- S. K. Acharyya
- Santanu Kumar Bhowmik
- Taraknath Pal
- N. C. Pant
- M. Ziauddin
- S. K. Biswas
- T. P. Gururaja Rao
- K. Krishnam Raju
- Debashish Ghosh
- B. K. Bandyopadhyay
- Kasturi Chakraborti
- Siladitya Sengupta
- Amit Mandal
- P. C. Bandopadhyay
- Utpal Chakrabarti
- N. R. Mohapatra
- M. Hanuma Prasad
- S. A. Chore
- L. L. Viswakarma
- M. K. Devarajan
- Subhash Bhandari
- Rachna Raj
- D. M. Maurya
- L. S. Chamyal
- S. N. Mahapatro
- A. K. Tripathy
- J. K. Nanda
- A. K. Huin
- Kasturi Chakraborty
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z All
Roy, Abhinaba
- Tectonothermal History of the Central Indian Tectonic Zone and Reactivation of Major FauIts/Shear Zones
Authors
1 Geological Survey of India, 27, Jawaharlal Nehru Road, Calcutta - 700 016, IN
2 Central Region, Seminary Hills, Nagpur - 440 006, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 55, No 3 (2000), Pagination: 239-256Abstract
Central Indian Tectonic Zone (CITZ), which divides the Indian subcontinent into Bundelkhand Block in the north and the Deccan Block in the south, is represented by a collage of different lithotectonic terranes ranging in age from Archaean to Recent. It comprises two parallel structural domains, namely the Son-Narmada (SONA) subzone in the north and the Sausar mobile belt (SMB) in the south. The ancestry of the SONA subzone is indicated by the Neoarchaean - Palaeoproterozoic ages yielded by the rocks of Mahakoshal fold belt; the Sausar belt, on the other hand, has yielded Meso- to Neo-proterozoic ages. The present response of CITZ to accumulation of stress and attendant seismicity is governed by the structures generated due to early tectonic history of rocks within it, particularly the development of number of E-W to ENE-WSW striking, brittle and ductile shear zones. While the Sausar belt has remained more or less stable since the late Precambrian, the SONA and Tapti lineament zones have been reactivated several times. Two prominent ENE-WSW trending deep fautts, termed the Son-Narmada North Fault (SNNF) and Son-Narmada South Fault (SNSF) have been episodically active from Neoarchaean onwards. The SNSF in particular has witnessed protracted reactivation well into the Phanerozoic. Intraplate seismicity in continents is commonly concentrated along ancient fault zones. Reactivation of faults or shear zones is favoured over new fault generation, since the SNSF is in a high shear stress orientation. Although the Sausar mobile belt is marked by a number of E-W trending parallel ductile shears, mass transfer processes such as silicification, recrystallization and grain growth during Precambrian appear to have healed them.Keywords
Structural Geology, Seismology, Tectonothermal History, Central Indian Tectonic Zone (CITZ), Maharashtra, Madhya Pradesh.- Evidence for Pre-Grenvillian High-Pressure Granulite Metamorphism from the Northern Margin of the Sausar Mobile Belt in Central India
Authors
1 Regional Petrology Laboratory, Geological Survey of India, Central Region, Nagpur - 440 006, IN
2 EPMA Laboratory, Geological Survey of India, Faridabad - 121 001, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 53, No 4 (1999), Pagination: 385-399Abstract
Distinct lithological associations, structural relationships, and petrographic and geothemobarometric results indicate the presence of reworked granulites of Pre-Grenvillian age from the northern margin of the Sausar mobile belt (SMB) in Central India. In rare residual zones, the mafic granulite-felsic migmatite gneiss association preserves early structures and high-P (∼ 10.5 kbar at ∼ 775°C) granulite histories which appear to pre-date and bear no relation to ∼ 1000 Ma old Sausar orogeny. An isothermal decompression through ∼ 5.5 kbar at 750°C from maximum recorded P-T condition, possibly signifies extensional collapse of the crust, previously thickened through collision. During the Sausar orogeny, the granulites were further tectonically interleaved with younger lithological associations. Widespread retrogression in amphibolite and finally in greenschist facies conditions have all but obliterated granulite mineralogy and structure except in rare low-strain domain.Keywords
Metamorphic Petrology, Granulites, High-P Metamorphism, Sausar Mobile Belt, Central India.- Volcanism in the Younger Dharwar Rocks Near Medur Dharwar District, Karnataka
Authors
1 Geological Survey of India, Jayanagar, Bangalore 560011, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 19, No 7 (1978), Pagination: 321-325Abstract
Andesite lavas are associated with pyroclastics and a few sheet-like porphyritic flows. The volcanism is considered to have been of mixed eruptions indicating sufficient fluidity and low gas content. The andesitic lavas are characterised by well developed pillow structure. They are interlayered with pyritiferous and carbonaceous argillites indicating volcanic eruption under submarine condition.
