• SUBBA RAO

Articles written in Journal of Earth System Science

• Longshore grain size trends in the Kakinada-Mulapeta Beach, east coast of India

Shore-normal and shore-parallel variations in grain size statistics of beach sand have been studied over a period of one year along the Kakinada-Mulapeta coast. The southern beaches of this coast have been accretionary while the northern ones erosional since 125 years. The grain size gradings, beach and nearshore processes help in identifying (i) the Groins-fishing harbour beach influenced predominantly by the tidal regime. (ii) the Mulapeta-Vakalapudi beach influenced by refracted wave regime and (iii) the Vakalapudi-fishing harbour beach affected by both wave and tidal regimes at relatively subdued levels.

• The nature of the basement in the Archaean Dharwar craton of southern India and the age of the Peninsular Gneiss

The Archaean Peninsular Gneiss of southern India is considered by a number of workers to be the basement upon which the Dharwar supracrustal rocks were deposited. However, the Peninsular Gneiss in its present state is a composite gneiss formed by synkinematic migmatization during successive episodes of folding (DhF1, DhF1a and DhF2) that affected the Dharwar supracrustal rocks. An even earlier phase of migmatization and deformation (DhF*) is evident from relict fabrics in small enclaves of gneissic tonalites and amphibolites within the Peninsular Gneiss. We consider these enclaves to represent the original basement for the Dharwar supracrustal rocks. Tonalitic pebbles in conglomerates of the Dharwar Supergroup confirm the inference that the supracrustal rocks were deposited on a gneissic basement.

Whole rock Rb-Sr ages of gneisses showing only the DhF1 structures fall in the range of 3100–3200 Ma. Where the later deformation (DhF2) has been associated with considerable recrystallization, the Rb-Sr ages are between 2500 Ma and 2700 Ma. Significantly, a new Rb-Sr analysis of tonalitic gneiss pebbles in the Kaldurga conglomerate of the Dharwar sequence is consistent with an age of ∼2500 Ma and not that of 3300 Ma reported earlier by Venkatasubramanian and Narayanaswamy (1974). Pb-Pb ages based on direct evaporation of detrital zircon grains from the metasedimentary rocks of the Dharwar sequence fall into two groups, 3300–3100 Ma, and 2800–3000 Ma. Stratigraphic, structural, textural and geochronologic data, therefore, indicate that the Peninsular Gneiss of the Dharwar craton evolved over a protracted period of time ranging from &gt; 3300 Ma to 2500 Ma.

• Magnetic anomalies over the Andaman Islands and their geological significance

The Andaman Islands form part of the outer-arc accretionary sedimentary complex belonging to the Andaman–Sumatra active subduction zone. The islands are characterized by thick cover of Neogene sediments along with exposed ophiolite rocks at few places. A regional magnetic survey was carriedout for the first time over the Andaman Islands with a view to understand the correlation of anomaly signatures with surface geology of the islands. The residual total field magnetic anomaly maps have revealed distinct magnetic anomalies having intermediate to high amplitude magnetic signatures andcorrelate with the areas over/close to the exposed ophiolite rocks along the east coast of north, middle and the south Andaman Islands. The 2D modelling of magnetic anomalies along selected E–W profiles across the islands indicate that the ophiolite bodies extend to a depth of about 5–8 km and spatiallycorrelate with the mapped fault/thrust zones.

• 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.

• The geological site characterisation of the Mandla region, Eastern Deccan Volcanic Province, Central India

