feart-09-635702 April 23, 2021 Time: 16:3 # 1 ORIGINAL RESEARCH published: 29 April 2021 doi: 10.3389/feart.2021.635702 Effects of Erosion and Deposition on Constraining Vertical Slip Rates of Thrust Faults: A Case Study of the Minle–Damaying Fault in the North Qilian Shan, NE Tibetan Plateau Qingri Liu1,2, Huiping Zhang1, Youli Li2, Feipeng Huang1, Xudong Zhao1, Jinghao Lei1,2, Weilin Xin2, Jianguo Xiong1* and Peizhen Zhang1,3 Edited by: 1 State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, China, 2 Key Fabien Graveleau, Laboratory of Earth Surface Processes of Ministry of Education, Peking University, Beijing, China, 3 Guangdong Provincial Université de Lille, France Key Laboratory of Geodynamics and Geohazards, School of Earth Science and Engineering, Sun Yat-sen University, Reviewed by: Guangzhou, China Xingwang Liu, Lanzhou National Observatory The height of a thrust-fault scarp on a fluvial terrace would be modified due to erosion of Geophysics, China R. Jayangonda Perumal, and deposition, and these surface processes can also influence the dating of terraces. Wadia Institute of Himalayan Geology, Under such circumstances, the vertical slip rate of a fault can be misestimated due to the India Andrew V. Zuza, inaccurate displacement and/or abandonment age of the terrace. In this contribution, University of Nevada, Reno, considering the effect of erosion and deposition on fault scarps, we re-constrained United States the vertical slip rate of the west end of the Minle–Damaying Fault (MDF), one of the *Correspondence: thrusts in the north margin of the Qilian Shan that marks the northeastern edge of the Jianguo Xiong [email protected] Tibetan Plateau. In addition, we tried to explore a more reliable method for obtaining the vertical fault displacement and the abandonment age of terraces with AMS 14C Specialty section: dating. The heights of the surface scarps and the displacements of the fluvial gravel This article was submitted to Structural Geology and Tectonics, layers exposed on the Yudai River terraces were precisely measured with the Structure a section of the journal from Motion (SfM) photogrammetry and the real-time kinematic (RTK) GPS. The Monte Frontiers in Earth Science Carlo simulation method was used to estimate the uncertainties of fault displacements Received: 30 November 2020 Accepted: 05 March 2021 and vertical slip rates. Based on comparative analysis, the dating sample from the fluvial Published: 29 April 2021 sand layer underlying the thickest loess in the footwall was suggested to best represent Citation: the abandonment age of the terrace, and the fluvial gravel layer could better preserve Liu Q, Zhang H, Li Y, Huang F, the original vertical fault displacement compared with the surface layer. Using the most Zhao X, Lei J, Xin W, Xiong J and Zhang P (2021) Effects of Erosion reliable ages and vertical offsets, the vertical slip rate of the MDF was estimated to be and Deposition on Constraining 0.25–0.28 mm/a since 42.3 ± 0.5 ka (T10) and 0.14–0.24 mm/a since 16.1 ± 0.2 ka Vertical Slip Rates of Thrust Faults: A Case Study of the Minle–Damaying (T7). The difference between the wrong vertical slip rate and the right one can even reach Fault in the North Qilian Shan, NE an order of magnitude. We also suggest that if the built measuring profile is long enough, Tibetan Plateau. the uncertainties in the height of a surface scarp would be better constrained and the Front. Earth Sci. 9:635702. doi: 10.3389/feart.2021.635702 result can also be taken as the vertical fault displacement. Furthermore, the consistency Frontiers in Earth Science| www.frontiersin.org 1 April 2021| Volume 9| Article 635702 feart-09-635702 April 23, 2021 Time: 16:3 # 2 Liu et al. Slip Rates of Thrust Faults of chronology with stratigraphic sequence or with terrace sequence are also key to constraining the abandonment ages of terraces. The fault activity at the study site is weaker than that in the middle and east segments of the MDF, which is likely due to its end position. Keywords: erosion and deposition, vertical slip rate, vertical offset, terrace abandonment age, Minle–Damaying Fault KEY POINTS image (e.g., QuickBird, WorldView, and Pleiades), and aerial photogrammetry (Matthews, 2008; Fraser and Cronk, 2009) are • The vertical slip rate of the Minle–Damaying Fault is re- gradually gaining usage relative to the traditional total station estimated to be 0.25–0.28 mm/a since 42.3 ± 0.5 ka at the outlet and the tape measure, it becomes more accurate and efficient of the Yudai River. to acquire topographic profile based on large-scale and high- • The topographic profile extracted from the top of the faulted precision topographic data. Although many studies on the fault gravel layer and