Transpressive Structures in the Ghadir Shear Belt, Eastern ......2019/05/16 · MOHAMED Abd El-Wahed*, SAMIR Kamh, MAHMOUD Ashmawy and ALY Shebl Geology Department, Faculty of Science,

Transpressive Structures in the Ghadir Shear Belt, Eastern Desert, Egypt:

Evidence for Partitioning of Oblique Convergence in the Arabian-Nubian

Shield during Gondwana Agglutination

MOHAMED Abd El-Wahed*, SAMIR Kamh, MAHMOUD Ashmawy and ALY Shebl

Geology Department, Faculty of Science, Tanta University, Tanta 31527, Egypt Abstract: Transpressional deformation has played an important role in the late Neoproterozoic evolution of the Arabian-

Nubian Shield including the Central Eastern Desert of Egypt. Ghadir Shear Belt is a 35 km-long, NW-oriented brittle-ductile

shear zone that underwent overall sinistral transpression during the Late Neoproterozoic. Within the Ghadir shear belt, strain

is highly partitioned into shortening, oblique, extensional and strike-slip structures at multiple scales. Moreover, strain

partitioning is heterogeneous along-strike giving rise to three distinct structural domains. In East Ghadir and Ambaut shear

belts, the strain is pure-shear dominated whereas, the narrow sectors parallel to the shear walls in West Ghadir Shear Zone

are simple-shear dominated. These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones.

The kinematic transition along Ghadir shear belt is consistent with separate strike-slip and thrust-sense shear zones. The

earlier fabric (S1), is locally recognized in low strain areas and SW-ward thrusts. S2 is associated with a shallowly plunging

stretching lineation (L2), and defines ~NW-SE major upright macroscopic folds in East Ghadir shear belt. F2 folds are

superimposed by ~NNW-SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional

deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation. F2 and F3 folds are

superimposed by ENE-WSW gentle F4 folds in Ambaut shear belt. The sub-parallelism of F3 and F4 fold axes with the shear

zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and

pure shear-dominant deformation in fold zones. Dextral ENE-striking shear zones were subsequently active at ca. 595 Ma,

coeval with sinistral shearing along NW- to NNW-striking shear zones. The occurrence of upright folds and folds with

vertical axes suggests that transpression plays a significant role in the tectonic evolution of Ghadir shear belt. Oblique

convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its

segments. Upright folds, fold with vertical axes and sinistral strike-slip shear zones developed in response to strain

partitioning. West Ghadir Shear Zone contains thrusts and strike-slip shear zones resulted from lateral escape tectonics

associated with lateral imbrication and transpressional in response to oblique squeezing of the Arabian-Nubian Shield during

agglutination of East and West Gondwana.

Keywords: Arabian-Nubian Shield, Ghadir shear belt, transpression, splay-dominated and thrust-dominated strike-slip shear

zones

E-mail: [email protected]

1 Introduction

Transpression is a combination of a strike-slip component and a shortening component orthogonal to the

deformation zone (Harland, 1971) and include a deformation developed between two undeformed blocks resulting from both simple and pure shear (Sanderson and Marchini, 1984; Fossen et al., 1994; Fossen and Tikoff, 1998; Jones et al., 2004). Transpression occurs in a wide variety of tectonic settings and scales during deformation of the Earth‘s lithosphere such as the Archean North Caribou greenstone belt (Gagnon et al., 2016), the Pan-African Kaoko belt in Namibia (Goscombe et al., 2003; Knopásek et al., 2005), Southern Uplands of SE Scotland (Tavarnelli et al., 2004), Kushtagi schist belt, India (Matin, 2006), salient-recess transition at the northern Gibraltar Arc (Barcos et al., 2015), Sanandaj–Sirjan metamorphic belt, Zagros mountains, Iran (Sarkarinejad et al., 2008; Shafiei Bafti and Mohajjel, 2015), Al Jabal Al Akhdar, Libya (Abd El-Wahed and Kamh, 2013), Central Asian Orogenic Belt (Li et al., 2016), Cauvery shear zone, southern Granulite Terrain, India (Chetty and Bhaskar Rao, 2006), Dom Feliciano Belt, Uruguay (Oriolo et al., 2016) Salem–Attur shear zone, south India (Kumar and Prasannakumar, 2009), Rengali Province, eastern India (Ghosh et al., 2016), and the Central Eastern Desert of Egypt (Fritz et al., 1996, 2002, 2013; Bregar et al., 2002; Shalaby et al.,2005; Abd El-Wahed, 2008, 2010, 2014; Abd El-Wahed and Kamh, 2010; Zoheir, 2008, 2011; Zoheir and Weihed, 2014; Abdeen et al., 2014; Abd El-Wahed et al., 2016; Makroum, 2017 Abd El-Wahed and Thabet, 2017; Hamdy et al., 2017) and Arabian Shield (Hamimi et al., 2013, 2014).

The East African Orogen (EAO) is an accretionary orogen that extends from Arabia to East Africa and into Antarctica. It was formed by the closure of the Mozambique Ocean. Rodinia was broken up and disintegrated

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between 900 and 800 Ma ago resulting in the Mozambique Ocean opening (Stern, 1994, 2017). The Arabian-Nubian Shield (ANS) was deformed and squeezed during the orogenic collision between East and West Gondwana (Fig. 1a), after the closure of the Mozambique Ocean during the Neoproterozoic (800–600 Ma). The Arabian Nubian-Shield (ANS) (Fig. 1b) constitutes the northern extension of the East African Orogen (EAO) and was exposed because of the uplift prior to the opening of the Red Sea (Stern, 1994, 2017). It is exposed in the Egyptian Eastern Desert, Sudan, western Saudi Arabia, Ethiopia, Uganda, Kenya, Eritrea, Jordan and Yemen.

In the Egyptian Eastern Desert, the Nubian Shield covers a significant area and appears as a belt parallel to the Red Sea coast for about 800 km between latitudes 22° 00/E and 28° 40/N (Fig. 1). These rocks are unconformably overlain on their western and eastern margins by the Nubia Sandstones and younger sediments.

Several models have been proposed to discuss the tectonic evolution of the ANS: (i) Infracrustal orogenic model, whereby ophiolites and island-arc volcanics and volcaniclastics were thrusted over an old craton consisting of high-grade gneisses, migmatites and remobilized equivalents during the Neoproterozoic (e.g. Akaad and Noweir, 1980; El Gaby et al, 1988; Abdelkhalek et al., 1992). (ii) Turkic-type orogenic model, whereby much of the ANS was formed in broad fore-arc complexes (Şengor and Natal'in, 1996). (iii) Hot-spot model, whereby much of the ANS, due to accretion of oceanic plateau was formed by upwelling mantle plumes (Stein and Goldstein, 1996). (iv) Arc assembly (arc accretion) model, whereby EAO juvenile crust was generated around and within a Pacific-sized ocean (Mozambique Ocean). This model was proposed first by Vail (1985) and Stoeser and Camp (1985), and modified by Stern (1994).


Fig. 1. (a) East African–Antarctic orogen (EAAO) is interpreted as main collisional orogen along which parts of proto–East and WestGondwana

collided to form Gondwana (after Stern, 1994; Jacobs and Thomas, 2002; Avigad et al., 2003, and references therein). Abbreviations: ANS—

Arabian-Nubian shield; Da—Damara belt; EF-European fragments; LH-Lutzow-Holm Bay; M-Madagascar; Z-Zambesi belt., (b) Relationship

between the Arabian–Nubian Shield (ANS) to the adjacent older continental crust. The border with the Saharan Metacraton, on the west, is the

effective contact between ANS and West Gondwana: a border, on the east, with putative East Gondwanan crust, is not certain. NED = North Eastern

Desert; CED = Central Eastern Desert; SED = South Eastern Desert; QSZ = Queih shear zone; MG= Meatiq gneisses; SG= Sibai gneisses; MG=

Hafafit gneisses; SHG= Shalul gneisses; KHSZ= Kashir-Houdien Shear Zone: SHS= Sol Hamed Suture; AHS= Allaqi-Heiani Suture; GGN= Gabal

Gerf Nappe; NKS= Nakasib Suture; KS= Keraf Suture; BS= Baraka Suture; AIS= Al Amar Suture; ADS=Ad Damm Suture; BUS= Bir Umq Suture;

YS= Yanbu Suture; HADRS= Hulayfah-Ad Dafinah-Ruwah; NBS= Nabatih Suture; HZFZ= Halaban-Zarghat fault zone; ARFZ= Al Rika fault zone

(compiled from Johnson et al., 2011; Abdeen and Abdelghaffar, 2011; Fritz et al., 2013; Abd El-Wahed et al., 2016).

The complicated structural pattern of the Eastern Desert Neoproterozoic rocks reflects a polyphase

deformational event referred to complex tectonic history. The Eastern Desert is dissected by a multidirectional fault pattern among them the NW, N-S and NE- trending faults. The interaction between these major fault systems resulted in a complex structural pattern.

