Crustal-Scale Duplex Development During Accretion of the Jiuxi Foreland Basin, North Qilian Shan

Understanding the propagation of shortening, especially the interaction of shallow and deep structural levels in space and time is important to understand the accretion process of a compressional orogen as well as to fully understand earthquake hazards to populated foreland basins. Here we combine evidence from geologic maps and stream-terrace surveys to construct a set of retrodeformable cross-sections of the western North Qilian Shan foreland. The uplifted, severely tilted Mesozoic and older rock units suggest the presence of both deep and shallow decollements in western and central part of our research area, and that these structures alternated activity since commencement of the latest phase of the North Qilian Shan uplift. Conversely, in the east, the absence of foreland fold-and-thrust belt and the moderately tilted Mesozoic rocks indicate the deformation is dominated by thick-skinned uplift. Based on our cross-sections, we estimate the long-term shortening rate of the Jiuxi foreland basin of 1.2-1.8 m/Kyr. Deformed foreland terraces show that, from west to east in our research area, active deformation switches between different structural levels. This trade-off between deformation styles in time and space shows that two decollement levels bound a crustal-scale duplex as the foreland is incorporated into the orogen. We suggest the complex and out-of-sequence deformation pattern may relate to pre-existing weakness within the basement rocks and is likely a common characteristic of the North Qilian foreland. This may impose an additional challenge for seismic hazard estimation of the region. Qilian Shan ("Shan" as mountains) and Hexi Corridor (basin) are located at the NE margin of the Tibetan Plateau and formed due to the northeastern movement of the Tibetan Plateau. The mountain front of the North Qilian Shan and the adjacent low hills have shown various deformation styles, that is, different depth levels of active faults, which proposes a challenge to understand the deformation history of the area. Our study shows that the deformation styles not only vary in space but also change with time, forming a duplex (imbricate faults of different depth levels) which controls the northeastward propagation of the mountain front. We suggest this variation may be resulting from the complex pre-existing weak structures within the bedrocks, which presents an additional challenge for seismic hazard estimation of the region. Analysis of the duplex suggests the western North Qilian mountain front has experienced compression at a rate of 1.2-1.8 m/Kyr. The North Qilian is likely dominated by out-of-sequence deformation, resulting from reactivating pre-existing structures Alternatively active crustal-scale duplex leads to various deformation styles of the North Qilian mountain front Long-term shortening rate of the mountain front and foreland area is 1.2-1.8 m/Kyr