Structural style and kinematic analysis of folding in the southern Dezful Embayment oilfields, SW Iran

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Author statement Masoumeh Vatandoust: Original draft preparation, Visualization Methodology, Software, Writing, Reviewing and Editing, Ali Faghih: Supervision, Conceptualization, Methodology, Software, Writing, Reviewing and Editing, Caroline M. Burberry: Validation, Reviewing and Editing Ghodratollah Shafiei : Data providing, Reviewing

ϭ

Structural style and kinematic analysis of folding in the Southern Dezful

Ϯ

Embayment oilfields, SW Iran

ϯ

Masoumeh Vatandoust1, Ali Faghih*1, Caroline M. Burberry2, Ghodratollah Shafiei3

ϰ

1

ϱ 2

ϲ

Department of Earth sciences, College of Sciences, Shiraz University, Shiraz, Iran

Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, United States 3

ϳ

National Iranian South oil Company (NISOC), Ahvaz, Iran

ϴ ϵ

Abstract

ϭϬ

Compression of a region containing multiple detachment layers produces large-scale

ϭϭ

disharmonic folding, where individual competent units separated by detachment fold

ϭϮ

independently within the succession. Here, we present a case study of the kinematic evolution of

ϭϯ

three subsurface structures (the Karanj, Paranj and Parsi oilfields) bounded by multiple

ϭϰ

detachments, located in the southern Dezful Embayment, a major petroleum province of the

ϭϱ

Zagros Fold-Thrust Belt in SW Iran. Interpretation of seismic sections and the construction of

ϭϲ

corresponding balanced cross-sections reveals different geometries along the folds and the

ϭϳ

complete decoupling of these structures from the surface geology. This is controlled by the

ϭϴ

reactivation of basement faults, and flow of the Gachsaran incompetent units (Miocene) as the

ϭϵ

upper detachment, and the Kazhdumi (Upper Cretaceous),Pabdeh-Gurpi and Dashtak Formations

ϮϬ

as the middle detachments, respectively. Detachment folding with limb rotation started in the

Ϯϭ

Middle Miocene and continued with the migration of the basal detachment horizon (equivalent

ϮϮ

Hormuz series) into the core of the anticlines.. Flow of both the basal and upper detachments

Ϯϯ

continued until no more material is available, whereupon the shortening is accommodated by

ϭ 

Ϯϰ

fault development. The development of a decoupled, disharmonic folding style was controlled by

Ϯϱ

the upper detachment surface through migration from the crests of the anticlines to the synclinal

Ϯϲ

areas and by the reactivation of the basement faults. from NW to SE part of the area The total

Ϯϳ

shortening amounts are changed between 21% to 12% .

Ϯϴ Ϯϵ

Keywords: folding, kinematics, oilfields, Dezful embayment, Zagros, Iran

ϯϬ ϯϭ

1. Introduction

ϯϮ

Fault-related folding plays a crucial role in the formation of structural traps in fold-thrust belts

ϯϯ

(Mitra, 1990; Kent and Dasgupta, 2004; Brandes and Tanner, 2014) as well as in the Dezful

ϯϰ

Embayment. The Dezful Embayment is one of the structural regions of the Zagros Fold-Thrust

ϯϱ

Belt (ZFTB; Fig. 1). The ZFTB is located in the central part of the Alpine-Himalayan orogenic

ϯϲ

system and extends for about 1800 km from SE Turkey to the straits of Hormuz, SW Iran (Alavi,

ϯϳ

2007; Fig.1). The belt formed due to oblique convergence between the Afro-Arabian and

ϯϴ

Eurasian lithospheric plates started in the Late Cretaceous (Takin, 1972; Stocklin, 1974;

ϯϵ

Berberian and King 1981; Koyi, 1988; Alavi, 2007; Pireh et al., 2015) and hence, resulted in

ϰϬ

significant crustal shortening (65–78 km (16–19%) )( Mouthereau et al., 2012) , faulting and

ϰϭ

folding of the cover and basement (Berberian, 1995; Hessami et al., 2001; Blanc et al., 2003;

ϰϮ

Nilforoushan et al., 2003; McQuarrie, 2004; Sepehr and Cosgrove, 2005; Alavi, 2007;

ϰϯ

Mouthereau et al., 2007).

ϰϰ

Large fault-related folds accommodate the main hydrocarbon oilfields in the Dezful Embayment

ϰϱ

(Allen, 2010; Verges et al., 2011; Mukherjee, 2014; Najafi et al., 2018). However, within these

ϰϲ

large folds, several detachment horizons including incompetent layers of shale and evaporites Ϯ 

ϰϳ

presumably control sub-surface deformation (Sherkati et al. 2006; Fig. 2) but the exact

ϰϴ

geometries of the folded layers remain indeterminate.

ϰϵ

Investigations of the structural style and kinematic evolution of faulting and folding can have far-

ϱϬ

reaching implications in oil and gas exploration and developments (Alipoor et al., 2019͖

ϱϭ

Vatandoust and Farzipour saein, 2017, 2019). In this regard, this study aims to determine the

ϱϮ

impact of different parameters (e.g. mechanical stratigraphy, basement faults, folding

ϱϯ

mechanism) on the structural and kinematic evolution of folds in the Dezful Embayment. To do

ϱϰ

so, we document the evolution and development of the three subsurface oilfields, the Karanj,

ϱϱ

Paranj and Parsi oilfields in the southern Dezful Embayment in four steps , viz.,1. seismic

ϱϲ

interpretation and construction of structural cross- sections, 2. calculation of the depth to the

ϱϳ

basal detachment which is involved in the folding, 3. analysis of the effect of the Izeh- Hendijan

ϱϴ

basement fault (IZHF) on the geometry of the anticlines, and 4. restoration and decompaction of

ϱϵ

the structural cross section in order to determining the kinematic evolution of folding by

ϲϬ

amplification mechanism.

