RESEARCH Structural Setting of Cretaceous Pull-Apart Basins And

Home , Gulf of Suez Rift, Quseir Formation, Red Sea Rift

RESEARCH

Structural setting of Cretaceous pull-apart basins and Miocene extensional folds in the Quseir–Umm Gheig region, northwestern Red Sea, Egypt

Mohamed Abd El-Wahed1,2, Mahmoud Ashmawy1, and Hossam Tawﬁ k1 1GEOLOGY DEPARTMENT, FACULTY OF SCIENCE, TANTA UNIVERSITY, TANTA 31527, EGYPT 2GEOLOGY DEPARTMENT, FACULTY OF SCIENCE, OMAR AL MOKHTAR UNIVERSITY, AL BEIDA, LIBYA

ABSTRACT

We examine the evolution of the northwestern Red Sea, Egypt, by study of the Quseir–Umm Gheig subbasin. The subbasin records two main tectonic events. The ﬁ rst event is related to development of Late Cretaceous synclinal basins due to sinistral movement along the reactivated Najd fault system. Evidence for this includes: (1) the Cretaceous basins are concentrated mainly in the central Eastern Desert, which represents the main inﬂ uence zone of the Najd fault system, (2) folds are not everywhere parallel to the faults and their axes are curvilinear, (3) the faults dislocated the axial plane of the synclines, (4) the Cretaceous basins occur in an en-echelon arrangement, (5) there is a difference of 20° between the orientation of the sinistral strike-slip shear zones and the associated en-echelon synclinal folds, (6) principal stress σ σ σ directions are delineated by subhorizontal 1 and 3 and subvertical 2, (7) sheared conglomerate is detected in the Nubia Formation, (8) minor overturned folds and minor NE-vergent thrusts occur in the Duwi and Dakhla Formations, and (9) there is a predominance of NE-SW normal faults in Cretaceous–Eocene sequences. The second event is related to the sinistral movement along the NNE-SSW Aqaba–Dead Sea transform and dextral movement along Queih and Hamrawin shear zones. This movement was synchronous with northeast extension of the Red Sea. The structures developed during this movement include: (1) NW-trending extensional faults, (2) extensional fault-related folds in Miocene-Pliocene deposits, and (3) buckle folds in Pliocene and post-Pliocene sequences. Buckle folds were developed during NW compression associated with sinistral movement along NNE-SSW strike-slip faults. Gypsiferous shale-rich beds in Miocene-Pliocene rocks played the main role in development of fault-related folds and buckle folds in the Quseir–Umm Gheig subbasin.

LITHOSPHERE; v. 2; no. 1; p. 13–32. doi: 10.1130/L27.1

INTRODUCTION 2006a, 2006b) such as the NW-trending shear magmatic expansion (Bohannon, 1989; Bohan- zone of the Najd fault system (Davies, 1984; non and Eittreim, 1991), (6) asymmetric rifting It is generally accepted that the main Red Stern, 1985). (Dixon et al., 1989), and (7) pull-apart basin(s) Sea extension started 30 m.y. ago during the The Cenozoic Red Sea rift belongs to a rift (e.g., Makris and Rhim, 1991). The major dif- late Oligocene–early Miocene and reactivated system that includes the East African rift in the ferences between the various models center on the steep NW-trending late Pan-African shear south and the Gulf of Aden and the Gulf of Suez the relative timing of updoming, rifting, and zones (McKenzie et al., 1970; Meshref, 1990; in the north (Bosworth et al., 2005; Guiraud et magmatism, and whether the rifting was active Moustafa, 1997; Purser and Bosence, 1998; al., 2005; Kinabo et al., 2007). These rifts were and driven by a mantle plume or passive and due Khalil and McClay, 2002, 2009). The initial rift initiated in the late Oligocene (Rupelian) to to lateral extension of the lithosphere leading to occurred in response to the NE separation of the Miocene in several small, en-echelon, approxi- reactive effects in the mantle (Ghebreab, 1998). Arabian plate from the African plate (Nubia), mately E-W– to ESE-WNW–trending basins in The models that invoke graben-horst formation and basins within the Red Sea rift were gener- the Gulf of Aden province (Fantozzi and Sga- along steep normal faults are supported by the ally asymmetric, 60–80 km wide half grabens vetti, 1998; Watchorn et al., 1998), and they earlier semibrittle stage of extension that corre- (Bosworth et al., 2005). The extension direction fragmented the Arabian-Nubian Shield (Marti- sponds to the predicted low-angle simple shear was N60°E during the late Oligocene to Mio- nez and Cochran, 1988). zone through the lithosphere (Ghebreab, 1998). cene (Bosworth and McClay, 2001). The forma- Several rifting mechanisms have been pro- In order to examine the evolution of Red tion of Cretaceous basins and orientation of rift- posed for the Red Sea (reviewed in detail by Sea continental rifting, Bosworth et al. (2005) related normal faulting were strongly controlled Ghebreab, 1998); they include: (1) prolonged distinguished three phases of rifting: (1) late by the presence of a preexisting Precambrian normal faulting (e.g., Lowell and Genik, 1972), Oligocene–early Miocene rift initiation; (2) early fault zone (Dixon et al., 1987; Bosworth, 1994; (2) lithospheric thinning by faulting and dike Miocene main synrift subsidence; and (3) middle Younes et al., 1998; Ghebreab, 1998; Ghebreab injection (Berhe, 1986), (3) diffuse extension Miocene onset of the Aqaba–Dead Sea trans- and Talbot, 2000; Younes and McClay, 2002; followed by brittle deformation (e.g., Martinez form. The Red Sea rift initially included the pres- El Shemi and Zaky, 2001; Khalil and McClay, and Cochran, 1988), (4) lithospheric simple ent Gulf of Suez, Bitter Lakes, and Nile Delta 2002, 2009; Gawthorpe et al., 2003; Bosworth shear (Voggenreiter et al., 1988), (5) combina- region on the continental margin of North Africa et al., 2005; Guiraud et al., 2005; Jackson et al., tions involving detachment faults and prolonged (Bosworth and McClay, 2001).

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A magnetic trend analysis carried out for ° ° ° ° the Gulf of Suez–Red Sea region from both 30 E 40 E 50 E60E 40°N Black Sea Caspian regional and residual magnetic maps (Meshref, Sea Eurasian 1990) indicates the presence of the following 40°N plate regional magnetic trends: (1) Gulf of Suez–Red Turkish Sea, Erithrean, or Clysmic (NW) trend of mid- plate Tertiary age, (2) meridional or East African (N-S) trend of Precambrian age, (3) trans-Afri- Anatolian-Persian plateau can, Qena-Safaga, Idfu-Mersa Alam, or Aualitic (NE-SW) trend, (4) Tethyan, Mediterranean, or Mediterranean Sea 30°N Sheikh Salem (E-W) trend of Paleozoic–Juras- Aqaba-Levant ° sic age, (5) Najd (WNW) trend of Precambrian 30 N Gulf of transform age, (6) Atalla (NNW) trend of Precambrian Suez age, and (7) Gulf of Aqaba, Dead Sea, or Aqaba Najd fault zone Najd fault zone (NNE) trend of mid-Tertiary age. Study The study area occurs in the Eastern Des- Area ert of Egypt between Quseir and Umm Gheig along the Red Sea coast (Fig. 1). The Quseir– Arabian Umm Gheig region contains the southernmost plate 20°N exposures of the pre–Red Sea rift stratigraphic 20°N section of the uplifted Egyptian continental mar- African gin. The well-exposed Cretaceous–Pleistocene plate Gulf of stratigraphic successions (Figs. 1 and 2) are Aden subdivided tectonically into two major categories (Said, 1990): the prerift sequence (Precam- brian to Eocene) and the synrift sequence (Oli- 0 500 km Indian gocene–Pleistocene). These stratigraphic units Ocean Afar 10°N were remarkably affected by the tectonic evolu- 10°N tion of rifting. The prerift structures are variably ° ° ° enhanced and disrupted by the synrift struc- 30 E40E50E tures. The structural architecture and tectonic Figure 1. Location of the Quseir–Umm Gheig subbasin and its relation to the Red Sea rift evolution of the northwestern part of the Red system and the Najd fault zone (modifi ed after Hempton, 1987). Borders of Najd fault zones Sea are still not fully understood. Understand- are adopted after de Wall et al. (2001). ing and reconstruction of the tectonic evolution of the northwestern Red Sea and evaluation of the Quseir–Umm Gheig subbasin, Cretaceous basins, and fault-related folds are the main NNW-vergent thrusts, open folds, imbricate N-S extension in the form of an escaping block objectives of this study. This study was based on structures, and thrust duplexes in the Pan-Afri- (Stern, 1994). detailed fi eld mapping using aerial photographs can nappe (low-grade volcano-sedimentary Many NW-striking strike-slip shear zones (1:40,000) and Landsat images (1:250,000) and rocks), (2) a ENE-WSW compression event cre- (Fig. 2) have been recognized in the Precam- analysis of fi eld and structural data collected ated NE-vergent thrusts, folded the NNW-ver- brian rocks of the central Eastern Desert, e.g., from both the prerift and synrift rocks. gent thrusts, and produced NW-trending major Meatiq, Sibai (Fritz et al., 1996, 2002; Abd El- and minor folds in the Pan-African nappe, and Wahed, 2008, 2009, and references therein), STRATIGRAPHY AND STRUCTURAL (3) sinistral shearing related to the Najd fault Hamrawin, and Queih shear zones (Abdeen et FRAMEWORK system developed along NNW- to NW-striking al., 1992; Moustafa, 1997; Abdeen and Greiling, strike-slip shear zones (660–580 Ma), marking 2005). Both sinistral and dextral movements are Stratigraphy of the Quseir–Umm Gheig the external boundaries of the core complexes. documented along the Hamrawin and Queih region (Fig. 3) and a brief description of the Sinistral shearing produced steeply dipping shear zones (Abdeen et al., 1992). Left-lateral Precambrian, pre-rift sequence and synrift sedi- mylonitic foliation and plunging folds in the slip on these faults was related to movement ments is presented in Table 1. NNW- and NE-vergent thrusts. along the Najd fault system, whereas right- The NW-striking strike-slip faults (650– lateral slip was related to rifting in the northern Precambrian Shear Zones 540 Ma) of the Najd fault system represents Red Sea in the late Oligocene (Moustafa, 1997). one of the major sinistral shear zones that cut The sequence of structural events in the Pre- through the Arabian-Nubian Shield (Moore, Quseir-Umm Gheig Subbasin and Duwi cambrian rocks of the central Eastern Desert of 1979; Stern, 1985, 1994; El-Rabaa et al., 2001; Accommodation Zone Egypt (Fritz et al., 1996, 2002; Shalaby et al., Johnson and Kattan, 2001). It developed in the 2005; Abd El-Wahed, 2007, 2008, 2009; Abd crust of central Arabia during the Proterozoic The term “accommodation zone” refers El-Wahed and Abu Anbar, 2009) is as follows: postorogenic stage as a result of convergence to a complex zone of faulting that accommo- (1) oblique island-arc accretion accompanied of a continental fragment from the east that dates along-strike change in both the fault dips by NNW-SSE shortening produced low-angle was accompanied by E-W compression and and in subbasin polarity within a rift system

