Investigation of tectonic uplift in Shekarab Mountain located in North of Birjand (Southern Khorasan) using morphotectonic evidence

Landscape evolution represents a morphotopographic balance resulting from interactive competition between tectonics and climate. The development of geomorphic features in response to tectonic uplift provides important information regarding nature, spatial and temporal distributions of tectonic forces in contractional and extensional tectonic regimes. The distribution of tectonic activities in regions experiencing tectonic uplift can be mapped through geomorphic proxie. Commonly, geomorphic indices are useful to classify areas as being very active, moderately active, or inactive. Aim of this research is separating of areas with differental tectonic uplift. Study area (Shekarab Mountain) is located in the East of Iran. In this research, for identifying rate of tectonic uplift geomorphic indices such as valley floor width–valley height ratio (Vf), hypsometric curve (Hc) and hypsometric integral (Hi), drainage basin shape (Bs) and Relative tectonic activity (Iat) were calculated.

For reaching to the aim of this research field work and Geomorphic indices were used. Indices of active tectonics may detect anomalies in the fluvial system or along mountain fronts. These anomalies may be produced by local changes from tectonic activity resulting from uplift or subsidence. The research design is to analyze several different indices in Shekarab Mountain.
Ratio of Valley Floor Width to Valley Height (Vf):Vf is defined as the ratio of the width of the valley floor to its average height (Bull and McFadden, 1977; Bull, 1978) and is computed by Vf=2Vfw/[(Eld–Esc)]+(Erd–Esc)] (1)  Hypsometric Integral (Hi):The hypsometric integral is an index that describes the distribution of elevation of a given area of a landscape. The index is defined as the area below the hypsometric curve and thus expresses the volume of a basin that has not been eroded. The simple equation that may be used to calculate the index is Hi = (average elevation – min. elevation) / (max. elevation – min. elevation). (2)
Index of Drainage Basin Shape (Bs):Bs (Ramirez-Herrera, 1998) expressed by the equation Bs=Bl/Bw (3)  Where Bl is the length of the basin measured from the headwaters to the mouth, and Bw is the width of the basin measured at its widest point.
Relative tectonic activity (Iat):Several index combined to provide information of relative degree of tectonic activity. To provide Iat index is obtained by averaging of different classes of geomorphic indices (S/n) and divided in to four classes. Class 1 is very high tectonic activity with values of S/n between 1 and 1.5; class 2 is high tectonic activity with values of 2>S/n>1.5; class 3 is moderately active tectonics with 2.5>S/n >2; and class 4 is low active tectonics with values of S/n>2.5 (El Hamdouni et al., 2008).

In this study, we have used 1: 25,000 topographic maps with 50 m contour intervals. This projection was the UTM zone 40 N. Values of Vf vary from a low of 0.3 for the west part of Shekarab Mountain where it is deeply incised into hard bedrock, to a high of 3.4 at east part of Shekarab Mountain. Hi index computed for each subbasin, ranges from 0.8 (Subbasin 1) to 0.3 (Subbasin 51). Computing of hypsometric integral (Hi) shows that actives part of Shekarab Mountain is west and north eastern part of Shekarab Mountain (subbasins 1, 24). Calculating hypsometric integral shows that in west and north eastern side of study area (subbasins 1, 24), rate of uplift are faster than rate of erosion. Bs was calculated for Shekarab Mountain, The highest value of Bs index is related to western basins of Shekarab Mountain. The highest class values for Iat mainly occur in the west and north eastern of Shekarab Mountain, while the rest of study area has classes of Iat suggesting moderate to low tectonic uplift. The distribution of indices defines areas associated with different rates of tectonic activity. Within the study area tow subbasin (subbasin 1 and 24) is about class 1 (very high relative tectonic activity). Result of this research shows that highest amount of tectonic uplift is about the west and north east side of study area, and in the areas of Shekarab Mountain that thrust fault were more concentrated tectonic uplift increased.

The values of morphometric indices valley floor width–valley height ratio (Vf), hypsometric curve (Hc), hypsometric integral (Hi), drainage basin shape (Bs) and Relative tectonic activity (Iat) compared with lineament and geological map. Calculating Af and T indices shows that most streams of study area is tilted toward the South. Computing of Hi index show that main basin is in youthful stage. Computing of SL index shows that highest amount of SL index is related to fault 3. Calculating of Vf index indicate that lowest value of Vf index is related to fault 3. Computing of geomorphic indices shows that fault 3 are more tectonically active than other faults of study area. Existing of thrust faults caused to pressure, tectonic uplift and V shape valleys in western and north eastern part of study area.