Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

C. Karunakaran ¹, Subimal Sinha Roy ¹

Affiliations
1 Geological Survey of India, Calcutta, IN

Abstract

'Pressure shadows' developed at the fringes of porphyroclasts of quartz, feldspar and rock fragments in gritty phyllite of Daling series, have been described in detail. Besides quartz, the 'pressure shadows' contain some secondary chlorite. They show a marked dimensional orientation and grain size variation vis-a-vis the distance from the controlling face. The mode of origin is suggested in the light of Riecke's principle.

Full Text

     

Subimal Sinha Roy ¹

Affiliations
1 Geological Survey of India, Calcutta, IN

Abstract

Blueschist facies assemblages are identified in the metagraywacke and associated metabasics occurring in the lower structural levels of the Daling-Darjeeling thrust block, which tectonically overlies the parautochthonous Gondwana-Buxa sequence. These rocks belong to an intermediate position between the pumpellyite-prehnite-quartz facies and the lawsonite-glaucophane-jadeite facies. A temporal and spatial relation of the blueschist facies metamorphism and the Barrovian regional metamorphism is demonstrated, and a transition from the blueschists to the greenschists from lower to higher structural levels is also indicated. The mineral paragenesis restrict the lower and upper limits of pressure-temperature magnitudes at about 300°C/5 kb and 400°C/7.5 kb respectively. The problem related to the age and geotectonic implications of blueschist metamorphism in these rocks are briefly discussed.

Full Text

     

Subimal Sinha Roy ¹

Affiliations
1 Geological Survey of India, Calcutta, IN

Abstract

The second generation deformation structures in the Daling metamorphic rocks and in the associated basement granitic mylonite in a part of the Sikkim Himalayas are represented by variably orientated coplanar minor folds of three morphologic types, and a stretching fabric, transverse to the thrust-front. Strain analysis on the dimensionally orientated deformed clastic grains in metagraywacke and semipelite indicates that the deformation during second folding phase was close to plane strain. Interrelation between folds, stretching fabric and thrusts suggests that thrusting did not take place parallel to the planes of maximum shear strain. Generation of these related structures is explained in a model of rotational three-dimensional progressive deformation. Ductile thrusting is suggested to have been caused by flattening process by high longitudinal strain, parallel to the principal deformation plane. The direction of thrusting is indicated by the stretching lineation, but the component of horizontal displacement of rocks may not be as large as usually postulated.

Full Text

     

Subimal Sinha Roy ¹

Affiliations
1 Centre of Earth Science Studies, P. B. No. 2235, Trivandrum 695010, IN

Abstract

The Ganga-Brahmaputra fluvial plains are separated from the Himalayan foothills by the extensive piedmont belt. Four longitudinal large-scale terraces that have sequentially developed are recognised ill the interfluvial region of the Tista and the Jaldhaka rivers. These terraces are composed primarily of Boulder Beds and Gravel Beds, and of subordinate Sand Beds, and show rapid facies variations. These deposits are dissected by streams into cut and fill type transverse small-scale terraces. The evolution of the Himalayan piedmont tJelt is consistent with a dynamic alluvial fan model in which the sediments are considered to have been derived through upliftdependent episodic debris-flows and rrtass emplacement. The vertical and lateral accretion of the fans is in consequence to shifting of the mountain front, and to flattening of the stream gradient in the mountains. The hinge-lines of the fans migrated toward south with time, while the transitional zones between the fanglomeratic and the floodplain facies, toward north. Several empirical relationships in the magnitude-frequency matrix between uplifting, terracing and fan-formation are suggested.

Full Text