Introduction

Rising mountains are different from tectonically inactive landscapes. We should scrutinize landforms and the geomorphic processes that create them in order to recognize if they are still active (Bull 2009). The landforms of the eastern part of Central Iran like straight mountain fronts, deformed Quaternary deposits, fault scarps, and triangular facets represent recent tectonic activity. The Cenozoic deformation in Iran is the result of the Arabia–Eurasia convergence that culminated with the Arabia–Eurasia collision at the Eocene–Oligocene boundary (Agard et al. 2011; Allen et al. 2004; Mouthereau et al. 2012). GPS vectors indicate a current NNE motion of the Arabian plate relative to Eurasia of 22 ± 2 mm year−1 (Vernant et al. 2004).

The Kalmard fault zone is considered as one of the major fault zones of Central Iran (Berberian and Mohajer-Ashjai 1977) (Fig. 1), which activity that led to the growth of the Ozbak-Kuh Mountains. The absence of definite earthquakes through the Kalmard fault zone in contrast to other parts of Central Iran, however limits the possibility of seismological evaluation of its current tectonic activity. Thus, geomorphological studies are important to evaluate earthquake hazards in tectonically active areas where the seismological record is absent (Keller and Pinter 2002). In this study, we are using morphometric indicators to evaluate the recent activity of the Kalmard fault zone and its impact on the adjacent Ozbak-Kuh Mountains which is placed at the north western part of Lut block, where Quaternary faulting has been recently documented (Nozaem et al. 2013; Calzolari et al. 2015, 2016). We carried out a structural and morphotectonic analysis to examine the involvement of Quaternary deposits during faulting.

Fig. 1
figure 1

Location of the study area in the Central Iran, (a) topographic map, blocks and faults of Iran and the middle east, (b) the structural map of Central East Iran and its crustal blocks (compiled from Berberian and King, 1981; Jackson and McKenzie 1984; Lindenberg et al., 1984; Haghipour and Aghanabati 1989; Alavi 1991). AZF = Abiz Fault; BDF = Behabad Fault, BKF = Biabanak Fault, CHF = Chapedony Fault, DRF = Doruneh Fault, GWF = Gowk Fault, KBF = Kuhbanan Fault, MAF = Mehdiabad Fault, MBF = Minab Fault, NAF = Nostratabad Fault, NHF = Nehbandan Fault, NNF = Na’in Fault, RJF = Rafsanjan Fault, SBF = Shahre-Babak Fault, TKF = Taknar Fault, KFZ = Kalmard Fault zone, ZRF = Zarand Fault, and ZTZ = Zagros Thrust Zone. Modification Panel (b) is after Ramezani and Tucker 2003. C) Hillshade image, structural map and faults rose diagram of Ozbak-Kuh area

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Geological and tectonic setting

The Ozbak-Kuh Mountains is a NNE–SSW oriented mountain range located in the Central Iran (Fig. 1), north of the Tabas Block that has been affected by faulting along the Kalmard fault zone, a major Quaternary Fault of Central Eastern Iran (Berberian and Mohajer-Ashjai 1977). Central Eastern Iran (also known as Central Iran Micro Continent) constitutes a mosaic of continental microblocks (Lut, Tabas, and Yazd) (Fig. 1) assembled along the Southern Eurasian margin during the long-lasting convergence history between Eurasia and various Gondwanian fragments (Alavi 1991; Bagheri and Stampli 2008; Berberian and King 1981; Davoudzadeh et al. 1981; Ramezani and Tucker 2003; Stöcklin 1968; Takin 1972). Major strike–slip fault zones bind these tectonic blocks and have peculiar stratigraphy, deformation style, and pattern of recent seismicity (Berberian and King 1981). A nearly 600 km long, arcuate and structurally complex fault-bounded belt known as the Kashmar–Kerman Tectonic Zone, where Neoproterozoic and lower Paleozoic rocks are exposed (Ramezani and Tucker 2003), separates the Tabas and Yazd blocks. The main characteristic of the Central Eastern Iran is the occurrence of prominent active fault zones like the Kalmard fault. This fault strand, has a length of 250 km, and is considered a major transpressional fault of the Kashmar–Kerman tectonic zone (Berberian and Mohajer-Ashjai 1977). The main rock units of the Ozbak- Kuh Mountain are early to late Paleozoic limestone, sandstone, and shale known as the Lalun, Niur, Padeha and Baharam formations, respectivelly. Locally, in some parts of the Ozbak-Kuh Mountains, Tertiary and Quaternary conglomerates, evaporites and sandstones are exposed.

Methodology

Morphometric indices are helpful in active tectonics studies as they provide a quick assessment of vast regions. Using multiple morphotectonic indices, we can evaluate a relative tectonic activity for the study area. These indices are known to be useful in active tectonics studies (Bull and McFadden 1977; Keller and Pinter, 2002). We categorized the level of rock resistance shown in Fig. 2a based on rock types and field observations: very low (alluvial deposits), low (older alluvial fan deposit and weakly cemented Neogene Conglomerates), moderate (shale, marl and gypsum), high (limestone, sandstone, dolomite, and well cemented conglomerate) and very high (diabase and associated rocks). In this article, we computed six geomorphic indices: the stream length gradient (SL), drainage basin asymmetry (Af), hypsometric integral (Hi), ratio of valley floor width to valley height (Vf), drainage basin shape (Bs), and mountain-front sinuosity (Smf). We, then, calculated a single index (Iat) from the six indices to characterize the relative degree of tectonic activity. Furthermore, Normalized steepness index (Ksn) has been used to show the effect of the main faults on the rivers and also to compare with the Iat index values. Finally, for assessing the outcome of all indices and completing the result of this study, we performed fieldwork and compared field data with our morphometric analyses.