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° ′ ° ′ 1990; Moustafa, 1997). Many authors divide G. Um 34 00 E 34 15 E Tagher the Gulf of Suez–northwestern Red Sea rift block 15 into four subbasins or half grabens separated by 26° tectonic accommodation zones (e.g., Moustafa 20 Synrift sedimentary rocks 45′N SAFAGA and Fouda, 1988; Jarrige et al., 1990; Moustafa, Mohamed N Prerift sedimentary rocks 1997; Khalil and McClay, 2002, 2009). The area Rabah 5 between Safaga and Quseir along the Red Sea block Precambrian rocks coast (Fig. 2) is occupied by the northern and

7 southern parts of two major subbasins, named Block-bounding normal faults 20 9 Gebel here Safaga and Quseir–Umm Gheig subbasins, Gassus Precambrian shear zone with respectively (Fig. 2). The two subbasins are sep- block sinistral movement Um El 25 arated by the Duwi accommodation zone. Each Huetat 21 Precambrian shear zone with 13 sinistral and dextral movement subbasin consists of a number of rift blocks. block 5 26° Syncline Safaga subbasin contains ﬁ ve rift blocks (Gebel 10 15 30′N G. Wasif Um Tagher, Mohamed Rabah, Gebel Gassus, El 15 30 block Um Huetat, and Gebel Wasif blocks) showing 10 15 0 10 20 km 25 constant NW dip and an average dip of 18°.

Quieh Detailed mapping shows that the Quseir– shear zone 10 Umm Gheig subbasin is delimited to the south R E D S E A by the NW-trending Queih and Hamrawin shear zones. Seven rift blocks down faulted against DAZ 15 Hamrawin Precambrian rocks with a NE dip (average dip shear zone of 20°) are located in the study area. These are 10 Wadi El-Nakheil 12 syncline 26° the Gebel Um Hammad–Gebel Duwi, Anz- G. Um 15′N Ambagi, Gihania, Gebel Atshan, Zug El Bahar, Hammad- 50 Anz- Study G. Duwi 15 Duwi Ambagi Gebel Hamadat, and Sharm El Bahari blocks syncline area block 15 block (Figs. 2 and 4). The Quseir–Umm Gheig sub- Meatiq shear zon 10 42 Gihania basin and its rift blocks are dominantly elon- 22 15 block gated northwestward. The main elongation Gebel of these rift blocks changes from NW-SE in 20 16 Meatiq e QUSEIR Quseir–Umm Gheig subbasin to approximately 17 15 N-S in Safaga subbasin. Directions of dip in 19 Cretaceous sediments of the Gebel Atshan and G. Atshan Hamadat fault blocks are different, e.g., NE, block 18 26° 29 E, SW, W, and NW, delineating the shape of Zog El 00′N 35 30 20 Bohar NNW-plunging Atshan and Hamadat synclines block 22 Sibai shea (Figs. 5A and 5B). Bedding in Eocene–Pleis- 25 20 tocene formations dips mainly NE, forming r zone Hamadat a NE-facing monocline dissected by exten- block 18 15 sional normal faulting and strike-slip faulting 20 (Fig. 5C and 5D). In general, the amount of dip 18 decreases northeastward, from 40° close to the Sitra shear z 15 20 Sharm Precambrian rocks to nearly horizontal close to Gebel El Bahari 20 the Red Sea shore. one 10 Sibai block 25° El-Sh 25 21 Dip angles in pre-Miocene and Miocene us 45′ h shear zo 10 sediments (Fig. 6) range between 15° to 25°

20 but locally reach up 40°, especially in the ne 13 Nubia Sandstone. In Pliocene rocks, these values decrease gradually to 7°–14°, then Figure 2. Principal structural elements and fault blocks in the Safaga-Quseir region, modiﬁ ed after continue to around 5° in Pleistocene raised Moustafa (1997). Axes of folds are after Moustafa (1997) and Khalil and McClay (2002). Queih and Hamrawin shear zones are after Younes et al. (1998) and Khalil and McClay (2002), respectively. beaches (Figs. 4 and 6). Meatiq and Sibai shear zones are after Fritz et al. (1996, 2002) and Abd El-Wahed (2008). Short bold The observed structures in the Quseir–Umm arrows show main dip direction of prerift sediments. DAZ—Duwi accommodation zone. Gheig subbasin include fault blocks and subblocks, major synclines in the Cretaceous– Eocene sequence, NE-SW and NW-SE normal ( similar usage to that of Bosworth, 1985; Jar- ferent structural style, such as between neigh- faults, a major monocline constituting the Quseir– rige et al., 1990; Faulds and Varga, 1998; Khalil boring half grabens of different dip directions, Umm Gheig subbasin, NNE-SSW, NE-SW, and and McClay, 2002, 2009; Younes and McClay, and they represent the areas through which NW-SE strike-slip faults, minor anticlines and 2002). They are also known as transfer zones. throw is changed from the bounding fault of synclines in Miocene rocks, and buckling and Transfer zones exist between two parts of dif- one half graben to that of the next (Jarrige et al., gypsum folds in Miocene-Pliocene rocks.

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Khalil and McClay, 2002, 2009), and they rep-

O resent landmarks in the central Eastern Desert

O ERA R Fm. LITHOLOGY of Egypt. They are described as extensional

E PPD fault-related folding (Khalil and McClay, 2002, Recent- 2009) and as Late Cretaceous basins nucleated Pleistocene Samadi as small pull-apart basins by reactivation of the Najd lineament (Bojar et al., 2002). They trend Shagara NNW to NW and plunge gently N and NW near their southern end and gently SSE and SE far- Pliocene ther north. Figure 3. Stratigraphy of the The Atshan syncline represents the southern Mersa Gabir northwestern Red Sea rift Alam continuation of the Wadi El-Nakheil syncline. system (compiled and modi- The Nakheil syncline is characterized by a ﬁ ed after Said, 1990; Purser

Samh curvilinear axial-trace where its strike changes Late and Bosence, 1998; Khalil and McClay, 2002). from NW-SE at the northern part to NNW-SSE Abu at its southern part (Fig. 4). The Atshan syncline Dabbab is asymmetric: the W-dipping limb is steeply dipping compared to the NE-dipping limb. The closure of the southern termination of the Middle Synrift Um Nakheil syncline plunges 20° to north, and a Mahara major NNE-SSW oblique-slip fault dislocated it from the NE-dipping limb of the Atshan syn- Miocene cline. The southern nose of the Atshan syncline Volcanics plunges 30° to north. Stereographic plots of the Early Atshan syncline and the southern termination Ranga of the Nakheil syncline (Fig. 6) show moderate Chalky limestone scattering of poles to bedding with NNW- and SSE-plunging shape of folds. Late Nakheil Reefal limestone The Hamadat fault block consists of three Oligocene C E N O Z I Middle Marl separate, doubly plunging asymmetric synclines bordered by a NW- to NNW-trending border Thebes Sandstone normal fault dipping steeply SW. The axes of Eocene Early these synclines plunge NNW and SSE, parallel Conglomerate to the major trend of the border fault (Figs. 4 and Esna 5). The W- and SW-dipping limbs are steeply Paleocene Evaporites inclined compared to the NE-dipping limbs. The Dakhla Limestone SW-dipping limb of the northern syncline dips with chert steeply (30°–70°) compared to the other synclines. The arrangement of these three synclines Duwi