ϲϭ ϲϮ

2.Geological setting

ϲϯ

This study was conducted in the Zagros Fold-Thrust Belt, which is located in SW Iran and

ϲϰ

formed after the closure of the Neo-Tethys Sea Miocene times (Alavi,, 2007, Pireh et al., 2015)

ϲϱ

in the Alpine- Himalayan orogenic system (Falcon, 1969; Frizon de Lamotte et al., 2011;

ϲϲ

Mouthereau et al., 2012). Ophiolite obduction and convergence began simultaneously in the Late

ϲϳ

Cretaceous (Agard et al., 2005; Saura et al., 2011) and were followed by a main folding phase in

ϲϴ

the Late Miocene forming the ZFTB (Homke et al., 2004; Emami, 2008; Fakhari et al., 2008;

ϲϵ

Khadivi et al., 2010).

ϯ 

ϳϬ

The ZFTB consists of three structural and stratigraphic areas (i.e., the foredeep, the Simply

ϳϭ

Folded Zone, and the Imbricate Zone), which range from its southwest to northeast parts.

ϳϮ

(Stocklin, 1968; Falcon, 1974; Berberian, 1995; Sepehr and Cosgrove, 2004; Fig. 1). Based on

ϳϯ

the along-strike changes in the structure and position of its deformation fronts and stratigraphy,

ϳϰ

the ZFTB has been divided into the Fars and Izeh zones, Dezful Embayment, Lurestan zone and

ϳϱ

Kirkuk Embayment (Stocklin, 1968; Falcon, 1974; Motiei, 1995; Sherkati and Letouzey, 2004;

ϳϲ

Lacombe et al., 2006; Casciello et al., 2009; Mouthereau et al., 2012; Fig. 1).

ϳϳ

The Dezful Embayment (Fig. 1) is a structural depression and a Late Cenozoic depocenter that

ϳϴ

developed as a result of uplift in the Zagros Fold and Thrust Belt near the Izeh folded zone in the

ϳϵ

northeast of the Mountain Front Fault (MFF) and consistent with sudden subsidence of that area

ϴϬ

(Falcon, 1974; Berberian, 1995; Sherkati et al., 2006; Van Buchem et al., 2006; Allen and

ϴϭ

Talebian, 2011, Saura et al., 2015; Pirouz et al., 2017). The deposits of the Dezful Embayment

ϴϮ

deformed progressively in an in-sequence manner (see similar concept reviewed in Mukherjee

ϴϯ

(2015) in collisional perspective)."

ϴϰ

That is why the sedimentary deposits of this region were folded and consequently, the foredeep

ϴϱ

basin migrated to the Persian Gulf (Hessami et al., 2001; Abdollahie Fard et al., 2006; Pirouz et

ϴϲ

al., 2011). The Dezful Embayment is surrounded by the Zagros Foredeep Fault (ZFF) to the

ϴϳ

southwest, the Mountain Front Fault (MFF) to the northeast and the Izeh- Hendijan Fault (IZHF)

ϴϴ

zone to the southeast, the Balarud Fault Zone (BFZ) to the west, and the Kazerun Fault Zone

ϴϵ

(KFZ) to the east (Fig. 1) (Alavi, 1994; Berberian, 1995; Sepehr and Cosgrove, 2005; Abdollahie

ϵϬ

Fard et al., 2006, Allen and Talebian, 2011). The Dezful Embayment is one of the most

ϵϭ

productive petroleum provinces worldwide which houses more than 45 large anticlinal oil traps

ϰ 

ϵϮ

(Bordenave and Hegre, 2005; Rabbani et al., 2010) such as the Karanj, Paranj and Parsi

ϵϯ

anticlines in the southern part of the Embayment which was chosen as the study area (Fig. 1).

ϵϰ

The stratigraphic column of the Dezful Embayment contains a series of competent and

ϵϱ

incompetent rock units varying in age from Upper Cretaceous to Neogene (Fig. 2). The

ϵϲ

competent units are divided into two groups as the lower competent units (the Bangestan Group

ϵϳ

(carbonate) and the Asmari Formation (carbonate)) and the upper competent units (the Aghajari

ϵϴ

Formation (sandstone) and the Bakhtiari Formation (conglomerates)). The competent units are

ϵϵ

detached by the incompetent units including the Kazhdumi and Pabdeh-Gurpi shales as

ϭϬϬ

intermediate detachments, and the Gachsaran evaporates, as upper main detachment (Fig. 2). The

ϭϬϭ

position of such intermediate incompetent layers is an important factor controlling both structural

ϭϬϮ

style and fold wavelength (Sherkati et al., 2005; Ruh et al., 2014). The competent strata are

ϭϬϯ

influenced by a series of anticlines and synclines associated with thrusts and reverse faults with

ϭϬϰ

NW-SE strikes within the Dezful Embayment (Fig. 3) (Sherkati and Letouzey, 2004; Allen and

ϭϬϱ

Talebian, 2011).

ϭϬϲ ϭϬϳ

3. Data and methods

ϭϬϴ

3.1. Seismic interpretation

ϭϬϵ

In order to better understand and visualize the structural analysis and kinematic evolution of the

ϭϭϬ

Karanj, Paranj and Parsi anticlines, several depth converted seismic sections (AA', BB', CC' and

ϭϭϭ

DD') oriented NE-SW (Fig. 3) and (EE’ FF’, GG’) oriented NW-SE were interpreted in Petrel

ϭϭϮ

(2013) software using well and check shots data came from National Iranian South Oil Company

ϭϭϯ

data bank. The maximum depth of the seismic data is ~6500m.

ϭϭϰ

3.2. Estimating the depth to basal detachment ϱ 

ϭϭϱ

Most of our information about the subsurface structures comes from the interpretation of the

ϭϭϲ

seismic reflection (e.g., Misra and Mukherjee, 2018). Although in most situations, the near-

ϭϭϳ

surface geometry of the structures can easily be interpreted by using the seismic-reflection data,

ϭϭϴ

the interpretation of the roots of these structures (e.g. depth of basal detachments) is almost

ϭϭϵ

impossible due to the deep influence of various factors including inhomogeneities of rocks in

ϭϮϬ

different directions (e.g., Mukherjee, 2017) repetition of stratigraphic layers affected by thrusting

ϭϮϭ

and structural and lithological boundaries with steep dip that causes subsurface complicacy

ϭϮϮ

(Carboni et al., 2019).

