Pre- and post-Messinian deformational styles along the northern Nile Delta Basin in the framework of the Eastern Mediterranean tectonic evolution

Abstract

The present work is focused on the structural deformation styles of the pre- and post-Messinian sequences of the Nile Delta Basin. The structural interpretations presented in this study shed critical insight about the evolution of the Nile Delta Basin through pre- and post-Messinian times, provides high reliability about its origin and development in the context of the Eastern Mediterranean tectonic history. The explanation for the seismic data covering three different gas fields (Baltim Field, Temsah Field and North Sinai Field) extend along the northern offshore portion of the Nile Delta Basin revealed significant variations in the deformational styles between the pre- and the post-Messinian sequences. The pre-Messinian rocks were mainly compressed in northwest-southeast direction creating folding-related compressional deformations which gradually decline eastward. The post-Messinian sequence exhibits differences in the deformational models between the examined fields. In Baltim field, it shows extensional features caused by the bulk northeast-southwest extensional deformation. However, in Temsah and North Sinai fields, this post-Messinian succession exposes north–south compressional deformation. These interpreted deformational directions are extremely well-matched with the regional structure configuration of the Nile Delta Basin and the eastern Mediterranean regions.

Introduction

The Nile Delta Bain is a giant gas provenance and has a long history of gas exploration from the Miocene up to the Pliocene sediments (Sarhan 2022, 2023; Shehata et al. 2023; Abdel-Fattah et al. 2023; Ewida and Sarhan 2023; Assal et al. 2023; Sarhan et al. 2022). The deltaic sequence of the Nile Delta represents the largest clastic accumulation on the northern passive margin of Africa (Badawy et al. 2015 and references therein). This margin is dominated by disrupted slivers of continental masses; such as the Eratosthenes Seamount (ESM), which are rifted from the Arabian continental margin since Late Cretaceous (Symeou et al. 2017; Jagger, et al. 2018; Segev et al. 2018) and currently came to the subduction zone of the Mediterranean arcs (Fig. 1; Roure et al. 2012; Günes et al. 2018), along the southern active margin of Europe (Gealey 1988; Abdel Aal et al. 2000; Okyar et al. 2005). The deltaic sequence of the Nile Delta section consists of two separate clastic successions about the unconformity surface of the so called “Messinian salinity crises” in the Mediterranean Sea region (Hsü et al. 1973; Krijgsman et al. 1999). They belong to the Oligocene–Miocene and Pliocene-Recent prograding delta systems that are commonly referred in many literatures as the pre- and post-Messinian deltaic systems; respectively (Summerhayes, et al. 1978; Sestini 1989; Marten et al. 2004). The Messinian salinity crises involve a unit of evaporite sequence that is primarily thicker within the deep centers of the Mediterranean basin; in contrast, it commonly disappears on the shelf and slope, instead it forms an erosional truncation surface dominated by numerous incised streams and reworked sediments (Ryan and Cita 1978; Barber 1981; Aal et al. 2001; Loncke et al. 2006).

Fig. 1

Modified after Abdel Aal, et al. (2000)

Bathymetry and structural map of the Eastern Mediterranean region involving the physiographic setting of the Nile Delta cone.

The Miocene tectonics in the northern African margin, concomitant with the accumulation of the Nile Delta succession, is dominated by a period of crustal rifting, which initiated the Afro-Arabian rift system (e.g. Gulf of Suez-Red Sea rift; Guiraud and Bosworth 1999; Bosworth and Mc Clay 2001). The fracture system of the Nile Delta substrate that accommodates its foundation is significantly linked with the Afro-Arabian rift system by the east–west trending Cairo-Suez shear zone (Fig. 1; Bosworth 1995; Hussein and Abd-Allah 2001; Shalaby 2017; Hammam et al. 2020). This shear zone represents the eastern termination of the Eastern Mediterranean south transform (EMST; Le Pichon et al. 2019), which crosses the southern extremities of the Nile Delta region. These fault systems inherited their trends through reactivation of preexisting faults of the Precambrian basements (Bosworth and Mc Clay 2001; Khalil and McClay 2002) particularly during the Oligo-Miocene rifting of the Afro-Arabian rift system (Guiraud and Bosworth 1999; Bosworth and Mc Clay 2001; Khalil and McClay 2002) and the Nile Delta region (Segev et al. 2018). The Early-Middle Miocene sediments—the rock units of pre-Messinian—of the Nile Delta syn-tectonically buried these fracture system, which are reactivated again in Messinian across the so called Nile Delta Hinge Zone (NDHZ; Fig. 1) (Sarhan et al. 2014; Sarhan 2015). The NDHZ is an east–west trending fracture zone passing through the mid-line of the onshore province of the Nile Delta; dominated by northward throwing growth faults (Harms and Wary 1990; Mosconi et al. 1996; Zaghloul et al. 2001). The pre-Messinian deltaic sediments entirely exhibit evidences of contractional deformation as they form fold-thrust belts across many parts of the distal provinces of the delta cone (Hamouda and El-Gharabawy 2019; Zucker et al. 2020); while they most likely show seaward dipping listric normal faults at its proximal parts (Zaghloul et al. 2001; Selim and Omran; 2012; Sarhan et al. 2014). In contrast, the post Messinian sequence is characterized by typical deltaic structures involving listric-growth faults and rotational block faults (Mosconi et al. 1996; Zaghloul et al. 2001; Selim and Omran; 2012; Sarhan et al. 2014), which are accommodated by seaward migration of the underlying mobile Messinian salt unit (Aal et al. 2001; Mascle et al. 2006; Loncke et al. 2006, 2010; Zucker et al. 2020).

We have considered, from different literatures across the NDHZ and surroundings (e.g. Sarhan et al. 2014; Abd-Allah et al. 2012; El Adl, et al. 2021; Leila et al. 2021, 2022) that the pre-Messinian successions are principally characterized by features of the active folding mechanism without clues of the presence of extensional features that would have been proximally synchronized to and behind of the fold-thrust belts at the delta toes. These pre-Messinian fabrics are seemingly pervasive across the whole Nile Delta cone even at it proximal parts, in contrast to the post-Messinian succession that is essentially dominated by extensional features. These features likely argue against the common evidences associated with the pure gravity-driven deformation regime on passive margins (e.g. Rowan et al. 2004; Morley et al. 2011).

Seismic interpretation is an essential tool for unraveling the subsurface geological framework and assessing hydrocarbon-reservoir potential in sedimentary basins (Sarhan 2017, 2019, 2020, 2021; Sarhan and Collier 2018; Shehata et al. 2018, 2019; Sarhan et al. 2017; Farouk et al. 2023; Sarhan and Selim 2023; Elbahrawy et al. 2023; Shehata and Sarhan 2022).

In this paper we describe the mode of structural evolution of the pre- and post-Messinian successions of the Nile Delta sediments based on seismic data interpretation. The possible reactivation of the preexisting bedrock-related crustal-scale fracture systems in modeling the pre- and post-Messinian delta successions are considered in the context of the structural evolution of the northern African passive margin. To achieve these goals, we draw out the structural patterns from two orthogonally oriented seismic lines and from time-depth maps of pre- and post-Messinian well-picked seismic horizons of selected three oil-fields; namely from west to east Baltim, Temsah and Northern Sinai (Fig. 2). These fields are located near to offshore parts of the Nile Delta shelf and along the eastern half of the seaward-facing circumference of the delta cone, which relatively situated about the region of the delta-slope break. The extracted structures are correlated and integrated with those occurring south and north of them.

Fig. 2

Location of the Baltim, Temsah and Northern Sinai gas-fields, used in this study for following the common structural features along the Nile Delta shelf margin. The attached line networks shows, with red-lines the two orthogonal seismic lines used for deciphering the structural elements pre- and post-Messinian erosional truncation surface

Geological setting

The Nile Delta cone is formed on the southern fringe of northerly extended African passive margin; along the southern border of the Eastern Mediterranean basin (e.g. Garfunkel 2004), which initiated rifting of the African plate since the Permo-Triassic up to the Lower Cretaceous (Walley 1998, 2001; Dolson et al. 2001; Montadert et al. 2014). The Eratosthenes Continental Block, capped with the Cretaceous carbonate platform, was rifted off from Arabia (Robertson 1998; Papadimitriou et al. 2018; Van Hinsbergen et al. 2020). It forms a conspicuous seamount on a common northeast-southwest trending submarine structural high along the Qattara-Eratosthenes line, between the Levant and the Herodotus basins (Fig. 1), Granot 2016; Hawie et al. 2013; Montadert et al. 2014). The whole Tethyan Ocean including the East Mediterranean Sea region underwent with the a period of North–South compression during the Upper Cretaceous until the Eocene forming the thrust belts of the Hellenic and Cyprus arcs (Fig. 1; Robertson and Dixon 1998). Rifting of the Arabia apart from the Africa in the Upper Eocene-Early Oligocene resulted in formation of the Suez-Red Sea rift system (Garfunkel 2004) that triggered with crustal uplifting of the Red Sea region (Shalaby 2017), which delivered large amounts of clastics that began to enter the Eastern Mediterranean basin during the early Miocene (Montadert et al. 2014). By the late Miocene in Tortonian, a substantial delta complex had already formed (Said 1981), which suffered great erosion due to falling of the sea-level during the Missinian salinity crisis (Barber 1981) that was followed by the Pliocene input of clastic sediments from the Nile river system (Aal et al. 2000; Leila and Moscariello 2019; Macgregor 2012), these sediments were deposited above the Messinian evaporitic deposits in a normal marine conditions (Ryan 2008).

