Neotectonics of Boroujerd Area , SW Iran by Index of Active Tectonics

Boroujerd area has located in the border zone of Zagros mountain and Sanandaj-Sirjan belt in the southwest Iran. Six geomorphic indices were calculated in the study area. Through averaging these indices we obtain 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 low 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 has located along faulted area, which shows 3 classes of relative tectonic activity (moderate level). Also, other values have located along folded area (low level). Therefore, middle part of study area (sub-basin No. 4) is showing the more active uplifting related to surroundings region (sub-basin No. 1, 2 and 3). In other words, sub-basin No. 4 has got the more active uplifting by quaternary movements of several faults such as Doroud fault.


Introduction
The study area is around Boroujerd city in the border zone of Zagros hinterland and Sanandaj-Sirjan belt in the south west Iran (Figure 1).This area is structurally and geographically belonging to Zagros Mountain.Its northeastern margin belongs to Sanandaj-Sirjan zone and its rest belongs to Zagros Mountain.These two zones have no similar geologic history.Sanandaj-Sirjan zone has comprised from some intrusive bodies in this area.The southwestern part is a folded mountain belt in which lies the highest parts of the mountain.The boundary of these two different zones is Silakhor plains covered by alluvium.
These two structural zones have structural, metamorphic, magmatic contrasts so that this province can be divided into two contrasting domains: The north-eastern parts of this area are territories defined by magmatic, thermic, metamorphic features and most of the rock sequences are metamorphic.Pellitic metamorphic rocks constitute low lands while marbles are feature forming.The south-western parts of this area are platform sequences of Paleozoic to Triassic that they are composed of sandstone, limestone.Rock sequences younger than Triassic are limited to Plio-Quaternary conglomerates which are formed as post-orogenic deposit.Dominant structural trends in Zagros are NW-SE in this area.From tectonics view, it contains the over thrust and simple fold belts of Zagros that formed on the northeastern part of Arabian plate's passive margin.Zagros hinterland is external platform (fold and thrust belt) of north margin of Arabian Craton (Figure 1).Vergence of folding in this hinterland is toward south and southwest.But, Sanandaj-Sirjan overthrust belt has been formed by metamorphic rocks of the northeastern part of Arabian plate.Late Cretaceous-Paleogene sequences in this belt have piled up on a wedge top part of Zagros, before regional metamorphism.Recently, pre-Cretaceous deformed and metamorphic rocks have exposed in this province by upthrusting of basement wedges [1]- [3].In this research, area is divided into 4 sub-basins and the following indices are calculated: stream-gradient index (Sl), valley floor width-valley height ratio (Vƒ), and mountain-front sinuosity (Smf), drainage basin asymmetry (Aƒ), hypsometric integral (Hi) and drainage basin shape (Bs).We use geomorphic indices of active tectonics, known to be useful in active tectonic studies [4]- [7]; 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), mountain-front sinuosity (Smf), drainage basin asymmetry (Aƒ), hypsometric integral (Hi) and drainage basin shape (Bs) are calculated in Boroujerd area by using of topographic data and DEM (Figure 2).On the other hand, the area was divided to four sub-basins tructural 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, sub-basins can be compared together.The study area is located between longitudes E48˚30' -49˚ and latitudes N33˚45' -34˚ in the Louristan province, south west Iran.Based on previous work on the salt diapirism [12]- [21] and neotectonics regime in Iran [22]- [26], Zagros in south Iran is the most active zone [27]- [36].Then, Alborz [37]- [69] and Central Iran [70]- [82] have been situated in the next orders.
Altitudes in this area reach to 3645 m on Garin mountain in the western part of Boroujerd, which it have about 2100 m difference respect to the Silakhore plain in the south eastern part of it.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.Silakhore plain is quaternary alluvium covered (Figure 3).

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, and the most important of which are: Digital Elevation Model (DEM), the drainage network and the sub-basins map of the Boroujerd area that have been extracted from DEM (Figure 4).DEM extracted from a digitized topographic map (with 10 m intervals).

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 [83] [84].[85] 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 four master rivers (Table 1) and then SL graphs have prepared for them (Figure 5).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 our results, 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 Eld, Erd and Esc are the altitudes of the left and right divisions (looking downstream) and the stream channel, respectively [84].[4] 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.So, we have considered suitable valleys in the study area (Figure 6).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 (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 2 and 3 classes and show U shape 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 sinuosity [3] is defined by: where Lj is the planimetric length of the mountain along the mountain-piedmont junction, and Ls is the straightline length of the front.The Mountain fronts of the study area have drawn in Figure 7 by and one of them in sub-basin No. 2 has shown in Figure 8. Smf is commonly less than 3, and approaches 1 where steep mountains rise rapidly along a fault or fold [84].Therefore, this index can play an important role in tectonic activity.Considering that mountain fronts sites are independent from basins places, chances are some of them have various fronts (Table 3).Values of Smf are readily calculated from topographic maps for 4sub-basins.

Asymmetry Factor (Aƒ)
This index is related to two tectonic and none tectonic factors.None tectonic factors may relate to lithology and rock fabrics.It is a way to evaluate the existence of tectonic tilting at the scale of a drainage basin.The index is defined as follows: ( ) where Ar is the right side area of the master stream basin (looking downstream) and At is the total area of the basin that can be measured by GIS software.To calculate this index in the area At and Ar are obtained using the sub-basins and the master river maps.Aƒ is close to 50 if there is no or little tilting perpendicular to the direction of the master stream.Aƒ is significantly greater or smaller than 50 under the effects of active tectonics or strong lithologic control.The values of this index are divided into three categories.1: (Aƒ < 35 or Aƒ > 63) 2: (57 < Aƒ < 65) or (35 < Aƒ < 43) and 3: (43 < Aƒ < 57), based on [11].
Among the obtained values (Table 4), the minimum value belongs to sub-basin No. 4 with 41.96 and the maximum value belongs to sub-basin No. 2 with 61.81 percents.Also, a map has prepared that it shows Asymmetry factor of study area (Figure 9).

