Abstract
The height of a thrust-fault scarp on a fluvial terrace would be modified due to erosion and deposition, and these surface processes can also influence the dating of terraces. Under such circumstances, the vertical slip rate of a fault can be misestimated due to the inaccurate displacement and/or abandonment age of the terrace. In this contribution, considering the effect of erosion and deposition on fault scarps, we re-constrained the vertical slip rate of the west end of the Minle–Damaying Fault (MDF), one of the thrusts in the north margin of the Qilian Shan that marks the northeastern edge of the 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 dating. The heights of the surface scarps and the displacements of the fluvial gravel layers exposed on the Yudai River terraces were precisely measured with the Structure from Motion (SfM) photogrammetry and the real-time kinematic (RTK) GPS. The Monte Carlo simulation method was used to estimate the uncertainties of fault displacements and vertical slip rates. Based on comparative analysis, the dating sample from the fluvial sand layer underlying the thickest loess in the footwall was suggested to best represent the abandonment age of the terrace, and the fluvial gravel layer could better preserve the original vertical fault displacement compared with the surface layer. Using the most reliable ages and vertical offsets, the vertical slip rate of the MDF was estimated to be 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 (T7). The difference between the wrong vertical slip rate and the right one can even reach an order of magnitude. We also suggest that if the built measuring profile is long enough, the uncertainties in the height of a surface scarp would be better constrained and the result can also be taken as the vertical fault displacement. Furthermore, the consistency 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.
Highlights
The vertical slip rate of a fault is a significant metric to quantify the intensity of tectonic activity (Tapponnier et al, 1990; Hetzel et al, 2002; Ai et al, 2017; Liu et al, 2017), reconstruct the behavior of the fault over time
The vertical displacement recorded by the geomorphic surface can be obtained by extracting the topographic profile perpendicular to the fault scarp (e.g., Hetzel, 2013; Wei et al, 2020)
As Laser Radar (Cunningham et al, 2006; Ren et al, 2016), high-resolution satellite optical image (e.g., QuickBird, WorldView, and Pleiades), and aerial photogrammetry (Matthews, 2008; Fraser and Cronk, 2009) are gradually gaining usage relative to the traditional total station and the tape measure, it becomes more accurate and efficient to acquire topographic profile based on large-scale and highprecision topographic data
Summary
The vertical slip rate of a fault is a significant metric to quantify the intensity of tectonic activity (Tapponnier et al, 1990; Hetzel et al, 2002; Ai et al, 2017; Liu et al, 2017), reconstruct the behavior of the fault over time It has been suggested that we should select sites less affected 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, 2019), or attempt to estimate the effect of the surface processes to correct the height of the scarp (Priyanka et al, 2017; Yang et al, 2018). How to choose more reasonable topographic profiles and obtain more accurate vertical displacements needs to be systemically understood
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