The Neogene-Quaternary delta-succession of the Nile Delta Basin is accumulated on the late Cretaceous carbonate platform and the associated Oligocene-related volcanic plugs (Bayoumi and Lotfy 1989; Guiraud and Bosworth 1999; Zaghloul et al. 2001). This succession is deposited with successive regression-transgression cycles with marginal to open marine environmental conditions (Hashem et al. 2010). They commonly form a thick sequence of siliciclastic rock units that are remarkably interrupted by the Messinian evaporites of Rosetta Formation, which intercalated with few shale and carbonate interbeds (Rizzini et al. 1976). These evaporites form a regional-scale unconformable unit straddling the siliciclastics of pre- to post-Messinian rocks. Salts of these Messinian rocks withdrew in some parts of the delta succession due to their rheology and the differential loading of the overburden (Selim and Omran 2012; Zucker et al. 2020) leaving a remarkable truncation surface between these two former silciclastic sequences (Omran 2004; Hamouda and El-Gharabawy 2019); otherwise, the Missinian channels were active incising the pre-Messinian rocks (Dolson et al. 2001, 2005; Leila and Moscariello 2019; Leila et al. 2020) forming a broad surface of Messinian erosional truncation surface at proximal reaches of the Nile cone (Pigott and Abdel-Fattah 2014; Leila and Moscariello 2019; Sultan et al. 2020).

The base of the pre-Messinian succession has not been reached by well-drilling that stopped generally in the middle of the Sidi Salim Formation of Middle Miocene (Fig. 3; Rizzini, et al. 1976). It is composed mainly of clayey mudstone with few intervals of dolomitic marls and rare interbeds of sandstones and siltstones (Rizzini et al. 1978; Kamel et al. 1998; Dolson et al. 2005). This rock unit underlies onshore by the Tortonian–Early Messinian thick layers of sandstones and conglomerates, with some clay interbeds of the Qawasim Formation and synchronously offshore by shale/sand turbiditic complexes of the Wakar Formation (Kamel et al. 1998; Khaled et al. 2014; Sarhan 2015). These units are capped by a few tens of meters thick of Messinian evaporites of the Rosetta Formation, intercalated with thin beds of clays (Leila and Moscariello 2019; Leila et al. 2020). The thickness of these Messinian evaporites in the Nile Delta cone increased substantially northeastward measuring up to 2 km in Levant basin (Manzi et al. 2005; Roveri et al. 2014). The post-Messinian sequence is called in the Nile Delta Basin as the post-rift megasequence (Sarhan et al. 2014; Sarhan and Safa 2019), resting unconformably on the underlying Messinian evaporites. It was deposited by prograding clastic sedimentation processes where the sediments were charged from distal fluvial system located to the south (El-Fawal et al. 2016). It includes from bottom to top the Kafr El Sheikh Formation of Early–Middle Pliocene, El Wastani Formation and Mit Ghamr Formation of Late Pliocene, Bilqas Formation of Quaternary age (Fig. 3; EGPC 1994). The sedimentary section of this sequence grade from the outer shelf marine shale of the Kafr El-Sheikh Formation, passing through the delta front-related mainly sandstones of the Wastani Formation, up to distributary mouth bars-related pebbly sandstones of the Mit Ghamr Formation (Mohamed 2004; El-Fawal et al. 2016) that is ended by Holocene transgression depositing few tens of meters of caly/sand interbeds of the Bilqas Formation. The Abu Madi Formation, locally underlies the Kafr El Sheikh Formation, is composed of thick layers of rarely conglomeratic sandstone with some intercalations of shales, which increased frequently atop beneath the shale of the Kafr El Sheikh Formation (Zaghloul, et al. 2001).

Fig. 3

Simplified stratigraphic column of the offshore Neogene-Quaternary succession in the Nile Delta Basin

Structural setting of the Nile Delta is controlled by the tectonic framework of the northern margin of the African plate that is actively subducted along Crete and Cyprus thrust belts since late Cretaceous (Fig. 1; Dixon and Robertson 1984; Sarhan and Hemdan 1994; Harrison et al. 2013; Van Hinsbergen et al. 2020). The Nile Delta cone is formed on the northern African passive margin, which extends northerly as a thinned continental crust; in contrast to the Egyptian mainland and the Eratosthenes Seamount (Cloetingh et al. 1980; Zaghloul et al. 2001; Luccio and Pasyanos; 2007). Crustal extension along this margin resulted in an E-W flexure zone, the Nile Delta Hinge Zone (NDHZ) that controlled the Paleogene–Neogene tectono-stratigraphic evolution of the Nile Delta (Fig. 1; Orwig 1982; Kamel et al. 1998). It is controlled by deep crustal faults inherited from the Early Mesozoic rifting phase (Mosconi, et al. 1996; Tari et al. 2012; Tassy et al. 2015) straddling the platformal carbonates of the south delta block to the north Nile Delta basinal facies (Sarhan and Hemdan 1994; Kamel et al. 1998). This zone is a common structural boundary between the so called Southern and Northern Delta basins, south and north of it; respectively (Fig. 1; Saleh 2012; Elsaka et al. 2020; Hassan et al. 2021). About 80 km apart south of the NDHZ, the East Mediterranean South Transform fault (EMST, Fig. 1; Garfunkel and Derin 1984; Ben-Avraham et al. 1987) is a prominent shear zone extending in the west-northwest direction along the northern Egyptian coastal margin with a sinistral sense of motion (Longacre et al. 2007; Gardosh et al. 2010). It continues easterly, passing through the southern half of the Nile Delta region (Fig. 1; Le Pichon et al. 2019); therein, it forms the southern boundaries of the Southern Delta Basin (Ibrahim et al. 2017; Hassan et al. 2021).

The continental shelf of the Nile Delta is bounded from its eastern and western margins by two main antithetic shear zones; the Temsah and Rosetta shear zones; respectively (Fig. 1; Abdel Aal et al. 1994, 2000; Barsoum et al. 1998; Hanafy et al. 2017). The Temsah shear zone extends in the northwest-southeast direction, with a dextral sense of motion, between the Eratosthenes Seamount (EMS) and the northern rim of the Nile Delta shelf. It commonly embraces in seabed extensional scars and mini-basins, which buried southerly along the eastern slope of the delta shelf by gravitationally collapsed shelf sediments (Fig. 1; Mascle et al. 2000, 2006; Loncke et al. 2006, 2008). The Rosetta shear zone rims the western margin of the delta shelf in the northeast–southwest direction with a sinistral sense of motion. It displays a large-scale structural relief, created by left-lateral transpressional movements along the Qattara-Eratosthenes line (Fig. 1; Abdel Aal, et al. 2000). This line is a common structural boundary between the Levant and Herodotus basins from the east to west; respectively (Segev et al. 2018). It involves activation of preexisting Late Cretaceous faults (Meshref 1990; Shalaby and Sarhan 2021) until Late-Middle Miocene. Accordingly the shelf slope along this line is steadily eroded with turbidity channels flowing to Herodotus basin with little evidences of Plio-Pleistocene reactivations (Fig. 1; Abd El-Fattah et al. 2021). This is in a contrast to the Temsah shear zone, which widely shows Pliocene wrench activities (Abdel Aal et al. 2000).

Data and results

Baltim oil field

The Baltim oil field occurs on the north–south oriented med-axial line of the Nile Delta cone extending from the delta onshore to some hundred kilometers offshore (Fig. 2). It spatially occupies an elongated concession overlaying a northward-running Abu Madi valley that incised its course during Messinian through the Early Miocene sediments (Issawi and McCauley 1992; Gargani et al. 2012). By the advent of Pliocene, it is backfilled beneath the thick (~ 4 km, Segev, et al. 2006) Pliocene–Quaternary sediments (Samuel et al. 2003; Abd-Allah et al. 2012; El Adl et al. 2021) during the flooding periods of Atlantic Ocean to the Mediterranean Sea basin (Garcia-Castellanos et al. 2020). The Baltim field and the whole area of the present day onshore delta consisted of a wide, elevated region almost devoid of Messinian salt deposits, which has been interpreted as a Messinian shelf or a detrital cone (Loncke et al. 2002; Abd-Allah et al. 2012; Abdel-Fattah and Tawfik 2015). The Neogene deltaic section is a siliciclastic sequence ranging from, base to top, the Sidi Salim up to Mit-Ghamr formations (Fig. 3). We picked up two well-discerned seismic horizons, representing the pre- and post-Messinian rock units, for structural mapping to understand the orientation of dominant structures before and after Messinian sea level drops in the Mediterranean Sea region. Structural mapping of the Messinian truncation surface was ignored because the map that would result might be structurally misinterpreted as it would map the topography of paleo-channels incising the Messinian erosional surface (e.g. Abdel-Fattah and Tawfik 2015; El Adl et al. 2021) rather than the dominant structural setting, which has become greatly obscured by erosional activities of these channels (Sarhan 2015).

