ALBERT

Description: The tectonic activity and potential for linkage of adjacent active faults are crucial for seismic assessment. As the two largest faults that bound the Weihe Graben (central China), the Qinling Northern Piedmont Fault (QNF, ~200 km) and the Huashan Piedmont Fault (HPF, ~150 km) are mainly responsible for seismic risk in this densely-populated area, where the 1556 M 8.5 Huaxian earthquake occurred with 830,000 fatalities. However, their tectonic activity and the degree of interaction remain poorly constrained, hampering an adequate seismic risk assessment of the Weihe Graben. Here, we integrate 23 new 10Be-derived catchment-averaged denudation rates of ~0.06–0.32 mm/yr with topographic metrics to evaluate the seismic risk. The results demonstrate that the landscape of the Qinling and Huashan Mountains is in transient state in response to the tectonic perturbations of the QNF and the HPF, with tectonic knickpoints formed along main streams and tributaries, and widespread unstable drainage divides. These two faults have comparable tectonic activity, and are potentially capable of generating earthquakes with the maximum magnitude of Mw ~7.7–7.9. Moreover, they have likely started linking, posing a greater seismic risk than previously estimated.

Description: High-resolution topographic datasets are being increasingly applied in studies of active tectonics for precise quantification of lateral and vertical offsets of landforms. Digital elevation models (DEM) acquired by airborne light detection and ranging (LiDAR) have also been used for morphological analysis of tectonic scarps to infer the seismic-slip history of active faults. However, the high cost of conducting LiDAR studies limits its widespread application. In contrast, the potential applicability of the low-cost and portable small unmanned aerial vehicles (sUAV) combined with structure-from-motion (SfM) techniques to investigate morphological features of fault-generated scarps and their paleoseismic implications has not been fully explored. Here, based on sUAV-acquired DEMs and field observations, we present a case study of the morphological features of fault scarps along the Seertengshan Piedmont Fault (SPF), a major boundary fault of the Hetao Graben, North China, and discuss their paleoseismic implications. The results demonstrate that low-angle (〈25°) scarp slopes along the SPF suggestive of a long period of erosion are consistent with the results of paleoseismological studies (i.e., the most recent surface-faulting event occurred 〉2000 years ago). The observed 1.8–2.5-m-high fault scarps with single slope sections probably represent the most recent incremental displacements associated with surface-rupturing earthquake(s). Topographic data acquired by sUAV are suitable for quantitative morphological analyses of fault-generated scarps in actively deforming regions and can contribute to assessments of seismic hazard.

Description: Normal fault linkage has significant impacts on uplift patterns and erosional processes in extensional regions. However, geomorphic process-based constraints on landscape response to normal fault linkage are still scarce. Here, we use landscape evolution models to examine how a landscape responds to the linkage of two normal faults. The results demonstrate that topography dynamically responds to the changes in uplift patterns that accompany fault linkage. Specifically, our results indicate that after fault linkage, (1) the steepest topography and the highest erosion rate shift from the center of each fault segment to the linkage zone; and (2) the main drainage divide evolves from an M-shape to a bow-shape. We apply these findings to the Langshan Mountains in northern China, and suggest that the two piedmont fault segments have linked and that a high geohazard risk exists near the linkage zone, where the steep, transient topography is experiencing intense erosion.

Description: The landscapes on Earth are shaped by the interaction of tectonics and erosion processes and thus is a recorder for past tectonic and climatic histories. A recently proposed geomorphologic index for fluvial landscape, χ, can indicate the drainage divide migration from the smaller-χ-value side to the other and provide a quick estimation of the dynamic state of drainage systems. Since then, this index has been frequently used to determine divide mobility and hence landscape evolution. Some factors, however, have substantial impacts on the interpretation of the χ value, including tectonic activities, rock erodibility, and precipitation differences. Here we set a series of numerical simulations implemented on the landscape evolution model of DAC (Divide and Capture). Each simulation includes a spatial variation of a particular factor to examine the relationship between divide migration and χ values. Our numerical modelling results indicate that the migration direction is related to cross-divide contrast in erosion rate rather than χ. We take the Wula Shan horst as a natural example to demonstrate a static drainage divide but with discrepancies of cross-divide χ values in a setting of spatially asymmetric uplift. Our results suggest that a straightforward relationship between cross-divide contrasts in χ values and the drainage divide motion may not exist when spatial and temporal variations exist in the rock uplift rate, lithology, and precipitation. We emphasize that researchers should take notice of the requisite assumptions and scope of the application when using this index in landscape analysis.

Description: Fold-and-thrust belts (FTBs) develop widely in and around active orogenic belts on Earth. With the accumulation of structural shortening, thrust-related folds can grow by increasing their amplitudes and lengths, providing insights into the structural evolution of FTBs. Investigations of fold growth patterns and landscape responses through time and space have important implications for hydrocarbon exploration and geo-hazard assessment. Evidence for lateral fold propagation (i.e., increase in length) usually comes from geochronological constraints based on differences in onset timings along strike. However, these traditional methods are time consuming and economically costly. Alternatively, previous studies have proposed the use of geomorphic features to investigate the lateral fold propagation, including spatial variations in the patterns and densities of river channels, the configuration of the topographic profile along the fold crest, and the occurrence of wind gaps. However, such analyses are based on morphological features of present-day landscapes, which still need process-based validations. Here, we conduct a series of landscape evolution models to reproduce fold growth and their associated geomorphic adjustments. Our results show that deflected rivers and inflected fold-crest elevation profile with decreasing gradient are reliable evidence of lateral fold propagation. The location of propagation is in accordance with the distribution of crest-profile slope breaks and the position where river deflection began. Then, we apply these findings to two natural examples located at the deformation fronts of the Tian Shan (NW China). Results suggest that 1) the Kashi anticline has experienced lateral propagation both westwards and eastwards; and 2) that the Yaken anticline has experienced accelerated tectonic uplift as well as eastwards propagation. These findings are supported by the constraints from geochronological dating and quantitative structural analysis. Future work that integrates folding mechanisms may provide novel insights into landscape response to complex deformation in active FTBs.