Early - middle Miocene Inversion of the Abu Jir Fault in the Western Iraq: a Possible Consequence of the Arabian Plate Northward Movement

Abstract


Introduction
This research investigates the northern segment of the Abu Jir fault to reveal the interaction between extensional and contractional structures during the Early-Middle Miocene.The structural style evolution of the Abu-Jir fault and Anah graben is of particular interest because of their inversion is due to shortening propagation from the north to the south, other extensional structures between the Arabia-Eurasia suture zone and the Anah graben necessitate inversion as well.However, this may not be the case as the published seismic profiles across the Tayarat and Khleissia grabens do not show inversion (Fouad and Nasir, 2009) (Fig. 1).The northern part of the Abu-Jir fault, which is the focus of this study, has not been investigated previously.For this study, surface geologic data were obtained and interpreted in a regional context to elucidate the timing and potential mechanism of the structural inversion of the northern part of the Abu-Jir fault.Fouad and Nasir (2009) During the Late Cretaceous, the northern margin of Arabia underwent an extensional phase due to the pull slab force of the subducting slab beneath Eurasia, leading to the formation of grabens and halfgrabens.These extensional structures were inverted by compressional forces during the late Tertiary, as manifested in the Anah inverted graben.The Abu-Jir fault and Anah Graben are considered the boundary between the inner, to the west, and outer, to the north and east, the platform of the Arabian Margin in Iraq (Fouad, 2015).The Abu-Jir fault zone is considered part of the Nabitah fault system that formed around 680 Ma.This system is considered to have roots in the basement of central Iraq, where the N-S trending discontinuities are bent along the NW-SE trending Najd fault system (Jassim and Goff, 2006).Kinematically, the Abu-Jir fault zone acted as a right-lateral strike-slip fault (Marouf, 1999;Fouad, 2000 and2015).The Abu-Jir fault zone extends from west of Razzazah Lake toward Haditha Lake.It consists of two parts: the northern and the southern parts.The southern part extends from the west of Razzazah Lake to Heet City and represents a negative flower structure associated with four sag ponds (Marouf, 1999;Fouad, 2007;Alhadithi, 2017;Alhadithi and Salih, 2017).The northern part of the Abu-Jir fault zone is between Heet and Anah, parallel to the Euphrates River, manifested as a transpressional structure (Marouf, 1999;Fouad, 2007).Farther north, the Abu-Jir fault intersects the Anah graben and divides it into two segments: eastern and western.

Geological Setting
Tectonically, the Anah Graben and Abu-Jir fault zone are located within the boundary of the subsided Zagros foreland basin to the east and the less subsided terranes to the west in Iraq.The Anah Graben and Abu-Jir fault zone is the boundary between the inner and outer platforms of Iraqi territory (Fouad, 2015).Surface structures are missing in the Iraqi Western Desert, except for some fault systems such as Hauran and Nukhaib Fault Systems (Al-Mubarak and Amin, 1983;Al-Bassam et al., 1997).
Stratigraphically, in the study area, the exposed section formations are the Anah Formation (upper Oligocene), Euphrates Formation (Lower Miocene), Fatha Formation (Middle Miocene), and Nfayil Formation, which is equivalent to the Fatha Formation (Figs. 2 and 3).The Anah Formation exposes as patches and consists of grey, brecciated, massive, and recrystallized detrital and coralline limestone (Bellen et al., 1959).The Euphrates Formation exposes along the Euphrates River from the Heet to Al-Qaim cities and consists of a lower unit of basal conglomerate, followed by 10 m of fossiliferous limestone.The upper unit of the formation consists of alternation of hard limestone and pseudoplastic limestone (Jassim et al., 1984).The Fatha formation crops out in two areas in the vicinity of the Heet and Dolab towns.Generally, the formation consists of cycles of green marl, bedded limestone, and gypsum.The first cycle is exposed in Dolab area, but three cycles are exposed in the Heet area.The Nfayil Formation is exposed in the south and east of the study area, and consists of green marl and grey fossiliferous limestone (Sissakian et al., 1997).Structurally, the syn-depositional structures of the Abu-Jir fault represented by the brecciated unit at the base of the Euphrates Formation (Lower Miocene) and its undulated uppermost part.

