"XIKANG-TYPE"FOLDS AND THEIR DEFORMATION MECHANIISM——A NEW FOLD TYPE IN OROGENIC BELTS

"Xikang-type" folds are developed in a very thick flysch sequence of the Triassic Xikang Group of the Songpan-Garze orogenic belt in southwestern China They are special syncleavage upright folds. Their characteristics are as follows; the plunge of fold hinge varies from+90° to -90°; there are many styles of composite folds such as rounded-concentric folds without cleavages, syneleavage chevron folds and transitional folds;many kinds of cleavavages such fracture cleavages, slaty cleavages and flow cleavages form a complex picture of refracted cleavages;there occur vertical stretching lineations on the flow cleavage planes. On the basis of mierostructural analysis,"Xikang-type"folds are the product of three different deformation mechanisms——flattening, pure shear and pure shear + simple shear. Dynamically they are superimposed folds formed under the action of three plates and two subvertical（S-N and E-W）compressive stresses. It is inferred that the depth of their formation is 10-15kin. In a word, they are a new fold type in the orogenic belt. As far as their tectonic characteristics, formation mechanism and magnitude are concerned, this type of folds can be completely compared with the Jura type and Pennine type of folds.