GEOLOGICAL JOURNAL Geol. J. 47: 30–40 (2012) Published online 28 September 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/gj.1329 Geomorphologic assessment of relative tectonic activity in the Maharlou Lake Basin, Zagros Mountains of Iran ALI FAGHIH1*, BABAK SAMANI 2, TIMOTHY KUSKY 3, SAMAN KHABAZI1 and REIHANEH ROSHANAK 1 1 Department of Earth Sciences, College of Sciences, Shiraz University, Shiraz, Iran 2 Faculty of Earth Sciences, Shahid Chamran University, Ahvaz, Iran 3 State Key Lab for Geological Processes and Mineral Resources, Three Gorges Research Center for Geohazards, China University of Geosciences, Wuhan, China Spatial differences of Quaternary deformation and intensity of tectonic activity are assessed through a detailed quantitative geomorphic study of the fault-generated mountain fronts and alluvial/fluvial systems around the Maharlou Lake Basin in the Zagros Fold–Thrust Belt of Iran. The Maharlou Lake Basin is defined as an approximately northwest–southeast trending, linear, topographic depression located in the central Zagros Mountains of Iran. The lake is located in a tectonically active area delineated by the Ghareh and Maharlou faults. Combined geomorphic and morphometric data reveal differences between the Ghareh and Maharlou mountain front faults indicating different levels of tectonic activity along each mountain front. Geomorphic indices show a relatively high degree of tectonic activity along the Ghareh Mountain Front in the southwest, in contrast with less tectonic activity along the Ahmadi Mountain Front northeast of the lake which is consistent with field evidence and seismotectonic data for the study area. A ramp valley tectonic setting is proposed to explain the tectonosedimentary evolution of the lake. Copyright © 2011 John Wiley & Sons, Ltd. Received 8 February 2011; accepted 4 August 2011 KEY WORDS neotectonics; tectonic geomorphology; geomorphic indices; faults; Maharlou Lake; Zagros Mountains 1. INTRODUCTION The study of landforms that appear to be controlled by the interaction between tectonic and geomorphic processes is the focus of tectonic geomorphology (Mayer, 1986). Landscapes in tectonically active areas result from a complex integration of the effects of crustal block motion, erosion and/or deposition by surface processes (Burbank and Anderson, 2001). Therefore, geomorphic investigation in regions of active tectonics is a powerful tool for studies of tectonic geomorphology. The quantitative measurement of landscape is based on the calculation of geomorphic indices using topographic maps or digital elevation models, aerial photographs or satellite imagery, and fieldwork (Keller and Pinter, 2002). Geomorphic indices are a tool for analyzing landforms and evaluating the degree of tectonic activity in a given area (Keller, 1986). The most characteristic landforms developed in semi-arid, tectonically-active zones are faultgenerated mountain fronts. These are large-scale tectonic landforms with long survival periods (>100 ka), in which *Correspondence to: A. Faghih, Department of Earth Sciences, College of Sciences, Shiraz University, Shiraz, Iran. E-mail: afaghih@shirazu.ac.ir; afaghihgeo@gmail.com the erosional and depositional history are linked to the related range-front fault (Mayer, 1986). Therefore, the geomorphologic analysis of mountain fronts, alluvial fans and related drainage networks would provide valuable information about the recorded tectonic history (Silva et al., 2003; Singh and Tandon, 2008; Figueroa and Knott, 2010). The aim of this study is to assess the relative intensity of tectonic activity of fault-generated mountain fronts around the Maharlou Lake Basin (Figure 1) through geomorphic analysis, and to explain the origin and evolution of this lake basin as a tectonic and morphologic feature in the Zagros Mountains of Iran. The lake is located 18 km southeast of Shiraz City in southwest Iran. Natural hazard assessment and disaster management depend on an understanding of active tectonics including studies of the patterns of deformation, landscape development, and the determination of rates of tectonic processes. In particular, earthquakes impact human societies with huge attendant economic consequences (Singh and Tandon, 2008). The fault-generated mountain fronts and fluvial system are the most characteristic landforms in this region. Therefore, geomorphological and morphometric analyses of these features may offer valuable information on the Copyright © 2011 John Wiley & Sons, Ltd. TECTONIC ACTIVITY IN THE MAHARLOU LAKE BASIN, ZAGROS MOUNTAINS 31 Figure 1. Geological map of the study area. The upper map shows tectonic subdivisions of the Zagros Orogeny (Zagros Fold–Thrust Belt, Sanandaj–Sirjan Metamorphic Belt and Urumieh–Dokhtar Magmatic Arc) and the location of the study area. Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. 47: 30–40 (2012) 32 a. faghih recorded tectonic history in the geomorphic features around the lake which may inform hazard assessments in the region. 2. GEOMORPHOLOGICAL AND GEOLOGICAL SETTING The Zagros Fold–Thrust Belt is part of the Alpine–Himalayan orogenic belt (Takin, 1972; Berberian and King, 1981) and lies on the northeastern margin of the Arabian Plate. Regional deformation (the Zagros Orogeny) arose from the LateCretaceous to Tertiary collision between the African–Arabian continent and the Iranian microcontinents. Crustal shortening led to thrusting and large-scale strike-slip faulting in the Zagros Orogeny (Alavi, 1994; Sepehr and Cosgrove, 2005; Sarkarinejad et al., 2008, 2009, 2010a, b). Postcollisional ET AL. crustal shortening is still active (Jackson and McKenzie, 1984; Talebian and Jackson, 2002; Allen et al., 2004; Regard et al., 2004; Tatar et al., 2004) with a N–S oriented convergence rate of approximately 20  2 mm yr 1 (Vernant et al., 2004; Molinaro et al., 2005). This fold–thrust belt is approximately 1800 km long and 200–300 km wide. It runs from eastern Turkey to the Strait of Hormuz, where it terminates against the Zendan Fault (Figure 2), which separates the Zagros Belt from the Makran accretionary prism (Molinaro et al., 2005; Regard et al., 2004). Southwest of the Zagros Thrust (Figure 1), the Neogene Period is represented by the Fars Group (Gachsaran, Mishan and Agha Jari formations) and Bakhtyari Formation. Shed from the gradually rising Zagros Range, red clastic conglomerates, sandstones and mudstones of the Razak Formation filled the basin from NE to SW during the Lower Miocene. In Fars Figure 2. Shaded SRTM topographic map which showing geodynamic setting of Iran and neighbouring regions. The rectangle represents the region shown in later figures. Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. 47: 30–40 (2012) TECTONIC ACTIVITY IN THE MAHARLOU LAKE BASIN, ZAGROS MOUNTAINS Province they interfinger with limestones and marls of the upper Asmari–Jahrum Formation and with evaporites and interbedded limestones of the Gachsaran Formation. From late Oligocene to early Miocene time, the Asmari Sea receded from the Zagros Basin. On the Fars platform, evaporates were deposited, comprising gypsum, anhydrite and halite beds of the Gachsaran Formation. Fars Group deposition ended with the Agha Jari Formation, a mudstone and sandstone sequence of continental nature. After the strong uplifting of the Zagros Range in the late Alpine phase, the Bakhtyari Conglomerates (Upper Pliocene in age) were deposited from rivers in fold belt valleys, with an irregular distribution pattern and preserved thickness (James and Wynd, 1965; Falcon, 1974; Motiei, 1994; Alavi, 1994, Bahrami, 1997). Maharlou Lake is located in the central part of the Zagros Fold–Thrust Belt (Figures 1 and 2; Alavi, 1994). It is an ephemeral saline lake that developed in an intra-continental basin (Sonnenfeld, 1991). In wet seasons, the lake expands to an area of 280 km2 (26 km long and 12 km wide). The lake is located in an elevated depression (1455 m above sea level) with a northwest–southeast trend. The average annual precipitation and minimum and maximum temperatures are 341 mm, 9.8  C and 25.6  C respectively, with the maximum precipitation occurring in January and December. The average annual evaporation rate is approximately 2391 mm. Average water depths in the northern part of the lake range between 1.5 to 2 m. Vegetation cover in the study area includes various species of Artemisia, Astragalus and liquorice. The lake receives water from direct precipitation and inflow from surface run-off, a few seasonal rivers and several karstic springs which collectively compensate for the high evaporation rates in the region (Dumas et al., 2003). There are no permanent rivers entering the lake, and the local drainage network is ephemeral, including the Nahre-Azam (the Khoshk), and Chenare-Rahdar (the Babahaji) rivers (Fayazi et al., 2007). The Khoshk River, has the largest discharge and plays the most significant role in the hydrochemical composition of the Maharlou Lake. The maximum discharge of the Khoshk River in wet and dry seasons is approximately 164 and 2.4 m3s 1, respectively. The Khoshk River is approximately 62 km long, 40 km of which passes through Shiraz City (Forghani et al., 2009). The lake’s hydrogeological properties, former lake level stands and