Prerift Shale suggests an en-echelon pattern. The northern syncline represents the larger one and consists Late Quseir Mafic volcanics of a Cretaceous–Oligocene sequence, where the Nakheil Formation exists in the trough of Cretaceous Granites

MESOZOIC the syncline (Fig. 5A and 5B). This formation Nubia Metavolcanics & is absent in the other two synclines. The north- metasediments ern and southern synclines are steeply plunging PRECAMBRIAN compared to the central one. The central syn- CRYSTALLINE BASEMENT Unconformity cline is highly eroded, and its axis is displaced from that of the southern syncline by the Wadi Essel strike-slip fault. There are two narrow Late Cretaceous Synclines tral movement along NNE-SSW and NE-SW transverse anticlines between the three synclines strike-slip faults. The arrangement of these syn- (Khalil and McClay, 2002). A stereographic plot Two main Cretaceous basins are pres- clines suggests en-echelon orientation. These of the Hamadat synclines (Fig. 6) shows strong ent in the study area: Hamadat–Zog El Bohar basin-shaped synclines are occupied by four scattering of poles for bedding due to the doubly basin and a part of Duwi-Nakheil-Atshan fault blocks (Gihania, Gebel Atshan, Zog El plunging character of folds. basin (Figs. 2 and 4). The axes of these basins Bohar, and Gebel Hamadat blocks) and four trend mainly NW-SE and N-S, subparallel to major asymmetric, doubly plunging synclines Wadi Aswad Subblock the main trend of the major shear zones in the in the Atshan and Hamadat areas (Figs. 4 and Precambrian rocks. Deﬂ ection of their axes 5). These synclines have been described previ- The Wadi Aswad subblock is a part of the from NW-SE to N-S may be assigned to sinis- ously by many authors (e.g., Moustafa, 1997; Zog El Bohar fault block (Figs. 2 and 4) and

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TABLE 1. BRIEF DESCRIPTION FOR THE ROCK SEQUENCE IN THE QUSEIR–UMM GHEIG REGION Formation Age Description Samadi Pleistocene–Recent Deposited in a reef environment; consists of carbonate to mixed carbonate-siliciclastic sediments. The recognized petrofacies are calcareous sandstone, skeletal mudstone, grainstone, wackestone, and micritic crusts (El Moursi and Montaggioni, 1994). The Pliocene-middle Pleistocene raised substratum comprises several down-stepping reefs, each of which is associated with a pebble beach and piedmont deposits arranged in four successive coarse alluvial fans (Freytet et al., 1993). Shagra Pleistocene Composed mainly of reefal limestones, sandstones, marls, calcareous grits, conglomerates, and gravel beds. Gabir Pliocene Composed mainly of sandstones with gravel intercalations and marked at its base by a deformed sandstone layer (Mahran, 1999). Samh Late Miocene–early Pliocene Composed of gray to yellowish green, poorly fossiliferous, gypsiferous sandy shale, siltstone, and fi ne-grained argillaceous sandstone (Said, 1990). At the base, it contains thin gypsum veinlets and minor pebbly sandstones, micritic limestone, pebbly limestones, and conglomerates. Um Gheig Late Miocene Mainly hard, compact dolomitic limestone. These limestone beds are generally dark yellow, cavernous; when freshly broken, they give marked oil odor, named oil-tainted limestone. Abu Dabbab Late Miocene Overlies unconformably the Um Mahara sediments and consists of nonfossiliferous, solid white gypsum, sometimes intercalated by masses and lenses of dolomite or semicrystalline limestone and rarely shales Synrift formations (El Akkad and Dardir, 1966; Said, 1990). In the study area, the Abu Dabbab Formation consists of gypsiferous claystone, sandy and clayey gypsum, and laminated gypsum. Evaporites form conspicuous high hills, exhibit- ing yellowish brown color with coralloid-like hackly weathering surface; extends hundreds of kilometers along the western margin of the Red Sea coast (e.g., El Sayed, 1986; Orszag-Sperber et al., 2001). Um Mahara Middle Miocene Consists of gypsiferous, fossiliferous dolomitic limestone and includes Mn, S, Pb-Zn, and pyrite mineraliza- tions (Said, 1990). It rests unconformably over the Ranga Formation, from which it is separated by a thin conglomerate bed. This formation decreases in thickness northward; its thickness is 60 m in Umm Gheig, and it reaches about 27 m in Wadi Essel. It forms the well-known Pb-Zn–bearing calcareous grit along Wadi Umm Gheig (Said, 1990). Ranga Early Miocene Composed of mixed conglomerates, sandstones, siltstones, and claystones (Mahran, 1999). They are exposed on both sides of Wadi Sharm El-Bahari, Wadi Sharm El-Qibli, and at the northern side of Wadi Wizr (Fig. 4). Generally, they are nonfossiliferous and seem to have been deposited subaerially by rivers, which drained from the elevated uncovered basement complex (Said, 1990). The Ranga deposits are capped by about 3–25-m-thick basaltic fl ows, south of Wadi Sharm El-Qibli, which seem to have been subjected to extensive weathering and erosion. These fl ows are concentrated in the area between Wadi Sharm El-Bahari and Wadi Wizr. According to Roussel (1986), the K/Ar whole-rock method on basalts gives an age of 24.9 Ma. Nakheil Late Oligocene Consists of lacustrine sediments interbedded with coarse breccia and pebbles derived from Eocene and Cretaceous sedimentary rocks. It represents the products of weathering and erosion and refl ects changes in depositional environment from shallow-marine to continental environment. This environment continued during deposition of the Ranga Formation. Thebes Early–middle Eocene About 50 m thick at Wadi Essel and south Wadi Aswad and composed mainly of thinly bedded limestone with fl int bands or fl int nodules and chert concretions (Abd El-Motaal, 1993). South of Wadi Um Gheig, no trace of this cover has been recorded. The contact between pre- and synrift series is dated as early to middle Eocene according to Strougo and Abu El-Nasr (1981). Thebes Formation is capped by a relatively planar erosion surface overlain by oligomictic conglomerates composed of residual chert nodules derived from the underlying Thebes Formation. Esna Shale Late Paleocene–early Eocene Exposed to the east of the study area at Gebel Duwi and its continuation at Gebel Atshan and composed mainly of gray laminated shale. It occurs as thin beds in Atshan and Hamadat fault blocks. Dakhla Maastrichtian–Paleocene Divided into two members composed of marl beds (Hamama Member, Maastrichtian) and shales (Beida Member, early Paleocene), respectively (Abd El Razik, 1967). A notable feature south of Wadi Aswad is the contact between the lower Dakhla and the upper Duwi Formations, which is marked by presence of oil (black) shales within and above the phosphatic beds (Said, 1990). Duwi Campanian–Maastrichtian Formed of several phosphatic bands interbedded with cherts, marls, shales, and silicifi ed and oyster limestones. Abd El-Motaal (1993) recorded this unit north and south of Wadi Zog El Bohar and in the area between Wadi Essel and Wadi Sharm El Bahari. Small outcrops of the Duwi phosphates are exposed in the area between Wadi Sharm El Bahari and Wadi Sharm El Qibli and completely absent south of Wadi Wizr (Fig. 4). Quseir Campanian Consists of multicolored, poorly fossiliferous shales and clays deposited under nonmarine to marginal marine conditions (Youssef, 1957; Said, 1990). These shales are predominantly variegated, vividly colored, and alternate in places with sand and siltstone beds. The lower part of the Quseir Formation is composed mainly of grayish to brownish, ripple-laminated siltstone and clayey siltstone with small lenses of fi ne sandstone. The

Prerift sedimentary Phanerozoic cover latter grades upwards into brownish green, bedded shales with thin ironstone layers. Nubia Pre-Campanian Unconformably overlies the Precambrian rocks and represents the oldest Phanerozoic sedimentary beds. Sandstone This formation has a wide age range extending from Cambrian to Cretaceous (Youssef, 1957; Issawi, 1973; Moustafa, 1987; Jackson et al., 2006a, 2006b). Some authors have restricted its age to Late Cretaceous in age (e.g., Van Houten et al., 1984; Kerdany and Cherif, 1990). In the study area, these sandstones reach about 200 m thick and are composed mainly of yellowish brown, highly dissected and frequently cross-bedded, fi ne- to coarse-grained sandstones (Said, 1990). The Precambrian basement rocks bound the western periphery of the study area (Fig. 2). These rocks are composed mainly of heterogeneous and poly- genetic crystalline igneous, metamorphic, and sedimentary rocks. In the study area, Precambrian rocks are mainly represented by ophiolitic mélange, metavolcanic rocks, Dokan volcanics, and younger granites with subordinate molasse sediments and later tectonic volcanics (Akaad and Abu El Ela, 2002; Abd El-Wahed, 2007, 2008, 2009). The lithological aspects and structural setting of the basement rocks played a signifi cant role in the Neogene rifting, since rocks lithological compositions infl uenced the quality and volume of clastic material delivered to the Neogene basins, while the structural pattern of the rift was Precambrian clearly inherited from the basement structures.