The structural features from longitudinal and transverse seismic lines

The longitudinal seismic line (Line No. 6850) extends in the N–S direction for about 40 km, parallel to the axial line of the Nile Delta cone (Figs. 2 and 4a). It reveals two major deformation styles about the Messinian erosional surface. The pre-Messinian units generally display three distinct horizons of strong seismic reflectors, all of them, particularly the lower one; extend relatively in a horizontal attitude with some undulations (Fig. 4a). The middle and upper reflectors display a major anticline at the southern end of the profile while the lower seismic horizon extends relatively flat (Fig. 4a). It represents the base on which the rock units are shortened horizontally forming shallow, with broad summits, pop-up structures underneath the core of the major anticline. Therein, these pup-ups and related thrust faults thicken the core of this major fold particularly in the region between the middle and lower strong reflectors (Fig. 4a). This highly horizontally compressed core is topped with extensional-related features including normal faults and the related, axial plane-parallel tensional vertical cracks (Fig. 4a). These faults form half grabens, which are tilted moderately northward. The seismic horizons in this region of predominantly extensional features show discrete continuous bands that fade upwards into a hazy zone of ill-developed bandings where the horizons are segregated into pieces that reflect a zone of highly fragmented rocks; typically form a discrete region of brecciation (Fig. 4a). The limbs of this major anticline are associated with S and Z parasitic fold types, associated with contractional duplexes and fan structures commonly showing top-to-major fold summit tectonic transportation that is locally dominated by neutral M-fold pattern (Fig. 4a). Northward of these contractional fabrics an isolated detachment anticline gradually fades upwards on the distinct upper undifferentiated horizon (Fig. 4a). Next to this anticline, the seismic horizons extend horizontally, however they display intra-formational rootless folds and duplexes, then they terminate again on a syncline associated with Z-, S-, and M-parasitic folds (Fig. 4a). The hinge zone-related axial plane cleavages of these folds exhibit vertical displacements that may sole deeper into a ramp segment of a basal detachment on which the detachment folds and the major anticline originate by shortening above its flat segment (Fig. 4b). The topmost seismic band underneath the Messinian truncation surface, north of the highly fractured hinge zone of the major anticline, represents a package of highly folded and imbricated thrust duplexes that displaced horizontally over a bedding-parallel flat-laying detachment. The orientation of these thrusts and related fold-asymmetries implies that the pre-Messinian rock units underwent shortening deformation with dominant top-to-south tectonic transportations. The Messinian truncation erosional surface is relatively flat with shallow undulations; however it profoundly erodes the summit of the major anticline deeper at two discerned time-span levels (the seismic line of Fig. 4a). The deeper one “the level of early truncation” forms a trough of a former wide, relatively deep, palo-channel. Within this channel, the seismic horizons are hazy and ill-developed; however they show in some parts some of flat and fragmented horizons that resemble stratified laminations of lacustrine or riverine sediments. The shallower “the level of later truncation” extends laterally to join the common regional level of the early truncation surface. It erodes deeply the sediments of the former channels forming a canyon immediately above the crest of the deeply buried major anticline. This canyon is concealed with laminations of sediments that are largely resembles to the underlying early backfilled riverine sediments. These sediments extend laterally to onlap the surface of Messinian truncation on banks of the former channel system. The post-Messinian sediments are relatively thick forming two main sequences of deltaic sedimentary packages, the lower one form a wedge-shaped that thins gradually northward (Fig. 4a, b). Its sediments exhibit laminations that slope monotonously gently northward, onlapping the Messinian surface of truncation without any evidences of continuing pre-Messinian deformations on this longitudinal profile. This wedge-shaped sequence is topped by thick units of sediments that commonly form seaward-dipping clinoforms.

Fig. 4

a Longitudinal seismic line (Line no. 6850) in Baltim Field (for its location see Fig. 2) and its interpretative illustration. Note that the pre-Messinian rocks are highly folded while the post-Messinian rocks form a carapace of prograding clinoforms. b Schematic diagram simplifying the common structure pre- and post-Messinian rocks. c The transverse seismic line (Line no. 2425) and its representative illustration along which the Messionan erosional surface commonly deformed in response to evolution of the deeply-seated structures of pre-Messinian rocks. d Compressional deformation led to emergence of pre-Messinian thrust packages, bending the Messionan erosional surface and deformation of the immediately overlaying post-Messinian rocks by giding. e At deeper levels, shortening of the pre-Messinian rocks results in upward buckling of tight anticlines, followed by normal faulting of the Messinian erosional surface that throw the post-Messinian rocks down to the core of adjoining syncline

The transverse line No. 2425 extends in the east–west direction for about 20 km. It crosses the northward trending major fabrics (Fig. 5). In pre-Messinian seismic horizones, the profile shows three major strong seismic reflectors (Fig. 4c). The bands between these reflectors commonly show a major anticlinal box fold, which displays a wide flat and thick hinge zone and narrow westward relatively steeply-dipping fore-limb (Fig. 4c). Two eastward-dipping listric thrust faults bound this major anticline on its eastern and western limbs, which are parasitically folded with S- and Z- fold types; respectively while its hinge zone is dominated by M-folds. This hinge zone is topped by a band of imbricated westward-dipping listric normal faults (Fig. 4c). The eastern termination of this hinge zone is progressively imbricated by eastward-dipping thrust packages and the related tight folds. The tight anticlines pierce upward through vertical faults forming a prominent flower structure (Fig. 4c) that arches the Messinian truncation surface convexly upward relative to the next neighboring shallow basin (Fig. 4d). The S-fold pattern, flower structure and blind thrusts on the eastern limb of the major anticline (Fig. 4c) is dominated by limb-parallel top-to-west tectonic movements that push the core of the eastern thrust package upward (Fig. 4d). This in turn forced the Messinian truncation surface to buckle up relative to the fringing basin at the toe of the pushed thrust package (Fig. 4d). This basin is filled with horizontally-laying, well-laminated Messinian turbidities. These turbidities unit, in a reference to the surfaces of blue continuous horizons (Fig. 4d), pinch gradually out easterly onlapping the emerged thrust package; and remarkably thicken westerly off the fringing basin. The synclinal drag fold and the shallow pop-up structure are common features formed within these turbidities (Fig. 4d). The structural setting of these features indicates that (1) the synclinal drag fold is seemingly formed by gravity gliding; downslope of the Messinian erosional surface truncating the emerged thrust package and (2) the shallow pop-up structure is formed within the basin by a local horizontal compression that seemingly originated from gravitationally collapsed, basin-ward gliding Messinian sediments at the toe of the basin margin, in response to active relative uplifting of the emerged thrust package.

Fig. 5

a Time-contour and interpretative maps of a well-picked pre-Messinian seismic horizon showing that the rock units are folded about two adjacent north-trending clinoriums; their common limbs is deformed with southward imbricated thrust fan structure at the southern ends of a northward trending left-lateral shear zone. b The time-contour and interpretative maps of a post-Messinian horizon show the dominancy of northwest-ward extensional deformation; the schematic cross-section explains the geometry of rollover folds on eastward-dipping normal faults

On the western end of this transverse profile, the former major anticline adjoins westerly an open syncline that is evolved geometrically into a westerly-tilted half-graben on an eastward moderately-dipping major normal fault (Fig. 4e). This fault originates via uplift of its western hanging wall block—relative to the adjoining syncline—which seemingly evolved through vertical buckling of a deeply seated tight anticline, associated with regional east–west shortening deformation (Fig. 4e). The Messinian sediments, deposited within this half-graben thickened and slightly tilted toward the hanging wall side of its eastward dipping imbricate normal faults (Fig. 4e), resembling the syn-rift sediments of rift basins. The post-Messinian rocks concealing these syn-rift sediments, form drape folds on the bounding normal faults (Fig. 4e). In contrast, the eastern limb of the adjoining syncline remains intact; however it is dominated by Z-type parasitic folds. This limb gradually thickens upwards; therein it is occupied with chaotic splays of seismic horizons that are likely features of disharmonic folds and related structures (Fig. 4e). Hence it swells up accommodating horizontal extension at upper levels, which locally results in formation of eastward dipping normal faults. These faults fade into the region of chaotic structures; therein the horizontally-laying horizons of the neighboring post-Messenian rocks diffuse laterally into this region (Fig. 4e).

The structural features from time-depth maps

The time-depth contour map of an undifferentiated surface of a strong seismic reflector of a pre-Messinian rock unit is picked up for mapping (Fig. 5a). This map shows two main contiguous regions of shallow and deeper levels occupying the western and eastern halfs of the Baltim field; respectively. The contour lines regionally curve in both regions showing two major anticlinorium and synclinorium (Fig. 5a). The contour lines at their hinge-lines are irregular that suggest superimposition of minor folds. The contour lines on the common limb of these two major folds trend in the north–south direction with slight undulations, which indicate that it contains a chain of many northeast-plunging minor folds; arrayed in an en-echelon pattern, of which its orientation reveal that the common limb has underwent a component of left-stepping strike-slip movements in the north–south direction (Fig. 5a). These strike-slip movements terminate southerly on southward verging thrusts at the toe of the southward-plunging anticlinorium, which typically form a pattern of strike-slip-related imbricate fan structural geometry. This pattern reveals that the pre-Messinian rocks have experienced shortening deformation in the northwest-southeast direction.

The time-depth contour map of the top of Kafr El Sheikh Formation regionally displays a regular contour pattern as their contour lines extend roughly in the east–west direction with some few undulations (Fig. 5b). The contour spacing is roughly homogenous; however it slightly decreases gradually northward where it becomes deeper. Few undulations of the contour lines reveal northward plunging anticline-syncline pairs (Fig. 5b). The regular, shallowly undulated contour patterns are principally truncated by northward trending, mostly northeast-dipping normal faults and related tensional cracks (Fig. 5b). As there is no evidence of post-Messinian horizontal shortening deformations documented in the previously described seismic lines; and bounding these folds with their hinge line-parallel normal faults (Fig. 5b), we suggest that they are typically a type of normal fault-related rollover folds (Fig. 5b; see the inset cross-section). The northwest orientation of these structural features reveals that the post-Messinian rock units have experienced extension in the northeast-southwest direction.

Temsah oil field

The Temsah Field is located in the northeastern part of the continental shelf of the Nile Delta Basin (Fig. 2). This field extends in the northwest-southeast direction parallel to the shelf edge.

