T. K. Biswal

Deformation pattern of the NW terrane boundary of the Eastern Ghats Mobile Belt, India: a tectonic model and correlation with Antarctica

Abstract Thrusts interspersed with lateral ramp and wrench structures mark the join between Proterozoic Eastern Ghats Mobile Belt and Archaean cratons of the eastern India. The join, referred to as Terrane Boundary Shear Zone, exhibits an arcuate geometry at the NW margin of the Eastern Ghats where the mobile belt shows the presence of northwesterly verging nappes. Based on this, the area has been described to represent the salient part of a fold thrust belt. Juxtaposition of the granulites over the low grade craton is largely attributed to thrusting along the terrane boundary shear zone. As the folds are disoriented and the granulites are retrograded along the thrusts, the terrane boundary shear zone is interpreted to be a retrograde shear zone. The shear zone hosts a number of synkinematic alkali intrusive rocks that constrain the age of thrusting to 1.4 Ga. In the correlative framework of Gondwanaland the terrane boundary shear zone is very likely to be contiguous with the Rayner-Napier boundary of Antarctica.

Deformation history of the NW salient of the Eastern Ghats Mobile Belt, India

Abstract The Eastern Ghats Mobile Belt of India displays fold-thrust belt structure with a prominent salient on its NW margin. The salient consists of Lathore nappe and Turekela klippe that have overthrust the craton to NW. The rocks of the nappes have undergone granulite facies metamorphism and multiple phases of folding corresponding to the Eastern Ghats orogeny, prior to thrusting. As a result the granulites along the thrust are retrograded to amphibolites and the axial traces of the folds have been truncated against the thrust plane. The basal decollement of the fold-thrust belt is represented by the Terrane Boundary Shear Zone that defines the tectonic margin between the craton and the mobile belt. It occurs as a ductile thrust affecting the cratonic basement as well as the mobile belt suggesting that the basement did not behave as a rigid body during thrusting. Thus the study area is comparable with the Caledonide fold-thrust belt. Further, two large lateral ramps namely Khariar and Paikamal lateral ramps have been developed on the decollement at the lithological contact between tonalite–trondhjemite gneisses/granite gneisses and the latetectonic potassic granites of the craton. Fault-bend folds associate with these ramps too. It is suggested that the salient structure of the fold-thrust belt is the combined result of (1) lateral ramps on the decollement and (2) differential displacement along the sole thrust due to lateral variation in detachment strength.

Emplacement kinematics of nepheline syenites from the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt, west of Khariar, NW Orissa: Evidence from meso- and microstructures

Nepheline syenite plutons emplaced within the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt west of Khariar in northwestern Orissa are marked by a well-developed magmatic fabric including magmatic foliation, mineral lineations, folds and S-C fabrics. The minerals in the plutons, namely microcline, orthoclase, albite, nepheline, hornblende, biotite and aegirine show, by and large, well-developed crystal faces and lack undulose extinction and dynamic recrystallization, suggesting a magmatic origin. The magmatic fabric of the plutons is concordant with a solid-state strain fabric of the surrounding mylonites that developed due to noncoaxial strain along the Terrane Boundary Shear Zone during thrusting of the Eastern Ghats Mobile Belt over the Bastar Craton. However, a small fraction of the minerals, more commonly from the periphery of the plutons, is overprinted by a solid state strain fabric similar to that of the host rock. This fabric is manifested by discrete shear fractures, along which the feldspars are deformed into ribbons, have undergone dynamic recrystallization and show undulose extinction and myrmekitic growth. The shear fractures and the magmatic foliations are mutually parallel to the C-fabric of the host mylonites. Coexistence of concordant solid state strain fabric and magmatic fabric has been interpreted as a transitional feature from magmatic state to subsolidus deformation of the plutons, while the nepheline syenite magma was solidifying from a crystal-melt mush state under a noncoaxial strain. This suggests the emplacement of the plutons synkinematic to thrusting along the Terrane Boundary Shear Zone. The isotopic data by earlier workers suggest emplacement of nepheline syenite at 1500 +3/−4Ma, lending support for thrusting of the mobile belt over the craton around that time.

