A Master Décollement Horizon below the Proterozoic Songshan Group in Songshan Area, Henan Province

The Songshan area of Henan Province is tectonically located in the southern part of the Sino-Korean craton. It is marked by the three groups of Precambrian separated by two unconformities. A master décollement horizon developed along the unconformity between the Dengfeng Group below and the Songshan Group above. The lower Songshan Group above the décollement horizon formed a nearly N-S trending folded belt. Therefore, Songshan Group structures have little relation with the Archaean Dengfeng Group below the décollement. The thickness of the master décollement horizon ranges from 4 to 30 m.Along this décollement horizon, the Denfeng Group amphibolite facies rock assemblage was strongly retrograded to that with superimposed greenschist facies mineral paragensis. The development of the décollment occurs near the brittle/ductle transition field. Our observations in various scales show that the formation of the décollement horizon is controlled by the rheological stratification which was induced by compositional layering of the crust. It is suggested that the weak interval or interface at the base of a stratigraphic sequence may be preferred structural level for the localization of sub-horizontal décollement zones. It is also clear that consideration of hydrolytic weakening is necessary to unravel the development of the décollement horizon.