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34° 10′E 34° 15′E 34° 20′E 17 13 Wadi Qusier El Qadim 20 Recent- Samadi 13 Pleistocene 20 17 15 13 22 16 N Gabir 16 15 Pliocene 18 20 18 20 15 Wadi El Nakheil 20 Samh 25 40 20 25 Late Umm Gheig 19 A` Abu Dabbab Miocene Um Mahara 20 Synrift

15 Middle 12 Basalt 24 22 20 Qusier

20 Early 22 20 Ranga Ambaji Coastal Faultfaul Wadi Ambaji 24 27 17 Oilgocene Nakheil 22 22

20 24 h 20 Wadi Aswad 26° s 20

t Thebes 20 Eocene ′ 22 05 N A 18 B` 22 Atshan Border Faultfault 24 Esna t 21 G. n 15 Paleocene Dakhla 29 22 20 17 2018 Prerift Duwi Late 22 15 30 Cretaceous Qusier 12 10 30 30 2828 Nubia A 22 28 Metamorphic 17 13 Precambrian and igneous 16 rocks 20

GMF

QMF 19 15 20 19 19 Wadi Zareib 20

18 15 15 Q 8 30 Zog El Bohar Co MF 15 26° 17 Hamadat Border Fault 00′N 28 R E0 D S E A4 km 25 2 40 28 20 15 40 as 25 28 t G. Hamadat al fault 40 25 30 Wadi Zog El Bohar 26 34 15

29 28 32

Q 10 26 MF Wadi Hamadat 38 GMF 33 B 26 20 20 29 30 20 23 23 10 Wadi 40 8 25 25 Essel 25 36 22 25 30 28 27 10 33 32 29 1717 20

28 15 25° 28 10 20 55′N

Major syncline 15 Strike and dip Major fractures in and plunge of fold axis Precambrian rocks Strike-slip faults Anticline Major shear zones Strike-slip faults (inferred) Fault-related folds in Precambrian rocks Normal faults Reverse faults Evaporite folds Buckle folds

Figure 4 (on this and following page).

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34° 20′E 34° 25′E 34° 30′E

17 R E D S E A 15 25° 10 ′ 20 C` N 55 N 16

30 40 15 35 25 15 10 45 35 22 22 17 40 18 15 20 17 18 18 Wadi Sharm El Bahari 40 2023 20 40 8 C 20 25 20 20 D`

QMF 19 Wadi Sharm El Qibli 40 28 25° 18 18 GMF 50′N 18 35 24

40 25 24 20 8

22 10

10 12 Wadi Wizr 20 15 15 15 Umm Gheig Border fault 18 15 15 15 15 12

D 25 G 16

QMF MF 25 25° 22 15 10 45′N 14 22 17 10 12 20GMF 15 12 18 12

Gheig

Southern extension Wadi Umm of Sibai shear zon 13 11

WadiUmm 11 13 20 12 0 2 4 6 km Gheig es 1814 12

Figure 4 (on this and previous page). Geological map of the Quseir–Umm Gheig subbasin (lithology compiled after Abd El-Motaal, 1993; Khalil and McClay, 2002; and this paper).

consists of Cretaceous–Eocene rocks that rest part of the subblock to N45°W/20°NE in the deformation compared to the Duwi and Thebes unconformably over Nubia Sandstone. This central part and to N20°E/22°SE in the south- Formations, which exhibit only brittle defor- sequence includes the Quseir, Duwi, Dakhla, ern part (Fig. 4). The northern and central mation. Both the northern and central parts and Thebes Formations. The contact between parts are separated from the southern part by a are characterized by the presence of minor the Duwi and Dakhla Formations is marked by NW-SE dextral strike-slip fault. This fault hin- imbricated NE-vergent thrusts (Fig. 8A) and black shale. This subblock forms an S-shaped ders the continuation of the Duwi and Dakhla NW-trending overturned (Fig. 8B) and recum- topographic feature clearly visible on Landsat Formations to the southern part of this rift sub- bent folds developed only in Dakhla shale. images (Fig. 7). The orientation of bedding block. The Dakhla Formation contains abun- The observed overturned fold is dissected by changes from N60°W/20°NE in the northern dant gypsum veins that exhibit high plastic a series of minor thrusts. These thrusts cut

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SW NE Geometry and Kinematic Data of the Faults A A′ Atshan syncline 22 500 (m) 24 A series of NE-SW–trending normal faults 27 30 Sinistral strike-slip fault is remarkably well-exposed in the Cretaceous– sl 0 20 19 X 15 Eocene sequence at Wadi Quseir El Qadim 500 (Fig. 8E) and Wadi Essel. Also, scarce NE- trending reverse faults are also observed in the 1000 Duwi Formation interbedded with chert at Wadi Essel (Fig. 8F). These faults strike N50–55°E 1500 and dip commonly 55–65°SE and less com- V=H monly to the NW (Fig. 6). Kinematic indicators 1 km such as rock facets, slickolites, and normal sepa- SW NE ration of stratigraphic units indicate dominantly normal slip. Several NE-trending normal faults B Hamadat syncline Dextral strike-slip fault B′ Sinistral strike-slip fault are also recorded in Pleistocene rocks of Wadi 500 (m) 20 28 30 45 55 X 32 X Sharm El Bahari, Wadi Umm Gheig, and around 22 20 15 sl 0 10 Quseir city. 500 Slickenside data from the NW-SE–trending normal faults recognized in the Miocene- 1000 Pleistocene rocks indicate dominantly dip-slip 1500 movement (Figs. 4, 5, and 6). The strike of 2000 faults and dominance of normal slip are concor- V=H dant with the direction of extension that led to 1 km the opening of the Red Sea. Faults in the Quseir– SW NE Umm Gheig region can be subdivided into three ′ C C groups related to three phases of extension: (1) 500 (m) Fault-related folds NW-SE–trending normal fault bordering the 35 X 35 30 25 25 17 15 10 Cretaceous–Eocene sedimentary basins, (2) two sl 0 systems of extensional normal faults, the Bor- der fault system and Coastal fault system (fault 500 terminology after Sharp et al., 2000), and (3) strike-slip and oblique-slip faults. The Cretaceous–Eocene basins occupied by 1000 the Gihania, Atshan, and Zog El Bohar blocks 1500 (Figs. 2 and 4) are bordered by two normal V=H faults linked together to form a V-shaped basin (Fig. 4). One of these faults strikes NNE-SSW 1 km and dips toward WNW, whereas the second SW NE fault strikes NNW-SSE and is inclined toward D D′ Fault-related folds ENE. NNE-SSW–trending normal faults are 500 (m) not observed in Miocene-Pleistocene rocks. 35 25 40 25 28 18 The Hamadat syncline is bordered by two sl 0 major NW-SE–trending normal faults forming one of the remarkable half grabens in the 500 central Eastern Desert of Egypt. The normal faults bordering these basins form a conspicu- Elevation1000 Elevation Elevation Elevation ous zigzag pattern marking the eastern edge of the Precambrian outcrop. The extensions of the 1500 normal faults are hindered by the effect of the V=H strike-slip faults. 1 km The Coastal and Border fault systems Figure 5. Regional cross sections across the Quseir–Umm Gheig subbasin (locations are shown in include the major normal faults separating Pha- Fig. 4). nerozoic rocks from NE and SW (Fig. 6) and are largely dissected by NE-SW, NNE-SSW, and ENE-WSW strike-slip faults. Khalil and across competent Duwi Formation and die out Paleocene shales underlying the Eocene suc- McClay (2002) determined the stratigraphic downward and upward (Fig. 8C). The over- cession and overlying the Cretaceous deposits. throws of both Coastal and Border fault systems turned folds in Dakhla shale can be interpreted On the other hand, the black shale and Thebes to be 0.5–2 km and 1.5–3.5 km, respectively, on as detached folds over a ductile décollement Formation are clearly dislocated by the NNW- the basis of topographic offset of Precambrian conditioned by the presence of a thick unit of striking normal faults (Fig. 8D). rocks and prerift strata.

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Two main master faults are traced in the Structural features Quseir–Umm Gheig region, one exists between

Age Bedding Folds Faults the middle and upper Miocene rocks, and the Formation N other is situated between the middle Miocene rocks and the older sedimentary rocks (e.g., lower Miocene, Cretaceous, and Precambrian rocks). They are named here Quseir (QMF) and Umm Gheig master faults (GMF), respectively.

