STRUCTURAL ANALYSIS FOR THE PLUNGE AREAS OF BEKHAIR, BRIFCA, DOHUK AND ZAWITA ANTICLINES NORTHERN IRAQ USING SEISMIC SECTIONS

The study area is located in Dohuk Governorate; Northern Iraq. The aim of this study is to concentrates on the structural analysis in the plunge areas of Bekhair, Dohuk, Brifca and Zawita anticlines by using seismic sections. The four anticlines have NW – SE trend, which coincide with the main trend of the Zagros folds. Three seismic sections (DK-16, DK-20, and DK-22) were used to emphasize the structural analysis and recognize the faults caused by folds development and affected on the plunge areas of the four anticlines. The results showed that the main axis of Bekhair anticline exhibits two small plunges (northern and southern domes) with a syncline in between. The northern dome near Zawita village, is developed as a fault bend fold over northeast-wards dipping forethrust. The dome extends eastwards in form of structure A, which is also developed as a fault bend fold, and is named Deralosh anticline in this study. The southern dome near Besire village, is developed over a planner northeast-wards dipping forethrust (Bekhair Thrust) as a fault propagation fold with reverse displacement of about 225 m. The syncline between the two domes also plunges eastwards to connect with the axis of the major syncline and continues plunging eastward. This study shows that Zawita anticline is composed of two structures; (A) and (B) and is separated by a saddle area. Structure B, which is named Benarinke anticline, is developed over a northeast-wards dipping forethrust; as a fault propagation fold along reverse displacement of about 150 m. Banye small anticline


Fig. 1: Geographic location of the study area
The area is not well studied studied previously; however, there are structural and stratigraphic studies on the anticlines surrounding the area.Ameen (1979) concluded that there are five Alpine Geoflextures, one of them is Bekhair anticline, whereas Al-Alawi (1980) divided Bekhair anticline into two parts: the first one is Spi Rais fold near Dohuk area and the second is Geri Baran fold near Sumail area.Numan and Al-Azzawi (1993) studied the vergence directions of the fold in the foreland region and concluded that there are two interpretations of vergence directions of folds.Al-Mosawi (2004) referred that Bekhair-Ishkaft fault, which crosses Bekhair anticline caused in a left lateral displacement to the axis and limbs of the anticline.Al-Hubaiti (2008) concluded that the axis of Bekhair anticline coincides with the Zagros Range (NW -SE) with some fluctuation.It changes its direction to the Taurus Range (E -W) from Zakho Gully to the northwestern plunge.Dohuk anticline was studied by Al-Kadhim (2010), he concluded that Dohuk anticline is double plunging asymmetrical anticline affected by suture listric fault confirmed by the northeastern vergence.
Al-Sumaidai (2010) studied Brifca anticline and concluded that the fold is asymmetrical mostly overturned and verge towards the southwest; except at the eastern part of the fold, which appears verging towards the south.Al-Kubaisi ( 2013) studied the analysis of some morphotectonic features on faulted anticlines and linear slope deposits in northwest Iraq.She believes that the contacts between the slope and Quaternary deposits and the folded rock units mark surface fault traces and the relative straight relation between them suggests that the underlying reverse faults have high angles of dip and those faults with their up and down thrown sides were elevated together to the ground surface passively above an underlying blind reverse faults.

THE AIM OF THE STUDY
Most of the previous studies on Bekhair, Brifca, Dohuk and Zawita anticlines believed that the plunge areas form very complex structure, and the plunges are arranged in very abnormal triple connections.The plunge area was not studied the details connecting areas of the plunge of these anticlines.This idea reflects the importance of the study.The aim of the study concentrates on determining the structural development of the plunges by using three seismic sections passing through the plunge areas.In addition, to construct a structural geological map of the area.

GEOLOGICAL SETTING
The study area is located within the Unstable Shelf, the High Folded Zone (Fig. 2).
The study area includes asymmetrical double plunging anticlines trending NW -SE.

