Subhajit Ghosh

Interfering folds in constrictional deformation

Abstract Experiments on buckle folding indicate that in pure constriction ( λ 2 = λ 3 λ 2 > λ 3 ), there is an association of domes and basins with nonplane noncylindrical folds. The areas of domes and basins are reduced with increasing deformation. There is a dominant fold trend perpendicular to λ 3 with a less dominant trend perpendicular to λ 3 . Folds at an angle to these two sets may also occur. The divergent folds link up in smooth arcs or join up at a dome or a basin. Over a relatively large domain the interference pattern produced by general constriction can be recognized by the association of domes and basins with nonplane noncylindrical folds, by occurrence of hair pin bends of hinge lines of open folds, by occurrence of amoeboid outcrop patterns and by absence of a consistent overprinting relation among different sets of folds. Layers subparallel to the λ 3 axis of general constriction may give rise to two or more sets of coaxial cylindrical folds with orthogonal axial surfaces.

Planar, non-planar and refolded sheath folds in the Phulad Shear Zone, Rajasthan, India

Abstract Progressive ductile shearing in the Phulad Shear Zone of Rajasthan, India has produced a complex history of folding, with development of planar, non-planar and refolded sheath folds. There are three generations of reclined folds, F 1 , F 2 and F 3 , with a striping lineation ( L 1 ) parallel to the hinge lines of F 1 . The planar sheath folds of F 1 have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices. L 1 swerves harmoniously with the curving of F 1 hinge line. There is a strong down-dip mineral lineation parallel to the striping lineation in most places, but intersecting it at apices of first generation sheath folds. Both the striping and the mineral lineation are deformed in U-patterns over the hinges of reclined F 2 and F 3 . Folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non-planar sheaths. The reclined folds of all the three generations were deformed by a group of subhorizontal folds. Each generation of fold initially grew with the hinge line at a very low angle with the Y -axis of bulk non-coaxial strain and was subsequently rotated towards the down-dip direction of maximum stretching. The patterns of deformed lineations indicate that the stretching along the X -direction was extremely large, much in excess of 6000 percent.

Boudinage and composite boudinage in superposed deformations and syntectonic migmatization

Abstract The banded gneisses of the Jasidih–Deoghar area of East India behaved as a complex geological multilayer, which evolved through the combined effects of deformation and leucosome emplacement. The early foliation and banding were transposed and retransposed by two generations of isoclinal folds. The intensity of foliation development, the layer-thickness ratios and the rheological contrasts within the gneisses changed both in space and in time. Under layer-normal compression these complex multilayers gave rise to a variety of boudins and of their pegmatite-filled separation zones. The geometrical relations with successive generations of folds can distinguish the closely associated pre- F 1 , syn- F 1 and syn- F 2 boudins. D 2 boudinage occurred at an advanced stage of F 2 folding so that boudinage-induced subhorizontal extension was considerably less than the total subhorizontal extension during D 2 . Composite boudinage occurred during progressive syntectonic leucosome emplacement in the course of which a group of boudinaged layers with a profusion of coarse quartz--feldspar leucosome became more competent than the adjoining rocks and underwent boudinage as a whole.

Rotation of long tectonic clasts in transpressional shear zones

Abstract The two-dimensional analysis of rotation of rigid ellipsoidal inclusions is not applicable to situations in which either the bulk deformation deviates from plane-strain or one of the principal axes of the inclusion is at an angle to the vorticity vector. Both these situations may occur in certain types of transpressional ductile shear zones. The model presented here shows how long cylindrical or ellipsoidal tectonic clasts (with the longest axis parallel to the walls of the shear zone but at an angle to the vorticity vector) rotate in transpressional shear zones. Long clasts, initially at a low angle to the vorticity vector and at a large angle to the stretching lineation, will rotate and tend to become subparallel to the stretching lineation. The rotation of the principal axes of the elliptical cross-section around the cylinder axis cannot be unlimited for any inclusion, including the inclusion with circular cross-section ( R =1); if the deformation is sufficiently large there is always a stable position of orientation. The model explains simultaneous occurrence of monoclinic rolling structures in sections parallel and perpendicular to the vorticity vector. Depending upon the initial orientation, the long axes of different clasts in the foliation plane may rotate clockwise and counterclockwise. Consequently, opposite senses of asymmetry of rolling structures may appear in sections parallel to the vorticity vector.

