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The Central Indian Ocean Basin, east of the southern Chagos Laccadive Ridge complex has evolved due to seafloor spreading in the N-S direction since the Late Cretaceous. The trace of the Rodriguez Triple Junction (RTJ) has been inferred in this basin, but its exact location and dynamics prior to C29o are not known. About 20,000 km of total magnetic intensity data along with satellite gravity mosaic were used to understand the dynamics of the RTJ in the Late Cretaceous. Synthetic seafloor spreading models endorse the presence of Late Cretaceous to early Palaeogene magnetic anomalies C34y to C24o in the study area. Half-spreading rates are initially low (32 mm/yr) between C34y to C33y, and increase up to 78 mm/yr by C28y. Thereafter a decreasing trend is seen (57 mm/yr). Towards the west in the study area, higher spreading rates (120 mm/yr) between C33y and C32ay, and distorted anomalies between chrons C32ay and C30y are observed. Detailed geophysical analysis suggests that the higher spreading rates are due to the presence of transferred crust, and the distorted anomalies are caused by the presence of fragmented tectonic blocks depicting microplate formation and rotation. Plate reconstruction models suggest that the RTJ at C34y time lay southwest of Comorin Ridge which coincided with the northern edge of the Conrad Rise. Until C33y, the RTJ left a linear trace parallel to the fracture zone trend on both the Indian and Antarctic plates, while it gradually moved eastwards on the African plate. The RTJ migrated westwards until C31y causing the transfer of a considerable amount of crust from the African plate to both the Indian and Antarctic plates. Thereafter, the RTJ rapidly reversed its migration direction towards the east until C29o causing the formation and rotation of a microplate on the Indian plate and lengthening of the Southwest Indian Ridge. The present study suggests that this episodic migration of the RTJ may be due to ridge propagation under the influence of the Marion hotspot which was in the vicinity during that period. This RTJ migration resulted in microplate formation, lithospheric transfer and thickened lithosphere similar to the inferences made at many triple junctions in the Pacific Ocean. Thus the origin of anomalous structures such as the Crozet Bank may be related to triple junction dynamics under the influence of Marion hotspot. Further, the counter clockwise rotation of the Southwest Indian Ridge between C33y and C29y may have a causal link with the formation and rotation of the microplate, which occurred during the same period. The present study thus facilitated the inference of the dynamics of the RTJ trace between C34y and C29o on the three plates.

International audience; The Central Indian Ocean, namely the Central Indian, Crozet, and Madagascar basins, formed by rifting and subsequent drifting of India (now Capricorn), Antarctica, and Africa (now Somalia). We gathered a comprehensive set of sea surface magnetic anomaly profiles over these basins and revisited location and identification of magnetic isochrons between C34ny (83.0 Ma) and C20ny (42.536 Ma) using the objective analytic signal technique. We present high-resolution magnetic isochrons for 29 periods based oñ 1,400 magnetic anomaly picks. From the conjugate sets of picks, we derive two-plate finite rotation parameters for both the Capricorn-Antarctica and Capricorn-Somalia motions. These finite rotations are compared to three-plate reconstructions of the plate boundaries between the Capricorn, Antarctica, and Somalia plates, constrained by the closure of the Indian Ocean Triple junction. In general, the three-plate reconstructions slightly overrotate the reconstructed isochrons with respect to the Capricorn-Antarctica and Capricorn-Somalia two-plate reconstructions. Conversely, the two-plate reconstructions for Somalia-Antarctica slightly underrotate the isochrons compared to the Capricorn-Antarctica-Somalia three-plate reconstructions. We suspect that the discrepancies between the two-plate and three-plate methods result from the recent seafloor deformation in the Capricorn-India diffuse plate boundary and/or from the contrasted nature and geometry of magnetic isochrons at different spreading rates (i.e., magnetic structure of the three spreading centers). Three-plate reconstructions better constrain the closure of the triple junction but spread any misfit among all three plate boundaries. When enough quality data are available, two-plate reconstructions may lead to more realistic plate motion estimates from which additional geological problems can be identified and solved.

High resolution paleomagnetic investigations were performed on a 50.08 m long sediment core (MD161/20) from Mahanadi basin, Bay of Bengal. Core yielded reliable paleomagnetic results for top 20 m below seafloor (mbsf) which spans about 80 ka. Based on the analysis of rock magnetic data, the core is subdivided into five distinct Zones: Zone 1 and Zone 2 cover top 20 mbsf and do not show any abrupt change in magnetic mineralogy, concentration and grain size. Zones 3 and 5 show significant reduction in χLF, χARM and SIRM due to dissolution of magnetic minerals. Zone 4 shows moderate values of χLF and SIRM. The low value of χARM suggests that magnetic signal is mostly carried by magnetic grains in PSD/MD state. The paleomagnetic data for the top 20 mbsf show four prominent geomagnetic excursions at ∼9 mbsf, ∼13.5 to 15 mbsf, ∼16.3 mbsf and ∼18 to 18.2 mbsf. The age-depth relationship is established using stratigraphic correlation between well-dated sedimentary core NGHP-01-19B and the core MD161/20. The ages of the observed excursions correspond to ∼18 to 20 ka, ∼42 to 49 ka, ∼54 to 57 ka and ∼69 to 70 ka. The excursions at ∼42 to 49 ka, ∼54 to 57 ka, and ∼67 to 70 ka is similar to the known excursions the Laschamp and the split Norwegian-Greenland Sea events (NGS-I and NGS-II). The excursion at 18–20 ka is not observed globally and may be related to lithological/sedimentological changes occurring during last glacial maxima (LGM). The virtual geomagnetic path (VGP) of Laschamp excursion traces clockwise loop. All excursions identified in present study fall in the periods of relatively low paleointensity.

We present high resolution paleosecular variation (PSV) records up to 8 cal. kyr BP from three piston cores, MD161/8, MD161/11 and MD161/13 acquired in the Krishna–Godavari (KG) basin, Bay of Bengal. During the Holocene period, high sedimentation rates are recorded at MD161/8 (38.8–248.3 cm/kyr), MD161/11 (137–336 cm/kyr) and MD161/13 (∼573 cm/kyr). Rock magnetic data analysis suggests that the remanence signal is carried by titanomagnetite/titanohematite grains in stable single domain (SSD)/pseudo single domain (PSD) state. The PSV records of MD161/11 and MD161/13 show good correlation in the uppermost sediments despite significant variation in the sedimentation rates; however, poor correlation of PSV records is observed for the core MD161/8 probably due to local effects. Paleoinclination records of MD161/8, MD161/11 and MD161/13 show a low between ∼2.4 and 2.0 cal. kyr BP, an increase between 2.0 and 1.4 cal. kyr BP and a decrease towards the present. To varying degrees these trends can be observed in the other Asian PSV records of Shuangchiling (SCL) and Biwa lakes. However, the magnitude of the observed inclination anomaly in KG basin is higher (∼40°) compared to those reported from SCL (∼25°) and Biwa (∼10°) lakes. Paleodeclination records of MD161/11 and MD161/13 show a decline between ∼4.0 and 2.9 cal. kyr BP, an increase between 2.9 and 2.1 cal. kyr BP, a substantial decrease between ∼2.1 and 1.0 cal. kyr BP and an increase towards the present. Similar trends can be observed in the other Asian PSV records of SCL and Biwa lakes with a minor age offset of 0.2–0.5 kyr. The available models CALS7k.2 and CALS10k.1 are evaluated for their capability in predicting the inclination and declination anomalies from the Asian regions. The CALS7k.2 model can predict most of the inclination anomalies while the CALS10k.1 is unable to predict many of them. The CALS7k.2 model shows that the observed inclination anomalies can be attributed to the presence of a strong non-axial dipolar magnetic field over the Bay of Bengal. More PSV records need to be constructed to better understand the anomalous non-axial dipolar field over the Bay of Bengal.

