ارزیابی فعالیت نسبی زمین ساختی و تحلیل جنبش ‏شناختی شمال پلدختر، غرب ایران

Extended Abstract



Introduction

Because the landscape contains an important archive of the rates and spatial distribution of deformation (Kirby and Whipple, 2012), tectonic geomorphology aims to quantify the geomorphic response of the landscape to active tectonics. The present-day topography of mountain ranges is the result of interaction between tectonic and erosional processes (Bishop 2007). Geomorphological analysis of mountain fronts provides significant clues for reconstructing the tectonic activity of range-bounding faults on variable timescales (103–106 years; Burbank and Anderson 2001; Keller and Pinter 2002; Bull 2007; Demoulin et al. 2015).

The topographical features, geological structures and frequent seismicity of the Zagros orogenic belt are the result of continuous northward collision between the Afro-Arabian plates and Central Iran (Berberian and King, 1981; Alavi, 1994). The Zagros mountain belt is still undergoing a crustal adjustment process due to the continuous northward movement of the Afro-Arabian plate. In this research, using morphometric indicators, we evaluate the recent activities of Posht-Jangel, Amiran, and Chahar-Qhale anticlines located in the Zagros Fold and Thrust belt. The studied area is in the south of Lorestan province. We performed a structural and morphotectonic analysis to study this area.





Methodology

In this research, several morphometric indices have been used to analyze the level of tectonic activity. The indices selected for this analysis are: the stream gradient index (SL), asymmetry factor (Af), basin shape ratio (Bs), hypsometric integral (Hi), valley floor width–valley height ratio (Vf), as well as transverse river sinuosity (S) (e.g. Hack 1973; Bull 1978; Keller and Pinter 2002). Finally, a single index (Iat) was calculated from these six indices for every drainage basin (El Hamdouni et al. 2008). All mentioned morphometric parameters for 19 basins were analyzed using ArcGIS software.



Results and Discussion

Morphometric indices are widely used in tectonic geomorphology research due to their low cost and relative ease of use. The studied area is located in Lorestan province and is part of the Zagros Fold and Thrust belt. As one of the most active tectonic areas in the world, the Zagros fold and thrust belt consists of northwest-southeast trending whaleback anticlines that are growing vertically and laterally. The main purpose of this research is to describe the tectonic activities in the study area. For this purpose, structural evidence and geomorphic landforms (triangular surfaces, wine-glass valleys, gorges, asymmetric valleys and knick points; Figures 6 and 7) were identified. The studied area has a variety of fold and fault structures with different geometries and dimensions that show the intensity of deformation and the level of tectonic activity. At the outcrop scale, oblique-slip thrust faults, brittle shear zones, thrust-related folds and chevron folds in the resistant layers of Sarvak Formation (Figure 4) are among the structures observed in the structural survey. Examination of field data and seismic evidence in the studied area indicates the presence of young movements of dextral (linement 1) and sinistral (linement 2) transverse-shear faults after the Pliocene.

Six geomorphic indices (stream gradient index (SL), asymmetry factor (Af), basin shape ratio (Bs), hypsometric integral (Hi), valley floor width–valley height ratio (Vf), and transverse river sinuosity (S)) were qualified to analyze 19 drainage basins. These parameters were combined in order to obtain the index of relative tectonic activity (Iat) using GIS. The average of six calculated geomorphic indices was used to evaluate the relative tectonic activity distribution in the study area. The average of six calculated geomorphic indices was used to evaluate the relative tectonic activity distribution in the study area: class 1 of very high tectonic activity (22.08%); class 2 of high activity (41.30%); class 3 of moderate activity (27.4%); and class 4 of low activity (9.22%). Iat values indicate high tectonic activity in the east and central of the study area.



Conclusion

In this research, based on the structural (transverse-shear faults, thrust faults and fault related folds) and geomorphic evidence (triangular and trapezoidal surfaces, gorges, nice points) and quantitative analysis of geomorphic indices (SL, Vf, S, Af, Hi and Bs) relative tectonic activity and stability of the studied area was estimated. Based on this, the relative tectonic activity zoning map of the studied area was prepared in four categories of very high, high, medium and low relative activity. These results indicate that the east and central areas of the study area have relatively high tectonic activity and the northwest parts of the basin show moderate to low activity. In general, the results of the indices and the analysis of the observed evidence indicate the dominance of tectonic activity in the region and its superiority over erosion.