The lavas occupy the central part of a major refolded antiformal anticline plunging to the west. They have undergone metamorphism only up to the green-schist facies of regional metamorphism. Carbonatisation of lavas is a common feature, particularly in andesite.
- Metamorphic History of the Sandur Schist Belt, Karnataka
Authors
1 Jayanagar, Bangalore-560011, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 20, No 4 (1979), Pagination: 179-187Abstract
The Sandur schist belt is predominantly characterised by greenschist facies of regional metamorphism, which increases to amphibolite facies near the periphery of the schist belt. The higher facies rocks are marked by the appearance of metamorphic index minerals like garnet, andalusite, staurolite and cordierite. Study of the 'Si - Se' tectonites suggests that metamorphism is progressive, where chlorite and micas grow 'synkinematically' and garnet, andalusite, staurolite and cordierite grow post-kinematically with reference to first phase of folding (F1). Static growth of garnet, andalusite, staurolite etc., following the first phase of folding (F1), and their subsequent rotation with reference to second phase of folding (F2) clearly indicates a time gap between F1 and F2. Formation of assemblages of higher grade metamorphic minerals along the border region of the schist belt, is possibly due to influx of additional heat from some source (? intrusi ve granite) at a late tectonic stage.- Polyphase Folding Deformation in the Hutti-Maski Schist Belt, Karnataka
Authors
1 Geological Survey of India, Jayanagar, Bangalore 560011, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 20, No 12 (1979), Pagination: 598-607Abstract
Three phases of folding deformation (F1, F2 & F3) are recognised in the Hutti-Maski schist belt. The first two phases (F1 and F2) are quite pronounced producing regional folds whereas the last one (F3) is of mild intensity. F1 folding is accompanied by the development of schistosity (S1) in metabasalt and metasedimentaries, mylonite banding (Sd) in gold quartz veins and minor folds on bedding plane (So) and vein quartz. Two major F1 folds, viz., a syncline and an anticline could be traced in the area on the basis of orientation of pillow structure in metabasalt. Gold mineralisation in Hutti-Maski schist belt is mainly controlled by first phase of folding (F1) and associated regional metamorphism. F2 fold shows clear evidence of superposition on F1. F1 axial trace is folded by F2 folds. F2 folding is accompanied by the development of crenulation or slip cleavage (S2), minor folds on S1 and Sd and shear fractures and faults. F2 fold is represented by two major folds, viz., an antiform and a synform having axial traces along NNW-SSE to NW-SE direction and plunge around 45° to 50° towards Nw to NNW. A time gap between F1 and F2 is marked by the intrusion of a granodiorite body. F3 is developed on the limbs of F2 folds, and is accompanied by the development of S3 cleavage and kink band or chevron folds. Axial plane of F3 fold trends N75°E-S75°W (approx.) with vertical dip. Fold axis (F3) shows very steep westerly plunge.- A Note on the Occurrence of Olivine Dolerite Dyke near Hutti, Raichur District, Karnataka
Authors
1 Geological Survey of India, Jayanagar, Bangalore-560011, IN
2 Hutti Gold Mines, Hutti, Raichur District, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 21, No 3 (1980), Pagination: 151-154Abstract
An olivine dolerite dyke with 15 to 20% olivine is described as cutting the meta-basalts of Hutti and the associated granites, and showing somewhat unusual chemical composition.- Study of a Deformed Conglomerate Horizon in the Sandur Schist Belt, Bellary District, Karnataka
Authors
1 Jayanagar, Bangalore-560011, IN
2 Cement Research Institute of India, New Delhi-110048, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 20, No 7 (1979), Pagination: 349-352Abstract
The deformed pebbles of the conglomerate horizon occurring to the SE of Hospet have their longest and the intermediate axis lying on the schistosity plane. The long axis shows a strong preferred orientation. The intermediate axis is parallel or subparallel to the regional fold axis. The strain is plane strain type with slight elongation parallel to the intermediate axis or the regional fold axis.- A Note on the 'Volcanic Conglomerate'/Pyroclast Horizon near Palkanmaradi in the Hutti Schist Belt, Karnataka
