Ductile Shearing and Large-Scale Thrusting in the Main Central Thrust Zone, Chur-Peak Area, Lesser Himachal Himalaya

In the Himalayan orogenic belt the rocks of the High Himalaya Crystalline Zone (HHCZ) have been thrust over the rocks of the Lesser Himalaya Zone along the Main Central Thrust (MCT). Around the Chur peak in the Lesser Himachal Himalaya the frontal part of the HHCZ is preserved in a half klippe. Analyses of structures from thin section to outcrop scales in this area show that a progressive ductile shearing has been superimposed on two generations of pre-shearing coaxial folds (F₁ and F₂ ). The F₁ and F₂ are tight to isoclinal and recumbent to gently plunging reclined/inclined folds with E or W axial trend. A penetrative cleavage (S₁) parallel to the axial planes of F₁ folds is the common planar structure, and a crenulation cleavage (S₂) parallel to the F₂ axial planes has sporadically developed. In the ductile shear zones the rocks have been extensively mylonitized and a plethora of small-scale compressional and extensional structures have developed. A set of very open and upright folds (F₃) has affected all these structures. The last episode of deformation is represented by a set of subvertical fractures some of which are normal faults. Detailed mapping in selected areas and stereographic analyses of small-scale structures in the shear zones show that the earlier interpretation of large-scale recumbent folding on stratification is not tenable. Four large-scale thrusts cut up the crystalline rocks of the area into a pack of four thin thrust slices in large scale. The MCT in this area can be considered to be a 5-6 km thick ductile shear zone in which two phases of preshearing coaxial folding and late-stage thrusting can be demonstrated.