Two distinctive tectonostratigraphic units characterize the Central Eastern Desert (CED) (Fig. 2). The first is the lower unit comprising high-grade metamorphic gneisses, migmatites, schists and amphibolites and is commonly referred to as the structural basement (El-Gaby et al.,1990; Loizenbauer et al., 2001; Abdeen, 2003) or as ‗‗lower tier‖ (e.g. Bennett and Mosley, 1987). The second commonly referred to as structural cover or the Pan-African nappes and is described as the upper unit including low-grade metamorphosed ophiolite slices (serpentinites, pillow lavas, metagabbros), arc metavolcanics, arc metasediments (e.g. El-Gaby et al.,1990; El Bahariya, 2012; 2018). Collectively, both of them were intruded by syn-tectonic calc-alkaline granites and metagabbros–diorite complex. The later stages of the crustal evolution of the CED is characterized by the eruption of the Dokhan Volcanic suite which is associated with the formation of molasse-type Hammamat sedimentary rocks that were deposited in non-marine, alluvial fan/river environments (Grothaus et al., 1979; Abd El-Wahed, 2010; Bezenjania et al., 2014). The crustal rocks, in turn, were intruded by a series of late to post-tectonic granites.


Ghadir shear belt is located in the extreme southern part of the Central Eastern Desert (CED). It lies approximately 30 Km southwest Marsa Alam city and extends between latitudes 24° 37" to 24° 57" N and longitudes 34° 39" to 35° 05" E (according to WGS84 Datum and UTM zone 36N projection) on the Red Sea coast (Fig. 2). It is limited from the East by the Red Sea and from the West by a NW trending prominent Red sea hills delineating the western limit of the drainage basins of the study area. To the north of the study area, Sukari gold mine area is located.

Fig. 2. Geological map of the southern part of the Central Eastern Desert of Egypt (modified after Klitzsch et al., 1987). 1;

core complexes 2; serpenitintes, 3; ophiolitic metagabbros, 4; metavolcanics and metasediments, 5; syn-tetonic intrusive metagabbros, 6; syn-tectonic

granite, 7; Dokhan volcanics, 8; molasse sediments, 9; felsites, 10; gabbros, 11; post to late tectonic granites; 12; ring complex, 13; Natash volcanics

and 14; trachyte plugs. GAK; Gebel Abu Khruq, HCC; Hafafit core complex, GOM; Wadi Ghadir ophiolitic mèlange, HM; Hamash gold mine, NSZ;

Wadi Nugrus shear zone, GS; Gabal Sikait, WDSZ; Wadi Abu Dabbab shear zone, WUSZ; Wadi El Umra shear zone, GS; Gebel Sukkari and

Sukkari gold mine, GUK; Gebel Um Khariga, IG; Igla molasse basin, DMD; Dubr metagabbro-diorite complex, GIA; Gebel Igl Al- Ahmar, HW,

Gebel Homrat Waggad, GY, Gebel El-Yatima, GUS; Gebel Umm Salim, US, Gebel Umm Saltit, GK; Gebel Abu Karanish, GM, Gebel Al Miyyat,

USZ; Um Nar shear zone, GUM; Gebel El-Umra, GK; Gebel Kadabora, GA; GH; Gebel El-Hidilawi, GU; Gebel Umm Atawi, GSH; Gebel El Shalul,

GR; Gebel El Rukham; SCC; Sibai core complex, GS; Gebel Sibai, WZ; Wadi Zeidon, WSSZ; Wadi Sitra shear zone, WKSZ; Wadi Kab Ahmed

shear zone, K; Kareim molasse basin. The major structures are after Akaad et al. (1993), Fritz et al. (1996), Helmy et al. (2004), Shalaby et al. (2005),

Abd El-Wahed (2008) and Abd El-Wahed and Kamh (2010).

This study deals with the structural pattern of the Neoproterozoic Ghadir shear belt, in order to investigate

transpressional structures in Ghadir ophiolite. Generally, in the areas to be discussed, two main deformation types were recognized: shortening perpendicular to the shear zones and lateral movements (sinistral and dextral strike-slip shear zones). On the basis of new integrated field and remote sensing techniques, we present the first detailed structural analysis in Ghadir shear belt. We discuss the relationship between the transpressional imbricate thrust system and the Najd Fault System in the Nubian Shield.

2 Regional Tectonic History of the Eastern Desert (ED) of Egypt


The ED is characterized mainly by the prevalence of a NW trending tectonic fabric marking the NW–SE sinistral shear zone of the Najd Fault System (NFS) (Abd El-Wahed and Kamh, 2010; Abd El-Wahed, 2014; Abdeen et a., 2014; Abd El-Wahed et al., 2016). Nappe transport directions reported from the ED show variation from top to the NE (e.g. El-Bayoumi and Greiling, 1984), top to the NW (e.g. Ries et al., 1983; Greiling, 1987), top to the SE (e.g. Kamal El Din et al., 1992), and top to the SW (e.g. Abdeen et al., 2002; Abdelsalam et al., 2003). Oblique convergence transpression, whether dextral or sinistral, is a current mechanism adopted by many authors (Fritz et al., 1996, 2002, 2013; Loizenbauer et al., 2001; Makroum, 2001; Bregar et al., 2002; Helmy et al., 2004; Shalaby et al., 2005; Abd El-Wahed, 2008, 2010; 2014; Shalaby, 2010; Abd El-Wahed and Kamh, 2010; Zoheir and Lehmann, 2011; Zoheir and Weihed, 2014; Abd El-Wahed et al., 2016; Stern, 2017; Abd El-Wahed and Thabet, 2017; Makroum, 2017; Hamdy et al., 2017; Hagaga et al., 2018; Hamimi et al., 2019) to explain and imply the deformation styles in the CED.

Simply and collectively, (Abd El-Wahed and Kamh, 2010) arranged the deformation events in the CED of Egypt as follows: (1) D1 linked to NNW-directed thrusts; (2) D2 related to NE- and SW-directed thrusts; (3) D3 attributed to sinistral movement along the NW-trending shear zones of the NFS; (4) D4 associated with dextral movement along NE-trending shear zones; and (5) D5 later events. The NNW-directed thrusts were produced as a consequence of compressional oblique island arc-accretion between (690 and 650 Ma) in Sibai dome (Bregar et al., 2002; Fowler et al., 2007; Augland et al., 2012) and 630 and 609 Ma in Meatiq and Hafafit domes (Andresen et al., 2009). The tectonic regime changed from the compressional arc accretion setting to the sinistral transpressional regime between 650 and 540 Ma.

In the CED, Exhumation of the core complexes (e.g. Meatiq, Sibai, El- Shalul and Hafafit) is related to the sinistral shearing along the Najd Fault System (NFS), which bounds them from the SW and NE (Fritz et al., 1996). Not only core complexes of CED but also the Hammamat molasse sediments are tectonically affected by the NFS (Abd El-Wahed, 2010).

Originally, the Najd Fault System and other NW-trending strike-slip faults in the ANS are considered post-accretionary structures (Abdeen et al., 1992) and were interpreted to be the result of the squeezing of the ANS between East and West Gondwana (Berhe, 1990; Stern, 1994; Abdelsalam and Stern, 1996; Abdelsalam et al., 2003; Abdeen and Greiling, 2005). The Najd Fault System (NFS) consists of brittle–ductile shears in a zone as much as 300 km wide and more than 1100 km long, extending across the northern part of the Arabian Shield and Extend to the ED. The structures in the Najd fault zone were developed in response to a sinistral transpressive tectonic regime, with the axis of maximum compressional stress oriented at oblique angles to the NW-trending orogenic front (Abd El-Wahed, 2014).

Different models were suggested to interpret one of the major landmarks of the CED, the series of gneiss domes or core complexes (e.g. Meatiq, Sibai, El- Shalul and Hafafit). Gneiss domes have either been interpreted as: (1) antiformal stacks formed during thrusting (e.g., Greiling et al., 1994), (2) core complexes during orogen-parallel crustal extension (e.g., Fritz et al., 1996; Bregar et al., 2002; Abd El-Wahed, 2008; Makroum, 2017; Hamdy et al., 2017), and (3) interference patterns of sheath folds (Fowler and El Kalioubi, 2002). Recently, there are two main tectonic models explain the formation and exhumation of these core complexes in the CED;

(1) Orogen-parallel extension model of Fritz et al., (1996) assigned the evolution of the core complexes to sinistral shearing along the NW-trending shear zones of the NFS.

(2) The second model attributed the formation of gneiss-cored domes to extensional tectonics by an overlap between NW–SE complex folding and NW–SE extension (e.g., Fowler et al., 2007; Andresen et al., 2010).