Materials and Methods
Fieldwork was conducted for detailed study of lithology and the geological structural features, such as joints, faults, and outcrop-scale folds.To achieve this research, the Stereonet (v.11.3.0)software was used to draw the attitude of the structural features.The main maps that were used were the tectonic and geological maps of Iraq, with a scale of 1:1,000,000.Additionally, Landsat satellite images were downloaded from Google Earth Pro and Sas-Planet.We utilized ArcGIS (v.10.8) to fit the maps derived from satellite images and layout a geological map for the study area.Furthermore, Adobe Photoshop (v.2020) was also used to process field photos and combined with Paint 3D to add symbols on photos.

Results
Heet pressure ridge in Sihailiya area was created due to activity of Abu Jir fault zone which is a part from the fault zone.Fieldwork observations in the ridge show that the upper part of the Euphrates Formation and the first cycle of the Fatha Formation contain a major listric positively inverted fault, which has a strike of 145° and the dip steeply upward with a net slip of 5 m.The listric fault was developed space between its blocks before the inversion that was filled by unconsolidated sediments below sea level as a sediment dyke.The hanging wall of the fault dips slightly toward northeast which contains a large amount of joint system.Depending on strike and bedding planes of the beds the joint system is hol-c, which developed due to the uplift of the hanging wall where the axis of maximum principal stress bisect acute angle of the system.The fault cuts the sequence of the Euphrates Formation downward (Fig. 4A and 4B).At some outcrops, many collapses or slump features are noticeable (Fig. 4C and 4D).These features were described as seismites that formed within the upper part of the Euphrates Formation between Heet and Al-Qaim cities due to Abu-Jir fault activities when the rocks were still soft (Sissakian et al., 2014).Some outcrops of the Euphrates Formation are manifested by many convex reverse faults, minor faults with positive flower structure that are flattened upward to develop a positive flower structure, making a local relief in comparison to the surrounding area.These faults are divergent and parallel to each other which have strikes of 145°, the dip toward northeast that decreases upward.They are formed in thick limestone beds (Fig. 4E).The flattened segments of the miner faults of the positive flower structure likely translated stress to become horizontal, which generate duplex features (Fig. 4F) and fault-related folds formed.hol-a joint system is shown in the duplex structure.Little displacement through the miner faults while the displacement in the duplex is within it.A local fault-bend fold was observed in the ridge, which might be formed when the fault surface was non-planar while the fault blocks bent as they ride over the fault surfaces.To generate a fault-bend fold, enough energy must be provided to overcome the friction on the fault, uplift the fold, and shear the fold material (Williams, 1987).A detachment fault develops within the 17 cm marly limestone layer under a fault-related fold.
We also observed that the uppermost of the Euphrates Formation is brecciated and undulated limestone units along the study area (Fig. 5).Sissakian et al. (2014) studied the genesis and deformation style of the soft sediments within the brecciated and undulated limestone units, which form the uppermost of the Euphrates Formation.In the area, these soft sediments extend on both banks of the Euphrates River, from Iraq-Syria border to the east and southeast toward Wadi-Hauran.These deformations were developed because they may indicate the seismic influence on the formation of breccia during sedimentation (Sissakian et al., 2014).Such a process implies that the deposition of the breccia is of syntectonic origin, most likely owing to tectonic activities that induced shock waves in the area of active deposition.The undulations vary in size and shape, ranging from less than 1 m to 10 m in width, having amplitudes ranging from a few centimeters to 5 m (Fig. 5).The deformational features in the brecciated units are synsedimentary structures or soft sediment deformation structures that formed before lithification of the sediments.The boudinage structures development within the brecciated unit implies a syn-sedimentary extension process, whereas compression wrinkles imply syn-sedimentary compression structures (Hongbo et al., 2006).Both types of structures are found at the top part of the Euphrates Formation's brecciated unit.In the field the upper part of the Euphrates Formation shows that the intensity of the undulation decreases towards the northwest.The undulations in the area of Anah (Al-Fahimi valley) is simpler compared to the Sihailiya, Dolab the surrounding areas.
Furthermore, in the field, we documented the presence of a fault-propagation fold in the lower part of the first cycle of the Fatha Formation in the Heet outcrop (Plat 2G).The fault-propagation fold is a style of fault-related folding, that occurs when a propagating thrust fault loses slip and ends up-section by transmitting its shortening to a fold that forms at its tip (Mitra, 1990;McClay, 2011).The slip reduces to zero at the fault tip at each instant during propagation and is absorbed by folding.The faultpropagation fold kinematic process is proposed as an explanation for the typical connection of asymmetric folds with one steep or even overturned fore-limb near the thrust fault and a less steep backlimb.It forms by compression in front of a fault tip during fault propagation (Suppe, 1983).When a fault steps up a ramp from a detachment level, it causes fault-propagation fold.Such kind of fold is formed by flexural flow mechanism.Flexural flow folding slip happens on individual grains inside a layer without the existence of apparent slip surfaces (Pluijm and Marshack, 2004).By flexural flow mechanism, ductile layers flow, whereas others remain brittle and buckle.Flexural flow necessitates moderate to high ductility contrast between layers.We observed flexural flow folding mechanism that developed in the marl and marly limestone units (Plat 2G).The fault in this fold has a strike of 020º toward NW.
Four detachments are distinguished that appear as cataclasis through the weak layers of the marly limestone.Individual cataclasis layer varies in thickness and in some cases is missing between two hard layers.Two of the detachments are represented by the floor and roof of the duplex respectively.The third detachment is cataclasis in a layer of marly limestone within the Upper Member of the Euphrates Formation, above which a fault-bend fold and a duplex were developed.The fourth one is a thick layer of marly limestone within the first cycle of the Fatha Formation.Finally, slickenlines are observed in the scarp of the ridge which indicates the uplift of the ridge related to the surrounding aera (Fig. 5-H).

