Dhruba Mukhopadhyay

Strain measurements from deformed quartz grains in the slaty rocks from the ardennes and the northern Eifel

Abstract Methods of linear correlation of strain axes have been used to determine the two-dimensional shapes of the deformed elliptical quartz grains in thin sections of the slaty rocks from the Hercynian tectonic belt in the Ardennes and the northern Eifel. It is found that in each sample the quartz grains from a particular lithological band can be considered to come from a single population having a certain axial ratio and a preferred orientation of the long axes parallel to the slaty-cleavage trace. It is reasoned that such a population could only be derived from an original population of nearly circular grains. Natural octahedral unit shear, a derivative of it ϵs, and the Lodes parameter have been used to describe the shapes of the three-dimensional strain ellipsoids. Throughout this tectonic belt the maximum compressive strain is more or less uniform in each rock type; the values of the other two principal strains show larger variations. In each rock type ϵs has a remarkably constant value, but the Lodes parameter ranges in value from 0 to nearly 1. All the ellipsoids fall in the constriction field.

“Eyed folds” in Precambrian marbles from southeastern Rajasthan, India

“Eyed folds” — acute domes and basins having elliptical cross sections and subparallel hinge lines at the two ends of the ellipses — on major and minor scales are common in the Precambrian marbles in part of southeastern Rajasthan, India. Closely associated with these structures are tight to isoclinal folds with rectilinear axes that are subparallel to the hinge lines of the “eyes.” These structures belong to the first phase of deformation. A later cleavage cuts across them, and minor second-generation folds occur on the limbs of the earlier structures. Where the “eyed” structures are bent by later folds, a boomerang-shaped pattern results. The axes of the earlier and later folds are everywhere mutually parallel to subparallel, but their attitudes vary from one part of the area to another. It is suggested that the “eyes” and “boomerangs” of this area were not formed by the superposition of later shear folds on earlier folds having rectilinear axes, but, rather, the “eyed folds” were formed by strong flattening during later deformation of earlier folds having original plunge culminations and depressions. Buckling accompanying this later compression was responsible for the formation of the boomerangs. The strong compressive strain rotated all the linear elements toward the X-axis of the strain ellipsoid, and this explains the parallelism of the early and the late linear directions.

Structural Geometry and the Time-Relation of Metamorphic Recrystallization to Deformation in the Precambrian Rocks near Simulpal, Eastern India

Three phases of deformation are recognized in the Simulpal area. The regional schistosity which was hitherto regarded as the earliest deformation structure in Singhbhum is shown to be a second generation structure preceded by a phase of isoclinal folding. The presence of a first generation easterly closing synclinorium, possibly complementary to the Singhbhum anticlinorium, is suggested. The main peak of regional metamorphism was attained after the second deformation and the earliest deformation was accompanied by only a low grade of metamorphism.

Determination of finite strain from grain centre measurements

A statistical method of determination of finite strain from measurements of distance and angle between neighbouring grain centres is described. The method depends on making a suitable transformation which linearises the relation between distance and angle. The finite strain is determined from the statistically determined parameters of the straight-line of best fit.