to shale tectonism in the KG basin have provided an environment conducive for the formation of gas hydrate deposits and gas flares/cold seeps. The analysis of high-resolution seismic data across the flares shows the presence of seismic chimneys and faults, which facilitated migration of deep-seated fluid/gas through the GHSZ. Focused fluid flow along the fault zone and the perturbation of methane + seawater phase curve due to salinity increase owing to the continuous formation of methane hydrate are the most likely mechanisms to explain methane gas migration.

Crustal architecture and rift tectonics across the Visakhapatnam Bay basin, central-east Indian margin: Insights from multichannel seismic and potential field data

The Visakhapatnam Bay (VB) basin is a passive margin rift basin located at the non-basinal segment of the Eastern Continental Margin of India (ECMI) and formed during the rift–drift events associated with the breakup of eastern Gondwanaland. In the present study, integrated analysis of potential field and multi-channel seismic reflection (MCS) data were carried out to understand the rift tectonics, crustal configuration and onshore–offshore structural continuity across this basin. The study revealed the following: (i) crustal models derived through joint gravity-magnetic modelling show limited stretching with 36–40 thick crust below the Eastern Ghat Mobile Belt (EGMB) thinning down to 16–20 km at the Ocean Continent Transition (OCT), (ii) extension of Charnockitic basement associated with the Eastern Ghat Mobile Belt (EGMB) into the offshore region. Comparison of the crustal configuration across the VB basin with that across the adjacent thick sedimentary area of the Krishna–Godavari shows that upper crustal configuration is significantly different in the VB area. The observed limited rift related structuration (horst-graben morphology), the mapped high angle break-away fault with large offset in the seismic data and narrow width (70–90 km) of extended crustal domain in VB basin suggests that this segment acts as transfer zone between Krishna–Godavari and Mahanadi rift zones. Further, long curve-linear trend of magnetic anomalies associated with Pudimadaka Lineament (PKL) demarcate the VB basin from the adjacent Krishna–Godavari rift zone.

Bacterial Diversity in the Metal-Rich Terrestrial Deep Subsurface Sediments of Krishna Godavari Basin, India

Krishna Godavari (KG) basin, located in the eastern continental margin of India, is a geological region well known for the abundance of economically important minerals. However, less is known about the microbial ecology of its subsurface sediments. The present study is the first report on the comprehensive culture-independent census of bacterial communities of deep subsurface of KG basin and their relationship with the geochemical environment. Elemental and mineralogical characterization of the sediments highlighted the presence of carbon and nitrogen deprived conditions along with the abundance of metalliferous minerals, especially rich in valuable elements like zirconium, vanadium, cesium, and rare earth elements. Diversity analysis based on Illumina MiSeq high-throughput sequencing platform revealed the predominance of Firmicutes (44.24%), Proteobacteria (34.17%), Bacteroidetes (15.18%), and Actinobacteria (3.81%) in the deep subsurface of this basin. ‘Abundant’ and ‘rare’ sub-communities analysis indicated that a large number of phyla like Acidobacteria, Armatimonadetes, Chloroflexi, and Deinococcus-Thermus were exclusively present as a rare community. Statistical analyses demonstrated that geochemical parameters, especially depth, pH, and metal content, showed significant influence on the microbial community structure. The present study should help future investigations for microbial mediated sustainable utilization of mineral-rich sediments of the region.

Geophysical evidence for marine prolongation of the Palghat-Cauvery shear system into the offshore Cauvery basin, Eastern continental margin of India

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.

India National Gas Hydrate Program Expedition 02 Summary of Scientific Results: Gas hydrate systems along the eastern continental margin of India

