W-TYPE SUPERPOSED FOLDS IN THE SHILU GROUP AND MACROFABRIC ANALYSIS

Based on the thematic structural mapping on a large scale, four structural elements and two groups of fold style have been sorted out from the Shilu Group.The results of detailed observations,statistical calculations,map preparation and analyses combined with large amounts of data from drill holes and profiles have led to the conclusion that the basic structural framework of the Shilu Group is mainly represented by w-type superposed folds created by deformation and superposion of two phases of folding The macrofabric analysis of the Shilu Group provides the following understandings.（1）The geometric analysis indicates that folds of the two phases making up the w-type structure have the following geometric characteristics: the folds of the first phase are cylindrical and those of the second phase are conical,while macro-axis uplifts（quasi-cylindrical folds）were formed in the inner arcs（cores） of the w-type superposed folds.（2）The kinematic analysis gives a kinematic trajectory of the fabric elements and reveals that the original extension orientation of the first-phase fold axes is roughly NNE.The symmetry analysis shows that the kinemetric plot and mean strain of the first-and second—phase folds are both characterized by monoclinic symmetry,but the symmetric plane of the first-phase folds is the a’c’plane,while that of the second-phase folds,the a"b" plane（the axial plane of the second phase folds）.There exist internal rotational components in the mean strain of the folds of the two phases.（3）The results of kinetic analysis shows that the principal compressive stress generating the first-phase folds is oriented NWW-SEE and exhibits to some extent the behaviour of simple shear and that the orientation of the principal compressive stress of the second fold phase is NNE-SSW.To sum up,a basic picture of the formation and evolution of w-type superposed folds is presented in the paper in the geometric,kinemetric and dynamie respects.