Folding style of the Dezful Embayment of Zagros Belt: Signatures of detachment horizons, deep-rooted faulting and syn-deformation deposition

Marine and Petroleum Geology 91 (2018) 501–518

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Research paper

Folding style of the Dezful Embayment of Zagros Belt: Signatures of detachment horizons, deep-rooted faulting and syn-deformation deposition

T

Behzad Derikvanda,∗, Seyed Ahmad Alavia, Iraj Abdollahie Fardb, Hossein Hajialibeigia a b

Geology Department, Shahid Beheshti University, Tehran, Iran Exploration Directorate, National Iranian Oil Company, Tehran, Iran

A R T I C L E I N F O

A B S T R A C T

Keywords: Zagros fold thrust belt Dezful Embayment Faulted detachment fold Roof-ramp geometry Disharmonic folding Growth strata

Defining the governing factors on the geometry of the Dezful Embayment folds, a major oil province in the Zagros Fold-Thrust Belt in SW Iran, reduces the risks inherent in hydrocarbon exploration. The goal of the current study is to find a reasonable relationship between folding and faulting in the northern part of the Dezful Embayment. The role of major and minor detachment horizons and deep-rooted faults on structural style is also a matter of investigations. To achieve these goals, geological surface information and the available well and 2D and 3D seismic data were used to construct a balanced structural cross-section. The area is subdivided into NE and SW sectors based on dissimilar physiographic features of the surface and structural and stratigraphic characteristics. In the both sectors, the Miocene Gachsaran Formation acted as the upper detachment horizon. In the NE sector, the role of the Gachsaran Formation as major detachment horizon in decoupled folding styles of overlying and underlying units is prominent. The Gachsaran Formation becomes less effective as a detachment horizon towards the SW as its thickness decreases and its facies change. The Triassic Dashtak Formation is most probably a major detachment horizon in the study area and evidence of another deeper detachment horizon (such as the pre-Cambrian-Cambrian Hormuz Series) is not apparent based on available data. The Lower Cretaceous Garau, Upper Cretaceous Gurpi and Paleocene-Eocene Pabdeh formations and the Oligocene Kalhur Member are subordinate detachment horizons. The role of major faults in the area, such as the Mountain Front Fault in driving thick-skinned deformation is taken into account along with the dominant thin-skinned deformation in the study area. The Kuh-e Asmari Anticline is the only emergent anticline exposing the Paleocene strata due to the activity of the deep-rooted Lahbari Thrust. This study reveals the commanding role of the upper detachment Gachsaran Formation and the overlying syn-deformation fluvial clastics of the Aghajari (Miocene) and Bakhtyari (Pliocene) formations on disharmonic folding styles.

1. Introduction The Zagros Fold-Thrust Belt (ZFTB) is a well-known natural laboratory of folding in southwest Iran. Because of its economic importance, this belt has long been of interest to geologists, especially structural geologists. The ZFTB hosts about 9% oil and 15% gas of worldwide reservoirs (Sherkati, 2005). A significant part of these reservoirs (45 oil fields) are located in the Dezful Embayment (Vergés et al., 2011). In this embayment, most of the hydrocarbon traps are anticlines (Bordenave and Hegre, 2005, 2010); therefore, identification of factors which affect the geometry of the anticlines is essential to exploration. Despite the existence of seismic profiles and well data, prediction of the structural style in the deeper areas for which seismic imaging is usually of poor quality is often difficult to unravel. Since an accurate

∗

prediction of the deep structure is a key to understand the evolution of the different structures while enhancing their restoration to the prefolding state, the characterization of the deformation style at different structural levels is of crucial interest. Therefore, the current study examined the geological structures of the northern part of the Dezful Embayment at the regional scale with the following aims: 1. describing the relationship between the folds and faults, 2. discovering the influence of the detachment horizons and syn-deformation deposits on the geometry of the folds and 3. determining the role of the deep-rooted faults in the structural style of the northern part of the Dezful Embayment and the southwest part of the Izeh zone (Fig. 1).

Corresponding author. E-mail address: [email protected] (B. Derikvand).

https://doi.org/10.1016/j.marpetgeo.2018.01.030 Received 6 April 2017; Received in revised form 21 January 2018; Accepted 23 January 2018 0264-8172/ © 2018 Elsevier Ltd. All rights reserved.

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Fig. 1. Structural map of central part of the ZFTB. The Dezful Embayment is separated from other zones by the Mountain Front Fault (MFF), Balarud Fault Zone (BFZ), Izeh Fault Zone (IFZ), Kazerun Fault Zone (KFZ) and Zagros Front Fault (ZFF). Zagros Main Reverse Fault (ZMRF), High Zagros Fault (HZF), Zagros Deformation Front (ZDF). Payoun (1), Toukak (2), Kamarun (3), Pare Siah (4), Kuh-e Asmari (5), Naft-e Safid (6), Khandagh (7), Kupal (8), Marun (9), Ahwaz (10), Band-e Karkheh (11), Aghajari (12), Changuleh (13) and Azadegan (14) anticlines.

2. Geological setting and mechanical stratigraphy of Dezful Embayment

horizon in this subzone (Sherkati and Letouzey, 2004; Sepehr et al., 2006). In contrast, in the southern subzone, folds are formed above a deeper detachment (Triassic Dashtak Formation) compared to the northern subzone (Sepehr et al., 2006). In this subzone, the Kazhdumi Formation played the role of a subordinate detachment horizon (Sherkati and Letouzey, 2004). As a result, the anticlines in the southern subzone are larger than those in the northern subzone (Sepehr et al., 2006). In the central part of the ZFTB, the Dezful Embayment is bounded by the Zagros Foredeep Fault (ZFF), Mountain Front Fault (MFF) and its segments, the Kazerun Fault Zone (KFZ), the Balarud Fault Zone (BFZ) and the Izeh Fault Zone (IFZ) (Fig. 1) (AlaviM, 1994; Berberian, 1995; Sepehr and Cosgrove, 2004). Excluding the Kazerun Fault Zone, none of the mentioned faults are outcropping at the surface and they have been identified from sedimentological and morphotectonic evidences and earthquake data (Falcon, 1961; Berberian, 1995; Pattinson and Takin, 1971; Sepehr and Cosgrove, 2004, 2005, 2007). These faults along with the Burgan-Azadegan, Hendijan and Kharg-Mish paleo-highs (Fig. 1) have affected sedimentation and tectonic evolution of the study area (Sepehr and Cosgrove, 2004; Sherkati and Letouzey, 2004; Abdollahie Fard et al., 2006). The Dezful Embayment was flexed in response to the uplift of the Zagros Belt in the hinterland side (northeast of the MFF) and as a result, the post-Oligocene foredeep basin (Falcon, 1974; Berberian, 1995; Sherkati et al., 2006; Van Buchem et al., 2006; Saura et al., 2015; Pirouz et al., 2017) was formed with the partial accumulation of syndeformation up to 5000 m siliciclastic the Aghajari and Bakhtyari formations. Therefore, this embayment is known as thickest part of the Mesopotamian foredeep basin (Sepehr and Cosgrove, 2004; Aqrawi