Nugrus Shear Zone (NSZ) (Fig. 2) is one of the major NW-trending shear zones forming the boundary separating the CED from the SED of Egypt and constitutes the boundary between Wadi Ghadir area (East) and Hafafit Core Complex (West). The Hafafit–Ghadir domain is interpreted as a back-arc accretion system that evolved above a NW-dipping subduction zone (Abd El-Naby and Frisch, 2006). The NSZ starts with a 750 m width near the meeting point of Wadi Nugrus and Wadi Sha‘it (Fowler and Osman, 2009), preserving its width at least as far as Gabal Sikait where its width decreased by 50 m (Harraz and El-Sharkawy, 2001). Lundmark et al. (2012) assigned an age of ≤595 Ma for left-lateral shearing of the Nugrus Shear Zone.

The NSZ was interpreted as a thrust accommodating westward or SW-ward ophiolitic material transport over the continental margin (El-Gaby et al., 1988; El-Bayoumi and Greiling, 1984; El-Ramly et al., 1984) or as roof thrust linked up NW-dipping thrust imbricates of gneissic rocks and allowing NW-ward displacement of low-grade CED metavolcanics over the gneisses (Greiling et al. 1994 and Greiling, 1997). Many workers considered NSZ as a Najd-related ductile strike-slip shear (e.g. Fritz et al.1996; Hassan, 1998a, b; Unzog and Kurz, 2000; Shalaby et al., 2005; Abd El-Wahed et al., 2016; Makroum, 2017; Hagaga et al., 2018; Hamimi et al., 2019). Fowler and Osman (2009) argued about the NSZ being a strike-slip shear zone as the NW trending, along strike, schistose shear foliations is ceased at the intersection of Wadi Nugrus with Wadi Sha‘it, which is supported by the absence of field evidences about extending the NSZ structure NW-wards as a strike-slip fault into the CED. Fowler and Osman (2009) described NSZ as an example of a low-angle normal ductile shear (LANF) due to approximate E–W strike, low-angle N-dip and a normal shear sense.

The NW-trending sinistral shear zones related transpression is conjugated and overprinted by the dominant dextral transpression along NE–SW trending shear zones (Shalaby et al., 2005; Abd El-Wahed and Kamh, 2010). Abd El-Wahed (2014) documented a huge NE-trending shear belt (up to 110 km in length) occuping the area between Wadi Barramiya and Wadi Sha‘it (Fig. 2) to the west (up to 60 km in width) and extends through


the whole width of the Central Eastern Desert to include the area between Wadi Mubarak and Wadi Ghadir on the Red Sea coast (up to 120 km in width) (Fig. 2). The 40–60 km wide, ENE–WSW trending Mubarak–Barramiya shear belt, deforms supra-crustal successions and structures associated with the NW-trending shear fabric (Fig. 2). The Mubarak–Barramiya shear belt is interpreted as a dextral transpressive shear zone ceased the extension of the NSZ and modeled in terms of a regional ‗flower structure‘ (Abd El- Wahed and Kamh, 2010).

3 Mapping Ghadir Shear Belt by Remote Sensing

Geological information acquired from remote sensing satellite platforms provides important perspective in

understanding surface lithological and structural processes. However, this information captured by satellite is all-inclusive, and for the usually complex outcropped lithological and structural units it is difficult to map them accurately and precisely with detailed field work. The integration of the processing of Landsat 8 image and field investigation is used to insight the deformation history of Ghadir shear belt. Subset of the Landsat 8 OLI scene (Path 173/Row 43, acquired on July 18, 2018) has been analyzed, and processed using ENVI 5.3 software to enhance the geological contacts and structural elements. Some pre-processing techniques were applied to the remotely sensed data used such as the atmospheric correction and re-scaling the raw radiance data from imaging spectrometer to reflectance data. After that, several techniques of satellite data processing have been applied on Landsat 8 OLI data for detailed mapping of the lithological units and structural elements exposed in Ghadir shear belt including false color composite, band rationing, Principal Component Analysis (PCA) and image classification.

The ophiolite of Wadi Ghadir constitutes a wide range of lithologies. Metamorphism, alteration and weathering also play a role in mixing and muting spectral signals of different lithologies. Hence, No single ‗perfect‘ technique is used to map a whole area body. Therefore, integration between various remote sensing enhancement techniques and GIS data is needed for successful lithologic mapping of the area. Thus, the Remote Sensing (RS) products for the current area geological mapping include True Color Composites (TCC), False Color Composite (FCC) and Principal Components Analysis (PCA) using landsat 7ETM+ Satellite images (obtained from NASA having an acquisition date of October 2000 (05-10-2000) consisted of nine bands and have Path 173/Row 043 with spatial resolution 30 meters).

Practically, true color image (3, 2 and 1 in RGB), was produced for the study area (Fig. 3a) giving a reasonable lithologic discrimination. Landsat ETM+ true color image, as the name suggests, is an image similar to that seen or perceived by the human eyes.

Several false color composites (FCC) are tested and chosen to better highlight and discriminate between different types of rocks in the area. Excellent results are given through bands of 7, 5 and 4 in RGB (Fig. 3b). In the latter FCC, Zabara granitic gneiss is eminently identified by a light orange to yellow color contrasting the dark color of mélange and serpentinites. Serpentinite rocks have a dark magneta to black color and form the most spectrally prominent rock unit for most combinations. The best ETM+ band ratios to discriminate serpentinites from other surrounding rocks are 5/1, 3/2 and 7/2 in R, G and B respectively. The distinctive mineral assemblages of serpentinites impact their spectral characteristics and enhance their discrimination using the Landsat band ratio images (Sultan et al., 1986; Kusky and Ramadan, 2002; Frei and Jutz, 1989). This was confirmed by the case study of mapping Oman Ophiolites (Abrams et al., 1988). Gad and Kusky (2006) during their work in Barramiya area, referred the distinctive spectral reflectance of serpentinites to the abundance of antigorite, lizardite, clinopyroxenite and magnetite in the mineral composition. The metavolcanic rocks are represented by light greenish yellow color, while gabbroic rocks are displayed by brown color varying from pale brown for metagabbro to reddish bright brown for gabbroic rocks.


Fig. 3. Band combinations applied to the Landsat 7 ETM+ Satellite image of the Ghadir shear belt in, a) 3, 2, 1 true color composite, and b) 7, 5, 4 false color composite in RGB sequence.


A light yellow to pale pinkish brown color represents the granitic rocks exposed at the eastern part of the study area due to prevailing of quartz and feldspar (75%) of the total rock composition. This light appearance is referred to the quartz and feldspar high reflectance in the selected bands in RGB. The two types of syn- and late-tectonic granites are also distinguishable where syn-tectonic granitic rocks are marked by pale pinkish brown contrasting the light yellow color appearance for the late tectonic granitic rocks. The color degree gradual difference is well-explained by gradual variance in the quartz percentage of the syn- to late-tectonic granites.

Principal Components Analysis (PCA) is a linear transformation technique related to Factor Analysis. Given a set of image bands, PCA produces a new set of images, known as components that are uncorrelated with one another and are ordered in terms of the amount of variance they explain from the original band set. PCA has traditionally been used in remote sensing as a mean of data compaction in various applications, including geologic mapping. These analyses were used to reduce redundancy in multispectral data. Practically, the PCA transformation was computed for the ETM+ data to produce the best PCA for the discrimination between rock units in the study area. PCA analysis results show that the first three PCs (PC1, PC2 and PC3) explain almost all the variance contained in the multi-spectral data. Respectively in RGB, (PC1, PC2 and PC3) and (PC2, PC1 and PC3) proved their useful role for the PCA in the study area lithologic discrimination and distinctiveness.

The former PCA colored composite (PC1, PC2 and PC3, in RGB) was applied for the whole study area (Fig. 4a) concluding extremely advantageous discrepancies among the study area different rock units where the granite rocks are displayed by a very distinguished blue color in the eastern part of the area while serpentinite rocks and mélange are indicated by the yellowish cyan color. The granitic gneiss rocks are represented by a golden yellow color (better highlights the topographic effects and provides an easier visual interpretation due to incorporating PC1 in the RGB color composites), while Gabbroic rocks are displayed by yellow color (pale for metagabbro and bright for fresh gabbro). Besides the latter PC composite, (PC2, PC1 and PC3, in RGB) also proved its useful role for the lithologic distinction (Fig. 4b), assisting in more identification, discrimination, interpretation and mapping of rocks exposed within the Ghadir shear belt.


Fig. 4. Principal Components for the Ghadir shear belt in RGB, a) PC1, PC 2 and PC 3, and b) PC 2, PC1 and PC3.

4 The Geological Setting

Wadi Ghadir area is covered mainly by the most common ophiolite in Egypt that is called Ghadir Ophiolite

(Fig. 5). The Ghadir ophiolite is one of the best preserved sections through late Proterozoic upper oceanic crust anywhere in the world (Kröner et al., 1992). Ophiolites of Egypt are mainly dismembered (Basta, 1983; Ries et


al., 1983; Zimmer et al., 1995; Farahat et al., 2004) and restricted to the Central and Southern Eastern Desert. These ophiolites are interpreted to be formed in back-arc (e.g. Basta, 1983; Berhe, 1990; El-Sayed et al., 1999; Takla et al., 2002; Abd El-Rahman et al., 2009a), or fore-arc setting (Azer and Stern, 2007).

