Relation between Coplanar Folds of Variable Orientation, Stretching Lineation and Thrust in the Daling and Associated Rocks from the Inner Tectonic Belt of Sikkim Himala

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.