Abstract The primary objectives of the India National Gas Hydrate Program Expedition 02 (NGHP-02) were to obtain new data on the occurrence of gas hydrate systems and to advance the understanding of the controls on the formation of gas hydrate accumulations in the offshore of India. In accordance with the ultimate overall goal of the NGHP effort to assess the energy resource potential of marine gas hydrates in India, particular focus was placed on the exploration and evaluation of gas hydrate occurrences at high saturations in sand-rich systems. NGHP-02 operations were conducted from 3-March-2015 to 28-July-2015 off the eastern coast of India and included logging while drilling (LWD) operations at 25 locations, and coring and wireline logging operations at 10 locations, in the Krishna-Godavari and Mahanadi Basins. The formation of highly concentrated gas hydrate accumulations, which are more suitable for energy extraction, requires the presence of relatively coarse-grained sediments with porosity needed to support the migration and accumulation of gas, and the nucleation of gas hydrate. The results of downhole logging, coring and formation pressure testing operations during NGHP-02 have confirmed the presence of extensive sand-rich depositional systems throughout the deepwater portions of the Krishna-Godavari and Mahanadi Basins. Two areas of the Krishna-Godavari Basin, referred to as Areas B and C, contain substantial gas hydrate accumulations in sand-rich systems and therefore represent ideal candidate sites for future gas hydrate production testing. This summary and technical report includes a comprehensive synthesis of the geologic, geophysical, geochemical, and physical property data acquired during NGHP-02 as it relates to the controls on gas hydrate occurrence, particularly with regards to sand-hosted accumulations. In the Mahanadi Basin, despite the confirmation of extensive reservoir capacity, gas supply at the NGHP-02 sites was insufficient to charge the reservoirs with gas hydrates. In the Krishna-Godavari Basin, extensive reservoir systems were confirmed with sediment grain-sizes ranging from coarse-silts to gravels. These reservoirs range from fully- to partially-filled with gas hydrate. The gas is determined to be from only microbial sources, and in part migrated into the reservoirs from deeper systems. The controls on gas hydrate occurrence are complex and varied; and include substantial reservoir heterogeneity and sufficient permeability throughout the reservoirs and seals that allowed pervasive fluid flow into and through the hydrate-bearing systems. These discoveries are the most significant confirmation of the exploration approach that focuses on direct detection of hydrate reservoirs supported by comprehensive petroleum systems analyses.

Multi-channel 2D seismic constraints on pore pressure- and vertical stress-related gas hydrate in the deep offshore of the Mahanadi Basin, India

Estimation of pore pressure and in-situ vertical stress magnitude is essential for understanding the geomechanical behavior of the gas hydrate sediments in deep offshore of the Mahanadi basin. The basin located at northern side of eastern continental margin of India (ECMI) contains gas hydrate in clay/silt sediments. The pore pressure and vertical stress are mapped on two 2D-multi channel seismic data of lines (MH-38A and MH-38B) with aid of information of three wells (namely NGHP-01-19, NGHP-01-09 and NGHP-01-08). Initially, the coefficients of best fit curve have been computed from velocity-effective stress plot for the individual wells and applied on the seismic velocity to transform into the effective stress. The vertical stress has been computed from both the seismic and log data. Therefore, the pore pressure is predicted by subtracting the effective stress from the vertical stress. The pore pressure have been mapped in gas hydrate stability zone (GHSZ) and sediments below bottom simulating reflector (BSR). The pore pressure and vertical stress gradient are 10.11 MPa/km and 10.67 MPa/km, respectively. The pore pressure and vertical stress from seismic data are closely matched at well location with excellent goodness of fit (R2) varying from 0.82 to 0.95. Normal pressure is observed in the gas hydrate bearing sediments but slightly high pressures are noticed below the BSR indicating presence of free gas. The pore pressure from seismic data will guide drillers for choosing the mud weight during well drilling and casing optimizing in other part of deep offshore in the Mahanadi basin.

Crustal architecture and Moho topography beneath the eastern Indian and Bangladesh margins – new insights on rift evolution and the continent–ocean boundary

