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Deducing Crustal-scale Reverse-Fault Slip Distribution from Folded River Terraces, Qilian Shan, China

Yiran Wang, Michael E. Oskin, & Youli Li

Published August 14, 2018, SCEC Contribution #8530, 2018 SCEC Annual Meeting Poster #244

Studies have shown that for many compressional orogens, such as the Andes, the North Los Angeles fault system, and the Longmenshan fault, rates of shortening from geodesy are faster than geologically determined rates. Lack of documented faulting or other processes that absorb this missing convergence limits our ability to understand of the evolution of compressional orogens and their seismic hazard potential. The Qilian Shan ('Shan' as mountains in Chinese) of northwest China is an actively shortening orogen developed along the northern margin of the Tibetan Plateau. Two generations of fill terraces (T1 and T2) are well preserved along Beida River from the foreland basin to at least 45 km within the Qilian Shan hinterland. Both terraces are offset by the North Qilian fault along the range front. Preservation of these terraces across the mountain range presents a unique opportunity to quantify additional fault slip absorbed by folding of the range interior. We mapped and surveyed fluvial terraces for a distance of 45 km upstream of the mountain front. Using the deformed terrace profiles and field observation of fault dip, we constrained forward and inverse elastic modelling of the North Qilian fault to deduce its geometry and estimate its slip distribution at depth. The T2 terrace profile reveals a long wavelength fold (~30km) with a largest vertical deformation of ~130m relative to T1. Elastic modelling indicates that this folding of North Qilian Shan is a result of a thick-skinned fault that dips at a high angle near the surface (~50o) and soles into a decollement at ~25km depth. Estimated slip at depth is ~10 times of the fault slip at the surface, indicating that the majority of slip is absorbed by folding of the range interior. This study indicates that for orogens like the Qilian Shan, most of the shortening is accommodated within the mountain range by folding, and surface displacement by reverse fault slip at the mountain front reveals only a fraction of the total shortening. This suggests a need to pay more attention to long-wavelength folding when estimating fault activity and evaluating earthquake hazard.

Key Words
Surface deformation, river terrace, fault geometry, elastic modeling

Wang, Y., Oskin, M. E., & Li, Y. (2018, 08). Deducing Crustal-scale Reverse-Fault Slip Distribution from Folded River Terraces, Qilian Shan, China. Poster Presentation at 2018 SCEC Annual Meeting.

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Earthquake Geology