The Wadi Ghadir area contains the first recognized ophiolitic mélange in the Nubian shield. The mélange is composed of fragments derived mainly from ophiolitic sheets, greywacke, siltstone, and mudstone units and granitic rocks (El-Bayoumi, 1980). Wadi Ghadir area represents a widely distribuated stretch of Neoproterozoic ophiolitic mélange constituted mainly of allocthonous ophiolitic fragments mixed firmly and incorporated in a sheared matrix, as well as, other different mappable units. The most extensive and widely disseminated unit of the area is the mélange assemblages (Fig. 5). The mélange is commonly interpreted to be allochtonous with respect to underlying paragneisses and/or orthogneisses and to have been generated by gravity sliding and/or thrusting and emplaced by SE-NW directed thrusting (Shackleton et al., 1980). The mélange would have been transported over distances of 300-500 kilometers (Ries et al., 1983).

El-Sharkawy and El-Bayoumi, (1979) and El-Bayoumi (1980) are the pioneers in recognizing the Neoproterozoic oceanic crust in Wadi Ghadir. They reported the occurrence and the description, for the first time, of a complete and genuine ophiolite sequence in Wadi Ghadir area. This section is well exposed and easily recognized to the northwest of Gabal Dob Nia (Fig. 5). In this section, all the ophiolite units from the layered gabbro at the base to the pillow basalts associated with deep-sea sediments at the top.

The other mappable units in the area include rhyolitic, andesitic, dioritic and granitic rocks. Dioritic rocks are foliated and depicted as a large fold structure having a NW trending axis in the southwestern part of the area (El-Sharkawy and El-Bayoumi, 1979). Granitic rocks occupy the eastern part of the area exhibit an intrusive contact with other units. They hold up a plentiful number of xenoliths of older rocks in various stages of interaction. Numerous later dykes of all directions and exhibiting different compositions cut and transect all the other rocks.


Fig. 5. Geological map of Wadi Ghadir area (compiled and modified lithologically after El Bayoumi, 1980; Basta, 1983; Abd El-Rahman et al., 2009b; Kamel et al., 2016). Structures are from the present study.

4.1 Granite gneisses

Zabara granite gneisses (633±5 Ma, Lundmark et al. 2012) occupy the southwestern part of the Ghadir area. They comprise granite gneiss intercalated with bands or lenses of hornblende gneiss and minor migmatites, together with cataclased granite, mylonites and schists (Kamel et al., 2016) that are metamorphosed at pressures of 6.8-7.7 kbars (Surour, 1995). Mineralization of emerald, rare element mineralization (Ta, Nb, Be, Zr, U, Th, Sn), cassiterite and fluorite in the Zabara area are mainly found in schist and believed to be related to hydrothermal fluids evolved from some of the leucogranites intruded into Nugrus shear zone (Harraz et al., 2005). Microscopically, granite gneiss consists of alternated felsic (quartz and plagioclase feldspars) and mafic (biotite) bands.

4.2 The serpentinized peridotites


The serpentinized peridotites constitute the lower-most unit of the ophiolite sequence of Wadi Ghadir. The ultramafics are the prevailing among the ophiolitic rocks and exhibit variable sizes from microscopic dimensions up to mountain size (Gabal Ghadir). They are mostly sheared and exhibit mesh structure. In most cases, they are generally serpentinized (with minor amounts of fresh relics of dunite, and pyroxenite) and in many places altered and transformed into talc-carbonate rocks of cream colours consisting principally of talc and magnesite with minor amounts of dolomite.

The metamorphic peridotites occur either as large mountain-size Klippers thrust onto the mélange or as small blocks (from few meters to centimeters in diameter) frequently incorporated in the mélange. The mountaineous masses of the serpentinized peridotites are rootless and seem to be dismembered parts representing the debris which was broken off from the obducted ophiolite plate and incorporated within the mélange (El-Bayoumi, 1980). The mountaineous masses of peridotites can be discriminated distinctly into two types:

(i) Massive peridotites: with black-coloured variety and partially altered. The masses of Gabal Lawi and Gabal Lewiwi (Fig. 5) are of this type. The largest peridotite mass at Gabal Lewiwi is elongated in shape with NE-SW trend and composed of harzburgite, wehrlite and lherzolite. The mass is altered to talc-carbonate rock at the base and becomes fresh upwards.

(ii) Foliated peridotites: they are extensively sheared and altered to foliated talc-carbonate rocks, but still have relics of fresh peridotites. The best examples of this type are the peridotite masses that are located at the junction between Wadi Ghadir and Wadi Lawi (Fig. 5). These masses form a domal core structure, where the foliation in the surrounding mélange dips away from it in all directions. They are composed of fresh peridotites (harzburgite, wherlite and dunite) surrounded by vast talc carbonate carapace.

Petrographically, the ultramafic masses contain partly metamorphosed dunites and peridotites, massive and schistose serpentinites, talc-carbonates and chromitites. Antigorite has several forms especially as fine-scales, or lamellar aggregates which exhibit plumose structure and mesh texture. Lizardite is the main constituent of the massive serpentinites. Besides the serpentine minerals, chromite, magnetite, talc, carbonates, chlorite, actinolite-tremolite and olivine are also present.

4.3 Ophiolitic metagabbro

The gabbroic rocks constitute the upper part of the cumulate sequence, below the sheeted dyke complex. Gabbros are not common as the ultramafics within the ophiolitic rocks, occurring as blocks which are heterogeneous in garain size and colour.

Abd El-Rahman (2009b) classified the plutonic rocks of Wadi Ghadir Ophiolite into Layered, coarse-grained massive and hypabyssal gabbros. Although they are structurally imbricated units, dipping moderately to the southwest, the layered gabbro is regarded as the basal unit, transitional upward into the massive coarse-grained gabbro and the hypabyssal gabbro at the top. This is best seen in the well exposed section along Wadi Miyah Saudi and Wadi Miyah al-Bayda junction (Fig. 5). The main mass of this complex occurs close to Gabal Dob Nia (Fig. 5) and stretched NW where the gabbro is clearly and obviously layered at the bottom and passes gradually upwards into coarse-grained rosette gabbro which in turn conduces upwards into microgabbro. Other mappable gabbro masses occur in Wadi Ghadir, Wadi Lawi and Wadi Lawiwi, as well as few other places.

The basal unit of gabbroic rocks is characterized by rhythmic layering with plagioclase-rich leuco-layers alternating with dark layers rich in altered pyroxene. The thickness of an individual layer reach up to 50 cm. Layering is often disrupted by deformation so that single layers could not be traced laterally for more than 20 meters.

Several other masses of massive and rosette gabbro were found in the area. They appear to be allochthonous blocks in the mélange. Commonly, the rosette gabbro grades downwards into layered gabbro and upwards into hypabyssal gabbro. This zonation can only be perceived on a large scale, but minutely, no regular contacts could be detected between the three types of gabbro.

The gabbroic complex is intruded by numerous diabasic dikes and sills with variable attitudes ranging from sub-horizontal at the base, parallel to the layered gabbro to subvertical near the top. As best seen in the gabbros of Wadi Miyah al-Bayda, subhorizontal dykes are occur near the base of the section, with a gradual increase in the amount of dip towards the southwest along Wadi Miyah al-Bayda until they attain a subvertical attitude near the sheeted dyke complex.

Gabbros of the Ghadir ophilite are tholeitic. They have Mg-numbers between 43 and 71, and their SiO2 content ranges from 46 to 51 wt%. They have the lowest incompatible trace element (REE, Zr, Nb) whereas, values of the transition metals, Sc and Cr are close to MORB values (Abd El-Rahman et al., 2009b).

4.4 The sheeted metadiabases dykes

Sheeted dykes are recognized and best seen in Wadi Ghadir area where they configurate the intact proper unit of the ophiolite section between the underlying cumulate sequence and the overlying pillow basalts. In other places, Sheeted dykes constitute dismembered blocks which were incorporated in the mélange, e.g. along Wadi al-Lawi where a huge block occurs with well recognized chilled margin. The dykes vary in thickness from about 50cm to about 2m (Fig. 6a and b).

There is a transitional zone between the sheeted dykes and the pillow basalts in which the contacts of the dykes become sapped and fade away and the rock exhibits incipient formation of pillows. Microscopically,

b


metadiabase is composed of euhedral lath-shaped plagioclase crystals set in a finer matrix of hornblende, with exhibiting blasto-diabasic texture.

Fig. 6. (a) and (b) Sheeted dykes from Wadi Miyah al-Bayda (c) Circular and oval shaped pillow lavas, Wadi Miyah al-Bayda, (d) Deformed

pillowed metabasalts from Wadi Miyah al-Bayda (e) Igneous breccia in which boulders of basic dykes are set and embedded in a matrix of white

colored igneous material, and (f) Basaltic dykes cross-cutting and dissecting the granitic rocks. The dykes are displaced by NE-striking normal faults,

looking NE.