The Zagros orogenic Belt is a part of the Alpine-Himalayan Belt that is the result of the opening and subsequent closing of the Neo-Tethys Ocean between Central Iranian and Arabian plates (Takin, 1972; Berberian and King, 1981; Dercourt et al., 1986; AlaviM, 1994; Stampfli and Borel, 2002). The convergence started with ophiolite obduction in the Late Cretaceous (Agard et al., 2005; Saura et al., 2011) and continued with a main folding phase in Late Miocene (Homke et al., 2004; Emami, 2008; Fakhari et al., 2008; Khadivi et al., 2010). The ZFTB is divided into several zones based on the along-strike changes in structural styles and position of the deformation front and stratigraphy (Mouthereau et al., 2012). These zones are from NW to SW: the Kirkuk Embayment, the Lurestan zone, the Dezful Embayment, the Izeh zone and the Fars zone (Motiei, 1995; Stocklin, 1968; Falcon, 1974; Sherkati and Letouzey, 2004; Lacombe et al., 2006; Casciello et al., 2009; Mouthereau et al., 2012). The Izeh zone is situated in the northeast of the Mountain Front Fault (MFF) and southwest of the High Zagros Fault (HZF) (Fig. 1). This zone corresponds to a narrow band between the Fars and Lurestan zones. The Bangestan and Khami Groups crop out in the resistant core of many of the Izeh zone folds (Abdollahie Fard et al., 2006; Sepehr et al., 2006). This zone is also subdivided into two northern and southern subzones (Sepehr et al., 2006). In the northern subzone, the folds are dominantly box and chevron geometries that formed above a relatively shallow detachment horizon within the Albian Kazhdumi Formation (Sherkati and Letouzey, 2004; Sepehr et al., 2006). Eocene marls of the Pabdeh Formation also formed an efficient detachment 502

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et al., 2010; Pirouz et al., 2017). The fluvial Aghajari Formation is the product of the erosion of uplifted folds in the hinterland side of the Zagros Belt caused by the Zagros orogeny. These sediments were transported to the foredeep of the Zagros Belt in form of multiple consecutive piggy-back basins (Abdollahie Fard et al., 2006; Pirouz et al., 2011). With southward propagation of deformation, the sedimentary deposits in the Dezful Embayment were folded and as a result, the foredeep basin transported to the Persian Gulf (Hessami et al., 2001; Abdollahie Fard et al., 2006; Pirouz et al., 2011). In the Dezful Embayment, evaporites of the Gachsaran Formation decoupled the small outcropping folds from the large buried folds which formed giant oil fields (Sepehr and Cosgrove, 2004; Sepehr et al., 2006; Sherkati et al., 2006; Abdollahie Fard et al., 2011). Sepehr and Cosgrove (2004) and Sepehr et al. (2006) suggested that the Dezful Embayment anticlines beneath the Gachsaran evaporites were formed by inversion of Permo-Triassic extensional faults in the basement. In contrast, Sherkati and Letouzey (2004) proposed a detachment horizon in the Lower Paleozoic level. Other studies suggest several detachment horizons within the stratigraphic section between the basement and the Gachsaran evaporites that affected geometry of the folds in this Embayment (Sherkati et al., 2006; Abdollahie Fard et al., 2006). Despite pervious fruitful studies, there are some ambiguities in the Dezful Embayment at large scale which have not been clarified yet. For instance some of the major issues are as follow: 1. impact of the lateral thickness changes of the Gachsaran Formation on the structural style of the Dezful Embayment, 2. governing factors on the outcropping Asmari and older formations in the core of the outcropping anticlines of the Dezful Embayment (i.e., the Kuh-e Asmari Anticline), 3. role of the basement faults on the present structural style of the Dezful Embayment. This research attempts to address the above-mentioned issues and provide possible solutions. The thick pile of sedimentary rocks of the ZFTB consists of four groups (Fig. 2) which deposited in different tectonic settings as summarized by Alavi (2004). The Paleozoic succession was formed by the marine and non-marine sedimentary rocks deposited in an epicontinental platform. These rocks are overlain unconformably by the Permian-Triassic succession (Szabo and Kheradpir, 1978; Berberian and King, 1981; Koop and Stoneley, 1982), which is related to continental rifting (Berberian, 1995; Sherkati and Letouzey, 2004; Sepehr and Cosgrove, 2004). This succession is constituted by transgressive basal siliciclastic rocks and overlying evaporitic carbonates. The lower Jurassic-upper Turonian units are shallow and deep-water carbonates with some siliciclastic and evaporite deposits which deposited on a shallow (Neo-Tethyan) continental shelf or passive continental margin (Berberian, 1995). Marine and continental sedimentary rocks were deposited in an active margin setting during Late Turonian to Recent, which overlie unconformably the older units. These units mainly comprise siliciclastic and carbonate deposits (Alavi, 2004). Fig. 2 show the stratigraphic column of the Dezful Embayment that presented corresponding formations of the aforementioned four groups. O'Brien (1950) divided the sedimentary sequences of the Zagros into five groups exhibiting different mechanical behavior: 1. basement (Precambrian), 2. lower mobile Group (Precambrian-Cambrian Hormuz Series), 3. competent Group (Cambrian to Early Miocene strata), 4. upper mobile Group (Miocene Gachsaran Formation) and 5. passive Group (Mishan, Aghajari and Bakhtyari formations). This classification is very general and recent researches have shown that subdivision of multiple mechano-stratigraphic levels may vary at different localities (Bahroudi and Koyi, 2003; Sherkati and Letouzey, 2004; Molinaro et al., 2005; Abdollahie Fard et al., 2006; Sepehr et al., 2006; Sherkati et al., 2006; Carruba et al., 2006; Farzipour-Saein et al., 2009; Jahani et al., 2009; Casciello et al., 2009; Abdollahie Fard et al., 2011; Vergés et al., 2011; Motamedi et al., 2012; Najafi et al., 2014). In the Dezful Embayment, there is a lack of evidence of the mobile Hormuz Series, but seismic profiles on the Azadegan Anticline in the Abadan Plain (Fig. 1) indicate deep steep reflectors as compared with