Authors
1 Samchi, BT
2 Jayanagar, Bangalore, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 23, No 10 (1982), Pagination: 506-510Abstract
Palkanmaradi 'Conglomerate' occurs as a conformable bed within the volcano-sedimentary sequence of the Hutti schist belt. Well-rounded pebbles of granodiorite/tonalite gneiss, microgranite/acid volcanic rock, angular to subangular fragments of amphibolite, are found embedded in a matrix having unusual chemistry representing probably an admixture of volcanic and sedimentary material. Along with the other rocks tbe conglomerate bed has suffered from three distinct phases of deformation and attained amphibolite facies of metamorphism.- Stratigraphy and Structure of the Sandur Schist Belt, Karnataka
Authors
1 12/ 1, Bireswar Dhole Lane, Calcutta-700 035, IN
2 377, 8th Block, 42nd Cross, Jayanagar, Bangalore-560 041, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 24, No 1 (1983), Pagination: 19-29Abstract
Sandur schist belt comprises predominantly of volcanic rocks with subordinate amount of metasediments. Mineralogy of the clastic sediments indicates a pre-existing sialic crust. The schist belt represents a fracture-controlled basin having an unstable tectonic set-up. Two distinct phases of folding deformation (SD1 and SD2), of which the earlier one (SD1) is quite pronounced producing the NW-SE trending regional structure of the schist belt, are noted. SD1 is associated with the development of macroscopic folds (F1), tight to isoclinal mesoscopic folds (f1 ), schistosity (S1 ), mineral lineation Im), fold mullion (In), and pebble lineation (Ip). SD2 is associated with the development of broad warps (F2) with axial traces along ENE-WSW direction, open to close mesoscopic folds (f2) and crenulation cleavage (S2).- Geochemistry and Environmental Significance of Banded Garnet Amphibole rock (BGA) Around Naurhiya, Bijawar Group in Sidhi District, Madhya Pradesh
Authors
1 Geological Survey of India, AMSE Wing, Nagpur, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 30, No 2 (1987), Pagination: 143-151Abstract
Occurrence of thin but persistent horizons of banded garnet-amphibole-rock (BGA) is reported from the Proterozoic Bijawar or Mahakoshal Group in Sidhi district, Madhya Pradesh. The physical appearance, mineralogy and chemistry of the rock are closely comparable to some metamorphosed manganiferous iron-formations. The rock occurs in association with a sequence of low-grade metasediments, mainly comprising wacke and argillites together with subordinate quartz arenite, banded magnetite quartzite, limestone, tuffs and basic volcanics, which bear similarity with the sediment-hosted Algoma type iron formation. Although there is a broad similarity in the composition of the Bijawar BGA rocks with the known B1F of the world, the former showsan appreciable enrichment in A12O3, MnO, MgO, CaO, Ba, Zn, Co, Ti, Y and V compared to the latter.- Tectonic Significance of Ultramafic and Associated Rocks Near Tal in the Bijawar Belt, Sidhi District, Madhya Pradesh
Authors
1 Geological Survey of India, Nagpur 440022, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 32, No 5 (1988), Pagination: 397-410Abstract
Serpentinised ultramafic bodies of peridotite-dunite composition occur as solid state intrusions in the Bijawar volcano-sedimentary sequence around Tal in Sidhi district, Madhya Pradesh. The associated basic volcanics and gabbros are predominantly tholeiitic in composition, although a slight enrichment in Na is recognised locally. However, apart from their juxtaposition, there is no petrological or chemical evidence to suggest that the uhramafic and gabbroic rocks were derived from a common magma. The ultramafics represent olivine-pyroxene cumulates, crystallised at depth and subsequently emplaced as crystalline mush (? diapiric intrusion) during the pre- to early-kinematic stage of basin development. This suggests the possible role of a mantle-derived magma to initiate basin depression. Near the cratonic margin of the basin, shelf sediments (orthoquartzite, carbonate-pelletal chert etc.) were deposited over the gneissic basement while the distal facies was mainly a turbidite. The rocks underwent polyphase deformation and low-grade greenschist facies metamorphism. The closing stage of Bijawar was marked by the emplacement of post-tectonic granite and occasional gabbroic diorite near the peripheral zones of the belt.- Precambrian Mafic-Ultramafic Magmatism in Central Indian Suture Zone