4.5. The pillowed metabasalts

Two outcrops of well exposed pillow lava occur along Wadi Miyah al-Bayda and Wadi Miyah Saudi. Morphologically, most pillows are well preserved, some are sheared and deformed. The pillows are aphanitic and grayish green in colour. They are rich in vesicles in their border zones. These vesicles are occasionally arranged in a zonal fashion parallel to periphery, and decrease from the border to the core. Most of these vesicles are now filled with chlorite and quartz. The pillows may be spherical, oval or elongated in shape. Others are still triangular or bunn shaped.

In Wadi Miyah al-Bayda (Fig. 5), where the most extensive and well displayed outcrop of the pillows, overlying the sheeted dykes is present. Individual pillows are circular or oval shaped (Fig. 6c and d) with massive core and a zone of vesicles near the periphery. Variolitic or schistose sheath encloses each pillow.

The second spot on the southern side of Wadi Saudi (Fig. 5), the pillowed mass is more altered with a highly schistose sheath. Some of these pillows exhibit porphyritic texture, others show vesicles. Fragments of pillow


basalt were also encountered broadly in the mélange, where the pillows are usually sheared and intensely deformed.

Abd El-Rahman (2009a) mentioned that Pillow lavas of Wadi Miyah al-Bayda (amygdaloidal pillows) plot in the tholeitic field, whereas the pillow lavas of Wadi Saudi (porphyritic pillows) plot in the calc-alkaline field. The latter are more enriched in Zr and other incompatible elements relative to the former. Both have similar chondrite-normalized REE patterns and have very low Mg-numbers (Abd El-Rahman, 2009a).

In Wadi Ghadir area, sediments of deep water origin are interlayering or resting on top of the pillow basalts, denoting that sedimentation occurred deeply on the sea floor before the emplacement of the ophiolite sequence.

Petrographically, the metabasalt is characterized by plagioclase phenocrysts disposed in fine grained volcanic glassy groundmass forming blastointersertal and porphyritic textures. Phenocrysts are usually plagioclase or hornblende amalgamated in a fine groundmass consisting of plagioclase, hornblend, augit relics and iron oxides, with a wide range of accessory minerals forming porphyritic texture.

4.6 The ophiolitic mélange

The pebbly mudstones are the dominant facies in the sedimentary matrix of the ophiolitic mélange units. They contain a large spectrum of blocks and clasts. The ophiolitic mélange in the study area are characterized petrographically by polymict assemblages of clasts and blocks: dismembered ophiolitic blocks, as well as, a large variety of sedimentary, metamorphic, volcanic and plutonic clasts. All of them are set in a pervasively deformed, fine grained matrix. Small blocks of the plagiogranites (trondhjemite) are scarcely observed in the Ghadir ophiolitic mélange.

El Sharkawy and El-Bayoumi (1979) divided the mélange of the study area into two facies: proximal and distal, in relation to the source of its ophiolitic components. The proximal facies is found mainly to the south of Wadi Ghadir and to the east of Wadi Lawi, whereas the distal facies occurs in the NW part of the area.

The proximal facies is composed of rolled and fragmented rock-debris of highly variable sizes in a matrix of scaly and schistose mudstone. The most abundant components in it, is serpentinized peridotite blocks. Second in abundance are disrupted, fragmented of variably sized dykes that are mixed and squeezed with talc carbonate rock. Other rock types recognized among the debris of the mélange are various types of volcanic rocks, greywackes, quartizites, chert, marble, shale, granite and other plutonic rocks, amphibolites and schistose rocks. Commonly, the pebbles and cobbles are stretched due to deformation.

The distal facies is composed of low grade schists, mostly of pelitic composition. It also contains pebbles of other rock types like those in the proximal facies, but of much less abundance and smaller sizes. Metamorphism and deformation of the distal facies might have been caused by over-riding by slices of proximal facies in the form of nappes, remnants of which are common in various parts of the area. All the contacts between the ophiolitic rocks and the sedimentary matrix are structural ones.

4.7 Metavolcanics

Metavolcanic rocks were found in the study area either as large outcrops or as xenoliths. The former is considered as separated blocks and best seen near the middle part of Wadi Ghadir and at the northwestern part of the study area. Petrographically, felsic, intermediate and mafic metavolcanics are represented in the study area. The felsic to intermediate metavolcanics, occupy a dispersed and limited areas, where mafic metavolcanic rocks are represented only at the western part of Gabal Ghadir. Metavolcanic rocks are widely affected by jointing in different parts of the study area.

4.8 Metagabbro-diorite

Diorite occurs as a foliated mass in the south central part of the area forming an anticlinal fold core (Fig. 5). The rock is highly altered and intensely foliated. It commonly incorporate patches of igneous breccia in which angular boulders of basic dykes are set and embedded in a matrix of white colored igneous material (Fig. 6e). Patches of this mass are gneissose due to the development of alternating quartz and mica-rich light colour bands. Lithologically, the pluton ranges from Metagabbro, diorite to quartz diorite in composition. These rocks consist essentially of plagioclase, hornblende, quartz, ilmenite, apatite and zircon. Tremolite, chlorite and muscovite are secondary minerals. Geochemically, the diorites are calc-alkaline, enriched in alkalies, Rb, Sr, Ba and particularly Zr; and depleted in FeO, MgO, Cr, Ni and Nb, which suggests their continental origin (Basta, 1983).

4.9 Granitic rocks

Ibrahim and Ali (2003) differentiate two types of magmatic granites in the Ghadir area: (i) Quartz diorite and gneissose granodiorite, which have a metaluminous character and were emplaced during pre-plate collision, (ii) perthitic-leucogranite and muscovite-biotite granite that contain large quantity of gabbroic and metavolcanic xenoliths in all stages of interaction and digestion by the granitic material. Also, Dykes of various composition and orientation are cross-cutting and dissecting the granitic rocks (Fig. 6f).

4.10 Lewiwi young gabbros

The Lewiwi young gabbros crop out at the foothills of Gabal Lewiwi as medium- to coarse-grained numerous isolated small masses (Bakhit et al., 2007). These masses were mapped as ophiolitic gabbro and serpentinites (El


Bayoumi, 1980). These masses are elliptical to rounded shaped, intruded by post tectonic granite. The gabbros intrude the adjacent serpentinites with sharp contacts and sometimes exhibit chilled margins.

5 Structural and Kinematic Analysis

We applied the information extracted from the satellite image about these elements in order to better

understanding and assisting in solving the complicated structural pattern of the Ghadir shear belt (Fig. 7a). To enhance the interpretability of the raster elevation, a shaded relief or hillshade, is often generated. Shaded relief is a visually pleasing representation of the terrain (Marston and Jenny, 2015) and is defined as the pattern of light and dark that a surface would show when illuminated from particular angles (ESRI, 2004). It evaluates the relationship between the position of the light source and the direction and steepness of the terrain. Shaded relief map of the study area (Fig. 7b) is generated as grid cell in shades of gray with values ranging from 0º to 254º increasing from black to white. The azimuth and altitude of the illumination source (sun) are 135° and 45°, respectively. The structural pattern of Ghadir shear belt is more complex due to the NE-SW trending tectonic structures in the northeast overprinting the NW-SE structural fabrics in the west of the study area.

Based mainly on the structural deformation features, the structural belt of the area is divided into three main shear belts named, East Ghadir Shear Belt (EGSB) from the eastern border of the study area to Wadi Zabara, West Ghadir Shear Zone (WGSZ) where the high strained area between Wadi Zabara and the western border, and finally Ambaut Shear Belt (ASB) to the northeastern part of the study area (Figs. 7a and b).


Fig. 7. Structural elements in the Ghadir shear belt on (a) Landsat 7 ETM+ false color composite image (bands 7, 4, 2 in R, G, B), (b)

Shade relief map of the study area derived SRTM DEM with an azimuth and altitude of the sun, 135°and 45°.

5.1 The East Ghadir Shear Belt (EGSB): imbricated thrust system


The EGSB contains syn-tectonic granites, syn-tectonic metagabbros, volcaniclastic metasediments, intermediate metavolcanics and late to post tectonic granites (Fig. 5). The EGSB is characterized by a series of thrust faults that developed under green schist facies conditions and imprinted a southstward vergence. The EGSB is characterized by zones up to several meters wide composed of steep shear zone foliation planes (S1). S1 foliations and their associated thrusts are deflected, folded and transposed into the foliation within the high strain zone of these shear zones. In some areas of the EGSB, the pre-existing S1 foliations are cut by a steep shear zone foliation that dips steeply away from the center of the shear zone (S2). Between the high strain zones the fabric is dominated by more gently dipping pre-existing S1 foliation planes. This pattern of deformation allowed us to determine the orientation of the boundaries of high strain zones. S1 foliation is the dominant fabric in the study area (Fig. 5). It is normally moderately dipping to the SW and NE (Fig. 8a) and parallel to the axial plane of F1 upright folds. Outcrop-scale F1 folds are common and affect earlier planar features. S2 mylonitic foliation is folded and crenulated around small-scale folds. Poles to S2 foliation show moderately developed clusters of foliation striking N30-55W and dipping 65-75 toward NE and SW (Fig. 8b).


