Application of landform planation index (LPI) in identifying planation surfaces and paleotopography of Gorgan loess plateau (northeast of Iran)

Since the theory of the erosion cycle and the peneplain was put forward at the end of the ۱۹th century(Davis ۱۸۸۹, b, ۱۸۹۹), the planation surfaces has become one of the basic paradigms and theoreticalparameters in geomorphology (Latifi, ۲۰۱۹). Identifying planation surfaces is the first step in understandingthe geomorphological evolution of an area (Latifi, ۲۰۲۰). When such planation surfaces formed in thedistant geologic past, they were often eventually buried by subsequent deposition, such that a regionallyextensive unconformity is preserved. When such low relief surfaces (pediment, peneplains, and regionalunconformities) are uplifted tectonically, they can form prominent markers in the landscape (Burbank andAnderson ۲۰۰۱). The identification of planation surfaces is important in computing tectonic uplift rates,exploring topographic limitations to landform formation, and reconstructing the evolutionary process ofupland plateaus (Fielding et al. ۱۹۹۴, Stanford et al. ۲۰۰۰, Zheng et al. ۲۰۰۲, Coltorti et al. ۲۰۰۷, Zhang etal. ۲۰۰۸, Polissar et al. ۲۰۰۹, Römer ۲۰۱۰, Li et al. ۲۰۱۴, Bessin et al. ۲۰۱۵). The recognition and reconstructionof planation surface are the foundation for exploring planation processes and its underlying formationmechanism with landform analysis techniques. Traditional methods extracted the planation surfaces bymanually delineating the planation boundaries on the topographical maps (Nash, ۱۹۸۸; Coltorti andPieruccini, ۲۰۰۰; Feng and Cui, ۲۰۰۲). However, peneplains and their outcropping strata vary over space;hence, the field observation method cannot completely and accurately analyze the characteristics ofplanation surfaces because this extraction and analysis method is highly subjective (Liu et al. ۱۹۹۹). In thelast two decades, with the development of the geographic information system (GIS) and digital elevationmodel (DEM), it has become possible for us to effectively recognize planation surfaces over a regional scale.Among these, the fuzzy logic method is one of the methods that has been proposed to evaluate variousgeomorphic parameters based on DEM to identify elevated planation surfaces (Zhang and Huang, ۱۹۹۵;Haider, ۲۰۱۵; Latifi, ۲۰۱۸; Latifi, ۲۰۱۹; Latifi, ۲۰۲۰). But this method is mostly used in high mountainousareas. With the increasing maturity of terrain characteristics expression based on DEMs, it provides a newway to propose a terrain index to express and extract the planation surface on a basis of full considerationof the planation characteristics (Lu et al., ۱۹۹۸; Qian et al. ۲۰۱۶; Xiong et al., ۲۰۱۳. Xiong et al., ۲۰۱۴a). Theaim of this paper is to identify planation surface of the gorgan loess plateau using peak-cluster assessmentmethod based on DEM in GIS. The proposed method consists of two major steps: (۱) peak extraction basedon DEMs and (۲) peak-cluster characterization for planation surface identification and quantitativeexpression. At last, Landform Planation Index (LPI) based on the slope gradient and relative terrain reliefof the summit surface is used for extracting and describing the characteristics of the planation surfaces. Thegorgan loess plateau (۳۷°۲۵′–۳۷°۴۵′N, ۵۵°۴′–۵۶°E) with a total area of ۲۱۷۳ km۲ located in the northeasternof Iran and is geologically located in the western part of the cope-dagh zone. The elevation of study arearanges from ۲۸ to ۱۲۳۷ m. A semi-arid continental climate with average annual precipitation ranges from۲۰۰ to ۵۰۰ mm exists in this area. Six selected test areas are distributed from east to west in loess plateau.These test areas represent different landform types with their varied morphological characteristics. Thedistribution of test areas is shown in Fig. ۱.