The Gop and Laxmi Basins lying off Northwest India have been assigned ambiguous crustal types and evolution mechanisms. The Chagos–Laccadive Ridge (CLR) complex lying along the southwest coast of India has been attributed to different evolutionary processes. Late Cretaceous seafloor spreading between India and Africa formed the Mascarene Basin, and the plate reconstruction models depict unequal crustal accretion in this basin. Re-interpretation of magnetic data in the Gop and Laxmi Basins suggests that the underlying oceanic crust was accreted contemporaneously from 79 Ma at slow half spreading rates (0.6 to 1.5 cm/yr) separating the Seychelles–Laxmi Ridge complex from India. The spreading ridge became extinct at 71 Ma in the central region between 19–20°N and 65.5–67°E, and at 68.7 Ma in the Gop Basin. Extinction progressed southwards with time until 64.1 Ma at ~ 14.5°N in the Laxmi Basin. This spreading probably limited the seafloor spreading in the northern Mascarene Basin, while normal to fast spreading continued in the south. The fracture zone FZ2 acted as a major boundary between the northern and southern Mascarene Basin, and may have also restricted the southward propagation of the Laxmi Basin spreading ridge. Plate reconstruction models generated in this study accommodate several microplates between Madagascar and India, and trace their movements over time within the India–Africa spreading corridor. Furthermore, these models suggest the formation of the Seychelles–Laxmi Ridge microplate by 79 Ma in a complex three plate system. This microplate may have existed up to 64.1 Ma and experienced an anticlockwise rotation of about 50°. Under the influence of the Deccan hotspot activity, the Laxmi Basin spreading ridge jumped southwestward splitting the Seychelles–Laxmi Ridge complex and initiating the Carlsberg Ridge. The presence of about 160 km of oceanic crust in the Gop and Laxmi Basins partially reduces the space problem faced in the earlier plate reconstruction models.

Gas hydrate resource assessments that indicate enormous global volumes of gas present within hydrate accumulations have been one of the primary driving forces behind the growing interest in gas hydrates. Gas hydrate volumetric estimates in recent years have focused on documenting the geologic parameters in the “gas hydrate petroleum system” that control the occurrence of gas hydrates in nature. The primary goals of this report are to review our present understanding of the geologic controls on the occurrence of gas hydrate in the offshore of India and to document the application of the petroleum system approach to the study of gas hydrates. National Gas Hydrate Program of India executed the National Gas Hydrate Program Expedition 01 (NGHP-01) in 2006 in four areas located on the eastern and western margins of the Indian Peninsula and in the Andaman Sea. These areas have experienced very different tectonic and depositional histories. The peninsular margins are passive continental margins resulting from a series of rifting episodes during the breakup and dispersion of Gondwanaland to form the present Indian Ocean. The Andaman Sea is bounded on its western side by a convergent margin where the Indian plate lithosphere is being subducted beneath southeast Asia. NGHP-01 drilled, logged, and/or cored 15 sites (31 holes) in the Krishna–Godavari Basin, 4 sites (5 holes) in the Mahanadi Basin, 1 site (2 holes) in the Andaman Sea, and 1 site (1 hole) in the Kerala–Konkan Basin. Holes were drilled using standard drilling methods for the purpose of logging-while-drilling and dedicated wireline logging; as well as through the use of a variety of standard coring systems and specialized pressure coring systems. NGHP-01 yielded evidence of gas hydrate from downhole log and core data obtained from all the sites in the Krishna–Godavari Basin, the Mahanadi Basin, and in the Andaman Sea. The site drilled in the Kerala–Konkan Basin during NGHP-01 did not yield any evidence of gas hydrate. Most of the downhole log-inferred gas hydrate and core-recovered gas hydrate were characterized as either fracture-filling in clay-dominated sediments or as pore-filling or grain-displacement particles disseminated in both fine- and coarse-grained sediments. Geochemical analyses of gases obtained from sediment cores recovered during NGHP-01 indicated that the gas in most all of the hydrates in the offshore of India is derived from microbial sources; only one site in the Andaman Sea exhibited limited evidence of a thermogenic gas source. The gas hydrate petroleum system concept has been used to effectively characterize the geologic controls on the occurrence of gas hydrates in the offshore of India.

During NGHP-Expedition-01, well logs were obtained for gas hydrate exploration in Krishna–Godavari (KG) offshore basin. These logs coupled with a suitable rock physics model can be used to understand the interaction between the sediment grains of unconsolidated marine sediments as well as with hydrate. In this paper, we study the friction-dependent effective medium model (EMM) to understand these grain interactions. The compressional (P) and shear (S) wave velocities of fluid saturated sediments are estimated using different friction parameters at Site NGHP-01-03, which represent the background fluid-saturated marine sediment, and are compared with the observed velocities derived from sonic logs. Our analysis shows that the shear velocity is overestimated for the Hertz–Mindlin contact theory [no-slip across the grain contact], but can be accurately estimated for the Walton's smooth contact model [zero friction across the grain contact]. It suggests that the background shear wave velocity need to be modeled without friction at the grain contact for unconsolidated marine sediments. Further, the friction-dependent EMM theory is tested at Site NGHP-01-07 which represents the load-bearing gas hydrate deposits in KG basin. The comparison between the gas hydrate saturations estimated from sonic and resistivity logs shows that saturations estimated from P-wave velocity match well with those estimated from resistivity and chloride anomaly and is largely independent of the frictional parameter. However, gas hydrate saturations estimated from shear wave velocity is overestimated in the absence of friction but agrees with the other estimates if an arbitrary small friction is included in the EMM. We further extended the friction-dependent EMM for multi-grain contact (clay + quartz + hydrate) in which the effective modulus of sediment matrix is estimated by accounting for all possible contact combinations among the grains like quartz–quartz (QQ), clay–clay (CC), clay–quartz (QC), quartz–hydrate (QH), clay–hydrate (CH), and hydrate–hydrate (HH). The gas hydrate saturations estimated from shear velocity assuming the same non-zero friction term are underestimated as compared to those estimated from P-wave velocity. Interestingly, the saturations estimated assuming zero-friction from both P- and S-wave velocities are comparable to each other and show a good match with those estimated from resistivity logs and chloride anomalies. The proposed EMM with zero friction and mixed grain contact is able to predict the velocities of fluid-saturated sediments as well as gas hydrate bearing sediments in KG offshore basin.

