Najd Fault System Research Articles

Structural influence on the evolution of the pre-Eonile drainage system of southern Egypt: Insights from magnetotelluric and gravity data

The Wadi Kubbaniya in the Western Desert of Egypt north of the City of Aswan has been interpreted as the downstream continuation of the Wadi Abu Subeira, comprising an ancient W- and NW-flowing river system originating from the Precambrian crystalline rocks of the Red Sea Hills which were uplifted during the Miocene in association with the opening of the Red Sea. This drainage system is thought to have been active before the onset of the N-flowing Egyptian Nile which started ∼6 Ma with the Eonile phase; an event that resulted in carving of ∼1000 km long canyon (the Eonile canyon) extending from the Mediterranean Sea in the north to Aswan in the south due to the Messinian Salinity Crisis. This study utilizes geophysical data to examine the role of regional tectonics and local structures in controlling the evolution of the pre-Eonile drainage system. Magnetotelluric (MT) and gravity surveys were conducted along two ∼5 km-long profiles across the NW-trending Wadi Kubbaniya. Two-dimensional (2D) inversion of MT data and gravity models indicate the Wadi Kubbaniya is filled with loosely-consolidated sandstone and conglomerate that extend to a depth of ∼150–200 m into Cretaceous sandstone formations which overlie Precambrian crystalline rocks. These results were evaluated in terms of two end-member models; an incision model in which the 150–200 m thick sedimentary rocks were considered as being deposited within an incised valley that was carved into bedrock, or a structural model in which the sedimentary rocks are considered as filling a NW-trending graben controlled by normal faults that deform the Cretaceous sandstone formations and the underlying Precambrian crystalline rocks. Geological observations as well as supporting seismic data favor the interpretation that the Wadi Kubbaniya is a NW-trending graben similar to other extensional structures found 400 km northwest along-strike of Wadi Kubbaniya. These structures are impressively parallel to the western shorelines of the Red Sea and the Gulf of Suez suggesting a regional tectonic link between them. Strain localization of these grabens (which are likely Miocene in age) might have been facilitated by inherited Precambrian and Jurassic – Early Cretaceous structures, such as the NW-trending Najd fault system, the most dominant regional structural grain in the Red Sea Hills of Egypt as well as the NW-trending grabens, such as the Kom Ombo graben located ∼25 km to the northeast of Wadi Kubbaniya.

Late Ediacaran to early Cambrian (Infracambrian) Jibalah Group of Saudi Arabia

ABSTRACT This paper is one of a series that document the Neoproterozoic – Cambrian rock units in the Middle East Geologic Time Scale. It is focused on the oldest sedimentary succession in Saudi Arabia, the late Ediacaran – early Cambrian (Infracambrian) Jibalah Group (ca. 585 to 530–520 Ma). The group crops out in disconnected, pull-apart basins (ca. 10–100 km long and up to 20 km wide) along the NW-trending, strike-slip Najd Fault System in the Arabian Shield. It was described and mapped in the 1960s to 1980s, and several formations were defined and named in two areas separated by ca. 400 km. The stratigraphic successions in these two areas have not been correlated, nor has their relationship to the subsurface been resolved. This paper reviews the nomenclature, type sections, lithologies and ages of the formations and members (sometimes units and/or facies) of the Jibalah Group. The Jibalah Group unconformably overlies the Ediacaran Shammar Group (ca. 620–585 Ma, consisting mainly of rhyolite or granitic plutons), or older Proterozoic rocks. The age of the intervening Sub-Jibalah Unconformity is here estimated at ca. 585 Ma based on radiometric data and regional correlations. The lower part of the Jibalah Group is defined in the northern Arabian Shield in the Mashhad area, where it consists of three formations, in ascending order: (1) undated Rubtayn Formation, divided informally into the “Volcanic Conglomerate Member” (up to ca. 700 m thick), “Polymictic Conglomerate Member” (up to ca. 1,500 m thick) and “Sandstone Member” (up to ca. 1,000 m thick); (2) poorly dated Badayi Formation consisting of andesite-basalt flows (ca. 150 m thick); (3) undated Muraykhah Formation (330–370 m thick) consisting of the informal “Cherty Limestone Member” (ca. 135 m thick), “Siltstone and Mudstone Member” (ca. 20 m thick) and “Dolomitic Limestone Member” (ca. 135–175 m thick). The Rubtayn, Badayi and Muraykhah formations in the northern Arabian Shield, by stratigraphic position and lithology, correspond to the Umm Al ‘Aisah Formation in the Najd pull-apart basins of the central Arabian Shield. In particular, the Cherty Limestone unit (300–500 m thick) of the Umm Al ‘Aisah Formation is correlated to the Muraykhah Formation, which represents a marine flooding event. Above the Muraykhah Formation, the uppermost part of the group is defined in the central Arabian Shield by the undated Jifn Formation (up to ca. 2,500 m thick). The Jibalah Group is unconformably overlain by the lower Cambrian Siq Sandstone Formation (Asfar Sequence), and the intervening Sub-Siq Unconformity (Angudan Unconformity) has an estimated age between ca. 530–520 Ma.