STRATIGRAPHY OF THE STUDY AREA
The exposed rocks of the sedimentary sequences range in age from Paleocene to Miocene (Fig. 3).These formations are from older to younger: 1. Kolosh Formation (Paleocene -Lower Eocene)

SEISMIC STUDY
The study area is mostly covered, only small areas are exposed as outcrops.By using seismic sections, the subsurface structures were revealed and their effect on the surface was studied.Seismic reflection survey, which is the most widely used and wellknown as a geophysical technique, is able to conclude good results, important explanation about causes of folding, fault-related folding, flat and ramp in addition to forethrust and backthrust must be known.

FAULT RELATED FOLD 1. Fault bend folding
A fault bend folds form where hanging wall strata move up over a stair step in a thrust fault and the footwall is completely inert and remains undeformed (Reference?).
The strata deformed in order to conform to changes in dip (bends) of the fault surface.
The folding occurs after the formation of the thrust (Fig. 4).

Fault propagation folds
Fault-propagation folds are common features in foreland basins and fold-and-thrust belts.Folds are developed over an upward propagating thrust ramp as a consequence of the horizontal displacement of the sedimentary pile above a decollement level (Deroin et al., 2012).Fold propagation folding occurs when a propagating thrust fault loses slip and terminates up-section by transferring its shortening to a fold developing at its tip.It means that the geometry and Kinematics of a fault propagation folds depend on the amount of displacement along a basal decollement level, the ramp angle and the slip to propagation ratio (Mitra, 1990) (Fig. 5).
Many folds form in association with the displacement of the fault planes and their propagation.The continental collision between Arabian and Iranian plates resulted in the formation of the Zagros Fold -Thrust Belt and its associated foreland basin.During convergence, pre-existing faults in the basement were reactivated and led to heterogeneous deformation, which segmented not only the Zagros Fold -Thrust Belt, but also its foreland basin into different compartments (Bahroudi and Koyi, 2004).
Foreland folds are characterized by simple folds obeying two kinds of trends, the first has NW -SE direction parallel to Zagros Range, whereas the second coincides with Taurus and has E -W trend.Also these folds have two types of vergence directions, some of them verging toward north and northeast, while the others have south and southwest vergence and the symmetrical folds are rare (Numan and Al-Azzawi, 1993).
Most of the foreland folds and thrust belts reflect the presence of complex basement structures that are usually described by obvious stratigraphic uplift.

INTERPRETATION OF SEISMIC SECTIONS IN THE STUDY AREA
One of the undirected seismic methods to determine the subsurface geological structure is the seismic reflector method.
The interpretation of the seismic method depends on the seismic data, which show the change in the velocity of the seismic reflected waves at different rock reflectors, which in turns depends on the elastic properties of these rocks.
Seismic data are commonly displayed as maps or cross sections in which the vertical scale is the two-way travel time.Three seismic profiles were chosen to be studied in the area (Fig. 3), they are DK-16, DK-20, and DK-22.

Line DK-16
Seismic profile DK-16 is a slightly curved line extending from the north (336649 E; 4092242 N) to southwest (329629 E; 4078777 N), and then towards the south (328975 E; 4064114N).The studied area is covered with the northern segment that extends for 15.7 Km (Fig. 3).Later development of the high angle dipping upper segment of the thrust and the back thrust resulted in the modification of the original fault bending fold.

