Articles written in Journal of Earth System Science
Volume 99 Issue 2 June 1990 pp 229-247
The northern fold belt away from the Singhbhum Shear Zone displays a set of folds on bedding. The folds are sub-horizontal with E-W to ESE striking steep axial surfaces. In contrast, the folds in the Singhbhum Shear Zone developed on a mylonitic foliation and have a reclined geometry with northerly trending axes. There is a transitional zone between the two, where the bedding and the cleavage have become parallel by isoclinal folding and two sets of reclined folds have developed by deforming the bedding—parallel cleavage. Southward from the northern fold belt the intensity of deformation increases, the folds become tightened and overturned towards the south while the fold hinges are rotated from the sub-horizontal position to a down-dip attitude. Recognition of the transitional zone and the identification of the overlapping character of deformation in the shear zone and the northern belt enable the formulation of a bulk kinematic model for the area as a whole.
Volume 104 Issue 3 September 1995 pp 433-446
Drag patterns of foliation are graphically constructed around very competent dykes under bulk strain of pure shear, simple shear and a combination of pure shear and simple shear. Four different types of drag patterns may be produced, depending on the nature of the bulk deformation and the initial orientations of the dyke and the foliation. The drag pattern can be symmetric or asymmetric, inward curving or outward curving. Both the magnitude and the sense of drag may vary along a dyke wall. A uniform sense of drag develops all along a dyke wall only in certain special situations. The type of foliation drag near a dyke may give us a rough idea of the nature of bulk deformation and the relative orientations of the dyke and the foliation with respect to the bulk strain axes.
Volume 106 Issue 4 December 1997 pp 185-196
The progressive deformation of the Singhbhum Shear Zone (SSZ) involved the initiation of a mylonitic foliation, its deformation by three generations of reclined folds and superposition of two later groups of folds, i.e., a group of asymmetric folds with subhorizontal or gently plunging axes and a group of gentle and open, transverse and more or less upright folds. The occurrence of sheath folds and U-shaped deformed lineations indicate that the reclined folds were produced by rotation of fold hinges through large angles. The total displacement along the SSZ was compounded of displacements along numerous mesoscopic shear zones. The cleavages in the shear lenses and the mesoscopic shear zones cannot be distinguished as C and S surfaces. They have the same kinematic significance and were produced by ductile deformation, although there were localized discontinuous displacements along both sets,-of cleavages. A mylonitic foliation had formed before the development of the earliest recognizable folds. Its time of formation and folding could be synchronous, diachronous or partly overlapping in time in the different domains of the SSZ.
Volume 125 Issue 5 July 2016 pp 1033-1040
Phulad Shear Zone (PSZ) of Delhi Fold Belt in Rajasthan is a northeasterly striking ductile shear zonewith a well developed mylonitic foliation (035/70E) and a downdip stretching lineation. The deformationin the PSZ has developed in a transpressional regime with thrusting sense of movement. The northeasternunit, i.e., the hanging wall contains a variety of rocks namely calc-silicates, pelites and amphibolites andthe southwestern unit, i.e., the footwall unit contains only granitic rocks. Systematic investigation ofthe granites of the southwestern unit indicate a gradual change in the intensity of deformation from adistance of about 1 km west of the shear zone to the shear zone proper. The granite changes from weaklydeformed granite to a mylonite/ultramylonite as we proceed towards the PSZ. The weakly deformedgranite shows a crude foliation with the same attitude of mylonitic foliation of the PSZ. Microscopicstudy reveals the incipient development of C and S fabric with angle between C and S varying from15◦ to 24◦. The small angle between the C and S fabric in the least deformed granite variety indicatesthat the deformation has strong pure shear component. At a distance of about 1 m away from the PSZ,there is abrupt change in the intensity of deformation. The granite becomes intensely foliated with astrong downdip lineation and the rock becomes a true mylonite. In mesoscopic scale, the granite showsstretched porphyroclasts in both XZ and YZ sections indicating a flattening type of deformation. Theangle between the C and S fabric is further reduced and finally becomes nearly parallel. In most places,S fabric is gradually replaced by C fabric. Calculation of sectional kinematic vorticity number (Wn) fromthe protomylonitic and mylonite/ultramylonite granites varies from 0.3 ± 0.03 to 0.55 ± 0.04 indicatinga strong component of pure shear. The similarity of the geometry of structures in the PSZ and thegranites demonstrates that the deformation of the two units is broadly synchronous and the deformationin both the units is transpressional.
Volume 131, 2022
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