Discussion
Strike slip movement of Abu Jir zone forms pressure ridge in the northern segment of the fault zone.The ridge consists of many normal and reverse fauls that good indication of the strike slip movement.Abu Jir fault zone display inversion of a listric fault that was likely active during the Early-Middle Miocene during deposition of the upper part of the Euphrates Formation and the lower part of the Fatha Formation.The age of this inversion event is indicated by presence of syntectonic sedimentation such as collapse or slump features, sediment dyke, as well as brecciated and undulated limestone beds.Such features were also reported by Fouad (1997 and2000) along most of the zone where sediments display syn-depositional (soft-sediments) deformational patterns, showing that the Anah-Abu Jir fault system was active during the middle Miocene (Fouad, 1997 and2000).The Abu Jir fault zone was inferred to have initiated as a normal fault during the late Cretaceous based on differential thickness of basin fill sediments (Fouad, 2007).
During Tertiary a regional compression in the northern Arabian plate boundary occurred, that is why the late Cretaceous ENE-WSW trending rift basins such as Palmyride (Chaimov et al., 1993), Sinjar (Daly, 1990), and western part of Anah (Fouad, 1997) inverted.The ongoing collision during Tertiary caused compressive stress normal to the ENE-WSW trending troughs, but exerted transpressive stress along the NW-SE trending fault zones such as Euphrates and Abu-Jir (Figure 1).Due to the stress orientation with respect to the regional N-S compressive field (Figure 1), the NW-SE trending Euphrates trough evaded structural inversion (Ruiter et al., 1994).The same reasoning applies to the Abu-Jir fault zone.Instead of being fully inverted, the Euphrates and Abu Jir fault zone underwent right-lateral strike slip movement.The associated flower structures, pressure ridges pull-apart and sag ponds provide further verification of the occurrence of right lateral movement on these fault zones.Additionally, in the study area a layer of basal conglomerates was preserved below the Euphrates Formation, suggesting an unconformity surface between the Oligocene formations and the Euphrates Formation (lower Miocene).This layer was formed as a result of the emergence of the Oligocene formations above sea level.Another broad region of exposure is noted and marked the end of the Oligocene time (Fouad, 2007).Anah Formation (upper Oligocene) crops out towards the northwest, indicating that an uplifting took place, whereas subsidence occurred to the southeast.
The abovementioned summary integrates the outcome of the research with the broader understanding of the northern Arabian tectonic evolution, yet this research highlight that the northwestern segment of the Abu Jir fault zone underwent transpression and inversion earlier than the Sinjar structure, which is farther north and closer to the collision zone.Detailed field investigation of the surface structures of the Euphrates and Fatha formations and integration with the regional evolution indicates a distinct extensional event during the early-middle Miocene, possibly due to either basin subsidence or strike-slip movement, which we will elaborate on them in the next sections.

