Eastern Margin Of India Research Articles

Chapter 19 The Sagaing Fault, Myanmar

The Sagaing Fault is amongst the longest and most active strike-slip faults in the world (e.g. Molnar & Dayem 2010; Robinson et al. 2010; Searle & Morley 2011). It accommodates more than half of the right-lateral motion between Sundaland and India within the diffuse plate boundary along the eastern margin of India, which occupies much of Myanmar (e.g. Vigny et al. 2003; Nielsen et al. 2004; Socquet et al. 2006) (Fig. 19.1). Acting as a ridge-subduction transform (Le Dain et al. 1984; Guzman-Speziale & Ni 1996; Yeats et al. 1997), the 1500 km long Sagaing Fault links major thrust systems in the north such as the Naga, Lohit and Main Central thrust zones near the eastern Himalayan syntaxis to the Andaman Sea spreading centre in the south (e.g. Win Swe 1970; Le Dain et al. 1984; Guzman-Speziale & Ni 1993; Curray 2005). Acting as a sliver-bounding lithospheric strike-slip partition inboard of a 3700 km long section of oblique subduction at the Sunda Trench, it links to the dextral West Andaman and Sumatran faults in the south (e.g. Curray et al. 1979; Nielsen et al. 2004; Curray 2005; Searle & Morley 2011). Fig. 19.1. Tectonic setting of the Sagaing Fault. Modified after Morley et al. (2011), Soe Thura Tun & Maung Thein (2012), Wang et al. (2014) and Morley & Alvey (2015). Topographic scarps, lineaments, earthquake clusters and gravity anomalies along the trace of the Sagaing Fault have long been recognized as indicating the presence of an important north–south-trending structure west of the Shan Plateau (e.g. La Touche 1913; Coggin Brown & Leicester 1933; Chhibber 1934; Dey 1968; Aung Khin et al. 1970) (Fig. 19.2a). Large parts of the fault were first …

Influence of fracture orientation distribution function on the estimates of gas-hydrate saturation of site 10 at Krishna-Godavari Basin, India

<p>We estimate the gas-hydrate saturation in fractured reservoirs of gas hydrate-rich marine sediments from Krishna-Godavari basin along the eastern margin of India. Effective medium modeling (EMM) is a combination of self-consistent approximation (SCA), differential effective medium (DEM) modeling and smoothing and it deals with the anisotropy associated with clay platelets’ orientation. It proves to be a good choice attributable to its dependency on physical parameters alone. Previous approaches appertain to the implementation of orientation distribution function (ODF) on clay platelets alone assuming a vertical ellipsoidal shape of gas hydrates whilst in the present study, we have implemented ODF on hydrate bearing fractures assigning an ellipsoidal shape to them as well. A bias of ~2-4% in the saturation estimates apparently streamlines the betterness of our method.</p>

Seismic Insights into Bottom Simulating Reflection (BSR) in the Krishna-Godavari Basin, Eastern Margin of India

The multichannel seismic data along one long-offset survey line from Krishna-Godavari (K-G) basin in the eastern margin of India were analyzed to define the seismic character of the gas hydrate/free gas bearing sediments. The discontinuous nature of bottom simulating reflection (BSR) was carefully examined. The presence of active faults and possible upward fluid circulation explain the discontinuous nature and low amplitude of the BSR. The study reveals free gas below gas hydrates, which is also indicated by enhancement of seismic amplitudes with offsets from BSR. These findings were characterized by computing seismic attributes such as the reflection strength and instantaneous frequency along the line. Geothermal gradients were computed for 18°C and 20°C temperature at the depth of BSR to understand the geothermal anomaly that can explain the dispersed nature of BSR. The estimated geothermal gradient shows an increase from 32°C/km in the slope region to 41°C/km in the deeper part, where free gas is present. The ray-based travel time inversion of identifiable reflected phases was also carried out along the line. The result of velocity tomography delineates the high-velocity (1.85–2.0 km/s) gas hydrate bearing sediments and low-velocity (1.45–1.5 km/s) free gas bearing sediments across the BSR.