Detailed rock magnetic measurements such as susceptibility (χ), susceptibility of anhysteric remanent magnetization (ΧARM), saturation isothermal remanent magnetization (SIRM), soft-isothermal remanent magnetization (soft-IRM) and hard-IRM (HIRM) were carried out on the topmost sediment samples of 73 gravity cores collected in the Krishna–Godavari (KG) offshore basin to understand the distribution of magnetic parameters and the associated diagenetic signatures related to upward methane flux. The variation in magnetic parameters in the KG offshore basin fluctuates around the average value (Χ = 105 × 10−8 m3 kg−1, ΧARM = 1.22 × 10−5 m3 kg−1, SIRM = 2040 × 10−5 Am2 kg−1, soft-IRM = 1807 and HIRM = 82) except at the location GC-07 (15.767°N, 81.814°E) where the magnetic parameters Χ, ΧARM and SIRM drop to ∼1/6th, ∼1/14th and ∼1/8th of the average near surface value indicating substantial loss in the concentration of primary magnetic iron oxide minerals. Anomalous low value of ΧARM/Χ and ΧARM/SIRM at GC-07 suggests that the magnetic remanence property is governed by coarser pseudo-single domain (PSD) or multi-domain (MD) grains. To understand the nature of the anomalous magnetic parameters at GC-07, complete downcore variation of magnetic parameters was also studied. The magnetic parameters were found to be even more depleted at all depths with susceptibility dropping (at 5.2 mbsf) to ∼1/200th of its average near surface value. Interestingly, this core shows a rapid increase in the concentration of dissolved methane from ∼0 nM at surface to ∼153 nM at around 5.2 mbsf where the sulfate concentration is also reduced to zero. The anaerobic oxidation of methane (AOM) and the sulfate methane transition zone (SMTZ) occur between ∼4.2 and 5.2 mbsf based on the present day sulfate concentration profile. The close association between the dissolution of magnetic iron oxide minerals and AOM at GC-07 suggests that the reduced magnetization is primarily due to the release of HS− by AOM. The magnetic iron oxide minerals are dissolved not only at SMTZ but also above the current depth of the SMTZ. It is likely that the methane flux was higher in the past leading to upward shift of the SMTZ, and the HS− released by AOM facilitated the dissolution of magnetic iron oxide minerals at shallower depths. Alternatively, the HS− diffusing upward from the current depth of SMTZ might have lead to dissolution at shallower depth. Therefore, additional geological/geochemical investigations are warranted at GC-07 to establish the dissolution mechanism of magnetic iron oxide minerals. The variability in different magnetic parameters within the slope basin and channels shows systematic decrease in the concentration of magnetic minerals and increase in the abundance of stable single domain (SSD) magnetic grains with water depth. The observed strong correlation between the silt fraction and the magnetic susceptibility for the slope basin and channels suggests that the variation in magnetic parameters is predominantly controlled by the sediment sorting/transport process as finer particles are deposited in deeper water.

Seismic attenuation of gas hydrate bearing sediments is estimated from the multi-channel seismic data acquired in Krishna-Godavari (KG) offshore basin in the vicinity of Site NGHP-01-10, where ~128 m of fracture-filled hydrate is confirmed by drilling/coring, to understand the effect of gas hydrate on the seismic attenuation. The seismic quality factor (Q), which is the inverse of seismic attenuation, is estimated from two methods – first, the spectral ratio method which utilizes the logarithmic ratio of the seafloor and bottom simulating reflector (BSR) spectra; and second, the peak frequency method which depends upon the variations of the peak frequency of the BSR spectrum as a function of traveltime. In the study area, the complex fault system has perturbed the seafloor resulting in noisy seafloor spectrum. As a result, the logarithm of the spectral ratio was also noisy and the Q-values obtained from spectral ratio method were found to be unstable. On the other hand, the peak frequency method is found to be relatively stable as it depends only on the peak frequency of the BSR which can be reliably obtained by conventional processing. This method yields an effective Q value of BSR which represents a weighted average of the interval Q-values of the water column and the gas hydrate bearing sediments. Since the Q-value of water column can be treated as a constant, the variations in effective Q-value can be interpreted in terms of the variations of Q in the gas hydrate bearing sediments for a constant thickness of water column. The estimated effective Q-values, along the inline and crossline seismic profiles, depend on several factors such as gas hydrate, free gas and the complex fault system. The combined interpretation of the quality factor and the interval velocity model helps to understand these effects in different zones of the seismic data. The attenuation of the background marine sediments is estimated from the analysis of crossline seismic profile. The gas hydrate bearing sediments, as inferred from the velocity model, show low attenuation as compared to that of the background sediments. In contrast, active migration of free gas across the fault system leads to increase in seismic attenuation within the gas hydrate stability zone. The complex fault zone may also lead to an increase in seismic attenuation. The blanked seismic zone shows a marginal decrease in seismic attenuation probably related to loadbearing form of gas hydrate. Our attenuation results are compared against the results obtained from waveform inversion, albeit with a higher resolution. The relative variation of Q estimated from the proposed method is close to the average Q-values estimated from full waveform inversion for most of the CDPs. The proposed methodology can be a useful technique to estimate seismic attenuation from multi-channel seismic data.

The seafloor and bottom simulating reflectors (BSRs) are interpreted from the 3D seismic data acquired in Krishna–Godavari (KG) offshore basin in the vicinity of sites drilled/cored during National Gas Hydrate Program (NGHP) Expedition-01. The shallow structures such as inner toe-thrust fault system, regional and local linear fault systems and mass transport deposits are inferred from attributes of seafloor time structure as well as from the seismic profiles. The geothermal gradient is estimated from the depths and temperatures of the seafloor and the BSR. The temperature at the BSR depth is estimated from the methane hydrate and seawater salinity phase boundary assuming that the BSR represents the base of the gas hydrate stability zone. The spatial variations in geothermal gradient (GTG) show a strong correlation with seafloor topography in the KG basin. The GTG decreases by ∼13–30% over the topographic mounds formed due to inner toe-thrust faults and recent mass transport deposits. The GTG decreases by only 5–10% over the mounds, likely due to defocusing of heat flux based on one-dimensional topographic modeling. Hence, the GTG perturbation due to topography alone cannot explain the observed GTG anomaly. The temperature profile beneath these mounds may not be in equilibrium with the surroundings either due to the recent upliftment of sediments along the inner toe-thrust faults or rapid deposition of sediments due to slumping/sliding. In contrast, an increase in GTG by 10–15% is observed in the vicinity of major fault systems. We presume that the likely mechanism for the increase in GTG is fluid advection from a deeper part of the basin. A detailed thermal modeling involving the effect of surface topography, high sedimentation rates, fluid advection and sediment thickening due to tectonics is required to understand the thermal profile in KG offshore basin.

Krishna–Godavari offshore basin, a part of the eastern continental margin of India is a proven petroliferous basin. Recent drilling in this area in search of gas hydrates reveals that the upper ∼300 m thick Quaternary–Recent strata comprised of nannofossil bearing rich clays and, fractures/faults are the suitable zones for gas hydrates accumulation. Therefore, the knowledge about the shallow geological environments and its architecture are significantly important in assessing the gas hydrates potential of this area. In order to enhance the geological understanding, the newly acquired high resolution seismic (HRS) reflection data in this gas hydrates prone area is interpreted. The processed seismic sections show a maximum penetration of 562 ms TWT (∼450 m) underneath the seabed with high resolution stratification. An attempt has been made to: (i) deduce the shallow geological environment from the reflection characteristics, and, (ii) assign tentative ages under the constraints of drilling/coring results. We further explained the observed folded structures on the surface and subsurface through a mechanism linked to shale tectonism and neotectonic activity.