Early Miocene extension
The tectonic development of the study area was marked by an extension during the end of the early Miocene.The existence of several structural features indicates an extensional phase, which is manifested clearly by the documented listric normal fault that cut the entire Euphrates Formation (early Miocene) and runs downward into the Sihailiya outcrop (Fig. 4A).Other indications are the presence of a sedimentary dyke between the two fault blocks, as its materials are from the carbonate rocks of the uppermost of the Euphrates Formation or the limestone of the first cycle of Fatha Formation that deposit before lithification, slumps, or collapse structures in the uppermost of the Euphrates Formation (Fig. 4) and local graben structure in the upper part of Euphrates Formation in Dolab area (Fig. 5C).
This phase of extension in the Abu Jir fault zone, which could represent the western flank of the Zagros foreland basin, occurred as a result of the foreland basin development.The foreland basin subsided flexural and asymmetrically with a depocenter near the fold-thrust belt to the east as a result of the Arabia-Eurasia collision (Koshnaw et al., 2020(Koshnaw et al., , 2021)).Such evolution tilted the stratigraphic sequence in the basin differentially and leads to the listric normal fault development and other extensional features on the western flank of the basin along the Abu-Jir fault zone (Alhadithi, 2013).Later, the middle-late Miocene, the Zagros foreland basin experienced rapid subsidence, just east of the Abu Jir fault zone (Jassim & Goff, 2006).Such rapid subsidence is evidenced by the accumulation of large quantities of sediments as observed by comparing the outcrops in the study area, in the west, with the Fallujah-1, and Melih Thirthar wells in the east of the Abu Jir fault zone (figure 1).In the wells the Euphrates, Jeribe and Fatha Formations have large thicknesses, whereas in the study area the Euphrates and Fatha Formations are less thick, with loss of the Jeribe Formation between them.Therefore, we consider the development of the listric normal as a result of the Zagros foreland basin evolution, particularly development of the forebulge segment due to rapid flexural subsidence of the foredeep area during the end of the late and middle Miocene.

Middle Miocene Transpressional Strike-Slip Movement
The regional tectonic evolution was characterized by counter-clockwise rotation of Arabia and compression as a result of ongoing Arabia-Eurasia oblique collision during the middle Miocene.In the study area, there are several structural features that indicate a transpressional phase of deformation along the Abu-Jir fault zone due to the strike-slip movement.During this period, the listric normal fault experienced a positive inversion and caused uplift of rocks on the hanging-wall of the fault (Fig. 4A).In the lower part of the Euphrates Formation, several systematic conjugate system joints of hol-c developed in the hanging wall of the inverted listric normal fault at the outcrop in Sihailiya (Fig. 4B).In these joints the direction of the main stress ( σ1) is upward and it bisects the acute angle between the two sets of joint system.Also, the presence of slickenlines on the hanging wall of the inverted fault indicates the occurrence of shear movement on the faults' surface, and the direction of the movement.These slickenlines are interpreted as a result of positive inversion that occurred during the strike-slip movement, which was acting along the Abu-Jir fault zone.The slickenlines are trending upward due to uplift of the ridge (Fig. 5H).Additionally, several secondary reverse faults developed in the lower part of the Sihailiya ridge, within thick limestone layers of the Euphrates Formation (Fig. 4E).These faults are convex and parallel to each other, and they are flattened upwards to form a typical positive flower structure on one side.Such configuration of positive flower structure was demonstrated by Harding et al. (1983) and the fault architecture is not always symmetric.The secondary faults may diverge toward only one side of the structure in some areas.Since these secondary faults are located within the western block of the Abu-Jir fault, they suggest that the western block moved northward concerning the eastern block.The uppermost of the Euphrates Formation shows structures that developed as a result of horizontal to sub-horizontal thrust faults, constituting duplex structures (Fig. 4F) and a fault-bend fold (Fig. 4A).
Based on our field data and observation we think that the Abu Jir fault zone consists of linkage of several fault segments.Relay ramp structures are developed between two successive segments, moreover the intensity of the deformation of the brecciated and undulated limestone units in the upper part of the Euphrates formation is higher in the Sihailiya and Dolab areas in comparison to the surrounding areas to the northwest (Fig. 5 2).This intensity variation indicates that not all segments of the Abu-Jir fault zone were experiencing the same intensity of deformation during the time of activity.Additionally, fieldwork shows that a fault-propagation fold developed in the lower part of the first cycle of Fatha Formation in the Heet outcrop (Fig. 5G), indicating that the Abu-Jir fault zone was still active after the deposition of the first cycle of the Fatha Formation.Furthermore, hydrocarbon seepages can also be seen, which are coming out through cracks and fractures in the gypsum layer of the upper part of the first cycle of the Fatha Formation in the Heet outcrop.Hydrocarbon seepages are abundant within the Abu-Jir fault zone in western Iraq, especially in the Fatha Formation (Al-Aslami, 2015 andAl-Khafaji et al., 2020).This seepage suggests that the activity of Abu-Jir fault zone during the middle Miocene have made a path for the migration of oil.

