عنوان مقاله [English]
Analysis of Factors Affecting the Terraces Sequences of the Jajrud River during the Late Quaternary
Due to the height of the Jajrud Basin, glacial traces, especially the accumulation of glaciers upstream of the basin, the extent of expansion of glacial flows below the basins of the Jajrud River in the construction of river terraces is not clear and does not show uniform height. On the other hand, the relationship between long-term glacial conditions in the sedimentary interference of the terraces can not be easily reconstructed and separated, and the stratigraphic sequence and sedimentological conditions of the river terraces do not show a specific order. It seems that the influence of ice flows has played an important role in the difference of terraces sequences from upstream to downstream of the study area, which needed to be investigated; Also, the sequence of terraces along the river route is not the same and it is not clear whether the changes in flow dynamics were the result of past climate change through the influence of glacial flows or the genesis of this period was the different morphodynamic performance of the interior along the basin.
Various researchers have studied the terraces, including Maghsoudi and Sharafi (2015)., Jafari and Abbasi 2019, Rezaei and Zarezadeh 2014, Darfashi et al., 2017, Yamani et al., 2015, Torabi Golsefidi and Karimian Eghbal 2002, Salehipour Milani et al., 2017and Fattahi et al., 2019) have researched and studied about terraces.
According to the background and history of studies conducted in Jajrud region, previous studies have been scattered and related to general geology, hydrology, etc. and there is no comprehensive study on the geomorphology of landforms in the region and their causes and terraces. And the connection of these colonies with the climatic and tectonic changes of the region in the past and the reconstruction of Quaternary climate change in connection with the morphogenesis of these colonies has not been done.
Methodology (Materials and methods or Data and methods)
The present study is mainly fundamental to investigate the role of tectonics and climate change on the formation and evolution of terraces in the Jajrud catchment, based on field research such as terraces morphometry and remote sensing methods to assess the tectonic effects on the region as well as sedimentological and stratigraphic evidence. The role of climate change on the formation and evolution of terraces has been studied. Topographic maps, geology, aerial photographs, satellite imagery and ArcGIS software are the main research tools. In the next step, to study the morphogenesis of the terraces, the length of Jajrud valley is divided into three main intervals. Then, by drawing transverse profiles and stratigraphic sections, sedimentary layers were morphometrically in the studied intervals. To study the tectonics of the study area, radar images were used to investigate the vertical displacement of the region using the SBAS time series method. Among these, 27 images of the Sentinel 1 satellite were used during the period of 10/14/2014 to 10/27/2016. have became. After selecting the desired pair of images, the desired interferogram maps have been prepared through the GMT software in the LINUX operating system.
Result and discussion
The findings of this study are reviewed in two parts. The first part examines the new tectonics using remote sensing methods by assessing the vertical displacement of the area, and then evaluates the effects of the new tectonics on the formation and evolution of the terraces; In the second part, the effects of climate change on the formation and evolution of terraces were investigated, using sedimentological and stratigraphic evidence.
In the first part, radar images are used to assess the tectonic state of the region. In order to study the vertical displacement of the region, the SBAS time series method is used. 27 images of Sentinel 1 satellite during the period of 10/14/2014 to 10/27/2016 have been used, which is shown in Table 1 of the specifications of the images used. After preparing the images, in order to evaluate the displacement of the region using the SBAS time series method, first, the temporal and spatial baseline status of the images was investigated and based on that, a pair of images was selected to form an interferogram (Table 2 and Figure 3). After selecting the desired pair of images, the desired interferogram maps have been prepared through the GMT software in the LINUX operating system. After preparing the interferogram maps, using the SBAS time series method, the area displacement map was prepared (Figure 4), which based on the results of the study area had a displacement between +103 and -28 mm, which in other words can be said that this The region has had a height of 103 mm and a subsidence of 28 mm during a period of 3 years (from 01/06/2016 to 12/21/2018). Examination of the final map indicates that the northern heights of the study area have elevations that can be attributed to tectonic factors. Then, the tectonic situation in the study intervals was investigated and the amount of vertical displacement in the river route and their profiles was calculated. Finally, the relationship between morphotectonics and morphogenesis of terraces was investigated. In the second part, the sedimentological and stratigraphic evidence of the terraces were analyzed from the perspective of the role of climate change.
The findings of this study show that in the study area, tectonics in the upstream part of the basin during the study period has shifted between +103 to -28 28 mm (06/01/2016 to 12/21/2018). The amount of uplift under the influence of active tectonics of the basin increases upwards. This difference in the amount of rise, in the height and sequence of the terraces is well reflected. Meanwhile, the arrival of paleo ice flows at the confluence of the main tributaries of Jajrud, as well as the occurrence of paleo landslides and the creation of a dam lake have been effective in the cross-sectional changes of the heights of the terraces and their irregularities along the main valley. Therefore, in addition to the role of tectonics upstream of Jajrud Basin, climate change downstream of the basin has also affected the formation and evolution of terraces.