Paleostress Analysis of the Brittle Deformations on the Northwestern Margin of the Red Sea and the Southern Gulf of Suez, Egypt 1

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1 ISSN , Geotectonics, 2017, Vol. 51, No. 6, pp Pleiades Publishing, Inc., Paleostress Analysis of the Brittle Deformations on the Northwestern Margin of the Red Sea and the Southern Gulf of Suez, Egypt 1 Kh. S. Zaky Minia University, Geology Department, Faculty of Science, Minia, Egypt zakyk@yahoo.com Received February 2, 2017 Abstract Shear fractures, dip-slip, strike-slip faults and their striations are preserved in the pre- and syn-rift rocks at Gulf of Suez and northwestern margin of the Red Sea. Fault-kinematic analysis and paleostress reconstruction show that the fault systems that control the Red Sea Gulf of Suez rift structures develop in at least four tectonic stages. The first one is compressional stage and oriented NE SW. The average stress regime index R' is 1.55 and S Hmax oriented NE SW. This stage is responsible for reactivation of the N S to NNE, ENE and WNW Precambrian fractures. The second stage is characterized by WNW dextral and NNW to N S sinistral faults, and is related to NW SE compressional stress regime. The third stage is belonging to NE SW extensional regime. The S Hmax is oriented NW SE parallel to the normal faults, and the average stress regime R' is equal The NNE SSW fourth tectonic stage is considered a counterclockwise rotation of the third stage in Pliocene-Pleistocene age. The first and second stages consider the initial stages of rifting, while the third and fourth represent the main stage of rifting. Keywords: Red Sea, Gulf of Suez, paleostress, stress regime, fractures, slip-faults DOI: /S INTRODUCTION The Gulf of Suez Red Sea Gulf of Aden rift system has long been recognized as having been formed by the separation of Arabia plate from Africa plate [e.g. 28, 29, 46] and is perhaps the best modern example of continental rifting and incipient ocean formation. The rift system was formed by the anticlockwise rotation of Arabia away from Africa about a pole of rotation in the central or south central Mediterranean Sea [41, 59, 61, 62, 67]. The movement of the Arabian plate away from Africa in a NE direction [23, 26, 54, 55, 59, 61] led to the opening of two young oceanic basins: the Red Sea, between Africa (Nubia) and Arabia, and the Gulf of Aden, between Somalia and Arabia. The early rift in the southern Gulf of Suez and Northwestern Red Sea was structurally and stratigraphically complex, and marked by distributed faulting on a variety of scales [20, 63]. All population of faults kinematic directions, in the southern Gulf of Suez and northwestern Red Sea, whether from major bounding fault complexes or smaller-scale faults, and of all ages, are compatible with ENE WSW to NNE SSW extension, with no evidence for significant rift parallel shortening [17]. 1 The article is published in the original. Understanding the paleostress analysis of the northwestern Red Sea and southwestern part of the Gulf of Suez are the main objective of this study. Knowledge of the stress field in a region is an important tool to understand local and regional effects induced by large-scale plate tectonics and the subsequent deformation field. Studies of continental stress fields have shown that principal stress axes can rotate through large angles over geologically short time period [18]. The rotation of the principal stress axes have been documented in the Rhine graben [50], the Basin and Range province of the western United States [93], Rio Grande rift [43], the eastern and western branches of the East African Rift System [21, 30, 32, 76, 85, 86]. In this work a great number of joint stations and small faults as well as some deformation bands are collected from pre- and syn-rift rocks, in five chosen regions. These regions are Gebel El Zeit on the western margin of the Gulf of Suez, El Quseir, Umm Gheig, Marsa Alam and Abu Ghusun regions on the northwestern side of the Red Sea (Fig. 1). The processing of the collected data have enabled me to determine the mean principal paleostress axes (σ1, σ2 and σ3 are respectively referred to as maximum, intermediate and minimum compressional stresses). 625

2 626 ZAKY 30 N 32 E Wadi Araba Syrian Arc Anticlines Gulf of Suez Eastern Desert River Nile 100 km Nasar Lake GAZ-AC M-AZ Sinai G. Zeit 35 Levant Shear Gulf of Aqaba RED SEA D-AZ Quseir Um Gheig Marsa Alam Abu Ghusun Ras Banas Fig. 1. Tectonic setting for the Gulf of Suez and northwestern Red Sea rift basin [20]. GAZ-AZ, M-AZ, D-AZ are Galala-Abu Zenima, Morgan and Duwi Accommodation Zones respectively. Solid circles are the locations of the studied areas. GEOLOGY The lithostratigraphy of the Gulf of Suez northwestern Red Sea rift includes three main units [64]. (1) The basement series which outcrops both along the marginal relief (shoulders) of the rift and within a number of large fault blocks along the margins. The Precambrian crystalline basement rocks consist of metavolcanics, metasediments, and granitoid intrusives [8, 79]. These rocks are related to several Precambrian orogenic cycles and lower to middle Paleozoic magmatic events (dikes and batholiths; cf. 22). The Pan-African tectono-magmatic event (about 500 Ma) played an important part in the structuring of this polyphased basement. The Cenozoic rift evolved partly by reactivation of the various categories of deformation within the basement. (2) The basement rocks are unconformably overlain by m thick section of pre-rift sediments that ranges in age from the Late Cretaceous to the Middle Eocene [58] (Fig. 2a). The lower part of the pre-rift section is the 130 massive thick bedded, siliciclastics Nubian sandstone. This is overlain by a m thick sequence of interbedded shales, sandstone and limestones of the Quseir, Duwi, Dakhla, and Esna Formations [6, 51, 91]. The uppermost pre-rift strata consist of m of competent, thick-bedded limestones and chert limestones of the Lower to Middle Eocene Thebes Formation [58]. Along the southern side of the Gulf of Suez the pre-rift sediments consist of Nubian sandstone, overlain by interbedded shale, sandstone and limestone (Fig. 2b). The base of Nubian sandstone, in places, consists of several tens of meters of interbedded red siltstone and sandstone that is undated, but may correlate in part with lithologically similar Paleozoic to Triassic strata in Sinai [9, 88]. The upper of Nubian consists of several hundred meters of quartzose, dominantly cross-bedded, non-marine sandstone that is referred to the Naqus [9] and/or Malha formations [e.g. 57]. The Malha is overlain by a widespread shallow marine sequence of Cenomanian to Eocene age, consisting of the Raha, Wata, Matulla, Brown Lime, Sudr, Esna, and Thebes formations [78]. (3) The Late Oligocene to Recent syn-rift strata unconformably overlie the Thebes Formation and vary in thickness from less than 100 m onshore to much as 5 km in offshore basin [45]. The lowermost syn-rift strata are coarse grained clastics (Nakheil and Ranga formations). Overlying these clastics are reef limestone, clastics and evaporates (Um Mahara, Sayateen and Abu Dabbab formations). Late Miocene carbonates and reefs and Pliocene to Recent syn-rift clastics overlie the evaporates in the coastal outcrops [64, 73]. In the southern side of the Gulf of Suez, the Late Oligocene to Recent syn-rift strata unconformably overlie the Thebes Formation. These sediments are divided into Nukhul Formation (clastics sediments), Rudeis Formation (Globigerina marl interbedded with limestone), Kareem Formation (Evaporites), Belayim Formation (anhydrite and salt), South Gharib Formation (massive halite) and Zeit Formation (interbedded evaporate, shale, sandstone and carbonate). These rocks are capped by extensive gravels and lesser Ostrea-Pecten coquinas and marls which are only broadly Pliocene to Pleistocene in age [49]. STRUCTURAL FRAMEWORK AND TECTONIC SETTING The Red Sea rift system was formed in the Late Oligocene Early Miocene in response to the NE separation of Arabia away from Africa [29, 59, 61, 62, 67, 74]. In the Late Middle Miocene, continued opening of the red sea became linked to sinistral offset along the Gulf of Aqaba Levant Transform (Fig. 3a) [13, 41, 61, 75, 82].

