关键词: 陡坎, 陡坎退化, 扩散模型, 形貌测年, 断层陡坎

Abstract:

A scarp is a common step-like landform in nature, which consists of a gently sloping plane connected to the upper and lower geomorphic surfaces of differing elevations. Common scarps include fault scarps, terrace scarps, lake shoreline scarps, shoreline scarps, volcanic ash cinder cones, etc. Scarps are often used as strain markers because of their linear characteristics and are favored in the study of active tectonics. However, it is difficult to directly constrain their ages. Instead, they are usually constrained by the ages of the upper and lower geomorphic surfaces. The scarp developed in loose deposits is controlled by a long process of low-energy degradation after a short collapse. This process can be modeled by the diffusion equation because the process can be considered as a slope process under the transport-limited condition. Under this condition, the slope can provide enough loose material for transport, that is, the material transport capacity is less than the material supply capacity. If process assumptions are sufficiently valid and rate constraints can be calibrated independently, the true age of scarps can be obtained. This method is called morphologic dating. This method has been included in many textbooks published overseas, but there have very little research on this method in China. Both linear and nonlinear models have been developed to describe scarp degradation. Linear diffusion models assume that the diffusion coefficient is a constant, whereas nonlinear transport models generally define the diffusion coefficient as a nonlinear function related to the topographic gradient. Compared to the linear transport models, nonlinear transport models can better explain the phenomenon of rapidly increasing deposition flux as the gradient approaches a critical value. In this paper, we review the study history of scarp degradation analysis and the concept model of scarp degradation. We focus on the establishment of the nonlinear model, the role of the different parameters in profile evolution, determining the best-fit age using a full-scarp nonlinear modeling procedure, and so on. Furthermore, we introduce the model of the nonlinear age chart, including the effect of far-field slope on morphologic dating of scarp-like landforms and two examples of the application of the chart, which shows that this method can correctly evaluate the ages of single-event scarps. Finally, we discuss the extension of the concept and method of the scarp degradation model, the applicability of the model, and repeated fault scarp morphological analysis. For nonlinear diffusion models, in addition to n equal to 2, two parameters (critical gradient (S_c) and diffusion constant (k)) need to be constrained. The critical gradient can be obtained from the young scarps in the study area, which roughly represents the initial state of scarp evolution, typically 0.6 to 0.7(30° to 35°). The diffusion constant needs to be characterized by a known age scarp. The slopes of the upper and lower geomorphic surfaces have an obvious influence on the morphology of a degraded scarp. These discussions indicate that both linear and nonlinear models can be used for the degradation analysis of single-event scarps, but a nonlinear diffusion model is recommended for young single-event scarps. The constant slip rate nonlinear model can be used to simulate the evolution history of<10ka high-slip rate active fault scarp. The multiple-event scarp model requires careful evaluation of the fault location and the amount of displacement per event. There are several assumptions in the scarp topography diffusion modeling, which require practice to verify its reliability. With advances in surveying technology, it is now possible to rapidly obtain high-resolution terrain data over broad areas from which numerous topographic profiles can be efficiently extracted. This provides a broad application prospect for scarp degradation analysis and morphologic dating.

Key words: scarp, scarp degradation, diffusion model, morphologic dating, fault scarp