The structural features from longitudinal and transverse seismic lines

The longitudinal profile no. L1156 extends in the northwest-southeast direction for about 35 km, parallel to the major orientation of pre-Messinian fold hinge-lines (Figs. 6 and 7). The pre-Messinian seismic horizons commonly display flat patterns with regular bandings (Fig. 6a). They relatively extend in a horizontal attitude; however they dip, at their ends, shallowly outwards implying that it crosses an open anticline with a broad hinge zone (Fig. 7). Discrete five strong undifferentiated seismic reflectors precisely distinguish five bands of variable thicknesses. The thicknesses of thin bands are relatively consistent along the profile; in contrast the thickest bands are variable as their thicknesses increase slightly toward the center of the hinge zone of the representative anticline (Fig. 6a). The seismic horizons within these bands show closely-spaced wrinkled rootless tight folds, with Z-, S- and M-parasitic fold patterns and related fore- and back-thrust faults, which occasionally bound some pup-up structures. The topmost seismic band that immediately underlies the surface of Messinian truncation exhibits disconnected and complexly tightly folded seismic horizons in contrast to the underlying bands, which are relatively continuous and relatively display shallowly undulated horizons (Fig. 6a). The surface of the Messinian erosional truncation tapping the upper seismic band is seemingly undulated displaying folds and pup-up structures (Fig. 6a). It is commonly truncated by southward dipping imbricate thrusts that commonly show top-to-northwest tectonic transportations (Fig. 6a). Further northwest, the Messinian erosional surface buckles significantly above a discrete V-shaped vertical zone of chaotic structural features (Fig. 6a). The seismic horizons within this zone exhibit an interference pattern of disharmonic tight folds, which are locally fragmented into separated pieces along surfaces of axial cleavage. Therefore, we deduced that this zone is likely a transpressional shear zone that seemingly has originated from horizontal squeezing and subsequent flow of its rock materials vertically upwards.

Fig. 6

a Longitudinal seismic line (Line no. L1156) in Temsah Field (for its location see Fig. 2) and its interpretative section. The pre-Messinian seismic horizons extends shallowly with signs on internal compressional deformation which are evolved northerly by a transpressional shear zone which buckle the overlying Messinian erosional surface. The post-Messinian rocks are shortened immediately above the Messinian surface and gradually lengthened upwards by extension. Salt migration at toes of normal faults drives their compressional and extensional deformations. b The transverse seismic line (Line no., T5350) explains the interaction between pre-Messinian tight folding and buckling and the post-Messinian compressional, followed by extenstional deformation

Fig. 7

a and b Time-contour (from Sarhan and Safa 2019) and interpretative maps of the lower and upper surfaces of the pre-Messinian Sedi Salim Formation showing concordant tight anticlines. Note that the orientation of the lower anticline is dislocated at the upper surfacen by lateral displacements. b The time-contour and interpretative maps of the upper surface of the post-Messinian Kafr El Sheikh Formation. The early developed northwest-trending folds are superimposed by a generation of east–west oriented major anticline. Shortening deformation and buckle folding are evolved westerly by eastward dipping normal fault zone (See the transverse line of Fig. 6b)

The post-Messinian seismic horizons are highly undulated that differ from the fairly flat-laying horizons of the pre-Messinian rocks (Fig. 6a). The rock units immediately above the surface of Messinian erosional truncation are intensively chaotically folded. Areas of weak reflectors above the Messinian erosional surface are recognized as the regions dominated by Messinian salts, similar to seismic facies of salts in neighboring fields (Fig. 6a). The seismic horizons of post-Messinian carapace, above these salts are strictly passively folded through vertical forces, triggered by salt tectonics (Fig. 6a). Salt rollers in cores of the footwalls of northwestward-dipping normal faults convexly bend the post-Messinian carapace upwards forming anticlinal-drag folds. Salts withdrawing and migration underneath hanging walls of these normal faults resulted in (1) generation of shear stresses on the underlying Messinian truncation surface that consequently led to its fragmentation with imbrications on northwestward-verging thrusts, and (2) vertical subsidence of the overlaying post-Messinian carapace that concavely bends upward welding the pre-Messinian rocks through the Messinian truncation surface (Fig. 6a). Northwest next to the Messinian transpressional shear zone, the salt layer has topped with a flat-laying, post-Messinian truncation surface that represent a detachment of the overlaying post-Messinian syncline (Fig. 6a). This syncline displays a tight trough at great depths, while it gradually has an open and thickened core at upper levels, which indicate that its core is advanced shallowly with syn-tectonic growth strata. Hence, the deformation in post-Messinian rocks has gradually changed from deep shortening to shallow lengthening. At deeper levels in regions adjacent to the fault toes, the post-Messinian bands are occasionally thickened their on hanging walls through intra-formational stacking of contractional duplexes (Fig. 6a). These post-Messinian folds decay gradually upwards until a well-developed flat-laying seismic horizon that is precisely traced as a surface of folding termination (Fig. 6a). Above this surface, the seismic horizons are highly flat-laying, however evidences of local contrctional deformation such as pup-up and imbricate thrusts are common (Fig. 6a). Horizontal extension at upper levels resulted in formation tensional fractures that terminate upwards on rollover synclines and downwards on, and sometimes penetrate deeper beyond, the surface of Messinian truncation.

The transverse seismic line no. T5350 extends in the northeast-southwest direction for about 20 km (Fig. 6b). It orthogonally crosscuts the northwestward elongated time-depth contour feature of a macroscopic-scale fold pattern (Fig. 7). The pre-Messinian seismic horizons display a pattern of a single major tight anticline. The eastward and westward dipping limbs of this fold and its hinge zone comprise of S-, Z- and M-parasitic fold types; respectively. The seismic horizons on both sides of this anticline have experienced a component of a horizon-parallel extension, causing these horizons to split into separate pieces like the symmetrically stretched boudins. On the hinge zone of this anticline, the fragmented horizons are commonly bounded by vertical surfaces of axial cleavages, some of which evolved into faults that displace these fragments vertically (Fig. 6b). The thickness of rock units is seemingly constant across this major anticline. In contrast, the presented portion of the core of the presumably adjoining eastern syncline is implicitly thickened by a wedge of accumulating parasitic folds (Fig. 6b). Horizontal shortening triggered folding results in penetrating the pre-Messinian anticline upwards through the overlying Messinian erosional truncation surface. In post-Messinian, the seismic horizons commonly show a carapace of fold chains (Fig. 6b). These folds are dominated by S- and Z-types of parasitic folds on their eastern and western fold limbs; respectively while the M-type folds commonly marks their hinge zones. Intra-formational folds are also formed locally on one of the westward-dipping fold-limb (Fig. 6b). Asymmetry of these folds reveals that they verge parasitically with top-to-summit tectonic movement of adjoining anticlinal, thus they considerably thicken hinge zones of involving folds (Fig. 6b). All folds of the post-Messenian horizons are later deformed by a wide zone of easterly dipping normal faults and the related tensional fractures (Fig. 6b). These faults penetrate deeper into the eastern limb of the deeply seated pre-Messinian anticline; therein they remarkably fade beyond the Messinian truncation surface. The well-defined post-Messinian folded horizons on the footwall side of this fault zone are remarkably underlain by a hazy region of ill-developed seismic horizons that are formerly defined as a region of predominating Messinian salts that immediately overlies the Messinian truncation surface (Fig. 6b). The top-surface of this region is remarkably flat-laying, truncating the overlying highly deformed horizons; therefore it is considered to be a detachment surface of the overlaying post-Messinian horizontally shortened folded horizons. In contrast, the post- and pre-Messinian folded horizons on the hanging wall side of this normal fault zone welded directly together, which suggest salt withdrawal under the effect of vertical forces originated from relative down-throw motion of the hanging wall faulted block (Fig. 6b).

The structural features from time-depth maps

Time-depth contour maps of the upper surfaces of all rock units constituting the Nile Delta sequence in the Temsah gas field as well as the top-surface of its substrate were previously constructed by Sarhan and Safa (2019). We re-evaluate these maps by tracing the structural features of three of them representing the upper surfaces of the delta substrate, i.e. bedrock of the delta sequence, and that of Sedi Salem and Kafr El Sheikh formations for the pre- and post-Messinian rocks; respectively (Fig. 7).

The time-depth contour map of the substrate underling the Miocene of the Nile Delta sequence (Fig. 7a) in Temsah field reveals a mid-major central high of an elongated closure, which indicates a major northwest trending doubly plunging anticline. The contours exhibit some few irregularities on both sides of the fold, which envisage slight sinistral dislocations of its fold hinge-line with reverse faults that diagonally oriented to its major trend (Fig. 7a). The contour lines decline rapidly northward of this central anticline; and extend in a direction parallel to the trend of its hinge-line that implicitly suggests an adjoining northwestward plunging syncline. The southern region, next to the central anticline, is predominated by northeast-trending normal faults bounding structural deeps that fairly oriented orthogonally to the major trend of the hinge line of the central anticline (Fig. 7a). The mutual relationship between these structural features reveals that the delta region has experienced, with the advent of Miocene, shortening deformation in the northeast-southwest direction.

The time-depth contour map for the upper-surface of Sedi Salim Formation shows that its contour pattern is relatively consistent with the deeply seated structural features of the pre-Miocene Nile Delta substrate (Fig. 7b). Consequently, this map comparatively displays, as like as deeper structures, central northwest-trending major anticline. However its contour patterns are significantly irregular showing a chain of east-northeastward plunging folds on its northern side that slightly oriented diagonally to its major trend. In contrast to the similar hinge-line of the deeper central anticline, the hinge-line of the central anticline of the current formation is extremely wavy and discontinuous, and seemingly to be dislocated on reverse-dextral faults (Fig. 7b). At the southern fringe of the mapped area, two local antithetic east–west dextral and northeast-southwest sinistral faults partially dislocate the hinge-line of the central fold laterally from its regional trend to plunge westerly (Fig. 7b). These antithetic faults and the re-orientation of fold hinges of central anticline and the diagonally oriented related reverse faults suggest that the Sedi-Salim Formation has been subjected to shortening deformation in the north-northwest—south-southeast direction during late early Miocene.