Tectonic Mechanism of the Abu Jir Fault Zone Inversion
The current study revealed the inversion of the Abu-Jir fault during the late Early Miocene and Early middle Miocene due to compressive stress that caused the strike-slip movement of the Abu-Jir fault zone.This compressive stress led to the western block of the Abu-Jir fault zone moving northward with respect to the eastern block.Such stress is possibly directed from the south to the north.This hypothesis is validated by studying the surface structural features in the Euphrates and Fatha Formations.We noticed that these deformational features are gradually decreasing towards the northwest within the study area.This observation is further supported by subsurface seismic data and segmentation of the Anah graben to the eastern and western parts.All the reflective seismic sections in the southern part of Abu-Jir fault zone show the southwestern part of the fault zone to appear as negative flower structure, subjected to a strike slip movement, whereas in the northeastern part, the normal faults and grabens were not subjected to strike-slip movements (Alhadithi, 2017).This indicates that the western block of the Abu-Jir fault zone was displaced in a shear movement relative to the eastern block and suggests that the regional force that caused the strike-slip movement of the Abu-Jir fault zone likely originated from the southern area, not from the north.The movement of the western block of the Abu-Jir fault zone towards the north with respect to the eastern block is also suggested by (Marouf, 1999).Additionally, the Anah structure is divided into two parts: the eastern subsurface graben, and the western inverted graben (Figure 1).The eastern extensional part is isolated from the western inverted part by intragraben lateral ramp generated by the meeting of the Abu Jir fault with the Anah structure.The activities throughout both, the Anah and Abu Jir structures, were generally connected and probably contemporaneous (Marouf, 1999).This relationship indicates that the inversion of the western part of Anah graben potentially took place during the late Early Miocene and Middle Miocene as well.
Moreover, the Al-Jazira area, farther north of the study area, includes several subsurface extensional structures, such as the eastern segment of Anah, Tayarat South, Tayarat North, Khleissia, and Tel Hajar Structures (Figure 1) were formed at the same time of Anah graben.Inspection of the reflection seismic profiles across these structures shows no signs of inversion in all the grabens, except the Tel Hajar structure.The Tel Hajar structure is farther north, closer to the Sinjar structure, and it was inverted considered Plio-Pleistocene inversion similar to the Sinjar structure or Alna anticline within Zagros-taurus foreland due to the north to the south compressional stress of the Arabia-Eurasia collision (Brew et al., 1999;Fouad &Nasir, 2009, andAlhadithi andAlhadithi, 2020).According to the current study, the inversion in the study area occurred potentially during the late Early Miocene and early Middle Miocene depend on the ages of syntectonic deposits.If the compressional stresses that caused the inversion of the Abu-Jir fault zone and Anah graben originated from the north as a result Arabia-Eurasia collision, like Sinjar and Tel Hajar structures, inversion of the all grabens in the Al-Jazira area is expected prior to the Anah and Abu Jir fault system, it must be emphasized that only the western part of Anah graben has been inverted.Therefore, we argue that the compressive stress that inverted Abu Jir fault is directed from the south to the north and cannot be linked the Pliocene-Pleistocene north-tosouth collision-related compressional phase.Alternatively, the Arabian plate kinematics with a northward motion that enhanced by mantle dynamic is a more viable hypothesis.On the other hand, there are some strctures to the south of western part of Anah graben e.g.Akkas subsurface structure which develops by basemet activates during the Ordivician not by inverted preexisting normal fault as the structures mentioned above that dose not reactive during Miocene (Alhadithi, 2021).Activity of Abu Jir -Anah graben during Miocene related to surrounding structures good indicator that Abu Jir -Anah boundary is at least cut the crust entirely that received the stress and reactive.
The main source for the force that has been influencing the northward movement of the Arabian plate since the Late Eocene is the generation of the Afar plume and its northward flow beneath western Arabia (Figure 4) (Ershov & Nikishin, 2004;Chang et al., 2011a and b;Faccenna et al., 2013).These authors used distribution of magmatism according to their ages, earthquake wave velocity interpretations, and numerical modeling to track to northward flow of hot mantle material beneath western Arabia, which is likely dragging the Arabian plate with itself to the north.Additionally, based on age of magmatism and numerical modeling, the arrival timing of the hot mantle material to the areas parallel to the location northern Abu Jir fault zone and Anah graben is comparable to the timing of the documented inversion in this study, which is early-middle Miocene (Ershov and Nikishin, 2004;Faccenna et al., 2013).This northward directed force, in addition to the slab pull force of the downgoing plate beneath Eurasia in the suture zone, has contributed largely to the inversion of the northern Arabian plate.Such differences in the timing and location of inverted structure in the northern Arabia imply that the north-to-south stress transmission, as a result of slab pull force and collision, became strong enough to invert extensional structures by Pliocene.The earlier inversion event in the study area and northward movement of the Abu Jir western block during the early-middle Miocene is more affected by the Arabian plate kinematics, which is influenced by the mantle dynamic.(Ershov & Nikishin, 2004;Chang et al., 2011a and b;Faccenna et al., 2013).Inset map shows the interpretation of the Anah graben inversion during the early-middle Mioncene (this study) in the south and Sinjar and Abdel Aziz inverted structures during Pliocene (Brew et al., 1999).