the survival of relict Pleistocene fish species (Djamali et al., 2009) indicate that the lake has been in existence since early Pleistocene times. The Lake Basin developed with a NW–SE- trend, and the floor of the Basin is infilled with Plio-Quaternary alluvial fan and alluvium deposits. Recent lacustrine deposits comprise predominantly well-laminated carbonates, evaporites and siliciclastic sediments. Lake centre deposits are evaporitic and in the lake margins are carbonates and siliciclastic sediments. (Lak et al., 2008). A study of the recent evolution of the lake Copyright © 2011 John Wiley & Sons, Ltd. 33 hydrochemistry has shown that the lake water has changed from a Mg–SO4–Cl type in 1970 to a Na–Mg–Cl–(SO4) type at the present time (Fayazi et al., 2007; Djamali et al., 2009). The margins of the lake are defined by a series of mountain ranges. The Ahmadi Mountains northeast of the lake and the Ghareh Mountains to the southwest were uplifted as doubly plunging anticlines. The mountain ranges are composed of Cenozoic sedimentary rocks. The main rock type which constitutes the mountain fronts in the study area is OligoMiocene limestone of the Asmari Formation (Figure 1). 3. METHODS Geomorphic indices represent a quantitative approach to geomorphic analysis. In this study we analyze the main morphological features of the mountain fronts, alluvial fans, and fluvial networks surrounding Maharlou Lake. The geomorphological and morphometric analyses were carried out in the field using 1:25 000 scale topographic maps and digital elevation models. Most of the geomorphic and morphometric parameters used in this study were developed by Hack (1973), Bull and McFadden (1977), Rantsman (1979), Wells et al. (1988), Silva et al. (2003) and El-Hamdouni et al. (2008). Six main parameters used in these analyses quantify the relationships between tectonics, lithology, sedimentation and erosion, and include mountain front sinuosity, facet, valley width/height ratio, drainage basin shape ratio, basin elongation ratio and alluvial fan topographic profiles. 3.1. Mountain front sinuosity index (Smf) Mountain front sinuosity index is defined as Smf ¼ Lms =Ls ; where Lmf is the length of the mountain front along the foot of the mountain where a change in slope from the mountain to the piedmont occurs; and LS is the straight line length of the mountain front (Bull and McFadden, 1977). This index reflects the balance between erosional processes and active tectonism along the mountain front. Active vertical tectonics (generally coincident with active faults or folds) tend to produce straight mountain fronts, however these become more sinuous as streams cut both laterally and into the front (Bull and McFadden, 1977; Keller, 1986; Keller and Pinter, 2002). Mountain fronts associated with active tectonics and active uplift are relatively straight with low values of Smf; but if the rate of uplift is reduced or ceases, then erosional processes along the mountain front produce a more sinuous front and thus lower value of Smf. Values of Smf are readily calculated from topographic maps or aerial photography, although they are scale dependant (Bull and McFadden, 1977). Small-scale maps (1:250 000) produce approximate values of Smf, while larger scale topographic maps due to their high resolution produce more accurate assessments of Geol. J. 47: 30–40 (2012) 34 a. faghih Smf. Mountain front sinuosity is also sensitive to the local climate, where the values need to be adjusted in areas of higher rainfall and erosion rates (e.g. Kusky et al., 2010). In this study, mountain fronts are defined as major faultbounded escarpments, and long escarpments were subdivided along-strike into discrete segments with similar geological and morphological characteristics (Figure 3). Following Wells et al. (1988), the following criteria were applied: (1) intersection with cross-cutting drainage large in scale relative to the front, (2) abrupt changes in the major morphological characteristics of the mountain front relative to adjoining front segments, and (3) changes in mountain front orientation. 