Fig. 2
figure 2

(a) Distribution of rock strength levels and SL index anomalies, (b) Eighty-seven catchments in the study area

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Morphometric analysis and indices

Morphometric indices

Morphometric indices are useful for studying areas experiencing tectonic deformation (Keller and Pinter 2002). In this study, we computed six geomorphic indices for every subbasin. Categorization of the result values for each catchment was into three classes, and then we summed and averaged these indices and ordered them into three classes of relative tectonic activity in the study area (El Hamdouni et al. 2008). We computed from digitized 1:25000 topographic maps and 30-m-resolution Digital Elevation Model of the study area. Subdivision of the study area is into 87 subbasins (Fig. 2b).

Stream length gradient index

Many processes shape the surface of planet Earth, but the action of running water is responsible for most subaerial landscapes. Uplift steepens stream gradients, which accelerates drainage basin erosion by making hill slopes steeper (Bull 2007). The SL index is one of the morphometric indices that indicate relationship between erosion and river flow procesess and this index correlates to stream authority (Hack 1973). Because the SL index is susceptible to variation in channel slope, it is an effective instrument for assessing river channels (Troiani and Della Seta 2008). The subsequent formula computes the SL equation (Hack 1973)

$$ \mathrm{SL}=\left(\Delta \mathrm{H}/\Delta \mathrm{Lr}\right)\ \mathrm{Lsc},\mathrm{where} $$
(1)

∆H is change in elevation, ∆Lr indicates the length of the reach between two adjacent contour lines, and Lsc is the total channel length from the divide to the midpoint of the reach; moreover, the SL index can be employed to appraise relative tectonic activity (Keller and Pinter 2002) because the SL anomalies result from frequent large tectonic displacements of the stream bed and variable rock mass strength (Bull 2007). We use a digital elevation model (DEM) 30 m, for computing this index along every longest flow path of each basin in the Arc GIS software. The three examples of longitudinal river profile, measured SL values and their relationship with faults in the study area have shown in (Fig. 3). To show the results, we computed the average value of this index (Fig. 4a). The maximum SL value is as high as 1000 in the study area. This index varies from 7 (Subbasin 3) to 1000 (Subbasin 72). Table 1 shows the classification of the SL value index.

Fig. 3
figure 3

Longitudinal river profiles and measured SL values for three basins in the study area

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Fig. 4
figure 4

The indices maps of the study area. (a) Stream Length- Gradient index map of study area. (b) Distribution map of Af classes. (c) Locations of sections for Vf calculations. (d) Thirty-five mountain-front segments for assessing the Smf index

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Table 1 Values of At (total subbasin area), the classes of SL (stream-gradient index), Af (drainage basin asymmetry), Vf (valley floor width–valley height ratio), Smf (mountain-front sinuosity), Hi (hypsometric integral) and Bs (drainage basin shape) and values and classes of Iat (relative tectonic activity)
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Drainage basin asymmetry (Af)

We applied the asymmetric factor to assess the tilting of the basins. Structural features, such as active tilting or bedding and foliation direction, play an important role and control the basin asymmetry (Pe’rez-Pen˜a et al. 2009). By using this method, we could calculate this index at the scale of a drainage basin (Hare and Gardner 1985; Keller and Pinter 2002). Definition of the asymmetric factor is as:

$$ A\mathrm{f}=100\ \left(A\mathrm{r}/A\mathrm{t}\right),\mathrm{where} $$
(2)

Ar is the area to the right of the main stream (looking downstream) and At is the whole area of the drainage basin. Af factor near of 50 shows that the basin has little or no tilting, while Af values greater or smaller than 50, indicate that the basin has affected by tectonic forces. In the study area, Af index varies from 16 (Subbasin 54) to 83 (Subbasin 6). Categorization of the Af index was into three classes: 1 (Af ≥ 65 or Af˂35), 2 (35 ≤ Af˂43 or 57 ≤ Af˂65), and 3 (43 ≤ Af˂57) (El Hamdouni et al. 2008; Dehbozorgi et al. 2010) (Fig. 4b; Table 1).

The ratio of valley floor width to valley height (Vf)

The Vf index is one of the morphometric indices that is susceptible to tectonic uplift (Bull 2007). This index is specified as the valley floor width to valley height ratio (Vf):

$$ \mathrm{Vf}={2\mathrm{V}}_{\mathrm{fw}}/\left[\left({\mathrm{E}}_{\mathrm{ld}}-{\mathrm{E}}_{\mathrm{sc}}\right)+\left({\mathrm{E}}_{\mathrm{rd}}-{\mathrm{E}}_{\mathrm{sc}}\right)\right],\mathrm{where} $$
(3)