3 PALEOSTRESS ANALYSIS OF THE BRITTLE DEFORMATIONS 627 (a) Era Cenozoic Mesozoic Period Recent Pleistocene Pliocene Miocene Eocene Oligocene Late Early Late Thebes Early Late Fm. Samadi Shagara Mersa Alam Abu Dabbab Syateen Um Mahara Ranga Volcanics Nakheil Esna Paleocene Dakhla Cretaceous Epoch Middle Middle Duwi Quseir Nubia Precambrian crystaline basement Samh Gabir Lithology PRE-RIFT SYN-RIFT Age Lithology Zones Recent Pliocene Miocene Late Middle Early 4 3 Oligocene M. Eocene E. Eocene Cenomanian Albian Paleozoic Pan African Ma 5 17 Mess 16 N11 15 N10 14 N N8 12 N N Lang. N5 7 6 N4 5 N3 Serravallian Tortonian Aquttanian Burdigalian Foraminifera Nannofossils N2 N1 25 Evaporites Limestone & chert Shale Marl Sandstone Conglomerate Mafic volcanics Granite Metavolcanics & metasediments Gulf of Suez stratigraphy Pleistocene to recent coral Reefs Pecten-Ostrea Gravel beds Zeit Fm. South Gharib Fm. Hammam Faraun member Belayim Fm. Kareem Fm. Rudeis Fm. Nukhul Fm. Abu Zenima Red beds Non-deposition & local erosion Thebes Fm. Sudr Fm. Esna shale Matulla/Wata/Raha Fms. Nubian sandstone Basement Salt Limestone Chalk Shale S.S. & shale Anhydite Marl Sandstone Cong. & shale Granite Metavolcanics & metasediments Fig. 2. (a) Summary stratigraphy of the northwestern Red Sea rift system. Data from [58, 74, 79]. Generalized stratigraphy and microfossil zonations of the southern Gulf of Suez [20]. The northwestern Red Sea Gulf of Suez rift consists of large scale half-graben that alternate in polarity along the basin axis (Fig. 3b) [15, 16, 27, 53, 66, 68]. These half-grabens are separated by complex accommodation zones. The accommodation zone refers to a complex zone of faulting that accommodates an along-strike change in both the fault dips and in subbasin polarity within a rift system [15, 38, 58, 77]. The accommodation zones are generally oblique to the rift trend [16, 27, 68, 69]. Cross-faults smaller than accommodation zones segment the rift at a variety of scales, producing apparent or real offsets in rift-trend normal faults [20]. These faults have E W, NE SW and NNE SSW orientations. The Gulf of Suez and Red Sea rift is not a purely neoformed Cenozoic structure [64], but a recombination and reactivation of pre-existing discontinuities during the Tertiary. The size, geometry and complexity of the blocks depend directly on the nature and density of the discontinuities susceptible to rejuvenation. The comparison with other rift systems (East African Gregory rift, South Atlantic African margin, Rhine graben, etc.) leads to a similar conclusion: the reactivation of numerous inherited structures and, correspondingly, the constitution of a zigzag fault pattern, may be a general rule in rift evolution. Red Sea Gulf of Suez rift comprises a complex structural pattern which directly influences depositional

4 628 ZAKY Suez 33 Rift Border Fault 35 N Intra Rift Fault Dip Domain 50 km ZAZ Gulf of Suez Gulf of Aqaba 29 (a) Eastern Desert Red Sea Hills BLACK SEA E 40 Eurasian 40 Plate N MEDITERRANEAN SEA 30 Aqaba Levant 30 Gulf of Suez Transform Figure b RED SEA African plate INDIAN Afar OCEAN Gulf of Aden MAZ Syn and Post Rift Pre-Rift Basement Hurghada Safaga DAZ Quseir RED SEA Fig. 3. (a) Plate tectonic setting of the northwestern Red Sea rift system. Principal structural elements of the northwestern Red Sea Gulf of Suez rift system. Bold arrows indicate the dominant stratal dip directions within the individual half-grabens subbasins [57]. processes [64]. Four major inherited tectonic trends comprise the structural framework of the rift [52]. These trends are N , N 10 20, N , and N The different fault trends are linked to compose the rift fault-block pattern. The N (Clysmic faults) are generally present and associated with another major, N (Aqaba Faults) or N (Duwi faults), resulting in the formation of a zigzag fault pattern [42, 52] which controls the Cenozoic structure.