Nubia They extend northwest for ~50 km in the area between Quseir and Wadi Umm Gheig. These master faults are dissected and dislocated by sinistral movements along strike-slip faults. The 58 poles, contours 0, 10, 20, 30%. secondary structures observed adjacent to these N N β master faults are fault-related folding of meter scale (10–300 m) and minor normal faults. A series of NW-SE–striking normal faults has Late Cretaceous been observed, including oppositely dipping normal faults (Figs. 9A and 9B), steeply dip- N ping (60°–80°) (Fig. 9C), scarce listric (fault dip Nakheil synclines decreases with depth) (Fig. 9D), convex upward N=43 Quseir and Duwi (fault dip increases with depth) normal faults, σ σ 55 poles, contours 0, 10, 20, 30%. 43 reading, contours 0, 3, 6, 9%. 3 1 and reverse faults (Fig. 9E). Footwall meter- N N scale anticlines and hanging-wall synclines β (Fig. 9D) were developed due to overlapping of σ NE- and SW-dipping macroscale normal faults 2 (Fig. 9C). The dips of intrablock normal faults NE normal faults increase toward the master faults. These faults 60 poles, contours 0, 3, 6, 9%. are either synthetic or antithetic to the mas- Hamadat synclines ter faults and form a series of horsts, grabens N=54 (Figs. 8A and 8B), and step-like normal faults

Paleocene and Eocene (Fig. 9F) of variable length. A clear example of Dakhla, Esna and Thebes Dakhla, Esna and 54 reading, contours 0, 3, 6, 9%. 55 poles,contours 0, 10, 20, 30%. a major horst and graben system exists along the N N N northern bank of Wadi Sharm El Bahari, where σσ β 1 Ranga Formation, Precambrian rocks, Nubia, Qusier, Um Mahara, and Abu Dabbab Forma- σ tions are faulted against each other (Figs. 4 and 2 5C). Displacements along these faults range

Miocene σ from a few centimeters to tens of meters. Kine- 3 Fault-related folds matic indicators, such as rough facets, slick- Um Gheig and Samh N=48 enlines, and normal separation of stratigraphic Um Mahara, Abu Dabbab, Um Mahara, 613 poles, contours 0, 20, 40, 60%. 48 reading, contours 0, 5, 10, 15%. 92 poles, contours 0, 3, 6, 9%. units indicate normal slip. The presence of a lis- N N N tric normal fault is interpreted in the cases where β the hanging wall is tilted more steeply than the footwall, whereas a convex-upward fault is inferred where tilting of the hanging wall is less than that of the footwall. Movements along listric normal faults created rollover or reverse

Evaporite folds drag folds. The geometry of fault-related folds Gabir and Samadi Gabir N=80 and minor drag folds is largely controlled by dip Pliocene and Pleistocene angle and dip direction of normal faults. Minor 287 poles, contours 0, 10, 20, 30%. 80 reading, contours 0, 4, 8, 12%. 46 poles, contours 0, 4, 8, 12%. gently dipping normal faults were also observed. Figure 6. Equal-area lower-hemisphere projection of the different structural elements from the rock These faults were developed originally as steep formations in Quseir–Umm Gheig subbasin. faults and then rotated to their present shallow dip by domino-like block tilting (e.g., Faulds et al., 2002) of their footwall and hanging wall. A Coastal and domino-style normal faults are ent rock units, especially between Cretaceous few of the minor reverse-separation faults were moderately to steeply seaward-dipping, and and Miocene and between lower Miocene and observed in Miocene and Pliocene rocks, but most show strike parallel to the Red Sea axis. middle Miocene rocks. Within the monocline, major reverse faults were not observed. These They include several master and intrablock major and minor normal faults strike NW-SE faults probably developed by tilting of some faults. They control the contacts between differ- and dip steeply 60°–80° to NE or SW (Fig. 6). antithetic SW-dipping normal faults.

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and Hamadat blocks, and then it changes its strike to WNW-ENE coinciding with northern extension of the Sibai shear zone in Precam- brian rocks to the west of the Hamadat fault block (Fig. 4). The ENE-trending Wadi Umm Gheig strike-slip fault delineates the southern extension of the Sharm El Bahari fault block. There is a NNE-SSW oblique-slip fault that cuts across Wadi Ambaji, dislocating Precam- brian and Cretaceous rocks, and relocating the axial plane of the Nakheil syncline and the Cretaceous–Eocene sequence of Gebel Atshan (Fig. 4). This fault has left-handed displacement (up to 500 m) and may be reactivated along an old NE-SW normal fault.

Fault-Related Folds

Defi nition and Classifi cation Faulds et al. (2002) summarized the mechanisms of normal fault-related fold development as follows: (1) movement on listric normal Figure 7. TM-Landsat 7 image showing S-shaped topographic feature of Aswad subblock and faults (i.e., rollover or reverse drag folds) (e.g., evaporite folds plunging toward the NE. Groshong, 1989; Dula, 1991; Xiao and Suppe, 1992), (2) displacement gradients on normal faults (e.g., Schlische, 1992, 1995; Janecke et NW-trending normal faults were observed in fault, and their second-order shear zones devel- al., 1998), (3) along-strike overlap of oppositely Quaternary sediments and recent terraces. Many oped in the middle Miocene. They follow the dipping normal faults within accommodation of these faults are related to recent earthquakes old Precambrian NE faults and major fractures. zones (e.g., Morley et al., 1990; Faulds and affecting the northwestern coast of the Red Sea These faults dislocated all the rock units from Varga, 1998), and (4) isostatically induced fl ex- (El Shemi and Zaky, 2001). Precambrian rocks to Pleistocene rocks and ures in footwalls of major normal faults (e.g., Several major and minor NE- to NNE- controlled the following structural and topo- Spencer, 1984; Wernicke and Axen, 1988). trending sinistral strike-slip faults and some graphic features: (1) the morphology of the Red Schlische (1995) identifi ed six categories NW- to NNW-trending dextral strike-slip faults Sea coast and the exposed Precambrian rocks, of fault-related folds (in Khalil and McClay, were observed (Fig. 4). Strike-slip and oblique- (2) the development of sharms at the entrance 2002): (1) hanging-wall fault-bend folds gener- slip faults appear to be younger than the oldest of the main wadis (large sharms [“bays”] in the ated by changes in fault dip (i.e., rollover anti- Coastal and Border normal faults and dextral study area, such as those at the entrance of Wadi cline and ramp synclines; e.g., McClay, 1990), strike-slip faults. Many of the NE-trending faults Ambaji, Wadi Sharm Al Bahari, Wadi Sharm Al (2) normal drag folds formed by frictional in the Quseir–Umm Gheig subbasin, initiated Qibli, Wadi Wizr, and Wadi Umm Gheig, are resistance along the normal fault plane (e.g., as normal faults in the old Cretaceous–Eocene largely developed at the point of intersection Twiss and Moores, 1992; Peacock et al., 2000), sequence, appear to have been subsequently between two conjugate strike-slip faults), and (3) reverse drag folds, where the hanging-wall reactivated as strike-slip or oblique-slip faults. (3) the main courses of the major wadis (the beds and the footwall beds fl ex the fault surface Three major dextral faults were observed, most prominent sinistral strike-slip faults in the downward and upward, respectively (e.g., Bar- named Wadi Ambaji, north Wadi Sharm Al Quseir–Umm Gheig subbasin are those run- nett et al., 1987), (4) transverse folds generated Bahari, and north Wadi Zareib dextral faults. ning along Wadi Aswad, Wadi Essel, and Wadi by displacement variation along the strike of the Dextral offsets of geomorphologic features Umm Gheig). The NE-trending Wadi Aswad fault (the axes of these folds are perpendicular and stratigraphic units across Wadi Ambaji and strike-slip fault (more than 16 km in length) to the fault surface), (5) fault-propagation folds Wadi Sharm Al Bahari strike-slip faults are gen- dislocates (offset over 1 km) the Precambrian produced by folding of a propagating fault- erally a little over 500 m. rocks, Nubia Sandstone, and the two normal tip line (e.g., Allmendinger, 1998; Hardy and In the Quseir–Umm Gheig subbasin, sinis- faults bordering the Atshan fault block. The McClay, 1999; Corfi eld and Sharp, 2000; Sharp tral strike-slip displacements dominate and ENE-trending Wadi Essel strike-slip fault is et al., 2000; Khalil and McClay, 2002; Willsey dislocate Coastal and Border normal faults ~22 km in length and accommodates ~1 km of et al., 2002; White and Crider, 2006), and (6) and dextral strike-slip faults. A series of major horizontal displacement. It dislocates the major compactional drape folds produced by differen- NE-trending steep strike-slip faults of this shears, normal faults, and lithological contacts tial compaction of sediments over a preexisting phase offsets structures in the Precambrian and in Precambrian rocks, stratigraphic units, and extensional fault scarp. Cretaceous–Miocene rocks and dissects the normal faults in the Cenozoic rocks. Also, this Kilometer-scale fault-propagation folds have Quseir–Umm Gheig subbasin into a number fault displaced the southern part of the Hamadat been recognized in the Gulf of Suez rift margin of fault subblocks. The main trends of these fault block from its northern part. The strike of (Moustafa, 1987; Gawthorpe et al., 1997; Sharp faults range between N20°E and N80°E. They this fault is ENE-WSW along Wadi Essel and in et al., 2000), Rhine graben (Maurin and Niviere, are related to the Aqaba–Dead Sea transform the Precambrian rocks between Zog El Bohar 2000; Lopes Cardozo and Behrmann, 2006),

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Figure 8. (A) Minor NE-vergent thrust in Dakhla shale, Aswad subblock, Wadi Aswad, looking NNW. (B) Meter-scale overturned fold in Dakhla shale of Aswad subblock dissected by minor NE-vergent thrusts, looking NW. (C) Minor NE-vergent thrust dislocating Dakhla shale with no effect on Thebes Formation, looking NW. (D) Minor NW-SE–striking normal faults dislocating black shale bed between Dakhla Formation (upper) and Duwi Formation (lower), looking NW. (E) NE-SW–oriented normal faults in Thebes Formation of Wadi Quseir Al Qadim, looking NE. (F) Nearly vertical NE-SW reverse fault in Duwi Formation of Wadi Essel, looking NE.