Vfw is the width of the valley floor, Eld and Erd are the altitudes of the left and right sides of the valley (facing downstream), respectively and Esc is the mean altitude of the valley floor of the stream channel (Bull and McFadden 1977; Bull 1978). V-shaped valleys with low Vf values < 1 develop in response to active uplift, and that broad U-shaped valleys with high Vf values > 1 indicate major lateral erosion, due to the stability of base level or to tectonic quiescence (Silva et al. 2003). Commonly, high Vf values indicate low uplift rates (Keller and Pinter 2002). Categorization of the Vf index was into three classes: with class 1 (Vf ≤ 0.5), for V-shaped valleys, class 3 (Vf ≥ 1), for U-shaped valleys, and class 2 (0.5 ≤ Vf˂1.0), for intermediate cases (El Hamdouni et al. 2008). The range of Vf is from 0.35 (Subbasin39) to 3.04 (Subbasin 52) (Tables 1 and 2). The computed Vf index illustrates that almost all of the valleys in the study area are V-shaped. The location of Vf cross sections has shown in (Fig. 4c).

Table 2 Vf table associated with lithology of the valley floor
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Mountain-front sinuosity (Smf)

This index demonstrates an equilibrium between river incision processes that tend to incise the embayment into a mountain front and active tectonics forces that tend to procreate straight mountain fronts (Bull and McFadden 1977; Keller 1986; Kaewmuangmoon et al. 2008). Bull and McFadden (1977) and Bull (2007) expressed this index as:

$$ \mathrm{Smf}={\mathrm{L}}_{\mathrm{j}}/{\mathrm{L}}_{\mathrm{s}} $$
(4)

Where, Lj is the planimetric length of a mountain front along the mountain-piedmont junction, and Ls is the overall length of the mountain front. Smf is generally less than 3 and approximately 1 where steep mountains upheave swiftly along the mountain fronts (Bull 2007). Computation of the mountain-front sinuosity index was for 43 mountain fronts (Fig. 4d; Table 3) and we categorized it into three orders: 1 (Smf˂1.1), 2 (1.1 ≤ Smf˂1.5), and 3 (Smf ≥ 1.5) (El Hamdouni et al. 2008). The minimum value of Smf index is 1.00, and it belongs to subbasins 10, 11, and 34 (Table 3). All the values of this index belong to class 1 (Tables 1 and 3).

Table 3 Values and classes of Smf (mountain-front sinuosity) for the considered mountain fronts
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Hypsometric integral (Hi)

The hypsometric integral demonstrates the repartition of the altitude in an area, especially a drainage basin (Strahler 1952). Description of the Hi index is as the area beneath the hypsometric curve. The equation used for computing the Hi index (Pike and Wilson 1971; Mayer 1990) is:

$$ Hi=\left( Average\ elevation\hbox{--} \mathit{\min}\ elevation/\mathit{\max}\ elevation\hbox{--} \mathit{\min}\ elevation\right). $$
(5)

In this index, we acquired the maximum, minimum, and average altitudes from digital elevation model (DEM) using the Arc GIS software. After computing the Hi index for all of the drainage basins, this index was ordered into three classes: class 1 with convex curves (Hi ≥ 0.5); class 2 (0.4 ≤ Hi˂0.5); and class 3 with concave curves (Hi˂0.4).The greatest value of hypsometric integral corresponds to subbasin 67 (0.58) (class 1).In the study area the Hi index ranges from 0.21 (class 3) (Subbasin 78) to 0.58 (class 1) (Subbasin 67) (Table 1).

Basin shape index (Bs)

Basins shape can change due to tectonic activity. The basin shape index illustrates discrepancy between basins with considerable elongation and basins with nearly roundish shape. The basin shape index or the elongation ratio (Ramirez-Herrera 1998) may describe the horizontal projection of a basin. The Bs index is determined as (Cannon 1976; Ramirez-Herrera 1998):

$$ \mathrm{Bs}=\mathrm{Bl}/\mathrm{Bw} $$
(6)

Where, Bl is the length of a basin computed from the most elevated point, and Bw is the width of the basin measured at its widest point. Calculation of the Bs index was for all drainage basins and we ordered it into three levels: 1 (Bs ≥ 4), 2 (3 ≤ Bs˂4), and 3 (Bs ≤ 3) (El Hamdouni et al. 2008). Computation of the Bs index was for all subbasins. Table 1 shows the results of this index. Bs index ranges from 0.9 (Subbasin 67) to 5.01 (Subbasin 12). About 60% of the studied subbasins belong to class 3 with nearly circular shapes (Table 1).

Calculation of Iat

Previous studies on relative tectonic activity based on geomorphic indices tend to focus on a particular mountain front or area (Bull and McFadden 1977; Rockwell et al. 1985; Azor et al. 2002; Molin et al. 2004). In this article, we utilize a number of morphometric indices to assess tectonics in the study area. To obtain more precise results, the six indices divided into four classes, and each class assigned a weighting value. These classes summed and averaged to obtain an Iat over the entire study area. Iat is defined as:

$$ \mathrm{Iat}=\left(\mathrm{SL}+\mathrm{Af}+{\mathrm{V}}_{\mathrm{f}}+\mathrm{Smf}+\mathrm{Hi}+\mathrm{Bs}\right)/6 $$
(7)

For describing the degree of active tectonics, we distributed this index into four categories: 1–very high (1.0 ≤ Iat˂1.5); 2–high (1.5 ≤ Iat˂2.0); 3–moderate (2.0 ≤ Iat˂2.5); 4–low (2.5 ≤ Iat) (El Hamdouni et al. 2008). The distribution of the four classes is shown in Fig. 5, and Table 1 shows the result of the classification for each subbasin.