ϭϮϯ

Since determining the depth to detachment is one of the requirements for constructing, balancing

ϭϮϰ

and restoring cross sections across fold and thrust belts or extensional terranes (Bulnes and

ϭϮϱ

Poblet, 1999) several methods have been developed to determine the depth to detachments in

ϭϮϲ

seismic cross sections (Chamberlin, 1910; Epard and Groshong, 1993; Groshong, 1994, 2015;

ϭϮϳ

Groshong et al., 2012; Schlische et al., 2014).

ϭϮϴ

In this study to estimate the detachment depth beneath the studied anticlines, the ADS method

ϭϮϵ

and the best-fit detachment depth graph technique have been used. The first method consists of

ϭϯϬ

plotting the excess area of each unit above multiple regional levels vs the depth of these levels

ϭϯϭ

(Chamberlin, 1910; Epard and Groshong, 1993; Groshong and Epard, 1994; Groshong, 2015;

ϭϯϮ

Wang et al., 2017, Carboni et al., 2019), and the best-fit detachment depth graph technique

ϭϯϯ

consists of estimating the detachment depth for several folded horizons using the Chamberlin

ϭϯϰ

(1910) and Bulnes and Poblet (1999) methods. Using these methods the detachment depths

ϭϯϱ

(excess area divided by shortening) for different horizons and cumulative stratigraphic

ϭϯϲ

thicknesses are calculated and then the results are plotted. Eventually, the intersection between

ϲ 

ϭϯϳ

the best-fit line through the plotted points and the y-axis shows the position of the detachment

ϭϯϴ

surface within the stratigraphic section.

ϭϯϵ

3.3. Restoration and decompaction

ϭϰϬ

Several balanced and restored cross sections (AA', BB', CC' and DD') were constructed after the

ϭϰϭ

interpretation of the seismic sections (Fig. 4) In general, the methods and procedures to be used

ϭϰϮ

in a restoration will mostly be chosen by its objectives. In the case of structural analysis and

ϭϰϯ

shortening amount determination, the kink method and flexural-slip folding mechanism

ϭϰϰ

algorithm were applied in constructing the cross sections in the unfolding modules of the Move

ϭϰϱ

2D (2016) software. To determine the kinematic evolution of the structures utilizing the

ϭϰϲ

amplification mechanism, successive cross-section restorations to the top of various stratigraphic

ϭϰϳ

layers have been performed using the 2D decompaction algorithms executed in the Move 2D

ϭϰϴ

(2016) software (Midland Valley). During restoration using this algorithm, the structures were

ϭϰϵ

put back to the undeformed state through removing the effects of faulting, folding and volume

ϭϱϬ

loss resulting from compaction and erosion with a kinematic algorithm assuming a flexural-slip

ϭϱϭ

folding mechanism. Assumption of the flexural-slip folding mechanism is based on the previous

ϭϱϮ

reports in the study area (Sherkati and Letouzey, 2004; Carruba et al., 2006; Khodabakhshnejad

ϭϱϯ

et al., 2015). In this study, sequential restorations were carried out on seismic section AA’,

ϭϱϰ

starting by removing effects of the most recent tectonic and sedimentary events. In this example,

ϭϱϱ

this is an ~2104 m (Vatandoust et al., 2020) erosion of the Aghajari and Bakhtiari Formations

ϭϱϲ

from 2Ma (Vatandoust et al., 2020) and requires reconstruction of eroded stratigraphy. As part of

ϭϱϳ

the cross-section preparation, the eroded parts of the Aghajari and Bakhtiari Formations have

ϭϱϴ

been reconstructed using the Horizons from Template tool in the Move 2D (2016) software. One

ϭϱϵ

of the assumptions of the decompaction method is that the initial porosity of the sediments is

ϳ 

ϭϲϬ

reduced by increasing burial depth (Schmoker and Halley, 1982; Mukherjee, 2018a, b; Dasgupta

ϭϲϭ

and Mukherjee, 2019).

ϭϲϮ

In this method, the exponential porosity/depth Athy's relation Ø=Øೊ exp-cy has been used, where

ϭϲϯ

Ø, Øೊ and c are the porosity at depth y, porosity at the surface and an empirically derived

ϭϲϰ

lithologically dependent coefficient, respectively (Sclater and Christie, 1980; Mukherjee, 2018c).

ϭϲϱ

According to Berra and Carminati (2010) the standard amount of Øೊ and c of various lithologies

ϭϲϲ

have been taken from the literature (Table 1).

ϭϲϳ ϭϲϴ

4. Results

ϭϲϵ

4.1 Structural features of the studied anticline

ϭϳϬ

4.1.1 Structural cross sections

ϭϳϭ

In this section, the fold structures and their geometrical characteristics are described based on

ϭϳϮ

the interpretation of the 2D seismic images (Figs. 5 and 6). The horizons identified and

ϭϳϯ

interpreted in the seismic sections across the studied anticlines range from the top of the Khami

ϭϳϰ

Group and the Kazhdumi Formation to Recent sediments (Figs. 5 and 6).

ϭϳϱ

The Karanj anticline according to the cross sections with interlimb angle (ࠧ) = 69ͼ ሺƍሻǡ ͺͷͼ

ϭϳϲ

ሺƍሻǡ͸ͺͼሺCCƍሻǡ96ͼ (DDƍ) is classified as an open anticline (Fleuty, 1964) and according to the

ϭϳϳ

classification of folded layers by Ramsay (1967), is a class 1B fold (Fig.7). The key formation

ϭϳϴ

for determining the fold order was Asmari Formation.

ϭϳϵ

The Paranj anticline has different axial surfaces and king fold shape in the section AAƍ and its

ϭϴϬ

shape turned gradually to the rouanded shape towards the SE part.

ϭϴϭ

The Parsi anticline with ࠧ = 78ͼሺƍሻǡ ͹ͺͼሺƍሻǡ 81.5ͼሺCCƍሻ ƒ† 80ͼሺƍሻ is classified as an

ϭϴϮ

open anticline based on the degree of interlimb angle(Fleuty, 1964). Based on the Ramsay

ϭϴϯ

classification (Ramsay, 1967), the Parsi anticline is also classified as a class 1B fold. Crestal ϴ 

ϭϴϰ

extensional fractures occur as main structural features in the Parsi anticline in the cross section

ϭϴϱ

BB’ in the SE part of the anticline and continue from the top Asmari Formation to the top Gurpi

ϭϴϲ

Formation (Fig. 5b).

