回曲构造

Omega-type structure

几年来在辽宁东部和山西中条山、康滇地轴、内蒙狼山对一些矿床、矿田和区域构造研究表明,老变质岩系普遍存在着回曲构造这样一种构造型式。这是两次(幕、期、阶段)主要塑性流动褶曲作用形成的一种迭加褶曲构造, 回曲构造尽管可以受到后期构造旋回影响,但没有改变这种构造格局,在地质图上能够很好地反映出来,有利于地层研究、普查和找矿勘探,对地球早期构造活动的研究也具有重要意义。

Through the investigation of the regional structures and the structures of ore deposits and ore fields in some Pre-cambrian metamorphic areas in China, the authors find out that there generally exists one type of superposed fold pattern, the omega (Greek Ω) type structure, in the Pre-cambrian metamorphic formations. The Ω-type structure is a structural pattern that strike folds are superposed by dip folds, and they are always in company with plastic faults. The plastic faults are not the common faults or ductile faults, and the dip folds are not plunging folds, either.The structures of Ω -type, therefore, are neither refolded folds nor cross folds. They may be small or large in extent and various in form. The Anshan Group and the Liaohe Group in the east of Liaoning contain a series of Ω-type structures respectively. In the areas of Mount Zhongtiao of Shanxi, the middle of Sichuan and the east of Yunnan, and Mount Lang of Inner Mongolia there are many good examples of such structures in the Pre-cambrian metamorphic formations.TheΩ -type structure probably becomes the characteristic structural pattern of Pre-cambrian metamorphic formations.Up to the present, dip folds have been generally regarded as plunging folds and thus these kind of structures as refolded folds or cross folds or gneiss domes. The basic reason for such confusions is that the b-axis in the stress field for any type of folding, including dip folding, has been taken for granted that it is horizontal . Actually it is mainly in high angle and even vertical in stress field for dip folding.