Fig. 5
figure 5

Distribution of Iat classes

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Appraisal of relative tectonic activity

The average of the six measured geomorphic indices (Iat) was used to evaluate the distribution of relative tectonic activity in the study area (El Hamdouni et al. 2008). The Iat index classes computed within the study area as follows: about 3% (7 Km2) is class 1 (very high relative tectonic activity); 27% (71 Km2) is class 2 (high relative tectonic activity); 50% (131 Km2) shows moderate values of tectonic activity (class 3), and 20% (52 Km2) has the lowest values of relative tectonic activity (class 4) in the study area (Fig. 5; Table 1). Almost all the values of Iat tend to be mostly high and partially moderate along the east Kalmard fault zone segment (F1) (Fig. 6a) and Iat values in the western segment of Kalmard fault zone (F2) (Fig. 6b) tend to be mostly moderate and partially high, but the central strip of the Ozbak-Kuh mountains has low values of tectonic activity (Fig. 5).

Fig. 6
figure 6

A The mountain front in the eastern segment of Kalmard fault zone (fault F1), Ozbak-Kuh area. (a) Prospective view from satellite image (source: http://earth.google.com). (b) Field photo of the straight mountain front along the fault F1. (c) Detail of the polished fault F1 surface of the fault shown in (b) with subhorizontal slickenlines with stereoplot (Schmidt net, lower hemisphere projection) showing the right lateral fault F1 data. B The mountain front in the western segment of Kalmard fault zone (fault F2), Ozbak- Kuh area. (a) Prospective view from satellite image (source: http://earth.google.com). (b) Field photo of the straight mountain front along the fault F2. (c) Polished subvertical fault F2 surface with subhorizontal slickenlines with stereoplot (Schmidt net, lower hemisphere projection) showing the right lateral fault F2 data

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Normalized steepness index (Ksn) and Knickpoints

The analysis of the longitudinal profile of rivers can provide useful information in association with climate changes and tectonic (Hack 1957; Whipple et al. 2013). The river profile is presented based on the relationship between the channel slope (S) and the upstream area of the basin (A) which is introduced as the law (Flint’s Law) (Flint 1974):

$$ S={k}_s{A}^{-\theta } $$
(8)

In this equation (s) is the amount of steepness, (ks) is the steepness index, (A) is area, and (θ) is the concavity index. Different values of the steepness index (ks) along the river indicate a change in the river’s creep rate due to the variability of the erosion of the channel floor sediments and the uplift of the bedrock (Kirby et al. 2003). The value of the steepness index (KS) and the rate of erosion or uplifting of the bedrock in a steady state is obtained from the following equation:

$$ {\mathrm{K}}_{\mathrm{S}}={\left(\mathrm{E}/\mathrm{K}\right)}^{1/\mathrm{n}} $$
(9)

In the above eq. (E) is bedrock uplift, (K) indicates the erosion coefficient that depends on the climatic and metamorphic conditions of the area and (n) is a positive exponent related to the dominant erosion process in the area. (Kirby et al. 2003; Wobus et al. 2006). Different rates of uplifting of the area in the regions with a stable river profile where river vertical digging balances the uplift is well illustrated by this equation (Kirby and Ouimet, 2011). There is a strong correlation between the amount of concavity (θ) and the steepness index (ks). Any change in the amount of the concavity index causes a wide variation in the value of the steepness index. To adjust the amount of concavity and compare the different longitudinal profiles the steepness index is normalized according to the reference concavity index (휃ref((Kirby and Whipple, 2001). In order to further validate the results the normalized steepness index (Ksn) is calculated with the reference concavity index value which is 0.45. In this study the steepness and concavity with the amount of reference concavity index are obtained in Matlab software. The results of the relative steepness index in the studied basins are shown in Figs. 7, 8, and 9 that concave index also indicates the uplift of the rivers in these basins.

Fig. 7
figure 7

Normalized steepness index (Ksn) maps on the faults of the study area. (a) Ksn map on the fault F1, (b) Ksn map on the fault F2, (c) Ksn map on the fault F3, (d) Ksn map on the fault F4, (e) Ksn map on the fault F5, (f) Ksn map on the fault F6

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Fig. 8
figure 8

Longitudinal river profiles on the faults F1-F3

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Fig. 9
figure 9

Longitudinal river profiles on the faults F4-F6

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A part of the longitudinal profile of the river which is more precipitous than adjacent parts is known as the knickpoint often seen along bedrock rivers and is more visible in the streams with flow in the form of water fall (Hayakawa and Oguchi, 2009; Hayakawa and Matsukura, 2003). The knickpoint are often formed abundantly in the precipitous section and near the sources of the rivers, and it is thought that the hydrological conditions affect their creation. Another factor contributing to the formation of knickpoints is active tectonic (Hayakawa and Oguchi, 2009). The effect of active tectonics on the rivers is surveyed in this study. The position of the faults on the longitudinal profiles, without considering their dip and strike, was done to investigate the relationships between the faults in creating the knickpoints (Fig. 10). In this study, likewise, the effects of the major and important faults (F1 to F6) on multiple longitudinal profiles were investigated (Figs. 7, 8 and 9).