The contour pattern of the time-depth contour map of the top-surface of Kafr El Sheikh Formation of post-Messinian is incongruent with those of the underlying pre-Messinian rock units (Fig. 7c). This is shown by the fact that the region of abnormally higher contour values exist on the east–west direction, which trend diagonally relative to the deeply-seated northwest-trending central anticline of former pre-Messinian rock units (Fig. 7c). Contour closures show two main diagonally intersecting trends of fold hinge-lines, which trend in the east–west and northwest-southeast directions (Fig. 7c). The contour line spaced narrowly on both sides of each closure, which indicate that the corresponding folds have steep limbs with narrow summits and troughs. Thus they geometrically form tight folds, which are seemingly bounded by northwest-trending hinge-line-parallel normal and reverse faults (Fig. 7c). From our investigation of transverse profile (Fig. 6b) we considered that both fault types are locally closely subsequent and/or contemporaneous.

North sinai oil field

The structural features from longitudinal and transverse seismic lines

The North Sinai Concession occupies the eastern flank of the Nile Delta cone about 65 km northeast of Port Said within the offshore part of the Nile Delta Basin (Ewida, and Darwesh 2010; Selim and Omran, 2012). This field is located further east of the Nile Delta cone; some tens kilomters north of its hinge zone and near to the fringe of the gravitationally collapsed delta-slope (Fig. 1).

The longitudinal profiled no. 6014 extends for about 18 km in the north–south direction (Fig. 2). The pre-Messinian rocks mostly show, at deeper levels thick seismic strips rather than those exhibiting thin horizons at shallow levels (Fig. 8a). The deeper seismic strips are relatively irregular exhibiting rough surfaces, which are occasionally serrated as they engulf and cusp into the surroundings (Figs. 8a, b). Some seismic strips mutually bulge into the adjoining one forming finger-like protrusions; others include some boudins (Fig. 8a). These features are pervasive in seismic strips of deeper levels while they gradually fade upwards. The mechanical heterogeneity of the interlayered sand/shale sequence of the pre-Messinian rocks at great depths, under the effect of the vertical overburden stress, led the incompetent shale to engulf into the competent sands forming inter-fingered cusp-like features (Fig. 8b). The seismic strips of the pre-Messinian rock units in the northern half of the profile are relatively flat-laying (Fig. 8a); however they show some slight undulations that evidently terminate on a discrete tight buckled anticline. The thickness of some seismic strips increased proportionally in the hinge zone of this fold while they are stretched along its both limbs (Fig. 8c). These thickened horizons are formed of a chain of conjugate array of tight minor folds (Figs. 8c, d). They extremely stiffen toward the summit of the fold that is advanced eventually to be tightly flattened to form axial plane foliation (Fig. 8e). Asymmetry of these tight folds and their conjugate array indicate that this fold is achieved by imbrication of summit-ward verging minor folds through horizontal shortening on the north–south direction (Fig. 8e). The seismic horizons next to this fold are slightly folded that terminate southerly by a pair of anticline/syncline tight chevron folds (Fig. 8a). These folds split into pieces through axial cleavages, which advanced into reverse faults on their hinge zones due to horizontal shortening in the north–south direction.

Fig. 8

a Longitudinal seismic line (Line no. 6014) in North Sinai Field (for its location see Fig. 2) and its interpretative section. The pre-Messinian seismic horizons are mostly flat except little regions of slightly folded, which evolved upward similarly folding the Messinian erosional surface. The lower section of post-Messinian carapace underwent shortening deformation possibly through salt migration and welding; as like as Temsah field, it gradually evolved upwards by extensional deformation. c Incompetent rocks protrude through the upper competent units as cusp- and finger-like features under vertical overburden stresses in pre-Messinian rocks. c and d Close up views for evolution of conjugate arrays of microscopic-scale tight folds in the central anticline, evolved with horizontal shortening; e is a representative diagram of their generation. f–h Micro-fabrics of the Messinian erosional surface; the asymmetry of tight folds and rotated imbricated fabrics suggest shortening about the central buckle anticline, which is the upper continuation of that of pre-Messinian units. g The transverse seismic line (Line no. 1668) shows that the Messinian rocks occur in a band straddling the pre-Messinian shortening to the post-Messinian stretching deformations. h Close-up of the pre-Messinian central anticline in which its core is composed of an accumulation of microscopic scale tight folds. i Close-up view of the Messinian band illustrating its micro-fabrics indicating horizontal stretching in reversal directions about the central anticline. j Illustrative diagram for evolving extensional deformations to the east due to salt migration on the Messinian detachment surface at the toes of normal faults and their contractional deformation west of central anticline for the post-Messinian rock units, these structures seemingly linked with horizontal shortening in pre-Messinian units

The Messinian erosional truncation surface is characterized by a band of thick seismic horizons of high reflectance in contrast to the equivalent thin truncation surfaces in Baltim and Temsah fields (Figs. 4 and 6). The structural pattern along this band is significantly concordant to the structural features in the pre-Messinian rocks (Fig. 8a). The northern part of this band exhibits continuous, relatively flat horizons with weak undulations, which shows discrete M- and Z-folds with extensive pieces of rotated boudins (Fig. 8f). The central part of this Messinian band is dominated by a chain of anticline/syncline pairs (Fig. 8g). The folded seismic horizons are discontinuous suggesting that they underwent early shortening and later lengthening deformations with advancing folding that resulted in thickening and thinning their hinge zones and limbs; respectively. The Z-type folds, imbricate fabrics, rotated boudins and inclined shear foliations on the fold limbs indicate top-to-their summit tectonic transportations (Fig. 8g). The M-type folds are pervasive in hinge zones of these folds, which buckled upward to penetrate the core of post-Messinian folds. The southern part of the Messinian bands displays a regional anticline with a broad hinge zone, thickened also with tight M-type folds (Fig. 8h). The seismic horizons are stretched on its limb in which the stretched fragments are later imbricated with a top-to-anticlinal summit sense of transportation (Fig. 8h). These features indicate that the structure of the Messinian bands is achieved by shortening in the north–south direction that is seemingly the upper continuation of the pre-Messinian deformation.

The post-Messinian sequence commonly shows an extensive inflation, which are cored with anticline/syncline pairs (Fig. 8a). The seismic horizons are fairly continuous; and their thicknesses remain constant across theses fold pairs. The parasitic Z-, M- and S-type folds occur essentially at great depths of this sequence, which outstandingly disappear gradually upwards, therein the folded seismic horizons turn into flat-laying (Fig. 8a). Thus, the post-Messinian fold pairs are formed by active buckle folding mechanisms through contractional deformation in the north–south direction. The homogeneity of these fold patterns about the Messinian seismic high-reflectance band indicates that the pre-Messinian contractional deformation that triggered the deeply seated structural features has continued through Messinian up to post-Messinian rock units. Buckle folding of the post-Messinian rocks accommodates extension at upper levels. The northern extremities of the post-Messinian rocks on the current seismic profile exhibit a large-scale syncline, which based on the flat-laying surface of the northern part of the Messinian band (Fig. 8a), which represents a detachment surface of this overlying strongly folded post-Messinian syncline. Both fold limbs show horizon-parallel reversal senses of transportation (Fig. 8a).

The transverse line no 1668 extends in the east–west direction for about 25 km (Fig. 2). The seismic horizons of the pre-Messinian sequence generally exhibit a texture of irregular, discontinuous and relatively thick gelatin-like bands (Fig. 8g). These bands are flat-laying with little undulations; however a central anticline of rounded hinge zone is formed in the center of the profile. Its core is thickened by an aggregation of mottled irregular features while its limbs are correspondingly thinned as they stretched during active folding processes (Fig. 8h). This fold is based on a flat-laying seismic band, which signifies that it is a type of detachment fold. The parasitic folds on fold limbs of this fold are extremely tightened (Fig. 8h); however their asymmetries reveal top-to-anticlinal summit tectonic transportations.

The Messinian seismic horizons show a thick band of two parallel strips of strong seismic amplitudes (Fig. 8i). This Messinian thick band is undulated forming a central anticline bounded on its both limbs by two broad synclines. This central anticline represents the upper continuation of the deeply-seated pre-Messinian detachment anticline (Fig. 8h). The seismic horizons of the pre-Messinian rocks immediately underneath the core of this central Messinian anticline are mottled with thickened rounded helical bands; in contrast they are stretched at the fold limbs. The asymmetry of sigmoidal and imbricated fabrics and the tight minor folds on the fold limbs reveal limb-parallel, reverse dip-slip movements toward the folds summit (Fig. 8h). These fabrics demonstrate that the aforementioned pre-Messinian buckle active folding has continued until the onset of the Messinian bands. In Messinian seismic bands, the summit of the central anticline is stretched horizontally forming separate sigmoidal pieces of boudin structures in contrast to the compressed summit of the underlying pre-Messinian anticline (Fig. 8i). The matrix coating these boudins form stair-stepping trails (Fig. 8i) that indicate normal dip-slip senses of movement parallel to the fold limbs of the central anticline. Hence, the Messinian bands are seemingly passively folded by bending, which envisaged that the vertical forces that pushed their bands up may have resulted from anticlinal buckling of the pre-Messinian rocks (Fig. 8j). The chevron and tight folds below the Messinian bands (Fig. 8i) support the prevalence of the horizontal shortening deformation immediately beneath the horizontally stretched Messinian rocks.