Detailed geological studies were carried out on the basaltic sequences along the Jabalpur–Niwas, Jabalpur–Chutka and Jabalpur–Mandla traverses covering an area of about 12 km $\times$ 15 km to characterise various basaltic lava flows and their behaviour on seismotectonics and geodynamic setting of their formation in the Mandla region of the Eastern Deccan Volcanic Province (EDVP). The studies involve an analysis of the satellite images for the identification of lineaments/faults and field geological studies consisting of geological controls such as ground check, thickness of fractures and orientation along the acknowledged lineaments/faults. The results of the present research comprising 65 lineaments/faults mainly belonging to two geometric groups, minor and major dominantly in the NW–SE and the NE–SW and altered strata varying lithology (weathered to compact basalts) are recognised in the study area. Based on their extent, 57 lineaments have been classified as minor (<100 km) trends in three different orientations, i.e., NNE–SSW, ESE–WNW and ENE–WSW, whereas 8 lineaments were classified as intermediate (300–100 km) trends in NNE–SSW. No major (>300 km) lineaments are noticed in the study region. The field geological investigations have facilitated the recognition of 10 flows with different characteristic features and a variety of volcanic structures such as columnar, vesicular, amygdaloidal, inflated pahoehoe lava flows and red bole interflow horizons have been documented. Basement rocks of these Deccan basalt lavas are represented by Tirodi Biotite gneisses, quartzite, quartz–mica schists and crystalline limestone in the SE part of the study area of the Mandla region. The present study will help evaluate the localised site characterisation for urban planning and setting up major civil structures.

• Alteration of primary Cr-spinel mineral composition from the Suru Valley ophiolitic peridotites, Ladakh Himalaya: Their low-temperature metamorphic implications

The present study focuses on the chemical modification of a Cr-spinel from the Suru Valley ophiolitic peridotites exposed near Trespone Village of Kargil district, Ladakh Himalaya. The Suru Valley peridotite is partially serpentinised with the preservation of the spinel and relics of olivine and pyroxene. These peridotites contain characteristic red-brown spinel grains of corroded grain boundaries. While analysing these grains for mineral chemistry, compositional variation was observed with Cr-rich cores rimmed by Cr-poor compositions. Secondary spinel compositions, i.e., ferritchromite and magnetite were observed along the margins and cracks of primary Cr-spinel grains. The primary Cr-spinel cores are identified as Cr-rich and are characterised by higher values of Cr$^{3+}$# (0.5–0.6) and lower values of Al$^{3+}$# (0.42–0.54). From primary spinel cores to altered rims it was observed that Cr$^{3+}$# and Fe$^{3+}$# increase while Mg$^{2+}$# decreases due to Mg$^{2+}$–Fe$^{2+}$ and Al$^{3+}$–Fe$^{3+}$ exchange with surrounding silicates of host peridotite during alteration. On the basis of present spinel mineral chemistry, metamorphic alteration conditions were transitional between greenschist and lower amphibolite similar to most of the Neo-Tethyan ophiolite peridotites.

• Prediction of wind-wave climate along Karnataka coast

Karnataka is a coastal state on the west coast of India along the Arabian Sea. The coast experiences a harsh wave climate during the southwest monsoons. Most of the coast is facing problems due to coastal erosion. Hence, in the present study, a numerical model has been set up using MIKE 21 Spectral Wave(SW) module to predict the wave climate. The wave climate along the Indian domain is simulated by wind speed datasets from Global Climate Model (GCM). Wind speed datasets from ERA-Interim is initially validated against in-situ measurement which had a correlation of 0.93. A hindcast study spanning26 years based on 38 GCMs from different modelling institutes was performed. A comparison of wind speed datasets showed CMCC-CM RCP 4.5 wind projections were closer to ERA-Interim reanalyzed dataset and was used to predict the wave climate. The performance of the MIKE numerical model driven by CMCC-CM RCP 4.5 wind Belds showed a correlation greater than 0.7 when validated against in-situ measurement. The numerical model simulations driven by wind speeds from CMCC-CM RCP 4.5 up to the year 2070 showed a gradual increase in the significant wave height which is indicative of the eAects ofclimate change on the wave climate along the Karnataka coast. The projected significant wave height for 2070, when compared with the present wave climate, indicated an increase in the range of 10–21% at the six locations. The predicted wave pattern based on numerical simulations indicated a shift in the peakvalues in the monsoon month of June along the coast. The predicted wave parameters with a 10-year return period can be used for the design of coastal structures along the Karnataka coast.

• # Journal of Earth System Science

Volume 131, 2022
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019