An analysis of the characteristics of the rock-structure association of the Dunhuang orogenic belt and their implications for tectonic setting

Located between the Beishan orogenic belt in the north and the Qilian orogenic belt in the south, the Dunhuang region possesses a crucial tectonic position. However, the geotectonic attribution of this region has long been controversial. In this paper, the authors investigated the characteristics of the rock-structure association of the Sanweishan-Hongliuxia area in the Dunhuang region, which includes the Sanweishan-Northern Hongliuxia Area (SNHA) and the Southern Hongliuxia Area (SHA). The observation in the SNHA shows that the intensively sheared metasedimentary rocks are well exposed as matrix which enclose the blocks with different lithological types and ages, and the structural slices with different lithological types are tectonically superimposed upon each other. This rock-structure association with the"blocks-in-matrix"fabric integrally shows the typical characteristics of mélange. In the SHA, the metamorphic grade of the exposed rocks decreases southward, and the northern part of the SHA is dominated by the micaquartz schist and felsic gneiss association, while the Late Carboniferous coal-bearing strata and carbonate succession regarded as being formed in a continental margin setting are exposed in the southern part of the SHA, which are overthrusted by the plagioclase amphibolite nappe. In the SHA, the structural forms from north to south are from the north dipping intense shear zone through the duplex to the imbricate thrust faults, accompanied by the southward decreasing deformation intensity. The characteristics of the rock-structure association of the SHA are consistent with those of the foreland fold-and-thrust belt. The authors thus hold that there exist two tectonic facies units, i.e., the Sanweishan-northern Hongliuxia mélange in the north and the Hongliuxia foreland fold-andthrust belt in the south of the Dunhuang region. The collision process within the Dunhuang orogenic belt should occur after the Late Carboniferous period.