Basin Shape Index (Bs)
Relatively young drainage basins in active tectonic areas tend to be more elongated than their normal shape to the topographic slope of a mountain.The elongated shape tends to evolve into a more circular shape [4].The horizontal projection of the basin shape may be described by the basin shape index or the elongation ratio, Bs [7].The calculation formula is: where Bl is the length of the basin measured from the headwater to the mount, and Bw is basin width in the widest point of the basin Bl.
To calculate this index in the area, Bl and Bw are obtained using the sub-basins and the master river maps then the values are divided into 3 classes: 1: (Bs > 4) 2: (3 < Bs < 4) 3: (Bs < 3), based on [11].According to Figure 10 and Table 5).The minimum value belongs to sub-basin No. 1 with 1 and the maximum value belongs to subbasin No. 2 with 2.4 (Class 3).

Hypsometric Integral Index (Hi)
The hypsometric integral (Hi) describes the relative distribution of elevation in a given area of a landscape particularly a drainage basin.The index is defined as the relative area below the hypsometric curve and it is an important indicator for topographic maturity.H max , H min and H ave are calculated on DEM.This index is calculated to all sub-basins in the area and the minimum value is 0.19 for sub-basin No. 2 and maximum value is 0.50 for sub-basin No. 3 (Table 6).The hypsometric integral reveals the maturity stages of topography that can, indirectly, be an indicator of active tectonics.In general, high values of the hypsometric integral are convex, and these values are generally > 0.5.Intermediate values tend to be more concave-convex or straight, and generally have values between 0.4 and 0.5.Finally, lower values (<0.4) tend to have concave shapes [11].We can consider class 1 for Hi > 0.5, class 2 for Hi between 0.4 and 0.5 and class 3 for Hi < 0.4 and so, sub-basin No. 3 shows younger topography.

Results and Discussion
The average of the six measured geomorphic indices (Vƒ, Smf, SL, Af, Bs and Hi) was used to evaluate the distribution of relative tectonic activity.Through averaging these six indices (Table 7).we obtain one index that is known index of active tectonics (Iat).The values of the index were 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 low relative tectonic activities in sub-basin No. 1, 2 and 3 and moderate relative tectonic activities in sub-basin No. 4 (Figure 11).

Conclusions
The calculated geomorphic indices are suitable for assessment of tectonic activity of the study area.The six geomorphic indices; stream-gradient index (Sl), valley floor width-valley height ratio (Vƒ) and mountain-front sinuosity (Smf), drainage basin asymmetry (Aƒ), hypsometric integral (Hi) and drainage basin shape (Bs) have been calculated in Boroujerd area.
Therefore, firstly the area was divided to sub-basins and for each one, indices were calculated, then all of the indices were divided into relative tectonic activity classes.Afterwards, the six measured indices for each subbasin were compounded and a unit index obtained as index of active tectonics (Iat).According to this index, there are both low and moderate relative tectonic activities levels.
Low relative tectonic activities level has been found in sub-basin No.

Figure 2 .
Figure 2. Digital elevation model of the Boroujerd area.

Figure 3 .
Figure 3.The Ghaleh Hatam fault (boundary of mountain and plain) in the north eastern part of Boroujerd city, view to the east.

Figure 4 .
Figure 4. Determination of sub-basins in the Boroujerd area based on Digital Elevation model (DEM).

Figure 5 .
Figure 5. Graphs of stream length-gradient index for 4 rivers.

Figure 6 .
Figure 6.Position map for measurement of the valley floor width to valley height ratio.

Figure 7 .
Figure 7. Position map for measurement of mountain-front sinuosity index.

Figure 8 .
Figure 8.A Mountain-front in the south western part of Boroujerd city (subbasin No. 2), view to the SW.

Figure 9 .
Figure 9. Asymmetry factor map of study areas.
The sub-basin No. 4 has situated in the middle part of study area and it has got several faults that shown in Figure 12.Also, based on [23], this area is a high seismic risk zone with following seismicity parameter: b = 1.06,M max = 7.2.Focal mechanisms of several earthquakes are dextral strike slip in relation to main recent faults of Zagros such as Doroud (Ms = 6.1, 2006).

Figure 10 .
Figure 10.Basin shape map of study area.

Figure 11 .
Figure 11.Relative Tectonic activity classification map of study area.This area is struck by moderate to high earthquakes with low frequency, long repeat time and 10 -15 Km focal depth.Intensity of earthquakes is in high levels.Sometimes, focal depths exceed to 70 Km which is an indication of initial stages of thick-skinned tectonics.The most serious seismic hazards in the study area are landslide in high regions, settlement in plain, surface faulting (Figure13).

Figure 12 .
Figure 12.Fault map of study area.

Figure 13 .
Figure 13.The Doroud fault, view to the south.
1, 2 and 3 and moderate relative tectonic activities level, has been found in sub-basin No. 4. It means that sub-basin No. 4 has got the more active uplifting by movements of several faults such as Doroud fault.

Table 1 .
Values of stream length-gradient index.

Table 2 .
Values of Vf index.

Table 3 .
Values of Smf index.

Table 4 .
Values of Af index.

Table 5 .
Values of Bs index.