عنوان مقاله [English]
Quantitative assessment of tectonically active regions can serve as an important tool for reconstructing tectonic history and development of landforms. In tectonic geomorphology studies, morphometric analysis refers to the quantitative measurement of landform such as drainage basin asymmetry, mountain front sinuosity, valley heights and widths, hypsometric integral. Morphometric characteristics of valleys formed on anticline limbs can act as basic reconnaissance tools to identify the level of tectonic activity. Study area, Danekhoshk anticline, is composed of same lithology, Asmari- Shahbazan unit and is located in the western part of Kermanshah province. Structurally it is part of Folded Zagros belt. Several studies reveal that tectonically active anticlines of Zagros belt are shortening and uplifting. The purpose of this paper is to examine the effect of tectonic uplifting on the morphometry of 103 valleys formed on the slopes of danehkhoshk anticline.
Material and method
To achieve the objective of this study, boundary of 103 valleys was determined based on Quickbird Satellite image as well as field works. Morphometric parameters of valleys such as Valley outlet width (Vow), Valley maximum width (Vmw), Valley length (VL) were derived based on Quickbird Satellite image. To determine the area (A) of valleys, borders of valleys were imported in Integrated Land and Water Information system (ILWIS) software and then area of valleys were extracted. After digitizing 20- meter contour lines from topographic maps of Iranian National Geography Organization, at a scale of 1:50 000, Digital Elevation Model (DEM) of study area were prepared. The valley floor width-to-height ratio ( ) was calculated as the width of the valley floor divided by the average height of the valley divides:
Where is width of valley floor, is elevation of the left valley divide, is elevation of the right valley divide, is elevation of the valley floor. The low values of are characteristic of tectonically active areas undergoing relatively rapid uplift and valley incision, whereas high values of are related to low tectonic activity. Geological data were derived from geological maps of National Iranian Oil Company at scales of 1:250 000 and 1/1000000. The strata dips (S) of anticline limbs were measured at mountain front by clinometer.
Danekhoshk anticline was subdivided into 9 tectonic zones based on dip of strata and width of limbs. Then, mean of mentioned parameters were calculated in each zone. Subsequently the relationship between means of morphometric characteristics of valleys and mean of strata dips were analyzed by preparing linear relationships between means of mentioned parameters.
Results and Discussion
Danehkhoshk anticline in Zagros belt is an uplifting anticline and rate of tectonic activity varies spatially within it. Decrease in the crest height and in the inclination of the limbs towards southeast and northwest shows that studied anticline propagates laterally toward southeast and northwest. Values of the strata dips (S) of anticline limbs measured at mountain front reveal that the steepness of limbs varies greatly within study area. Results of this study reveal that there is meaningful positive relation between structural slopes (S) and the ratio of valley maximum width to valley outlet width (Vmw/Vow), with appropriate correlation coefficient (91%). Also there is positive relation between structural slopes and the ratio of valley maximum width to valley length (Vmw/VL), with appropriate correlation coefficient (75%). Data analysis represents that there is positive relation, with fairly low correlation coefficient, between the structural slopes and valley area (A) as well as valley maximum width (Vmw). There is no meaningful relation between structural slope and valley length. The inverse relation, with low correlation coefficient, exists between structural slope and Valley outlet width.
This study has focused on the effects of variations of strata dips (S) of anticline limbs as representative of tectonic activity on the morphometric characteristics of valleys formed on anticline limbs. Steep slopes of anticlines such as zones 6 and 3 have wide valleys with narrow outlets that can be considered as wine- glass valleys. Results of linear relationship between strata dips (S) and morphometric parameters of valleys show that the ratio of valley maximum width to valley outlet width (Vmw/Vow) and ratio of valley maximum width to valley length (Vmw/VL) are perfectly affected by tectonic or strata dips of anticline limbs. Therefore, Vmw/Vow and Vmw/VL parameters can be considered as two adequate geomorphological indexes of valleys for determining degree of active tectonic activities of uplifting anticlines of Zagros.
Keywords: Danekhoshk, tectonic, Valley morphometry, Zagros, ratio of valley maximum width to valley outlet width.