Characteristics of bottom-simulating reflectors for Hydrate-filled fractured sediments in Krishna–Godavari basin, eastern Indian margin

The bottom-simulating reflector (BSR) is weak and patchy on the seismic section in the Krishna–Godavari basin, eastern Indian margin, where massive gas-hydrates have been recovered at Site 10 of the Indian National Gas-hydrates Program Expedition 01 (NGHP-01). The depth of the BSR near this site is around 160 m below the sea floor (mbsf). The average reflection coefficient from the BSR is −0.06, significantly smaller than the common global values of −0.1 to −0.2. The BSR shows a strong lateral variation in amplitudes along the seismic line due to the presence of faults. The methane solubility is modeled using a theoretical model of the gas-hydrates system, and methane concentrations from the pressure core show that the distribution of free gas below BSR is not uniform. A combination of synthetic seismogram analysis and rock physics modeling leads to the conclusion that weak and patchy BSRs are primarily caused by lateral discontinuities induced by the gas-filled fractures below BSRs. The free gas zone is thin and it shows segmented characteristics on the seismic section and acoustic impedance profile that we inverted. Fault zones increase the permeability and therefore trap gas in associated fractures that can scatter seismic energy and create low velocity zones.

A kinetic-model approach to quantify the effect of mass transport deposits on pore water profiles in the Krishna–Godavari Basin, Bay of Bengal

Mass transport deposits (MTDs) in the Krishna–Godavari basin from the eastern Indian margin are common sedimentary features over the modern continental slope. Quantitative understanding of the scale and age of these MTDs is relevant to understanding their triggering mechanisms and environmental feedbacks, as well as for interpreting pore water chemical profiles. Pore water profiles in sediments recovered during the 2006 Indian National Gas Hydrate Project (NGHP-01) expedition suggest that MTDs are present at seven of the ten sites cored in the Krishna–Godavari basin. Kinetic modeling of the S-shaped pore water sulfate and ammonium profiles from these sites provides quantitative estimates of the individual MTDs thickness, time elapsed after the event, rate of organic matter-fueled sulfate reduction, and time required to reach a new steady state. Model results suggest that the MTDs at the seven study sites are 8–25 m thick and are 300–1600 years old. Within the MTD sections, the organic matter-fueled sulfate reduction rates are 130–1200 mmol/m2/yr and the time needed to reach a new steady state ranges from 2000 to 3800 years. In comparison to depth-integrated sulfate reduction rates estimated in other regions, our estimates are relatively high reflecting the much thicker sulfate reduction zone that results from the MTDs. A positive correlation is observed between water depth and the MTD activity index defined as thickness of MTD package divided by its emplacement age. This correlation suggests the close relationship between MTD activity and water depth which agrees with previous studies of MTDs in this region and elsewhere. We caution against using the pore water data from sites experiencing significant MTDs to infer metabolic processes and to quantify steady-state reaction rates as our model results confirm the remarkable influence of transient MTDs on pore water profiles.

Gas hydrate and free gas saturations using rock physics modelling at site NGHP-01-05 and 07 in the Krishna–Godavari Basin, eastern Indian margin

Several approaches exist for the estimation of gas hydrate and free gas depending on the nature and distribution of gas hydrate into the sediments. Here, we apply various rock physics models to the P-wave sonic velocity logs that were collected during the Indian National Gas Hydrate Program (NGHP) Expedition 01 at sites NGHP-01-05 and NGHP-01-07 for the resource estimate of gas hydrate. Using the weighted equation, it is found that maximum up to13% and 12% volumes of the pores of regional sediments above bottom simulating reflector (BSR) of sites NGHP-01-05 and NGHP-01-07 respectively are occupied by gas hydrates. Alternatively, gas hydrate saturations computed for these sites using effective medium models are found maximum up to 16% and 22%. We have further used the velocity porosity transforms to estimate the free gas saturations below the BSRs for both the sites. The saturation of free gas in the sediments at both the sites is found in the range of 0.5–2.2%.

Palaeomagnetic, geochronological and geochemical study of Mesoproterozoic Lakhna Dykes in the Bastar Craton, India: Implications for the Mesoproterozoic supercontinent

Palaeomagnetic analysis of the Lakhna Dykes (Bastar Craton, India) yields a palaeopole at 36.6°N, 132.8°E, dp=12.4°, dm=15.9°, and the UPb zircon age obtained from one of the rhyolitic dykes is 1466.4±2.6Ma (MSWD=0.21, concordia age based on two analyses with identical Pb/U ages), similar to previously published UPb ages. Major and trace element analyses of the Lakhna Dykes show shoshonitic and high-K calc-alkaline affinities consistent with a subduction related characteristics suggesting an active continental margin setting. This is in keeping with the Palaeo- to Mesoproterozoic tectonic environments in the eastern Indian margin. The new 1460Ma Indian palaeopole was used to test possible palaeopositions of India within the Mesoproterozoic supercontinent Columbia. Of the four palaeomagnetically permissible reconstructions, juxtaposing western India against south-west Baltica is geologically the most reliably constrained and best fitting model. Our preferred reconstruction implies a long Palaeo- to Mesoproterozoic accretionary orogen stretching from south-eastern Laurentia through south-western Baltica to south-eastern India. Breakup of India and Baltica probably occurred in the Late Mesoproterozoic, but additional constraints are needed.

