STRUCTURAL AND METAMORPHIC EVOLUTION OF THE ROCKS OF THE JUTOGH GROUP, CHUR HALF-KLIPPE, HIMACHAL HIMALAYAS - A SUMMARY AND COMPARISON WITHTHE SIMLA AREA

The rocks of the Jutogh Group in the Himachal Himalayas and their equi valents elsewhere are now considered to represent a several km thick c rustal scale ductile shear zone; the so called Main Central Thrust Zon e. In this article we present a summary of structural and metamorphic evolution of the Jutogh Group of rocks in the Chur half-klippe and com pare our results with those of Naha and Ray (1972) who worked in the a djacent Simla klippe. The deformational history of the Jutogh Group of rocks in the area around the Chur-peak, as deduced from small-scale s tructures, can be segmented into: (1) an early event giving rise to tw o sets of very tight to isoclinal and coaxial folds with gentle dip of axial planes and easterly or westerly trend of axes, (2) an event of superimposed progressive ductile shearing during which a plethora of s mall-scale structures have developed which includes successive generat ions of strongly non-cylindrical folds, several generations of mylonit ic foliation, extensional structures and late-stage small-scale thrust s, and (3) a last stage deformation during which a set of open and upr ight folds developed, but these are regionally unimportant. The struct ure in the largest scale (tens of km) can be best described in terms o f stacked up thin thrust sheets. Km-scale asymmetric recumbent folds w ith strongly non-cylindrical hinge lines, developed as a consequence o f ductile shearing, are present in one of these thrust sheets. The duc tile shearing, large-scale folding and thrusting can be related to the development of the Main Central Thrust Zone. The microstructural rela tions show that the main phase of regional low- to medium-grade metamo rphism (T approximate to 430-600 degrees C and P approximate to 4.5-8. 5 kbar) is pre-kinematic with respect to the formation of the Main Cen tral Thrust Zone. Growth zoned garnets with typical bell-shaped Mn pro files and compensating bowl-shaped Fe profiles are compatible with thi s phase of metamorphism. Some of the larger garnet grains, however, sh ow flat compositional profiles; If they represent homogenization of gr owth zoning, it would be a possible evidence of a relict high-grade me tamorphism. The ductile shearing was accompanied by a low-greenschist facies metamorphism during which mainly chlorite and occasionally biot ite porphyroblasts crystallized.