that with sufficient length from the faulted tend to focus on the resolution or precision of the topographic loess layer are suggested to be used to constrain the vertical data (Blakely et al., 2009; McPhillips and Scharer, 2018; Hetzel fault displacement. et al., 2019), inadequate evaluation of surface processes may lead • The age of the fluvial sand layer underlying the thickest loess in to errors or mistakes in the measurement of fault displacement. the footwall of a thrust fault is suggested to best represent the It has been suggested that we should select sites less affected abandonment age of the terrace. by surface processes to extract profiles (Palumbo et al., 2009), calculate the vertical slip rate based on the offset of the top of the fluvial gravel layer in the trench (Liu et al., 2014; Ren et al., INTRODUCTION 2019), or attempt to estimate the effect of the surface processes to correct the height of the scarp (Priyanka et al., 2017; Yang The vertical slip rate of a fault is a significant metric to quantify et al., 2018). More often, however, researchers acknowledged the intensity of tectonic activity (Tapponnier et al., 1990; Hetzel the underestimation of the vertical displacement but did not et al., 2002; Ai et al., 2017; Liu et al., 2017), reconstruct the discuss much about it (Blakely et al., 2009; McPhillips and behavior of the fault over time (Zheng W. J. et al., 2013; Xiong Scharer, 2018; Hetzel et al., 2019). In different geomorphic et al., 2017; Hetzel et al., 2019), evaluate the seismic risk (Ren conditions, how to choose more reasonable topographic profiles et al., 2019; Lei et al., 2020), and understand the regional active and obtain more accurate vertical displacements needs to be deformation (Yang et al., 2018; Liu et al., 2019; Ren et al., 2019; systemically understood. Zhong et al., 2020). The estimation of vertical slip rates mostly Another challenge in constraining the vertical slip rate depends on two factors: the magnitude of the offset and the age is chronology. Taking the Qilian Shan area as an example, of offset landmarks (Burbank and Anderson, 2011). However, geomorphic dating tends to be affected by loess cover and accurate determination of vertical slip rates is not easy because erosion. If we assume that the loess overlying the terrace surface the complex surface processes including erosion and deposition begins to deposit immediately after the abandonment of the would affect the estimation of the original displacement and the terrace, there will be no significant gap between the age of the representation of the collected dating samples. bottom of the loess and that of the top of terrace deposition For thrust faults, the vertical displacement recorded by (Pan et al., 2003; Xu et al., 2010; Lu et al., 2018; Lu and Li, the geomorphic surface can be obtained by extracting the 2020). However, in most cases, the deposition of the loess layer topographic profile perpendicular to the fault scarp (e.g., Hetzel, usually lags behind the abandonment of the terrace, thus taking 2013; Wei et al., 2020). However, as the fault scarp degrades the bottom age of the loess as the abandonment age of the terrace with time (Avouac, 1993; Hetzel, 2013; Jayangondaperumal et al., will affect the estimated deformation rate (Stokes et al., 2003; 2013), the hanging wall can be eroded and acts as the sediment Küster et al., 2006; Lu and Li, 2020). For example, when dating source for the footwall (Stewart and Hancock, 1988). In addition, the same terrace of the Xie River, OSL age from the bottom of the stream that formed the terrace may shed sediments onto the the loess is 12.7 ± 1.4 ka (Xiong et al., 2017), while 14C age footwall (Hetzel et al., 2004; Hetzel, 2013). Fluvial aggradation is from the floodplain sand layer is 16,405 ± 210 cal a BP (Lei also possible in the footwall due to the adjoining stream (Priyanka et al., 2020). The age gap between the two results is ∼4 ka. et al., 2017), and the footwall is also more likely to accumulate The latter is the age of the uppermost fluvial sediment and is aeolian sediments (Avouac and Peltzer, 1993). Therefore, it is close to the abandonment age of the terrace, while the OSL age a consensus that vertical offsets derived from scarp profiles is younger due to the lagging deposition of loess mentioned may underestimate the real displacement of a fault especially previously. In the middle and western Qilian Shan, the loess in areas with heavy loess cover (Wallace, 1980; Peterson, 1985; deposition with a thickness less than 1.5 m is mostly younger Stewart and Hancock, 1988; Hetzel, 2013; Liu et al., 2017; Yang than 13 ka by OSL dating (Stokes et al., 2003; Küster et al., et al., 2018). In recent years, as Laser Radar (Cunningham 2006).