Authors
1 Geological Survey of India, Eastern Region, Kolkata - 700 091, IN
2 Geological Survey of India, Southern region, Hyderabad - 500 068, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 72, No Spl Iss 1 (2008), Pagination: 123-140Abstract
The Precambrian crust of Central India comprising Bundelkhand craton in the north and Bastar craton in the south were accreted along the ENE-WSW trending Proterozoic Central Indian tectonic zone (CITZ). According to this plate tectonic model, a northward dipping subduction system leading to continent-continent collision during Mesoproterozoic, explains the growth and assembly of the different lithotectonic units within the CITZ. They are identified as (1) the suture zone represented by the Ramakona-Katangi granulite belt, (2) the active continental margin (magmatic arc) represented by the Betul belt and (3) the back arc region represented by the Mahakoshal belt, each of which is characterized by conspicuous mafic and ultramafic magmatism. They have been strongly deformed and variously metamorphosed depending upon their tectonic locations with respect to convergent type plate tectonic setting. The grade of metamorphism varies from granulite facies in the suture zone to greenschist and amphibolite facies in the magmatic arc and back arc regions. Emplacement of significant proportion of mafic and ultramafic rocks in the form of volcanics and intrusives in different tectonic domains, within an overall convergent type plate tectonic setting is indicative of pronounced mantle activity and related magmatism. The paper deals with the petrological and geochemical characteristics of the mafic-ultramafic rocks that are emplaced during the Mesoproterozoic collisional orogeny. In general, they show a distinct tholeiitic trend irrespective of their tectonic locations. But for diorite that occurs in association with the mafic-ultramafic rocks in the Betul belt, the typical calc-alkaline andesitic volcanism, so characteristics of arc type magmatism is altogether absent in the area. Ubiquitous presence of phlogopite and conspicuous LREE enrichment in the mafic-ultramafic complex of the magmatic arc region is suggestive of a possible metasomatised mantle source rock. Mantle metasomatism through plate subduction is quite likely in convergent type tectonic setting.Keywords
Mafic, Ultramafic, Tholeiite, CITZ, Collision, Proterozoic, Betul Belt, Mahakoshal Belt, Madhya Pradesh.- Synchronous Development of Mylonite and Pseudotachylyte in Ductile Shear Zone: An Example from the Chitradurga Eastern Margin Shear Zone, Karnataka
Authors
1 Geological Survey of India, Eastern Region, Salt Lake City, Kolkata-700091, IN
2 Geological Survey of India, CGL, CHQ, Kolkata-700016, IN
3 Geological Survey of India, Antarctica Division, Faridabad-121001, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 72, No 4 (2008), Pagination: 447-457Abstract
The eastern margin of the Chitradurga schist belt is marked by a NNW-SSE trending sub vertical crustal scale ductile shear zone. The kinematic indicators indicate a predominant sinistral sense of strike-lip movement along the shear zone. Syntectonically emplaced granitic rocks are converted to mylonites and ultramylonites as a result of crystalplastic deformation in the shear zone. In contrast, there are localized zones of brittle failure with attendant functional heat generation exemplified by the development of thin but conspicuous bands and veins of pseudotachylytes, which are emplaced either subparallel with or transgressing the C-planes of the mylonites. From our field and petrographic studies it is interpreted that these two coexisting rock types, namely the mylonite and pseudotachylyte, which are the results of contrasting deformational mechanisms, have generated near synchronously in a progressively developed ductile shear zone. The pseudotachylytes represent the brief interlude of sudden increase in strain rate in an overall ductile regime.Keywords
Ductile, Brittle, Shear Zone, Mylonite, Pseudotachylyte, Chitradurga, Karnataka.- New Field Evidence of Coseismic Coastal Uplift during the December 2004 Earthquake, North Andaman Island
Authors