Fig. 8. Structural data from East Ghadir shear belt: (a) and (b) Equal area lower hemisphere for poles to S1 and S2 foliations, (c) Mineral

lineation along thrust planes, (d) Mineral lineation along strike-slip shear zones (e) Equal area lower hemisphere for poles to foliation from anticlinal

pericline fold in diorites, and (f) Poles to fold axes (F1).

The area to the east of Gabal Ghadir and that between Wadi Lawi and Wadi Zabara (Fig. 2) is defined by a

series of map-scale relatively open, high-amplitude, NNW-trending upright anticlines and synclines. The major


folds are spaced at approximately 10 km intervals and have wavelengths of up to 5 km. The axis of this fold curve from north to NW and plunge 30°/N10-30°W (Fig. 8c). Axial planes of these folds mostly have moderate to steep dips to the NE. Minor folds are abundant in schists (Figs. 9a, 9b and c) and less common in metavolcanics and absent in metagabbros. Tight folds occur in schists and volcaniclastic metasediments whereas low amplitude folds with parallel geometry occur in sheared metavolcanics. The fold axes appear to swing from N- to NNW-trending and change from inclined plunging folds to recumbent folds with reclined geometry where overprinting produces complex refolding.

Fig. 9. (a) NW-plunging open folds in volcaniclastic metasediments, (b) and (c) Overturned folds in volcaniclastic metasediments, (d) strike-slip

shear zone in metavolcanics (e) Subvertical foliation in ductile strike-slip shear zone (f) Subhorizontal and moderately plunging lineations, (g) Thrust

sheets forming an imbricate structure, (h) Pop up structure between a pair of oppositely moving conjugate thrusts, (i) and (j) NE-striking normal faults

in syn-tectonic metagabbros and granites.



Fig. 10. Diorite dome which enhanced by the band ratio composites of (a) (1/4, 3/6, 5/7) and (b) (7/5, 2/3, 5/6) of Landsat 8 image.

Diorites to the south of EGSB occurs as a foliated mass forming an anticlinal pericline fold (Figs. 8e and 10).

It takes a domal shape trending NW-SE and exhibits outward dipping directions. It occupies a considerable area of about 31.4 km

2 and has a well-defined boundary with 21.5 km perimeter which can be easily discriminated

by the tonal contrast between diorites and other rock units in the processed images. A close relationship exists between the thrust faults in volcaniclastic metasediments and metavolcanics and the folded diorite rocks (Fig. 5). The movement occurred along the large and gently dipping thrust faults. The thrusts propagated through the rock from east to west, with rocks at shallow depths sometimes becoming folded ahead of the advancing thrust fault. In this way, the characteristically narrow elongate folds of the diorite rocks are thought to have formed. The fold has steeply dipping beds on their western side and more gently dipping beds on their eastern flank.

A strong brittle-ductile strike-slip shear zones (Fig. 9d) occur along Wadi Ghadir forming mainly fine-grained highly sheared metavolcanics, gabbroic rocks and mylonite schists. They show steeply dipping to subvertical foliation (Fig. 9e) and shallowly south-easterly plunging lineation. Sub-horizontal slickensides overprinted by steeply dipping striae are common (Fig. 9f).

Several sinistral strike slip shear zones were observed in EGSB (Fig. 9c), the main shear zone exist between Gabal Ghadir and Gabal Dob Nia (Fig. 5). It is of about 250 m width, striking N50W and characterized by steeply-dipping foliation, subhorizontal slickenlines and formation of mylonite and cataclasites within the volcaniclastic metasediments and the gabbroic rocks. The core of the shear zone is a sheared gabbroic rocks with a well-developed L-tectonite exhibited by segregation of recrystallized quartz and feldspar-rich domains. The steeply dipping mylonitic foliation has a sigmoidal trend within the strike slip shear zones. Foliation deflects into the shear zones, indicating a sinistral sense of shear. Foliation in the shear zones strikes NW and dips 45–85°NE, and lineation plunges 15–30° toward NW (Fig. 9e). Although the shear zones are meter wide, mylonite zones are less than 20 cm in width (Fig. 9d). Foliation outside the shear zones dips gently to steeply northeastward, and lineations are shallowly plunging northward, although they are significantly variable (Fig. 9f).

Furthermore, away from the high strain zone, there are imbricate thrusts dipping mainly to NE. The shear zone is expresses as a steeply dipping foliation, moderately to steeply plunging mineral lineation (Fig. 8d), surrounded by an imbricate thrust structure (Fig. 9g). Several indicators of oblique thrusting suggest that the shear zone is a transpressional zone with both thrusting and strike-slip movements. In addition, some ductile shear zones have been brittlely reactivated and display fault breccia with fragments of mylonitic material at their core.

The thrusts in the EGSB show a typical sequence of propagation where volcaniclastic metasediments, metavolcanics rocks and schists form piggy-back thrust system. This led to the development of pop-up (Fig. 9h) and flower-like structures where block of rocks or a tectonic wedge moved upward antithetically between fore (main) thrust and back thrust. The uplifted hanging wall block between a pair of oppositely moving conjugate thrusts is the pop-up (Fig. 9g). Such structures are usually associated with flower-like cleavage and tile-like piling of subsidiary thrust sheets forming an imbricate structure.

Two systems of high angle NW–SE and NE–SW of coeval hectometric to kilometric brittle normal fault zones, as well as hectometric joints are widely distributed within granites and metagabbro-diorite (Figs. 9i and j). The NE–SW faults dominate and dip 50–75° with variable dip sense and slickenlines show a main dip-slip displacement with variable throw. They represent the latest brittle deformation event in Wadi Ghadir area. The different structures that developed in syn-tectonic granites include extensional faults, oblique reverse and strike-slip faults that markedly displace dikes in granitic rocks.

5.2 The West Ghadir Shear Zone (WGSZ): transpressional structures

The WGSZ consists of an NNW-SSE striking zone of deformation that connects the EGSB to the NNW-striking structures in Hafafit core complex (HCC). The WGSZ is highly strained and comprises an association of highly tectonized mafic metavolcanics, serpentinites, talc-carbonate and metagabbros-diorites embedded in a matrix of volcaniclastic metasediments and schists. The high strain zone of WGSZ represents a shear zone of ca. 5-10km in width, composed mainly of mylonites, protomylonites and phyllonites, predominant sinistral movement with brittle–ductile reactivations.

Foliations in WGSZ range from sub-vertical (S2, Figs. 11a, b) to steeply dipping toward NE and ENE (S1, Fig. 11c and 12 a) with an average orientation being N17W. Foliation is generally parallel to the main trend of the shear zone that varies in intensity, spacing and homogeneity (Figs. 11a-c). S1 foliation (Fig. 12a) is mainly defined by segregation of quartz/feldspar-rich microlithons from phyllosilicate-rich foliation planes in volcaniclastic metasediments, and by the alignment of actinolite in metavolcanic rocks. Prevalence of rock structure increases toward narrow shear zones and in some cases their cores display highly recrystallized and lineated units defined by ―rods‖ of recrystallized quartz and feldspars (e.g. mylonite and L-tectonite in granitic gneiss) that moderately plunge toward SE (L1). In gneisses, quartz typically consists of polycrystalline aggregates of homogeneous fine-grained elongated crystals. These aggregates locally develop an oblique foliation. Feldspar porphyroclasts present microfracturing and cataclasis as well as recrystallization only


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localized in tails. Within the high strain zones, the strike of subvertical mylonitic foliation (S2; Figs. 11a and b) and trend of subhorizontal elongation lineation (L2; Figs 11e, f and 12d) are parallel to the trace of the shear zone. Foliation planes show slight variations in dip direction (to the north and to the south) and in inclination values. The plunge of lineations also varies, and may be gently to the south, horizontal, or gently dipping to the north.

Fig. 11. (a) and (b) Subvertical foliation (S2) within the hgih strain zone of West Ghadir shear zone, (c) ENE-dipping foliation (S1) in West Ghadir

shear zone (WGSZ), (D) Moderately plunging lineation (L1) along thrust planes, (e) and (f) Subhorizontal lineation (L2) in the high strain zone of

WGSZ, (g) Thrusting of volcaniclastic metasediments over Gabal Zabara granite gneisses, (h) ENE-dipping thrust mark the tectonic contacts between

volcaniclastic metasediments and tserpentintes, (j) Pop up structure between a pair of oppositely moving conjugate thrusts, (d) (e) and (f) Lineation

plunging gently to SE in mylonite schists.

