• Shalivahan Srivastava

Articles written in Journal of Earth System Science

• The South India Precambrian crust and shallow lithospheric mantle: Initial results from the India Deep Earth Imaging Experiment (INDEX)

We present here the most comprehensive study of the thickness and composition ($Vp/Vs$ ratio) of the South India Precambrian crust and the nature of shallower mantle inferred from analysis of teleseismic receiver functions from 70 broad-band seismic stations operated as a part of the India Deep Earth Imaging Experiment (INDEX). South India could be broadly divided into regions with thin crust (32–38 km) and thick crust (38–54 km). Thin crust domains include the East Dharwar Craton (EDC), Cuddapah basin and Madurai/Kerala Khondalite Block. The thicker crust domain includes the Western Dharwar Craton (WDC) and northern part of Southern Granulite Terrain. The WDC shows progressive increase in thickness from 38 km in north to 46–54 km in south, compared to an almost flat Moho beneath the EDC. Compositionally, most of the crustal domains are felsic to intermediate ($Vp/Vs$ ∼ 1.69–1.75) except the mid Archean block in the southern WDC where it is mafic ($Vp/Vs$ &lt; 1.81). Considering erosion depth in the WDC, we argue for Himalaya like ∼70 km thick crust beneath it during the Archean. Variation in crustal thickness does not have a first-order influence on regional topography in South India and suggests significant role for the crustal composition. We also present evidence of mid-lithospheric low velocity at ∼85–100 km beneath South India.

• Lithologic boundaries from gravity and magnetic anomalies over Proterozoic Dalma volcanics

Dalma volcanics (DVs) has intruded the older Singhbhum Group of Metapelites. Despite DVs being rich in mineralisation, its boundaries are not clearly demarcated. Gravity and magnetic surveys have been attempted for mapping the boundaries in DVs. These surveys were made in the northern fringeof the DVs over an area of ∼0.70 km2 along 13 parallel lines at 50 m spacing. The data was acquired at ∼25 m spacing. The surveys were taken for determination of lithological boundaries, depths and nature of causative source using Euler depth solutions and radially averaged power spectrum (RAPS).Residual anomaly maps of gravity and magnetic intensity show the same trend as that of Bouguer gravity anomaly and total magnetic intensity anomaly map indicating towards shallow sources. The magnetic map in general follows the same pattern as that of gravity anomaly maps. The map shows coincident highgravity and magnetic anomalies. These anomalies together with resistivity signatures confirm that the northern fringe of DVs hosts volcanogenic massive sulphide settings. The Euler depth solution delineated the lateral boundaries and nature of the source. It seems that the source is of spherical nature lying withina depth range of 25–40 m. The obtained lithological (vertical) units from RAPS are between Lower DVs, Upper DVs and Singhbhum Group Metapelites at depths of ∼15, ∼25 and ∼40 m, respectively. The metallogeny is associated with the Upper DVs and the corresponding delineated lithological (vertical) unit is indicative of the top of the ore body. Good agreement is observed with the geological successionfrom the drilling data and resistivity data. The findings suggest that the northern fringe of DVs could be a preferred target for drilling.

• Audio-magnetotelluric investigation of sulfide mineralization in Proterozoic–Archean greenstone belts of Eastern Indian Craton

Greenstone belts are well known for gold occurrences at different regions of the world. The Dhanjori basin in the eastern Singhbhum region shows major characteristics of a rifted greenstone belt. Initially, we conducted 14 audio-magnetotelluric (AMT) measurements for a profile of ~20 km in the frequencyrange of 1 kHz to 10 Hz over this rather complex geologic environment covering Dhanjori Volcanics (DhV) and Kolhan Group (KG). Subsequently, gravity and magnetic surveys were also conducted over this AMT profile. The purpose of the survey was to identify and map conductive features and to relate them to metallogeny of the area along with the mapping of the basement of Dhanjori basin. The strike analysis showed N30◦W strike for DhV for all the frequencies and for sites over KG domain in the frequency range of 100–10 Hz, but for KG domain, the obtained strike in 1 kHz to 100 Hz is N45◦E. As the combination of transverse electric (TE), transverse magnetic (TM) and tipper (Tzy) can recoverthe electrical signature in complex geological environment, we discuss the conductivity model obtained from TE+TM+Tzy only. The inversion was carried for the regional profile with 14 sites and for 7 sites over KG domain. Conductivity model shows two well resolved conductors, one each in KG and Quartz Pebble Conglomerate Dhanjori (QPCD) domains respectively showing common linked concordant features between these regional and KG profiles. The conductors are interpreted as sulfide mineralization linked with QPCD group of rocks which may host gold. These conductors are also horizontally disposed due to the intrusive younger Mayurbhanj Granite. These intrusives correlate well with the gravitymodeling as well. The thickness of the Dhanjori basin at the central is about 3.0 km, similar to that from gravity modeling. The conductivity model also indicates the presence of shallow conductors, but could not be resolved due to lack of high frequency data. However, the results from the close-by drill site indicate the presence of shallow sulfide mineralization hosting gold. The deep level conductors delineated from AMT studies are associated with gravity high and low magnetic. ICP-AES results of Dhanjori samples show significant concentration of gold ~5.0 g/t, which is of economic consideration. Thus, it can be inferred that the conductors have evidences of sulfide mineralization which host gold.

• Interpretation of resistivity data using 3D Euler deconvolution and Radially Averaged Power Spectrum

Different electrode configuration in resistivity measurements over the same geologic structures generally produce different anomaly patterns. This is related to the position of the structure concerning the electrodes. Given the above, a unified approach has been proposed using the concept of analytical signal to interpret the resistivity data as electric potential follows Laplace’s equation. We interpret the data sets using Euler deconvolution and Radially Averaged Power Spectrum (RAPS) to determine both lithological boundaries and units and compare the results with inverted resistivity section. We analysed the resistivitydata using electrical resistivity tomography (ERT) technique over conducting dyke and vertical fault for Wenner and dipole–dipole arrays, respectively. The obtained structural indices for dyke and fault are 1.0 and 0.6, respectively. The results from Euler depth solutions match well with the inverted resistivity section. Subsequently, two field examples one each over ground water and mineral exploration were analysed. The delineated lineaments over ground water exploration matches fairly well with the available results. In addition, some additional lineaments are also mapped. These new features could be azone of interest for a detailed survey. The depths from Euler depth solutions and RAPS are in agreement. However, no depth information was available earlier. The data for mineral exploration have been acquired by ERT technique with a profile length of $\sim 500 \rm{m}$ over Dhanjori Basin, Jharkhand, India. The Euler depth solution and RAPS indicated the presence of two interfaces at an average depth of 8 and 20 m and 7 and 21 m, respectively. The first interface is present all along the profile, whereas the second interface liesbetween a part of the profile. The location and depth of these are in broad agreement with the resistivity sections obtained from ERT/AMT and borehole data. A borehole in the vicinity of the survey area indicated the presence of two interfaces. The first interface coincides with the soil depth and the second interface coincides with the sulphide mineralization.

• # Journal of Earth System Science

Volume 129, 2020
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019