Fig. 10
figure 10

Knickpoints location on the geological map in the Ozbak-Kuh area

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Field work data

In the Ozbak-Kuh area, a number of major faults including F1 to F6 (Fig. 1c) are considered as the Kalmard fault zone strand. Based on field studies, the rose diagram of the strand of Kalmard fault zone (F1 to F6) was made in which the average trend is N38E (Fig. 1c). These faults present some geomorphic evidences which can show their potential activity in the future.

Fault F1

This fault is known as the eastern segment of the Kalmard fault zone in the Ozbak-Kuh area is located in the east of the studying area (Fig. 1c). Considering that fault F1 separates Quaternary sediments from Paleozoic rocks, it is considered as a mountain front fault in the eastern parts of the Ozbak- Kuh Mountains. Fault F1 coincide with a straight mountain front that has been classified as class 1 in the classification of mountain-front sinuosity index. The collected data indicates a strike of 062° and a dip of 85° to the SE with a rake angle of 20° (from southwest) (160° clockwise with respect to the right hand rule strike and positive dip). Regarding the slicken-lines on the fault plain, the movement of this fault is identified as right lateral strike-slip with a normal component (Fig. 6a).

Fault F2

Fault F2 represents the western segment of the Kalmard fault zone and is situated in the western part of the Ozbak-Kuh Mountains (Fig. 1c). Similarly to the fault F1, it separates Quaternary from Paleozoic units and coincides with a linear front. The fault F2 has an orientation of 042/70 SE and a rake of 12° (from northeast). Based on the slicken-lines on the fault plain, the movement of this fault is right lateral strike-slip with a reverse component (Fig. 6b).

Fault F3

Fault F3 with a length of 5 km (Fig. 1c) and a dip of 80° to the northwest has an average strike of 040°. Movements of this fault have placed limestones and shales of the Niur Formation (Silurian) in contact with sandstones and dolostones of the Padeha Formation (Devonian). The observed S-C structures in the vicinity of this fault and the slicken-lines with a rake of 10–15° associated with riedel fractures point to a right lateral strike-slip mechanism with a reverse component. The collected data on fault plains and the locations of the slicken-lines are shown in a Streograph (Fig. 11).

Fig. 11
figure 11

Characteristics of fault F3. (a) Field photo of the fault F3. (b) fault sarface slickenlines and synthetic riedel shear fractures indicating the right-lateral kinematics along the fault F3 plane. (c) S-C structures related to fault F3. (d) Stereoplot showing the collected fault F3 data, striae as arrows

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Fault F4

Fault F4 with an approximate length of 12 km (Fig. 1c) has an average dip of 77° toward the southeast and an average strike of 050°. With regard to the data collected in numerous parts of the fault plain during the field observation such as fault steps, riedel fractures, slicken-lines, and S-C structures, this fault presents a right lateral strike-slip fault kinematics with a reverse component (Fig. 12).

Fig. 12
figure 12

Characteristics of fault F4. (a) IRS P5 Satellite image. (b) Field photo of the fault F4. (c) Slickenlines and riedel shear fractures on the fault F4 plane. (d) S-C planes representing the right-lateral kinematics along the fault F4. (e) Stereoplot (Schmidt net, lower hemisphere projection) showing the fault F4 planes, striae is marked by arrows

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Fault F5

Fault F5 with a relative length of 8 km (Fig. 1c) separates gray limestones of the Bahram formation (Devonian) from the red sandstones of Lalun formation (Cambrian). The aforementioned fault has an average strike of 035° and dips 82° toward the northwest. The observed slicken-lines on the fault plain have an average rake of 20 degrees which are associated with extensional fractures. Moreover, the S-C structures observed in the vicinity of this fault, demonstrate a right lateral strike-slip kinematics with a reverse component. The mentioned reverse component has caused the juxtaposition of Cambrian sandstones over Devonian limestones (Fig. 13).

Fig. 13
figure 13

Attributes of fault F5. (a) Field photo and stereoplot showing the fault F5 data, striae as arrows. (b) slickenlines and extension fractures indicating the right-lateral kinematics along the fault F5. (c) S-C structures associated with fault F5

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Fault F6

This fault has a length of 5 km (Fig. 1c). Fault F6, separates the Sardar formation (Carbonifer’s shale and sandstones) from the Padeha formation (Devonian’s sandstones). The movement of this fault is right lateral strike-slip with a reverse component.