Sedimentary basins of Yemen: their tectonic development and lithostratigraphic cover

This paper describes the updated stratigraphy, structural framework and evolution, and hydrocarbon prospectivity of the Paleozoic, Mesozoic and Cenozoic basins of Yemen, depicted also on regional stratigraphic charts. The Paleozoic basins include (1) the Rub’ Al-Khali basin (southern flanks), bounded to the south by the Hadramawt arch (oriented approximately W–E) towards which the Paleozoic and Mesozoic sediments pinch out; (2) the San’a basin, encompassing Paleozoic through Upper Jurassic sediments; and (3) the southern offshore Suqatra (island) basin filled with Permo-Triassic sediments correlatable with that of the Karoo rift in Africa. The Mesozoic rift basins formed due to the breakup of Gondwana and separation of India/Madagascar from Africa–Arabia during the Late Jurassic/Early Cretaceous. The five Mesozoic sedimentary rift basins reflect in their orientation an inheritance from deep-seated, reactivated NW–SE trending Infracambrian Najd fault system. These basins formed sequentially from west to east–southeast, sub-parallel with rift orientations—NNW–SSE for the Siham-Ad-Dali’ basin in the west, NW–SE for the Sab’atayn and Balhaf basins and WNW–ESE for the Say’un-Masilah basin in the centre, and almost E–W for the Jiza’–Qamar basin located in the east of Yemen. The Sab’atayn and Say’un–Masilah basins are the only ones producing oil and gas so far. Petroleum reservoirs in both basins have been charged from Upper Jurassic Madbi shale. The main reservoirs in the Sab’atayn basin include sandstone units in the Sab’atayn Formation (Tithonian), the turbiditic sandstones of the Lam Member (Tithonian) and the Proterozoic fractured basement (upthrown fault block), while the main reservoirs in the Say’un–Masilah basin are sandstones of the Qishn Clastics Member (Hauterivian/Barremian) and the Ghayl Member (Berriasian/Valanginian), and Proterozoic fractured basement. The Cenozoic rift basins are related to the separation of Arabia from Africa by the opening of the Red Sea to the west and the Gulf of Aden to the south of Yemen during the Oligocene-Recent. These basins are filled with up to 3,000 m of sediments showing both lateral and vertical facies changes. The Cenozoic rift basins along the Gulf of Aden include the Mukalla–Sayhut, the Hawrah–Ahwar and the Aden–Abyan basins (all trending ENE–WSW), and have both offshore and onshore sectors as extensional faulting and regional subsidence affected the southern margin of Yemen episodically. Seafloor spreading in the Gulf of Aden dates back to the Early Miocene. Many of the offshore wells drilled in the Mukalla–Sayhut basin have encountered oil shows in the Cretaceous through Neogene layers. Sub-commercial discovery was identified in Sharmah-1 well in the fractured Middle Eocene limestone of the Habshiyah Formation. The Tihamah basin along the NNW–SSE trending Red Sea commenced in Late Oligocene, with oceanic crust formation in the earliest Pliocene. The Late Miocene stratigraphy of the Red Sea offshore Yemen is dominated by salt deformation. Oil and gas seeps are found in the Tihamah basin including the As-Salif peninsula and the onshore Tihamah plain; and oil and gas shows encountered in several onshore and offshore wells indicate the presence of proven source rocks in this basin.

تمددي في جنوب شرق الدرع العربي قبه نايس جبل كرش: معقد تحولي

A number of gneiss-cored domes and antiforms are exposed along the regional strike-slip Najd fault system in the Arabian Shield and the eastern desert of Egypt. The mode of origin is still controversial, although plausible comparisons with modern metamorphic core complexes were made in some well-studied areas. The Kirsh dome is located within the major Ar Rika shear zone and consists of a core of orthogneiss/migmatite and an envelope of paragneisses with locally abundant kyanite-bearing quartzites. The dome is surrounded by the low-grade metasediments of the Murdama Group and is bound from the south by a low-angle dip-slip fault. Beyond the southern strand of the Ar Rika Fault is the Kibdi Basin which hosts unmetamorphosed sediments belonging to the Jibalah Group; this group occupies scattered pull-apart basins closely associated with releasing bends along the Najd fault system. Little dating has been done on the gneiss domes of the Arabian Shield; however, recent dates from similar structures in the eastern desert and Sinai range from 580 to 620 Ma. A similar, albeit younger 40Ar/39Ar age of 557 ± 15 Ma was obtained from a biotite paragneiss south of Jabal Kirsh; this age difference probably represent the time interval it took the Kirsh rocks to cool below the biotite closure temperature and would place a lower age limit for the dome. The Kirsh dome occupies an extensional zone between left-stepping faults; movement within this zone might have caused enough decompression to trigger fluid-absent melting in the middle crust especially as the rocks cross the biotite dehydration solidus. Diapiric ascent aided by strike-slip dilatancy pumping led to the emplacement of the Kirsh rocks in their present position within the Murdama Group metasediments.

