Neotectonics of Tabas Area , Central Iran by Index of Active Tectonics ( IAT )

In this research, Tabas area, which is located in central Iran, was selected as the study area and three geomorphic indices were calculated for its structural fronts. Through averaging these three indices, we obtained index of active tectonics (IAT). The values of the index were divided into classes to define the degree of active tectonics. Therefore, relative tectonic activity was calculated and their values were classified and analyzed in two groups. Regions were identified as high and moderate levels. In analyzing data and combining them with tectonic setting, the results were often associated and justified with regional geology. Our results show that the highest value is located along Shoutori fault, which shows 2 class of relative tectonic activity (high level). Also, moderate values are located along Ereshk, Ezmeighan and Jamal faults (moderate level). According to these results, Shoutori fault is the most active fault in the study area and this situation is compatible with its position as a mountain front fault.


Introduction
The study area is the north part of Tabas area [1] in the east-central Iran basin [2].Dominant structural trend in east-central Iran province is N-S (Figure 1).From tectonics view, it contains an ancient island arc on the west of the Lut Plain-Gonabad province that has accreted to this along Nayband fault system (western border of the Lut microcontinent) by eastward subduction in Pre-Cambrian [3].In this study, area is divided into 4 structural fronts and the following indices are calculated: stream-gradient index (SL), valley floor width-valley height ratio (Vƒ), and mountain-front sinuosity (Smf).We use geomorphic indices of active tectonics, known to be useful in active tectonic studies [4]- [7].This methodology has been previously tested as a valuable tool in different tectonically active areas, namely SW USA [8], the Pacific coast of Costa Rica [9], central Zagros, Iran [10].

Materials and Methods
The calculated geomorphic indices are suitable for assessment of tectonic activity of the study area.The geomorphic indices such as: stream-gradient index (SL), valley floor width-valley height ratio (Vƒ) and mountain-front sinuosity (Smf) are calculated in Tabas area by using topographic data.On the other hand, the area was divided into four structural fronts and for each one, indices were calculated, then all of the indices were combined to obtain index of active tectonics (IAT) by new method [11].Therefore, structural fronts can be compared together.
Altitudes in this area reach to 2600 m on Shoutori Mountains, which have about 400 m difference respect to the Tabas plain.Geomorphologically, the ridges and valleys in the area under study are mainly due to the rocks variations in the lithology and assisted by faults presence in the area that offer varying degrees of resistance to the degradation processes.Topographically, the down faulted Tabas plain is Neogene gypsiferous soil covered (Figure 2).Slope wash following occasional rains forming badland topography is mainly observed in the low weathered strikes valleys in the Shotouri Mountains (Figure 3).The Tabas plain has limited to the west by an N-S trending fault and risen in a sequence of N-S trending folds of main Jurassic sandstone shale and marl-gypsum units to a prominent escarpment.In the cores, the folds partly expose Triassic permanent carbonate units, granite extrusion and Ravar Salt intrusion (Infra-Cambrian).In the southern part, the Jurassic marls and evaporate are covered by red beds and Cretaceous shelf limestones.The Tabas plain is filled with Neogene clastics and covered by dasht and playa deposits.It is limited to the E and NE by the Shotori Mountain as a horst.This horst, first uplifted in the Cimmerian tectonic phase, have several 1000 m of Paleozoic-Triassic platform sediments and a cover of Middle-Upper Jurassic limestones, shales and marls [77].

Results and Discussion
To study the indices, there is a formula which we turn to describe each one of indices; it is necessary to have some primary maps to calculate the indices.

The Stream-Gradient Index (SL)
The rivers flowing over rocks and soils of various strengths tend to reach equilibrium with specific longitudinal profiles and hydraulic geometrics [78] [79] defined the stream-gradient index (SL) to discuss influences of environmental variables on longitudinal stream profiles, and to test whether streams has reached equilibrium.The calculation formula is in this manner: where (∆H/∆L) is local slope of the channel segment that is located between two contours and L is the channel length from the division to the midpoint of the channel reaches for which the index is calculated.This index is calculated along the master river (Table 1) and then SL contour map is prepared (Figure 4).The SL index can be used to evaluate relative tectonic activity.An area on soft rocks with high SL values can be indicated for active tectonics.Based on [11], SL value is classified into 3 categories, which are: class 1 (SL > 500), class 2 (300 < SL < 500), and class 3 (SL < 300).The more values of SL is greater than 300, thus there are in 2 and 3 classes.