The post-Messinian seismic horizons are mostly continuous and well matched, however they are extremely stretched into separate pieces in the region directly overlies the Messinian seismic bands (Fig. 8g). They are strongly folded above the Messinian bands, the associated Z-, M- and S-parasitic folds indicate that they are formed by horizontal shortening in the east–west direction. The patterns of these folds and the underlying Messinian folded bands are greatly discordant (Fig. 8g). They totally form the core of a thick band of flat-laying horizons at shallow depths, which abruptly bend concavely as a drag synclinal fold on the hanging wall of a westward-dipping normal fault zone. The imbricate fabrics and S-parasitic folds on the western limb of this syncline show a limb-parallel top-to-west reverse-slip sense of shear movements (Fig. 8g). The horizons on the hanging wall of this normal fault zone are stretched fairly on the fault surface against the strongly stretched horizons on its footwall. The common slip across the hanging wall of the normal fault is a clockwise. Accordingly, this major normal fault is formed by a local extension caused by relative vertical motions between footwall uplift and hanging wall subsidence through a regional buckle folding associated with horizontal shortening in the east–west direction (Fig. 8g). The seismic horizons on the western limb of the syncline are substantially stretched and discordantly extend on the flat-laying bands of the Messinian horizons. Therefore these post-Messinian buckle folds are formed independently to the underlying horizontally stretched Messinian bands. The independent deformations across the Messinian seismic bands indicate that these bands are considered as the basal detachment for the highly deformed post-Messinian rocks (Fig. 8j).

The structural features from time-depth maps

The time-depth contour map for an undifferentiated pre-Messinian seismic horizon shows that the contours commonly display an irregular pattern (Fig. 9a). Nonetheless, they trend regionally in the east–west direction in the southern half of the mapped area. In this part of the mapped area, the contour lines relatively form a broad region of a flat-topped structural high (Fig. 9a). Within this region, some contour lines display curvilinear and closed patterns, which commonly elongate in the northeast-southwest direction. We demonstrate that this region is built up of a chain of broad folds and domal structures (Fig. 9a). As shown formerly that the pre-Messinian fabrics are essentially formed by shortening deformations (Fig. 8), with little evidences of related extensional fabrics, the contour-line irregularities on the footslope of this region of structural high imply southward dipping thrusts (Fig. 9a). Northward, next to this region, small-scale folds of a northeasterly orientation are arranged in an en-echelon fold pattern (Fig. 9a). They are diagonally bounded by imbrication of right-stair stepping thrusts, which commonly dip southerly. Accordingly, the central part of the mapped area is likely dominated by a right-lateral strike-slip related transpressional shear zone (Fig. 9a). North of this shear zone, the contour lines seemingly show a rectangular-like closure elongated in the northwest direction (Fig. 9a). It has a flat-laying, wide-spacing center of the deepest elevation, which verifies a basin-like broad syncline. It is flanked from the east and west by northward-trending major synclines (Fig. 9a). Hinge-lines of these synclines are curved along the northern margin of the central shear zone (Fig. 9a). Geometry of the region of broad syncline implicitly reveals that it has remarkably squeezed along the northern margin of the central shear zone via shortening in the west-northwest-east-southeast direction, almost followed by northward tectonic escape with indistinct shallow folding (Fig. 9a).

Fig. 9

a Time-contour and interpretative maps of a well-picked pre-Messinian seismic horizon showing en-echelon folds trending in the north-northeast direction; diagonally to the northeast imbricated thrusts. Their right-stepping en-echelon pattern defines a northeast central dextral shear zone. b The time-contour and interpretative maps of a well-picked post-Messinian units explain east–west trending folds, their dextral offsets is on northwest, mostly eastward dipping, normal faults

The time-depth contour map for a horizon representing the structure of the post-Messinian rocks shows that the contours lines trend irregularly in the north–south direction; and they considerably descend deeply eastward (Fig. 9b). They are extremely curved about; occasionally exhibit some distinct closures that elongate entirely in the east–west direction (Fig. 9b). These features reveal that the post-Messinian rocks are structurally dominated by east–west trending folds and domal structures. Regular breaks in contour lines pervasively show pronounced linear dislocations of the former folds, with a dextral sense of motions across normal faults; dipping mostly in the northeast direction (Fig. 9b). Orientation of these linear breaks and the sense of motions about them demonstrate that the post-Messinian rocks underwent shortening deformation in the north-northwest—south-southeast direction.

Discussion

The Miocene up to Pleistocene deltaic section in the onshore and offshore regions on the Nile Delta district shows a complex pattern of extensional/contractional deformations (e.g. Sarhan et al. 2014; Leila and Moscariello 2019), although the Nile Delta cone are accumulated on the northern African passive margin (Guiraud and Bosworth 1999; Abdel Aal et al. 2001; Abd-Allah 2008; Abd-Allah et al. 2012; Tassy et al. 2015). The structural fabrics in sediments immediately above the Messinian erosional truncation and the next post-Messinian mega-sequence (Sarhan 2015) originated basically through (1) gravitational collapses via slope-driven deformation (Abd-Allah et al. 2012; Gvirtzman et al. 2015) onshore part of the delta cone, where thick Messinian salts are comparatively missing, and (2) substrate Messinian salts withdrawal, followed by gravitational collapses in the Nile deep-sea fan (Loncke et al. 2006; Selim and Omran 2012). All literatures tackled with these spatially related deformational processes excluding the possible influences of tectonic stresses in the deformation of the Miocene, pre- and post-Miessinian, Nile Delta sequences. However, little information is available about the influence of the Eastern Mediterranean tectonic stresses, forming and/or reactivating the deep seated structures, on the deformation of the Nile Delta Miocene sediments. In this context, a review on regional structural and deformation regimes in the Eastern Mediterranean region is summarized in the following section to envisage how far the compressional forces driving convergences along the southern thrust belt of Cyprus affect the deformation of the Nile Delta sediments.

Review of discussions on regional crustal deformations

The Nile Delta cone is formed on a northeast-trending buried Nile Delta-Eratosthenes ridge (Segev et al. 2018) that extends along the course of the more elevated geanticline of the Eratosthenes Seamount (ESM) (Abd-Allah et al. 2012; Skiple et al. 2012; Aboulela et al. 2022). It is a prominent boundary following the course of Qattara-Eratosthenes line (QEL, Fig. 1) between the Levant and Herodotus basins in the Eastern Mediterranean Sea region (Hussein and Abd-Allah 2001; Jagger et al. 2018). The ESM is a continental mass, rifted off from the Afro-Arabian passive margin, and came to subduct beneath the Crete-Cyprus thrust belt since late Cretaceous along the southern active margin of Eurasia (Abdel Aal et al. 2000). Currently, along this margin next to the ESM, there are no active volcanic arcs and well-developed trenches, which indicate that this subduction has fairly faded with limited underthrusting of Africa along the southern active margin of Eurasia (Woodside 1977); resulting in uplifting of Cyprus by late Miocene (Schattne 2010).

The inspection of colored bathymetric maps published by different literatures (e.g. Loncke et al. 2006; Mascle et al. 2006) for the ESM (Fig. 10a), let us to find out the following morphotectonic features: (1) it is an isolated flat-topped submarine hill that seems to vertically pierce through the upper seabed rock units, (2) its foothill-slope facing the Levant basin is relatively smooth, however it is regularly entrenched by steep gullies without evidences of slump scars, (3) its western slope against the Herodotus basin is extremely rugged as it is collapsed with east–west trending normal faults, peculiarly following the common orientation of the northerly adjoining Cyprus thrust belt, (4) these normal faults transect the flat summit of the ESM in a staircase geometry, (5) A second generation of southwestward-dipping imbricated normal faults cross-cut the former ones; fault surfaces of these later generation commonly sole, in strike-slip fan geometry, to a major southward-trending sinistral fault following the QEL, (6) this fault forms a conspicuous submarine fault-scarp at the toe of the northwestern rugged ESM foothill slope, (7) it is buried southwest by the salt-based post-Messinian seabed units, therein the summit of salt ridges are collapsed, and (8) some of these ridges display arcuate foreheads overlooking the ESM fault-scarp; others exhibit patterns of an en-echelon array showing sinistral and dextral senses of motions on the northern and southern boundaries of salt-based masses of the post-Messinian seabed sediments; respectively. Geometry of these masses implicitly indicates their movements to the ESM fault-scarp. These morphotectonic features of the ESM substantiate its geanticline geometry (e.g. Galindo-Zaldivar et al. 2001).

Fig. 10

a The bathymetry three dimension of the Eratosthenes Seamount (from Mascle et al. 2006) and its interpretative structural features showing physiographic evidences of a sinistral fault at its northwest boundary postdating its buckling-related east–west normal faults (b). Orientation of this sinistral fault is on the course of the Qattara-Eratosthenes line (Fig. 1). b Interpretative section across the Eratosthenes Seamount (Matched from Fig. 9 of Gao et al. 2020) that explains its formation by buckle folding which synchronously followed by formation of the eastern Herodotus sub-basin during Early Miocene

The inspection of published seismic sections across the ESM (e.g. Robertson 1998; Skiple et al. 2012; Peace et al. 2012; Gao et al. 2020) shows that it is cored with intensely folded horizons with subtle trails of tight parasitic folds, which are vertically dissected by axial plane cleavages (Fig. 10b). In contrast, the seismic horizons at its summit show an open antiform, which is dominated by northward dipping normal faults, overlooking the southward propagating Cyprus thrust belt (Fig. 10a; Robertson 1998; Galindo-Zaldivar et al. 2001; Gao et al. 2020). Thus, we think that, the ESM geanticline is originated by active, buckle folding mechanisms; perhaps triggered with subduction-related horizontal crustal shortening at deeper levels. The Herodotus eastern sub-basin (Fig. 10b; Gao et al. 2020) is a small basin formed on the western rugged slope of the ESM, which is dominated by active normal faults. The thicknesses of pre-Messinian (Early to Middle Miocene; Skiple et al. 2012) rock units increase substantially toward the center of the basin while thicknesses of post-Messinian rocks remain constant across it. Thus buckling of the ESM was ended by Late Miocene. However the current submarine fault-scarp and the salt-based seabed migration toward the Herodotus eastern sub-basin may attest that they have some pronounced activation even during the Pliocene–Quaternary period.