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Figure 9. (A) Oppositely dipping NW-SE–oriented normal faults forming a minor graben in middle Miocene rocks, Wadi Wizr, looking SE. (B) Oppositely dipping normal faults forming minor horst in Pliocene rocks, Wadi Sharm El Qibli, looking NW. (C) Steeply dipping normal fault in Pliocene rocks, Wadi Sharm El Qibli, looking NW. (D) Listric normal fault with the development of hanging-wall open syncline in middle Miocene rocks, Wadi Sharm El Bahari, looking NW. (E) Listric reverse fault in middle Miocene rocks, Wadi Sharm El Bahari, looking NW. (F) Normal step faults in the upper Miocene rocks, Wadi Zarieb, looking NNW.

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northwestern Red Sea (Khalil and McClay, affected by folding but only acquire the shape of ated with NW-striking reverse faults. The best 2002); Gulf of California (Willsey et al., 2002), the underlying fold and were clearly fractured development of these folds was recorded in the Smørbukk area, North Sea (Corfi eld and and brecciated. The open anticline (Fig. 10A) an outcrop along the contact between middle Sharp, 2000), southwest Iceland (Grant and is asymmetric; its SW-dipping limb dips at 40°, and upper Miocene rocks along the southern Kattenhorn, 2004), Sirt Basin, Libya (Fodor and its NE-dipping limb dips at 24°. It con- bank of Wadi Sharm El Bahari, where they et al., 2005), and Modoc Plateau, northeastern tains a thick bed of shale in its core, followed are present as a series of oppositely dipping, California (White and Crider, 2006). Fault- upward by thin layers of sandstone, sandy shale, NW-trending normal faults and a few reverse propagation folds have received much attention shale, and reefal limestone. The other anticline faults. Minor reverse drag folds (i.e., rollover) due to the frequent occurrences of hydrocarbons is also asymmetric as well; its SW limb dips at developed when the beds constituting the associated with the folds themselves and the ~35°, and its NE limb dips at 80° changing to hanging wall bent downward relative to those underlying fault blocks. 15°. The upper Miocene–lower Pliocene rocks of the footwall due to upward movement of the exposed around these folds are characterized hanging wall (Fig. 10C). The normal drag folds Fault-Related Folds in Quseir–Umm Gheig by predominance of NW-trending oppositely (Fig. 11D) were formed when footwall beds Subbasin dipping normal faults. The NE-dipping limb is were dragged down and hanging-wall beds Extensional fault-propagation folds are abun- dislocated by a minor NE-dipping normal fault, were dragged up along the fault surface as a dant in accommodation or transfer zones (e.g., whereas the SW-dipping limb is displaced by a result of frictional resistance along the normal East African rift and Gulf of Suez, northwestern SW-dipping normal fault (Fig. 10B). fault plane. Red Sea). The Miocene–Pliocene rocks of the A macroscale NW-trending hanging wall Quseir–Umm Gheig subbasin are dissected and syncline was recorded in the lower Miocene Evaporite Folds dislocated by a series of NW-SE–trending nor- multicolored sandstone and minor shales of mal faults. Both oppositely dipping, listric and the Ranga Formation between Wadi Sharm The evaporite deposits are remarkable fea- convex-upward normal faults are observed. The El Bahari and El Qibli. It is a strongly open tures along the Red Sea coast. They are distrib- length of these faults ranges between a few tens symmetric fold with wavelengths up to 50 m uted as separated lobes deposited in isolated of centimeters to several kilometers. The throw (Fig. 10C) developed in the hanging wall of the grabens and consisting of gypsum beds inter- of these several-kilometer-long faults decreases Umm Gheig master fault. Dip measurements bedded with sand, clays, marls, shales, and rapidly toward the tip, suggesting elliptical slip on the lower Miocene rocks along the northern carbonates. The major and minor NE-trending distributions. Fault-parallel monoclinal folds are fl ank of Wadi Sharm El Bahari (Fig. 4) indi- folds in the upper Miocene and Pliocene Red present beyond the tip, and axes are offset into cate 7 km extension for this syncline, although Sea evaporites show characteristics of typi- the hanging wall like those described from the its central part was dislocated by the Sharm cal buckle folding. Sehim (1994) studied and Modoc Plateau, northeastern California (White El Bahari strike-slip fault and obliterated by regionally traced these buckle folds in the area and Crider, 2006). Normal-fault propagation weathering processes. Several minor NW-SE– between Gebel El Zeit in the north and Wadi in the Quseir–Umm Gheig subbasin is accom- trending normal faults were recorded in the Umm Gheig in the south (~250 km along the panied by fl exing of Miocene–Pliocene beds limbs of this syncline. In the upper Miocene Red Sea coast). beyond the Quseir master fault-tip line, resulting rocks, some other minor synclines were also Naturally strained shale-sulfate multilayers in development of several kinds of extensional observed associated with the hanging wall of and minor gypsum folds include chevron, sym- fault-related folds. These folds together consti- listric normal faults (Fig. 9D). These hanging- metrical, asymmetrical, open, tight (Fig. 11E), tute an along-strike fault-parallel monoclinal wall synclines are typical of fault-related folds overturned, and noncylindrical isoclinal folds. fold between middle and upper Miocene rocks. developed by change in dip of fault by depth. The disposition of these minor folds and the Three types of NW-trending extensional A few minor drape folds were also observed arrangement of their hinge lines between the fault-related folds were observed in the study in upper Miocene rocks. They are character- limbs of alternate domes and basins resemble area: (1) forced folds or extensional fault-prop- ized by development of the steeply dipping syncline-anticline alternation in enterolithic agation folds, (2) compactional drape folds, and and intermittently reverse faulted monoclinal gypsum folds, which developed by vertical (3) normal drag folds. Forced folds were formed fl ank, accompanying the major normal faults expansion during rehydration (e.g., Gandin et between a series of oppositely dipping normal between the middle and upper Miocene rocks. al., 2005). Folding may be open, and it often faults that characterize the horst and graben tec- The main characters of these folds are that they develops into small thrusts. The presence of tonics of the Quseir–Umm Gheig subbasin. are bounded by normal faults on one side and thin shale layers between gypsum beds acts Several footwall anticlines and hanging-wall reverse faults on the other side (Fig. 11A); as a lubricant facilitating folding. These folds synclines were observed within this fault-par- however, sometimes they are bounded by nor- are only preserved in shale-sulfate multilayers, allel monoclinal fold and were observed to be mal faults either on both sides, and the dis- and they are completely absent in the overlying well-developed mainly in the Samh Formation placement of the normal fault is parallel to the and underlying beds. The axes of these folds (late Miocene–early Pliocene), especially in the steeply dipping fl ank. Sometimes, the steeply trend mainly NE-SW. The major buckling area between Wadi Sharm El Bahari and Wadi dipping fl ank is broken and displaced when the folds include both mode 1 and 4 of Ghosh et Wizr. Two meter scale, NW-SE–trending fault- normal faults were reactivated by recent move- al. (1993). The nonplane, noncylindrical folds related folds were observed along a small stream ments or present-day earthquakes (Fig. 11D). consist of a number of parallel arc-lengths (~1 on the southern bank of Wadi Sharm El Qibli. The presence of a few drape folds in upper and 5 km) characterizing both upper Miocene They represent the footwall anticlines of the Miocene rocks explains the occurrence of and Pliocene rocks, and they are easily traced master fault. Their outcrops consist of gypsif- some reverse faults in the same places as the from correctly processed Landsat and conven- erous sandy shale and fi ne-grained argillaceous normal faults. tional aerial photographs. They exist as a num- sandstone capped with reefal limestone of the Both reverse and normal drag folds were ber of anticlines and synclines plunging toward Gabir Formation. Reefal limestones were not sporadically observed and usually associ- the northeast (Figs. 7 and 11F).

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Buckling

Buckling is a well-known folding process that results from compression parallel to the layers (Ramberg, 1964), and it gives rise to regular waveforms, the lengths of which depend on both thickness and viscosity or strength con- trast between layers (Lan and Hudleston, 1996; Espina et al., 1996; Woodward, 1997; Sengupta et al., 2005). Buckling is common in a single layer and multilayers. In the study area, the well-exposed nonplane, noncylindrical buckle folds are described from Wadi Quseir El Qadim, Wadi Aswad, and Wadi Essel. Buckle folds of domes and basin patterns are best developed in the large evaporite lobe between Wadi Essel and Wadi Sharm El Bahari. They occur as marked domes (Fig. 11G) and half domes formed by vertical upward movement of evaporites that consequently pushed up the overlying Pliocene and Quaternary sediments. The observed domes occur close to the Red Sea shoreline, where they are of variable dimension and represent hazardous places for building construction and urban development.