The prominent mineral lineation on foliation planes is defined by elongated quartz and actinolite crystals and

show a girdle distribution with mean orientation plunging SE at moderate to steep angles in oblique shear zones and thrusts (L1) and plunging SSE at a low angle to subhorizontal (L2) in high strain zone (Figs. 11e, f, 12c and d). Asymmetric minor folds, boudins and porphyroclasts, shear band and S/C fabrics are well-developed on horizontal planes (orthogonal to foliation and parallel to lineation) and consistently indicate sinistral kinematics.

Thrust faults are the most common structures in the WGSZ. The talc-carbonate and steeply dipping lenses and sheets of serpentinites are emplaced along thrust planes and elongated in NNW–SSE direction parallel to the


penetrative foliation of the volcaniclastic metasediments matrix. The thrust contacts between, granite gneisses, tectonized serpentinites and other mélange components (Fig. 11g) is always decorated and marked by mylonite schists. The thrusts in the WGSZ show a typical sequence of propagation where each successive thrust develops in the footwall of the previous thrust in piggy-back thrust manner (Fig. 11h). This led to the growth of flower-like and pop-up structures (Fig. 11j).

Fig. 12. Structural data from West Ghadir Shear Zone: (a) Equal area lower hemisphere for poles to S1 foliation showing moderately

developed clusters with non-uniform distribution, (b) Equal area lower hemisphere for poles to S2 foliation, (c) Mineral lineation (L1) along the planes

of thrusts, (d) Mineral lineation (L2) along strike slip shear zones, and (e) Poles to fold axes (F2)


Map-scale (up to 10 km long) reclined folds (Fig. 13a-c) occur close to the eastern boundary of WGSZ. The folds have fold axes plunging down the dip of the axial surface. The axial plane dips between 55° and 65° and the pitch of the hinge line on the axial plane is more than 80°. The core of the folds contains serpentinites whereas, their limbs consist of metavolcanics rocks and volcaniclastic metasediments (Fig. 13b). The foliation in the overturned folds dip in the same direction on both sides of the axial plane because one of those limbs being rotated. The forelimb of the synform get overturned with development of reverse shears sub-parallel to back limbs (Fig. 13b). Map scale Upright fold exist in the southern bank of Wadi Lawi (Fig. 5) with axis trending NE-SW perpendicular to the axes of major recumbent and minor folds in EGSB and WGSZ. Also minor folds with vertical axes occur within the high strain zones of WGSZ.

Fig. 13. (a) Landsat 7 ETM+ Satellite image showing map-scale (up to 10 km long) reclined folds close to the eastern boundary of West Ghadir

shear zone, (b) panorama showing part of the reclined fold in West Ghadir shear zone, and (c) sketch showing attitude of reclined folds.

Asymmetric, westerly verging folds and asymmetric boudins on sub-vertical planes (orthogonal to both foliation and sub-horizontal lineation) indicate reverse slip kinematics (with top-to-the-SW movement sense) where down-dipping lineation on thrust planes becomes important. However, the thrust-related structures far outweigh in intensity than sinistral strike slip features suggesting an overall dominance of thrusting over strike slip shearing in WGSZ. Intense shearing along the WGSZ resulted in a predominated schistose rocks of the sheared lowermost hanging wall part.

Minor F2 folds (Fig. 14a) are abundant in WGSZ, with fold axes plunging at low to steep angles to SSW (Fig. 14b) and the prevailing sub-vertical foliation being axial planar to the fold structures. Quartz veins have been folded and strongly stretched parallel to the main shear plane resulting in development of boudions, en-echeoln structures (Fig. 14c), asymmetric recrystallized tails and meso-scale fish structures, all indicating sinistral slip. Meso-scale overturned folds developed within the closure of the major overturned synform (Fig. 14d).


Fig. 14. (a) and (b) Asymmetric minor folds and folded quartz veins in volcaniclastic metasediments, (c) folded and strongly stretched quartz veins

with development of boudions and meso-scale fish structures, (d) SSE-plunging meso-scale overturned fold developed within the closure of the major

overturned syncline.


Fig. 15. Structural data from Ambaut shear belt: (a) and (b) Equal area lower hemisphere for poles to NW- (S2) and NE-striking (S3)

foliations showing moderately developed clusters, (c) Mineral lineation along the planes of thrusts, and (d) Mineral lineation along strike-slip shear

zones.


Fig. 16. Ambaut Shear Belt: (a) Thrusting of volcaniclastic metasediements over metavolcanics along SE-dipping thrust and development of

steeply dipping mylonitic foliation and mylonite schists within dextral strike slip shear zone, (b) Shallowly south-westerly plunging mineral lineation

and moderately to steeply plunging down-dip mineral lineation, (c) Stretched pebbles in metamudstone, (d) Thrusting of alc-carbonate serpentinites

over volcaniclastic metasediements along SE-dipping thrust.

L–S tectonic fabrics are pervasively developed along the WGSZ, where both subvertical foliation and sub-horizontal stretching lineation are well developed. The foliation, formed by the preferred orientation of biotite,


quartz, and feldspar ribbons, is nearly parallel to the regional fabric. Stretching lineation, formed by elongation of quartz, feldspar, and biotite, long tails of porphyroclasts. The S-L tectonite fabric in granitic gneisses change to mylonite fabric in the high strain zones where S/C structures, asymmetric recrystallized tails on porphyroclasts and asymmetric northerly verging folds dominate in subvertical planes (orthogonal to foliation and sub-parallel to lineation) indicating a strong component of thrust sense top-to-SW shear. The effects of this shearing are strongest to the south of Gabal Lawi and along Wadi Um Abid where there are a southwest verging major thrusts in the WGSZ. These thrusts (Fig. 16a) placed low-grade rocks (metavolcanics rocks, volcanicastic metasediments and serpentinites) over amphibolite-grade basement gneisses (grantic genisses and hornblende gneisses of HCC).

Within the high strain zone of WGSZ, tectonite fabrics, rods, mullions, boudin pods, elongate enclaves, and persistent linear features all plunging gently to the SSE. Mesoscopic folds on sub-vertical planes (orthogonal to foliation and parallel to stretching lineation) typically show asymmetric northwesterly verging geometry with axial planes inclined southwest. Minor fold axes plunge gently to moderately due NNW or SSE. The strong, steeply dipping planar fabric is defined by elongated amphibole crystals separated by recrystallized quartz (and plagioclase) ribbons; quartz exhibits strong undulose extinction and subgrain development. A few sigmoidal-shaped augens composed of highly strained quartz (±plagioclase) indicate sinistral displacement and represent strain focusing during deformation.

The metamorphic grade contrast along Nugrus shear zone (NSZ) and WGSZ constitutes the main estimated displacement along it. The footwall of WGSZ is Migif- Hafafit gneisses deemed to be of high temperature – low pressure amphibolite facies (El-Ramly et al., 1984, 1993). Numerically, the metamorphic conditions were estimated by (Asran and Kabesh, 2003) as 720–740 ºC for Migif-Hafafit amphibolites and 800–820ºC for associated migmatites, both under pressures of 6–7 kbars. The pressure conditions were confirmed as 6–8 kbars by (Abd El-Naby and Frisch, 2006). On the other hand, Numerical estimations of the greenschist facies in the Central Eastern Desert (CED) show about 450 ºC at 4 kbars pressure (Fritz et al., 2002). This gives a 3 ± 1 kbar pressure difference across the NSZ corresponding to a loss of section measuring 10 ± 3.5 km.

5.3 The Ambaut Shear Belt (ASB): an imbricate thrust belt

The metavolcanics, volcaniclastic metasediments and ophiolitic metagabbros wedge out to the northeast where sinistral shearing along the WGSZ is partitioned into a number of splays forming an imbricate thrust fan and thrust duplex characterizing the frontal ramp of the WGSZ. The Ambaut Shear belt (ASB) is elongated in a NE direction and bounded by EGSB and WGSZ and occupied mainly by volcaniclastic metasediments, highly tectonized serpentinites, intermediate metavolcanics, metagabbro–diorites and intruded by syn-tectonic granites. The dominant foliations (S3) strike NNE-SSW close to the WGSZ but curve into a NE-SW direction around Wadi Umm Tundubah and Wadi Ambaut (Fig. 5). Chlorite and actinolte schists are strongly foliated, with the foliation defined by the alignment of chlorite, sericite, quartz and talc in schist and aligned actinolite and chlorite in metavolcanic rocks. Intensity of cleavages varies throughout the ASB from volcaniclastic metasediments to metavolcanic rocks. Poles to NW-striking S2 foliation show moderately developed clusters (Fig. 15a). Also, poles to S3 foliation showing moderately developed clusters and tend to strike N20°–65°E with dips of 52°–65° toward NW and SE (Fig. 15b). Steep crenulation cleavages (65°–85°) with an approximate NE-SW strike are developed parallel to the axial planes of NE-plunging major folds in intensely foliated schists. In narrower and discrete high-strain zones, steeply dipping mylonitic foliation (S3) (Fig. 16a) has a sigmoidal trend within the strike slip shear zones and folded into small-scale folds. Moderately to steeply plunging down-dip mineral lineation (Fig. 15c, 16b and 16c) along planes of intense mylonitic foliation in mylonite schists is defined by quartz ribbons and feldspar porphyroclasts and amphibole needles. Shallowly south-westerly plunging mineral lineation are confined to steeply dipping strike-slip shear zones (Fig. 15d).