ϭϴϳ

In the cross section AAƍ (Fig. 8) the presence of wide anticlines and tight synclines in the upper

ϭϴϴ

units and wide synclines and tight anticlines in the lower units of the competent units located

ϭϴϵ

between two main detachment surfaces (Gachsaran and Dashtak formations) above and below

ϭϵϬ

suggest concentric detachment folding according to Dahlstrom (1969) and Sherkati (2005). Also,

ϭϵϭ

the operation of a minor detachment level above the Ilam- Sarvak Formations originating from

ϭϵϮ

Pabdeh-Gurpi Formations have caused the formation of rabbit ear structures (Sherkati, 2004,

ϭϵϯ

2006) in the left limb of the Paranj anticline in this section (Fig. 8). The variable thickness (~ 0-4

ϭϵϰ

km) Gachsaran Formation separates the Karanj, Paranj and Parsi anticlines below from a

ϭϵϱ

syncline in the overlying Mishan and Aghajari formations.

ϭϵϲ

The best-fit function for the data plotted on the detachment depth against cumulative

ϭϵϳ

stratigraphic thickness graph intersects the cumulative thickness axis at the depth of 10 km (Fig.

ϭϵϴ

9a). According to the work of Sherkati et al. (2005, 2006) on Dezful Embayment this depth

ϭϵϵ

corresponds to the depth of the Paleozoic sequences. Furthermore, the best-fit function for the

ϮϬϬ

data plotted on the excess area against depth graph for the horizons intersects the depth axis

ϮϬϭ

(detachment depth) between 11-12 km depth corresponding to the base of Lower Paleozoic

ϮϬϮ

sequence (Top of the Hormoz equivalent as a basal detachment in the study area) (Fig 9b). Since

ϮϬϯ

the maximum depth to which seismic profiles are shown in the study area is ~ 6 km, we were not

ϮϬϰ

able to display the basal detachment on the profiles and cross sections.

ϮϬϱ

According to the Jamison graphs (Jamison 1987) (Fig. 10), the forelimb of folds became

ϮϬϲ

thickened (~10%) and anticlines become tighter from the NW to the SE. The total shortening

ϵ 

ϮϬϳ

amounts from NW to SE part of the area (from AAƍ to DDƍ) are changed between 21% in section

ϮϬϴ

AAƍ and 15% in sections BBƍ, CCƍ and 12% in section DDƍ which was accommodated by the

ϮϬϵ

thrusts. The shortening changes may be due to the changes of slip amount at the fault surface.

ϮϭϬ

The amount of slip decreases from the core of fault which is generally common at the thrust

Ϯϭϭ

faults (Maerten et al., 2002; Mueller, 2017).

ϮϭϮ Ϯϭϯ

4.1.2 Along-strike variation in the studied oilfields

Ϯϭϰ

According to the interpretation of the 2D seismic sections and balanced cross sections of the

Ϯϭϱ

Karanj, Paranj and Parsi oilfields within the Dezful Embayment, SW Iran, the geometry of the

Ϯϭϲ

anticlines varies from NW to SE of the study area, that is, along strike of the anticlines. From

Ϯϭϳ

NW to SE, the Karanj anticline appears as a popup structure in the NW to the central part of the

Ϯϭϴ

study area. In the SE part, the Karanj anticline does not display the same backthrust and popup

Ϯϭϵ

geometry.

ϮϮϬ

According to the Jamison graphs (Jamison 1987) (Fig. 7), the forelimb of folds became

ϮϮϭ

thickened (~10%) and anticlines become tighter from the NW to the SE. A 3D structural model

ϮϮϮ

constructed from the balanced cross-sections provides better insights on the structural style of the

ϮϮϯ

anticlines (Fig. 11a). The anticlines become tighter with interlimb angle (ࠧ) varies ~100°-70°

ϮϮϰ

from the NW to the SE (Fig. 10). The axis of the anticlines has a curved shape in particular

ϮϮϱ

along the Karanj anticline, and the highest amount of deformation has occurred in the central part

ϮϮϲ

of the structure which causes the structural culmination in this region (Fig. 11a). This may be

ϮϮϳ

due to the different amount of displecment along fault surface, which may also explain the

ϮϮϴ

structural culmination in this region (Fig. 11a). The Paranj anticline has a different attitude of the

ϮϮϵ

axial plane (such as dip and number of axial planes) from NW to the central part. In the NW part,

ϮϯϬ

four axial planes can be recognized. This part shows box fold shape on seismic section (Fig. 11a) ϭϬ 

Ϯϯϭ

and the existence of two thrust faults with opposite vergence between the Karanj/Paranj and

ϮϯϮ

Paranj/Parsi anticlines may have subsided the Paranj anticline (Fig. 5). Furthermore, two oblique

Ϯϯϯ

reverse faults are extended in the Paranj anticline from central to the SE part of the area (Fig.

Ϯϯϰ

11a) and in SE part they created new structural closure near to the Karanj anticline (Fig. 11a).

Ϯϯϱ Ϯϯϲ

4.1.3. Basement-involved shortening

Ϯϯϳ

There are many tear fault systems in the central part of the ZFTB, which have often been seen as

Ϯϯϴ

inherited basement-level faults (e.g., Falcon, 1969; Barzegar, 1994; Berberian, 1995; Talbot and

Ϯϯϵ

Alavi, 1996; Hessami et al., 2001a; Bahroudi and Talbot, 2003; Authemayou et al., 2005;

ϮϰϬ

Mouthereau et al., 2006, 2007; Alavi, 2007). The High Zagros Fault (HZF), the Mountain Front

Ϯϰϭ

Fault (MFF) and the Zagros Foredeep Fault (ZFF) are in the first group of basement faults (Fig.

ϮϰϮ

1). The N-S trending basement faults and the Izeh-Hendijan Fault (IZHF), the Kharg-Mish Fault

Ϯϰϯ

(KMF), and the Kazerun Fault(KZF) formed in the latest Proterozoic and early Cambrian in the

Ϯϰϰ

Arabian basement (Beydoun, 1991) and the Triassic and Late Cretaceous, respectively.