Implications of the geochemistry of the Pelitic Granulites of the Delhi Supergroup, Aravalli Mountain Belt, Northwestern India

Abstract An isolated granulite terrain occurs within the linear zone of schists and amphibolites of the Proterozoic Delhi Supergroup, in the Aravalli mountain belt of NW India. The geochemistry of the pelitic granulites of such terrains suggests the arkosic to shaley nature of the protolith. These rocks are characterized by a high maturity index, low to intermediate Fe2O3 + MgO values (5.78%), high La Sc (8.3), Th Sc (4.0) and Ni Co (1.4) ratios and, low Ti Zr (16), Sc Cr (0.16) and Sc Ni (0.16) ratios which imply a passive continental margin nature of the basin. The high K2O value (3.0%), high K 2 O Na 2 O ratio (1.96), LREE enrichment, flat HREE pattern, consistent negative Eu anomaly, high La cn Yb cn (10.6) and Th Sc (4.0) ratios and low La Th (2.4) ratio, low Ni (30 ppm) and Cr (47 ppm) contents, and uniform Zr Hf ratio largely reflect continental provenance and probable Proterozoic age. These rocks also differ from Archaean granulites of the fold belt with regards to K2O and Th U values and deformational history.

Fold-Thrust-Belt Structure of the Proterozoic Eastern Ghats Mobile Belt: A Proposed Correlation Between India and Antarctica in Gondwana

Abstract The Proterozoic Eastern Ghats Mobile Belt along the east coast of India shares a thrusted lower contact with the surrounding cratons. The thrust, known as the Terrane Boundary shear zone, is associated with two large lateral ramps resulting in a curved outline on the northwestern corner of the mobile belt. The Eastern Ghats Mobile Belt is divided into two lithotectonic units, the Lathore Group and the Turekela Group, based on their lithological assemblages and deformational history. On the basis of published data from a Deep Seismic Sounding (DSS) profile of the Eastern Ghats crust, the Terrane Boundary Shear Zone is considered to be listric in nature and acts as the sole thrust between craton and mobile belt. The Lathore and Turekela Groups are nappes. With this structural configuration the NW part is described as a fold thrust belt. However, the thrusting postdates folding and granulite metamorphism that occurred in the Eastern Ghats, as in the Caledonide type of fold thrust belt of NW Scotland. The Terrane Boundary Shear Zone is interpreted to be contiguous with the Rayner-Napier boundary of the Enderby Land in a Gondwana assembly.

Tectonic Significance of Fissure Veins Associated with Pseudotachylites of the Kui-Chitraseni Shear Zone, Aravalli Mountain, NW India

Abstract Kui-Chitraseni Shear Zone of the Precambrian Aravalli Mountain, NW India is marked by a nearly 400 m-wide NE-SW trending belt of cataclasites developed on the granites and metasediments of the Delhi Supergroup. The cataclasites are criss crossed by pseudotachylite and fissure-veins. While the pseudotachylites are aphanitic to microlitic, the fissure veins are characterized by syntaxial-bitaxial as well as unitaxial growth of elongate quartz grains. Textural study of the fissure veins suggests crystallization of quartz grains in a dilational environment in hydrous condition. Contrarily the pseudotachylites bear the evidence of frictional melting of rocks under anhydrous conditions. Since in many instances it is observed that the fissure veins have replaced the pseudotachylites following the crack seal mechanism, the formation of fissure veins is considered to be latekinematic to pseudotachylites. Hence it is interpreted that the pseudotachylites and fissure veins were formed in two distinct tectonic events. While pseudotachylites were formed in a compressional setting resulted from thrusting along the shear zone, the fissure veins have been formed in an extensional setting during a late stage normal faulting. In light of this tectonic rejuvenation along the Kui-Chitraseni Shear Zone, various lineaments present within the Aravalli Mountains need further study for understanding the evolution of the region.

