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 décollements 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 décollement 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.