The thick sedimentary cover (≤18 km) along the Eastern Continental Margin of India (ECMI) and over the Bay of Bengal has hindered understanding of the mechanisms of rifting that operated on the eastern Indian and Bangladesh margins. Analysis of multichannel seismic reflection data, together with 3D gravity inversion and 2D gravity forward modelling, illuminates basement configuration, crustal thickness and Moho topography, thereby revealing the modes of rift evolution and the location of the continent–ocean boundary (COB) along these margins. The basement, particularly in the shelf and slope regions of eastern peninsular India, is traversed by nearly coast-perpendicular graben, and their seaward continuity is delineated up to 125 km from the coastline. Three different types of continental margin are present along the ECMI: (1) sheared rift on the southern segment of the ECMI, up to 14° N latitude (offshore the Southern Granulite Terrain); (2) hyper-extended rift in the central segment between 14° N and 17° N (offshore the Dharwar Craton); and (3) hypo-extended rift on the northern segment between 17° N and 20° N (offshore the Eastern Ghats Mobile Belt). The graben terminations, crustal thickness and Moho topography of both the ECMI and offshore Bengal Basin clearly suggest that the COB runs nearly parallel to the coastline of peninsular India, but takes an orthogonal turn offshore in the Bengal Basin and connects the palaeo-continental shelf and the Rajmahal–Sylhet Line onshore in the Bengal Basin. Crustal thickness (8–10 km) and depth to the Moho discontinuity (≤28 km) beneath the Bangladesh margin are unusual because the crustal thickness increased through interaction of the Kerguelen plume with the existing oceanic lithosphere. The Moho deepened largely due to the load of the Bengal Fan sediments. The absence of rifted crustal blocks on the Bangladesh margin and the continuity of the COB into the onshore Bengal Basin together imply that the present Bangladesh region was under marine conditions at least until the beginning of Bengal Fan sedimentation (i.e. no less than 23 Ma).

Preface: Marine gas hydrate reservoir systems along the Eastern Continental Margin of India: Results of the National Gas Hydrate Program Expedition 02

• This “Preface” is the fifth in a series of JMPG supported gas hydrate expedition related special issues. • This “Preface” reviews the contents and organization of the papers included in this Special Issue. • This “Preface” acknowledges the contributions made to this special issue.

Estimate of gas hydrate saturations in the Krishna-Godavari basin, eastern continental margin of India, results of expedition NGHP-02

Abstract A total of 25 drill sites were established in the Krishna-Godavari (KG) and Mahanadi Basins along the eastern margin of India during the India National Gas Hydrate Program Expedition 02 (NGHP-02) with the goal to identify and characterize gas hydrate occurrences in sand-rich reservoir systems. The objective of this study is to characterize the gas hydrate bearing zones linked with two seismic inferred horizons (R1 and R2) and estimate gas hydrate saturations at two sites in Area B of the KG Basin: Sites NGHP-02-22 and NGHP-02-23. The upper seismic-imaged reservoir section (reflector R1) contains both pore-filling and fracture-filling gas hydrate, while the lower seismic-imaged reservoir section (reflector R2) has only pore-filling gas hydrate. Since electrical resistivity seems to be the most pronounced physical response to the presence of gas hydrate, we have analyzed the resistivity log data using Archie's empirical equation from two sites based on an isotropic reservoir model. In order to assess the gas hydrate saturation, we have also performed isotropic velocity modelling using the three-phase Biot equation (TPBE) and density log derived sediment porosities. At Site NGHP-02-22, the average gas hydrate saturation estimated using the resistivity log data and P-wave velocity modelling are 29% and 17%, respectively in the depth interval of 207–290 mbsf whereas at Site NGHP-02-23, the average gas hydrate saturation obtained from resistivity log data and P-wave velocity modelling are 25% and 60%, respectively in the depth interval of 271–288 mbsf. Estimated results are further validated with the pressure core derived gas hydrate saturations to ensure their reliability at the respective drill sites. It was found that P-wave velocity log-inferred gas hydrates show a slight mismatch with the pressure core derived hydrate saturations while, the electrical resistivity log based result depicts a major mismatch with the pressure core derived hydrate saturations. The reason for this discrepancy is likely due to isotropic resistivity analysis of the reservoir section having anisotropic properties.