Ϯϰϱ

So far, the existence and orientation of the IZHF has been investigated in the neighboring

Ϯϰϲ

anticlines of the study area (e.g. Falcon, 1969; Sherkati and Letouzey, 2004; Ahmadhadi et al.,

Ϯϰϳ

2008; Vatandoust and Farzipoure Saein, 2017) (Fig. 1).

Ϯϰϴ

Based on the specific shape of the Karanj anticline in the seismic profile (see Fig 3 for location)

Ϯϰϵ

the partial coupling of the sedimentary cover (competent strata) and the basement (Fig. 12a) in

ϮϱϬ

the Karanj anticline indicate the position of the IZHF basement fault. This is compared with the

Ϯϱϭ

Jackson and Hudec (2017) model, which has defined the coupling between basement and cover

ϮϱϮ

as a mechanical connectivity between basement faulting under and overburden faulting above the

Ϯϱϯ

detachment. Fully coupled faults have a strong penetration throughout the salt layer and

ϭϭ 

Ϯϱϰ

topography mimics basement relief (Fig. 12b). The most usual outline is partial coupling,

Ϯϱϱ

wherein the basement fault does not penetrate the salt and is overlain by a monocline draped over

Ϯϱϲ

the master basement fault (Fig. 12c). Decoupling ensues because the salt flows to fill in the relief

Ϯϱϳ

generated by basement faulting (Fig. 12d) and is promoted by small basement-fault offset, slow

Ϯϱϴ

faulting, and thick or low-viscosity evaporites. Therefore, it can be concluded that the monocline

Ϯϱϵ

in the Karanj anticline can mark the position of the IZHF and the increasing deformation from

ϮϲϬ

central to the SE part of the studied anticlines (figs.10 and 11) could be affected by the existence

Ϯϲϭ

and reactivation of this basement fault (Fig. 1).

ϮϲϮ Ϯϲϯ

4.2 Kinematic evolution of the study area

Ϯϲϰ

4.2.1. Amplification mechanism

Ϯϲϱ

In order to analyze the kinematic evolution of the anticlines a cross-section (Fig. 5a) from the

Ϯϲϲ

studied anticlines was restored by parallel kink method because dip of layers in folds changes at

Ϯϲϳ

small distance as previously done in the Dezful Embayment by Sarkarinejad et al. (2017).

Ϯϲϴ

Restoration done to determine the timing of fold development and to measure the required

Ϯϲϵ

parameters for the amplification mechanism analysis (Butler and Lickorish, 1997; Poblet et al.,

ϮϳϬ

2004, Valero et al., 2015). Restoration has been applied by the “flattening on horizon” method

Ϯϳϭ

using Move (2013) software (Petex). Figure 13 shows the results of the decompaction of the

ϮϳϮ

seismic section AAƍ. The folding of the Asmari and older formations and flow of the Gachsaran

Ϯϳϯ

incompetent units, as a result of folding, were started at the time of deposition of the Gachsaran

Ϯϳϰ

Formation in middle Miocene time. To analyze the amplification mechanisms responsible for the

Ϯϳϱ

development of the Karanj, Paranj and Parsi anticlines, an indirect technique has been employed

Ϯϳϲ

(Butler and Lickorish, 1997; Poblet et al., 2004, Valero et al., 2015). Following Poblet and

ϭϮ 

Ϯϳϳ

McClay (1996), four different geometric and kinematic models may be used to evaluate the

Ϯϳϴ

kinematics of individual folds with a detachment mechanism in which a homogeneous competent

Ϯϳϵ

unit detached over a ductile unit. In Model 1 (Mitchell and Woodward, 1988) the dip of the fold

ϮϴϬ

limb is kept constant and limb lengthening results in fold amplification. In Model 2 (De Sitter,

Ϯϴϭ

1956) limb length is constant and limb rotation is the main mechanism for anticline growth. The

ϮϴϮ

basis of Model 3 (Dahlstrom, 1990) is the law of conservation of area for both the competent and

Ϯϴϯ

the ductile unit, and both mechanisms of limb rotation and limb lengthening are responsible for

Ϯϴϰ

shortening. Model 4 (Blay et al., 1977) indicates that both limb rotation and limb lengthening

Ϯϴϱ

accommodate fold amplification, but the point of intersection of the axial surface is fixed such

Ϯϴϲ

that it occurs on the detachment surface (Fig. 14a).

Ϯϴϳ

In this technique, at first the shortening, forelimb dip, axial plane dip and interlimb angle of the

Ϯϴϴ

anticlines in the present-day deformed section (Fig. 13) and in each of the stepwise restored

Ϯϴϵ

sections are measured. The results are plotted versus shortening, and the best-fit functions of the

ϮϵϬ

plotted data constructed (Fig. 14b).

Ϯϵϭ

Comparing the shape of the best-fit functions with functions for the same parameters obtained

ϮϵϮ

from forward models of simple folds resulting from horizontal compression of Poblet and

Ϯϵϯ

McClay (1996), demonstrate that the kinematic evolution of the Karanj, Paranj and Parsi

Ϯϵϰ

anticlines are in accordance with Model 2 (De Sitter, 1990) (Fig. 14a). This model is

Ϯϵϱ

characterized by fold limbs with variable dip and constant length and the mechanism of fold

Ϯϵϲ

growth is purely limb rotation (Fig. 14a).

Ϯϵϳ Ϯϵϴ

4.2.2. Geometry of syn-tectonic sediments

ϭϯ 

Ϯϵϵ

In the field of fault-related folding, kink-band migration (active-hinge folding) and progressive

ϯϬϬ

limb rotation (fixed-hinge folding) are two important mechanisms of folding (Shaw et al., 2005).

ϯϬϭ

The geometry of syn-tectonic sediments are affected by these two folding mechanisms. During

ϯϬϮ

folding by kink-band migration, material moves across the axial surfaces and therefore the width

ϯϬϯ

of the limbs increase, but the dip is kept constant (Suppe et al., 1992). This can be clearly

ϯϬϰ

observed in the case of fault-bend folding (Fig. 15a). The axial surfaces in the pre-growth strata

ϯϬϱ

(Asmari, Pabdeh, Gurpi and Ilam-Sarvak formations) are parallel.

ϯϬϲ

Migration of material through the axial surfaces progressively extend the rock volume. At the

ϯϬϳ

end of the folding process in the fault-bend fold type, deformation is distributed along the fold

ϯϬϴ

limb (Salvini and Storti, 2004). Once the syn-tectonic sedimentation exceeds the uplift of the

ϯϬϵ

growing anticline, a narrowing kink band with converging axial surfaces develop (Suppe et al.