In Pliocene–Quaternary period, the about 25 km thick and buoyant crust of the ESM (Ben-Avraham and Nur 1986; Kempler and Ben-Avraham 1987) hinders its subduction beneath the Cyprus thrust belt. Thus the buried ridge along the QEL, which terminated northward by the ESM and southward by the edge of the African platform remains high against the Herodotus basin; and thus unable to subduct beneath the Cyprus thrust belt. We have shown from the rugged slope of the ESM that, its western margin with the Herodotus basin is a normal-sinistral diagonal fault, which is subsequent to the earlier (Early-Middle Miocene) ESM buckle folding-related east–west trending normal faults. This fault penetrates deeper (Rybakov et al. 2008) and thus, it can be considered as a transform fault boundary on the northwestern margin of what is known in the Eastern Mediterranean region as the Levant-Sinai microplate (Mascle et al. 2000). The Nile Delta hinge zone (NDHZ) extends easterly; separating the Levant basin from Sinai as a result of rifting during Early Mesozoic time (Fig. 1; Orwig 1982; Kamel et al. 1998). The crust of this basin is regarded either as a highly stretched continent (Woodside 1977; Hirsch et al. 1995; Vidal et al. 2000) or as oceanic (Makris et al. 1983; Ginzburg and Ben-Avraham 1987; Ben-Avraham et al. 2002; Segev et al. 2006). Recently, comparisons on magnetic and gravity data in the Eastern Mediterranean Sea region have deduced that it is a hybrid crust comprising continental slivers intervened by oceanic patches (Segev et al. 2018). These continental slivers are widely common near to the rifted continental margins until the ESM, while the oceanic crust seems to be widely distributed northwards (Jagger et al. 2018). Accordingly, the entire Phanerozoic sedimentary fill averaging about 15 km is in the center between the ESM and the Lebanese coastal line (Gardosh and Druckman 2006). The ~ 4 km thick Nile derived sediments accumulated on the Mediterranean seafloor of the Levant basin, south of the ESM since the Pliocene (e.g., Segev et al. 2006) is exclusively linked with subsidence of the Levant against Sinai margin (Gvirtzman et al. 2008), and in turn along the NDHZ (e.g., Tibor et al. 1992). Thus the development of a flexural crustal fore-bulge due to subduction of Levant lithosphere underneath the Cyprus thrust belt is inhibited by this thick load of the Nile Delta sediments that with the effect of ESM buoyancy resulted in absorbing this subduction, which transitionally substituted by collision (Schattner 2010).

Deformation regime of the Nile Delta region

The pre-Miocene, bedrock-crustal deformation regime

Formation of the Nile Delta cone is largely controlled by tectonic evolution of preexisting faults of pre-Cambrian basements (Longacre et al. 2007), which continued also upward deforming the sub-Nile Delta, Late Cretaceous up to Eocene platform bedrock units (Abdel Aal et al. 1994; Sarhan and Hemdan 1994; Zaghloul et al. 2001). The Late Cretaceous tectonic inversion affected the northern margin of the Egyptian platform forming a set of Syrian Arch-related, northeast-trending asymmetrical folds (Garfunkel 1998; Bosworth et al. 1999), followed by subsequent erosion of emerged lands (Abdel Aal et al. 2000; Loncke et al. 2002). The slope of the northern Egyptian platform margin is steep following the present-day coastal-line of the northern margin of the Western Desert and continued subsurface, inland across the southern fringe of the Nile Delta region (Zaghloul et al. 2001). This platform margin defines the Eastern Mediterranean Sea Transform (EMST; Le Pichon et al. 2019; Fig. 1), which—by its dextral motion since Late Cretaceous (Zaghloul et al. 2001)—resulted in developing tilted faulted blocks on westward-dipping imbricate normal faults along the passive margin of the northeast Africa (Meshref 1990). This fault system forms a rugged terrain that accommodates foundation of the Miocene sediments of the Nile Delta cone (Marzouk 1988; Said 1990). North of the EMST, about 70 km apart, the continental crust underneath the Nile Delta cone thins abruptly about its hinge-zone from 31 to 27 km with a rapid decrease in depth to Moho from 36 to 33 km and progressive increase in thicknesses of its sedimentary cover from ~ 3 to ~ 7 km (Fig. 11a; Ibrahim et al. 2017; Elsaka et al. 2020; Hassan et al. 2021). Thus we may envisage crustal necking, and/or decoupling, of the thinned crust from the mother African plate immediately north of the Nile Delta hinge-zone (NDHZ), which may have experienced a component of lateral motion to satisfy juxtaposition of the stretched crust against the mother one (Fig. 11a).

Fig. 11

a Schematic section showing the structural variations across the EMST and NDHZ in the Nile Delta cone. Steep gradients of substrates linked with severe folding of pre-Messinian rocks between the EMS and NDHZ while shallow gradients seaward next to the NDHZ led to developing shallow bands, which are slightly folded in locally, therein they pierce the post-Messinian carapace. The attached interpretative sections are redrawn from seismic profiles of Figs. 3 of, Leila and Moscariello (2019); and Figs. 3 and 7 of Sarhan et al. (2014) and Sarhan (2015) respectively. b A representative diagram illustrating the deformation by gravitational gliding of pre-Messinian rocks between the Baltim to North Sinai fields over an eastward tilting substrate of a half graben. c Thinnest post-Messinian rocks south of the NDHZ are deformed by gliding on shallow seaward-sloping Messinian erosional surface. The thickest post-Messinian rocks next to the NDHZ favors deformation by gravity spreading due to a combined action of their vertical load and salt-seaward escaping

The pre-Messinian “Early Miocene” deformation regime

The Late Cretaceous compressional deformation regime associated with the generation of fold-related wrench corridor of the Syrian Arc System in northern Egypt is altered in the Oligo-Miocene with severe crustal rifting, which resulted in the development of the Afro-Arabian rifts; such as the Red Sea and the Gulf of Suez rifts (Guiraud and Bosworth 1999; Bosworth and Mc Clay 2001; Khalil and McClay 2002). This rifting phase seemingly continued to the Nile Delta region, which is spatially located at the northwestern termination of the Gulf of Suez rift (Sarhan et al. 2014; Abd El-Fattah et al. 2021). However many arguments for complex compressional and extensional deformation patterns of the Miocene up to Pleistocene Nile Delta sediments about the surface of Messinian salinity crises are shown on seismic profiles, presented by different literatures in northern Egypt (e.g. Sarhan 2015; Leila 2019; Leila and Moscariello 2019)-and this study.

The Early Miocene, pre-Messinian sediments in the South Delta Basin (SDB) are accumulated with considerable thicknesses, south of the NDHZ (Zaghloul et al. 2001; Hassan et al. 2021). The isopach maps of the pre-Messinian rock units, which includes the Sedi Salem and Qawasim formations show gradual and regular increases in their total thicknesses northward from zero to about 1500 m on the northern fringe of the EMST to the NDHZ; respectively (Fig. 12a; Sarhan and Hemdan 1994; Zaghloul et al. 2001). We estimated about 30 m/km of the surface slope of the basement (Fig. 11a), based on its relief map, constructed by Rybakov and Segev (2004). Hence these sediments form a section of wedge-shaped geometry, which pinches-out along the EMST and thickens gradually toward the NDHZ. Accordingly, the bedrock substrate of these rock units is implicitly expected to be tilted steeply northward (Fig. 11a; e.g. Ibrahim et al. 2017), beginning from the platform marginal listric normal faults along the EMST (Hassan et al. 2021) to the NDHZ. It has been considered from published seismic lines in the SDB (Fig. 11a; e.g. Sarhan et al. 2014; Sarhan 2015; Leila 2019; Leila and Moscariello 2019) that the Early Miocene rock units deposited within this basin have experienced severe active folding. The horizontal compressional forces required for these folding mechanisms are seemingly gravity-driven (e.g. Tingay et al. 2011; Loncke et al. 2006; Clark and Cartwright 2009; Brun and Fort 2011; Rowan et al. 2012) and/or by tectonic activities along the bounding EMST and NDHZ (Sestini 1989; Aal et al. 2001; Saleh 2013; Aboulela 2022). The steep slope of the bedrock substrate of the wedge-shaped Early Miocene Nile Delta section, which thickens downslope toward the NDHZ, and their intense internal deformation satisfy the conditions of combined action of gravity gliding and spreading deformations; respectively. The augmentations of geothermal gradients along the EMST and NDHZ and the higher residual gravity anomalies and magnetic susceptibilities along them (Morgan et al. 1976; Saleh 2013; Aboulela 2022) indicate that they commonly mark zones of active rifting, however geophysical potential surveys document that they have a dextral component of movements (Saleh 2013; Aboulela 2022), associated with rapid necking of the crust immediately north of the NDHZ (Ibrahim et al. 2017; Elsaka et al. 2020; Hassan et al. 2021) and their juxtaposition against the thickest northern African platform margin. Therefore, we argue that the intense active folding of the SDB pre-Messinian, Early Miocene sediments is basically achieved by a combination of gravity-driven deformation mechanisms that seemingly enhanced by dextral motions along the EMST and NDHZ.

Fig. 12

Data collected from Mascle et al. (2006); Loncke et al. (2006); Folkman and Mart (2015) and Givertizman et al. (2015)

Schematic structural maps illustrating changes in deformation regimes from pre- to post-Messinian times, (a–c); respectively) in the Nile Delta region and their relation to regional tectonics in the Eastern Mediterranean. See text for explanation.