Gas hydrate reservoir identification, delineation, and characterization in the Krishna-Godavari basin using subsurface geologic and geophysical data from the national gas hydrate program 02 expedition, offshore India

Abstract During the National Gas Hydrate Program Expedition-02 (NGHP-02), logging while drilling, conventional coring and pressure coring, wire-line logging, vertical seismic profiling and formation pressure measurements were carried out in deep water portions of the Krishna-Godavari (K-G) and Mahanadi Basins along the eastern continental margin of India. A total of 42 research holes were completed for comprehensive data collection and measurements related to gas hydrate reservoir and host sediments properties with an objective to identify producible gas hydrate in sand-rich reservoirs that would be suitable for a future pilot production test. Shipboard and post cruise evaluation of physical properties measurements, pressure core dissociation studies and gas analysis of recovered core samples have confirmed the occurrence of sand hosted gas hydrate deposits in three operational areas named during NGHP-02 as Area B, Area C and Area E in the deep-water portion of the K-G Basin. The energy resource potential of the discovered gas hydrate deposits in the K-G Basin depends on many factors, including the quantity of available gas trapped as gas hydrate in the basin. Therefore, this study was undertaken to identify, characterize and delineate the discovered and confirmed gas hydrate prospects in the K-G Basin. Based on the integrated analysis of regional seismic data and NGHP-02 acquired borehole data, a total of 8 sand-rich reservoir constrained gas hydrate prospects have been identified and mapped in the K-G Basin (Areas B, C, and E) as described in this report.

Origin of low-chloride fluid in sediments from the eastern continental margin of India, results from the National Gas Hydrate Program Expedition 02

Abstract National Gas Hydrate Program Expedition 02 was conducted in 2015 by D/V Chikyu in the western Bay of Bengal, India. Analyses of interstitial water in sediment from four drilling sites along the eastern and western transect in the Krishna–Godavari Basin show that dissolved chloride (Cl−) concentrations decreased with depth from seawater values (>550 mM) near the sediment surface to 300–400 mM at 350–400 m below seafloor (mbsf), suggesting the upward advection of low-Cl– fluid. Excursions of Cl− concentrations toward low values in gas hydrate zones were attributed to the presence of fresh water released by dissociation of gas hydrate during core recovery. At Sites in the western transect, Cl− concentrations below the gas hydrate reservoir (around 280 mbsf) were ca. 60 mM and ca. 45 mM lower than those from above the reservoir, and δ18O and δD values were 1–2‰ and 0.7–1.3‰ higher than those from above the reservoir, respectively. These results suggest that a fraction of migrating low-Cl– fluid is trapped in the gas hydrate zone, causing decreases of δ18O and δD in the residual water due to isotopic fractionation during gas hydrate formation. From mass-balance calculations, we estimated that 10–20% of the advected fluid is trapped at the gas hydrate zone. At all sites, the δ18O and δD values below the gas hydrate zone, representing relatively unaltered low-Cl– fluid, were 0.5–1‰ higher and ca. 13–15‰ lower than those of seawater, respectively. This trend is consistent with water derived from the dehydration of clay minerals, which generally occurs in deeply buried sediments. The contribution of clay mineral dehydration is relatively large at the western transect compared to that in the eastern transect. Our results imply that fluid advection contributes to the accumulation of gas hydrate in the study area.

Integration of rock physical signatures with depositional environments: A case study from East Coast of India

Rock physical crossplots from different geological setup along eastern continental margin of India (ECMI) represent diversified signatures. To characterize the reservoirs in rock physics domain (velocity/modulus versus porosity) and then connecting the interpretation with geological model has been the objectives of the present study. Petrophysical logs (total porosity and volume of shale) from five wells located at sedimentary basins of ECMI have been analyzed to quantify the types of shale such as: laminated, dispersed and structural in reservoir. Presence of various shale types belonging to different depositional environments is coupled to define distinct rock physical crossplot trends for different geological setup. Wells from three different basins in East Coast of India have been used to capture diversity in depositional environments. Contact model theory has been applied to the crossplot to examine the change in rock velocity with change in reservoir properties like porosity and volume of shale. The depositional and diagenetic trends have been shown in the crossplot to showcase the prime controlling factor which reduces the reservoir porosity. Apart from that, the effect of geological factors like effective stress, sorting, packing, grain size uniformity on reservoir properties have also been focused. The rock physical signatures for distinct depositional environments, effect of crucial geological factors on crossplot trends coupled with established sedimentological models in drilled area are investigated to reduce the uncertainties in reservoir characterization for undrilled potentials.