1 Geological Survey of India, Eastern Region, DK-6, Salt Lake City, Kolkata - 700 091, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 71, No 6 (2008), Pagination: 871-874Abstract
This report documents ground uplift of a part of the eastern coast of North Andaman Island during the December 2004 earthquake. Emergence of a part of fringing coral reef above the high tide level has been recorded from the Kahpur beach. The newly emergent part of the coral reef is -75m long, -25m wide and trends ESE paralleling the trend of the shore line. The study records appearance of a -600m long and 200m wide tombolo joining the Smith Island to the Ross Island off the eastern coast of North Andaman Island. Before the December 2004 earthquake it was a sand bar that emerged only during the low tides. Vertical movement of 0.4m is calculated for the Kahpur beach while that between the Smith and Ross Islands is 0.7-0.8m.Keywords
Coral Reef, Tombolo, December 2004 Earthquake, Kahpur, Smith Island, Ross Island, North Andaman Island.- Sedimentary Processes and Facies of Upper Pleistocene Alluvial Fans in the Purna Valley Basin of Central India
Authors
1 Geological Survey of India, Seminary Hills, Nagpur - 440 006, IN
2 Geological Survey of India, Eastern Region, Kolkata, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 69, No 5 (2007), Pagination: 916-924Abstract
Alluvial fans of Upper Pleistocene age occur as transverse depositional systems along the fault-Defined Satpura mountain front that marks the northern margin of Purna Valley Basin. Our findings indicate that the fans were built by three main sedimentary processes : Debris flow, sheetflood and stream flow. The debris flow fans are the smallest in size and comprise ill-Sorted, matrix supported conglomerates showing evidence of deposition on slopes. The sheetflood fans are larger than debris flow fans in size and are composed predominantly of laterally extensive, tabular, planar-Bedded couplets of relatively coarse and fine grained sediments. The stream flow fans are the largest in size and represent channelized flows with sheet-Like, clast supported, massive gravel beds as the fundamental depositional unit.
It emerges from this study that two essential factors controlled the sedimentary processes acting on the alluvial fans. These are : The size of the drainage basins and the tectonic geomorphology of the mountain front. The debris flow and sheetflood fans are restricted to the linear, tectonically active part of the mountain front dissected by small streams only. The stream flow fans developed adjacent to that part of the mountain front which is tectonically less active, erosionally embayed and drained by rivers with large catchments.
Keywords
Alluvial Fan, Facies, Sedimentary Processes, Mountain Front, Upper Pleistocene, Purna Valley Basin, Central India.- Granulite Facies BIF from the Betul Supracrustal Belt, Central India
Authors
1 Geological Survey of India, Seminary Hills, Nagpur - 400 006, IN
2 Geological Survey of India, 0p.MP-I, Bhopal - 462016, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 62, No 5 (2003), Pagination: 635-640Abstract
No Abstract.- Ductile Shearing and Syntectonic Granite Emplacement along the Southern Margin of the Palaeoproterozoic Mahakoshal Supracrustal Belt: Evidence from Singrauli Area, Madhya Pradesh
Authors
1 Geological Survey of India, Central Region, PO. Seminary Hills, Nagpur - 440 006, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 59, No 1 (2002), Pagination: 9-21Abstract
A prominent E-W trending, sub-vertical to steep south dipping, ductile shear zone occurs along the southern margin of the Palaeoproterozoic Mahakoshal supracrustal belt of Central India. Shear zone structures have been documented from the supracrustal rocks as well as from the intrusive granites (s.l.) from an area near Singrauli. The supracrustal belt away from the shear zone displays a set of subhorizontal upright folds on E-W to ENE-WSW striking axial planes. The intensity of deformation increases southward; the folds become tight to isoclinal with slight overturning towards the north while the fold hinges arc rotated from the subhorizontal position to sub-vertical or down-dip reclined attitude. Mylonitic foliation within the shear zone is sub-parallel to the axial plane schistosity of the dominant fold-set outside the shear zone. The granitic rocks show both magmatic and solid-state fabrics which are part of a continuum of emplacement and