Evidence of quaternary deformation

Deformation related to above faults is documented by the general presence of Quaternary fracturing (Fig. 14a–d), deformed Quaternary deposits (Fig. 15), straight mountain front (Figs. 4d and 6; Tables 1 and 3) and our Iat analysis (classes 1, 2) (Fig. 5; Table 1). The presence of faults and fractures in the Quaternary deposits seem to be very important, because they can indicate the active tectonics of this region. Fractured river terraces and consolidated alluvium were surveyed through field studies. Consequently, we measured the direction of the 109 Quaternary fractures occurring in the different parts of the study area on the Kalmard fault zone (Fig. 14). According to field investigations, the average trend of the Kalmard fault strand (F1 to F6) is as N38E (Fig. 1c). Based on structural studies and field evidences, we constructed the Rose Diagram of Quaternary fractures cutting the apex zone of some terraces (Fig. 14e). Accordingly, the direction of their averages is N26E, which is parallel to the common direction of Kalmard Fault zone. Quaternary deformation evidences are well exposed in some parts of the area as the elevation of the consolidated alluvial terraces reaches to 15 m due to a deep cut through the channel course. We can see an illustrative example around the Gushkamar village (Fig. 14a) where some high-steep fractures cut both the Quaternary alluvium and the underlying Lower Cretaceous deposits. Likewise, Around the Maadan Qaleh village (Fig. 14b), Paleozoic rocks have been down cut by the river and have made a strath; so that these rock units are cropped out through the walls and floor of the river, and the Quaternary alluvial terraces lied on them. The faults and fractures of the Kalmard fault zone have ruptured both the Paleozoic and the overlying Quaternary rock units. Furthermore, in some parts of Ozbak-Kuh, it can be seen that the Quaternary alluvial terraces have an average strike of 020° and dips 32° toward the northwest, are tilted 32° northwestward (Fig. 15). This represents the effect of F6 fault in Quaternary unit’s deformation in this part of study area. Furthermore, some morphotectonic landforms such as straight mountain fronts (Figs. 4d and 6), well-developed triangular facets (Fig. 16a), deep and narrow gorges (Fig. 16b) incised into mountain fronts along the fault strand and knickpoints (Fig. 16c) were measured and investigated in the field studies.

Fig. 14
figure 14

Quaternary faulting and fracturing along the Kalmard fault zone in different parts of Ozbak-Kuh area. Fractures is marked by arrows. (a) Faults and fractures affecting the Quaternary deposits at Gushkamar village. (b) Truncated Paleozoic rock units and fractured Quaternary alluvial deposits (terraces) due to the Kalmard fault zone activity near Ma’dan Qal’eh village. (c) Fracturing of consolidated Quaternary deposits through steeply incised drainages in Gazu village. (d) Quaternary faulting and fracturing in the Kalmard fault zone at Niur village. (e) Rose diagram and Contour diagram of the cumulative fracture strike data in the Quaternary deposits of the Kalmard fault zone

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Fig. 15
figure 15

Young deformed Mio-Pliocene-Quaternary alluvial deposits. (a) Prospective view from satellite image (source: http://earth.google.com) is showing the relationship between fault F6, river, terraces and recent alluvium deposits. (b) Field photo of deformed recent alluvium near Niur village in the southern part of the study area

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Fig. 16
figure 16

Morphotectonics landforms in the study area. (a) Triangular facets along the fault F1 (the eastern segment of Kalmard fault zone). (b) A deep gorge cutting the limestone units in Gazu village. (c) A knickpoint on the longitudinal profile of a stream due to fault movements near Ma’dan Qal’eh village

