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 F4. Detailed study shows that various structures found in the area were formed during two deformational events. - D1and D2. During the initial phase of (DI1), 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.

Keywords

• Deformation events, Structural history, granulite terrain, planar structures