deformation in the shear zone. In the initial stages of solid-state deformation, which gave rise to protomylonites, coarse K-feldspar porphyroclasts behavcd in a brittle manner in an overall bulk ductile deformation in the matrix. With increase in intensity of deformation accompanied by the introduction of fluids, the deformation mechanism in feldspar changed from brittle to ductile. Mylonitic foliation is invariably associated with a pervasive stretching lineation. Shear-sense indicators suggest oblique-slip movement with a large reverse-slip component and a small sinistral strike-slip component. The geometry of the folds and their sequence of development are similar in the rocks within and outside the shear zone. Concordance between the granite plutons and the supracrustal rocks is due to syntectonic emplacement. Low pressure-medium temperature metamorphism in the Mahakoshal supracrustal belt contemporaneous with granite intrusion suggests depth of emplacement about 10- 12 km. The bulk kinematic model involves N-S to NNESSW subhorizontal compression over which was superimposed in the southern part a sinistral shearing movement during progressive transpressional deformation.Keywords
Ductile Shear Zone, Mylonites, Syntectonic Granite, Shear-Sense Indicators, Mahakoshal Group, Palaeoproterozoic, Central India.- Formation and Erosion of Holocene Alluvial Fans along the Narmada-Son Fault near Rajpipla in Lower Narmada Basin, Western India
Authors
1 Geological Survey of India, Central Region, Nagpur, IN
2 Department of Geology, M. S. University of Baroda, Vadodara-390002, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 60, No 4 (2002), Pagination: 477-478Abstract
No Abstract.- Coexisting Ultramylonite and Pseudotachylyte from the Eastern Segment of the Mahanadi Shear Zone, Eastern Ghats Mobile Belt
Authors
1 Geological Survey of India, Operations: Orissa, Bhubaneswar, IN
2 Geological Survey of India, Eastern Region, Kolkata, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 74, No 6 (2009), Pagination: 679-689Abstract
Pseudotachylytes occur associated with mylonite and ultramylonite in the Mahanadi shear zone (MSZ) in the Eastern Ghats Mobile Belt (EGMB). The MSZ is about 200 km long curvilinear high strain zone trending WNW-ESE in its eastern part that splays out in the west. In Kantilo-Ganian segment of MSZ in northern EGMB, an interbanded sequence of granulite facies lithoassemblage has undergone ductile shearing. Kinematic studies of mylonite and ultramylonite indicate MSZ to be a NE-dipping, extensional type ductile shear zone. Non-coaxial metamorphic growth of garnet and presence of truncated sillimanite-fish in ultramylonite suggest high temperature regime during shearing. Pseudotachylytes in MSZ occur as millimetre thick layers to decimetre thick zones containing fragments of mylonite, ultramylonite and lithic clasts. Pseudotachylyte generation veins are mostly sub-parallel to C-planes and the injection veins cross-cut at high angle to these. The presence of an isotropic glassy matrix, injection features, corroded grains and dendritic microlites can be evidences for the existence of a melt phase. The composition of pseudotachylyte matrix (by EPMA) indicates silica deficiency with higher normative hypersthene, plagioclase and lower quartz compared with average whole rock composition for host. Absence of overprinting of mylonitic fabric on pseudotachylytes indicates their formation by brittle failure without passing through a plastic deformation and thus a two stage development for mylonite-ultramylonite and pseudotachylyte generation is suggested.Keywords
Mahanadi Shear Zone (MSZ), Mylonite, Ultramylonite, Pseudotachylyte, Kantilo, Ganian, EGMB.References
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- First Report of Trace Fossils from Palaeogene Succession (Namunagarh Grit) of Andaman and Nicobar Islands
Authors
1 Geological Survey of India, Eastern Region, Salt Lake City, DK-6, Kolkata - 700 091, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 73, No 2 (2009), Pagination: 261-267Abstract
The Namunagarh grit of Eocene age in South Andaman island is a submarine fan deposit where sedimentation occurred in deep marine environment mainly by turbidity currents. The mudstone and fine-grained sandstone of the turbidite sequence yielded three ichnogenera viz Thalassinoides, Teichichnus and Lorenzinia.