DISCUSSION

Relation between Najd Fault System and Cretaceous Basins

An important structural characteristic of faults belonging to the Najd fault system is the formation of second-order strike-slip motion, which was often accompanied by extensional movements responsible for development of pull-apart basins (El-Rabaa et al., 2001). Such basins act as sites for deposition of molasse sediments. The Najd fault system is a major component in the geological framework of the Pre- cambrian rocks in Egypt and Saudi Arabia and is regarded as the last signiﬁ cant structural event that affected these rocks (Moore, 1979; Stern, 1985, 1994; El-Rabaa et al., 2001; Johnson and Kattan, 2001; Abd El-Wahed, 2007, 2008, 2009; Abd El-Wahed and Abu Anbar, 2009). NW-trending sinistral shear zones bound the core complexes (e.g., Sibai, Meatiq, Hafaﬁ t) in the central Eastern Desert of Egypt and represent the northwest contiguity of the Najd fault system (Fritz et al., 1996, 2002; Abd El-Wahed, 2007, 2008, 2009). The Cretaceous synclinal basins in the study area are described as extensional fault-related Figure 10. (A) NW-plunging open anticline in middle Miocene rocks of Wadi Sharm El Qibli, looking folding resulted from along-strike displace- NW. (B) NW-trending tight anticline in middle Miocene rocks of Wadi Sharm El Qibli. The limb of ment variations on the individual fault segments the fold is dislocated by oppositely dipping NW-SE–striking normal faults, looking NW. (C) Mac- during late Oligocene–Miocene extension of roscale NW-trending hanging-wall syncline in lower Miocene multicolored sandstone, Wadi Sharm the Red Sea (Khalil and McClay, 2002). On El Bahari, looking WNW. the other hand, they have been interpreted as pull-apart basins developed during Cretaceous

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sinistral strike-slip tectonics with subhorizontal faults occurs without dragging of the sedimen- can be interpreted as a part of a breached mono- σ σ 1 (ENE/WSW) and 3 (NNW/SSE), and sub- tary cover, the stress component parallel to the cline formed as a result of fault-propagation σ vertical 2, as evidenced from fi ssion-track and faults is only responsible for the lateral displace- folding. These fault-related monoclinal folds paleostress data (Bojar et al., 2002). ment, i.e., of the slip, whereas the perpendicular are formed by warping of middle Pliocene beds According to Khalil and McClay (2002), component is responsible for the development beyond major-scale master normal faults. folding of the Upper Cretaceous–lower Oligo- of thrusts, anticlines, and synclines striking par- Normal master faults nucleated at depth cene sequence was produced by folding of a allel to the faults (e.g., Ali Kassim et al., 2002). within the Precambrian rocks and then propa- propagating fault-tip line where half grabens The parallelism between synclinal folds and the gated upward into Phanerozoic cover and produced synclines. This hypothesis does not major shear zones depends mainly on orienta- caused the development of master fault–related σ satisfactorily explain all of the tectonic features tion and intensity of 1 (ENE to WSW). The monoclinal folds within the overlying Miocene– of the Cretaceous basins for the following rea- curvilinear shape and the sudden change in ori- Pliocene sequence. Layer-parallel slip occurred sons: (1) it is diffi cult to accept the existence of entation of the Nakheil syncline axis may have in shale and sandy shale units at depth, and a as many half grabens as synclines, and each half developed later and can be interpreted as a drag normal fault, which splays from the master fault, graben contains more than one syncline; (2) the structure related to sinistral shearing during fi nal deformed limestone and sandstone units. The Cretaceous basins contain Nubia Sandstones, stages of deformation. presence of thick shale beds in the study area which have a wide range of ages, from Cam- (3) Major NW-SE–trending Precambrian increased competency of bending of beds. Also, brian to Cretaceous (Moustafa, 1987; Jackson et shear zones (such as Sibai, Meatiq, Hamrawin, all the recorded meter-scale fault-related folds al., 2002, 2006a, 2006b), and this indicates that Queih shear zones) occur in the neighborhood are observed in shale-rich beds. The ductile the nucleation of the basins might have been ini- of the synclinal basins. A fi nal phase of Neo- character of shale- and sandy shale–dominated tiated earlier than Cretaceous time; (3) the main proterozoic basin formation resulted in deposi- units of the Abu Dabbab and Samh Formations Red Sea extension started in the late Oligocene– tion of molasse sediments in isolated, pull-apart played the main role in the formation of fault- early Miocene and not in Early Cretaceous, as basins (e.g., Umm Seleimat, Queih, Kareim, related folds in the Quseir–Umm Gheig subba- cited by Khalil and McClay (2002); (4) some Atawi, El-Miyah, and Igla Basins) caused by sin. Layer-parallel slip and ductile fl ow occur in of the folds axes are curvilinear, such as those strike-slip movements on faults of the Najd fault shale and sandy shale units, whereas sandstone of Nakheil and Atshan synclines; and (5) the system (Abd El-Wahed, 2004, 2009). and limestone units are faulted, fractured, and strike-slip faults dislocated the axial plane of the (4) There is a difference of 20° between the brecciated. The same setting has been described Nakheil syncline. orientation of the sinistral strike-slip shear zones previously from the western margin of the Dead The development of Cretaceous basins can and the associated en echelon synclinal folds. Sea rift (Gross et al., 1997), from the Sirt Basin, be better explained in the context of strike-slip (5) En echelon arrangement of Cretaceous south-central Libya (Fodor et al., 2005), and the tectonics. The axes of the basins and the syncli- basins and en echelon pattern of synclines are Gulf of Suez (Jackson et al., 2006a, 2006b), and nal folds trend mainly NW-SE and NNW-SSE observed in the same basin (e.g., Hamadat). In has been explained as due to the ductile fl ow of parallel to the major normal faults bordering progressive simple shear deformation, there is mudstone units. The monocline that developed them. An analysis of the geological map of the usually a 45° angle between the principal stress early in the process was later breached by meter- study area allows us to recognize the typical axis of the strain ellipsoid and the main shear scale folds through continued upward propaga- features of a wrench zone in the Quseir–Umm plane; this orientation facilitates the formation tion of the master fault at the different stages Gheig subbasin during Late Cretaceous time. of en echelon folds. of monocline development. Generally, fl ex- The present-day normal faults bordering the (6) The principal stress axes are subhorizon- ing increases in the area between Wadi Sharm σ σ σ synclinal basins were originally developed dur- tal 1 and 3, and subvertical 2, as presented by El Bahari and Wadi Wizr and then dies out to ing wrench tectonics and enhanced during rift Bojar et al. (2002). the north and south with thinning of shale beds tectonics. The basins and the axes of synclinal (7) Sheared conglomerates from the Nubia and thickening of limestones and sandstone folds show an en echelon arrangement; their Formation were incorporated into a major verti- beds. The disappearance of the fault-related orientation in respect to the main wrench fault cal sinistral shear zone, including vertical folia- monocline and its associated fault-related syn- and the dislocation of their axial planes by faults tion and horizontal lineation (Bojar et al., 2002). clines and anticlines is due to thinning of shale indicate a left-lateral displacement along the (8) Minor overturned folds and minor NE- and sandy shale beds in the Miocene–Pliocene main wrench fault. vergent thrusts occur in Dakhla shale (Paleo- sequence. Therefore, variations in deformation The main features that support the develop- cene) of the Aswad subblock. style were largely controlled by variations in ment of the Cretaceous pull-apart basins during (9) There is a predominance of NE-SW nor- shale and sandy shale intercalations. sinistral wrench tectonics are: mal faults in Cretaceous–Eocene sequence. The fl exing disturbed the gentle regional (1) All the Cretaceous basins are concen- dip of the Miocene–Pleistocene sequence and trated in the area between Safaga and Umm Tectonic Regime of the Fault-Related led to the development of large, gently dipping Gheig (Fig. 2) in the central Eastern Desert, Folds in Miocene–Pliocene Rocks parts and small, moderately to strongly dipping which represents the main infl uence zone of the segments. The change from gently dipping to Najd fault system (de Wall et al., 2001; Abd El- Fault-related folds in Quseir–Umm Gheig steeply dipping limbs is abrupt and marked by Wahed, 2007, 2008, 2009). subbasin were formed synchronous with the development of meter-scale fault-related syn- (2) The synclinal fold axes are parallel to main episode of Red Sea extension and include clines and anticlines. These folds are associated the major faults. Transpressive deformation a series of meter-scale folds produced in exten- with a series of NW-SE–trending macro- and along vertical faults has two components: one sional tectonic environments. These include: (1) micro-scale normal faults developed mainly in is parallel to the strike-slip faults, and the other extensional fault-propagation folds, (2) com- the hanging walls of the master faults. Some of is perpendicular (Mount and Suppe, 1987; Ali paction drape folds, and (3) normal drag folds. these normal faults may have been overturned Kassim et al., 2002). If the movement along Meter-scale folds adjacent to large normal faults or rotated to appear as steeply dipping reverse

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Figure 11. (A) Drape fold in upper Miocene rocks of Wadi Wizr, looking NW. (B) Steeply dipping ﬂ ank of the drape fold broken and displaced in Pliocene rocks between Wadi Ambaji and Wadi Aswad, looking NNW. (C) Reverse drag fold in middle Miocene rocks, Wadi Sharm El Bahari, looking WNW. (D) Nor- mal drag fold formed when footwall beds are dragged down and hanging-wall beds are dragged up, looking NW. (E) NE-plunging evaporite chevron folds in Pliocene rocks of Wadi Aswad, looking NE. (F) Major NE-plunging anticline belonging to nonplane, noncylindrical buckling folds of Ghosh et al. (1993), in Pliocene rocks between Wadi Ambaji and Wadi Aswad, looking NE. (G) Buckle dome in upper Miocene rocks of Samh Formation between Wadi Essel and Wadi Sharm El Bahari. The conical hills in the background are the upper Miocene evaporite of Abu Dabbab Formation, looking NE.