Calc-silicates from Wadi Solaf region, Sinai, Egypt

Calc-silicates have proved to be important rock types to place constraints on the fluid behaviour in high grade metamorphic rocks. Here we describe amphibolite facies calc-silicate rocks of Wadi Solaf, Egypt which is one of the highest grade basement complexes of Egypt and was exhumed in close connection with the Najd fault system – one of the largest pre-Mesozoic fault systems on the Earth. Calc-silicates formed around 7–8 kbar in temperature range of 600–720 °C and can be classified into three groups (CS 1, CS 2 and CS 3). CS 1 and CS 2 are characterized by the presence of garnet porphyroblasts which contain concentrically arranged wollastonite inclusions. CS 3 has a similar paragenesis but is characterized by the presence of clinozoisite/epidote and the absence of wollastonite as well as a pervasive late overgrowth of prehnite. Garnet in CS 1 lies along a grossular–andradite solid solution, while the garnet composition of CS 2 and CS 3 is hydrogrossular and hydroandradite. There is a positive correlation between Al 2O 3 and Na 2O as well as between Al 2O 3 and Fe 2O 3 from CS 2 through CS 1 to CS 3 possibly indicating a successive increase in fluid flow between the different calc-silicate types. During the peak metamorphism, the XCO 2 is 0.02–0.08 and 0.2–0.44 for the CS 1 and CS 3, respectively. The mineral assemblage of CS 1 and CS 2 buffered the composition of the fluids along the reaction: q + cc = wo + CO 2. During post-peak metamorphism the rocks were intruded by syn-tectonic granites. CS 2 records the contact metamorphic conditions. The rocks reached a peak contact metamorphism conditions at temperature 790–828 °C and XCO 2 = 0.22–0.41. Finally, the rocks reached a temperature range 296–311 °C during the cooling path.

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

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 first 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 influence 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 directions are delineated by subhorizontal σ1 and σ3 and subvertical σ2, (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.

The northern dome of Wadi Hafafit culmination, Eastern Desert, Egypt: Structural setting in tectonic framework of a scissor-like wrench corridor

Although the gneissic domes in Eastern Desert of Egypt have been studied recently in a considerable detail; their origin remains controversial. Basically four schools of thought exist: one argues for an origin parallel crustal extension, a second suggests emplacement within antiformal stacks, a third envisages young emplacement within a core of a sheath fold and finally some authors believe that the emplacement is due to overlap of regional folds and extension parallel to the fold axes. The Wadi Hafafit Culmination is one of these domes and occupies the southern part of the Central Eastern Desert of Egypt. The culmination is cored with five separated gneissic domes ranging in composition from orthogneiss to paragneiss. They are overthrust by a low-grade, volcano-sedimentary association constituting the Pan-African cover nappes. Detailed structural mapping of the northern dome reveals that, the gneisses are vertically emplaced through the cover rock units. This is based on field evidence which shows that the gneisses experienced vertical flattening associated with exhumation corresponding to coaxial deformation. It is suggested that the emplacement of gneissic core occurred during accretion of the Pan-African nappes. Later, strike-slip shear zones of Najd Fault System and the associated subsidiary shear arrays postdate emplacement of the dome. The gneisses contiguously underlie the Pan-African nappe assemblages through discrete low-angle left-lateral thrust-dominated shear zones from the east. Ongoing accretion of nappe assemblages on the gneisses increases the density contrast between the overlying denser nappe and the underlying lighter quartz-rich gneisses, leading to squeezing the gneissic materials in oblique convergence regime. As a consequence, the gneisses are suggested to have up-domed vertically through the nappe rock units.

Exhumation during oblique transpression: The Feiran–Solaf region, Egypt

AbstractThe Feiran–Solaf metamorphic complex of Sinai, Egypt, is one of the highest grade metamorphic complexes of a series of basement domes that crop out throughout the Arabian‐Nubian Shield. In the Eastern Desert of Egypt these basement domes have been interpreted as metamorphic core complexes exhumed in extensional settings. For the Feiran–Solaf complex an interpretation of the exhumation mechanism is difficult to obtain with structural arguments as all of its margins are obliterated by post‐tectonic granites. Here, metamorphic methods are used to investigate its tectonic history and show that the complex was characterized by a single metamorphic cycle experiencing peak metamorphism at ∼700–750 °C and 7–8 kbar and subsequent isothermal decompression to ∼4–5 kbar, followed by near isobaric cooling to 450 °C. Correlation of this metamorphic evolution with the deformation history shows that peak metamorphism occurred prior to the compressive deformation phase D2, while the compressive D2 and D3 deformation occurred during the near isothermal decompression phase of the P–T loop. We interpret the concurrence of decompression of the P–T path and compression by structural shortening as evidence for the Najd fault system exhuming the complex in an oblique transpressive regime. However, final exhumation from ∼15 km depth must have occurred due to an unrelated mechanism.

