• P RAMA RAO

Articles written in Journal of Earth System Science

• Gravity anomalies over the Central Indian Ridge between 3°S and 11°S, Indian Ocean: Segmentation and crustal structure

High-resolution shipboard geophysical investigations along the Indian Ocean ridge system are sparse especially over the Carlsberg and Central Indian ridges. In the present study, the shipboard gravity and multibeam bathymetry data acquired over a 750 km long section of the Central Indian Ridge between 3°S and 11°S have been analysed to understand the crustal structure and the ridge segmentation pattern. The mantle Bouguer anomalies (MBA) and the residual mantle Bouguer anomalies (RMBA) computed in the study area have shown significant variations along the ridge segments that are separated by transform and non-transform discontinuities. The MBA lows observed over the linear ridge segments bounded by well-defined transform faults are attributed to the thickening of the crust at the middle portions of the ridge segments. The estimates of crustal thickness from the RMBA shows an average of 5.2 km thick crust in the axial part of the ridge segments. The MBA and relative RMBA highs along the two nontransform discontinuities suggests a thinner crust of up to 4.0 km. The most significant MBA and RMBA highs were observed over the Vema transform fault suggesting thin crust of 4 km in the deepest part of the transform fault where bathymetry is more than 6000 m. The identified megamullion structures have relative MBA highs suggesting thinner crust. Besides MBA lows along the ridge axis, significant off-axis MBA lows have been noticed, suggesting off-axis mantle upwelling zones indicative of thickening of the crust. The rift valley morphology varies from the typical V-shaped valley to the shallow valley floor with undulations on the inner valley floor. Segments with shallow rift valley floor have depicted well-defined circular MBA lows with persistent RMBA low, suggesting modulation of the valley floor morphology due to the variations in crustal thickness and the mantle temperature. These are supported by thicker crust and weaker lithospheric mantle.

• Frequency characteristics of geomagnetic induction anomalies in Saurashtra region

Magnetovariational studies were carried out along four different EW profiles in Saurashtra region in different phases, during January 2007–March 2012. Transient geomagnetic field variations (X, Y horizontal field and Z vertical field components) recorded along these profiles are analyzed to infer the electrical conductivity distribution of the region. The vertical field transfer functions which depict the characteristics of electrical conductivity distribution are presented in the form of induction arrows. From the spatial distribution of these arrows, it is inferred that the sediments filling the offshore basins have more conductivity than those basins in Saurashtra region. Z/H pseudo sections along the four profiles in conjunction with tectonics and other geophysical methods permit to infer that the conductivity anomaly in the eastern part of the profiles is associated with the crustal/lithosphere thinning. The possible cause for these anomalies may be explained in terms of partial melts associated with mafic intrusions, related to Deccan and pre-Deccan volcanism. High resistive block related to underplating mantle material has been reflected in 1D models of long period magnetotelluric data and its thickness reduces from west to east. Lithosphere–asthenosphere boundary varies from 80 to 100 km.

• Structural framework of the Wagad uplift and adjoining regions, Kutch rift basin, India, from aeromagnetic data

The Kutch sedimentary basin formed during the Late Triassic breakup of Gondwanaland is characterised by horst and graben structures consisting of several east–west trending uplifts surrounded by low-lying plains. The eastern part of the basin has a diverse landscape comprising the Wagad uplift, Banni plain, Island Belt uplift and the Rann of Kutch. This area is bounded by major faults like the South Wagad Fault (SWF), Gedi fault and the Island Belt Fault. The lineaments/faults present in the region at different depth levels and the propagation of these features through the different sedimentary layers are studied using the semi-detailed aeromagnetic data collected over the basin. The aeromagnetic anomaly map depicts several major E–W, NE–SW and NW–SE oriented lineaments/faults, which probably represent structural trends associated with different stages of evolution of this rift basin. Power spectral analysis of the differential reduced to pole magnetic data indicates the presence of four magnetic interfaces. The slopes identified from the 1D power spectra were used for designing matched bandpass filters for isolating and enhancing the magnetic signatures present within those interfaces. Different edge detection techniques were used to delineate the magnetic contacts/faults/lineaments present in those interfaces. In addition, we have computed the radially averaged power spectrum of 121 subset grids each with a dimension of $\rm{20 km \times 20 km}$ from which three magnetic interfaces were delineated and compared with the stratigraphic sequence of the Wagad uplift and adjoining regions. A major NE–SW fault is delineated from this analysis and suggests that this fault has depth persistence as it dislocates the different magnetic interfaces. Integration with stratigraphic data suggests that this fault was formed prior to the deposition of Miocene Kharinadi formation. We have interpreted that this fault, forming the eastern limit of the Banni basin, might have formed during the passage of the Indian plate over the Reunion hotspot. Based on the results of the aeromagnetic data analysis and other published data, we propose a generalised evolutionary model for the study region.

• Dimensionality and directionality analysis of magnetotelluric data by using different techniques: A case study from northern part of Saurashtra region, India

Magnetotelluric (MT) data has been collected along 32 stations along E–W profile in northern part and eight LMT (long period MT) stations in north-central part of Saurashtra region. Dimensionality analysis is carried out prior to MT modelling for obtaining the subsurface dimension as well as the direction of the underlying substructures. To estimate the subsurface dimensionality from MT data, different techniques Swift skew, Bhar’s skew, normalized weights, phase tensor (PT) analysis and Wall’s rotational invariant approach have been applied. These results suggest 1D structure for lower periods (0.01–1 s) and 3D structure for higher periods (1–10000 s) along two different profiles indicating that the study area is highly heterogeneous. Regional strike has been estimated through phase tensor (PT) and Groom–Bailey (GB) techniques suggests N40$^{\circ}$ E regional strike direction that correlates well with the Delhi–Aravalli tectonic trend. 2D modelling of MT/LMT data sets brings out different resistivity and conductivity blocks. Basaltic magmatic intrusion and its recrystallization have resulted in resistivity blocks with conductivity anomalies (trapped fluids) in between them. It has been reflected as 3D structures at higher periods. Different sedimentary basins at shallow depth are observed as 1D structure in dimensionality analysis.

$\bf{Highlights}$

$\bullet$ Magnetotelluric (MT)/long period Magnetotelluric (LMT) survey is carried out in northern part of Saurashtra. Different dimensionality techniques were used to assess the structural dimensionality of the electrical conductivity of the earth and were compared.

$\bullet$ Analysis of MT sites by using various methods indicates the electrical conductivity structure is less complex at the shallowest depths with mixed 1D and 2D cases that are affected by galvanic distortion. Both MT/LMT denote complex 3D nature from middle and lower depths.2D inversion of MT/LMT data brings out large-scale heterogeneities in the crust. This is attributed to different resistive and conductive blocks present at mid-crustal depths and extending up to lower crustal depths and correlates with dimensionality analysis.

• 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
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• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019