5 PALEOSTRESS ANALYSIS OF THE BRITTLE DEFORMATIONS 629 STRUCTURAL ANALYSIS Fault pattern of the northwestern Red Sea and southwestern Gulf of Suez (Fig. 4a) indicate the main trends are NW SE ( ), WNW ESE ( ), NNE SSW (0 20 ) and NE SW (40 70 ) (Fig. 4b). In the following paragraphs I will shed light on the brittle deformations in the chosen regions. Gebel El Zeit Region Gebel el Zeit (Fig. 5) is a large, intra-rift fault block that exposes granitic basement, an excellent section of pre-rift strata and a thin (~150 m) of syn-rift rocks [9, 16, 20, 36, 63]. The pre-rift rocks in Gebel el zeit consist of Nubian sandstone overlain by interbedded shale, sandstone and limestone. The base of the Nubian sandstone consists in several tens of meters of interbedded red siltstone and sandstone that is undated, but may correlate in part with lithologically similar Paleozoic to Triassic strata in Sinai [9, 88]. These rocks are dissected by a conjugate joint system (Fig. 6a). The two sets of the joint system are striking ENE and NNE (Fig. 6b). The previous joint system is recorded also in the upper of the Nubian sandstone which consists of several hundred meters of quartzose, cross-bedded, non-marine sandstone that is referred to the Naqus [9] and/or Malha formations [57]. A number of left- and right-lateral strike slip faults are also recorded in the Paleozoic rocks (Fig. 7). These faults are striking ENE WSW and NNE SSW respectively. The joint system and the strike-slip faults in the Nubian sandstones are sealed by younger shallow marine sequence of Cenomanian to Eocene rocks and disrupted by syn-rift structures. The syn-rift structures are represented mainly by NW SE normal faults (Fig. 8). These faults dip in NE and SW directions. The NW SE small normal faults are focused in interbedded shale, sandstone and carbonates rocks under and over the evaporate rocks (Rudeis and Zeit formations respectively). Most of the NW SE normal faults in pre- and syn rift rocks at gebel el Zeit consist of fault core and damage zones. The fault core, which accommodates most of the displacement, ranges from ~5 cm to ~1 m (Fig. 8). The thickness of the fault core increases with increase the displacement. Damage zones in Nubian sandstone are represented by deformation bands, while in low-porosity and non-porous rocks (shale, marl and limestone) are represented by drag folds and fractures (Figs. 8d, 8e). Deformation bands are millimeter-thick structures that result from strain localization processes in highly porous granular media. They are characterized by grain reorganization due to grain sliding, rotation and/or fracturing associated with dilation, shear and/or compaction mechanisms. Compaction within cataclastic bands causes a reduction in porosity and (a) Ras Gharib 100 km Hurghada Safaga Quseir Marsa Alam Ras Honkorab Abu Ghusun Ras Benas Fig. 4. (a) Fault pattern of the Gulf of Suez and the northwestern Red Sea [64]. Rose diagram showing the main trends of the fault patterns after this paper. permeability as compared to the host rock [e.g., 39, 40]. The deformation bands in Nubian sandstone at gebel el zeit are striking NW SE and dip NE and SW (Fig. 9a). The thickness of these bands is ranging from 1 to >5 mm (Fig. 9b). El Quseir Region El Quseir region (Fig. 10) contains the southernmost exposures of the pre-rift stratigraphic section of the uplifted Egyptian continental margin. The pre-rift rocks (Precambrian to Eocene) and the syn-rift rocks (Oligocene Pleistocene) are well exposed in the area. Two conjugate joint systems are the first brittle deformation which recorded in the pre-rift rocks (Nubian sandstone) at Gebel Duwi. The first one has N E and N E joint sets (Fig. 11a). This system is interrupted by a younger joint system according to the field relationships. The younger joint system has N W and N 0 10 W joint sets (Figs. 11b, 11c). The N 10 W joint zone is dominant and interrupted all the older deformations (Figs. 11d, 11e). The pre- and syn-rift rocks were affected by the tectonic evolution of rifting. The NNW SSE to NW SE oriented normal faults are dissecting the pre-rift rocks (Fig. 12a). These faults dip towards NE and SW directions. Fault surfaces are exposed and slickensides are recorded along some of these faults (Fig. 12b). Some N

6 630 ZAKY Gulf of Suez Pliocene to recent Pre-rift and Miocene syn-rift strata Crystalline basement km Fig. 5. Surface geological map of the southwestern margin of the Gulf of Suez rift [16]. Faults in the offshore and areas of Pliocene to Recent cover are projected to the surface after [17]. N N 65 E N10 E (a) Fig. 6. (a) Conjugate joint system in Paleozoic Triassic Nubian sandstone at G. El Zeit. N 65 E is the main joint set, while the N10 E is the subsidiary set. Rose diagram showing the main trend of the two joint sets.