The role of the Najd Fault System in the tectonic evolution of the Hammamat molasse sediments, Eastern Desert, Egypt

The Hammamat molasse sediments of the Eastern Desert of Egypt were deposited in isolated basins formed during an initial stage of orogen parallel N–S extension (650–580 Ma) in the Neoproterozoic time. Supply of sediments to the molasse basins began after the eruption of Dokhan volcanics (602–593 Ma), exhumation of core complexes (650–550 Ma), and intrusion of late tectonic granites (610–550 Ma). The late Neoproterozoic structures in the molasse sediments include: (1) NNW-directed thrusts due to NNW–SSE shortening (650–640 Ma), indicated by the presence of NE-, ENE-, and WSW-trending folds and NNW-directed thrusts. (2) SW- and NE-directed thrusts due to ENE–WSW constriction during oblique convergence and arc accretion at around 640–620 Ma. Many of the map- and mesoscopic-scale NW-trending folds in the core complexes, the molasse sediments, and the Neoproterozoic nappes in the Eastern Desert are related to this event. Sinistral shearing along the Najd Fault System (650–540 Ma) resulted in the development of subvertical foliation, subhorizontal stretching lineation, and NW-trending tight folds overprinting earlier folds. Stretched pebbles are oriented NW–SE and WNW–ESE in the molasse basins localized within the Najd Fault System, and NE–SW in the basins outside the influence zone of this NW-trending fault system. Strain estimated using pebbles from nine molasse basins indicate that the amount of strain differs from one basin to another and from one place to another within the same basin. Weak tectonic strain (Rs = 2.16–2.24) is obtained from post-orogenic basins; moderate strains are reported from foreland basins (Rs = 2.37–3.18), whereas moderate to high tectonic strains are recorded from the intermontane basins (Rs = 2.40–4.36). The obtained tectonic strain and K values indicate that the flattening strain prevails in the post-orogenic and foreland basins, whereas as both constrictional and flattening strains are recorded in intermontane basins. Strain variation from one basin to another and within the individual basin is attributed to presence of thrust and sinistral shear zones. Away from the deformed zones, the pebbles show no significant stretching. Two phases of thrusting and an episode of transpressional sinistral shearing are the latest structure features of the East African orogeny in the Arabian–Nubian Shield.

Synoblique convergent and extensional deformation and metamorphism in the Neoproterozoic rocks along Wadi Fatira shear zone, Northern Eastern Desert, Egypt

The Wadi Fatira area occurs at the southern margin of the Northern Eastern Desert (NED) of Egypt and is occupied by highly sheared metavolcanics tectonically alternated with banded iron formations and intruded by Barud tonalite–granodiorite, post-tectonic gabbroic and granitic intrusions. Detailed structural investigation showed that the schists and migmatitic amphibolites are formed by shearing in metavolcanics and syntectonic Barud tonalite–granodiorite due to movement along the Wadi Fatira shear zone (WFSZ). This shear zone starts as a NW–SE striking fault along Wadi Barud Al Azraq and the Eastern part of Wadi Fatira and turns to a E–W trending fault to the north of Wadi Fatira. Microstructural shear sense indicators such as asymmetric geometry of porphyroclasts such as σ-type and asymmetric folds deforming fine-grained bands which are frequently found around porphyroclasts indicate sinistral sense of shearing along the WFSZ. This shear zone is characterized by transitions from local convergence to local extension along their E–W and NW–SE trending parts, respectively. The NW–SE part of the WFSZ is of about 200 m in width and characterized by synmagmatic extensional features such as intrusion of synkinematic tonalite, creation of NE–SE trending normal faults, and formation of migmatitic amphibolites and schlieric tonalites. This part of the shear zone is metamorphosed under synthermal peak metamorphic conditions (725°C at 2–4 kbar). The E–W compressional part of the WFSZ is up to 3 km in width and composed of hornblende, chlorite, actinolite, and biotite schists together with sheared intermediate and acidic metatuffs. Contractional and transpressional structures in this part of the WFSZ include E–W trending major asymmetrical anticline and syncline, nearly vertical foliation and steeply pitching stretching lineations, NNE dipping minor thrusts, and minor intrafolial folds with their hinges parallel to the stretching lineation. P–T estimates using mineral analyses of plagioclase and hornblende from schists and foliated metavolcanics indicate prograde metamorphism under medium-grade amphibolite facies (500–600°C at 3–7 kbar) retrogressed to low-grade greenschist facies (227–317°C). The foliation in Barud tonalite–granodiorite close to the E–W part of the WFSZ runs parallel to the plane of shearing and the tonalite show numerous magmatic flow structures overprinted by folding and ductile shearing. The WFSZ is similar to structures resulted from combined simple shear and orthogonal shortening of oblique transpressive shear zones and their sense of movement is comparable with the characteristics of the Najd Fault System.