comparatively gently-dipping reflectors in the upper parts. These steep reflectors may be associated with the salt diapirism of the Hormuz Series (Abdollahie Fard et al., 2006). The folded reflectors at the lower part of Paleozoic strata in the core of the Chachmeh Kush Anticline have also been attributed to the movement of the Hormuz Series (Vergés et al., 2011). On the other hand, Eo-Cambrian evaporite deposits or Cambrian shales are also considered by many researchers (Sherkati and Letouzey, 2004; Farzipour-Saein et al., 2009; Vergés et al., 2011) to be basal detachment horizons involved in the deformation of the Dezful Embayment, Izeh and Lurestan zones. The nearest outcrop of Cambrian shales to the Dezful Embayment is along the hangingwall of the High Zagros Fault (Fig. 1) (Gavillot et al., 2010). The stratigraphy characteristics of the Paleozoic units are not distinguished in the SW of the Izeh zone and Dezful Embayment because of the lack of outcrops and deep well data (Sherkati and Letouzey, 2004). Based on limited outcrops in the High Zagros, the Paleozoic rocks are considered as competent units in the previous studies (Sherkati and Letouzey, 2004; Abdollahie Fard et al., 2006; Sherkati et al., 2006; Carruba et al., 2006). The evaporites of the Dashtak Formation (Middle Triassic) (Fig. 2) have played in most parts of the Zagros as a prominent detachment horizon during the deformation process (Sherkati et al., 2006; Abdollahie Fard et al., 2006; Sepehr et al., 2006; Vergés et al., 2011; Motamedi et al., 2012; Najafi et al., 2014). Towards the northeast of ZFTB, the evaporites of the Dashtak Formation change to the carbonates of the Khanehkat Formation (Szabo and Kheradpir, 1978; Setudehnia, 1978) that does not act as a detachment horizon (Sherkati et al., 2006). In the Central Frontal Fars, the geometrical variations of anticlines were attributed to changes in the thickness of the Dashtak evaporites (Najafi et al., 2014). In the west of the Razak Fault (on the Gavbandi paleo-high), the relatively thin basal detachment horizon (Hormuz Series) caused the activation of the Dashtak Formation which formed short wavelength anticlines compared to those formed above the thick Hormuz Series in the east of the fault (Motamedi et al., 2012). In the study area, the Dashtak Formation has been reported in the Kuh-e Asmari 2 well (See Fig. 3 for location). The variation in lateral thickness and facies in the Jurassic-Cretaceous sequences across the Izeh and Kazerun faults prove their activity during Jurassic-Cretaceous times (Setudehnia, 1978; Sepehr and Cosgrove, 2004). These variations are marked by changes from the carbonates of the Surmeh Formation and Khami Group in the Fars zone to the Adaiyeh, Mus, Alan, Sargelu, Najmeh, Gotnia and Garau in the Lurestan zone and Dezful Embayment (Setudehnia, 1978; Sepehr, 2001; Sepehr and Cosgrove, 2004). In the study area, soft Triassic-Cretaceous sequences are potentially incompetent units (Fig. 2). Therefore, the carbonates of Ilam-Sarvak and Asmari formations acted as competent units in the folding process. Changes of the competent/incompetent thickness ratio are an effective factor on fold wavelength. The folds of the Dezful Embayment and Lurestan Zone have a shorter wavelength than compared to the Fars Zone folds because of reduction of this ratio (Colman-Sadd, 1978; Blanc et al., 2003; Sepehr et al., 2006; Casciello et al., 2009). Other detachment horizons are (Fig. 2): the Paleocene-Eocene Pabdeh and Gurpi formations (Abdollahie Fard et al., 2006), the Kalhur Member of the Asmari Formation (Vergés et al., 2011) and the Gachsaran Formation (main upper detachment horizon) (Sherkati and Letouzey, 2004; Sepehr et al., 2006; Abdollahie Fard et al., 2006; Abdollahie Fard et al., 2011). Using well data, the Gachsaran Formation in the Dezful Embayment was divided into 7 members (Motiei, 1995). Members 2–5 of the Gachsaran Formation are considered as incompetent because of the presence of salt and red and green marl. The syn-deformation deposits (Aghajari and Bakhtyari formations) represent the uppermost part of stratigraphy column of the Dezful Embayment. As competent units, these deposits actively influenced the mechanical balance and kinematic evolution of the folds of the Dezful Embayment (Abdollahie Fard et al., 2011).

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Fig. 2. Stratigraphic column of the northern part of the Dezful Embayment based on surface and well data. Competent units are separated by several detachment horizons in different parts of sedimentary columns. Paleozoic lithostratigraphic units are presented based on regional geology (Modified after Alavi, 2004; Abdollahie Fard et al., 2006).

field data, 6. restoration of the structural cross-section and re-checking of the seismic data and maps and 7. finalizing the structural crosssection and analysis.

3. Materials and methods 3.1. Methods The current study is based on four data sets, including: 1. 2D and 3D seismic data, 2. well data, 3. data from field observations and 4. geological maps. The seismic and well data were provided by the National Iranian Oil Company (NIOC). The surface data were inferred from the 1:100,000 NIOC geological maps and field visits. To better understand the structural style of the study area, a structural cross-section has been constructed almost perpendicular to the structures (Figs. 1 and 3). Steps of this research are as follows: 1. review of seismic data, which included four 3D cubes and more than one hundred 2D seismic lines. 2. selecting representative seismic profiles, 3. defining the location of a regional structural cross-section, 4. visits and field observation of surface structures using available maps. The field trip was supported by NIOC; 5. constructing the structural cross-section based on the seismic and

3.2. Definition of interpreted horizons In this study, four time-migrated seismic profiles are used to introduce the structures in the northern part of the Dezful Embayment. Eight main horizons (tops of the different formations) were interpreted in each seismic profile. These horizons, from younger to older, are including: Mn: Mishan Formation (Middle-Upper Miocene): marine marls with some thin intercalations of limestone. This horizon marks the base of the Aghajari Formation as main syn-deformation deposits. Gs: Gachsaran Formation (Middle Miocene): alternating anhydrite, halite, marl, and limestone. This formation is well-known as upper detachment horizon in the Dezful Embayment. 504

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Fig. 3. Geological map of the study area. The Asmari and Pabdeh formations are outcropping in the core of the Kuh-e Asmari Anticline. In NE sector, the Lahbari, Mordeh Fel and Andakan thrusts emplaced the Gachsaran Formation on top of younger stratigraphic units. The Aghajari and Bakhtyari formations are covering the SW sector. Dashed black lines mark trace of the MFF and IFZ. See Fig. 1 for location.

As: Asmari Formation (Oligo-Miocene): fractured neritic limestones. This unit represents the main reservoir in the study area. This horizon also displays geometry of the folds beneath the Gachsaran evaporites. Sv: Sarvak Formation (Cenomanian): thin-bedded to massive neritic limestones. This formation is the second reservoir and one of the thick competent units in the Dezful Embayment. Dr: Dariyan Formation (Aptian): limestones with minor siliceous and argillaceous beds. This unit form one of the competent units in the stratigraphic column. Gr: Garau Formation (Lower Cretaceous): black shales and marly limestones. It is a source rock and one of the detachment horizons in the study area. Gt: Gotnia Formation (Upper Jurassic): anhydrite intercalated with shale and limestone. This unit marks top of the Jurassic strata. Dk: Dashtak Formation (Triassic): evaporites intercalated with dolomite and shale. This evaporitic formation represents an effective detachment horizon. These horizons were correlated with well data in the different part of the study area. In addition to the main horizons, in each seismic profile, several horizons were interpreted within the Aghajari (Aj) and Bakhtyari (Bk) formations to show the pattern of the growth strata.

4.1. Izeh zone To determine the effect of the MFF on the geometry of the structures and structural relationships on both sides of this fault, folds in the southern part of the Izeh zone on the hangingwall of the MFF were investigated (Figs. 1 and 3). In this part of the study area, the outcropping units are Lower Cretaceous (Khami Group) to Oligo-Miocene (Asmari Formation) (Fig. 3). The folds display dominantly geometry of whale-back anticlines with smooth sigmoidal axes and are generally asymmetric with steeply to overturned dipping southern flanks (Fig. 3). These folds are generally characterized by high amplitude and large wavelength. The Payoun and Toukak anticlines are separated by a tight syncline with an overturned northern flank (Fig. 4). The Toukak and Kamarun anticlines, in an en-echelon arrangement, are located south of the Payoun Anticline (Fig. 3). In the overlapping area between these two anticlines, 2D seismic data show that the Pabdeh and Gurpi formations act as detachment horizons (Fig. 5).

