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
Srivastava, Deepak C.
- The Deformation and Strain Analysis of the Gandemara Conglomerate from the Archaean Iron-Ore Group Rocks, District Singhbhum, Bihar
Authors
1 Department of Earth Sciences, University of Roorkee, Roorkee 247667 (U.P.), IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 26, No 4 (1985), Pagination: 233-244Abstract
The Archaean I.O.G. metasediments exposed near Gandemara have undergone three successive deformational phases, D1, D2 and D3 of varying intensities. The quartzite pebbles from oligomictic conglomerate arc considered as the markers for finite strain analysis. Strong correlation between major and minor axes of the ellipses (sections of the pebbles along XY, YZ and XZ planes) and good linear fitting to the axial-ratio plots imply that these markers came from the same population. High values of fluctuation 'φ' and nearly symmetric Rf/φ patterns (IYSM values) indicate large initial axial-ratios (Ri >Rs) and random predeformational fabric respectively. The axial-ratios of strain ellipses (Rs) in XY, YZ and XZ sections are 1.5, 1.83 and 2.5 respectively and these correspond closely with the β values. The three dimensional strain analysis reveals that most of the pebbles have undergone flattening (γ=0.2 to 0.4) corresponding to the low amounts of strain (εs =0.6 to 1.0). Available structural relations and evidences from strain analysis strongly suggest that the conglomerate has been dominantly deformed during D1 phase and subsequently modified during D2 phase.
- Multiple Deformation and Interference Structures in the Archaean Iron Ore Group Rocks Around Barda-Ulugutu Area, Singhbhum District, Bihar
Authors
1 Department of Earth Sciences, University of Roorkee, Roorkee, U. P., IN
2 Indian School of Mines, Dhanbad, Bihar, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 26, No 7 (1985), Pagination: 453-464Abstract
Systematic structural analysis of the Archaean Iron Ore Group rocks around Barda and Ulugutu throws new light on the tectonic pattern and structural evolution of the area. The polyphase tectonics (D1-D3) is evident by the style and over-printing relationship between three types of s-planes (SO-S2), folds (F1-F3), and related lineations. Folds (F1) showing complex geometries, related lineations showing variable plunges and slaty cleavages (S1) have developed synchronously during the earliest tectonic phase (D1). The concomitant evolution of a new set of structures and deformation of preexisting D1 structures took place during the second phase (D2). The numerous smallscale culminations - depressions of F2 folds are attributed to the variable orientations of s-surfaces prior to F2 folding. Critical field evidences suggest the evolution of both F1 and F2 folds through similar deformational plans including buckling and post-buckle flattening. The concentric warps (F3) are envisaged to have developed under flexural conditions during D3 phase in local domains.
En-echelon arrangement of elliptical interference structures are ascribed to the nonorthogonal axial surfaces of F1 and F2 folds, and axial migrations during F2 folding. The co-existence of type-1 and type-2 patterns is due to F2-flexure of alternating tight and open folds (F1) within the same layer. That the bedding (sO) has been successively subjected to layer parallel compression is implied by buckling nature of repeated folding. Therefore, finite strain-ellipse of the whole area corresponds to constrictional field (1>λ1>λ2)·
- Ductile Shearing and Large-Scale Thrusting in the Main Central Thrust Zone, Chur-Peak Area, Lesser Himachal Himalaya
Authors
1 Department of Earth Sciences, University of Roorkee, Roorkee - 247667, U.P., IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 50, No 1 (1997), Pagination: 5-24Abstract
In the Himalayan orogenic belt the rocks of the High Himalaya Crystalline Zone (HHCZ) have been thrust over the rocks of the Lesser Himalaya Zone along the Main Central Thrust (MCT). Around the Chur peak in the Lesser Himachal Himalaya the frontal part of the HHCZ is preserved in a half klippe. Analyses of structures from thin section to outcrop scales in this area show that a progressive ductile shearing has been superimposed on two generations of pre-shearing coaxial folds (F1 and F2 ). The F1 and F2 are tight to isoclinal and recumbent to gently plunging reclined/inclined folds with E or W axial trend. A penetrative cleavage (S1) parallel to the axial planes of F1 folds is the common planar structure, and a crenulation cleavage (S2) parallel to the F2 axial planes has sporadically developed. In the ductile shear zones the rocks have been extensively mylonitized and a plethora of small-scale compressional and extensional structures have developed. A set of very open and upright folds (F3) has affected all these structures. The last episode of deformation is represented by a set of subvertical fractures some of which are normal faults. Detailed mapping in selected areas and stereographic analyses of small-scale structures in the shear zones show that the earlier interpretation of large-scale recumbent folding on stratification is not tenable. Four large-scale thrusts cut up the crystalline rocks of the area into a pack of four thin thrust slices in large scale. The MCT in this area can be considered to be a 5-6 km thick ductile shear zone in which two phases of preshearing coaxial folding and late-stage thrusting can be demonstrated.Keywords
Structural Geology, Main Central Thrust, Himachal Himalaya.- Mesoscale Fractures as Palaeostress Indicators: A Case Study from Cauvery Basin
Authors
1 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667, IN
2 Department of Earth Sciences, Pondicherry University, Pondicherry - 605 014, IN
3 Reliance Industries Ltd, Navi Mumbai - 470 110, IN
4 Department of Geology, National College, Tiruchirapalli - 620001, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 70, No 4 (2007), Pagination: 571-583Abstract
This paper presents the results of field studies, and palaeostress analyses of the mesoscale fractures and veins in Cauvery basin. It shows that different sedimentary sequences (119-64 Ma) are cut by tensile structures that belong to two successive phases of post-Palaeocene brittle tectonics (I) an early phase during which bedding parallel fractures and veins were developed due to horizontally directed maximum compression and vertical extension, and (II) a late phase of hydraulic fracturing in a tectonic regime of vertically directed maximum compression. Dynamic analyses imply triaxial and axial compression deviatoric states of palaeostress during first and second phases of fracturing, respectively. Very high pore-fluid pressure and low differential stress, during the second phase of fracturing, resulted into hydraulic brecciation as a consequence of simultaneous extension in different orientations. We suggest that the development of the mesoscale extensional (mode I) fractures in Cauvery basin is due to reactivation of large-Scale normal faults in the basement rocks.Keywords
Extensional Fracturing, Hydraulic Fracturing, Pore-Fluid Pressure, Cauvery Basin, Tamil Nadu.- Hourglass Structure: an Evidence of Buckle Folding
Authors
1 Geological Survey of India. Op: TNPK, Chennai, IN
2 Earth Sciences, IIT Roorkee, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 67, No Spl Iss 5 (2006), Pagination: 696-697Abstract
No Abstract.Full Text
- Origin and Ductile-Shearing of the Microgranitoid Enclaves in the Granulite-Granitoid Terrane around Devariya-Bandanwara Area, Central Rajasthan
Authors
1 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667 (UA), IN
2 Wadia Institute of Himalayan Geology, 33, Gen Mahadev Singh Road, Dehra Dun - 248 001 (UA), IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 63, No 6 (2004), Pagination: 587-610Abstract
Two distinct types of enclaves, namely, gneissic enclaves and microgranitoid enclaves occur abundantly in the granitoid rocks belonging to the granulite-Granitoid terrane of the Banded Gneissic Complex Field relationships, and petrographic and microstructural studies indicate that the gneissic enclaves are the xenoliths of the older rocks but the microgranitoid enclaves are formed by the process of magma mixing and magma mingling.
Both microgranitoid enclaves and the host granitoids are cut by numerous normal fault type and strike-Slip type of ductile shear zones. Stress analysis by the fault-Slip method and, strain analysis using the feldspar megacrysts as strain markers, reveal that the normal fault type of shear zones are the extensional structures formed by non coaxial and flattening type of deformation. A critical comparison of the concentrations of major and trace elements, in the unsheared and the sheared microgranitoid enclaves, implies the addition of SiO2 Al2O3, Na2O, K2O, Pb, Rb and Sr, and loss of Cr and Cu during the ductile shearing. The slopes of isocon lines, passing through the plots of the immobile elements, such as Y and TiO2 indicate that 18-64% gain in volume has occurred during the ductile shearing of the microgranitoid enclaves.
