Abstract
The subsurface fault geometry is the base for understanding a process of crust deformation and mountain building. Based on kinematic models for fault-related folds, a geomorphic method is recently applied to estimate the subsurface fault geometry, while the validation on its reliability is lacking. In this study, we surveyed a suit of river terrace surfaces across an active fold at the north front of the Qilian Shan. According to the deformation geometry of the terraces, the fold deformation is interpreted by a listric fault fold model, and based on this kinematic model, the fault geometry underlying the fold is estimated. In comparison between the estimated fault geometry and a seismic reflection profile, we found that the decollement depth and the back thrust are highly consistent with each other. Although some small fault bends or internal shearing cannot be estimated solely by the terrace deformation, the overall fault geometry is successfully revealed by the terrace deformation. Using this fault geometry and the terrace dating results, the region deformation kinematics are re-evaluated, which suggest that the dip slip (in a rate of 1.8 ± 0.4 mm/a) along the decollement is mainly accommodated by two structures, one is the blind-back-thrust fault within the piggy basin in a dip-slip rate of 0.9 ± 0.3 mm/a and another is the thrust and fold at the west portion of the Yumu Shan range.
Highlights
The subsurface geometry of an active fault is the base for estimating the deformation kinematics (e.g., Whipple et al, 2016), to calculate the crust shortening rate (e.g., Lavé and Avouac, 2000), and to understand a process for mountain building (e.g., Hu et al, 2019b)
According to the deformation pattern of terraces (Figure 4), the coupled deformation and related fault geometry can be separated into two parts: The first part is the Dahe Anticline deformation, relating to a back thrusting fault derived from the decollement (Zuza et al, 2016; Hu et al, 2017) and the second part is the monocline with north-ward tilting terraces on the south of the anticline and probably related to the thrusting on the base fault with a south-ward tilting shape (Hu et al, 2017)
This evidence probably suggests that the North Qilian Shan Fault (NQF) had been active along this boundary before the thrust fault propagating to the north below the piggy-back basin and to the Yumu Shan
Summary
The subsurface geometry of an active fault is the base for estimating the deformation kinematics (e.g., Whipple et al, 2016), to calculate the crust shortening rate (e.g., Lavé and Avouac, 2000), and to understand a process for mountain building (e.g., Hu et al, 2019b). Fault Geometry Estimated by Terraces popular method to acquire the subsurface fault geometry is a seismic reflection survey (e.g., Gao et al, 2013; Wang et al, 2013). It can supply relatively reliable data on sedimentary basins with layered strata (e.g., Wu et al, 2019), the results usually have great uncertainties in regions involving basement rocks (e.g., Gao et al, 2013). It is highly necessary to verify the geomorphic method for the estimation of the subsurface fault geometry
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