Deducing Crustal‐Scale Reverse‐Fault Geometry and Slip Distribution From Folded River Terraces, Qilian Shan, China

Abstract

The deep structure of active reverse faults is generally difficult to constrain from surface observations and may conceal shortening within the hinterland. Here we investigate the geometry of active, crustal-scale reverse faulting from deformation of an unusually extensive set of river terraces preserved along the Beida River through the northwestern Qilian Shan. Two generations of fill terraces (T1 and T2) are well preserved from the foreland basin to at least 45 km within the hinterland. Optically stimulated luminescence and 10 Be dating methods indicate that T1 was abandoned at 24 ± 3 kyr B.P.; T2 was abandoned at 144 ± 30 kyr B.P. The T2 terrace profile reveals a long wavelength fold (~30 km) with a largest vertical deformation of~120 m relative to T1. Both kinematic and elastic modeling of the deformed terraces indicate that folding is a result of movement along a steep reverse fault (~50 o ), which merges into ã 10°décollement at 15-17 km depth. Estimated slip at depth is 1.8 times higher than fault slip at the surface, indicating a substantial proportion of shortening is absorbed by folding of the range interior. The shortening rate since abandonment of T2 is 1.4 ± 0.4 mm/yr, which is consistent with previous research and makes up 23 ± 10% of the geodetic shortening rate across Qilian Shan orogen. Our results support that crustal-scale reverse-faulting accommodates shortening and northward propagation of the North Qilian mountain front and that the total shortening rate is distributed between multiple faults within the Qilian Shan and Hexi Corridor.

Late Miocene to Quaternary Development of the Jiujing Basin, Southern Beishan Block, China: Implications for the Kinematics and Timing of Crustal Reactivation North of Tibet

“…The calculated slow strike-slip rates and long recurrence intervals determined for faults within the Beishan and Alxa blocks are consistent with the low geodetic crustal velocities and limited seismicity (e.g., [10,55]), suggesting that the Beishan-Alxa Blocks should be classified as a low-strain region where tectonic loading may be complexly partitioned amongst a currently unknown number of potentially active faults [65]. This is in strong contrast with the higher rates of Quaternary deformation along the Qilian Shan and Altyn Tagh Fault to the south (e.g., [66][67][68]).…”

Constraining Late Quaternary Crustal Shortening in the Eastern Qilian Shan From Deformed River Terraces

“…An increasing number of studies quantify shortening rates along the Frontal Thrust system and provide insights into the active tectonics in the area during the Late Quaternary. From west to east of the Yumen Fault, an estimated slip rate yielded a horizontal component of 0–2.2 ± 0.5 mm/year since ~10 kyr (Hetzel et al, 2006); Liu et al (2019) report a shortening rate of 1.9 ± 0.5 mm/year since ~22 kyr; a shortening rate of 1.3 ± 0.3 mm/year since ~45 kyr is obtained (Liu et al, 2017); Wang et al (2020) obtain a millennial shortening rate of 1.4 ± 0.4 mm/year since ~144 kyr. Crustal shortening rates along the western, middle, and eastern Fodongmiao‐Hongyazi Fault are constrained to be 1.0–1.4 mm/year since ~207 kyr, 0.7–1.2 mm/year since ~46 kyr, and 0.8 ± 0.2 mm/year since ~16.7 kyr, respectively (Yang, Yang, Huang, et al, 2018; Yang, Yang, Zhang, et al, 2018).…”

“…During the past decade, various studies have been conducted in the Qilian Shan and its piedmonts (e.g., Champagnac et al, 2010; Hetzel et al, 2004, 2006, 2019; Hu et al, 2015; Tapponnier et al, 1990; Wang et al, 2020; Zhang et al, 2012). Based on these works and the increasingly detailed GPS data (e.g., Liang et al, 2013; Wang & Shen, 2020; Zhang et al, 2004; Zhao et al, 2015; Zheng, Zhang, He, et al, 2013), we now have an opportunity to discuss the spatial pattern of crustal shortening of the Qilian Shan.…”

Test on the Reliability of the Subsurface Fault Geometry Estimated by Deformed River Terraces Along the Bailang River, North Front of the Qilian Shan (North West China)

“…In the profit from the development of kinematic models for fault-related folds (Suppe, 1983;Suppe and Medwedeff, 1990;Erslev, 1991;Hardy and Poblet, 1994;Wickham, 1995;Allmendinger, 1998;Mitra, 2003;Cardozo, 2008;Hardy and Allmendinger, 2011;Poblet and Lisle, 2011;Brandes and Tanner, 2014), scholars try to estimate the active fold geometry using deformation patterns of geomorphic markers (Thompson et al, 2002;Gold et al, 2006;Scharer et al, 2006;Wilson et al, 2009;Burgess et al, 2012). Based on kinematic models for fault-related folds geometry and geomorphic deformation, the geometry of the related fault can also be estimated (Hu et al, 2015(Hu et al, , 2017(Hu et al, , 2019bLiu et al, 2019;Wang et al, 2020;Zhong et al, 2020), which provides a more convenient way to investigate the subsurface fault geometry. Due to the uncertainty derived from the selection of the fold model and from the assumption of the fault dip close to the surface (e.g., Hu et al, 2015), the reliability of the estimated fault geometry is still questionable.…”