Keywords
Microgranitoid Enclaves, Magma Mixing and Mingling, Stress and Strain Analyses, Ductile Shear Zone, Volume Change, Isocon, Devariya Bandanwara Area, Central Rajasthan.- Hourglass Structure: An Evidence of Buckle Folding
Authors
1 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 66, No 6 (2005), Pagination: 673-677Abstract
The Nimbahera shale beds show an hourglass map pattern due to juxtaposition of an anticline and a syncline along a common NNE striking, upright axial plane. Several possible hypotheses, such as superposed folding, sheath folding, variation in shortening, variation in orientation of pre-Existing S-Surfaces and coalescence of fold waves are tested for explaining the development of the hourglass structure. Evidence from geometrical characteristics of the folds, associated mesoscopic scale structures and the published experimental results suggest that the hourglass structure has developed due to the linkage of the two out-Of-Phase fold waves that propagated towards each other during the folding of the Nimbahera shale beds by buckling.Keywords
Hourglass Structure, Buckle Folding, Nimbahera Shale, Chittaurgarh, Rajasthan.Full Text
- Deformation Pattern in the Precambrian Basement around Masuda, Central Rajasthan
Authors
1 Department of Earth Sciences, University of Roorkee, Roorkee 247667, Uttaranchal, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 57, No 3 (2001), Pagination: 197-222Abstract
Detailed mapping of the Precambrian rocks around Masuda in central Rajasthan reveals that the basement is made up of a granulite-granitoid terrane and a sneared gneiss-migmatite terrane. The gneiss-migmatite terrane mainly comprises a multitude of the monzogranite gneisses, amphibolites and migmatised biotite schists. These gneissic-migmatitic rocks are repeatedly mylonitised and folded at scales ranging from thin section to map. The granulite-granitoid terrane consists of massive bodies of granulite and granitoid hosting numerous enclaves of gneiss, monzogranite porphyry and mafic-ultramafic rock.
Two successive groups of folds (F1 and F2), each containing two or more sets of co-axial folds are common in the gneiss-migmatite terrane. Whereas the interference between different fold sets belonging to the individual groups has resulted in type 3 interference patterns, the superposition of F2 group folds on F1 group folds has resulted in type 2 interference patterns. Several lines of evidence suggest that F1 and F2 groups of folds have evolved as sheath folds during two phases of ductile shearing, respectively. It is primarily due to the isoclinal nature of the late folds that both the early and late folds display similar style and coplanar relationship in most of the area.
A critical comparison reveals that the gneiss-migmatite terranes in the southern and central Rajasthan are different from each other with respect to style of deformation. It is proposed that the basement-cover contact in central Rajasthan may have acted as a dicollement surface during a major part of the early deformation in the cover rocks.
Keywords
Sheath Folds, Ductile Shear Zone, Structural Analysis, Deformed Lineations, Banded Gneissic Complex, Rajasthan.- Strain Estimation from Single Forms of Distorted Fossils - A Computer Graphics and MATLAB Approach
Authors
1 Schlumberger Asia Services Limited, Goregaon(E), Mumbai-400067, IN
2 Department of Earth Sciences, IIT Roorkee, Roorkee-247 667, IN
3 Department of Physics, IIT Roorkee, Roorkee-247 667, IN
4 Shell Technology India, Bangalore-560 048, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 75, No Spl Iss 1 (2010), Pagination: 89-97Abstract
Most of the existing methods of strain analysis can estimate strain in a single form of distorted brachiopod, or trilobite provided independent evidence, such as the association of the fossil with cleavage and/or stretching lineation is available for inferring the direction of maximum principal strain. This article proposes a simple computer graphics based method and its MATLAB code that determine the minimum amount of strain in a single distorted fossil form even if data for inferring the maximum principal strain direction are lacking. Our method is a rapid computer-graphics alternative to some of the existing analytical methods.
In a distorted fossil form of original bilateral symmetry, the relative senses of angular shears along the hinge line and the median line are mutually opposite to each other. It follows, therefore, that the maximum principal strain direction lies within the acute angle between the hinge and the median lines in the plane of the fossil. Using this principle, our method performs several simulations such that each simulation retrodeforms the distorted fossil by assuming a particular orientation, lying within the acute angle between the hinge line and the median line, as the potential direction of the maximum principal strain. Each simulation of retrodeformation yields a potential strain ratio. The distribution of all the potential strain ratios, obtained by assuming different orientations as the potential directions of the maximum strain, is typically a parabola-like curve with a distinct vertex that corresponds to the minimum amount of strain in the distorted fossil. An entirely computer graphical approach is somewhat time-intensive because it involves a large number of retrodeformational simulations. We, therefore, give a MATLAB code, namely, the Minstrain, that rapidly retrodeforms the fossil and determines the minimum strain with precision.
Keywords
Strain, Distorted Fossil, Lineation, Cleavage, MATLAB.References
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