4.2. NE sector Outcrops of the NE sector are mainly Fars Group strata which include the evaporatic Gachsaran, marly Mishan, clastic Aghajari and conglomeratic Bakhtyari formations (Fig. 3). The seismic profiles show that the outcropping anticlines have gentle amplitudes and short wavelengths but, unlike the anticlines, the synclines show generally higher amplitudes and longer wavelengths (Figs. 3, 5 and 6). These synclines are mainly filled by the growth strata of the Aghajari and Bakhtyari formations (Figs. 3, 5 and 6). There are two dominant sets of thrust faults: One set is on the southern flanks of small outcropping anticlines

4. Geometry of structures Based on geological characteristics, the study area has been divided into three parts: Izeh zone, NE sector and SW sector. The structural styles of these parts have been presented separately.

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Fig. 4. Tight syncline between the Payoun and Toukak anticlines, showing NE dipping axial plane and an overturned NE flank. See Fig. 3 for location.

Fig. 5. (a) Uninterpreted and (b) interpreted seismic profile of the southern flank of the Payun Aanticline and Toukak, Pare Siah and Kuh-e Asmari anticlines. See Fig. 3 for location. The Payun, Toukak and Pare Siah anticlines form an imbricated system. Growth strata within the upper member of the Gachsaran Formation indicate that folding started in Middle Miocene time. Ramp-flat geometry of thrust fault in the southern flank of the Kuh-e Asmari Anticline led to repetition of the Cretaceous to Oligo-Miocene units. MFF (Mountain Front Fault), MFT (Mordeh Fel Thrust), LT (Lahbari Thrust), Bk (Bakhtyari), Aj (Aghajari), Mn (Mishan), Gs (Gachsaran), As (Asmari), Pd (Pabdeh), Sv (Sarvak), Dr (Daryan) and Gr (Garau).

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Fig. 6. (a) Uninterpreted and (b) interpreted seismic profile of the Naft-e Safid and Khandagh anticlines. See Fig. 3 for location. At surface, the Naft-e Safid Anticline is a small fold with a faulted southern flank. Beneath the Gachsaran evaporites, the Naft-e Safid Anticline is thrusted on top of the northern flank of the Khandagh Anticline. Bk (Bakhtyari), Aj (Aghajari), Mn (Mishan), Gs (Gachsaran), As (Asmari), Sv (Sarvak), Dr (Daryan) and Gr (Garau).

of a buried subsurface syncline (Figs. 5 and 6). The seismic profiles in this part of the study area cross the Pare Siah, Kuh-e Asmari, Naft-e Safid and Khandagh subsurface anticlines. These anticlines are asymmetric folds with southward vergence, as their southern flanks are cut by thrust faults (Figs. 5 and 6). Based on seismic data, we propose that these thrust faults are rooted in the Garau-Dashtak detachment horizon (Lower Cretaceous to Middle Triassic). In this sector, the Kuh-e Asmari Anticline has larger wavelength than other anticlines at the Asmari level (Figs. 5 and 6). Therefore, this anticline is the result of folding either above a deeper detachment horizon or associated with a basement fault. Migration of the Gachsaran evaporites from the axis of the Pare Siah and Kuh-e Asmari anticlines and their movement toward the neighboring syncline caused formation of the salt welds between member 1

(such as Naft-e Safid and Dimeh Darb anticlines) (Figs. 3 and 6) and another set thrusted the Gachsaran Formation on top of the younger stratigraphic units in the cores of larger outcropping anticlines (Fig. 3) such as the Lahbari (Dezful Embayment), Mordeh Fel (Fig. 7a) and Andakan thrusts (Fig. 3). Due to the plastic behavior of the Gachsaran detachment horizon, buried subsurface anticlines (at the Asmari and deeper levels) of the NE sector have a different geometry compared to the outcropping anticlines. In the Dezful Embayment, these anticlines are mainly whaleback anticlines which form giant oil fields (Bordenave and Hegre, 2005). The axes of the buried subsurface anticlines are shifted relative to the axis of the outcrop; in some locations, outcropping synclines are above the deeper anticlines and vice versa on the northern flank of the Naft-e Safid Anticline, Dimeh Darb Anticline (Fig. 3) can be seen on top 507

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Fig. 7. (a) The Gachsaran Formation thrusted on the Bakhtyari Formation by Mordeh fel Thrust. (b) Outcrop of the Asmari Formation in the Kuh-e Asmari Anticline. See Fig. 3 for location. (c) Outcrop of the Mishan Formation in the core of the surface Aghajari Anticline. The southern flank of the anticline is cut by several small thrust faults. See Fig. 1 for location.

(Dahlstrom, 1990; Letouzey et al., 1995; Sherkati and Letouzey, 2004; Vergés et al., 2011) on the northern flank of the Pare Siah Anticline (Fig. 10b). This rabbit-ear fold shows hinge-ward vergence with a small thrust fault in its SW flank (Fig. 10b). While this thrust fault cuts the Asmari Formation, it also affects the upper parts of the Gachsaran Formation (Fig. 10b). In the study area, outcrops of both the Asmari and Pabdeh formations are only seen in the Kuh-e Asmari Anticline (Figs. 3 and 7b). The seismic profile (Fig. 5) is crossing the NW plunge of the Kuh-e Asmari Anticline where the Asmari Formation is not outcropping (Fig. 3). The Kuh-e Asmari Anticline was affected by the activity of two thrust faults (Fig. 5). The southern thrust fault cuts the southern flank of the anticline and uplifted the Mesozoic-Paleogene sedimentary units of the Kuh-e Asmari Anticline. This fault exhibits large displacement that displaced the Upper Jurassic formations at the level of the Gachsaran Formation (Middle Miocene) (Fig. 5). The northern thrust fault, with a ramp-flat geometry, continued to the surface and formed the Lahbari Thrust (Figs. 3 and 5). The northern flank of the Kuh-e Asmari Anticline has also been cut by a back-thrust fault (Fig. 5).

and the base of member 6 of the Gachsaran Formation (Fig. 5). The plastic units of the Gachsaran Formation appear in the form of diapirs (Fig. 8a) and syn-deformation deposit affecting this diapirism and somehow controlling it (Fig. 8b and c). In addition to disharmonic folding in the section view, the role of the Gachsaran detachment horizon is evident in the disharmonic pattern in the map view. For instance, several smaller folds can be detected in the north of the Kuh-e Asmari Anticline (Fig. 9). These folds show versatile patterns such as box-type (Fig. 9a), lift-off (Mitra and Namson, 1989; Mitra, 2003) and isoclinal bullet-form anticlines (Fig. 9b) (as described by Sherkati et al., 2005; Sepehr et al., 2006). These small anticlines are separated by tight synclines with out-of-syncline thrusts (Fig. 9b). The Payoun, Toukak and Pare Siah anticlines have been thrusted on each other and form an imbricated system (Fig. 5). Similar configurations can be seen in the Naft-e Safid and Khandagh anticlines (Fig. 6). The Asmari and Sarvak carbonate reservoirs were separated by soft units of the Kalhur Member and Pabdeh and Gurpi Formations in the NE sector. The Kalhur Member of the Asmari Formation played a detachment role as evidenced by parasitic folds formed above this member in the Changuleh oil field (Vergés et al., 2011) (Fig. 10a). In the NE sector, the mobility of the Kalhur member, Gurpi and Pabdeh formations can be observed in the development a rabbit-ear fold