Geothermal modeling for the base of gas hydrate stability zone and saturation of gas hydrate in the Krishna-Godavari basin, eastern Indian margin

The passive eastern Indian margin is rich in gas hydrates, as inferred from the wide-spread occurrences of bottom-simulating reflectors (BSRs) and recovery of gas hydrate samples from various sites in the Krishna Godavari (KG) and Mahanadi (MN) basins drilled by the Expedition 01 of the Indian National Gas Hydrate Program (NGHP). The BSRs are often interpreted to mark the thermally controlled base of gas hydrate stability zone (BGHSZ). Most of the BSRs exhibit moderate to typically higher amplitudes than those from other seismic reflectors. We estimate the average geothermal gradient of ∼40°C/km and heat flow varying from 23 to 62 mW/m2 in the study area utilizing the BSR’s observed on seismic sections. Further we provide the BGHSZ where the BSR is not continuous or disturbed by local tectonics or hidden by sedimentation patterns parallel to the seafloor with a view to understand the nature of BSR.

Effective medium modeling of gas hydrate‐filled fractures using the sonic log in the Krishna‐Godavari basin, offshore eastern India

Indian National Gas Hydrate Program Expedition 01 has established that clay‐rich marine sediment from the Krishna‐Godavari (KG) basin in the eastern Indian margin hosts one of the richest gas hydrate deposits in the world. Resistivity at‐bit images and pressure cores reveal that the gas hydrate morphology in clay‐rich sediment varies from complex vein structures (grain displacing) to invisible pore filling. Existing rock physics models, which relate acoustic data to in situ gas hydrate concentrations, generally assume isotropic pore‐filling gas hydrate, which yields misleading concentration estimates for fractured fine‐grained sediments. The anisotropic KG basin sediment presents additional complications. Here we apply differential effective medium theory to incorporate grain‐displacing morphologies by which gas hydrate is included as vertical ellipsoids with aspect ratios ranging from those of thin veins up to those of nodules in an elastic anisotropic background. We have estimated gas hydrate concentrations from sonic velocities at hole 10D in the KG basin considering three basic gas hydrate morphologies: (i) pore filling, (ii) grain displacing, and (iii) a combination of grain displacing and pore filling. Average gas hydrate saturations for these three cases are 35–42%, 27–30%, and 33–41% of the total porosity, respectively, in the depth range 60–140 m below seafloor (mbsf). Saturation is highest at ∼67 mbsf for any morphology but the values differ between morphologies. For the pore‐filling morphology, the maximum gas hydrate saturation of 56% is 18–22% higher than the grain‐displacing morphology and 2–9% higher than the combined morphology. Estimates differ by ±6% of the sediment volume with rotations of gas hydrate veins from vertical to horizontal.

Petrogenesis of basalts for Sangxiu Formation in the central segment from Tethyan Himalayas: Plume-lithosphere interaction

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 125-7

Rapid emplacement of the Kerguelen plume–related Sylhet Traps, eastern India: Evidence from 40Ar‐39Ar geochronology

We report for the first time 40Ar‐39Ar plateau ages for the Sylhet Traps of eastern India. Our results provide concordant ages for two samples, vertically separated by ∼200 m, from a tholeiite lava flow sequence. The ages are indistinguishable at 2σ confidence level indicating a rapid emplacement of these lavas. The weighted mean of the plateau ages associated with least errors, 116.0 ± 3.5 Ma, most likely represents the age of eruption. Clearly, the Sylhet Traps are contemporaneous with the Kerguelen plume generated Rajmahal and Bengal Traps. Our results in conjunction with the existing age data in the Rajmahal‐Bengal‐Sylhet igneous province suggest that the latter experienced widespread, and rapid emplacement of flood basalts at ∼118 ± 2 Ma. Such a large‐scale volcanism would have required a direct involvement of the Kerguelen plume, suggesting that the Kerguelen hotspot was located close to the eastern Indian margin during its initiation.