3.2. Facet A facet is a triangular to polyhedral shaped hillslope situated between two adjacent drainage structures within a given mountain front escarpment (Ramírez-Herrera, 1998). Facets are interpreted as variably degraded remnants of fault generated footwall scarps (Wallace, 1977). Two indices related to facet development were used in this study: (a) the percentage of faceting along mountain fronts (FCl), (b) the percentage of dissected mountain fronts (Fd) as described by Wells et al., (1988). The FCl defines the proportion of a mountain front that has well-defined triangular facets, using the ratio of the cumulative lengths of facets to overall mountain front length which can be expressed as: ET AL. FCl ¼ Lf =LS where Lf is the cumulative length of facets and LS is the overall mountain front length. Tectonically active fronts display prominent, large facets that are generated and/or maintained by recurrent faulting along the base of the escarpments, i.e. high percentage faceting (Wells et al., 1988). Fd defines the proportion of a mountain front that has been dissected into distinct facets (Bull, 1978). Most tectonically active mountain fronts tend to be less dissected, i.e. low Fd values (Bull and McFadden 1977; Wells et al., 1988). This index is defined as: Fd ¼ Lmfd =LS Where Lmfd is the length of the dissected mountain front and LS is the straight line length of the mountain front (Bull and McFadden, 1977). 3.3. The valley width/height ratio (Vf) Vf is defined as the ratio of the width of the valley floor to its average height (Bull and McFadden, 1977; Bull, 1978). Comparison of the width of the floor of a valley with its mean height provides an index that indicates whether the stream is actively downcutting or is primarily eroding laterally into the adjacent hill slopes. This index can be expressed as: Vf ¼ 2Vfw =½ðEld ESC Þ þ ðErd ESC ފ where Vf is the ratio of valley floor width to valley height; Vfw is the width of the valley floor; Eld is the elevation of the divide on the left side of the valley; Erd is the elevation on the right side; and ESC is the average elevation of the valley floor. Valley floors tend to become progressively narrower upstream from the mountain front in larger drainage basins for a given mountain range (Ramirez-Herrera, 1998). For this reason, in this work the transverse valley profiles were located 0
5 km and 0
5 to 1 km upstream from the mountain front in small and large drainage basins respectively as prescribed by Silva et al. (2003). Vf values was calculated for the main valleys that cross mountain fronts using crosssections drawn from the digital elevation model and the 1:25 000 topographic map of the study area. 3.4. Drainage basin shape ratio (BS) Figure 3. Digital elevation model of the Maharlou Lake and surrounding mountain fronts. The numbers show the mountain front segments for assessing the Smf index in the study area. Copyright © 2011 John Wiley & Sons, Ltd. In tectonically active mountain ranges, basins have typical elongate shapes which become progressively more circular with time after cessation of mountain uplift (Bull and McFadden, 1977). The horizontal projection of basin shape may be described by the basin shape ratio, BS (Cannon, 1976; Ramírez-Herrera, 1998) expressed by: Geol. J. 47: 30–40 (2012) TECTONIC ACTIVITY IN THE MAHARLOU LAKE BASIN, ZAGROS MOUNTAINS BS ¼ Bl =Bw where Bl is the length of the basin, measured from its mouth to the most distant drainage divide, and Bw is the width of the basin measured across the short axis. 35 activity of recent uplift along this front. Alternatively, Smf values vary from 1.16–1.37 along the Maharlou Fault which revealed less active tectonism (Figure 4). 4.2. Facet 3.5. Basin elongation ratio (Re) The basin elongation ratio (Bull and McFadden, 1977), is one of the proxy indicators of recent tectonic activity. This parameter (Re) is calculated as a ratio of the drainage basin area (A) to the maximum basin length (L), i.e. the distance between the two most distant points in the drainage basin which is expressed by:  pffiffiffiffiffiffipffiffiffi Re ¼ 2 A : p =L where A is the drainage basin area and L is the maximum length of the basin. 3.6. Alluvial fan topographic profiles Tectonic uplift creates elevated terrain and provides increased potential energy to the agents of erosion such as fluvial systems. Alluvial fan morphology sheds light on fault activity and reflects the rate of the source mountain uplift (Gürbüz and Gürer, 2008). The plan view morphologies and the longitudinal profiles of eight alluvial fans around the Maharlou Lake were used to define control mechanisms on fan development. 4. RESULTS 4.1. Mountain front sinuosity In this work, eight mountain fronts were evaluated. Obtained values of Smf range from 1.03 to 1.37 (Figure 4). The lowest values of Smf are associated with the southwest border of Maharlou Lake with the Ghareh Fault (Tables 1 and 2). The Smf values calculated for the Ghareh Mountain Front vary from 1.03 to 1.08, which point to a relatively high The obtained FCl (35.3%) and Fd (42.6%) values revealed a high percentage faceting and a low percentage of dissected mountain front along the Ghareh Mountain Front than the Ahmadi Mountain Front (FCl = 21.6%; Fd = 65.2%) which suggests a relatively higher degree of tectonic activity along the Ghareh Fault than the Maharlou Fault (Figure 4). Tectonically active fronts display a high percentage of triangular faceting and tend to be less dissected (Bull and McFadden, 1977; Wells et al., 1988). 