Valley Floor Width-Valley Height Ratio (Vƒ)
Another index sensitive to tectonic uplift is the valley floor width to valley height ratio (Vƒ).This index can separate v-shaped valleys with small amounts from u-shaped valleys with greater amounts.The calculation formula is in this manner: ( ) where Vƒw is the width of the valley floor, and Ald, Ard and Asc are the altitudes of the left and right divisions (looking downstream) and the stream channel, respectively.[4] [78] found significant differences in Vƒ between tectonically active and inactive mountain fronts.Also, they found significant differences in Vƒ between tectonically active and inactive mountain fronts, because a valley floor is narrowed due to rapid stream down cutting (Figure 5).
Valleys upstream from the mountain front tend to be narrow and Vƒ is usually computed at a given distance upstream from the mountain front.We set a distance to 1 -2 km, and within the mountain range.The Vƒ of the main transverse valleys in the study area was calculated by cross-section that was drawn from the topographic map.
Vƒw value is obtained by measuring the length of a line which cuts the river and limits to two sides of a contour through which the river crosses.Values of Ald, Ard, and Asc are measured using the drawn profile (Table 2).Based on [11], Vƒ values are divided into 3 classes: 1 (Vƒ < 0.3), 2 (0.3 < Vƒ < 1), and 3 (Vƒ > 1).Therefore, all of the valleys are in class 3 and show u-shaped valleys.

Mountain-Front Sinuosity Index (Smf)
This index represents a balance between stream erosion processes tending to cut some parts of a mountain front and active vertical tectonics that tend to produce straight mountain fronts.Index of mountain front si- nuosity [3] is defined by: Smf= Lj Ls where Lj is the planimetric length of the mountain along the mountain-piedmont junction, and Ls is the straight-line length of the front.The mountain fronts of the study area are drawn by faults and folds (Figure 6).Smf is commonly less than 3, and approaches 1 where steep mountains rise rapidly along a fault or fold [73].Therefore, this index can play an important role in tectonic activity.Considering that mountain fronts sites are independent from basins places, and there are four various fronts (Table 3).Values of Smf are readily calculated from topographic maps for 4 mountain fronts in 8 segments.

Results and Discussion
The average of the three measured geomorphic indices (Vƒ, Smf and SL) was used to evaluate the distribution of relative tectonic activity.Each of the indices was divided into 2 and 3 classes (Table 4).Through averaging these three indices, we obtain one index that is known index of active tectonics (IAT).The values of the index are divided into four classes to define the degree of active tectonics: 1-very high (1 < IAT < 1.5), 2-high (1.5 < IAT < 2), 3-moderate (2 < IAT < 2.5), 4-low (2.5 < IAT) [11].Thus, there are high relative tectonic activities along Shoutori fault and moderate relative tectonic activities along Ereshk, Ezmeighan and Jamal faults.

Conclusions
The calculated geomorphic indices are suitable for assessment of tectonic activity of the study area.The geomorphic indices such as: stream-gradient index (SL), valley floor width-valley height ratio (Vƒ) and mountain-front sinuosity (Smf) are calculated in Tabas area.Therefore, firstly the area was divided to four structural fronts and for each one, indices were calculated, then all of the indices were divided into 2 classes.Afterwards, 3 measured indices for each front were compounded and a unit index obtained as index of active tectonics (IAT).According to these indexes, there are both high and moderate relative tectonic activities levels.
High relative tectonic activities level has been found along Shoutori fault and moderate relative tectonic activities level has been found along Ereshk, Ezmeighan and Jamal faults.It means that Shoutori fault is the most active fault in the study area and this situation is compatible with its position as a mountain front fault.

Figure 2 .
Figure 2. The Tabas plain, view to the east.

Figure 3 .
Figure 3.The Shoutori Mountain, view to the northeast.

Figure 4 .
Figure 4. Sl Contour map of study area.

Figure 5 .
Figure 5.The narrow valleys in the Shoutori Mountain, view to the northeast.

Figure 6 .
Figure 6.The Shoutori Mountain front, view to the northeast.Also, based on [23], this area is a high seismic risk zone with following seismicity parameter: b = 0.90, M max = 7.5.Focal mechanisms of the many earthquakes are dextral strike slip faulting with reverse component in southern part and reverse faulting (Figure 7) with dextral strike slip component in northern part such as Tabas (Ms = 7.4, 1978) and western part such as Zarand (Ms = 6.5, 2004).Tabas area experiences moderate to high earthquakes with low frequency, long repeat time and down to 15 Km focal depth.Intensity of earthquakes is in high levels.The most serious seismic hazards in the study area are landslide in high regions, settlement in plain, surface faulting.The seismic records and the major structural front traces of study area have shown in Figure 8.

Figure 7 .
Figure 7.The Tabasfault, view to the east.

Figure 8 .
Figure 8. Seismic records and the major structural front traces of study area.

Table 1 .
Values of stream length-gradient index.

Table 2 .
Values of Vf index.

Table 3 .
Values of Smf index.