Origin of Cretaceous continental tholeiites in southwestern Australia and eastern India: insights from Hf and Os isotopes

Rifting between eastern India and western Australia in the Early Cretaceous (∼140–133 Ma) resulted in the formation of the eastern Indian Ocean. Tholeiitic flood basalts erupted on the southwestern margin of Australia (Bunbury basalts at ∼132 and ∼123 Ma) and the eastern margin of India (Rajmahal Traps at ∼118 Ma) shortly after this time, while the Kerguelen Plateau began to form in the nascent ocean basin (∼119 Ma). We studied the Hf and Os isotopic systematics of the continental basalts in order to better understand the relationship between these volcanic events and the Kerguelen plume. The objective of this work was to determine the mantle sources of the continental tholeiites and the nature and quantity of continental contamination. Both the Hf and Os isotopic compositions of these basalts vary with indicators of continental contamination (SiO 2, (Th/Ta) PM, (La/Nb) PM). There is no obvious correlation of γ Os( T) with Sr or Nd isotopic ratios, but rough correlations are present between Os and Hf isotopes and 208Pb/ 204Pb; in contrast, ε Hf( T) shows strong correlations with all isotopic systems and indicators of continental crust assimilation. We demonstrate that the mantle sources of these continental tholeiites cannot originate purely from the depleted asthenosphere. An enriched mantle source is required for both of these basalt suites and the geochemical characteristics of this source are broadly consistent with those inferred to be present in the Kerguelen plume during Early Cretaceous time. The Os isotopic results suggest that at least some continental contamination is present in all samples from both the Bunbury basalts and the Rajmahal Traps. In the Rajmahal Traps, this contaminant appears to be derived from the continental crust and there is little to no evidence for involvement of subcontinental lithospheric mantle (SCLM). The Bunbury basalts have consistently unradiogenic Os isotopic compositions and high ε Hf( T) for a given ε Nd( T), characteristics ascribed to the SCLM. Previous investigations of these basalt suites have concentrated on major and trace elements and Sr, Nd and Pb isotopes; none of these results suggested the involvement of SCLM in the Bunbury basalts. Here, we emphasize that the study of Hf and Os isotopes provide additional, useful tools for deciphering mantle sources and continental contaminants involved in the petrogenesis of continental tholeiites.

40Ar/39Ar Geochronology of the Rajmahal Basalts, India, and their Relationship to the Kerguelen Plateau

During the mid-Cretaceous, extensive magmatism occurred in the Indian Ocean to form volcanic portions of the southern and central Kerguelen Plateau, Elan Bank and Broken Ridge. Basalt was erupted also along the rifted margin of eastern India (Rajmahal). We investigated the ages of these Indian basalts using Ar-40/Ar-39 incremental-heating experiments on whole rocks. Our results are consistent with the hypothesis that the lava pile of similar to230 m thickness in the Rajmahal Hills, Jharkhand, and alkalic basalts in the Bengal Basin were emplaced at similar to118 Ma. Dykes intruded to the SW of the Rajmahal Hills appear to be 2-3 Myr younger than these lavas. Magmatic activity in eastern India therefore was contemporaneous with the final stage of volcanism at Ocean Drilling Program Site 1136 on the Southern Kerguelen Plateau (119-118 Ma), but older than final magmatism at Sites 749 and 750 on the Southern Kerguelen Plateau (112-110 Ma), Site 1137 on Elan Bank (108 Ma) and Site 1138 on the Central Kerguelen Plateau (100 Ma). By combining these age data with plate reconstructions that take into account the motion of hotspots in a convecting mantle, we suggest that eruption of the Rajmahal basalts, formation of the Southern Kerguelen Plateau, and Elan Bank's separation from India are best explained by the presence of the Kerguelen hotspot close to the eastern Indian margin just after 120 Ma.

Geophysical studies over the continental margins of the east coast of India

Results of six profiles of continuous echosounding, magnetic and gravity data collected across the continental margin of the east coast of India between 10° and 20° N in the Bay of Bengal aboard R.V. “Chain” in 1973 are presented. Continuity of surface and subsurface features from profile to profile is indicated. Two major structural elements — viz. the “Marginal High”, situated at the foot of the continental slope and extending along the east coast of India, and the “Marginal Basin”, wide in the south becoming narrower towards the north — have been delineated. Based on the topography and the gravity and the magnetic anomalies, the eastern margin of India may be divided into two parts, viz. the southern and northern margins. The southern margin is broader and more stable than the northern margin, over which lava flows occur.