ϯϭϬ

1992; Shaw et al., 2005). During folding via the limb rotation mechanism, the limb dip

ϯϭϭ

progressively increases (Fig. 15a; Poblet and McClay, 1996; Shaw et al., 2005). In this case, the

ϯϭϮ

rock volume within the axial surfaces is not varied during folding. In the case of limb rotation,

ϯϭϯ

the fold has a narrow, strongly deformed area around the axial surface. The deformation

ϯϭϰ

decreases from the axial surface area towards the limbs (Salvini and Storti, 2004). This type of

ϯϭϱ

folding mechanism is an important for the development of

ϯϭϲ

McClay1996). In the case of Karanj and Parsi anticlines the thickening and fanning pattern of the

ϯϭϳ

members 5- 7 of the Gachsaran Formation (Abdolahie Fard et al., 2011)can be observed towards

ϯϭϴ

the crest of the anticline (Fig. 15b). The fanning pattern of growth strata reveals the start of

ϯϭϵ

folding simultaneous with deposition of Gachsaran Formation and a progressive limb rotation

ϯϮϬ

mechanism of folding (Shaw et al., 2005) (Fig. 15a), as described by the amplification

ϯϮϭ

mechanism method (Figs 13, 14). With continuing folding members 1-4, the major incompetent

ϭϰ 

detachment folds (Poblet and

ϯϮϮ

units within the Gachsaran Formation began to migrate simultaneously with the deposition

ϯϮϯ

(Fig.15b).

ϯϮϰ ϯϮϱ

5. Discussions

ϯϮϲ

Detachment folds are formed due to shortening of rock assemblages above a detachment surface,

ϯϮϳ

a low-angle fault, which may be (sub) parallel to a sedimentary horizon. (Poblet et al., 1997;

ϯϮϴ

Rowan, 1997; Peacock et al., 2000; Scharer et al., 2014). This type of folding is developed in

ϯϮϵ

competent rocks cored by enough mobile material to fill the developing structure (Homza and

ϯϯϬ

Wallace, 1995; Stewart, 1996). In the absence of incompetent and mobile material to fill the

ϯϯϭ

amplifying fold, the folding process stops and the shortening may be compensated by faulting

ϯϯϮ

(Stewart, 1996).

ϯϯϯ

The results of this work revealed that the subsurface Karanj, Paranj and Parsi anticlines, located

ϯϯϰ

very close to the Mountain Front Fault formed on the equivalent Hormoz series as a basal

ϯϯϱ

detachment and Dashtak or Pabdeh-Gurpi formations as intermediate detachments. These

ϯϯϲ

detachments have been previously introduced in other parts of the Dezful Embayment (Sherkati

ϯϯϳ

et al., 2005; Jahani et al., 2009). The presence of two main detachment levels (Gachsaran and

ϯϯϴ

equivalent Hormuz series) above and below the competent units caused the concentric folding

ϯϯϵ

according to Dahlstrom (1969).

ϯϰϬ

5.1. Kinematic evolution 

ϯϰϭ

The presence of growth strata in the upper Gachsaran Formation can be observed towards the

ϯϰϮ

anticline that indicates the start of folding simultaneously with the deposition of the Gachsaran

ϯϰϯ

Formation in Middle Miocene time synchronous with the Zagros orogeny as previously

ϯϰϰ

mentioned by Sherkati and Letouzey (2005) in the other parts of the Dezful Embayment. This

ϭϱ 

ϯϰϱ

time has already proven using magnetostratigraphic, sedimentology and low-temperature

ϯϰϲ

thermochronometry of Miocene detrial sediments ~ 19.7- 14.8 Ma (Khadivi, 2010).

ϯϰϳ

In the earlier stages of the folding process, the migration of the incompetent formations toward

ϯϰϴ

the core of anticlines produced detachment fold. The folding was followed by a change in

ϯϰϵ

structural style due to progressive deformation and increasing shortening. Limb rotation and/or

ϯϱϬ

hing migration took place during folding (Poblet and McClay, 1996). In the case of the Karanj

ϯϱϭ

and Parsi anticlines the pattern of Gachsaran Formation evolution reflects the progressive limb

ϯϱϮ

rotation resulting from the origin of these symmetric anticlines (Fig. 15).

ϯϱϯ

In all cases, the underlying detachment horizons flow into the core of the fold until no more

ϯϱϰ

material is available (Sherkati and Letouzey, 2004; Sherkati et al., 2005) whereupon the

ϯϱϱ

shortening is accommodated by fault development. The upper detachment gradually thickens

ϯϱϲ

above the shortening competent layers and flows into the synclines that are first formed between

ϯϱϳ

the symmetric anticlines. As shortening is then accommodated by faulting, and the synclines are

ϯϱϴ

overridden by the faulted anticline limbs, the overlying detachment horizon thickens and the

ϯϱϵ

layers above, the Mishan and the Aghajari Formation, may then in turn be folded as the Zagros

ϯϲϬ

Orogeny progresses and the upward ductile flow of members 1-4 of the Gachsaran Formation

ϯϲϭ

with continued shortening. These processes leads to the propagation of a thrust fault in the upper

ϯϲϮ

competent strata and finally leads to outcropping of the Gachsaran Formation on the Earth

ϯϲϯ

surface (figs.13 and 16). This suggests that folding of compentent layers between detachments in

ϯϲϰ

a prolonged orogenic event may be a pulsed or long-lived event that propagates up through the

ϯϲϱ

sedimentary succession as older layers fold, forcing deformation in overlying, mobile, younger

ϯϲϲ

layers.

ϭϲ 

ϯϲϳ

Faulting of fold limbs and the development of faulted detachment folds usually occurs due to

ϯϲϴ

high strains on the fold limbs during limb rotation (Mitra, 2002b)͘So, as it occurred in the Karanj

ϯϲϵ

Anticline, if the two limbs are symmetrical and undergo approximately the same amount of

ϯϳϬ

shortening, the propagation of faults through steep limb segments occurs simultaneously on both

ϯϳϭ

limbs of the anticline. This results into a symmetrical pop-up structure bounded by two faults

ϯϳϮ

dipping toward each other (Fig. 17a). According to the model of detachment folding presented

ϯϳϯ

by Mitra (2002), finally, one of these two faults connects with the basal detachment and controls

ϯϳϰ

future asymmetric growth of the structure, whereas the other fault terminates against the main

ϯϳϱ

fault (Fig. 17a).

