Structure,mechanical properties and evolution of the lithosphere below the northwest continental margin of India

International Journal of Earth Sciences - The continental breakup history at the northwest continental margin of India remained conjectural due to lack of clearly discernable magnetic anomaly...     

Static deformation of two welded monoclinic elastic half-spaces due to a long inclined strike-slip fault

Static deformation of two monoclinic elastic half-spaces in welded contact due to a long inclined strike-slip fault situated in one of the half-spaces is studied analytically and numerically. Closed-form algebraic expressions for the displacement at any point of the medium are obtained. The variation of the displacement at the interface with the horizontal distance from the fault is studied. The effect of anisotropy on the displacement field is examined. It is found that while the anisotropy of the source half-space has a significant effect on the displacement at the interface, the anisotropy of the other half-space has only a marginal effect.     

Light hydrocarbons geochemistry of surface sediment from petroliferous region of the Mehsana block,North Cambay Basin

A study was carried out to test the usefulness of surface geochemical methods as regional evaluation tools in petroliferous region of the Mehsana block, North Cambay Basin. A suite of 135 soil samples collected from the depth of 2.5 m, were analyzed for adsorbed light gaseous hydrocarbons and carbon isotopes (δ¹³C_methane and δ¹³C_ethane). The light gaseous hydrocarbon analysis show that the concentration ranges 402 ppb, 135 ppb, 70 ppb, 9 ppb and 18 ppb of C₁, C₂, C₃, iC₄ and nC₄, respectively. The value of carbon isotopic ranges of methane −29.5 to −43.0‰ (PDB) and ethane −19.1 to −20.9‰ (PDB). This data, when mapped, indicates patterns coinciding with major known oil and/or gas field of Sobhasan/Linch in this study area. The existence of un-altered petroliferous microseeps of catagenetic origin is observed in the study area. A regional study, such as the one described here, can provide important exploration facts concerning the regional hydrocarbon potential in a block. This method has been confirmed and can be applied successfully in frontier basins.     

Contamination of shallow aquifers by arsenic in upper reaches of Tista river at Siliguri–Jalpaiguri area of West Bengal, India

Arsenic is present in groundwater at Siliguri–Jalpaiguri area, West Bengal, India. This is the place where Tista river descending from the Himalayas meets the alluvial plain. The area represents alluvial fan and floodplains of Tista, Mahananda-Balasan, Jaladhaka and its tributaries. In the river sediment samples, para- and ferro-magnetic minerals within 0.3–0.05 mm fraction contain 9–80 ppm of arsenic. The study indicates that iron bearing minerals viz. biotite, hornblende as well as iron coated grains of the sediment are major contributors towards arsenic budget. Though magnetite as a mineral shows maximum arsenic content (22 ppm), it is volumetrically not of much significance. Measurement of groundwater collected from tube wells shows up to 0.05 ppm of arsenic. These arsenic contaminated tube wells occur in a linear fashion along the course of the rivers. Moreover, localization of contaminated tube wells coincides with the change of channel gradient as observed in longitudinal section. The study enumerates a cause–effect relationship of arsenic occurrence with river gradient and fluvial sedimentation.     

Distribution of biochemical constituents in the surface sediments of western coastal Bay of Bengal: influence of river discharge and water column properties

Biochemical composition of surface sediment samples from off major and minor rivers along the east coast of India revealed that spatial distribution of sediment organic carbon (SOC) composition was mainly governed by differential characteristics of discharged water and associated biogeochemical processes in the water column. The northwest (NW) region of coastal Bay of Bengal was influenced by discharges from Ganges river while peninsular (monsoonal) rivers influenced the southwest (SW) region. The NW region characterized by low nutrients suspended particulate matter (SPM), high phytoplankton biomass in the water column and high SOC while contrasting to that observed in the SW region. The isotopic ratios of SOC (?22 ‰) in the NW region were close to that of organic matter derived from phytoplankton (?23 ‰) suggesting in situ production is the major source whereas terrigeneous source contributed significantly in the SW region (?19.6 ‰). Though low in situ biological production in the SW region, relatively higher total carbohydrates (TCHO) were found than in the NW and insignificant difference of total and free amino acid concentrations between NW and SW were resulted from faster removal of organic matter to the sediment in association with SPM in the SW region. Higher proteins concentrations than total amino acids indicate that nitrogenous organic matter is preserved in the former form. The protein to TCHO ratio was lower in the SW suggesting significant contribution of aged and non-living organic matter in this region.     

Metamorphic reactions in dry and aluminous granulites: a Perple_X <Emphasis Type="Italic">P–T</Emphasis> pseudosection analysis of the influence of effective reaction volume

In high-Mg, Al metapelites, monophase sapphirine corona occur around spinel–corundum aggregates in monomineralic cordierite layers, and bi-phase orthopyroxene–sillimanite aggregates replace locally warped sapphirine in polygonized cordierite aggregates. P–T phase topologies computed (Perple_X software) using compositions of cordierite-rich layers that host the reaction textures did not match the assemblages for the discontinuous reactions spinel + corundum + cordierite → sapphirine and sapphirine + cordierite → orthopyroxene + sillimanite. Instead, the reaction assemblages were reproduced using P–T pseudosection analysis for micro-domain reaction volumes estimated from compositions of product phases in the volume proportion they occur. The results are consistent with known phase relations deduced using Schreinmakers P–T grids. Apparently, the compositions of cordierite-rich layers that hosted the reaction textures were inadequate chemical proxies for determining P–T–X relations of phase-boundary controlled reactions influenced by compositions of the nearest-neighbor minerals in the proportion they react (effective composition), and not in the proportion they existed in the layer/bulk rock. In other words, P–T–X phase topologies and reconstructed P–T paths in dry and aluminous rocks may be best understood by thermodynamic modeling of reactions using effective reaction volume compositions rather than the bulk composition of the rock or the mineralogical layer that host the reaction textures.     