Crustal evolution and recycling in a juvenile continent: Oxygen isotope ratio of zircon in the northern Arabian Nubian Shield

Crustal recycling patterns during the evolution of the Neoproterozoic Arabian-Nubian Shield (ANS) were defined using the oxygen isotope ratio of zircon [ δ 18O(Zrn)]. Evidence for early (~ 870–740 Ma) crustal recycling in the northernmost ANS (southern Israel and Sinai, Egypt) is given by laser fluorination analysis of bulk zircon separates, which yield higher than mantle δ 18O(Zrn) values of several island arc complex (IAC) orthogneisses (6.9 to 8.2‰) and also from the average δ 18O(Zrn) value of 6.4‰ determined for detrital zircons (~ 870–780 Ma) from the Elat-schist; the latter representing the oldest known rock sources in the region. These results indicate prolonged availability of surface-derived rocks for burial or subduction, melting, and assimilation at the very early stages of island arc formation in the ANS. Other IAC intrusions of ~ 800 Ma show mantle-like δ 18O(Zrn) values, implying that not all magmas involved supracrustal contribution. Much younger (650–625 Ma) deformed syn-collisional calc-alkaline (CA1) intrusions are characterized by δ 18O(Zrn) values of 5.0 to 7.9‰ indicating continued recycling of the felsic crust. The main sample set of this study comprises rocks from the mostly granitic, post-collisional calc-alkaline (CA2: ~ 635–590 Ma) and alkaline (AL: ~ 608–580 Ma) magmatic suites. Despite having distinct geochemical characteristics and petrogenetic paths and spans of magmatic activity, the two suites are indistinguishable by their average δ 18O(Zrn) values of 5.7 and 5.8‰ pointing to the dominance of mantle-like δ 18O sources in their formation. Nonetheless, grouping the two suites together reveals geographical zoning in δ 18O(Zrn) where a large southeastern region of δ 18O(Zrn) = 4.5 to 5.9‰ is separated from a northwestern belt with δ 18O(Zrn) = 6 to 8‰ by a ‘6‰ line’. It is thus suggested that all CA2 and AL magmas of the northernmost ANS were derived from mantle-like δ 18O reservoirs in the mafic lower-crust and the lithospheric-mantle, respectively. However, while in the northwestern belt these magmas intruded a thick crustal section and assimilated ~ 15–35%, high- δ 18O IAC+CA1 material, magmas in the southeastern region intruded a thinner crust and little or no contamination occurred. The proposed NW–SE variance in crustal thickness during the late Neoproterozoic fits well with the geometry of the fan shaped rifting model proposed by Stern [Stern, R.J., 1985. The Najd Fault System, Saudi Arabia and Egypt: a late Precambrian rift related transform system. Tectonics 4, 497–511.] for this region. Deep parts of the lithosphere were beginning to rift at ~ 630 Ma, allowing the asthenospheric mantle to rise and transfer heat to the lithosphere. This resulted in vast melting of the mafic lower-crust to produce the batholithic CA2 magmas. Later (~ 610 Ma) percolation of lithospheric-mantle melts (possibly along deep seated lithospheric-scale faults) introduced AL magmas to shallow levels of the crust. Intrusion of CA2 and AL mantle-like δ 18O parent magmas into the thinned southeastern crust did not involve assimilation of older crust whereas similar intrusion into the thicker northwestern crust resulted in mild assimilation of high- δ 18O pre-635 Ma crust. An important implication from our results is that petrogenesis of some high- δ 18O AL magmas of the northernmost ANS involved assimilation of supracrustal material. Felsic intrusions of the AL suite were previously described as A-type granites derived solely from mantle melts with no crustal components. Our results contribute to the “A-type petrogenesis debate” by showing that their formation can involve recycling of crustal material.

Sub-surface geometry of Ar Rika and Ruwah faults from gravity and magnetic surveys

Ten global positioning system (GPS)–gravity profiles were conducted to provide sub-surface geometry of two sections of the Najd Fault System (NFS) Ruwah and Ar Rika faults, six in the Afif and four in the Al Muwayh area about 500 and 650 km west of Riyadh, respectively. GPS surveys were collected in differential GPS (DGPS) mode, allowing a large area to be covered in limited time. DGPS is utilized for the advantages of accuracy, economy, and speed. Output DGPS location coordinates were used in free-air and Bouguer reductions; terrain corrections were applied using a 3-arcsecond digital elevation model; finally, isostatic and decompensative corrections were applied. Integration of the resulting decompensative isostatic residual anomalies and aeromagnetic map has mapped the NFS very accurately. Modeling the gravity field crossing the Ruwah fault zone revealed that it is associated with low gravity anomalies probably due to a complex of lower density crushed rocks and modeled the geometry of the subsurface structure of Ar Rika fault as an inclined fault with reverse movement that would imply a compression component (post-dated the shearing) parallel to the plane of the cross-section.