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faults. Rotation of normal faults occurred with Cretaceous. This movement was accompanied NW-SE–trending border normal faults marking the progressive rotation of strongly dipping seg- by reactivation of sinistral movement along the the contacts between the prerift sediments and ments of the fault-related monocline. Normal NW-SE–trending major shear zone in the Pre- the Precambrian rocks. drag folds usually developed in the hanging cambrian rocks. Such movement was accom- In the middle–late Miocene transition, the walls of these rotated normal faults. Additional plished by formation of the en echelon–oriented Red Sea switched from rift-normal movement breaching increased displacement along the pull-apart basins (Fig. 12A). These basins were to highly oblique extension parallel to the trans- master fault and may have led to detachment of ﬁ lled with a thick Cretaceous–late Oligocene form (Bosworth et al., 2005). This probably strongly dipping segments from gently dipping sequence composed of Nubia (Cretaceous?), produced minor compression and uplift in the parts (see Fig. 10F). Quseir (Campanian), Duwi (Campanian–Maas- northernmost Gulf of Suez (Patton et al., 1994), trichtian), Dakhla (Maastrichtian–Paleocene), and, to some extent, isolated the northern Red Origin of Evaporite Folds and Buckling Esna (late Paleocene–early Eocene), Thebes Sea from invasion of marine waters of the Med- (early–middle Eocene), and Nakheil (late Oli- iterranean. This was accompanied by sudden The NE-trending evaporite folds in the upper gocene) Formations. The axes of these basins changes in sedimentation processes to prevalent Miocene–Pliocene sequence indicate NW-SE trended mainly NW-SE and were deﬂ ected evaporites of the Abu Dabbab Formation. A compression. Sehim (1994) discussed the mech- to N-S due to the effect of sinistral movement series of NW-trending coastal normal faults was

anisms of these folds and concluded that the along NNE-SSW strike-slip faults (D2). Devel- developed during Miocene–Pliocene time. Nor- strain axes of these folds are parallel to those opment of Late Cretaceous basins and deforma- mal master faults developed at depth within the of the NNE-SSW strike-slip faults. This con- tion of the Late Cretaceous–Eocene sequence Precambrian rocks and then propagated upward clusion is supported in the present study by the during wrench tectonic events are supported by: into the overlying Miocene–Pliocene sequence following features: (1) the axes of these folds (1) arrangement of the Late Cretaceous basins and caused development of the fault-related trend NE-SW and moderately plunge (20–30°) in an en echelon system, (2) the axes of the monoclinal fold. Macro- and micro-scale nor- toward the NE; (2) the folds are asymmetric, synclinal basins, oriented at low angle to the mal faults splayed from the master faults, and σ σ their limbs have dip angles between 15° and wrench fault, (3) subhorizontal 1 and 3 and fault-propagation folds developed within the σ 30°, and the folds extend along the entire Red subvertical 2 orientations, (4) the presence of monoclinal folds. Sea coast, forming a series of anticlines and syn- NE-vergent thrusts and minor overturned folds During the late Miocene–Pliocene transition, clines; (3) the intensity of folding increases with in Dakhla shale of the Aswad subblock, and an additional movement occurred along the NNE- increasing thickness of gypsiferous shale beds (5) the presence of NE-SW normal faults in the SSW sinistral strike-slip faults and their second- in the upper Miocene–Pliocene sequence where Cretaceous–Eocene sequence. order shears. This movement was accompanied

the presence of shale increases the ductile ﬂ ow The D2 deformation event was related to the by NW-SE compression that commenced with of the beds and facilitates folding; (4) folds are rifting of the Red Sea initiated in the late Oligo- development of NE-SW folds in upper Miocene not restricted to evaporite and gypsiferous shale cene–early Miocene. The synrift sediments rest evaporites and lower Pliocene rocks (Fig. 12B). beds but extend to the overlying clastic sedi- unconformably over the lower–middle Eocene Also, buckling of Pliocene and post-Pliocene ments; and (5) folds are also observed in areas limestone of the Thebes Formation. The lower sediments occurred during this event. Buckling with no evaporites but with the presence of shale. Miocene rocks are mainly conglomerates and and NE-SW evaporite folds are important struc- According to the structural features observed in varicolored sandstones deposited in an alluvial- tural features throughout the Quseir–Umm Gheig this study and those recorded by Sehim (1994), plain environment. subbasin and the entire Red Sea coast. the evaporite folds and the buckle folds are syn- In the early Oligocene, rifting was initiated In the Quseir–Umm Gheig subbasin, the tectonic folds developed during deposition of in the form of several small, en echelon E-W– presence of extensional faults, extension gashes, the upper Miocene–Pliocene sequence. These to ESE-WNW–trending basins in the Gulf of joints in Pleistocene rocks, and uplift of upper folds may have developed due to NW-SE com- Aden province (Fantozzi and Sgavetti, 1998; Pleistocene and Holocene coral terraces indi- pression associated with differential movement Watchorn et al., 1998). In late Oligocene–early cates that movement along the NNE-SSW along sinistral NNW-SSE strike-slip faults and Miocene time, rifting had extended to Afar Aqaba–Dead Sea transform trends and normal their second-order shears. and all over the Red Sea system (Bosworth et extensional faults is still active to the present al., 2005; Guiraud et al., 2005). The N-trend- time (Fig. 12C). Active strike-slip faulting in CONCLUSIONS ing Pan-African weak zones or stress guides Quseir–Umm Gheig subbasin generally poses played a role in the redirection of the Red Sea a greater earthquake hazard than the movement Three major tectonic events constitute the rift propagation. During the middle Miocene, on normal faults. deformation history of the Quseir–Umm Gheig the Red Sea was subjected to NW compres-

subbasin: D1 represents the oldest deforma- sive movement (Jarrige et al., 1986; Richert et ACKNOWLEDGMENTS tion event and was related to Late Cretaceous al., 1986). This movement generated dextral sinistral movement along the reactivated Najd transpression along NW-trending Queih and The authors would like to express their deep

fault system; D2 includes NE extension of the Hamrawin shear zones and sinistral movement thanks to Adel R. Moustafa, Department Red Sea in late Oligocene–early Miocene and along the NNE-SSW Aqaba–Dead Sea trans- of Geology, Ain Shams University, Cairo, a middle Miocene deformation event associated form (Fig. 12B). Both synthetic and antithetic Egypt, for his valuable and constructive com- with NW-SE dextral and NNE-SSW sinistral conjugate strike-slip faults were developed and ments on an earlier version of the manuscript.

strike-slip faults; and D3 is a late Pliocene–early oriented at low and high angles to the wrench We are also grateful to Samir Kamh, Department Pleistocene deformation event associated with fault, respectively. They include NNE-SSW, of Geology, Tanta University, Tanta, Egypt, for development of buckling and evaporite folds. NE-SW, and ENE-WSW synthetic and anti- his kind support in the ﬁ eld. The manuscript was

During D1, the study area was subjected to thetic faults. This event was associated with the substantially improved by the helpful comments ENE-WSW compressive movement in the Late NE extension of the Red Sea and reactivation of of James P. Evans and an anonymous reviewer.

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0 20 km N A NW eh shear zone Qui one Hamrawin shear z

pull-apart basin Duwi-El Nakheil Zog El Bohar- Hamadat Pull-apart basin Meatiq shear zone

Meatiq G. G. Sibai Sibai shear zone El-Shush shear zone

Sitra shear zone

Late Cretaceous

σ Synrift and post rift rocks 1 Block-bounding normal faults Precambrian shear zone Prerift sedimentary rocks σ 3 Syncline Precambrian rocks Strike-slip faults

Extension N B Zog El Bohar Sharm Evaporite QUSEIR block El Bahari and buckle Extension block folds Atshan block Hamadat Figure 12. Block diagrams show- block No rmal ing major events contributing to fa σ u 1 R lts 2 the evolution of the Quseir–Umm Sibai shear zone Gheig region (see text for details). i M G. Siba σ3 zone El-Shush shear R1

Normal faults

Miocene-Pliocene

Extension AGA R E D SAF S E A

G. G. Umgher Gassu a s El-Nakheil Atshan T hamedh blo block Mo ck syncline block Raback Anz-Ambaji blo N El t block Um eta Gihania ck SEIRZog C Hu block blo El Bohar Sharm Quieh shear zone QU block El Bahari hear zone Hamrawin s block G. Wasif block Hamadat NW Duwi b - syncline lock

G. Ummmadwi Ha . Du G ck lo b Meatiq shear zone

eatiq G. M ai shear zone G. Sibair zone Sib h shea El-Shus

NW Sitra shear zone

Recent Monoclinal fold

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