4.3. SW sector The main anticlines of the SW sector are the Aghajari, Marun, 508

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Fig. 8. (a) Salt diapir resulting from deformation of the Gachsaran evaporites. See Fig. 3 for location. (b) Growth strata in the upper part of the Gachsaran Formation in northwestern part of the salt diapir. (c) Growth strata in the upper part of the Gachsaran Formation in southwestern part of the salt diapir.

older units (Fig. 11). Also, the Garau Formation is locally thickened in the core of the Kupal and Marun anticlines and affects their geometry (Fig. 11). In this sector, the Jurassic strata served in a manner similar to that of a competent unit and was sandwiched between the Garau and Dashtak detachment horizons (Fig. 11). The Dashtak evaporites acted as an efficient detachment in the Izeh (Sepehr et al., 2006), Lurestan (Farzipour-Saein et al., 2009; Vergés et al., 2011) and Fars zones (Motamedi et al., 2012; Najafi et al., 2014; Farzipour-Saein and Koyi, 2016).

Kupal, Ahwaz and Band-e Karkheh anticlines and this sector is separated by the ZFF from the Abadan Plain (Fig. 1). The wide synclines of the SW sector are mostly covered by alluvial deposits (Fig. 3). The outcropping units in this sector are primarily the Aghajari and Bakhtyari formations (Fig. 3), although the Mishan and Gachsaran formations are observed in faulted flanks of some anticlines, such as the Aghajari Anticline (Fig. 7c). The anticlines of the SW sector are elongated with their lengths exceeding 60 km (e.g., Aghajari, Kupal, Marun and Ahwaz anticlines) (Fig. 1). The synclines in this sector are wider (10–15 km) than those of the NE sector (Figs. 11 and 12). Alike the NE sector, these synclines are also filled by syn-deformation deposits (Aghajari and Bakhtyari formations) (Figs. 11 and 12). A difference in folding style above and beneath the Gachsaran detachment horizon and accumulation of these evaporites in synclines on both sides of the major subsurface anticlines can be observed (Figs. 11 and 12). The outcropping anticlines have low amplitudes and short wavelengths compared to the buried subsurface anticlines (Fig. 11). In some cases, the outcropping anticlines display an axis shift relative to the buried subsurface anticlines (Figs. 11 and 12). The folds of the SW sector beneath the Gachsaran Formation are long whale-back anticlines which display slightly asymmetric to asymmetric geometry with rounded hinges and faulted flanks in form of steep fore- and back-thrusts rooted in the Dashtak Formation as an intermediate detachment horizon (Figs. 11 and 12). At the Asmari level, these anticlines have larger wavelength than those of the NE sector. In the SW sector of the study area, subsurface data shows that the Dashtak Formation played the role of intermediate detachment horizon which decoupled the Upper Triassic-Oligo Miocene succession from the

5. Structural cross-section To better understand the relationship between the structures at the regional scale and identify factors controlling their geometry, a nearly 150-km-long balanced and restored structural cross-section has been constructed (Fig. 13). The Geophysical data were provided by Geophysics Department of the NIOC. The seismic profiles in the study area have acceptable quality down to the Triassic units. Excluding the Kuh-e Asmari 2 well, well data are limited to the Lower Cretaceous-Recent succession. Using well data, the interpreted horizons were converted from time to depth domain. In the deeper horizons, the structural crosssection has been constructed using data from the Lurestan and Izeh zones (stratigraphic thickness from well data and surface stratigraphic columns)with using Kink method (Suppe, 1983), balancing conventional methods (Dahlstrom, 1969; Harrison and Bally, 1988; Woodward et al., 1990; Koyi, 2000) and transect D provided by Sherkati et al. (2006). In the construct of a structural cross-section, the basic assumptions are line length and area preservation during the folding 509

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Fig. 9. (a) A box fold in map view in the northeastern flank of the Kuh-e Asmari Anticline. (b) Small detachment fold and Lift-off detachment fold in map view formed above the Gachsaran evaporites. An out of syncline thrust is formed in the core of syncline between two folds. See Fig. 3 for location.

the Jurassic-Cretaceous units across the MFF (Fig. 13). Thickness changes of the Bangestan and Khami group are observed on both sides of the MFF (Fig. 13). The Jurassic-Cretaceous carbonates (Faliyan and Surmeh formations) in the Izeh zone were replaced by deep marine shales and marly limestones (Garau Formation) and alternations of shales and anhydrite (Gotnia, Sargelu, Alan and Adayah formations) in the Dezful Embayment (Fig. 13). The structural cross-section reveals that the NE sector is structurally more complicated than the SW sector. Thrust faults, imbricated systems and pop-up structures are common in the NE sector (Fig. 13). In this sector, the Gachsaran Formation and lower Cretaceous-Middle Triassic units acted as upper and intermediate detachment horizons, respectively. The folds with a southward vergence are slightly asymmetric to asymmetric at the Asmari level (Fig. 13). Beneath the intermediate detachment horizon, the geometry of the folds has been affected by the activity of the MFF, Lahbari Thrust and the basal detachment horizon (Hormuz Series or its equivalent) (Fig. 13). In the SW sector, the short wave-length, low amplitude outcropping anticlines are separated by wide synclines (Fig. 13). Beneath the Gachsaran Formation (upper detachment horizon), the anticlines have rounded with a slightly asymmetric geometry as both flanks are cut by steep faults (Fig. 13). In this sector, the Dashtak Formation played the role of the intermediate detachment horizon and the Garau Formation acted as another detachment horizon (Fig. 13). In both sectors, large volumes of the Aghajari and Bakhtyari formations (growth strata) were deposited within the wide synclines (piggy-back basins) synchronous with deformation (Fig. 13). For instance, in a syncline in south of the Khandagh Anticline, the thickness of the growth strata is about 4 km (Fig. 13).

process (Dahlstrom, 1969; Moretti and Callot, 2012). In a compressional regime, change of rock volume is negligible during deformation. Since bed thickness and surface area of any bedding plane remain constant (Dahlstrom, 1969). In this research for the geometric validity of the structural cross-section, the assumption of line length preservation is considered for competent units such as the Asmari Formation to Khami Group and Paleozoic units. The assumption of area preservation is also applied for intermediate (Garau to Dashtak formations) and basal (Hormuz Series or its equivalent) detachment horizons. The magnetic maps show that the basement of the Dezful Embayment is located at 12–15 Km with a dip to the NE (Kugler, 1973; Morris, 1977; Aqrawi et al., 2010). In accordance with the thickness of the sediments in the Zagros foreland basin (10–14 km) (James and Wynd, 1965; Stocklin, 1968; Colman-Sadd, 1978; Vergés et al., 2011; Pirouz et al., 2017), the dip of the basement is about 1° to the NE (Carruba et al., 2006; Saura et al., 2015; Pirouz et al., 2017). Therefore, the thickness of the sedimentary cover is considered about 12–14 km in the different parts of the structural cross-section (Fig. 13). In the hinterland side of the structural cross-section, the deformation pattern is mainly affected by the activity of the MFF (Fig. 13). This fault significantly uplifted its hangingwall, juxtaposing Cretaceous-Paleogene (Fig. 13) and Neogene (Figs. 3 and 13) strata on its the hangingwall and footwall, respectively. A nearly 6 km difference of structural elevation, based on lower boundary of the Bangestan Group, across the MFF can be deduced by comparison of the synclines depths on both sides of this fault, from point A to point B (Fig. 13). On the hangingwall of this fault, the Payoun Anticline displays similar folding styles in both the outcropping and deeper parts (Fig. 13). The structural cross-section shows thickness and facies changes of 510