7 PALEOSTRESS ANALYSIS OF THE BRITTLE DEFORMATIONS 631 (a) Fault core (c) (d) Fig. 7. (a) Strike-slip faults in Paleozoic Triassic Nubian sandstone at G. El Zeit. Left-lateral strike-slip fault. Sinistral fault zone. (c) Left lateral strike-slip fault with clear fault core. (d) Dextral strike-slip fault. of N 50 W oriented normal faults have two groups of slickensides (Fig. 12c). The first slickensides are nearly horizontal (5/N 50 W) and indicate to right-lateral movement. The second slickensides (60/S 40 W) have recorded superimposed normal to the first group and indicate to dip-slip movement. Two sets of deformation bands were recorded along some of NW trending normal faults in Cretaceous Nubian sandstone at Gebel Duwi (Fig. 13). The two sets are striking in a N E and N W directions. The N 40 W basaltic dyke is dissecting the pre-rift rocks at El Quseir region (Fig. 14a). These magmatic rocks are probably related to the early stage of rifting and it is dissected by two joint sets (Fig. 14b). The first set is striking N W and the second one is striking N E (Fig. 14c). WNW ESE oriented faults (Hamrawein and El Queih) traverse the study area [7, 69] (Fig. 10). Both left- and right-lateral movement are documented along these faults. Left-lateral slip on these faults was related to movement along the Najd Fault system, whereas the right-lateral slip was related to rifting in the northern Red Sea in the Late Oligocene [69]. The Hamrawein fault forms the Sudmain transfer zone [69] or Duwi accommodation zone [58] between the oppositely tilted half-grabens in the northwestern Red Sea region (Fig. 1). The syn-rift rocks (Miocene Pleistocene) are dissected by a great number of NNW to WNW oriented normal faults (Fig. 15). The WNW ESE oriented normal faults are common in the Pliocene Pleistocene rocks (Shagara Formation). Some of NNW oriented normal faults have oblique slickensides, which indicate that these faults were reactivated during a recent extension movement. Umm Gheig Region The sedimentary rocks in Umm Gheig area are mainly syn-rift (Miocene to Pleistocene) rocks. These

8 632 ZAKY (а) Fault core (~5 cm thick) SE Hanging wall Deformation bands NW (c) NW Nubian sandstone (d) Miocene rocks Rudeis Fm. Interbedded shale & s.s. (e) SE Hanging wall Damage zone Deformation bands NW Fig. 8. Normal faults in pre- and syn-rift rocks at G. El Zeit. (a) NW SE normal fault in Paleozoic Triassic Nubian sandstone. Low angle normal fault in Cretaceous rocks. Displacement is about 1.5 m. (c) Major NW SE normal fault separate the shallow marine rocks (interbedded of shale, sandstone and limestone) on the left side and Nubian sandstone (the Naqus and/or Malha formations) on the right side. Fault core is about 1 m. (d) NW SE normal fault in the Miocene rocks. The damage zones along the fault planes are represented by drag folds. (e) Deformation bands in the hanging wall damage zone along NW SE normal fault in the Paleozoic Triassic Nubian sandstone. rocks are unconformably overlies the Precambrian basement rocks. The pre-rift sedimentary rocks are not represented in this area. The dip angle of these rocks is ranging from 10 to 20 in southeast direction (Fig. 15). The NW SE trending normal faults are exposed in the syn-rift rocks (Fig. 16). Slickenside data from these normal faults indicate dominantly dip-slip movement. The lead-zink deposits are controlled by the main

9 Deformation bands PALEOSTRESS ANALYSIS OF THE BRITTLE DEFORMATIONS 633 N W E (a) S SE NW Fig. 9. (a) Lower hemisphere, equal area projection shows the orientation of the deformation bands at Gebel El Zeit. Field photograph showing the NW SE bands and their thicknesses.

10 634 ZAKY El Quwyh Shear Zone Coastal Fault System N KF Hamrawin Shear Zone AF RED SEA W El Hakheil G. Duwi El Quseir Pliocene & Quaternary Miocene Late Oligocene Eocene Cretaceous Paleocene Precambrian Basement Pre-rift Syn-rift Major extensional fault Extensional fault Precambrian shear zone Basement fault zone Anticline Syncline Dip & Strike of strata 20 km G. Hamadat HF ZF Fig. 10. Simplified geologic map of the Quseir region (Gebel Duwi-Gebel Hamadat), northwestern Red Sea [57]. KF, NF and HF indicate the Kallahin, Nakheil and Hamadat fault segments of the Border fault system. AF and ZF indicate the Anz and Zug El Bahar fault segments of the Coastal fault system. NW SE normal fault between the Precambrian basement rocks and Syn-rift rocks (Umm Mahara Formation). The major and minor normal faults strike N30 40 W and dip steeply to NE or SW. The hanging wall in some of NE dipping normal faults has minor normal and reverse faults (Fig. 17). Marsa Alam Region The syn-rift rocks in Marsa Alam area begins with the Middle Miocene (Um Mahara Formation). These rocks rest unconformably on the Precambrian basement rocks (Fig. 18). The pre-rift sedimentary covers are not recorded in this area. The strata of the syn-rift rocks are striking in a NW SE direction with a dip angle ranges from NE. The syn-rift rocks are dissected by a number of NW SE normal faults (Fig. 19). These faults dip in NE and SW directions with angle ranges from 65 to 80. Abu Ghusun Area The syn-rift sediments in the Abu Ghusun area, (Fig. 20a), begins with red clastic deposits sandstone and conglomerates [57, 64]. These sediments lie directly on the Precambrian basement rocks. The red clastic deposits are overlain by evaporates and related anoxic sediments (A2). Group B are open marine deposits unconformably overlying Group A strata or the basement. They are late Burdigalian to Langhian (and possibly Serravallian) in age. Geology of this area is discussed details in [63, 64]. Group A1 is dissected by a systematic joint set. This set is striking N W. A large number of NW SE trending normal faults (Fig. 20b) dissects the syn-rift rocks. These faults interrupted the WNW joint set. A number of WNW right lateral strike-slip faults are recorded in this area. The dextral strike-slip faults cut the syn-rift rocks and basement rocks. The NW and

11 PALEOSTRESS ANALYSIS OF THE BRITTLE DEFORMATIONS 635 (а) N N (c) N W Joint Set (d) N N 0 10 W Joint Set 80 /N 80 E Joint Set (e) N 75 /N 80 W Joint Set Fig. 11. (a, b) Rose diagrams showing the main directions of the conjugate joint systems in Nubian sandstone at Gebel Duwi. (c) Field photograph shows the N W and N 0 10 W joint sets. (d, e) Field photographs show the predominant N 10 W joint zone. WNW faults are linked to compose the rift fault-block pattern. PALEOSTRESS ANALYSIS Paleostress analysis is relevant to faults that have slickensides or to fracture without slip lines [11, 31]. In this work, paleostress analysis was accomplished using Win-Tensor program following the procedures explained in [31]. Win-Tensor is a paleostress analysis program developed by [30]. The program has two options: Right dihedron Method, which is an interactive approach, and Rotation Optimization Method that accounts for angular deviations between the observed slip lines and the modeled shear on each plane. Using both methods, the state of paleostress of the study area was reconstructed from the small faults and shear joints which collected in the field. Fault-Slip Data and Shear Fracture Collection Fault-slip data (fault planes with associated slickenside lineation) and shear fractures have been collected at five areas along the Gulf of Suez and northwestern Red Sea. The kinematic indicator that can be used in paleostress analysis [11, 33, 74] have been measured and described: fault planes with slickenlines and slip sense shear fractures conjugated shear fracture