Singhbhum Shear Zone: Structural transition and a kinematic model

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.

Automatic shoreline detection and future prediction: A case study on Puri Coast, Bay of Bengal, India

Abstract Shoreline prediction models have the capability of integrating geoinformatics within them. The present study is conducted on the 142 km-long coastline of Puri district, India. It aims to analyze the change in coastline due to erosion/accretion and provide best estimate of future shoreline positions based on past shorelines. A simple mathematical model, End Point Rate (EPR), has been used to calculate the rate of change of shoreline and its future positions, based on empirical observations. The erosional/accretional scenario has also been analysed by delineating the shoreline from Landsat imageries of 1972, 2001 and 2010. It is found that the northern part of Puri, in the vicinity of Kushabhadra estuary and Chandrabhaga beach undergo high rates of erosion. Based on the delineated shoreline, the short term (2015) and long term (2025) shoreline positions have been predicted.

The kinematic history of the Singhbhum Shear Zone

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.

Hinge migration and hinge replacement

Abstract The axial migration angle (ф) is defined as the angle between the current hinge line and a marker line which was parallel to the initial hinge line. It is generally assumed that the marker line is deformed in an entirely passive manner whereas the fold axis traces out the orientations of the long axis of the sectional strain ellipse. This concept is modified when the competence contrast between a layer and its embedding medium is taken into consideration. Large values of ф may be obtained in constrictional deformations; however, because of strong hinge line arcuations of the nonplane noncylindrical folds, the concept of hinge migration will not be meaningful. Hinge migration is greatly influenced by the competence of layers. In inextensible layers the magnitude of hinge migration may become very large even in flattening and plane strain. On the other hand, hinge migration will be very small or negligible if buckle folding is associated with large layer-parallel homogeneous strain. The concept of hinge migration and hinge replacement have been critically analysed for development of Types 0, 1 and 2 fold interference patterns.

Tectonic deformation of soft-sediment convolute folds

Abstract The structures of the Precambrian Ghatsila–Galudih fold belt of eastern India have been studied in great detail but in the absence of suitable strain markers there is no estimate of strain for these rocks. However, tectonically deformed soft-sediment convolute folds are plentiful in these rocks. These spectacular structures morphologically resemble sheath folds but have been produced by tectonic deformation of lobes of soft sediment contortions. An estimate of the magnitude of bulk tectonic strain (√ λ 3 =0.31) across the axial plane cleavage, along with the assumption of no volume change during tectonic deformation, gives us crucial information about the pretectonic shape of the convolutes that can be utilised to determine strain along the X and Y axes (√ λ 1 =2.6, √ λ 2 =1.24). The shapes of inclusion trails in garnet porphyroblasts within the mica schists indicate that the axis of rotation is parallel to the Y -axis of bulk strain. The bulk deformation involves a non-coaxial general flattening, with λ 1 > λ 2 >1> λ 3 , with the vorticity vector approximately parallel to the λ 2 axis.

Forest cover change prediction using hybrid methodology of geoinformatics and Markov chain model: A case study on sub-Himalayan town Gangtok, India

In the Himalayan states of India, with increasing population and activities, large areas of forested land are being converted into other land-use features. There is a definite cause and effect relationship between changing practice for development and changes in land use. So, an estimation of land use dynamics and a futuristic trend pattern is essential. A combination of geospatial and statistical techniques were applied to assess the present and future land use/land cover scenario of Gangtok, the subHimalayan capital of Sikkim. Multi-temporal satellite imageries of the Landsat series were used to map the changes in land use of Gangtok from 1990 to 2010. Only three major land use classes (built-up area and bare land, step cultivated area, and forest) were considered as the most dynamic land use practices of Gangtok. The conventional supervised classification, and spectral indices-based thresholding using NDVI (Normalized Difference Vegetation Index) and SAVI (Soil Adjusted Vegetation Index) were applied along with the accuracy assessments. Markov modelling was applied for prediction of land use/land cover change and was validated. SAVI provides the most accurate estimate, i.e., the difference between predicted and actual data is minimal. Finally, a combination of Markov modelling and SAVI was used to predict the probable land-use scenario in Gangtok in 2020 AD, which indicted that more forest areas will be converted for step cultivation by the year 2020.