ϯϳϲ

This stage can be observed on the cross- sections, which pass through the central part of the

ϯϳϳ

anticlines (cross section CCƍ) (Fig. 17b). As shown in Figure 8, the pop up structure in the Karanj

ϯϳϴ

anticline continues from NW to the central part of the anticline and the mechanism of folding

ϯϳϵ

changes to a fault propagation geometry both in the Karanj and Parsi anticlines by developing

ϯϴϬ

thrusts in the steeper forelimb from the NW to the central part. A significant character of this

ϯϴϭ

cross section is the generation and development of a double thrust in the forelimb and a potential

ϯϴϮ

trishear zone in the Parsi anticline (high dip of the bedding reduces the quality of seismic

ϯϴϯ

images) (Fig. 17b). Thrusts formed in the middle detachment and grew upward during

ϯϴϰ

progressive folding from a faulted detachment fold (Mitra 2002, 2003). The Parsi anticline

ϯϴϱ

shows a trishear fault-propagation fold model (Fig. 17b). In this model slip on the fault is

ϯϴϲ

dissipated within a triangular deformation zone (Fig. 17c) (Erslev, 1991; Erslev and Mayborn,

ϯϴϳ

1997). The deformation is concentrated within a narrow area immediately above the fault and

ϯϴϴ

spreads to a wider zone in higher stratigraphic units. The deformation zone also can widen with

ϯϴϵ

progressive deformation, with the intensity of deformation decreasing away from the fault on

ϭϳ 

ϯϵϬ

either side (Erslev, 1991; Erslev and Mayborn, 1997; Hardy and Ford, 1997; Almendinger,

ϯϵϭ

1998). The trishear model provides a simple mechanism for distributing shear from the tip of a

ϯϵϮ

fault to a widening zone. The distribution of deformation along the Paranj anticline is different

ϯϵϯ

from the other two anticlines. Due to the operation of the thrust faults on both side as well as

ϯϵϰ

more overburden thickness, the Paranj anticline has subsided and lies deeper than other two

ϯϵϱ

anticlines.

ϯϵϲ ϯϵϳ

6. Conclusions

ϯϵϴ

The structural analysis and kinematic evolution of this study demonstrated that, consistent with

ϯϵϵ

previous studies, the structural style and kinematic evolution of a sequence including a viscous

ϰϬϬ

layer in addition to the shortening could be controlled by different parameters. These parameters

ϰϬϭ

are including the flow of the viscous layer above and below the folded strata, presence and

ϰϬϮ

reactivation of the basement faults. For example structural analysis of the subsurface anticlines

ϰϬϯ

of the Karanj, Paranj and Parsi oilfields in the Dezful Embayment, SW Zagros, Iran reveals that

ϰϬϰ

different mechanisms of folding from detachment to fault propagation folding were acting in the

ϰϬϱ

progressive development of these anticlines. Based on our kinematic analysis, these anticlines

ϰϬϲ

formed as detachment folds on equivalent Hormuz series as a basal detachment horizon in the

ϰϬϳ

earlier stages of deformation. The results of amplification mechanism analysis in addition to the

ϰϬϴ

growth strata in the Karanj and Parsi anticlines reveal that the onset of folding was in the Middle

ϰϬϵ

Miocene time and the main active process in the evolution of folding was limb rotation. The

ϰϭϬ

present day geometry of the anticlines in the study area largly influenced by the flow and

ϰϭϭ

migration of the upper detachment horizon (Gachsaran Formation) during the folding, which in

ϰϭϮ

itself resulted in disharmony between the upper and lower incompetent units. This suggests that

ϰϭϯ

folding of compentent layers between detachments in a prolonged orogenic event may be a ϭϴ 

ϰϭϰ

pulsed or long-lived event that propagates (toward SW) up through the sedimentary succession

ϰϭϱ

as older layers fold, forcing deformation in overlying, mobile, younger layers and decoupling the

ϰϭϲ

subsurface deformation from structures visible at the surface.

ϰϭϳ ϰϭϴ

Acknowledgements

ϰϭϵ

The research is supported by the Shiraz University Research Council (SURC) grant, which is

ϰϮϬ

gratefully acknowledged. The authors thank the Exploration Directorate of the National Iranian

ϰϮϭ

South Oil Company (NISOC) for providing the data. Constructive comments by Soumyajit

ϰϮϮ

Mukherjee and two anonymous reviewers improved the scientific content of the manuscript.

ϰϮϯ ϰϮϰ ϰϮϱ

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ϳϮϵ ϳϯϬ ϳϯϭ

Figure caption:

ϳϯϮ

Fig. 1. Structural map of the ZFTB showing location of the Karanj, Paranj and Parsi oilfields in

ϳϯϯ

the southern Dezful Embayment (Modified after NISOC).

ϳϯϰ

Fig. 2. Lithostratigraphic column illustrating the alignment of competent and detachment units of

ϳϯϱ

the sedimentary progression of the ZFTB in the southern Dezful Embayment . (Modified after

ϳϯϲ

Abdollahie Fard et al., 2006).

ϳϯϳ

Fig.3. The Geological map of the study area at a scale of 1: 100,000 showing the locations of the

ϳϯϴ

studied anticlines.

ϳϯϵ

Fig.4. Flow chart of seismic interpretation work flow using Petrel 2013 software.

ϳϰϬ

Fig.5. a) AA‫ މ‬interpreted seismic profile and balanced and restored cross-section from the NW

ϳϰϭ

part of the studied anticlines in the Dezful Embayment.b) BB‫ މ‬interpreted seismic profile and

ϳϰϮ

balanced and restored cross-section from the central part of the studied anticlines in the Dezful

ϳϰϯ

Embayment.

ϳϰϰ

Fig.6. a) CC‫ މ‬interpreted seismic profile and balanced and restored cross-section from the central

ϳϰϱ

part of the studied anticlines in the Dezful Embayment. b) DD‫ މ‬interpreted seismic profile and

Southern

Dezful

Embayment,

Ϯϵ 

SW

Iran,

Mar

Pet

Geol.

