• OM PRAKASH

Articles written in Journal of Earth System Science

• On the seismic structure of Indian upper-mantle regions

Several velocity models on upper mantle regions of the world have been postulated during the last two decades. There has been a broad agreement amongst seismologists that upper mantle has got two transition zones, though the models differ in detail. These zones have been found to occur around ‘400 km’ and ‘650 km’ depth ranges with varying thicknesses of the zones. A limited number of such studies have been made on the upper mantle structure of the Indian subcontinent. High positive velocity gradients were reported to exist around the above depth range. Evidence for lateral heterogeneities has also been found. We address some problems like refinement of Indian upper mantle velocity models specially after considering the effect of scattering and attenuation on the short periodP-waves. The study of proper positioning of the cusps of the travel-time branches and their extension is essential as well. In our opinion, analysis of such problems would help in the better understanding of the nature of propagation of seismic waves and mechanism of earthquakes. Complexity of seismic signatures observed is another major problem and may also be taken into consideration.

• An integrated approach of GIS, RUSLE and AHP to model soil erosion in West Kameng watershed, Arunachal Pradesh

Soil erosion has always been a major environmental problem in many parts of the world including the northeastern region of India. An increase in the rate of soil erosion has tremendous implications on land degradation, biodiversity loss, productivity, etc. Hence, assessment of soil erosion hazard and its spatial distribution is essential to serve as a baseline data for effective control measures. The present study uses revised universal soil loss equation (RUSLE) and analytical hierarchy process (AHP) approach integrated with geospatial technology for modeling soil erosion hazard zone of West Kameng watershed of Arunachal Pradesh, Northeast India. The assessment showed that the erodibility factor of soil ranged between 0 and 0.38 t/ha/MJ/mm and slope length and steepness factor increases with increase in slope angle. Lower normalized difference vegetation index (NDVI) values depict vegetation cover and higher values represent the rocky area or barren land. Spatial distribution of conservation support practice on soil loss indicated the variability (0–1) where lower value represents the higher conservation practice.The predicted average soil erosion rate was 124.21 t/ha/Yr. Normalized eigen vector values ranged between 0.03 and 0.20. The areas with more slope, relative relief, drainage density, lineament density, and frequency have shown comparatively higher eigen vector values, and it has been noticed that the strength of these eigen vectors reduces with a decrease in the values of the parameters. The spatial soil erosion potential map was delineated using eight geo-environmental variables (LULC, geomorphology, slope, relative relief, drainage density, drainage frequency, lineament density, and lineament frequency). The soil hazard map showed that the moderate soil erosion has the maximum(57.71%) area cover followed by high erosion class (26.09%)which depicts that most of the watershed areas are moderate to high vulnerable to soil erosion. The efficiency of the AHP was validated applying area under curve (AUC) method which result 84.90% accuracy in the present study. Based on the findings, it is being recommended that present watershed requires adequate control procedures on a priority basis to conserve soil resources and reduce flood events and siltation of water bodies.

• Multistage magmatic intrusion in Narmada–Tapti region, India: Insights from geopotential modelling

The present-day crustal structure of tectono-magmatic regions is the product of dynamic interactions of crust and mantle materials. The Narmada–Tapti region is a mosaic of tectono-magmatic signatures and is characterized by active seismicity, deep-seated faults, shear zones, and high heat flow, suggesting it to be a zone of crustal weakness. The availability of ample seismic and magnetotelluric datasets and inherent complexity drew our attention to image the crustal structure in the third dimension using high-resolution gravity data. The derived 3D crustal density model shows that the Deccan trap extends from 200–1700 m partly below the 90–150 m thick Quaternary sediment exposed in some pockets. The sub-trappean Mesozoic sediment is present at a depth of 250–2400 m followed by the basement. Our 3D model further shows that the high gravity values in residual anomalies are due to high-density magmatic intrusions between 1.5 and 9 km depth. The gravity high in regional anomaly is modelled with a broad dome-shaped high-density (3.02 g/cm3) underplated layer between 14 and 38 km depth. The spatial correlation of delineated high-density lower crustal body with the high-velocity and high conductivity zones mapped by earlier workers in this region indicates the possible presence of mantle magma intrusion in the realm of Deccan volcanism. Analysis of isostatic residual anomaly indicates that the region beneath Narmada–Tapti is not in local isostatic equilibrium. Analysis of the isostatic residual anomaly, root depth, and crustal thickness from the 3D model further ascertains the modification of the crust due to the interaction of mantle plume material. The gravity effect of residual geoid up to 50 km corroborates the high-density magmatic material distribution at two different places, i.e., one at Navsari near the west coast and the other is Junapani near Khandwa. The region has signatures of upliftment and together with the crustal-scale basic magmatic intrusion, satisfies both high gravity anomalies and positive residual geoid undulation. The residual geoid undulations are bounded by major tectonic faults and together with the magmatic underplate at the crustal base indicate that these faults were activated during the Deccan magmatism.

$\bf{Highlights}$

$\bullet$ Narmada-Tapti region has a weak crustal architecture with crustal and sub-crustal magmatic intrusions, dyke swarms, atypical geophysical signatures, and crustal upliftment.

$\bullet$ 2½D crustal density modelling along available seismic sections using high-resolution gravity data in Narmada-Tapti region.

$\bullet$ Three-dimensional crustal-scale density structure with multistage magmatic intrusion in the Narmada-Tapti region, central India.

$\bullet$ Positive Bouguer and isostatic anomalies and geoid undulation over the Narmada-Tapti region provide extra arguments for densification of the crust through multistage magmatic intrusions caused by the Deccan magmatism.

$\bullet$ About 250–2400 m thick Mesozoic sediments delineated at a depth of about 500–3000 m illustrates the potential for hydrocarbon exploration in the Narmada-Tapti region.

• # Journal of Earth System Science

Volume 131, 2022
All articles
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Posted on July 25, 2019

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