Organoclastic degradation and anaerobic oxidation of methane (AOM) are known to influence sulfate, methane and bicarbonate concentrations of sediment pore-waters. In this work we have analyzed the pore-fluid composition of three piston cores (36–39 m long) collected from water depths ranging from 1429 to 1691 m. We attribute the observed linear sulfate concentration profiles and (ΔCa + ΔAlk)/ΔSO42− ratios dominantly to anaerobic oxidation of methane (AOM) induced sulfate consumption. The gas rich layers just below the base of hydrate stability zone (BGHSZ) is the possible source of the enhanced diffusive flux of biogenic methane ( δ 13 C CH 4 ranging from −99.7 to −106.3‰ VPDB). Occurrences of disseminated gas hydrate in the Mahanadi Basin within the hydrate stability zone (210–220 m thick) supports the link between linear sulfate profile, high rate of AOM and hydrate occurrence.

The nature and origin of the subsurface 85°E Ridge in the Bay of Bengal has remained enigmatic till date despite several theories proposed by earlier researchers. We reinterpreted the recently acquired high quality multichannel seismic reflection data over the northern segment of the ridge that traverses through the Mahanadi offshore, Eastern Continental Margin of India and mapped the ridge boundary and its northward continuity. The ridge is characterized by complex topography, multilayer composition, intrusive bodies and discrete nature of underlying crust. The ridge is associated with large amplitude negative magnetic and gravity anomalies. The negative gravity response across the ridge is probably due to emplacement of relatively low density material as well as ∼2–3 km flexure of the Moho. The observed broad shelf margin basin gravity anomaly in the northern Mahanadi offshore is due to the amalgamation of the 85°E Ridge material with that of continental and oceanic crust. The negative magnetic anomaly signature over the ridge indicates its evolution in the southern hemisphere when the Earth’s magnetic field was normally polarized. The presence of ∼5 s TWT thick sediments over the acoustic basement west of the ridge indicates that the underlying crust is relatively old, Early Cretaceous age. The present study indicates that the probable palaeo-location of Elan Bank is not between the Krishna–Godavari and Mahanadi offshores, but north of Mahanadi. Further, the study suggests that the northern segment of the 85°E Ridge may have emplaced along a pseudo fault during the Mid Cretaceous due to Kerguelen mantle plume activity. The shallow basement east of the ridge may have formed due to the later movement of the microcontinents Elan Bank and Southern Kerguelen Plateau along with the Antarctica plate.

In this paper, we investigate a spatially homogeneous and anisotropic Bianchi type-V cosmological model in a scalar-tensor theory of gravitation proposed by Harko et al. (Phys. Rev. D 84:024020, 2011) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. To obtain a determinate solution, a special law of variation proposed by Berman (Nuovo Cimento B 74:182, 1983) is used. We have also used the barotropic equation of state for the pressure and density and bulk viscous pressure is assumed to be proportional to energy density. It is interesting to note that the strings in this model do not survive. Also the model does not remain anisotropic throughout the evolution of the universe. Some physical and kinematical properties of the model are also discussed.

We carried out detailed magnetic measurements of the core (MD161/8) located in the vicinity of Site NGHP-01-10, where ~128 m of hydrate is confirmed by drilling/coring, to understand the diagenesis of magnetic minerals in a gas hydrates/cold seep environment. The rock magnetic measurements along with SEM-EDS and XRD analyses show a zone of reduced magnetic susceptibility (zone 2) where most of the magnetic minerals are dissolved. The enhanced concentration of chromium reducible sulfur (CRS) in this zone suggests an intense pyritization process while isotopically depleted authigenic carbonates indicate sulfate reduction via anaerobic oxidation of methane (AOM). Therefore, the dissolution of magnetic minerals is attributed to the HS - released during AOM that has resulted in the reduction in the magnitude of magnetic parameters. Within zone 2, a zone of enhanced susceptibility (zone 2a) is observed between 17.68 and 23.6 mbsf, and is located beneath the present day sulfate-methane transition zone (SMTZ). The frequencydependent magnetic susceptibility and low temperature magnetic measurements suggest the abundance of fine grained superparamagnetic (SP) sized ferrimagnetic particles. The SEM-EDS and XRD analyses show the presence of greigite which occurs in interstices between the pyrite crystals. Such occurrence of greigite in sediments has important implications in the interpretation of paleomagnetic records. We evaluated the likely mechanism for the greigite formation in KG offshore basin and our data suggest that the formation of greigite may be related to either paleo-SMTZ or anaerobic oxidation of pyrite. It is unlikely that the formation of greigite can be explained by the downward diffusion of sulfide below the current depth of SMTZ. However, further investigations are required to ascertain the mechanism for the

[1] For the first time, we report the amplitude variation with angle (AVA) pattern of bottom-simulating reflectors (BSRs) beneath fracture-filled gas hydrate deposits when the effective medium is anisotropic. The common depth point (CDP) gathers of two mutually perpendicular multichannel seismic profiles, located in the vicinity of Site NGHP-01-10, are appropriately processed such that they are fit for AVA analysis. AVA analysis of the BSR shows normal-incidence reflection coefficients of −0.04 to −0.11 with positive gradients of 0.04 to 0.31 indicating class IV pattern. The acoustic properties from isotropic rock physics model predict class III AVA pattern which cannot explain the observed class IV AVA pattern in Krishna-Godavari basin due to the anisotropic nature of fracture-filled gas hydrate deposits. We modeled the observed class IV AVA of the BSR by assuming that the gas hydrate bearing sediment can be represented by horizontally transversely isotropic (HTI) medium after accounting for anisotropic wave propagation effects on BSR amplitudes. The effective medium properties are estimated using Backus averaging technique, and the AVA pattern of BSRs is modeled using the properties of overlying HTI and underlying isotropy/HTI media with or without free gas. Anisotropic AVA analysis of the BSR from the inline seismic profile shows 5–30% gas hydrate concentration (equivalent to fracture density) and the azimuth of fracture system (fracture orientation) with respect to the seismic profile is close to 45°. Free gas below the base of gas hydrate stability zone is interpreted in the vicinity of fault system (F1).

Indian scientists carrying marine geophysical research at different Earth Science institutions have made significant advances during the last six years (2012-2015) for understanding evolution of continental margins of India and deep sea regions. The investigations were mostly focused on continental margins of India encompassing deep-water regions, mid-ocean ridges, aseismic ridges and Andaman Sea including the back-arc basin. The studies were aimed at unraveling the tectonic and physical processes associated with evolution of the margins, ocean basins, volcanic structures, etc. In addition, industryrelated research for mapping of energy resources such as gas hydrates and hydrocarbons was also carried out in rifted basins on Eastern Continental Margin of India. The investigations were successfully carried out with integration of several geophysical datasets available at National Institute of Oceanography, India, National Geophysical Data Center, USA and petroleum industries, and by acquiring new geophysical data and geological sampling. In this report we describe major outcomes of the investigations focused on specific geological aspects of the Indian Ocean. Investigations carried out by individual works are also included in the bibliography.

