DEFORMATION EVENTS AND STRUCTURAL HISTORY OF THE HIGH GRADE GRANULATE TERRINE AROUND POLUR, THIRUVANNAMALA DISTRICT TAMILNADU, SOUTH INDIA.

Abstract

Detailed Study   of an area of 700sq. km reveals that the complex structural pattern observed is the result of overprinting and interference of structures generated during two different deformation episodes imprints of both penetrative and non-penetrative secondary planar and linear structures and folds of different generations are present throughout the area. At places F1 folds are intrafolial and rootless. Due to the tightness of F₁ fold So and S₁ are effectively parallel, except in F₁ fold closures where S₁ cuts across  So. Map scale F₂ folds identified Kommandal antiform and Padagam synform, are characterized by thickening of the hinges and thinning of the limbs indicating that the rocks were sufficiently ductile to allow flowage during deformation. F₃ folds were characterized by an E-W to NW-SE axial trend with constant orthogonal thickness. All the pre F₃ planner and linear fabrics are affected by F₃ folds. Interference of F₃ with F₁ and F₂ has resulted in plunging and eyed folds. F₄ folds are expressed as extremely broad open flexures that have resulted in regional variations in the strikes of the litho units. Faults observed in the area have a constant NW-SE trend and post dates major folds. However, faults trending E-W affect F₃ folds and pre date F_4.

Detailed study shows that various structures found in the area were formed during two deformational events. - D₁and D₂. During the initial phase of (D_I1), tightly appressed, mapable F₁ folds with synchronous development of penetrative foliation S₁, axial planer to F₁ were formed. The co-axial nature and geometrical similarities of F₁ and F₂ suggest their formation during progressive deformation. This often gives rise to hook-shaped interference pattern. The second event D₂ indicates a stress field different from that of D₁. During the first phase of (DII₁) fracture cleavage S₂ were formed. The evolutionary history of the area worked out on the basis of structural analysis can serve as a model for similar of high-grade granulate terrains in other parts of the world.