12 636 ZAKY (a) (c) Fig. 12. (a) 50 /S 55 W small normal fault in Nubian sandstone at Gebel Duwi, Hammer as scale. 63 /N 70 E slickensides along 65 /N 70 E dip-slip normal faults in Nubian sandstone. (c) Field photograph showing 10/N 50 W slickensides along 74/S 40 W fault plane. These slickensides are overprinted by 70/S 40 W slickensides in Nubian sandstone at Gebel Duwi.

13 PALEOSTRESS ANALYSIS OF THE BRITTLE DEFORMATIONS 637 (а) S 20 E Deformation bands S 30 W Deformation bands Fig. 13. Deformation bands in Nubian sandstone along NW SE normal fault at Gebel Duwi. systems as well as shear planes with associated enechelon tensile joints. Additional qualitative parameters have been carefully recorded in the field in order to provide a complete description of the fault-slip data: the fault-rock type, cross-cutting relations and striation superposition. At each site or sub-site, the entire data set is further subdivided into subsets when multistage brittle history is observed. Stress Tensor Determination Fault-slip data allows to reconstruct the 4 parameters of the reduced tectonic stress tensor: the orientation of the three orthogonal principal stress axes σ1, σ2, σ3 with σ1 σ2 σ3 0 and the ratio of principal stresses R = (σ2 σ3)/(σ1 σ 3) with 0 R 1 which expresses the magnitude of σ2 relative to the magnitudes of σ1 and σ3. These four parameters are first estimated with an improved version of the Right Dihedra method based on [12]. They are more precisely determined with an iterative rotational stress optimization which minimizes the slip deviation between observed slip line and resolved shear stress and favors slip on the fault planes [30]. For shear fractures, slip along the plane is favored by minimizing the resolved normal stress magnitude (witch combined with the friction coefficient, impedes slip) and maximizing the resolved shear stress magnitude. The quality of the results is evaluated using the quality ranking parameter QR for fault-slip data inversion as defined in [81]. The horizontal principal

14 638 ZAKY N 40 W (a) N (c) Fig. 14. (a) N 40 W basaltic dyke dissect the pre-rift Nubian sandstone at El Quseir region. Field photograph shows two joint sets in the basaltic rocks. Compass as scale. (c) Rose diagram shows the strikes of the joint sets. stresses S Hmax or S Hmin are computed using the 4 parameters of the reduced stress tensor following the method of [60]. The stress regime index R' is determined on the basis of the stress ratio R and the most vertical stress axis in the forms of a continuous scale from 0 (radial extension) to 3 (constriction), with R' = R for normal faulting regimes (0 1), R' = 2 R for strike-slip regimes (1 2) and R' = 2 + R for thrust faulting regimes (2 3) [31]. The 1σ standard deviations for the S Hmax /S Hmin and stress regime R' are determined using the uncertainties associated to σ1, σ2, σ3 and R.

15 PALEOSTRESS ANALYSIS OF THE BRITTLE DEFORMATIONS 639 Pre-rift Syn-rift Recent- Pleistocene Pliocene Late Middle Samadi Gabir Samh Miocene Umm Gheig Abu Dabbab Um Mahara Late Nubia Cretaceous Precambrian Basement Dextral strike-slip faults Sinistral strike-slip faults Normal faults Inferred faults 10 Strike and dip Fault-related folds 15 6 km W. Umm Gheig Fig. 15. Geological map of Umm Gheig area. Lithology and structures compiled after [5, 58, and this work]. Stress Tensor Results After processing all sites, 27 stress tensors have been obtained, representing the fault-slip data and shear fractures. These tensors are reported in the Table 1 and detailed in Fig. 21. This significant data set allows to identify 4 different stress stages and to determine their relative chronology. The correlation proposed is based on an integrated evaluation of the results, their spatial and temporal relations, fault cross-cutting relationships and striation superposition and overall consistency in terms of stress field evolution. Stage 1: NE SW Compression The first stage is related to a general NE-SW tectonic compression and is expressed by shear fractures (Figs. 6a, 11a) and strike slip faults (Fig. 7). These structures dissected the Triassic-Paleozoic rocks at Gebel El Zeit area and the Nubian sandstone (Upper Cretaceous at Gebel Duwi). The shear fractures are trending NNE SSW and ENE WSW, and are steeply dipping. The strike-slip faults are recorded in the Paleozoic rocks at Gebel El Zeit, and in basement rocks at northwestern side of Gebel Duwi. The basement rocks are not studied detailed in this work. At Gebel El Zeit the faults are mainly ENE WSW leftlateral faults and secondary NNE SSW right lateral faults, while at Gebel Duwi they are NW to WNW leftlateral strike-slip faults and NW reverse faults. The rake (seismological notation equivalent to the pitch angle with indication for the slip sense) show dominantly strike-slip faulting, consistently with the stress regime index R' which averages 1.55 (Table 1). The 4 stress tensors obtained have a compressional stress regime and S Hmax oriented NE SW. It has been defined chronologically in the multi-stage sites 13 and 14 at Gebel Duwi, and single-stage outcrops sites 1 and 2 at Gebel El Zeit. Stage 2: NW SE Compression The brittle deformation in this stage is characterized by moderately to steep shear fractures and strike slip faults (>60 ). The fault planes are dominantly strike slip. The shear fractures and strike-slip faults oriented NW SE to WNW ESE and N S. The NW SE faults are dextral slip faults and they probably as a result of reactivation of the NW SE left-lateral slip faults in the first stage. The stress tensor show compressional stress regime with R` value around The S Hmax directions are NW SE with average angle 146. Stage 3: NE SW Extension This stage is defined by about 200 high angle normal faults recorded at 13 different sites. The S Hmax directions are consistently oriented NW SE parallel to the major fault trend. The average stress regime is pure normal faulting (R' ~ 0.26). These normal faults are parallel to the Red Sea Gulf of Suez rift. The multidirectional extension has recorded in site (12), El Quseir region, with stress regime R' = The paleostress axes σ2 and σ3 are horizontal and oriented 337 and 247 respectively.