4.3. Valley floor width-to-height ratio Valleys dissecting the Ghareh Mountain Front display Vf values (calculated for valley segments located 1 km upstream of the mountain front) ranging between 0.2–0.45 whereas those of Maharlou Mountain Front vary between 0.32–0.76 (Figure 4 and Tables 1 and 2). Most Vf values in this study are relatively low, revealing that most valleys of the study area are V-shaped. U-shaped valleys generally have high values of Vf, whereas V-shaped valleys have relatively low values. Because uplift is associated with incision, the index is thought to be a surrogate for active tectonics where low values of Vf are associated with higher rates of uplift and incision (El-Hamdouni et al., 2008). 4.4. Drainage basin shape ratio The resulting values of BS calculations range from 1.3 to 4.8 (Figures 4 and 5). The highest values are along the SW border of the Maharlou Lake Basin. The index reflects differences between elongated and more circular basins. High values of BS are associated with elongated basins, generally associated with relatively higher tectonic activity (Keller and Pinter, 2002). Low values of BS indicate a more circular-shaped basin, generally associated with low tectonic Figure 4. Geomorphic and morphometric parameters and relative tectonic activity in each mountain front in the study area. Arrow tip shows the increasing intensity of tectonic activity. AMF and GMF letters indicate Ahmadi Mountain Front and Ghareh Mountain Front, respectively. Six used parameters are mountain front sinuosity index (Smf), facet (FCl), dissected facets (Fd), the valley width/height ratio (Vf), drainage basin shape ratio (BS) and basin elongation ratio (Re). This figure is available in colour online at wileyonlinelibrary.com/journal/gj Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. 47: 30–40 (2012) 36 a. faghih ET AL. Table 1. Morphometric data of the Maharlou Fault Maharlou Fault Station Lmf (m) LS (m) Smf Eld (m) Erd (m) ESC (m) Vfw (m) Vf 1 2 3 4 6307.6 13718.2 12892.9 11775.2 5437.5 11337.3 9767.3 8595.1 1.16 1.21 1.32 1.37 1685.5 1650.2 1710.5 1585.2 1682.8 1680.7 1696.8 1591.9 1618.8 1581.9 1660.5 1545.9 21 32.6 26.7 32.4 0.32 0.39 0.62 0.76 Table 2. Morphometric data of the Ghareh Fault Ghareh Fault Station Lmf (m) LS (m) Smf Eld (m) Erd (m) ESC (m) Vfw (m) Vf 1 2 3 4 8368.4 9903.3 6421.5 11742.1 8124.6 9431.7 6058.1 10872.3 1.03 1.05 1.06 1.08 1686.2 1636.8 1625.3 1711.1 1709.3 1684.9 1654.2 1741.6 1616.9 1591.1 1549.5 1642.9 16.2 18.1 35.2 37.5 0.2 0.35 0.39 0.45 4.5. Basin elongation ratio The Re values have been calculated for nine small drainage basins located on the NE and SW borders of the lake in the Ghareh and Ahmadi Mountain Fronts. These results range from 0.435 to 0.689, pointing to a slightly active uplift of the Maharlou Fault than the more active Ghareh Fault (Figures 4 and 5). Drainage basins in arid and semiarid climates tend to show Re values ranging from <0.50, through 0.50–0.75 to >0.75 for tectonically active, slightly active and inactive settings, respectively (Cuong and Zuchiewicz, 2001). 4.6. Alluvial fan topographic profiles Figure 5. Shaded SRTM topographic map which shows alluvial fans and fluvial systems used in the morphometric analysis. activity. Rapidly uplifted mountain fronts generally produce elongated, steep basins; and when tectonic activity is diminished or ceases, widening of the basins occur from the mountain front up (Ramírez-Herrera, 1998; El-Hamdouni et al., 2008). Elongated drainage basins characteristically occur in the southwestern part of the Maharlou Lake Basin, suggesting a relatively higher degree of tectonic uplift in this part of the study area. Copyright © 2011 John Wiley & Sons, Ltd. The fans generally present a profile with a similar mean slope. They have concave upwards profile with slopes that gradually decrease from the apex down. Most of these profiles are segmented into two parts with constant, but different, slopes. These slope breaks were determined in the field and some of them were identified by their observable fault scarps. Therefore, the slope breaks with dashed lines in the longitudinal fan profiles may be caused by tectonic activity (Figure 6). 