Conclusions
The northern segment of the Abu-Jir fault zone was subjected to extension and contraction during the early and middle Miocene in response to foreland basin subsidence and the Arabian plate's northward movement, particularity the western block.On the northwestern side of the Abu-Jir fault, the upper Oligocene Anah Formation is exposed, whereas on the southeastern side of the fault it is deeper and followed by thick marly limestone layers of the lower Miocene Euphrates Formation.These variations in stratigraphic level and thickness indicate activation of the Abu-Jir fault as a normal fault and subsidence of the southeastern side as a result of the Zagros foreland basin development.Later during the early middle Miocene, transperssional stress associated with a strike-slip movement affected the Abu-Jir fault zone.This contractional phase led to formation of positive flower structures, a series of minor duplex structures, and movement of the Abu-Jir fault's western block northward relative to its eastern block.These contractional structures suggest Abu-Jir fault inversion and are manifested clearly in the upper part of the Euphrates Formation and the lower part of the Fatha Formation.This age of inversion of the Abu-Jir fault zone is older than the age of the northern inverted Sinjar and Tel Hajar structures, suggesting limited or no relation with the Pliocene-Pleistocene shortening propagation from the north.Based on structural interpretation, field relationships, regional tectonic synthesis of the northern Arabia during the early-middle Miocene, and the coeval arrival of the Afar plume-related mantle flow from the south to the north, the main force that inverted the Abu-Jir fault is inferred to be, the Arabian plate basal dragging by mantle flow that affected the western part.

Fig. 1 .
Fig.1.Location map and structural features in the study area.Some features fromFouad and Nasir (2009)

Fig. 4 .
Fig. 4. Regional map of the Middle East showing the flowing path of the mantle material in the upper mantle, beneath the western Arabia, since the latest Eocene (Ershov & Nikishin, 2004; Chang et al., 2011a and b; Faccenna et al., 2013).Inset map shows the interpretation of the Anah graben inversion during the early-middle Mioncene (this study) in the south and Sinjar and Abdel Aziz inverted structures during Pliocene (Brew et al., 1999).