16 640 ZAKY SE Lead-Zinc Mine (a) NW Fig. 16. Field photographs shows, (a) major NW SE normal fault dissect the Middle Miocene rocks Um Mahara Formation at Umm Gheig region. The Lead-Zinc Mine is located along this fault. NW SE small normal fault in Late Miocene Pliocene rocks. Stage 4: NNE SSW Extension The last stage is represented by WNW ESE dipslip normal faults. These faults cut the Pliocene and recent rocks. The average stress regime is of pure normal faulting (R = 0.22), and the direction of horizontal extension (S Hmin ) lies in general oblique to the rift. The S Hmix directions are oriented NNE SSW to N S. This consistent with the late Pleistocene results at Gebel El Zeit (NNE SSW S Hmin, R' = 0.3 data reprocessed from [19]. DISCUSSION The succession of brittle tectonic stages and their stress field characteristics provide additional constraints to refine the knowledge on the tectonic development of the northern Red Sea Gulf of Suez. The first stage in the northern Red Sea and southern Gulf of Suez recorded a NE SW compressional stress regime. This stage is responsible for the reactivation of the basement systematic joint systems which recorded by [16, 34, 90, 92] and developed the ENE WSE and NNE SSW shear fractures and ENE WSW and NNE SSW left-lateral and right lateral-strike slip faults in the Paleozoic Triassic Nubian sandstone at Gebel El Zeit and Cretaceous Nubian sandstone at Gebel Duwi. This stage is documented by: 1 minor overturned folds in Dakhla shale (Paleocene) at El Quseir region [5]; 2 sheared conglomerates from the Nubia Formation were incorporated into a major vertical sinistral shear zone, including vertical foliation and horizontal lineation [14]. They mentioned that the principal stress directions with sub-horizontal σ1 (ENE/WSW) and σ3 (NNW/SSE), and a sub-vertical σ2 have been found from paleostress analyses using fault plane solution techniques and orientation

17 PALEOSTRESS ANALYSIS OF THE BRITTLE DEFORMATIONS 641 (a) (c) (d) Fig. 17. Field photographs showing: (a) NW SE normal fault dissect the Middle Miocene rocks Um Mahara Formation. (b, c) Small-scale reverse faults within the hanging wall of the NW SE normal fault. (d) Small-scale normal fault in the same hanging wall of the NW SE normal fault.

18 642 ZAKY RED SEA Plio Quaternary Wadi Igla Pliocene (Shagna Fm.) Miocene (Um Mahara & Abu Dabbab Fms.) Precambrian rocks Rift border faults Normal faults Strike and dip Marsa Alam Wadi Alam km Wadi Samadai 17 Ras Samadai Fig. 18. Simplified geological map of the Marsa Alam region after this work. (а) 55 /N 35 E 80 /N 30 E 70 /N 35 E 65 /S 50 W 65 /N 30 E 65 /N 35 E 4 m 3 m (c) 80 /S 20 W Fig. 19. Field photographs showing: (a, b) NW SE normal faults in Mersa Allam Formation. These faults dip in NE and SW directions with an average angle ~65. The displacements along these faults range from 0.5 m to more than 5 m. (c) NW SE normal fault in Shagara Formation. 4 m

19 PALEOSTRESS ANALYSIS OF THE BRITTLE DEFORMATIONS 643 Table 1. Paleostress sites and parameters for the corresponding reduced stress tensors Location σ1 σ2 σ3 S Hmax Regime Stage N R QR site stratigraphy and structure pl az pl az pl az Ori 1σ R' 1σ Reg 1 Paleozoic sandstone at G. El Zeit, shear joints SS C 2 Paleozoic sandstone at G. El Zeit, strike-slip faults SS C 13 Basement rocks at NW G. Duwi, Quseir region. Strike-slip and reverse faults SS E 14 Cretaceous Nubian sandstone at G. Duwi, shear joints SS C 13 Basement at G. Duwi, El Quseir region SS D 14 Cretaceous Nubian sandstone at G. Duwi, shear joints SS C 17 Pre-rift sedimentary rocks at el Quseir area, strike-slip faults SS C 23 Early Miocene rocks at Abu Ghusun, strike-slip faults SS E 4 Miocene rocks at G. El Zeit, normal faults NF C 5 Late Miocene Pliocene rocks at G. El Zeit, normal faults NF C 6 Late Miocene rocks at W. Queih north El Quseir, normal faults NF C 8 Late Miocene rocks at W. Abu Hamra north El Quseir, normal faults NF C 9 Late Miocene rocks at W. Hamrawein north El Quseir, normal faults NF C 11 Pre-rift sedimentary rocks at north El Quseir, normal faults NF C 12 Pre-rift sedimentary rocks at west El Quseir, normal faults NF C 18 Basaltic rocks at G. Duwi, El Quseir, shear joints SS E 19 Middle Miocene at Umm Gheig, normal faults NF D 24 Early Miocene at Abu Ghusun, normal faults NF C 25 Late Miocene Pliocene rocks at W. Ranga south NF C 26 Abu Ghusun, normal faults NF C 7 Pliocene rocks at W. Queih north El Quseir, normal F NF E 10 Pliocene rocks at W. Siatin north El Quseir, normal F NF C 24 Pliocene rocks at Umm Gheig, normal faults NF C Pliocene rocks at north Marsa Alam, normal faults NF D Pliocene rocks at south Marsa Alam, normal faults NF C 27 Late Pleistocene rocks at G. El Zeit, normal faults reprocessed after Bosworth and Taviani NF D N means number of fault & fracture data used for the stress tensor calculation; σ1, σ2, and σ3 are stress axes, R means stress ratio, S H means horizontal principal stress directions with max for S Hmax and min for S Hmin and 1σ standard deviation. Regime with stress regime index R', 1σ standard deviation for R' and regime qualification as in the World Stress Map: NF, normal faulting, SS strike-slip; QR means quality rank: A, excellent, B, good, C, medium, D, poor.