Doi:

ϳϰϲ

balanced and restored cross-section from the SE part of the studied anticlines in the Dezful

ϳϰϳ

Embayment.

ϳϰϴ

Fig.7. Fold classification diagram (Ramsay,1967) based on t’Į/Į. Were t’Į is equal to

ϳϰϵ

perpendicular thickness between two isogon of the folded layer and Į is the local dip of the layer.

ϳϱϬ

shapes with yellow, red and blue colour belongs to the fold parameter measurements in the cross

ϳϱϭ

AA’, BB’ and CC’ respectively.

ϳϱϮ

Fig.8. a) concentric detachment folding style of competent strata between two detachment

ϳϱϯ

surface and corresponding structures by Dahlstrom (1969) and Sherkati (2005). b) document for

ϳϱϰ

concentric detachment folding in the studied area (rabbit ear fold in the Paranj and disharmonic

ϳϱϱ

folding in the Parsi anticlines).

ϳϱϲ

Fig.9. a) The graph of the best-fit detachment depth for the Karanj, Paranj and Parsi anticlines

ϳϱϳ

and b) Excess area against depth graph (methods of Epard and Groshong, 1993, Groshong and

ϳϱϴ

Epard, 1994; Bulnes and Poblet, 1999; Groshong, 2015; Wang et al., 2017, Carboni et al., 2019).

ϳϱϵ

Fig.10. Geometric analysis of the folds (Jamison 1987). Location of anticlines is marked.

ϳϲϬ

Fig.11. a) the 3D structural model constructed using 7 cross sections (illustrated on the figure. 1)

ϳϲϭ

illustrating the thrusting of the Parsi anticline on the Paranj anticline and the change in folding

ϳϲϮ

mechanisms from NW to SE part. b) The underground contour map of the top Asmari Formation

ϳϲϯ

in the Karanj, Paranj and Parsi anticline.

ϳϲϰ

Fig.12. Different type of the contractional coupling between basement and overburden. a) hardly

ϳϲϱ

coupled faults are hard connected across the salt layer.b) Decoupling take places due to the salt

ϳϲϲ

flow in the relief developed by basement faulting.c) partial coupling which is overlain by a

ϳϲϳ

monocline draped over the master basement fault (modified after Jackson and Hudec, 2017), d)

ϯϬ 

ϳϲϴ

interpreted seismic section along the axis of the Karanj anticline showing the structural

ϳϲϵ

culmination and partial coupling over the IZHF.

ϳϳϬ

Fig.13. Sequential restoration of the studied anticlines on the cross section illustrated in Fig. 5

ϳϳϭ

(see section line A- Aƍ in Fig. 1 for location).

ϳϳϮ

Fig. 14. a) forelimb dip, axial plane dip and interlimb angle plots against shortening for the study

ϳϳϯ

anticlines, using the parameters measured in the deformed and restored sections of cross section

ϳϳϰ

AAƍ, constructed using limb-rotation fold amplification mechanisms (model 2). b) illustration of

ϳϳϱ

the kinematics evolution of detachment fold in depandance on two parameters including : limb

ϳϳϲ

dip and limb length. Models 1 and 2 evolves with the limb migration (changeble limb lenght and

ϳϳϳ

constant limb dip) and limb rotation (constant limb length and changeble limb dip) respectively,

ϳϳϴ

while, the model 3 and 4 evolves both limb migration and limb rotation (variable limb rotation

ϳϳϵ

and variable limb dip) (Poblet and McClay, 1996).

ϳϴϬ

Fig.15. Seiscmic profile BBƍ. Top of Asmari Fm shown with the green line and top of Gachsaran

ϳϴϭ

Fm, shown as the yellow line was flattened to the pre-folding situation to better visualize the

ϳϴϮ

growth strata in the Gachsaran Formation with the lithology column of the Gachsaran formation.

ϳϴϯ

Fig. 16. The outcrop of the Gachsaran formation in the surface through a thrust fault originating

ϳϴϰ

from the subsurface Parsi anticline.

ϳϴϱ

Fig. 17. a) Schematic represent faulting of a symmetric detachment fold. The result of continued

ϳϴϲ

limb rotation and compression is the formation of faults in the forelimb and backlimb of the fold.

ϳϴϳ

Eventually these faults reconnect with the detachment and a pop-up may occur (Mitra, 2002). b)

ϳϴϴ

Cross section CCƍ and its related seismic profile illustrating the pop up structure on Karanj

ϳϴϵ

anticline and the trishear on the forelimb of Parsi anticline. (As: Asmari Fm, Pd: Pabdeh Fm, Gu:

ϯϭ 

ϳϵϬ

Gurpi Fm, IL-Sv: Ilam-Sarvak Fm, Kz: Kazhdumi Fm) c) Evolution of a fault- propagation fold

ϳϵϭ

by the trishear mechanism (Mitra, 2002, modified after Erslev and Mayborn, 1997).

ϳϵϮ

Table1: The standard amount of Øೊ and c of various lithology according to Berra and Carminati

ϳϵϯ

(2010).

ϳϵϰ ϳϵϱ ϳϵϲ

ϯϮ 



Table 1 Compaction Coefficient (c)

Porosity at the surface (ij0)

0

0.3

Berra and Carminati , 2010

Sandstones

0.2

0.49

Sclater and Christie, 1980

Pelites(i.e. siltstones and shalestones)

0.51

0.63

Sclater and Christie, 1980

Micritic limestones (chalk)

0.71

0.7

Sclater and Christie, 1980

Limestones

0.518

0.513

Schmoker and Hally, 1982

Calcarenites (grainstones)

0.25

0.445

Goldhammer, 1997

Dolomites

0.216

0.303

Schmoker and Halley, 1982

Gypsum

0.51

0.63

Berra and Carminati, 2010

Lithology

Conglomerates



References

• • • •

The Karanj, Paranj and Parsi oilfields are bounded by multiple décollements. The geometry of these oilfields is controlled by the reactivation of basement faults. The flow of the Gachsaran Formation influenced the geometry of the oilfields. Fault-related folding plays a crucial role in the formation of structural traps.

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