Pore water sulfate consumption in marine sediments is controlled by microbially driven sulfate reduction via organo-clastic and methane oxidation processes. In this work, we present sediment pore fluid compositions of 10 long sediment cores and high resolution seismic data from the Krishna–Godavari (K–G) basin, Bay of Bengal. Our results show occurrence of transient (S and kink types) and steady state (quasi-linear) sulfate concentration profiles which are attributed partly to the anaerobic oxidation of methane ( δ 13 C CH 4 : −84.8 to −100.1‰ VPDB) and organo-clastic sulfate reduction. Influence of AOM on alkalinity is evident from the presence of authigenic carbonate layers with highly depleted carbon isotope ratios in core MD161-8. The authigenic carbonates represent the paleo-SMTZs and suggest marked fluctuation in vertical methane flux. Our geophysical data show the acoustic signatures of upward fluid migration from shallow sub-surface, whereas, coring during NGHP expedition-01 confirms the presence of sub-surface gas hydrate deposits in K–G basin which can be linked to deep methane sources. The geochemical analysis suggests that shallow methane source can be attributed to high burial flux of labile organic matter due to high sedimentation rate. Sampling sites with high methane flux from the shallow gas source are characterized by quasi-linear sulfate concentration profile and a shallow sulfate methane transition zone (SMTZ) and may not be necessarily linked to deeper gas hydrate deposits. In contrast, the deep methane source results in a transient kink type sulfate profile and a deeper SMTZ. We have observed a close link between the occurrence of gas hydrate and the S/kink type sulfate profile. We interpret the short lived ‘kink’ in the sulfate profiles as a result of recent enhancement in vertical methane flux possibly driven by reactivation of fault-fractures systems which provide the conduits for fluid flow.

KALACHAND SAIN, MAHESWAR OJHA, NITTALA SATYAVANI, G.A. RAMADASS, T. RAMPRASAD, S. K. DAS and HARSH GUPTA CSIR National Geophysical Research Institute, Uppal Road, Hyderabad 500 007 National Institute of Ocean Technology, Velachery-Tambaram Main Road, Chennai 600 100 CSIR-National Institute of Oceanography, Dona Paula, Goa 403 004 Ministry of Earth Sciences, Prithvi Bhavan, Lodhi Road, New Delhi 110 003 Email: kalachandsain@yahoo.com

Drilling/coring activities onboard JOIDES Resolution for hydrate resource estimation have confirmed gas hydrate in the continental slope of Krishna-Godavari (KG) basin, Bay of Bengal and the expedition recovered fracture filled gas hydrate at the site NGHP-01-10. In this paper we analyze high resolution multi-channel seismic (MCS), high resolution sparker (HRS), bathymetry, and sub-bottom profiler data in the vicinity of site NGHP-01-10 to understand the fault system and thermal regime. We interpreted the large-scale fault system (>5 km) predominantly oriented in NNW-SSE direction near NGHP-01-10 site, which plays an important role in gas hydrate formation and its distribution. The increase in interval velocity from the baseline velocity of 1600 m/s to 1750–1800 m/s within the gas hydrate stability zone (GHSZ) is considered as a proxy for the gas hydrate occurrence, whereas the drop in interval velocity to 1400 m/s suggest the presence of free gas below the GHSZ. The analysis of interval velocity suggests that the high concentration of gas hydrate occurs close to the large-scale fault system. We conclude that the gas hydrate concentration near site NGHP-01-10, and likely in the entire KG Basin, is controlled primarily by the faults and therefore has high spatial variability. We also estimated the heat flow and geothermal gradient (GTG) in the vicinity of NGHP-01-10 site using depth and temperature of the seafloor and the BSR. We observed an abnormal GTG increase from 38 °C/km to 45 °C/km at the top of the mound, which remarkably agrees with the measured temperature gradient at the mound (NGHP-01-10) and away from the mound (NGHP-01-03). We analyze various geological scenarios such as topography, salinity, thermal non-equilibrium of BSR and fluid/gas advection along the fault system to explain the observed increase in GTG. The geophysical data along with the coring results suggest that the fluid advection along the fault system is the primary mechanism that explains the increase in GTG. The approximate advective fluid flux estimated based on the thermal measurement is of the order of few tenths of mm/yr (0.37–0.6 mm/yr).

The Krishna–Godavari (KG) offshore basin is one of the promising petroliferous basins of the eastern continental margin of India. Drilling in this basin proved the presence of gas hydrate deposits in the shallow marine sediments beyond 750 m water depths, and provided lithologic and stratigraphic information. We obtained multibeam swath bathymetry covering an area of about 4500 km 2 in water depths of 280–1800 m and about 1260 line km of high resolution seismic (HRS) records. The general lithology of midslope deposits is comprised of nannofossil-rich clay, nannofossil-bearing clay and foraminifera-bearing clay. The HRS records and bathymetry reveal evidence of slumping and sliding of the upper and midslope sediments, which result in mass transport deposits (MTD) in the northwestern part of the study area. These deposits exhibit 3–9.5 km widths and extend 10–13 km offshore. The boundaries of the MTDs are often demarcated by sharp truncation of finely layered sediments (FLS) and the MTDs are characterized by acoustically transparent zones in the HRS data. Average thickness of recent MTDs varies with depth, i.e., in the upper slope, the thickness is about 45 m, while in the lower slope it is about 60 m, and in deeper offshore locations they attain a maximum thickness of about 90 m. A direct indication for slumping and mass transportation of deposits is provided by the age reversal in 14 C AMS dates observed in a sediment core located in the midslope region. Seismic profiling signatures provide indications of fluid/gas movement. We propose that the presence of steep topographic gradients, high sedimentation rates, a regional fault system, diapirism, fluid/gas movement, and neotectonic activity may have facilitated the slumping/sliding of the upper slope sediments in the KG offshore basin.

Increased oil and gas exploration activity has led to a detailed investigation of the continental shelf and adjacent slope regions of Mahanadi, Krishna–Godavari (KG) and Cauvery basins, which are promising petroliferous basins along the eastern continental margin of India. In this paper, we analyze the high resolution sparker, subbottom profiler and multibeam data in KG offshore basin to understand the shallow structures and shallow deposits for gas hydrate exploration. We identified and mapped prominent positive topographic features in the bathymetry data. These mounds show fluid/gas migration features such as acoustic voids, acoustic chimneys, and acoustic turbid layers. It is interesting to note that drilling/coring onboard JOIDES in the vicinity of the mounds show the presence of thick accumulation of subsurface gas hydrate. Further, geological and geochemical study of long sediment cores collected onboard Marion Dufresne in the vicinity of the mounds and sedimentary ridges shows the imprints of paleo-expulsion of methane and sulfidic fluid from the seafloor. To understand the origin of the mounds and their relationship with gas hydrate/cold seep formation, we analyzed the multi-channel seismic reflection (MCS) data close to the mounds. The MCS data show that the subsurface layers beneath the mounds are folded. Below the folded overburden, we observe zones of no coherent reflections probably originating from Miocene sequence. Since the study area is located in shale tectonics regime where Miocene sequences are known to be overpressured, we interpret the zones of no coherent reflections as Miocene shale diapirs. The upward movement of shale diapirs has folded the overburden layer and resulted in the formation of numerous faults/fractures. These faults act as permeable pathways for fluid/gas movement facilitating the formation of gas hydrate and cold seeps close to shale diapiric mound.