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6. Discussion Researches introduce mechanical behavior of sedimentary succession (Sherkati and Letouzey, 2004; Molinaro et al., 2005; Sherkati et al., 2006; Abdollahie Fard et al., 2006, 2011; Sepehr et al., 2006; Casciello et al., 2009; Abdollahie Fard et al., 2011; Vergés et al., 2011; Motamedi et al., 2012; Najafi et al., 2014) and activity of the basement faults (Berberian and King, 1981; Berberian, 1995; Molinaro et al., 2005; Sherkati et al., 2005) as main factors which control the folding style of the ZFTB. The impacts of these two factors on the folding style of the study area are discussed as follow: 6.1. Effect of mechanical stratigraphy on structural geometry In the study area, the sedimentary succession contains several competent units separated by detachment horizons at different depths. The different mechanical behavior of these units caused both vertical and lateral changes in folding style during the folding process. Our cross-section constructed based on field observations, seismic and well data, shows that the Gachsaran evaporites played a major role in the formation of the structural pattern as mentioned by pervious researches (Sherkati et al., 2006; Abdollahie Fard et al., 2006; Sepehr et al., 2006). This role is much more considerable in the NE sector compared to SW sector, where its thickness has decreased (Fig. 14). For example, in the Abadan Plain (Fig. 1), the relatively thin Gachsaran Formation may not be considered as a detachment horizon (Abdollahie Fard et al., 2006). The decrease in thickness of the Gachsaran Formation is the result of a wedge-shaped foredeep basin (DeCelles and Giles, 1996; DeCelles, 2011) of the ZFTB. The folds at the Mishan (above the Gachsaran Formation) and Asmari (beneath the Gachsaran Formation) levels have different shapes, sizes and lengths. The aspect ratio (R) (amplitude/width ratio of the folds) and shape parameter (L) (distribution of the fold curvature) (Srivastava and Lisle, 2004) of the folds of the study area (sixteen anticlines) were calculated for the Mishan and Asmari levels. At the Mishan level, the folds display chevron to sinusoidal shapes with gentle interlimb angles (Fig. 15). At the Asmari level, in contrast, the shape of the folds changes from sinusoidal to rounded or semielliptical with open interlimb angles (Fig. 15). In the Lurestan zone alike the study area, the folds range in shape from chevron to rounded or semielliptical with gentle to tight interlimb angles (Vergés et al., 2011). In addition, the maximum aspect ratio is 1.5 in the Lurestan zone, while in the study area the aspect ratio does not exceed 0.5 (Fig. 15). Based on these differences, it seems that the intensity of deformation in the study area is less than the Lurestan zone, in agreement with their position ahead and behind the MFF. As mentioned, the withdrawal of the incompetent units of the Gachsaran Formation and their accumulation primarily in the southern flanks of the anticlines led to disharmonic folding in the study area (Fig. 13). This process was boosted by deposition of the syn-deformation growth strata of the Aghajari and Bakhtyari formations. The migration of the Gachsaran evaporites led to accommodation for sedimentation in form of piggy-back basins (Fig. 13) and consequently the Aghajari and Bakhtyari formations had been deposited within these basins. On the other hand, the gradually compacted Aghajari and Bakhtiari overburdens accelerated the withdrawal of the Gachsaran evaporites. Finally the salt welds as shown on Figs. 5 and 6 have been formed between overlying and underlying units. The large volume of the growth strata within the piggy-back basins also affects the mechanical balance of the folded strata (Abdollahie Fard et al., 2011). The southern flank of the anticlines has steepened due to the accumulation of thick siliciclastic growth strata (Aghajari and Bakhtyari formations) at the neighboring synclines (Fig. 13). In the final folding stages, these synclines are considered to play as buttresses against the southwestward advance of deformation and the generation of both fore- and back-thrusts (Fig. 13). Therefore, some of the anticlines show typical

Fig. 10. (a) Formation of parasitic folds of the Changuleh oil field on the evaporites of the Kalhur Member. See Fig. 1 for location. (b) Minor rabbit-ear fold along the north flank of the Pare Siah Anticline. See Fig. 5 for location. Bk (Bakhtyari), Aj (Aghajari), Mn (Mishan), Gs (Gachsaran), As (Asmari) and Sv (Sarvak).

Based on transect D provided by Sherkati et al. (2006), along the whole Dezful Embayment part of the cross-section, above the basal detachment horizon, the folds in the Paleozoic units are slightly asymmetric with thrust faults in their southern flanks (Fig. 13). Some of them show double vergence thrust faults and thus creating pop-up structures (Fig. 13). The structural cross-section has been restored to the undeformed state using the Asmari formation as a horizontal data, thus reproducing the geometry of the area at Early Miocene times (Fig. 13). An average of about 15% shortening is measured for the study area. In the hinterland side, the magnitude of shortening is high and gradually decreases toward the foreland side. The structural cross-section is almost parallel to the structural crosssection D of Sherkati et al. (2006), but there are some differences between them. In fact, the transect D of Sherkati et al. (2006) has been modified in this study using the new geophysical data. In the new structural cross-section, most of the (some of them are still deeply rooted, especially in the internal parts) outcropping thrust faults are interpreted as a result of the Gachsaran evaporites activity as nicely imaged in the provided seismic profiles (Figs. 5, 6, 11 and 12), but Sherkati et al. (2006) proposed them as faults rooted down into the Dashtak evaporites. In addition, the Garau to Dashtak formations were considered as intermediate detachment horizons throughout the Dezful Embayment by Sherkati et al. (2006), whereas, these formations are only intermediate detachment horizon in the NE sector and the Dashtak Formation solely played the role of an intermediate detachment horizon in the SW sector (Fig. 13). Based on transect D provided by Sherkati et al. (2006), the Kuh-e Asmari Anticline crops out as a result of thinskinned deformation above a flat detachment on top of the basement. In contrast, in the new structural cross-section (Fig. 13), we propose that the outcrop of this anticline this anticline partly due to a structural step associated to a basement thrust, and therefore, the Asmari anticline formed by both thin- and thick-skinned deformations. 511

