• Rajesh Sharma

      Articles written in Journal of Earth System Science

    • Two- and three-dimensional gravity modeling along western continental margin and intraplate Narmada-Tapti rifts: Its relevance to Deccan flood basalt volcanism

      Somdev Bhattacharji Rajesh Sharma Nilanjan Chatterjee

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      The western continental margin and the intraplate Narmada-Tapti rifts are primarily covered by Deccan flood basalts. Three-dimensional gravity modeling of +70mgal Bouguer gravity highs extending in the north-south direction along the western continental margin rift indicates the presence of a subsurface high density, mafic-ultramafic type, elongated, roughly ellipsoidal body. It is approximately 12.0 ±1.2 km thick with its upper surface at an approximate depth of 6.0 ±0.6 km, and its average density is {dy2935} kg/m3. Calculated dimension of the high density body in the upper crust is 300 ±30 km in length and 25 ±2.5 to 40 ±4 km in width. Three-dimensional gravity modeling of +10mgal to -30mgal Bouguer gravity highs along the intraplate Narmada-Tapti rift indicates the presence of eight small isolated high density mafic bodies with an average density of {dy2961} kg/m3. These mafic bodies are convex upward and their top surface is estimated at an average depth of 6.5 ±0.6 (between 6 and 8km). These isolated mafic bodies have an average length of 23.8 ±2.4km and width of 15.9 ±1.5km. Estimated average thickness of these mafic bodies is 12.4±1.2km. The difference in shape, length and width of these high density mafic bodies along the western continental margin and the intraplate Narmada-Tapti rifts suggests that the migration and concentration of high density magma in the upper lithosphere was much more dominant along the western continental margin rift. Based on the three-dimensional gravity modeling, it is conjectured that the emplacement of large, ellipsoidal high density mafic bodies along the western continental margin and small, isolated mafic bodies along the Narmada-Tapti rift are related to lineament-reactivation and subsequent rifting due to interaction of hot mantle plume with the lithospheric weaknesses (lineaments) along the path of Indian plate motion over the Réunion hotspot. Mafic bodies formed in the upper lithosphere as magma chambers along the western continental margin and the intraplate Narmada-Tapti rifts at estimated depths between 6 and 8 km from the surface (consistent with geological, petrological and geochemical models) appear to be the major reservoirs for Deccan flood basalt volcanism at approximately 65 Ma.

    • Earthquake-induced soft sediment deformation (SSD) structures from the Bilara limestone formation, Marwar basin, India

      Partha Pratim Chakraborty Rajesh Sharma Pramod Kumar

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      The Neoproterozoic Bilara limestone Formation of the Marwar Group, Rajasthan, India exposes metres-thick layers of soft sediment deformation (SSD) structures at different stratigraphic levels which could be traced over hundreds of metres on the outcrop scale. The SSD structures include disharmonic folds, low-angle thrusts, distorted laminae, fluidisation pipes, slump and load structures, homogeneities, diapirs, etc. Whereas SSD structures suggesting tensional stress, viz., intrastriatal graben, fluidisation, slump, etc. dominate in the lower part of the Bilara succession, features implicating compression, viz., folds, low-angle thrust are prevalent in the uppermost part. Since SSD structures are mostly confined within the algal laminites, we interpret that enhanced micritic fluid pressure below early cemented algal carbonate played a major role in laminae deformation. Depending on the degree of lithification and pore-water pressure, deformation features formed either plastically or led to diapiric injection at enhanced pore water pressure. Separated by near-horizontal underformed strata, the SSD layers, traceable over hundreds of metres, are interpreted as products of seismic shacking. Considering the time frame of the Marwar basin, i.e., the Precambrian–Cambrian transition, the SSD horizons present within the Bilara succession may hold the potential for the correlation with SSD structures reported from the time-correlative stratigraphic successions present in erstwhile adjoining tectonic terrains, e.g., China, Siberia, etc.

    • Carbonaceous material in Larji–Rampur window, Himachal Himalaya: Carbon isotope compositions, micro Raman spectroscopy and implications

      SHRUTI RANA RAJESH SHARMA SANTOSH KUMAR

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      This work focuses on the natural graphitic carbonaceous material (GCM) distributed in metasedimentary and crystalline rocks in and around Larji–Rampur tectonic window, Himachal Himalaya. The GCM, associated with the ore mineralization, is mostly flaky, however, it is also granular and amorphous. The micro Raman spectroscopy of representative samples confirms that the studied GCM is mostly disordered graphite and rarely poorly ordered graphite, but well crystalline ordered graphite is also present. The carbon isotope compositions reflecting the source of carbon in GCM at various locations attribute that the carbon was mostly sedimentary organic carbon which has been metamorphosed to disordered graphite, however, the ${\delta}^{13}$C of the inorganic carbon contents in metabasalts from Bhallan signify the involvement of fluid possibly derived from the mantle. Limited ${\delta}^{13}$C$_{inorganic}$ data in a range from 0 to -11%, points to the heavier carbon probably derived from the diagenetic carbonates or dissolved organic matter. Overall, the carbon isotope compositions of GCM from the Larji–Rampur window reject diversity in carbon source and mixing of carbon reservoirs, which can adequately be explained by the Proterozoic marine carbon cycling. A close linkage in the depositional processes of GCM with ore mineralization in the area is also invoked.

      $\bf{Highlights}$

      $\bullet$ The graphitic carbonaceous material (GCM) is present in and around Larji–Rampur tectonic window, Himachal Himalaya, at places associated with ore mineralization.

      $\bullet$ Micro Raman spectroscopy confirms the presence that this GCM is mostly disordered graphite though the ordered graphite is also present uncommonly.

      $\bullet$ The ${\delta}^{13}$C values vary widely from –1.5‰ to –33.5‰. The ${\delta}^{13}$C compositions are heterogeneous and complex carbon systematics is apparent. In addition to the predominant sedimentary organic carbon form Proterozoic marine carbon, it was also derived from carbonate source, carbon from the fluids, and rarely but possibly from the mantle source.

      $\bullet$ A close linkage in the formation and evolution processes of the GCM with the ore mineralization is also invoked.

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