The isopach maps of the pre-Messinian, Early Miocene rock units in the region north of the NDHZ displays eastward oscillatory increases of their total thicknesses from few hundred meters, near to Qattara-Eratosthenes line, to about 1500 m further east (Fig. 12a; Sarhan and Hemdan 1994). Hence their deposition is seemingly controlled by eastward shallowly tilted half-grabens, extending between the Baltim to Northern Sinai fields (Fig. 11b). The flat-laying seismic horizons of these sediments—and their few knocks of upright tight folds—north of the NDHZ, particularly at the Temsah and North Sinai fields (Figs. 6 and 8), in contrast to those displaying highly deformed and tightly folded features south of it, imply that they have a weak internal deformation, which is essentially constrained by dominant slope-driven gravity gliding deformations. The common trajectories for orientations of the fold hinge-lines of the pre-Messinian, Early Miocene rock units in the Baltim, Temsah and North Sinai fields (Figs. 5, 7 and 9) reveal that they spatially display large-scale asymmetrically folded features, about the north-northwesward-trending axial surfaces (Fig. 12a). Thus the slope-driving forces producing the gravity-gliding deformation of these sediments generate shortening roughly in the east–west direction following the general orientation of the slope of the bedrock substrate (Fig. 11b). This shortening direction is widely active further east along the southeastern margin of the Levant basin where the offshore Sinai Messinian turbidity channels are offset sinsitrally by northwest-trending fault zones (Fig. 12a; Folkman and Mart 2015).

The Messinian and post-Messinian “Plio-Pleistocene” deformation regime

The Messinian salinity crisis dramatically affected the Mediterranean Sea at the end of Miocene, which resulted in accumulation of significant quantities of both layered and massive salt deposits within the basin (Loncke et al. 2006; Mascle and Mascle 2019). The isopach map of the total thicknesses of the lower and upper units of Messinian evaporites and clastics; respectively (Fig. 12b; Loncke et al. 2006) shows a substantial increase of their total thicknesses within the Herodotus basin (~ 2000–5700 ms) while they record lesser thicknesses in Levant basin (~ 900–1300 ms). Hence, we suggest that the oceanic crust of the Herodotus basin (Granot 2016) would subside, against the Levant continental masses (Netzeband et al. 2006; Jagger et al. 2018) along the Qattara-Eratosthenes line (Segev et al. 2018). Gravitational loading of these sediments and the stayed Messinian seawater, which dropped for about 600–1500 m (e.g. Ben-Moshe et al. 2020; Gvirtzman et al. 2022) from the present day sea-level isostatically influence its subsidence regime.

Therefore the Levant continental lithosphere would rebound by flexural-isostatic uplift along the Qattara-Eratosthenes line that represents the common tectonic boundary of it with the Herodotus basin (Segev et al. 2018). This may explain the general eastward titling of the Nile Delta substrate and emerging the delta cone subaerially that permit the Messinian River Nile distributary channels to start active erosion via incising their courses through the delta surface (e.g. Loncke et al. 2006; Cross et al 2009; Leila 2019; Gvirtzman et al. 2022). The isopach map of the Messinian sediments in the Nile Delta cone (Fig. 12b; Sarhan and Hemdan 1994; Zaghloul et al. 2001) demonstrates that its section is deposited within a central embayment that extremely pinch-out on the EMST and opens seaward, bounded on its both sides by preexisting Nile Delta marginal faults. Gradual increasing of its total thickness toward the footwalls of these faults suggests their reactivation as growth faults; and thus its sedimentary section is deposited syntectonically.

These features are noticeably reported in the Messinian section on seismic lines crossing the NDHZ (Fig. 11a, e.g. Sarhan et al. 2014; Sarhan 2015). Such Messinian embayments reported on the Eastern Mediterranean margins are interpreted as megaslides (Jaggr et al. 2018; Leila et al. 2022), which include an alternation of chaotic seismic facies and high amplitude reflectors onlapping the paleoslopes (Sarhan et al. 2014).

The post-Messinian, Plio-Pleistocene sedimentary prism—with the underlying Messinian clastics—directly onlaps the Messinian erosional truncation surface, while it overlies the mobile Messinian salts; onshore and offshore of the Nile Delta cone; respectively (Fig. 11c, Abdel Aal et al. 2001; Loncke et al. 2006). The northward gradual increase in its thickness to about 1700 m at the NDHZ (Fig. 12c, Sarhan and Hemdan 1994), and its abnormal thickening (about 4000 m; Segev et al. 2006) immediately north of the NDHZ (Abdel Aal et al. 2000; Loncke et al. 2002; Gaullier et al. 2000) indicate that the growth faults triggering the former Messinian megaslides were substantially active north of the NDHZ with a little hinterland uplifting along the EMST. The seaward sloping of the Messinian erosional truncation surface underneath the post-Messinian sedimentary prism and its overalping the Messinian evaporites at its toe (Zucker et al. 2020) induced structures typical to gravity-driven spreading and gliding of the mobile salt layer (salt tectonics) below the Nile Delta cone (Fig. 11c). Differential loading of its thickest sedimentary prism launched lateral flow of the Messinian evaporites causing a thinning of the salt deposits below the central part of the cone and an outward thickening up to 2.8 km (e.g., Loncke et al. 2002) of it as salt rollers, ridges and buckle folds (Costa and Vendeville 2002) in the Nile deep sea-fan (NDSF; Fig. 11a).

Structural imaging of the seabed of the Eastern Mediterranean Sea basins, based on its bathometry (e.g. Mascle et al. 2000, Mascle et al. 2006; Loncke et al. 2006, 2008) has evidenced about 70 km width and 150 km length northwest-trending dextral shear zone, we called it as Temsah shear zone, extending between the toe of the Nile Delta shelf from the south and the margin of the ESM to the north (Fig. 13). We used the published images of the aforementioned literatures for redrawing this shear zone to shed more lights on its origin, and to understand how far it does take part in deformation of the post-Messinian rocks of the Nile Delta cone. The shear zone terminates southerly on the foot-slope of the Nile Delta shelf sediments where it is buried under a region of the northeastward collapsed continental shelf margin. Normal faults of this region terminate at their toes with downslope-concavely facing contractional scars, outlining northward verging salt ridges (Fig. 13). Next to these contractional scars; and on the Nile Deep Sea Fan (NDSF) the shear zone contains polygonal mani-basins, which are outlined by elongated salt ridges, topped with collapsed summits (Figs. 11a and 13). These mini-basins disappear on the eastern side of this shear zone, where they are exchanged by north–south scars of extensional gashes; their asymmetries reveal the dextral motion along the shear boundary (Fig. 13). They seemingly represent trails of collapsed elongated salt ridges. North of this zone, and next to the Qattara-Eratosthenes line, the supra-salt seabed sheet shows imbrications of four salt moving pulses; their frontal outliers show salt migrations toward the center of the Herodotus basin (Fig. 13).

Fig. 13

Structural map of the supra-salt overburdens, i.e. Mediterranean seabed, in the region of Temsah shear zone between the Nile Delta-Eratosthenes Seamount. (Redrawn from figures of Mascle et al. 2000 and Loncke et al. 2006). The Qattara-Eratosthenes line separates the extensional features related salt-ridges on Levant margin from those of compressional deformation in Herodotus basin. Note that the westward moving supra-salt caps are in the form of imbricated pulses, the later one burdens the Qattara-Eratosthenes line; therein the salt caps are collapsed

It is interestingly noting that the upper salt pulse concealed the Qattara-Eratosthenes line; however its frontal outlier is highly collapsed (Fig. 13), which imply that (1) this line is slightly inactive during movement of the later salt pulse; and (2) because of this line terminates on the western fault-scarp of the ESM, of which its reactivation accommodates the ESM uplifting, we believe that the salt migration was started by buttressing of this seamount for the northerly collapsed delta slope and the subsequently migrating salt-sheet. Thus the dextral motion of the Temsah shear zone is related to horizontal shortening in the north–south direction that perhaps is induced by ESM buttressing of the northward moving salt sheet, triggered by collapsed shelf margin at the toe of the delta slope.

The Nile Delta-Eratosthenes geanticline ridge (Segev et al. 2018) separates two different salt-related deformation styles; compressive wave-like salt ridges and extensional gash-like summit-collapsed salt ridges, in the Herodotus and Levant basins, immediately west and east of its regional trend; respectively (Fig. 13). We considered that, eastward tilting of the Nile Delta-Eratosthenes ridge induced extensional gashes due to a unidirectional gravitational gliding of the supra-salt sheet toward the Levant basin. In contrast, the supra-salt carapace glides downslope gravitationally toward the center of the Herodotus basin as explained by the elevation gradient along its western and eastern margins, dominated by the Mediterranean ridge and the Nile Delta-Eratosthenes geanticline ridge; respectively (Zucker et al. 2020), thus the compressional forces are bilateral that is favorable to induce contraction ridges.

Conclusions

The pre-Messinian rock units in the Nile Delta cone commonly underwent compressional deformation on the northwest-southeast direction; common evidences of subsequent extensional fractures and normal faults are rarely detected. It is controlled by combined deformations of gravitational gliding and spreading with some evidences of deformation from lateral tectonic movements along the EMST and NDHZ.

The post-Messinian rocks are commonly dominated by extensional features due to bulk compressional and extensional deformations, trending roughly in the north–south and east–west directions; respectively. It explains the role of dextral motions of the Temsah shear zone, perhaps resulted from buttressing the ESM for the northward migrating Messinian salts from the toe of the delta cone of which exceptionally compatible with the regional structure setting of the Nile Delta block and the Eastern Mediterranean basins.