Document Type : Full length article
Department of physical Geography, Faculty of Geographical Sciences and planning, , university of Isfahan, Isfahan, Iran
Department of physical Geography , Faculty of Geographical Sciences and planning, , university of Isfahan, Isfahan, Iran
Department of physical Geography, Faculty of Geographical, , university of Tehran, Iran
River terraces represent a history of river stratigraphy and provide valuable information for understanding the interactions of tectonics, erosion, and climate change. The high altitude of the Jajrood basin has resulted in extensive glacial remnants, especially the accumulation of moraines upstream of the basin. The extent of moraine depositions under the upstream basins of the Jajrood River is not evident in the structure of river terraces and they do not have a uniform elevation. Moreover, the relations of old glacial conditions in the sedimentary interference of terraces cannot be easily reconstructed and discriminated, and there is no regular order in the stratigraphic sequence and sedimentological conditions of the river terraces. The moraine deposition seemingly has had a major role in the differences in river terrace sequences from upstream to downstream of the study area, which calls for further investigation and is also addressed in this study. Many studies have delved into the analysis of the evolution of the river terraces. Other important areas of study include paleontology and sedimentology and their effects on the canal sustainability against flow dynamics. This study aimed to explore the climatic and neo-tectonic developments of the Jajrood River Basin and the role they have played in creating terrace sequences.
Materials and methods
This pure research investigated the role of neo-tectonic developments and climate change on the formation and evolution of terraces in the Jajrood River Basin. The research was based on analytical calculations and reports prepared through surveys as well as remote sensing methods to examine the effects of tectonics in the area. In addition, sedimentological evidence was studied to see how climate change has affected the formation and evolution of these terraces. The primary research tools were topographic and geological maps alongside aerial photographs and satellite images. Other fieldworks such as terrace morphometry using GPS measurements and sedimentological analysis helped to add insight to the findings. Then, the data was analyzed in ArcGIS. Here, the Jajrood Canyon was divided into three sections to better examine the morphogenesis of the terraces. Next, the transverse profiles and stratigraphic sections were drawn up to investigate the sedimentary strata in each section through morphometry, and then the evolution of the terraces was analyzed and reconstructed. The tectonics were studied using radar images to determine vertical displacement through the small baseline subset (SBAS) time series. Here, 27 Sentinel-1 images were used for the period from Oct. 14, 2014, to Oct. 27, 2016. which was performed using Generic Mapping Tools (GMT) in Linux OS. After preparing the interferogram maps, a map of the displacement in the study area was generated using the SBAS method (Zhao, 2013). Moreover, changes in the climate were studied using sedimentological and stratigraphic evidence.
Result and discussion
The research findings can be classified into two parts. First, neo-tectonics was investigated through remote sensing methods and the analysis of vertical displacement across the region. Then, the effects of these neo-tectonic processes on the genesis and evolution of terraces were examined. In the second part, the effect of climate change on terrace developments was explored using sedimentological and stratigraphic evidence.
In the first part, the region's tectonics was analyzed using radar imagery. In doing so, the vertical displacement was measured using SBAS time series and 27 Sentinel-1 imagery during the period from Oct. 14, 2014, to Oct. 27, 2016. The specifications of the research images are presented in Table 1. The images were selected based on the research purpose and the baseline of the images relative to each other. The VV polarization was used for all images since co-polarizations exhibit a stronger backscatter. Some sensors have different polarizations, and images with different polarizations can inform interpretations to a great extent.
After preparing the images, for measuring displacement using the SBAS method, first, the temporal and spatial baseline of the images was examined and image pairs were selected for interferogram generation (Table 2 and Figure 3), which was performed using Generic Mapping Tools (GMT) in Linux OS. After preparing the interferogram maps, a map of the displacement in the study area was generated using the SBAS method. In the end, the role of morphotectonic relations in the morphogenesis (i.e., origin and development) of the terraces were examined.
These findings suggest that terraces in the Jajrood Canyon are highly heterogeneous in terms of sedimentological structures, stratigraphy, and altitude. For instance, the T3 to T1 terraces, respectively, were located 130, 90, and 80 m above the river. These terraces have also experienced three intermittent processes. These three river terraces were created through the combined effects of climate change, tectonic uplifts, and the formation of dam lakes. The results of SAR interferometry (InSAR) and fieldworks also confirm the effect of active tectonic uplift differences along the main canal. These differences reflect the differences in their morphogenetic processes. The altitude of the terraces at the Oushan River tributary (Section 1) is nearly 130 meters. However, this section's altitude downstream (near Hajiabad Village) is estimated at 90 m. This difference cannot be merely due to baseline discrepancies. Evidence indicates that a sedimentary interference originating in the lake due to a past landslide downstream of the study area (Hajiabad landslide) is the cause of the higher altitude of the terraces in this section. The terrace sequences were not the same in any of the three sections. To be more precise, there are three identifiable terrace levels in Section 1, two in Section 2, and one in Section 3. In addition to the differences in the tectonic baseline, three factors—namely climate change, moraine, and the formation of a landslide-dam lake downstream—were identified for the genetic diversity, sequence differences, and terrace sequences throughout the three sections. In addition to morphometric differences, there were great differences in the genesis of the terrace sediments. The river has contributed the most to the formation and structure of terraces and their genesis. Nevertheless, the interference of landslide-dam lake deposits, moraines, and alluvial deposits, consecutively or simultaneously, have affected particularly sections 1 and 2 through differences in flow dynamics. Meanwhile, greater uniformity can be seen upstream, particularly in the Garmābdar basin (Section 3), such that upstream terraces in this section are predominantly glacial.