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Fig. 11. (a) Uninterpreted and (b) interpreted seismic profile of the Kupal and Marun anticlines. See Fig. 3 for location. The Kupal outcropping Anticline (above Gachsaran Formation) is a gentle fold compared to an asymmetric Marun Anticline with faulted southern flank. Beneath the Gachsaran evaporites both of the anticlines are asymmetric fold with thrust faults at the southern flanks. Northern flank of the folds have affected by back -thrusts. Folded reflectors (Dashed blue line) beneath the Dashtak Formation can be attributed to activity of the Hormuz Series. Bk (Bakhtyari), Aj (Aghajari), Mn (Mishan), Gs (Gachsaran), As (Asmari), Sv (Sarvak), Dr (Daryan), Gr (Garau), Gt (Gotnia) and Dk (Dashtak). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

this formation are rarely observed (Fig. 14). In addition, a reduction in the magnitude of tectonic shortening towards the foreland (from the NE sector to the SW sector) should be considered as another factor that controls the frequency of the surface faults. The subordinate detachment horizon (Kalhur Member and Pabdeh and Gurpi formations) generated rabbit-ear folds in the flanks of major anticlines (e.g., the northern flank of the Pare Siah Anticline, Fig. 10b). The rabbit-ear folds were possibly formed by flexural-slip or shear mechanisms (Dahlstrom, 1990). Minor folds formed in the flanks of the main fold with vergence toward the main fold hinge caused by heterogeneity in the stratigraphy column. In the SW sector, the evaporites of the Kalhur Member are replaced by sandstones of Ahwaz Member and as a result, impact of the Pabdeh and Gurpi formations on folding style is less pronounced in this sector (Figs. 11–13). In the northwest of the Kazerun and Izeh fault zones, the Jurassic–Cretaceous sequence mostly consists of shales and evaporites (James and Wynd, 1965; Setudehnia, 1978; Sepehr and Cosgrove, 2004) which together with the Dashtak Formation formed an efficient intermediate detachment horizon (Blanc et al., 2003; Sherkati et al., 2006). In the NE sector of the study area, these formations played the

pop-up geometry similar to those described by Mitra (2002). The growth strata suggest that migration of the evaporites was started during deposition of the upper member of the Gachsaran Formation as its thickness reduces at the flank of outcropping anticline (Figs. 5, 6, 11 and 12). These growth strata were related to the first step of the folding process in Middle Miocene (Sherkati et al., 2005; Abdollahie Fard et al., 2006). The migration process was continued during the subsequent folding as shown by growth strata in the Aghajari and Bakhtyari formations (Figs. 5, 6, 11 and 12). The diverse patterns of the outcropping structures north of the Kuhe Asmari Anticline (Fig. 9) may be related to considerable lateral facies variations within the Gachsaran Formation, as there are some evidences of early Zagros folding during deposition of the Gachsaran Formation (Abdollahie Fard et al., 2006) and even during deposition of the Asmari Formation (Saura et al., 2011). The equivalent of these patterns (Fig. 9a) can be seen in the section view of the Kupal Anticline (yellow rectangles in Fig. 11). In the NE sector, the surface thrust faults are rooted in the upper detachment horizon. But in the SW sector, because of the decrease in thickness of the Gachsaran Formation, surface thrust faults rooting in 512

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Fig. 12. (a) Uninterpreted and (b) interpreted seismic profile of the Marun and Ahwaz anticlines. See Fig. 3 for location. Beneath the Gachsaran Formation, the Ahwaz Anticline has a rounded shape with its flanks cut by high angle reverse faults. Above these evaporites, a low angle thrust fault cuts a small anticline. Bk (Bakhtyari), Aj (Aghajari), Mn (Mishan), Gs (Gachsaran), As (Asmari), Sv (Sarvak), Dr (Dariyan) and Gr (Garau).

folds and their distribution in map view suggested that the Dezful Embayment is detached above an efficient basal detachment horizon (Vergés et al., 2011). In this study, the structural cross-section of Fig. 13 shows a structural style characterized by a low shortening value (15%), low tectonic wedge angle (< 1°), open symmetric to slightly asymmetric folds with a rounded geometry and formation of back-thrusts in the northern flank of some folds. Based on experimental modeling (Calabrò et al., 2002) and critical Coulomb wedge theory (Davis and Engelder, 1985; Costa and Vendeville, 2002), this structural style is similar to a fold-thrust belt formed above thick ductile units.

role of an intermediate detachment horizon (Fig. 13). But in the SW sector, only the Dashtak Formation acts as the intermediate detachment horizon that controlled the geometry of the overlying folds up to the Gachsaran Formation (Fig. 13), as also observed in the Fars zone (Motamedi et al., 2012; Najafi et al., 2014; Farzipour-Saein and Koyi, 2016). This interpretation is proved by changes of the fold wavelength along the structural cross-section. At the Asmari level, the folds of the SW sector generally have larger wavelength than those of the NE sector (excluding Kuh-e Asmari Anticline) (Fig. 16). As there is a direct relation between the fold wavelength and the depth of the detachment horizon (Sepehr et al., 2006; Casciello et al., 2009; Farzipour-Saein et al., 2009), beneath the Gachsaran evaporites, the folds of the SW sector were formed above a deeper detachment horizon compared to those in the NE sector. In both sectors, the folds above the intermediate detachment horizon with a rounded geometry and thrust faults with low displacement at the forelimb and footwall synclines have geometries similar to the symmetric and asymmetric faulted detachment folds (Mitra, 2002). But geometry of the study area folds show differences with models of the faulted detachment folds proposed by Mitra (2002). These models have steep to overturned forelimbs but, in this area the dip of forelimbs range from 20 to 40°. Main reason for such differences can be the presence of the massive and thick competent neritic limestones of the Asmari and Sarvak formations in the Dezful Embayment (Motiei, 1995), especially in the SW of the embayment (Abdollahie Fard et al., 2006), which facilitated development of thrust fault in the forelimb (Casciello et al., 2009; Farzipour-Saein et al., 2009; Vergés et al., 2011). The Kuh-e Asmari Anticline indicates a different geometry compared to the other folds of the study area. The southern flank of this anticline is interrupted by a thrust fault with a ramp-flat geometry (Fig. 17a). We interpret this geometry as a detachment fold affected by a roof-ramp thrust fault (Fig. 17b). The role of the deep Hormuz series as detachment horizon is still ambiguous in the study area due to low quality of seismic profiles at depth. For instance, some of the seismic profiles show uncertain deepseated structures (Fig. 11), but the relevant seismic image might be obscured due to multiple contamination or applying inappropriate time-domain seismic migration algorithms. However, the large scale

6.2. Effect of basement faults on structural geometry Normal and strike-slip faults were possibly developed during extensional phase around southern margin of the Neo-Tethys Ocean in Jurassic-Early Cretaceous time (Berberian and King, 1981; Sepehr and Cosgrove, 2004; Aqrawi et al., 2010). In the study area, the facies and thickness changes on both sides of the MFF (Fig. 13) can be attributed to this extensional phase. During Late Cretaceous time, the Zagros basin has been located in compressional setting (Alavi, 2004; Saura et al., 2015) and its sedimentation and deformation has been controlled by the MFF (Sepehr and Cosgrove, 2004). Earthquake data along the MFF propose a high-angle reverse fault (Berberian, 1995; Talebian and Jackson, 2004; Nissen et al., 2011; Allen et al., 2013). The structural cross-section demonstrates harmonic folding on the hangingwall of the MFF (Payoun Anticline) (Fig. 18a). Field data suggests that displacement along this fault was governing the deformation of this area since significant decoupling along main incompetent units was not observed. Seismic data and the structural cross-section show that an imbricated system formed across the MFF (Figs. 5 and 18a). A similar structure is also observed in the southern foothill of the Mongasht Anticline (see Fig. 4D in Sherkati et al., 2006). The formation of these structure across the MFF, as a steep inverted normal fault (Jackson, 1980; Berberian, 1995; Sepehr and Cosgrove, 2004; Letouzey and Sherkati, 2004), can be justified by the tectonic-inversion model presented by Coward (1994) and Bonini et al. (2012) (Fig. 18b). Therefore in the study area, the thrust fault on the southern flank of the Payun 513

Fig. 13. Balanced and restored structural cross-section on the northern part of the Dezful Embayment and the southern part of the Izeh zone. See Fig. 1 for location. For more details, see text.