Typically shallow marine Thalassinoides burrows co-occur with deep water burrows represented by Teichichnus and Lorenzinia, indicating a dominant control of substrate, availability of food and rate of sedimentation against bathymetry on distribution of trace fossils. Trace fossils are particularly abundant in the mudstones pointing to a strong lithofacies control. A well-oxygenated muddy bottom with a low rate of sedimentation is envisaged.
An order in the succession of traces could be deciphered. Simply constructed burrows of Thalassinoides appeared initially followed by complex structures of Teichichnus and ornate Lorenzinia.
Keywords
Trace Fossils, Eocene, Namunagarh Grit, South Andaman Island.- Geology and Geochemistry of Basic Volcanics From the Sakoli Schist Belt of Central India
Authors
1 Geological Survey of India, Central Region, Nagpur - 440006, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 50, No 2 (1997), Pagination: 209-221Abstract
The Mafic volcanic suite exposed mainly in the eastern part of the Sakoli schist belt comprises the lower part of the Sakoli sequence. The samples are tholeiitic in composilion having moderate M-values (45-62) and lower abundances of Ni, Co and Cr indicative of evolved nature of the magma. Certain major and trace element characteristics and LREE enriched REE patterns suggest magma derivation by low degrees of partial melting of komatiite-like sources.Keywords
Sakoli Schist Belt, Geochemistry, Komatiites, Maharashtra.- Tectonic Amalgamation of Crustal Blocks along Gadag-Mandya Shear Zone in Dharwar Craton of Southern India
Authors
1 Geological Survey of India, 15 Kyd Street, Kolkata - 700 016, IN
2 Presidency University, Kolkata, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 80, No 1 (2012), Pagination: 75-88Abstract
The crustal scale Shear Zone that can be traced from Gadag in the north to Mandya in the south in Dharwar Craton of southern India is considered as the boundary between two subcratonic blocks namely the Eastern Dharwar Craton (EDC) and the Western Dharwar Craton (WDC) in published literature. The present study on the Gadag-Mandya Shear Zone (GMSZ) in the Javanahalli-Hagalvadi sector has brought out a detailed account on the disposition, geometry and kinematics of the shear zone, and also the distinctive structural patterns of the two adjacent supracrustal belts, namely the Chitradurga schist belt (CSB) in the west and Javanahalli schist belt (JSB) in the east. The JSB has an overall N-S striking and gentle easterly dipping geometry, the structural features of which are indicative of a predominant non-coaxial deformation and westward transportation of the supracrustal assemblage. In contrast, deformation in the CSB, which is defined mainly by a flattening type of strain, has produced an overall verticality of the structures (dominant foliation, axial planes of regional folds).
Difference in metamorphic grade is apparent from the mineral assemblages in the rocks of these two belts, while the JSB showing an amphibolite facies CSB is restricted to green schist facies mineral assemblages.
The 2 to 3 km wide, N-S striking, prominent ductile shear zone, GMSZ, has affected both CSB and JSB rocks and also the syntectonically emplaced Bukkapatna granite. The geometric and kinematic evidences from both the belts and the GMSZ point to an E-W horizontal compressional regime. The gentle dipping, amphibolite grade JSB and its underlying gneisses acted as an allochthon which got amalgamated with the subvertical, green schist grade CSB by a top-W thrusting along the GMSZ. The subvertical CSB impeded further up-dip movement of the eastern block, resulting in the subsequent development of N-S trending sinistral strike slip shear along the contact of the two contrasting belts to accommodate the continued E-W compression. The linear Bukkapatna Granite, synkinematically emplaced along GMSZ shows evidences of both magmatic stage and solid state deformation in the shear zone.
Keywords
Amalgamation, Shear Zone, Syntectonic Granite, Javanahalli, Chitradurga, Karnataka.References
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