20 644 ZAKY (a) N W. Gemal Neogene & Quaternary Basement Normal faults Dextral strike-slip faults Sinistral strike-slip faults Limeament 10 km Ras Honkorad N Abu Ghusun W. Ranga Fig. 20. (a) Geological map of Abu Ghusun region [64]. Rose diagram showing the trend of the normal faults, which dissect the syn-rift rocks. of tension gashes. These results are in accordance with [10] who suggested that there is evidence for growth of NW trending basins during the Cretaceous and for reactivation of the Najd lineament [87]. The Najd fault system is a major component in the geological framework of the Precambrian rocks in Egypt and Saudi Arabia and is regarded as the last significant structural event that affected these rocks [1 4, 35, 56, 65, 83, 84]. The second stage is responsible for the WNW & NNE shear fractures in the Cretaceous Eocene prerift rocks and in Oligo-Miocene basaltic rocks at Gebel Duwi. The WNW right-lateral and NNW to N S left-lateral faults in pre-rift and early Miocene rocks are a result of the NW SE compressional stage. Giraud et al. [44] state that a new pulse of deformation occurred across much of north Africa, during the late Eocene, corresponding to a significant phase of shortening in the Alpine fold belt in Europe, and in Greece and Turkey [17]. The second stage in this study is equivalent to the first stage of rifting after [64]. This stage is recorded in the wadi Sharm el Qibli (south of Quseir) by fracturing a 24.9 Ma basalt flow and surrounding red sediments (top Oligocene). These rocks show sinistral movement of the north-south to N 30 trending Aqaba fault; dextral movement of the N 105 to N l30 trending Duwi fault and normal movement of the northwest-southeast clysmic faults. Some strike-slip fault planes have recorded superimposed normal movements related to the second phase [52]. The first stage of rifting which displays specific strike-slip movement, is related to a stress system characterized by northwest-sout-east compression and northeast-southwest extension (respectively σ1 and σ3 in the horizontal plane) [64]. The setting of the volcanics (tholeiitic basalt flows, dikes) is probably related to tension gashes generated in the strike-slip regime (northwest-south-east trending gashes, parallel to the horizontal direction of compression [70]. Stress stage 3 corresponds to the most NW SE normal faulting observed in the northwestern side of the Red Sea and Gulf of Suez. This stage is characterized by normal faulting without any evidence of strikeslip movement. It is equivalent to the second stage of rifting [64]. It is characterized by paleostress tensors with vertical maximum stress. A true extensional regime (σ1 vertical, σ3 oriented northeast- southwest)

21 PALEOSTRESS ANALYSIS OF THE BRITTLE DEFORMATIONS 645 Stage 1 σ1: 07/051 σ2: 83/231 σ3: 00/321 R: 0.13 F5: 4.3 σ1: 00/044 σ2: 89/290 σ3: 01/134 R: 0.62 F5: 1.3 Site 1 Schmidt low Weight mode n/nt: 9/9 Site 2 σ1: 06/239 σ2: 83/027 σ3: 04/149 R: 0.58 F5: 17.3 QRw: E QRt: E σ1: 25/044 σ2: 64/213 σ3: 04/312 R: 0.47 F5: 5.6 Site 13 Site 14 Stage 2 Schmidt low Weight mode n/nt: 28/28 σ1: 13/153 σ2: 77/318 σ3: 03/062 R: 0.7 F5: 8.7 QRw: D QRt: D σ1: 11/324 σ2: 78/163 σ3: 04/055 R: 0.62 F5: 10 QRw: D QRt: D Site 13 Site 14 Schmidt low Weight mode n/nt: 7/7 σ1: 10/144 σ2: 80/332 σ3: 01/235 R: 0.99 F5: 7.5 σ1: 29/137 σ2: 60/334 σ3: 08/231 R: 0.58 F5: 50.8 QRw: E QRt: E Site 17 Site 23 Fig. 21. Fault-slip data and stress inversion results. Lower-hemisphere Schmidt stereoplot of the fault-slip data and corresponding stress tensor. Horizontal stress symbol as in legend at left side of every site. Details in Table 1.

22 646 ZAKY Stage 3 σ1: 81/076 σ2: 03/328 σ3: 09/238 R: 0.2 F5: 0.9 σ1: 86/180 σ2: 03/325 σ3: 02/055 R: 0.38 F5: 2 Site 3 Site 4 σ1: 86/098 σ2: 03/327 σ3: 03/237 R: 0.2 F5: 0.5 σ1: 86/177 σ2: 03/332 σ3: 02/062 R: 0.2 F5: 1 Site 5 Site 6 σ1: 86/109 σ2: 03/320 σ3: 02/229 R: 0.2 F5: 1.1 σ1: 83/208 σ2: 02/310 σ3: 07/040 R: 0.2 F5: 0.4 Site 8 Site 9 σ1: 88/157 σ2: 02/326 σ3: 00/056 R: 0.3 F5: 0.6 σ1: 90/133 σ2: 00/337 σ3: 00/247 R: 0.05 F5: 0.4 Site 11 Site 12 Fig. 21. (Contd.)