Groundwater flow and solute transport at the Mourquong saline-water disposal basin, Murray Basin, southeastern Australia

Saline groundwater and drainage effluent from irrigation are commonly stored in some 200 natural and artificial saline-water disposal basins throughout the Murray-Darling Basin of Australia. Their impact on underlying aquifers and the River Murray, one of Australia's major water supplies, is of serious concern. In one such scheme, saline groundwater is pumped into Lake Mourquong, a natural groundwater discharge complex. The disposal basin is hydrodynamically restricted by low-permeability lacustrine clays, but there are vulnerable areas in the southeast where the clay is apparently missing. The extent of vertical and lateral leakage of basin brines and the processes controlling their migration are examined using (1) analyses of chloride and stable isotopes of water (²H/¹H and ¹⁸O/¹⁶O) to infer mixing between regional groundwater and lake water, and (2) the variable-density groundwater flow and solute-transport code SUTRA. Hydrochemical results indicate that evaporated disposal water has moved at least 100 m in an easterly direction and that there is negligible movement of brines in a southerly direction towards the River Murray. The model is used to consider various management scenarios. Salt-load movement to the River Murray was highest in a "worst-case" scenario with irrigation employed between the basin and the River Murray. Present-day operating conditions lead to little, if any, direct movement of brine from the basin into the river. Electronic Publication     

Kinematics of the Gondwana basins of peninsular India

The Gondwana successions (1–4 km thick) of peninsular India accumulated in a number of discrete basins during Permo-Triassic period. The basins are typically bounded by faults that developed along Precambrian lineaments during deposition, as well as affected by intrabasinal faults indicating fault-controlled synsedimentary subsidence. The patterns of the intrabasinal faults and their relationships with the respective basin-bounding faults represent both extensional and strike-slip regimes. Field evidence suggests that preferential subsidence in locales of differently oriented discontinuities in the Precambrian basement led to development of Gondwana basins with varying, but mutually compatible, kinematics during a bulk motion, grossly along the present-day E–W direction. The kinematic disparity of the individual basins resulted due to different relative orientations of the basement discontinuities and is illustrated with the help of a simple sandbox model. The regional E–W motion was accommodated by strike-slip motion on the transcontinental fault in the north.     

Variations in long term wind speed during different decades in Arabian Sea and Bay of Bengal

A study has been carried out by comparing the extreme wind speeds estimated based on NCEP/NCAR reanalysis data for 100 years return period using Fischer Tippet-1 (commonly known as Gumbel) and Weibull distributions for three locations (off Goa, Visakhapatnam and Machilipatnam) in the north Indian Ocean. The wind dataset for Goa is compared with that from ERA-40 data. For higher wind speeds (12–20m s⁻¹), NCEP wind speed has higher percentage of occurrence than that of ERA-40. Analysis has shown slight upward trend in the annual maximum wind for location off Machilipatnam with an increase of 1.2 cm s⁻¹ per year and a decreasing trend of −1.3 cm s⁻¹ per year in the case of Goa. The Weibull distribution with shape parameter 2 fits the annual maximum wind data better than FT-1 distribution.     

Hydrologic characteristics of floods in Ganges–Brahmaputra–Meghna (GBM) delta

In the middle of 2007, a severe flood affected the People’s Republic of Bangladesh. This is a natural disaster that takes people’s lives, destroys livestock, infrastructures and communication systems and, damages crops and fish ponds. Despite many adverse impacts, the flood situation is an accepted phenomenon to the citizens of Bangladesh, due to the immense increase of soil fertility due to the flood, plus, the recharge of aquifer, ecosystem and fish. The flood of 2007 was the 5th major flood of the last 20 years when more than thirty-five percent of the area of the country was inundated with flood water. As in the past, the flood of 2007 had its own significance. The geography of the country contains a floodplain delta of three major river basins: the Ganges, the Brahmaputra and the Meghna (GBM). The mean monthly rainfall plot from the TRMM satellite data has shown that for both the Meghna and Brahmaputra basins, the rainfall was higher during July 2007 than any other months of the last 2 years. This excess rainfall had accumulated in the Brahmaputra and Meghna rivers and carried downstream to Bangladesh. This was the main cause of the flooding in 2007. The first crossing above the danger level of the river waters was observed at Durgapur station of the Someswari and at Sunamganj station of the Surma on the nineteenth of July, 2007 inside Bangladesh. In terms of magnitude of the peak and duration of the flood, the Brahmaputra was higher in 2007 than during 2004. However, the Ganges river water level never crossed the danger level during flood of 2007. The Meghna was lower during the flood peak for the duration of the flood in 2007. The year–to-year variability in both the magnitude and duration of the flood suggests changes in rainfall and landuse pattern of the catchment.