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Fig. 14. Plot of surface faults on thickness map of the Gachsaran Formation in the north part of the Dezful Embayment. This map is created using well data and top and base maps of the Gachsaran Formation prepared by NIOC. Mountain Front Fault (MFF), Izeh Fault Zone (IFZ), High Zagros Fault (HZF), Zagros Deformation Front (ZDF). Payoun (1), Toukak (2), Kamarun (3), Pare Siah (4), Kuh-e Asmari (5), Naft-e Safid (6), Khandagh (7), Kupal (8), Marun (9), Ahwaz (10), Band-e Karkheh (11) and Aghajari (12) anticlines.

Fig. 15. Plot of the shape parameter (L) versus aspect ratio (R) (Srivastava and Lisle, 2004) of the Mishan and Asmari levels for folds of the study area.

Fig. 16. Diagram show changes of the fold wavelengths in the study area. Excluding the Kuh-e Asmari anticline, other anticlines of the NE sector have smaller wavelength then those in the SW sector. Pare Siah (PS), Kuh-e Asmari (KAS), Naft-e Safid (NS), Khandagh (KH), Kupal (KL), Marun (Mn) and Ahwaz (Az) anticlines.

Anticline is a main branch of the MFF and thrust faults in the southern flank of the Toukak and Pare Siah anticlines are the footwall shortcut thrusts of the MFF (Fig. 18a). In the NE sector, two earthquakes of (5 June 1977 at Ms 5.8 and 18 September 1985 at Mb 5.2) with thrust focal mechanisms at a depth of 12 km have been attributed to the activity of the Lahbari Thrust (Figs. 3 and 5) (Berberian, 1995). The seismic profiles indicate that the outcropping Lahbari Thrust is connected to the Kuh-e Asmari Anticline at its SW flank and rooted along the Garau-Dashtak detachment horizon (Figs. 5 and 13). In addition, another thrust fault is observed in the deeper part, the deep-rooted Labhari thrust, rooted at the Hormuz Series or its equivalent (Figs. 5 and 13). This fault has caused large displacement bringing that the Jurassic-Cretaceous strata on top of the Gachsaran Formation (Middle Miocene) (Figs. 5 and 13). It appears that the mentioned earthquakes should be associated with the activity of

this deeper fault. However, as the Kuh-e Asmari Anticline has a larger wavelength than other anticlines of the NE sector (Fig. 16), it can be interpreted as a basement-involved fold, as discussed above. In conclusion, the structural styles of the NW sector of the study area show evidence of both thin and thick-skinned deformation.

7. Conceptual model for evolution of the Kuh-e Asmari Anticline A conceptual model has been proposed for evolution of the Kuh-e Asmari Anticline in Fig. 19. In the early stages of deformation, a low amplitude and large wavelength fold was formed above the basal detachment horizon (Fig. 19a). As the shortening increased, the deep515

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Fig. 17. (a) Pop-up structure of the Kuh-e Asmari Anticline caused by movement of a thrust fault with the ramp-flat geometry and a back-thrust. See Fig. 5 for location. In this anticline similar fold is a result of activity of the Kalhur Member, Pabdeh and Gurpi formations. (b) Roof-ramp model for detachment folds presented by Suppe (2011). Bk (Bakhtyari), Aj (Aghajari), Mn (Mishan), Gs (Gachsaran), As (Asmari), Sv (Sarvak), Dr (Dariyan) and Gr (Garau).

Fig. 18. (a) Northeastern part of the structural cross-section of Fig. 13. The Payoun Anticline is a harmonic fold on the hangingwall of the MFF. The Payoun, Toukak and Pare Siah anticlines have formed an imbricated system. (b) Inversion model for primary normal fault with high-angle dip presented by Coward (1994). In this model, a part of the deformation is compensated as footwall shortcuts.

intermediate detachment horizon. The final geometry of the Kuh-e Asmari Anticline can be compared to a roof-ramp structure as described by Suppe (2011) (Fig. 19c). This geometry concentrated the strain on the back limb and led to formation of a back-thrust (Fig. 19d).

rooted Lahbari thrust (the southern fault in Figs. 5 and 13) affected the southern flank of the initial fold and the intermediate detachment horizons (Garau to Dashtak formations) were activated in form of large rabbit-ear folding on the northern flank of the main fold (Fig. 19b). Afterward, a thrust fault with a ramp-flat geometry originated from the 516

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Fig. 19. Conceptual model for evolution of the Kuh-e Asmari Anticline. (a) Initial fold. (b) Activation of the intermediate detachment horizon and formation of a large rabbit-ear fold. (c) Breaching forelimb of the initial fold by thrust fault originated from intermediate detachment horizon (d) Evolution of anticline in units above intermediate detachment horizon and formation of roof-ramp structure (Suppe, 2011). Due to increasing displacement on the thrust fault and formation of back-thrust.

References

8. Conclusion The conclusions of the current study are as follows:

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• In the study area, the thickness reduction of the Gachsaran eva• • •

• •

porites (as the upper detachment horizon) from the NE to the SW can be considered as one of main factors for southwestward reduction of the structural complexity. The upper detachment horizon caused versatile patterns, such as diapirs in section view, lift-off and bullet-shaped structures in map view. In the SW sector, thrusting diminished as the thickness of the detachment units of the Gachsaran Formation and the shortening decreased compared with the NE sector. Other detachment units, such as the Kalhur Member and Pabdeh, Gurpi and Garau formations acted as subordinate detachment horizons and increased the structural complexity in the NE sector of the study area. The Dashtak Formation may act as a major detachment horizon in the study area. The role of the Hormuz Series as a basal detachment horizon has been reported by many researchers, but is not obvious based on available data. The movement of the Mountain Front Fault formed large harmonic folding and imbricated system in the hangingwall and footwall, respectively. In addition, the deep-rooted Lahbari Thrust is introduced as a main element for generation of the Kuh-e Asmari Anticline in the northern part of the Dezful Embayment. The present day configuration of structures in the study area is largely dependent on the interaction of competent and incompetent units. At later stages of deformation, the growth-strata of the Miocene-Pliocene deposits have also had a major role on the geometry of the structures.

Acknowledgment The authors thank the Exploration Directorate of the National Iranian Oil Company (NIOC) for providing data and facilities including interpretation and field trips.

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