Thrusting and multiple folding in the Neoproterozoic Pan-African basement of Wadi Hodein area, south Eastern Desert, Egypt

Detailed field mapping and structural studies of the area around the mouth of Wadi Hodein, some 20 km west of Shalatein at the Red Sea coast in the south Eastern Desert of Egypt, revealed four phases of structural deformation (D1–D4) affecting the Neoproterozoic Pan-African basement rocks. D1 is related to arc–arc collision and is represented by ENE–WSW oriented megascopic upright open folds associated with low angle thrusts and mesoscopic tight, overturned and recumbent F1 folds. Kinematic indicators indicate thrusting towards the SSE. D2 is represented by NNW–SSE oriented megascopic and mesoscopic folds, which are tight, verge towards the WSW and display a left-stepping en echelon pattern. D3 includes major NNW–SSE trending sinistral shear zones that show subordinate reverse fault components and dip steeply towards the ENE. These sinistral shear zones are comparable with the Najd Fault System, as they display a similar sense of movement and relationships to earlier structures. Therefore, they are interpreted to be the continuation of the Najd Shear System in southern Egypt. D2 and D3 are related to accretion of east and west Gondwana. D4 is represented by E–W oriented dextral faults with left-stepping segments. The first three deformation events are in agreement with the general evolutionary model for the East African Orogen in the Arabian–Nubian Shield that begins with NNW–SSE shortening, followed by ENE–WSW compression and subsequent deformation by the NNW–SSE striking Najd Fault System. The E–W dextral faults may be the conjugate shear fractures to the D3 NNW–SSE oriented sinistral wrench faults or are related to a subsequent event, D4. NW–SE oriented gold-bearing quartz veins originated during D1 and were subsequently deformed by D2–D4 events.

Tectono sedimentary evolution of the Umm Ghaddah Formation (late Ediacaran-early Cambrian) in Jordan

The terrestrial Umm Ghaddah Formation of late Ediacaran-early Cambrian age was deposited in NE–SW elongated intracontinental rift system basins and sub-basins bounded by active listric half-graben faults. Basin fill consists of conglomerate facies association A, deposited in a fault-controlled transverse alluvial fan system that drained northwestward and graded laterally into sandstone facies association B, deposited by a braided river system flowing northeastward axial to the rift basin. The alluvial fan facies association was deposited by rock falls and non-cohesive debris flows of sediment gravity flow origin, and by sheetflood processes. The Umm Ghaddah Formation is dominated by a large-scale fining upward succession interpreted to reflect a gradual cessation of the Pan African Orogeny. Within this large-scale trend there are also minor fining and coarsening upward cycles that are attributed to repeated minor tectonic pulses and autocyclic shifting of the system. The distribution pattern of the Umm Ghaddah Formation and the underlying Ediacaran Sarmuj Conglomerates, Hiyala Volcaniclastics and Aheimir Volcanics in Jordan and adjacent countries in isolated extensional half-grabens and grabens formed during the extensional collapse phase of Arabia associated with the Najd Fault System seems to be unrelated to the present day Wadi Araba-Dead Sea transform fault system.

Thrusting and transpressional shearing in the Pan-African nappe southwest El-Sibai core complex, Central Eastern Desert, Egypt

The Wadi El-Shush area in the Central Eastern Desert (CED) of Egypt is occupied by the Sibai core complex and its surrounding Pan-African nappe complex. The sequence of metamorphic and structural events in the Sibai core complex and the enveloping Pan-African nappe can be summarized as follows: (1) high temperature metamorphism associated with partial melting of amphibolites and development of gneissic and migmatitic rocks, (2) between 740 and 660 Ma, oblique island arc accretion resulted in Pan-African nappe emplacement and the intrusion of syn-tectonic gneissic tonalite at about 680 ± 10 Ma. The NNW–SSE shortening associated with oblique island arc accretion produced low angle NNW-directed thrusts and open folds in volcaniclastic metasediments, schists and isolated serpentinite masses (Pan-African nappe) and created NNE-trending recumbent folds in syn-tectonic granites. The NNW–SSE shortening has produced imbricate structures and thrust duplexes in the Pan-African nappe, (3) NE-ward thrusting which deformed the Pan-African nappe into SW-dipping imbricate slices. The ENE–WSW compression event has created NE-directed thrusts, folded the NNW-directed thrusts and produced NW-trending major and minor folds in the Pan-African nappe. Prograde metamorphism (480–525 °C at 2–4.5 kbar) was synchronous with thrusting events, (4) retrograde metamorphism during sinistral shearing along NNW- to NW-striking strike-slip shear zones (660–580 Ma), marking the external boundaries of the Sibai core complex and related to the Najd Fault System. Sinistral shearing has produced steeply dipping mylonitic foliation and open plunging folds in the NNW- and NE-ward thrust planes. Presence of retrograde metamorphism supports the slow exhumation of Sibai core complex under brittle–ductile low temperature conditions. Arc-accretion caused thrusting, imbrication and crustal thickening, whereas gravitational collapse of a compressed and thickened lithosphere initiated the sinistral movement along transcurrent shear zones and low angle normal ductile shear zones and consequently, development and exhumation of Sibai core complex.