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Discussion

The present study focuses on Ozbak-Kuh area (Kalmard Fault Zone, Central–East Iran) where no earthquakes have been recorded. However, there exist several evidences related to recent tectonic activity like knickpoints, ruptured terraced alluvium and active landforms. Without doubt, the creation of tectonic landforms in the Ozbak-Kuh area is largely rooted in the recent movements along Kalmard fault zone strand in this part of Iran. The Kalmard fault zone is a 250-km long zone that consists of NE–SW striking fault segments, runing on Kashmar–Kerman tectonic zone in the Central Iranian plate. The present-day stress state obtained from earthquake focal mechanisms shows that the North of Central–East Iran Blocks (NCEIB) is presently subjected to a transpressional tectonic regime (R′ = 1.78 ± 0.26), with a N37 ± 7°E direction of horizontal principal compression (Naimi-Ghassabian et al. 2015). The resulting stress tensor suggests that compression is homogeneous in a NE–SW orientation (N38 ± 8.8°E), and the expression of the combination of thrust and strike–slip faulting has caused by the relatively high stress ratio R′ (2.00 ± 0.31) (Naimi-Ghassabian et al. 2015). This stress state is consistent with the direction of convergence between the Arabian and Eurasian plates and leads to the recent active tectonics of the study area. The structural and geomorphic investigation revealed the evidence of prominent Quaternary transpression kinematics along the NE–SW segments of Kalmard fault zone. Faulting mainly localizes along the range fronts, likely reactivating preexisting structures in the bedrock, and suggesting a strong control operated by older geological structures on the post-Neogene fault pattern in Central Iran (Walker and Khatib 2006). Although there are no absolute ages of alluvial fans, we can regionally compare them to similar deposits from Central Iran. Thus, we used the regional scenario and some more detailed data on age of the deformation in neighbor areas based on optically stimulated luminescence dating of three generations of alluvial fans and fluvial terraces. We have used it as a reference for provisional age correlations which reconstructed by Calzolari et al. (2016) for the Central eastern Iran. These ages are related to the recent alluvial deposits that have cut by faults. Three age clusters for aggradation phases can be recognized at ~6, ~25, and ~53 ka, respectively, which correspond the regional ages obtained for Central Iran (Walker and Fattahi, 2011). Deposition of the Shesh–Taraz river terrace along the Doruneh Fault (situated in the northeastern part of the study area) has the age of 20.1 ± 1.5 ka (end of the Last Glacial Maximum) and Fattahi et al. (2007) obtained it using an IRSL (infrared-stimulated luminescence) method. Forthermore, the age of the younger faulted deposits (~5 ka) constrains the minimum age of right lateral faulting in the Kuh-e-Sarhangi (KSF) and Kuh-e-Faghan Fault (KFF) systems (situated in the north of Ozbak-Kuh) to the Holocene at the northern edge of Lut Block (Calzolari et al. 2015). The OSL and structural data set on the Quaternary deposits along the KSF-KFF fault zone shows that strike-slip tectonics and related localized uplift occurred post ~50 ka through Holocene, with a general progression of fault zone localization and slip along eastward propagating KFF (Calzolari et al. 2016). Three generations of Quaternary alluvial fans and river fill terraces associated with the dismantling of the fault-related topographic relief were recognized and dated by means of OSL at ~53, ~25, and ~6 ka, respectively (Calzolari et al. 2015). Forthermore, right lateral strike-slip tectonics was represented for the Kuh-e-Faghan Fault (KFF) (east of the Kuh-e-Sarhangi Fault) by Calzolari et al. (2015). The study has done for the first faulting/exhumation episode based on the structural and depositional architecture of the Neogene deposits in the Kuh-e-Faghan Fault (Calzolari et al. 2016). They are correlated with the regionally defined Late Quaternary aggradation and abandonment phases previously recognized over Central Eastern Iran in other studies. Such dates constrain to the Holocene the minimum age of faulting along the KSF-KFF right lateral strike-slip system at the northern edge of Lut Block (Calzolari et al. 2015). Also, post Neogene right lateral strike-slip faulting has documented at the north-western edge of the Lut Block in the Kashmar-Kerman Tectonic Zone by Nozaem et al. (2013). Other recent dating studies in Iran show similar ages correlations, as for some incised fan surfaces in the Sabzevar area (to the north of the study area) dated at 9–13 ka (Fattahi et al. 2006), and for two different generations of Quaternary fans in the Zagros–Makran Transfer Zone (southeast Iran) dated at 5–9 ka and 13.6 ± 1.1 ka by in situ-produced 10Be (Regard et al. 2005, 2006). As a result, given the aforementioned correlations and the studied structures, we can say that if regional aggradation occurred at 53, 25 or 6 kyr and our supposed Quaternary deposits have been faulted after surface abandonment, faulting will be younger than 53 25 or 6 kyr, depending on the age of these faulted Quaternary deposits. Quaternary fracturing with NE–SW trending is clearly attributed to the movements of Kalmard fault zone and the Quaternary tectonic activity of this fault zone in Ozbak-Kuh area is documented in this research (Fig. 14e). Since the effect of faults in Quaternary unit’s deformation in the study area is obvious, this deformation can be regarded as another evidence of the tectonic activity which is younger than Mio-Pliocene at Kalmard fault zone. The segments of Kalmard fault zone are the most typical case according to the distribution of Iat. Measurements of the index of relative active tectonics (IAT) classes present that there exists high to moderate amount of deformation in the study area except for the range interior with class 4 (Fig. 5), where the most of geomorphic indices as well as wide valleys (Table 2) and low elevation areas suggest low tectonic activity. This could be related to the inactive folds and faults associated with old deformation. The higher values of Iat with low levels of tectonic activity distribute mainly in the central parts of the study area (basins 22, 29, 32, 35, 61, 70, 71). The lower SL and Hi values, concave hypsometric curves, and higher Vf values in these basins reflect the weak role of tectonic ativity at this part. The combination of the rock strength map (Fig. 2a) with the distributions of computed SL index classes in the area (Fig. 4a) shows that the high and moderate classes mostly correspond to the considerable faults. The anomalous higher SL index in the east and southeastern (basins 62, and 72) and western (basins 53, 74, and 78) parts of the study area were affected by faults F1 and F2 and the rivers which intersect with a high or perpendicular angle. The rock group (resistence) is the same around the anomalous SL which can be interpreted as tectonic signals. Iat is high throughout fault F1 which is the eastern segment of Kalmard fault zone in the Ozbak-Kuh mountains (Fig. 5), which corresponds to structures and morphotectonics landforms in different parts of the study area. The asymmetric factor (Af) clearly shows the tectonic tilting as a consequence of fault movements at the scale of a drainage basin. Due to the tectonic uplift and different movements of the faults, the basins have abnormal SL values, high Hi and so convex hypsometric curves, low Vf values and and Af values. High values of the Hi index generally clear that most part of the uplands have not been eroded, and suggest the younge landscapes which can be created by active tectonics. Some basins in the southeastern part of the study area, such as 8 and 87, which developed above Quaternary alluviums, show high Hi values. Meanwhile, the standard deviations for Hi index values are high (0.078). Furthermore, this impact of the lithology is not enough to explain the variations in the relatively higher or lower Hi values (Chang et al. 2015). The values of Af indicate widespread drainage basin asymmetry related to tectonic processes.

Valley floor widths increase with watershed size, erodibility of rock type, and with decrease of uplift rate. Valley heights decrease with the passage of time after cessation of uplift, but not nearly as fast as valleys widen. In this studty, the values of Vf show that the quick river incision has created narrow valleys as a result of Quaternary tectonic base level fall and tectonic uplift.