Seismic profile DK-20
Seismic profile DK-20 extends from the north (341380 E and 4087872 N) towards southwest (326923 E and 4071604 N) with a total length of 21.85 Km.The profile DK-20 runs east of profile DK-16 also across the plunge of Bekhair, Brifca and Dohuk structures in the central part of the studied area (Fig. 3).
As the seismic profile DK-20 has been interpreted, five main horizons were recognized (Fig. 7), only Khurmala and Kolosh horizons are exposed in Brifca structure.The fault defining Dohuk and Brifca recognized from both the surface and in the section.
The seismic section DK-20 shows that the southern dome of the Bekhair plunge dies out west of this profile (doesn't appear in the section), whereas the northern dome plunge of Bekhair anticline continues eastwards.It is called here Structure A, and is named Deralosh anticline.
The interpretation of this profile also clarifies that Dohuk anticline is developed on short and thrust bifurcated from the Brifca main thrust and the Brifca and Dohuk anticlines are developed as fault propagation folds evolved over relatively planner fault.
In comparison with the seismic section DK-16, the displacement on the Dohuk thrust becomes much smaller of about 0.03 sec ~ 45 m, whereas it is of about 0.5 sec ~ 750 m on Brifca structure.
This correlation between the displacement of Brifca and the southern dome of Bekhair anticline in line DK-16 (Fig. 6) and their plane view in the geologic map (Fig. 3) suggests that the Brifca fault is separated and differs from the Bekhair fault.
Accordingly, a Lateral ramp is possibly separating them.
On the other hand, the thrust fault defining structure A or Deralosh anticline is not well recognized with depth, the possible moderate angle dipping lower segment is not clear, but the gentle dipping middle segment and the upper high angle dipping segment are clear.
Accordingly, it is suggested that this structure is similar to the northern dome of Bekhair and also developed as fault bending fold modified by a relatively high angle thrust segment.

Fig. 2 :
Fig. 2: Tectonic map of Iraq shows the location of the study area (Jassim and Goof, 2006)

Fig. 3 :
Fig. 3: Geological map of Dohuk area shows the strata and locations of seismic profiles in the study area (Sissakian, 1994)

FigFig. 5 :
Fig. 4: Progressive developments of a fault-bend folding (Suppe, 1985) Figure (6) shows the interpretation of this part of the profile DK-16.Five horizons are recognized from subsurface data and well Sheikh Adi-(SA-1B) which is located at (339517 E and 4075639 N) in the eastern part of Brifca anticline.These horizons are from older to Younger; Garago, Qamchuqa (Lower Cretaceous), Bekhme/ Kometan (Upper Cretaceous), Shiranish (Upper Cretaceous), Khurmala/ Kolosh (Paleocene -Lower Eocene), Pila Spi (Upper Eocene) and Fatha (Middle Miocene) appear in the synclines.The interpretation of the profile (DK-16) shows that Dohuk and Bekhair and even the anticline north of Bekhair have developed over northeast dipping thrusts.A back thrust affected the northern limb of Dohuk anticline is up to (0.4 sec ~ 600 m) that resulted in thrusting of the Pila Spi over thick Bakhtiari sequence.The displacement on the Bekhair thrust is about (0.15 sec.~ 225 m) that brought the Pila Spi over the Fars sequence (Fatha and Injana) and locally on the Bakhtiari Formation.Line DK-16 also shows that the plunge of Bekhair consists of two domes; the southern Dome developed on the Bekhair main thrust, whereas the northern Dome is developed over northeast dipping fault that formed beneath the yellow horizon (the yellow horizon probably defines the top of Jurassic) (Fig. 6).Northern and southern domes are separated by a small syncline developed on the tip of thrust defining the northern dome.Thrusts defining Dohuk and Bekhair structures are relatively high angle running deep into the sedimentary sequence.Thrust defining the northern dome consists of three parts; two with relatively moderate dip separated by a gentle dipping segment.Back thrust influencing the northern limb of the Northern Dome is developed from the bend between the gentle and the lower moderate dipping segments of the thrust forming dome.Folding and displacement of the main Dohuk and Bekhair structures on a relatively high angle thrusts suggest that these structures are developed as a fault propagation folds.In comparison, the wide relatively flat crestal zone of the northern dome developed over the gentle dipping segment suggests that the northern dome is developed as fault bend fold.

Fig. 8 :
Fig. 8: Interpreted seismic section along profile DK-22 DISCUSSION AND CONCLUSIONS A Structural geological map was drawn for the study area (Fig. 9).It shows the locations of anticline and syncline axes, and thrust faults which determined by the interpretation of the seismic sections.This study detects that the anticline, which was believed by the Iraq Geological Survey (GEOSURV) as Zawita anticline(Sissakian, 1994), is composed of structure (A)

Fig. 9 :
Fig. 9: A structure geological map of the study area compiled from the field data, interpreted seismic sections (Sissakian, 1994)