Phosphorus Accumulation Associated with Intense Diagenetic Metal-Oxide Cycling in Sediments along the Eastern Continental Margin of India

Sequential phosphorus extractions were carried out to understand phosphorus cycling and enrichment in surface sediments along the eastern continental margin of India. Phosphorus associated with authigenic (P aut ) and biogenic (P bio ) phases is high by a factor of 2-10 in the continental shelf sediments compared to slope and deep-sea sediments. Phosphorus associated with Fe oxides (P Fe ) is enriched by a factor of 2-5 in the continental slope and rise sediments (500-3000 m water depth) compared to shelf sediments. Fe-Mn oxy(hydroxides) formed during early diagenesis adsorb phosphate from the water column or pore waters, thereby enriching the P Fe fraction in the continental slope sediments. These results are in contrast with those from the Arabian Sea, where wide and intense mid-depth oxygen minimum zone (150-1200 m water depth) releases P Fe to pore waters and enhances P aut accumulation in the continental slope sediments.

Structural features and proto-type basin reconstructions of the Bay of Bengal Basin: A remnant ocean basin model

Remnant ocean basin is a key to understand the plate suturing and subsequent uplift and erosion of orogen. The Bay of Bengal Basin (BOBB) provides a typical example to analyze the remnant ocean basin structures, evolution, and relationships between depositional filling and uplifting of the Himalayan Orogen. Thirty-nine seismic profiles as well as interval velocities of well BODC3 were used to compile isopach maps of the basin. Among the seismic data, 26 seismic profiles were applied to establish 8 cross sections. The cross sections suggest the basin is asymmetric, bounded to the west by the eastern continental margin of India (ECMI) with graben-horst and to the east by the Sunda convergence margin dominated by trench-arc system. The BOBB is characterized by a prominent down flexure structures caused by huge amount of Bengal fan turbidite sediments accumulation. Our isopach maps and chronology data collected from adjacent regions reveal the initial development and fast southward growth of the Bengal fan were related to the early and major stage uplift and erosion of the Himalayan Orogen, respectively. The BOBB has experienced a critical transition from an ocean basin to a remnant ocean basin at Late Oligocene. Such basin structures and evolution features indicate the BOBB provides whole records of oblique convergence of the India and Asia plates, and the early and major stage evolution of the Himalayan Orogen.

Effective elastic thickness along the conjugate passive margins of India, Madagascar and Antarctica: A re-evaluation using the Hermite multitaper Bouguer coherence application

Gondwana correlation studies had rationally positioned the western continental margin of India (WCMI) against the eastern continental margin of Madagascar (ECMM), and the eastern continental margin of India (ECMI) against the eastern Antarctica continental margin (EACM). This contribution computes the effective elastic thickness (Te) of the lithospheres of these once-conjugated continental margins using the multitaper Bouguer coherence method. The results reveal significantly low strength values (Te∼2km) in the central segment of the WCMI that correlate with consistently low Te values (2–3km) obtained throughout the entire marginal length of the ECMM. This result is consistent with the previous Te estimates of these margins, and confirms the idea that the low-Te segments in the central part of the WCMI and along the ECMM represents paleo-rift inception points of the lithospheric margins that was thermally and mechanically weakened by the combined action of the Marion hotspot and lithospheric extension during the rifting. The uniformly low-Te value (∼2km) along the EACM indicates a mechanically weak lithospheric margin, probably due to considerable stretching of the lithosphere, considering the fact that this margin remained almost stationary throughout its rift history. In contrast, the ECMI has comparatively high-Te variations (5–11km) that lack any correlation with the regional tectonic setting. Using gravity forward and inversion applications, we find a leading order of influence of sediment load on the flexural properties of this marginal lithosphere. The study concludes that the thick pile of the Bengal Fan sediments in the ECMI masks and has erased the signal of the original load-induced topography, and its gravity effect has biased the long-wavelength part of the observed gravity signal. The hence uncorrelated flat topography and deep lithospheric flexure together contribute a bias in the flexure modeling, which likely accounts a relatively high Te estimate.