5. DISCUSSION 5.1. Geomorphology The geomorphic and morphometric analyses carried out on the mountain fronts, alluvial fans and fluvial system around Maharlou Lake have significant value in providing information on the relative rate of tectonic uplift. Geomorphological Geol. J. 47: 30–40 (2012) TECTONIC ACTIVITY IN THE MAHARLOU LAKE BASIN, ZAGROS MOUNTAINS 37 Figure 6. Longitudinal profiles of some studied alluvial fans along the mountain fronts (Ahamdi Mountain Front in this figure) bounding Maharlou Lake. Slope breaks indicated by dashed lines interpreted as the location of the Maharlou Fault. features such as fault scarps and triangular facets along mountain fronts reflect recent tectonic activity. Indices of active tectonics may detect anomalies in the fluvial system or alluvial fans along the mountain fronts. These anomalies may be produced by local changes from tectonic activity resulting from uplift or subsidence (El-Hamdouni et al., 2008). Alluvial fans are ubiquitous features of mountainous range-fronts worldwide. Tectonic activity is now commonly recognized as the primary controlling factor in dictating alluvial fan properties such as location, setting and morphology, primarily through tectonic influences on drainage basin relief and fan accommodation space (Allen and Hovius, 1998; Allen and Densmore, 2000; Densmore et al., 2007). Alluvial fan morphology is an indicator of active tectonics because the fan form reflects varying rates of tectonic processes such as uplift of the catchment on mountains along a fault or tilting of the fan surface. Several factors, in particular tectonics, climate, and geomorphic history affect the geomorphology of alluvial fans. Within the context of the geomorphic setting, fan morphology reflects fan processes and evolution. In this study, slope breaks indicated by dashed lines are interpreted to be caused by fault activity along the mountain front associated with the Ghareh and Maharlou faults (Figure 6). Slope morphology of scarps produced by faulting is a useful geomorphic indicator of active tectonics (Keller and Pinter, 2002). The geometry of faults and the structure of the basin surrounding the lake indicate a ramp valley tectonic setting, with the lake developing in the topographic depression created by synclinal buckling (Figure 7). The ramp valley (Willis, 1928) developed in the structural accommodation zone created between the two thrust faults with opposing vergences. Other examples of similar ramp valley basins have been described by Bally (1982), Mann et al. (1991), Cobbold et al. (1993), and Lavenu et al. (1996). Mann et al. (1991) accounted for Miocene–Pliocene basin structures in Hispaniola by applying a similar tectonic evolution that developed along the restraining bend between the North American and Caribbean Figure 7. Simplified cross-section showing the two fault-generated mountain fronts bounding the Maharlou Lake Basin that developed in a ramp valley tectonic setting. (See Figure 1 for location of section line). Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. 47: 30–40 (2012) 38 a. faghih Plates. Lavenu et al. (1996) proposed that a compressional setting prevailed in the Ambato-Latacunga area, which corroborates a full-ramp setting (Cobbold et al., 1993). These authors assume that both the east-verging Victoria Fault in the west and the west-verging Pisayambo Fault in the east overthrust the basin margins. Seismotectonic studies carried out by Andalibi and Oveisi (1999) within the study area confirm the presence of the Maharlou and Ghareh faults. A full-ramp basin model (Cobbold et al., 1993), in which the opposite verging Maharlou and Ghareh faults drive differential uplift of the Lake Basin borders, most appropriately describes the tectonosedimentary setting of the lake. 5.2. Geomorphic indices Several authors have tried to categorize tectonic activity of regions into different tectonic classes as measured by geomorphic indices. El-Hamdouni et al. (2008) introduced three active tectonics classes corresponding to Smf values; class I (Smf < 1.1), class II (1.1 ≤ Smf <1.5), and class III (Smf ≥ 1.5). Smf values lower than 1.4 indicate tectonically active fronts (Rockwell et al., 1985; Keller, 1986) while higher Smf values (>3) are normally associated with inactive fronts in which the initial range–front fault may be more than 1 km away from the present erosional front (Bull and McFadden, 1977). Mountain fronts associated with active uplift are relatively straight with low values of (Smf). For slightly active and inactive regions, the Smf values tend to be between 1.4–3.0 and 1.8 to >5, respectively. When the rate of uplift is reduced or ceases, erosional processes will begin to