CONTRIBUTIONS OF FOLD-ACCOMMODATION FAULTS IN FOLD SCARPS TO THE CALCULATION OF SHORTENING INCREMENT:A CASE STUDY FROM QIULITAGE ANTICLINE( SOUTHERN TIAN SHAN,CHINA)

Fold-accommodation faults,secondary faults subordinated to the principal fold,are of much significance to accommodate strain variation in different parts of the rock during the evolution of folding. They are generally found in groups. And each of them has limited displacement and does not connect with the main detachment. After the geological survey in the East Qiulitage anticline zone,we find that the secondary faults accompanying fold scarps in this area are out-of-syncline thrusts and also give an instance of secondary faults occurring later than the folding. The fact that the secondary faults in fold scarps force the hanging wall to move upward relative to the footwall not only makes the terrace tilting and increases the slope of fold scarps,but also affects the authenticity in calculating regional shortening increment. The theoretical results show that if we do not consider the increased fold scarps height influenced by the secondary faults,the shortening increment is 51. 42 m. Otherwise,the value will be 45. 23 m and the difference between them is 6. 19 m. Because the deviation is 13. 7% of the total shortening increment,the contributions of fold-accommodation faults to the calculation should not be ignored. The fold scarps in the northern and southern flanks of the East Qiultiage anticline depend on same bedrock type and formation mechanism. But three levels of fold scarps were found in the cross section of less than 300 meters in horizontal distance. This fact indicates that the active kink band of northern part is more closed because of higher compressive stress and faster lifting,which produce a large number of secondary faults in the fold scarps only in the northern flank. Therefore,the study of secondary faults is of significance in understanding of regional tectonic evolution and interaction between folds and faults. But there still exist many problems: 1) Limited by the observing scope,discontinuous distribution of secondary faults and variations of displacement along fault,we may underestimate the influence of secondary faults and the theoretical result should be the minimum. 2)What is the quantitative relationship among the increased height of fold scarps,the transfer slip and the dip of secondary faults? 3) If secondary faults only grow in active kink band,how will they affect fold scarp? More examples of fold-accommodation faults are needed for further research.