Rejuvenation of The Pre-Cambrien Najd Fault System and Its Importance In The Oil Province of Saudi Arabia

Structural interpretation covering the Central Saudi Arabia was carried out through Landsat, Radar images, topographic maps and seismic data. Existing geology maps provide materialized evidence for this study. A strong northwest trending lineament system has been mapped in the region and is interpreted as being rela- ted to the reactivated Najd Fault System. Elongate hills, strait-going creeks, offset along valley and ridges, and pull-apart basins are evident, indicating rejuvenation of the Najd transtensional sinistral movement. Geological map shows evidence indicating a northwestward dragging on the sedimentary cover along a NW trending regio- nal lineament in the region which extend from South Ghawar to An Nafud Basin in Saudi Arabia. Northeast trend- ing structures in Saudi Arabia are related to the pre-Permian structuring and Oman-Masirah stress regime. The Ghawar (Saudi Arabia), Dukhan (Qatar) and other north-northwest trending anticlines were reshaped by the reju- venated Najd stress regime. The northwest trending sinistral Najd Fault was initiated in Precambrian and was reactivated at various times, probably during Paleozoic and late Jurassic times, and continuously, though of vari- able intensity, from late Cretaceous to the late Tertiary time. The Carboniferous, Late Cretaceous and Tertiary events are overprinted on one another in the region. The Oman stress regime and the stress regime that is rela- ted to oblique obduction of the Masirah Ophiolite are the principal controlling event for the anticline structures in Central Arabia. Dominant structural elements of the area are N-S anticline axis that suggests approximate E-W compressional stress direction, which is consistent with Oman and reactivated Najd tectonic regimes. The effect of the Tertiary orogenic overprint of the Oman-Masirah stress regime in eastern Arabia was to favor a slight renewed movement along the old basement faults such as the Najd Trend. Reactivated Najd system, Oman prin- cipal horizontal stress regime and obduction of the Masirah Ophiolite onto the Arabian continent have produced a combined effect on the structures in Eastern Arabian Plate. The Zagros stress regime may have produced Mineral Res. Exp., Bull., 132, 73-82, 2006 little effect in the region.

A Quantitative structural study of late pan-african compressional deformation in the central eastern desert (Egypt) during Gondwana assembly

The Arabian-Nubian-Shield (ANS) is composed of a number of island arcs together with fragments of oceanic lithospere and minor continental terranes. The terranes collided with each other until c. 600 Ma ago. Subsequently, they were accreted onto West Gondwana, west of the present River Nile. Apart from widespread ophiolite nappe emplacement, collisional deformation and related lithospheric thickening appear to be relatively weak. Early post-collisional structures comprise not only extensional features such as fault-bounded (molasse) basins and metamorphic core complexes, but also major wrench fault systems, and thrusts and folds. The Hammamat Group was deposited in fault-bounded basins, which formed due to N-S to NW-SE directed extension. Hammamat Group sediments were intruded by late orogenic granites, dated as c. 595 Ma old. A NNW-SSE-oriented compression prevailed after the deposition of the Hammamat Sediments. This is documented by the presence of NW-verging folds and SE-dipping thrusts that were refolded and thrusted in the same direction. Restoration of a NNW-SSE- oriented balanced section across Wadi Queih indicates more than 25% of shortening. Transpressional wrenching related to the Najd Fault System followed this stage. The wrenching produced NW-SE sinistral faults associated with positive flower structures that comprise NE-verging folds and SW-dipping thrusts. Section restoration across these late structures indicates 15 17% shortening in the NE-SW direction. At a regional scale, the two post-Hammamat compressional phases produced an interference pattern with domes and basins. It can be shown that the Najd Fault System splays into a horsetail structure in the Wadi Queih area and loses displacement towards N and NW. The present study shows a distinct space and time relationship between deposition of Hammamat Group/late-Pan-African clastic sediments and late stages of Najd Fault wrench faulting: Hammamat deposition is followed by two episodes of compression, with the second episode being related to Najd Fault transpression. Therefore, the Hammamat sediments do not represent the latest tectonic feature of the Pan-African orogeny in the ANS. The latest orogenic episodes were the two successive phases of compression and transpression, respectively. It is speculated that extension during (Hammamat) basin formation was sufficiently effective to reduce the thickness of the orogenic lithosphere until it became gravitationally stable, and incapable of further gravitational deformation.

The Wadi Mubarak belt, Eastern Desert of Egypt: a Neoproterozoic conjugate shear system in the Arabian–Nubian Shield

The Wadi Mubarak belt in Egypt strikes west–east (and even northeast–southwest) and crosscuts the principal northwest–southeast trend of the Najd Fault System in the Central Eastern Desert of Egypt. The belt therefore appears to be a structural feature that formed postdate to the Najd Fault System. In contrast, it is shown here that the deformation in the Wadi Mubarak belt can be correlated with the accepted scheme of deformation events in the Eastern Desert of Egypt and that its geometry and apparently cross-cutting orientation is controlled by a large granite complex that intruded prior to the structural evolution. Structural correlation is facilitated by a series of intrusions that intrude the Wadi Mubarak belt and resemble other intrusions in the Eastern Desert. These intrusions include: (1) an older gabbro generation, (2) an older granite, (3) a younger gabbro and (4) a younger granite. The structural evolution is interpreted to be characterized by early northwest directed transport that formed several major thrusts in the belt. This event is correlated with the main deformation event in the Eastern Desert, elsewhere known as D2. During this event the regional fabric of the Wadi Mubarak belt was wrapped around the El Umra granite complex in a west–east orientation. The Wadi Mubarak belt was subsequently affected during D3 by west–east and northwest–southeast trending sinistral conjugate strike–slip shear zones. This event is related to the formation of the Najd Fault System. Detailed resolution of superimposed shear sense indicators suggest that D3 consisted of an older and a younger phase that reflect the change of transpression direction from east-southeast–west-northwest to eastnortheast–westouthwest. The El Umra granite complex is dated here with single zircon ages to consist of intrusion pulses at 654 and 690 my. These ages conform with the interpretation that it intruded prior to D2 and that the structural pattern of the Wadi Mubarak belt was initiated early during D2.