Geophysical and geochemical surveys were carried out off Goa, central west coast of India, to understand the genesis and distribution of shallow gases in marine sediments. Shallow gas charged sediments within the water depths of ∼ 15 to 40 m are reported all along the west coast and are characterized by gas masking, high amplitude and reverse polarity reflections in high resolution seismic (sparker) data, attributed to the presence of free gas. These high amplitude reflections (gas front) are observed within a Holocene fine grained, wedge shaped sediment package overlying the maximum flooding surface. The gas front lies between 1.2 and 5 m below the seabed and demarcates the transition from gas in the dissolved phase to bubble phase when the gas concentration exceeds the saturation level. The observed and extrapolated sulfate methane transition zone (SMTZ) lies between 0.7 and 2.25 m below the seabed and it is related to the depth of the gas front. Methane concentration reaches saturation below the SMTZ, and produces a bubble phase which lowers the p-wave velocity and produces high amplitude reflection observed in the seismic data. Depleted carbon isotope ratios as low as − 88.9‰ suggest a microbial origin for the methane in the study area. High sulfate reduction rate, high concentrations of pore water methane, phosphate and ammonium in the shallowest site (Sasu123/1 at 14.7 m water depth) suggest an enhanced availability of reactive organic matter required for microbially mediated biodegradation processes. High gas flux in the shallow waters is also indicated by the shallowing of the gas front and the widening of the gas masked zone towards shore. The present investigations do not testify the origin of Holocene shallow gases either from buried channel deposits or the Pleistocene sediments.

SUMMARY Analysis of 3100 km of newly acquired marine magnetic data constrained by satellite and shipborne free air gravity anomalies in the corridor between the 86°E fracture zone and Ninetyeast Ridge, north of the equator reveals the evolutionary history of the Late Cretaceous crust characterised by anomaly 34 through 31 (83.5-68.7 Ma) under complex tectonic settings. Seafloor spreading model studies suggest that the crust, particularly between the chrons 33R and 33 (79.0 - 73.6 Ma) was formed with variable and slightly higher half-spreading rates (4.8 - 7.1 cm/yr) than the crust of similar age either in the regions west of 86°E fracture zone, east of the Ninetyeast Ridge or the Southern Crozet Basin. Further, the interpretation of magnetic anomalies suggests the presence of fossil spreading ridge segments and extra oceanic crust on the Indian plate that has been transferred from the Antarctica plate by discrete southward ridge jumps. These ridge jumps are caused by thermal instability of the spreading center as the Indian plate moved northward over the Kerguelen mantle plume. The present study indicates that the spreading ridge-plume interaction is the prime mechanism for these ridge jumps, which have occurred since 75.8 Ma. The newly identified magnetic anomalies 34 through 31 and the inferred ~N3°E trending fracture zones refined the plate reconstruction models for that period.

We report some main results of multidisciplinary investigations carried out within the framework of the Indian National Gas Hydrate Program in 2002–2003 in the Krishna–Godavari Basin offshore sector, east coast of India, to explore indicators of likely gas hydrate occurrence suggested by preliminary multi-channel seismic reflection data and estimates of gas hydrate stability zone thickness. Swath bathymetry data reveal new evidence of three distinct geomorphic units representing (1) a delta front incised by several narrow valleys and mass flows, (2) a deep fan in the east and (3) a WNW–ESE-trending sedimentary ridge in the south. Deep-tow digital side-scan sonar, multi-frequency chirp sonar, and sub-bottom profiler records indicate several surface and subsurface gas-escape features with a highly resolved stratification within the upper 50 m sedimentary strata. Multi-channel seismic reflection data show the presence of bottom simulating reflections of continuous to discrete character. Textural analyses of 76 gravity cores indicate that the sediments are mostly silty clay. Geochemical analyses reveal decreasing downcore pore water sulphate (SO42−) concentrations (28.7 to

The presence of gas hydrates along the Indian continental margins has been inferred mainly from the bottom simulating reflection/reflector (BSR) and the gas hydrate stability zone thickness map of India. Multidisciplinary investigations have been carried out in the Krishna‐Godavari offshore area along the eastern continental margin of India which is known for its hydrocarbon potential. Processed multibeam data provided a high resolution seafloor mosaic with a fine scale geomorphology. Deep tow digital side scan sonar, multifrequency chirp sonar and 3.5 KHz sub‐bottom profiler records depict various kinds of gas escape features over the regions where BSRs are prominent. Geochemical analyses of the 5 m‐long cores show a general decrease trend in the porewater sulphate concentration, while the gas chemistry reveals an increase trend of methane concentration with core depth. Total Organic Carbon varies from 0.6 to >2.0% and CaCO3 from 5.0 to >29%. Observed geophysical, geochemical and microbial proxies sugges...

The voluminous gravity and magnetic data sets demand automatic interpretation techniques like Naudy, Euler and Werner deconvolution. Of these techniques, the Euler deconvolution has become a popular choice because the method assumes no particular geological model. However, the conventional approach to solving Euler equation requires tentative values of the structural index preventing it from being fully automatic and assumes a constant background that can be easily violated if the singular points are close to each other. We propose a possible solution to these problems by simultaneously estimating the source location, depth and structural index assuming nonlinear background. The Euler equation is solved in a nonlinear fashion using the optimization technique like conjugate gradient. This technique is applied to a published synthetic data set where the magnetic anomalies were modeled for a complex assemblage of simple magnetic bodies. The results for close by singular points are superior to those obtained by assuming linear background. We also applied the technique to a magnetic data set collected along the western continental margin of India. The results are in agreement with the regional magnetic interpretation and the bathymetric expressions.

Spatially homogeneous and anisotropic Kantowski-Sachs space-time is considered in the scale covariant theory of gravitation proposed by Canuto et al. (Phys. Rev. Lett. 39:429, 1977) in the presence of bulk viscous fluid containing one dimensional cosmic strings. A determinate solution of the field equations of this theory is presented using the special law of variation proposed by Bermann (Nuovo Cimento B74:183, 1983). We have also used the proportionality of shear scalar to the scalar expansion of the space time. The solution obtained represents a bulk viscous string cosmological model in this theory. Some physical and kinematical properties of the model are also discussed.

Five dimensional FRW space-time is considered in the presence of bulk viscous fluid in the frame work of Brans and Dicke (Phys. Rev. 124:925, 1961) scalar–tensor theory of gravitation. To obtain determinate solution of the field equations we have used a power law between scalar field and the scale factor of the universe. Radiating flat, closed and open models are also presented. Physical properties of the models are also discussed.

Results obtained from compilation and reinterpretation of about 21,200 line km of bathymetry, magnetic and satellite gravity data between 10°S to 10°N latitudes and 75 to 90°E longitudes south off Sri Lanka are presented here. Magnetic data and the synthetic seafloor spreading model reveal the presence of Mesozoic anomaly sequence M11 through M0 south of Sri Lanka. The oldest magnetic anomaly M11 (134 Ma) occurs between 110 and 140 km away from the Sri Lankan coast. The seafloor created during the Early Cretaceous is estimated to have evolved with variable half-spreading rates ranging from 5.5 to 1.53 cm/yr. The trends of the fracture zones inferred from the offsets in the magnetic anomalies have been constrained using the satellite gravity mosaic. The Cretaceous Magnetic Quiet Zone (CMQZ, 121–84 Ma) crust between the isochrons M0 and A34 has an unequal width and widens from about 170 km in the west (80°E longitude) to about 500 km towards east (85°E longitude). Plate reconstruction models for 160 (Fit), 134 (M11), 121 (M0) and 84 Ma (A34) are generated under the constraints of newly identified magnetic anomaly isochrons and fracture zones. The mismatch in the A34 reconstruction is attributed to the emplacement of the Ob, Lena and Marion Dufresne seamounts at the spreading ridge axis and/or the frequent ridge jumps in the Middle Cretaceous during the major plate reorganization.