23 PALEOSTRESS ANALYSIS OF THE BRITTLE DEFORMATIONS 647 σ1: 83/327 σ2: 07/153 σ3: 01/063 R: 0.7 F5: QRw: E QRt: E σ1: 81/233 σ2: 00/142 σ3: 09/052 R: 0.3 F5: 0.2 QRw: D QRt: D Site 18 Site 19 σ1: 78/233 σ2: 01/140 σ3: 12/050 R: 0.2 F5: 0.5 QRw: C QRt: D σ1: 79/247 σ2: 04/134 σ3: 10/043 R: 0.2 F5: 0.8 Site 24 Site 25 σ1: 80/253 σ2: 05/135 σ3: 09/044 R: 0.3 F5: 1 Site 26 Stage 4 σ1: 74/022 σ2: 01/115 σ3: 16/205 R: 0.41 F5: 21.3 QRw: E QRt: E σ1: 81/174 σ2: 06/305 σ3: 07/036 R: 0.27 F5: 0.7 Site 7 Site 10 Fig. 21. (Contd.)

24 648 ZAKY σ1: 87/008 σ2: 01/127 σ3: 03/217 R: 0.05 F5: 0.1 QRw: C QRt: D σ1: 87/030 σ2: 00/300 σ3: 03/210 R: 0.07 F5: 0.1 QRw: D QRt: D Site 20 Site 21 σ1: 83/233 σ2: 02/124 σ3: 07/034 R: 0.21 F5: 0.5 σ1: 86/197 σ2: 00/290 σ3: 04/020 R: 0.3 F5: 2.6 QRw: D QRt: E Site 22 Site 27 Fig. 21. (Contd.) controlled the formation of tilt-blocks, predominantly bounded by northwest-southeast Clysmic faults [71]. A multidirectional extension was recorded at the end of this tectonic stage. Montenat et al. [64] consider this extension in the northwestern Red Sea as a new tectonic extension stage. This phase replaced the NW SE extension tectonic stage due to flexure of the margin toward the basin axis. This stage is only recorded at El Quseir region and not recorded in the other studied regions. The NE SW extension stage was marked by the development of domino-style tilted fault blocks with growth the syn-rift strata in the hanging walls of the active extensional faults [17]. The NNE SSW extensional stage 4 is responsible for WNW ESE pure dip-slip normal faults in the Pliocene Pleistocene sediments. This tectonic stage represents a counterclockwise rotation of the NE SW tensional regime of the third stage. The borehole breakout data of the southern Gulf of Suez after [19] indicate that the present-day regional S Hmin orientation is N 10 E, and this is supported by analysis of telesismic earthquake data by [47]. The change of extension direction is recorded by many authors in the Gulf of Suez and other areas. In the gulfs of Suez and Aqaba, many workers have evidenced changes in the extensional stress direction (e.g., see [19, 24] for Suez and [24, 37] for Aqaba). Some of these stress reorientations are Late Quaternary, and thus possibly related to the slowing down of the African and Arabian plates [80]. In the Gulf of Suez, the direction of extension rotated anticlockwise before 125 ka, becoming N S at present instead of NE SW earlier [18], possibly related to the increasing locking of Africa and Arabia against Eurasia. Stress reorientations also occurred earlier, in the Middle to Late Miocene, probably related to the development of the Gulf of Aqaba Dead Sea transform fault boundary in response to the collision of Arabia with Anatolia [48]. Interestingly, stress reorientations also occurred during the Quaternary in the Kenyan rift, far to the south of the Red Sea. After Ma, extension rotated clockwise from E W to NW SE [18, 21, 87]. An earlier clockwise rotation also occurred by about 2.6 Ma [18]. Similar observations were done in the western branch of the East African rift (Tanganyika Malawi rift) [30, 76]. Two possible interpretations for this Late Pleistocene rotation of the stress field are: (1) a change in the dynamics of the Afro-Arabian Rift System affected this plate boundary over a region spanning 4000 km,

25 PALEOSTRESS ANALYSIS OF THE BRITTLE DEFORMATIONS 649 or (2) motions along one of the distant African plate boundaries changed dramatically, resulting in stress field changes throughout the plate. Gravitational collapse is another possible cause for post-rift, or postrift shoulder uplift, extension differing from the synrift regional stress field. This has been shown for the Gulf of Aqaba [37] and the eastern border of the Ethiopian plateau [25]. The polyphased rifting of the Gulf of Suez and Red Sea basin, with gradual change from compressional to extensional tectonics, may be compared with other rift evolutions [64]. For example, the East African rift, located close to the Red Sea and initiated approximately at the same time, recorded Early Oligocene Early Miocene strike-slip tectonics: northeast-southwest compression (σ1 horizontal) associated with northwest-southeast extension (σ3) [24]. Subsequently, a true extensional regime developed (σ1 vertical and northwest- southeast direction of extension); later kinematic evolution of both rifts reveals relevant analogies [53]. The Late Eocene Oligocene North European rift recorded a similar evolution. At the beginning of the Late Eocene, the Rhine graben was initiated in a compressional strike-slip regime (submeridian horizontal σ1) which induced a slight eastwest extension (σ3 horizontal). This extension became progressively stronger, resulting in the graben opening at the beginning of the Oligocene [89]. At the end, I think that all previous stages are responsible for the present architecture of the northwestern Red Sea and Southern Gulf of Suez. CONCLUSIONS Four tectonic stages are recorded in the northwestern margin of the Red Sea and southwestern side of the Gulf of Suez. The first tectonic stage is related to a NE SW compression stress and is expressed by shear fractures and strike-slip faults. The average stress regime index R is 1.55 and S Hmax oriented NE SW. This stage is responsible for the reactivation of the Precambrian fractures. These fractures are mainly ENE, NNE and WNW (Najd fault system). The second stage is belonging to a NW SE compression stress regime. The stress tensor show compressional stress regime with R' value around The S Hmax directions are NW SE with average angle 146. This stage reactivated the WNW ESE right-lateral strike-slip faults (Duwi trend) and NNW to N S left-lateral strike-slip faults (Aqaba trend). The NE SW extensional tectonic stage is the third one in this study. This stage is characterized by normal faulting without any evidence of strike-slip movement. The S Hmax directions are consistently oriented NW SE parallel to the major fault trend. The average stress regime is pure normal faulting (R = 0.26). The multidirectional extension is recorded at the end of this stage due to flexure of the margin toward the basin axis. 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