Post-amalgamation basins of the NE Arabian shield and implications for Neoproterozoic III tectonism in the northern East African orogen

Abstract The development and structure of Neoproterozoic III volcanosedimentary basins in the northeastern part of the Arabian shield record periods of uplift, erosion, extension, subsidence, compression, and strike-slip faulting that postdated the completion of terrane amalgamation in the northern East African orogen (EAO) and predated the initiation of a passive margin on the northern flank of the Gondwana supercontinent. The oldest basins, dating from 650 Ma, include as much as 8000 m sandstone, conglomerate, bimodal volcanic rocks, and limestone, and were deposited in a large area (approximately 72,000 km 2 ) of locally fault-controlled subsidence (Murdama basin) and in narrow grabens (Bani Ghayy basins). The regional unconformity at the base of the basins bevels the newly amalgamated terranes and exposes greenschist-, and locally amphibolite-, and granulite-grade metamorphic rocks, indicating possibly >10 km uplift and erosion in parts of the region prior to deposition. The basins were closed and inverted by folding at about 650 Ma and unconformably overlain by rocks of the Jurdhawiyah group and Hibshi formation, which were deposited between 640 and 620 Ma in fault-controlled basins following a further phase of uplift and erosion. The Jurdhawiyah and Hibshi basins closed and inverted during subsequent north–south shortening and north- and south-vergent reverse faulting. A final phase of Neoproterozoic III basin formation resulted in deposition of the Jibalah group (580–570 Ma) in small, isolated, pull-apart basins caused by strike- and dip-slip movements on faults of the Najd fault system. The Murdama and Bani Ghayy basins contain shallow-marine clastic and carbonate deposits indicating that seaways penetrated the core of the EAO soon after terrane amalgamation, which implies that much of the region was at a low elevation soon after terrane amalgamation and orogeny. This limits the extent to which gravity-driven extensional collapse may have been a factor in the Neoproterozoic III tectonic evolution of the region, unlike other parts of the EAO where orogenic extensional collapse was significant. The Jurdhawiyah, Hibshi, and Jibalah basins were probably isolated fault-controlled lakes. The northerly trend of Murdama and Bani Ghayy folds implies bulk east–west shortening, which is consistent with the model of east–west Gondwana convergence conventionally applied to the EAO. East–west convergence conceivably accounts, additionally, for the creation of the Jurdhawiyah and Hibshi basins as a result of concomitant northward extension or tectonic escape. It would also account for the simultaneous creation of the Murdama and Bani Ghayy basins, themselves, linking the two as foreland basins at subsided and extended parts of a newly amalgamated crust in the center of the study area that was downflexed by the overthrusting of an ophiolite complex and other terranes from the east.

Structural and tectonic evolution of the Neoproterozoic Feiran–Solaf metamorphic belt, Sinai Peninsula: implications for the closure of the Mozambique Ocean

The Feiran–Solaf metamorphic belt of the southern Sinai Peninsula consists predominantly of amphibolite facies metapelitic, arenaceous, and minor calcareous rocks deformed during Neoproterozoic (850–550 Ma) orogenesis and intruded by dioritic– tonalitic–granitic plutons. The belt is divided into the Solaf Zone in the southeast and the Feiran Zone in the northwest. Twenty-five detailed structural cross-strike transects reveal that intense polyphase deformation resulted in complex fold geometries and fabric relationships. On outcrop scale, the progressive D 1 deformation is recognized as early, NW-striking, intrafolial isoclinal folds (F1), an associated steeply dipping gneissic (S 1) fabric, and the development of calc-mylonite zones along the NE margin of the belt. Tight, NW-striking, recumbent similar folds (F 2) characterize the late D1 deformation, whereas NE-striking open (F3) folds represent D2. Macro-scale folds of the F1 and F2 account for the map-scale structural pattern shown by the lithologic units. F 3 is represented mostly by meso-scale folds. The early SE- and NW-plunging F1 and F2 fold axes may be related to accretion of intraoceanic sediments deposited between arcs and/or microcontinents, whereas the late NE-trending upright F 3 folds may be related to post-accretionary tectonic events. In particular, the F 3 folds and associated mineral stretching lineations, which trend NW–SE, may be related to the Najd fault system. © 2003 Elsevier Science B.V. All rights reserved.