Multibeam swath bathymetry data acquired over an area of about 30 000 km2, together with magnetic and single channel seismic data, have been analyzed to understand the tectonic evolution of the Andaman basin, northeast Indian Ocean. Swath bathymetry data revealed several morphotectonic features that divide the basin into a complex western part comprising arc-parallel seamount chains and N–S-trending fault systems and a relatively smooth eastern part. A SW–NE-trending spreading ridge bisects the basin. An offset of about 11.8 km at 94°21′E longitude divides the spreading ridge into SW and NE segments. High-resolution bathymetry and magnetic data enabled us to identify two finer-scale segments over the SW part of the ridge that display distinct topographic fabric compared to the NE segment. Thin sediment cover and presence of axial seamounts characterize SW segments of the ridge whereas the NE segment has thick sediment cover and well-maintained rift valley. Kinematics of basin evolution, derived from the identification of the seafloor spreading magnetic anomalies, suggest very slow initial opening rates of about 1.6 cm/yr and an increase in the spreading rate to about 3.8 cm/yr from anomaly 2 to present, although this interpretation relies only on magnetic anomalies in the two westernmost segments. It is suggested that the true seafloor spreading started about 4 Ma in the Andaman backarc basin, rather than the 11 Ma postulated previously. We infer based on the integrated analysis of swath bathymetry, magnetic and seismological data that the basin has evolved as a consequence of extrusive tectonics that prompted extension and rifting along the plane joining the Sagaing and Semengko fault systems. Extension by seafloor spreading during the past 4 Myr resulted in the formation of the deep Andaman backarc basin. This phase has also experienced westward propagation of the spreading center.

Analyses of multichannel seismic reflection, gravity, magnetic and bathymetry data along a regional profile across the central-western continental margin of India have revealed the depositional pattern of sediments, crustal structure and tectonics. The four most distinct and varied crustal regions of the margin are palaeo-shelf edges, shelf margin basin, Prathap and Laccadive Ridges and the Arabian Basin. The shelf margin basin is carpeted by ∼4.5 km maximum thick aggraded and prograded Paleocene to Holocene sediments. Six major seismic sequences of the sediments of the margin are identified and their ages are assigned on correlation with drill-well results. Development of the sequence boundaries is attributed to the events of rifting of western India, eustatic sea-level changes, Indian and Eurasian plate collision and Himalayan orogeny. Tilted fault blocks (half-grabens) located almost equi-distance from the igneous construct of the ‘Prathap Ridge’ in the shelf margin basin suggest a failed rift associated with stretched continental crust of the basin. 2-D model studies of gravity and magnetic anomalies, constrained by the seismic results, have revealed 6 to 27 km thick crust across the margin. The Laccadive Ridge crust limited by two volcanic intrusives and a steep scarp at its western end is ∼16 km thick. It gradually thins towards offshore and juxtaposed with early Tertiary normal oceanic crust ∼6 km thick of the Arabian Basin. The crustal thickness and velocity and density structure of the ridge are comparable to that of the Laxmi Ridge, a continental sliver. The inferences and abrupt change in magnetic and gravity anomaly signatures across the western end of the Laccadive Ridge mark the zone of transition from continental to oceanic crust.

Analyses of about 6000 km of processed magnetic data in the central Bay of Bengal using Analytical Signal Processing and Werner Deconvolution techniques revealed that the depth to top of the magnetic basement varies between 5 and 12 km from the sea surface, where the water column thickness is about 3.4 km. These inferred depths are comparable to the reported acoustic basement depths. The basement map derived from magnetic interpretation defines the general configuration of the central Bay of Bengal. The N10–12° W trending subsurface 85° E Ridge buried under 2 to 3 km thick sediments is a prominent tectonic feature. Offshore basins characterised by deeper magnetic basement (∼ 9 km) and 100–200 km wide are present on either sides of the ridge. These basins were filled with 6–8 km thick lower Cretaceous to recent sediments. Integrated geophysical study depicts that the magnetic basement is characterised by NW-SE, NE-SW, NNE-SSW, N10-12° W and E-W trending structural features that are associated with the lower Cretaceous ocean floor. The Analytical Signal Processing and Werner Deconvolution techniques proved to be effective in determining the depth to the basement in areas covered by thick sediment overburden and characterized by a complex geologic/tectonic framework.

The timing of the early separation of India from the contiguous Antarctica–Australia is still an unresolved problem although it is well established that Antarctica and India formed a single Indo-Antarctic platform prior to the fragmentation of eastern Gondwanaland. Inadequate age information either in the form of magnetic anomaly isochrons or dating of oceanic rocks from the conjugate margins of Antarctica and India perhaps led several authors to propose incomplete plate reconstruction models particularly for the early separation of India from Antarctica. Analysis of magnetic and satellite-derived gravity data in the Enderby Basin, East Antarctica, reveals the presence of seafloor spreading type linear magnetic anomalies and eight new fracture zones. The observed magnetic anomalies can be interpreted as the younger sequence of Mesozoic anomalies M11–M0. Half-spreading rates range from 6.5 to 2.8 cm/yr and are comparable with those measured in the Bay of Bengal. These similarities in the Mesozoic magnetic anomaly sequence and in the spreading rates provide evidence that these two basins are conjugate and contemporary. A consistent plate reconstruction model can be derived from the identified conjugate patterns of Mesozoic magnetic anomalies and fracture zones. The occurrence of the oldest magnetic anomaly M11 close to the coasts in these two offshore basins unequivocally suggests that the break-up of India from Antarctica occurred before ∼134 Ma.

Study of bathymetric data for the distal Bengal Fan reveals the presence of positive topographic features resembling seamounts. These features exhibit a relief of about 800 m above the surrounding seafloor at 4,400 m and are associated with large‐amplitude magnetic and gravity anomalies. Magnetic model studies suggest that one of the seamounts evolved along with the oceanic crust represented by the magnetic isochron A32 (∼ 72 My), while the other seamount was emplaced over the Cretaceous Quiet Zone Crust with reversed polarity. The seamounts studied provide an environment for possible locations of polymetallic encrustations. Morphology of the seamounts depends on several factors, including physical and chemical composition of the magma, eruption style, sediment thickness, maturity of underlying crust, and time of origin. Studying the seamounts together with integrated geophysical and physical data gave useful indications of their evolution and the existence of possible mineral deposits.

Swath bathymetric, gravity, and magnetic studies were carried out over a 55 km long segment of the Central Indian Ridge. The ridge is characterized by 12 to 15 km wide rift valley bounded by steep walls and prominent volcanic constructional ridges on either side of the central rift valley. A transform fault at 7°45′S displaces the ridge axis. A mantle Bouguer anomaly low of −14 mGals and shallowing of rift valley over the middle of the ridge segment indicate along axis crustal thickness variations. A poorly developed neovolcanic zone on the inner rift valley floor indicate dominance of tectonic extension. The off-axis volcanic ridgs suggest enhanced magmatic activity during the recent past.