form a sinuous front that becomes more irregular with time (Keller, 1986). According to Silva et al. (2003), linear mountain fronts with Smf < 1.5 are the main geomorphic and structural character of regions with active tectonics (class I). Irregular mountain fronts with Smf values ranging from 1.8 to 2.30 characterize class II regions. Results in this study indicate that the Ghareh and Maharlou Fault scarps belong to the class I of relative tectonic activity of Silva et al. (2003) with associated uplift rates of >0.08 m/ka which is also consistent with regions classified by high tectonic activity by Rockwell et al. (1985) and Keller (1986). In addition, the Ghareh Fault and Maharlou Fault belong to the tectonic classes I and II of El-Hamdouni et al. (2008) regarding the Smf values respectively. Tectonically high active mountain fronts display prominent and large facets, whereas tectonically less active fronts display fewer and dissected facets (Bull, 1978). High values of percentage of faceting and dissected escarpments that are associated with the Ghareh Mountain Front characterize the high tectonic activity. In the same way, these values are inverted in the Maharlou Mountain Fronts due to less tectonic activity. Copyright © 2011 John Wiley & Sons, Ltd. ET AL. The same conclusion is derived from analysis of valley floor width and valley height ratios (Vf) in the study area. This index differentiates between broad-floored valleys, with relatively high values of Vf, and V-shaped canyons with relatively low values. Low values of Vf reflect deep valleys of actively incising streams, commonly associated with the uplift (Keller and Pinter, 2002).The Vf values associated with Maharlou Mountain Front are relatively higher than those associated with the Ghareh Mountain Front. The elevation of the hills associated with the Ghareh Mountain Front is greater than that associated with the Maharlou Mountain Front; this reflects the larger displacement on the Ghareh Mountain Front. Thus, the width of the valleys on the hills associated with the Ghareh Mountain Front is less and also the elevations of the valley walls are high resulting in a low Vf value. El-Hamdouni et al. (2008) classified Vf values into three classes: I (Vf ≤ 0.5); II (0.5 ≤ Vf < 1.0) and III (Vf ≥ 1). According to Silva et al. (2003), V-shaped valleys (Vf < 0.6) characterized by active incision and U-shaped valleys (Vf: 0.3 – 0.80) characterized by valley floor aggradation. Corresponding to the obtained Vf values in the study area (Figure 4) the Ghareh Fault is categorized in class I of ElHamdouni et al. (2008) and classes I and II of Silva et al. (2003). The Maharlou Fault belongs to classes I and II of ElHamdouni et al. (2008) and class II of Silva et al. (2003) with relation to Vf values. In addition, the data resulting from basin shape and elongation ratio analyses carried out on the small basins around the lake also reveal that the Ghareh Fault on the SW border of the lake is more active than the Maharlou Fault on the NE border of the basin. In summary, the quantitative geomorphic and morphometric analyses on the mountain fronts surrounding the lake reveal that the Ghareh Mountain Front is more active than the Ahmadi Mountain Front. Seismotectonic studies (Andalibi and Oveisi, 1999) indicate that the deepest part of the lake is located along its southwest border (i.e. adjacent to the Ghareh Mountain Front). Andalibi and Oveisi (1999) argued that the tectonically-generated accommodation space developed by the Ghareh Fault had more influence on the lake formation than that generated by the Maharlou Fault. 6. CONCLUSIONS Tectonic geomorphology of orogenic belts has become one of the principal tools in the identification of active faults, seismic-hazard assessment and the study of landscape evolution. Geomorphic analysis of two mountain fronts surrounding Maharlou Lake within the Zagros Fold–Thrust Belt reveal marked differences between the fronts. Fault geometry in the study area is characterized by mountain front faults on both sides of Maharlou Lake. The results of the geomorphic analysis show that the southwestern mountain front boundary (Ghareh Fault) is more active than the northeastern Geol. J. 47: 30–40 (2012) TECTONIC ACTIVITY IN THE MAHARLOU LAKE BASIN, ZAGROS MOUNTAINS front (Maharlou Fault) which is consistent with field evidence and seismotectonic data for the study area. This study indicates that Maharlou Lake developed in a ramp valley tectonic setting that developed in the structural accommodation zone between the two opposing basin bounding faults. 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