Assessment of groundwater salinity in Nellore district using multi-electrode resistivity imaging technique

Multi-electrode resistivity imaging survey with 48 electrodes was carried out to assess the extent of salinity inland, in the shallow subsurface in Nellore district, Andhra Pradesh, in the Eastern Ghats Mobile Belt (EGMB) region. Resistivity data were recorded using Wenner–Schlumberger configuration at nine sites along a profile of about 55 km in length, laid perpendicular to the coast. An average spacing of 6 km is maintained between each site. Assessment of groundwater salinity in the study area was made by joint interpretation of the two-dimensional (2D) geoelectrical models of all the sites together with the geochemical analysis results of water samples and geology. At sites closer to the coast, 2D geoelectrical models of the subsurface indicate low resistivities (2–50 Ωm) in the depth range from surface up to 15 m. Such low resistivities are due to the high salinity of the groundwater. Geochemical analysis results of water samples at six locations close to the electrical resistivity survey sites also suggest high salinity and high concentrations of total dissolved solids and other chemicals at sites closer to the coast. Away from the coast, the resistivities in the depth range from surface up to 15 m vary in the range of 50–150 Ωm. Accordingly, the chemical analysis of water samples collected at these sites also showed relatively low levels of salinity and salt concentrations in them. However, away from the coast, the resistivities vary in the range of 150–1500 Ωm in the depth range from 20–40 m. While the aquaculture and agriculture activities may contribute to high salinity at the sites closer to the coast, the presence of deep-seated paleochannels aiding in transporting seawater inland, and water–rock interactions are suspected to be the chief causes for notable salinity at places away from the coast at shallow depths. We opine that the high salinity at shallow depths, coupled with the deep-seated paleochannels transporting seawater, could pose problems to probe further depths particularly using electromagnetic induction methods in the study region.

Characterization of hydrocarbon-bearing fluid inclusion in sandstones of Jaisalmer basin, Rajasthan: A preliminary approach

The quartz grains from the sandstone of Jaisalmer, Pariwar and Goru Formations of the Jaisalmer basin, Rajasthan, India, exhibits a variety of primary and secondary fluid inclusions. Most of them are hydrocarbon bearing fluid inclusions. Laser Raman studies indicate that the primary fluid inclusions were mostly having aliphatic hydrocarbons with lower degree of maturity, while the secondary fluid inclusions were generally with aliphatic as well as aromatic hydrocarbons with higher degree of maturity. This inference was consistent with their fluorescence characteristics. The homogenization temperatures of primary monophase CH4 rich fluid inclusions varied from −80°C to −100°C, whereas the primary biphase fluid inclusions (CH4-CO2) homogenized between +80°C and +150°C. The secondary petroleum rich monophase fluid inclusions were having homogenization temperature between −80°C to −90°C, whereas the secondary biphase fluid inclusions homogenized between +130°C and +180°C. Most of the secondary biphase fluid inclusions were having the mixtures of H2O-CO2-NaCl, and were identified on the basis clathrate formation and they got homogenized between +140°C and + 250 °C. The three past events of migration of petroleum inferred in the host rock which were marked by the presence of characteristic secondary fluid inclusions. They were identified on the basis of cross-cutting relationships of different trails of fluid inclusions in the quartz. The cement generation in the basin might have been occurred in two stages as per the fluid inclusion petrography.

Geoinformatic Modeling of Groundwater Resource Mapping of Shear Zone Regions- A Case Study on Attur Valley, Tamil Nadu, India

The general tendency of mapping groundwater resource using remote sensing and Geographic Information System (GIS) techniques involve assigning higher weightage to geomorphology. But this cannot be used as a thumb rule everywhere, especially an area where many ductile and brittle zones are prevalent. The influence of texture and structure of sheared rocks might play a control over retaining and permitting groundwater to flow. Attur valley is characterized by the presence of many shear zones and faults and hence the rocks are highly fissile within the shear zones. The present study tries to establish a new ranking and weightage scheme and hence a new spatial model for groundwater resource mapping in shear zone area like Attur Valley. This spatial model can be verified with field data such as water level data, pump test and resistivity data.

Neoproterozoic Magmatism and Metamorphism in Northwestern Indian Shield: Implications of Rodinia-Gondwana Tectonics

Swamy, S.N. and Kapoor, P.N. (1999) Identification of potential source rock facics and preparation of spatio-temporal model for Barakar Formation equivalent sediments/ Raghavapuram Shale in Kakinada bay offshore. Unpub. ONGC report. Vairavan, V. (1993) Tectonic history and hydrocarbon prospects of Palar and Pennar basins, India. Proc. I1 seminar on Petroliferous basins of India, In: Biswas, S.K. et al. (Eds.), Indian petroleum publishers, Dehradun, v. 1, pp. 389 396.