The submerged 85°E Ridge in the Bay of Bengal trends approximately N-S between 19°N and 6°N latitudes. Off the southeast coast of Sri Lanka it takes an arcuate shape and seems to terminate with the northward extension of the Afanasy Nikitin seamounts situated around 2°S latitude. The ridge is characterized by positive magnetic (100–400 nT) and negative free-air gravity (

In fine-grained, faulted sediments, both stratigraphic and fault-induced structural variations can simultaneously determine the gas hydrate distribution. Insights into hydrate distribution can be obtained from P-wave velocity (VP) and attenuation (QP -1 ) character of the gas hydrate stability zone (GHSZ). In this paper, we apply frequency-domain full-waveform inversion (FWI) to surface-towed 2D multichannel seismic data from the Krishna-Godavari (KG) Basin, India, to image the fine-scale (100 X 30 m) VP and QP -1 variations within the GHSZ. We validate the inverted VP model by reconciling it with a sonic log from a nearby (~250 m) well. The VP model shows a patchy distribution of hydrate. Away from the faults-dominated parts of the profile, hydrates demonstrate stratigraphic control which appears to be permeability driven. The QP -1 model suggests that attenuation is relatively suppressed in hydratesbearing sediments. Elevated attenuation in non-hydrate-bearing sediments could be driven by the apparent pore-fluid immiscibility at seismic wavelengths. The VP and the QP -1 models also suggest that fault zones within the GHSZ can be hydrate- or free-gas-rich depending on the relative supply of free gas and water from below the GHSZ.

[1] A combination of diffusion and advection in fine grained sediments can create a patchy Bottom Simulating Reflector (BSR) which has little to no apparent correlation with the overlying hydrate distribution. Using 2D seismic data from faulted, clay-rich sediments in the Krishna-Godavari (KG) basin, we show both the hydrate distribution and the BSR structure are fault controlled. Our demonstration hinges upon a kinematically accurate P wave velocity (VP) model which is estimated using a composite traveltime-inversion, depth-migration method in an iterative manner. The flexibility of the method allows simultaneous usage of traveltimes from multiple, discontinuous reflectors. The application begins with a simple VP model from time processing which is reflective of a diffusive, continuous, hydrate– and free gas–bearing system. The application converges in three iterations yielding a final VPmodel which is suggestive of a patchy distribution of hydrates and free gas possibly developing through a combined diffusive-advective system. The depth image corresponding to the final VP model can be interpreted for faults that suggest ongoing tectonism. The BSR appears to be truncated at active faults zones. Both the final VP model and the corresponding depth image can be reconciled with the hydrate distribution and BSR depth at logging/coring sites located ∼250 m away from the line by projecting the sites along the strike direction of the regional faults.

The analysis of 8200 line km of total magnetic intensity data in the Bay of Bengal, northeastern Indian Ocean, revealed the presence of approximately N30°E-trending seafloor spreading type magnetic anomalies. These anomalies resemble the Mesozoic anomaly series (M11–M0) reported elsewhere. The oldest anomaly (M11, 132.5 Ma) identified close to the east coast of India is followed by the younger series of Mesozoic anomalies towards the offshore. Some of the anomalies are offset by 60–80 km. The configuration of the offsets of the isochrons allowed us to propose approximately N120°E-trending oceanic fracture zones. The Mesozoic crust of 132.5–118 Ma is estimated to evolve with an average half-spreading rate of 3.5 cm/yr, except for the ocean floor between the M9 and M4 magnetic isochrons. This part of the crust appears to be affected by the buried subsurface 85°E Ridge.

The study of magnetic, gravity and bathymetric data over a segment of Carlsberg Ridge between 3° and 4°N latitude, and 63°30′ and 64°30′E longitude revealed the presence of a prominent N57°W trending rift valley. The depth of the valley varies from 3250 to 4400 m and its width varies from 15 to 19 km. Analysis of the magnetic data leads to the identification of the Brunhes/Matuyama boundary (0.73 Ma) across the ridge axis. The Jaramillo event commonly associated with the fast spreading centres is not reflected in the present study. The late Pliocene-Recent oceanic crust across part of the Carlsberg Ridge evolved with an average spreading rate of 1.25 cm yr −1 . The rift valley shallows in the southeast, narrows in the middle and is deeper and wider toward the northwest. The occurrence of large amplitude magnetic and gravity anomalies in association with the bathymetric deep in the northwest within the rift valley is suggested to be a zone of future segmentation of the ridge axis. A new transform fault appears to offset the trend of bathymetric and gravity contours in the northwestern part of the study area.

A study of the bathymetric and linear magnetic anomalies between Dronning Maud Land, Antarctica and the South West Indian Ridge (SWIR) revealed a Mesozoic sequence of linear magnetic anomalies, M0 to M12 (108–126 Ma), a fracture zone offset (∼- 130 km), and the Cretaceous magnetic quiet zone between isochrons M0 (108 Ma) and 34 (80 Ma). The study sheds light on: 1. (1) the evolution of an anomalous “stretched” pattern of seafloor anomalies during magnetic isochrons M0 and 34, and the associated offset (> 10°) in some segments of the northeastern part of the SWIR; 2. (2) continuation of a proposed buried fracture zone to the south, where it abuts the Astrid Ridge off Dronning Maud Land. The history of spreading is related to declining dynamic pressures at the plate margin due to the Kergeulen-Heard mantle plume during Cretaceous time (∼-110 Ma).

Large quantities of gas hydrate were discovered while drilling in the Krishna-Godavari (K-G) basin along the Indian east coast in 2006. Drilling also showed significant inhomogenity in hydrate distribution. Using the unified imaging approach of Jaiswal an

Bathymetric and gravity data totalling 2000 line kilometres on the continental margin off Goa and Mulki, India, have been studied and prominent NNW-SSE and ENE-WSW trending free-air gravity anomalies varying between −60 and +25 mGal have been identified. Using constraints from seismic refraction and reflection and well log results the thicknesses of the layers are estimated by computing two-dimensional crustal structure models for the free-air gravity anomalies. The study suggests that (1) there are three layers varying in thickness from 0.5 to 4.0 km. 0.5 to 12.0 km and from 14.0 to 26.0 km with densities of 2.3, 2.6 and 2.9 g/cm 3 which are interpreted as sedimentary, granitic and mafic/basaltic layers respectively, that (2) there is a maximum thickness of 4.0 km of sediment over the shelf margin basin, and that (3) the Moho occurs at a depth of 24–30 km.

A regional scale bathymetric map has been constructed for the 79°E fracture zone (FZ) in the Central Indian Basin between 10°15′S and 14°45′S latitudes and 78°55′E and 79°20′E longitudes using the high-resolution capabilities of the Hydrosweep multibeam swath bathymetric survey system. The bathymetric map clearly reveals the 79°E FZ, depicting a ridge and trough topography with an elevation difference of over 300 m. Prominent E-W bathymetric lineations and several new features such as seamounts are conspicuous on the map. The E-W bathymetric lineations bend at the fracture zone toward south resulting in oblique extensional features. These features are more prominent in the middle and northern portion of the study area and are less significant in the southern portion. The prominence of these features in the north and their subtle presence in the south are probably related to the strong and weak shear coupling at the RTI, respectively, during the various stages of FZ evolution. Gravity and magnetic data were analyzed together with the bathymetric data. The observed free-air gravity across the 79°E FZ is modeled in terms of the lithospheric thickness, taking into consideration the thermal evolution of the oceanic lithosphere. A lithosphere thickness of about 100 km is inferred from these studies.

A Detailed study of magnetics and bathymetry was carried out in the Central Indian Basin between latitudes 9°S and 16°30′S and longitudes 73°E and 79°30′E. The observed east-west trending magnetic anomalies are identified as anomalies 21 to 25. The magnetic lineation pattern revealed the presence of a new fracture zone at 75°45′E trending N12°E. This fracture zone runs parallel to the 86°E fracture zone and appears to change its direction south of 20°S latitude. The differential offsets observed in the identified magnetic anomalies across the new fracture zone are explained in terms of the evolution of the triple junction. The study also confirms the strike of the 79°E fracture zone and its southern extension through the study area.