Determination of the double-layer structure in orogenic belts and its geological significance

Abstract

At present, the study of accretionary orogenic belts and collisional orogenic belts has achieved numerous insights and improvements. However, continental subduction, which often occurs during the oceanic and continental transition, has not received enough attention for a long time, such as what kind of structural deformation characteristics it has and how it affects the evolution of the orogenic belt. This paper studied two Cenozoic orogenic belts (the Taiwan orogenic belt and the Yarlung Zangbo River orogenic belt) and one Mesozoic orogenic belt (the Qiangtang orogenic belt) in order to clarify the unique structural deformation characteristics of continental subduction and its interaction with orogenic processes. It is found that the subduction of continental crust often forms a double-layer structure in the orogenic belt. The upper part is a set of thrust imbricate composed of Smith strata, and the lower part is a set of subduction complexes with a “blocks in the matrix” structure. The upper and lower parts of the double-layer structure are similar, mainly slope facies–submarine fan facies rocks and little shelf facies rocks. Due to the similar deformation time, the double-layer structure should be a structural system formed in different depths by the subduction of the same passive continental margin. We suppose that the subduction of the slope–submarine fan is the main factor for the formation of the double-layer structure. The subsequent continental shelf subduction could induce the collision and thus lead to strain’s gradual propagation to the craton’s interior, resulting in the foreland fold-thrust. Also, the double-layer structure is often destroyed during the collision, so the deeply underplated continental subduction complex can be exhumed to the shallow level. Therefore, this study also emphasizes the importance of continental subduction and the exhumation of subducted crustal rocks in the evolution of orogenic belts.

Keywords

• double-layer structure
• orogens
• continental subduction
• qiangtang orogenic belt
• yarlung zangbo river orogenic belt
• taiwan orogenic belt