The straight or gently curving nature of most faults or folds allows evaluation of the degree of erosional modification of a structural landform. However, erosion dominates landscape evolution after cessation of uplift and creates a sinuous mountain–piedmont junction, especially where lithologic resistance to erosion is weak. Streamflow becomes the dominant process shaping the mountain-front landscape in tectonically quiescent settings. Streams downcut quickly to their base level of erosion by removing small amounts of rock, and then slowly widen their valley fl oors by removing large amounts of detritus derived from hillslopes. The result is a highly sinuous mountain–piedmont junction (Bull 2007). In the study area, tectonic activity along the oblique strike-slip faults have made the linear nature of the fronts. So, the sections of mountain fronts between the subbasins are eroded at slow rate, which have not allowed the embayments to be developed. Smf index commonly is less than 1.1, and maintains the raising mountains along the range-bounding faults.

The knickpoints in the study area are divided into two general categories corresponding to the lithology boundaries and locating on the fault lines. Since most of the faults of the study area are strike slip faults with minor dip slip component, that caused different lithological units with different strengths juxtaposed one another, the knickpoints on the fault lines can not show the current activity of these faults specifically.

The longitudinal river profiles with upward concave shapes show long-term, dynamic equilibrium between uplift and erosion; convex and concave river profiles with knickpoints or knickzones in the middle reaches reflect long-term predominance of erosional processes. However, convex-up rivers profiles are presented in the areas where surface uplift is dominant (Kirby and Whipple 2001).

The results of the longitudinal profiles of the rivers that crossed the fault F1 indicate that this fault has influenced Ksn values so that the rivers show higher amounts where intersect with the fault. At the southwestern part of the fault F1 the high Ksn values can indicate the activity of this fault. The highest Ksn index belongs to the basin 45 with a value of 33.2 (Figs. 7a and 8). The highest values of this indice correspond with high values of Iat index in different parts of this fault. At all parts of the fault F2 from northeast to southwest, the rivers indicate low to moderate Ksn values in the intersection with this fault (Figs. 7b and 8). Ksn index values through this fault indicate a well correlation with moderate values of Iat index. The Ksn index of the rivers on the fault F3 are medium to high values which belong to middle and southern parts of this fault. Like fault F1, here the Ksn values correspond well with high relative tectonic activity in the study area. The high values of Ksn in the longitudinal profile can confirm the recent activity of this fault (Figs. 7c and 8). Ksn values related to the fault F4 can indicate the activity of this fault. This fault caused increasing of Ksn values on its northeastern and middle parts (basin 62) which represents a well corresponding with high Iat values near Huk village. Ksn values of this fault are nearly medium (basins 77, 73, 62, 33, 51) (Figs. 7d and 9). The Ksn valus through fault F5 are variable so that the highest ones belong to middle and northern parts of it. Based on Iat map, the highest Ksn values correspond with the high values of relative tectonic activity (Figs. 7e and 9). The high Ksn values in southwestern part of fault F6 correspond with high Iat values in southwestern of Gushkamar village (Figs. 7f and 9).

Conclusion

Evaluating morphometric indices have been considered as one of the most important methods through which the relative active tectonics of regions can be assessed effectively. Due to the lack of both seismic record and proper works on active tectonics, we applied this method to the Ozbak-Kuh area in the Kalmard fault zone to identify geomorphic anomalies and evaluate relative tectonic activity, and then corroborated the achieved quantitative results in the field. Since the mean direction of Quaternary fractures is parallel to the general strike of Kalmard fault segments (F1 to F6), it is documented that this fault zone is active in Quaternary. This result is completely correspondent with geomorphic results. There has been the measurement of seven geomorphic indices, such as the stream-gradient index (SL), drainage basin asymmetry (Af), hypsometric integral (Hi), valley floor width–valley height ratio (Vf), drainage basin shape (Bs), mountain-front sinuosity (Smf), and the combination of the above indices (Iat). Based on Iat, we divided the study area into four classes of relative tectonic activities. We found the values of SL, Hi, and Bs to be high along fault F1 (eastern segment of the Kalmard fault zone). The values of Af show high drainage basin asymmetry related to tectonic tilting. The values of Smf suggest that mountain fronts are tectonically active, and the values of Vf show that some valleys are narrow and deep, suggesting a relative high rate of incision. Satellite images, digital elevation models, accurate drainage system data and field observations of the Quaternary structures, the collection of new structural and geomorphic data along the Kalmard fault zone have been used to document a main NE–SW striking, dextral right-lateral brittle deformation zones, which is active in Quaternary. About 80% of the study area has Iat values of classes 1, 2, and 3 indicating moderately to highly active tectonics. Classes 1 and 2 of Iat, indicative of the most active tectonics, occur mainly in the eastern segment of the Kalmard fault zone (fault F1). Class 3 of Iat corresponding to moderately active tectonics, occurs mainly in the western segment of the Kalmard fault zone (fault F2). These classes also correspond well to the areas with young deformed or fractured Quaternary deposits, prominent scarps, triangular facets, deep valleys, and narrow gorges. Furthermore, calculation of normalized steepness index (Ksn) represents significant high values in the eastern and southeastern parts of the study area which correspond well with high values of Iat index. Class 4 of Iat mainly takes place in the center parts of the study area, where all geomorphic indices suggest low tectonic activity. This could be related to structures associated with old deformation in the Ozbak-Kuh area. The dispersal of hypsometric integral, elongation of drainage basins, high anomalies of SL index, and other indices anomalies, represent that the main morphotectonic deformations are a consequence of recent activity. The results of this study imply that the Kalmard fault zone is active in Quaternary.