STRUCTURAL EVOLUTION OF THE HIGH HIMALAYA CRYSTALLINE THRUST SHEET IN THE CHUR HALF-KLIPPE, LESSER HIMALAYA, HIMACHAL PRADESH

Abstract

The High Himalaya Crystalline Zone (HHCZ) representing a 25-30 km thick thrust sheet rides over the rocks of the Lesser Himalaya Zone (LHZ) along the Main Central Thrust (MCT), the most important intracontinental thrust in the Himalaya. The rocks of the HHCZ are the most highly deformed and metamorphosed rocks in the Himalaya. Around the Chur peak in the Lower Himachal Himalaya the frontal part of the HHCZ thrust sheet, locally called the Jutogh Group, is exposed in a half-klippe. Large-scale map pattern together with detailed analyses of structures from thin section to outcrop scales have been used to elucidate the deformational history of these rocks. The frontal part of the HHCZ thrust sheet in this area can be considered to be a 5-6 km thick ductile shear zone in which ductile shearing is most intense in three restricted horizons. In this zone, however, two generations of pre-shearing folds (F\ and F2) which are tight to isoclinal and recumbent to gently-plunging reclined/inclined with E or Waxial trend can be recognized. A penetrative cleavage (Sj) parallel to the axial planes ofFj folds is the dominant planar structure and a crenulation cleavage (S2) parallel to the F2 axial planes is sporadically developed. The F[ and the F2 folds are coaxial resulting in type-3 interference patterns. In the ductile shear zones the structures of the early (i.e. F1-F2) generations have been extensively modified. Structures developed during progressive ductile shearing include: (1) mylonites, mylonitic foliations and S-C composite planar fabrics, (2) folds on layering and crenulations on pre-existing cleavage surfaces, (3) interference structures such as sheath folds, type 2 and type 3 patterns, (4) several generations of crenulation cleavage, (5) extensional structures, such as extension crenulation cleavage, foliation boudinage, rotated boudinage and pinch-and-swell structures. Small-scale thrusts giving imbricate or schuppen structures are also common in the shear zones. A set of very open and upright folds (F3) have affected all these structures. The last deformation episode is represented by a set of subvertical fractures some of which are faults with a normal sense of displacement. Four large-scale thrusts can be recognized, viz., the Chail thrust, the Jutogh thrust, the Rajgarh thrust and the Chur thrust, the last two are reported for the first time. These four thrusts cut up the crystalline rocks of the area into four thin thrust slices giving rise to imbricate structure in large scale. A granite body (the Chur granite) occurs in this area at the highest topographic and structural levels and belongs to the Lesser Himalaya granite belt. It has been shown that the Chur granite is a thrust slice which suggests that there is another, probably basementinvolved, thrust in the HHCZ sitting at a higher structural level than the MCT. The main phase of regional progressive metamorphism is completely pretectonic with respect to the ductile shearing. It is argued that the inversion of metamorphic zones in this area is due to imbrication of thrust slices derived from a terrain with normal Barrovian metamorphic zones.