The ASB is marked by a series of map-scale relatively open NE-trending anticlines and synclines spaced at approximately 2-4 km intervals and have wavelengths of up to 3 km. Axes of these folds are plunging moderately to the NE and less commonly SW whereas, axial planes are moderately to steeply dipping to the NW. Minor folds with low amplitudes are common in schist, volcaniclastic metasediments and sheared metavolcanic rocks.

The ASB is marked by a series of oppositely southwestward (Fig. 16d) and northwestward dipping imbricate thrusts marked by SE and NW-side-up thrust movements and form pop-up or flower like structures. These thrusts mark the sheared contacts between metavolcanics and metagabbro–diorite and the overlying schists and volcaniclastic metasediments.

6 Shear Kinematics

The Ghadir shear belt consists of three structural units: the East Ghadir shear belt (EGSB), West Ghadir shear

zone (WGSZ) and Ambaut shear belt (ASB). There is a clear transition along Ghadir shear belt from low strain zone in the east to high strain zone represented by WGSZ. EGSB expresses as an imbricate structure, consisting of several parallel thrusts, rather than a near-vertical strike-slip shear zone. EGSB is a simple piggyback imbricate thrust system with a unidirectional sense of thrust propagation to the southwest (Fig. 11b). The imbricate thrust slices in EGSB is overprinted by NNW-trending sinistral strike-slip shear zone of WGSZ that


changed to an imbricate thrust fan and flower-like structure to the north and the northeast occupying the ASB. Also, there is a marked change along the ASB from sinistral strike-slip shearing along WGSZ to dextral shearing along the main stream of Wadi Umm Taundubah to thrusting in the northern bank of Wadi Ambaut. The kinematic transition along Ghadir shear belt is consistent with separate strike-slip and thrust-sense shear zones. Abundant shear criteria give a top-to-the-southwest sense of thrusting in EGSB (Figs. 17a-d). Intensity of deformation dipping angle of thrusts and plunging of mineral lineation in EGSB increase toward and close to WGSZ.

Fig. 17. (a-d) Shear criteria showing a top-to-the-southwest sense of thrusting in East Ghadir shear belt, (a), (b) and (c) Asymmetric s-type

ultramafic and serpentinite porphyroclasts, (d) Folded and faulted quartz vein, (e) Deflection of foliation in metasediments indicating sinistral sense of

shearing, West Ghadir shear zone, (f and g) Asymmetric serpentinite porphyroclasts in mylonite schists indicating sinistral sense of shearing, West

Ghadir shear zone, (h and I) Asymmetric serpentinite porphyroclasts in metasediments indicating dextral sense of shearing.

The WGSZ is the northern part of the major Nugrus sinistral strike-slip shear zone separating Ghadir shear

belt to the northeast from the Hafafit core complex in the southwest. Several kinematic indicators of reverse sinistral (i.e., oblique) slip suggest that the WGSZ is a transpressional imbricate zone with both thrusting and strike-slip movements. The high strain zone within the WGSZ is a vertical ductile deformation zone of about 10 km width and having a strike direction of N17-20°W. The main shear sense indicators include deflected foliation (Fig. 17e), asymmetric porphyroclasts (σ type; Hanmer and Passchier, 1991) (Figs. 17f-g and 18a-f), asymmetric pressure shadows, asymmetric boudins (Goscombe and Passchier, 2003), S/C textures (Fig. 17g) and segmoids. The porphyroclasts have mantle of finer materials and extends out into two winged structures that orient parallel to a mylonite foliation, the wings often showing a characteristic curvature (Figs. 18a-f). Sigmoids


are lozenge-shaped lenses differ in composition from the host rock and also, lack the central rigid grain of mantled porphyroclasts (Passchier and Coelho, 2006). The sigma structures in Granite gneisses indicate top up to NW characterizing a sinistral compressional transport. Also, asymmetric folds, with fold axes at a high angle to foliation, may indicate the sense of shear (Carreras et al., 2005). The observed macro- and microstructures within the high strain zone are all indicative of sinistral movement through WGSZ. Also, the observed kinematic indicators involve oblique slip vector on shear zones parallel to strongly plunging down-dip slip lineations, indicating top-to SW reverse movements. The authors propose a distinct explanation: a previous west-to southwestward thrusting followed by transpressional sinistral shearing that formed a NW shallowly plunging stretching lineation.

Fig. 18. Shear sense indicators from West Ghadir Shear Zone. (a-f) and Ambaut Shear Belt (g-h). Low-grade mylonite derived from

granite (a-e) and metavolcanics (f-h). Porphyroclasts are composed of feldspar and quartz. The matrix contains fine-grained plagioclase, quartz,


https://structuredatabase.wordpress.com/ductile-shear-sense-indicators/#ref

https://structuredatabase.wordpress.com/ductile-shear-sense-indicators/#ref

muscovite and biotite. Sense of shear is sinistral (a-f) and dextral (g-h) as can be deduced from stair stepping to the left across the porphyroclasts of

quartz are with undulose extinction and apparent recrystallization to very small grains mainly concentrated in the strain shadows.

A fan-like structure exists at the northern termination of WGSZ and consists of oppositely dipping foliations

to the SE in its southeastern limb, and to the NW and ESE in the northwestern splays. The transpressive character of deformation in the WGSZ is shown by the coexistence of strike-slip and compressive structures.

The Ambaut shear belt is characterized by a bidirectional thrust system consisting of both thrusts and back thrusts, pop-up structures and flower-like structures. Two sets of striations are developed with southwestward plunge angles of 7° and 60°, respectively, implying that the ASB underwent at least two stages of oblique movements. Both reverse and strike-slip components occurred in the ASB. Steeply NW- and NE-dipping foliation and Pop-up structure indicate southeast and northwest-side-up reverse movements along fore- and back-thrusts respectively. Small scale high strain zones are marked by S/C fabrics, asymmetric deflections of foliation planes, asymmetric tails on porphyroclasts (Figs. 17h-I and 18g-h), boudinaged and folded quartz veins indicating sinistral sense of movement overprinted by dextral shear sense.

7 Discussion

7.1 Tectonic model for deformation

The oldest phase (D1) in the Central Eastern Desert is an early shortening phase associated with arc collisions (740–660 Ma) and characterized by NW-directed imbrication of Pan-African nappes (Fig. 19a) with the development of SSE-plunging stretching lineations (Greiling et al., 1994). D2 is SW- and NE-directed thrusting associated with terrane assembly where regional nappe transport toward the SW is common in the Eastern Desert of Egypt especially around core complexes and major shear zones (Greiling, 1997; Fritz et al., 1996; Shalaby et al., 2005; Abdeen and Abdelghaffar, 2011; Abd El-Wahed, 2008, 2014). D3 is a sinistral transpression associated with continued oblique convergence (Fritz et al., 1996; Makroum, 2001, 2017; Abd El-Wahed, 2008, 2014; Abd El-Wahed and Kamh, 2010; Abdeen et al., 2014; Abd El-Wahed et al., 2016; Makroum, 2017). D4 is dextral transpression in a conjugate with D3 (Shalaby et al., 2005; Abd El-Wahed and Kamh, 2010; Abd El-Wahed et al., 2016). D5 include later events.


Fig. 19. Block diagrams showing structural evolution of Ghadir shear belt (explanations in the text).

Structures within the Ghadir shear belt can be described in terms of three main ductile to semi-ductile

deformational episodes (D1–D3) associated with the collision between East and West Gondwana. D1 structures (Fig. 19a) include SW-propagated imbricate thrusts and produced NW-trending major F1 folds in vlocanicalstic


metasediments and metavolcanic rocks. S1 foliation is associated with movement along southwestward imbricate thrust system and accompanied by moderately SE- plunging mineral lineation. In EGSB, D2 deformation is occurred at ~640–560 Ma ago and manifested by the development of the steeply dipping, NW-striking shear zones, NW-trending major F2 with NW-plunging axes, F2 minor asymmetric folds of different scales, F2 upright fold trending NE-SW, S2 axial planar cleavage, L2 subhorizontal to moderately plunging mineral stretching lineation.

D2 formed an arcuate-shaped structure constituting the West Ghadir shear zone (~595 Ma) and Ambaut imbricate thrust belt (ASB). D2 structures (Fig. 19b) formed during the NNW-trending sinistral shearing along WGSZ and its arcuate thrust fan in ASB. The high strain zone within the WGSZ is expressed as steeply dipping strike-slip shear zones with dominantly sub-vertical foliations and sub-horizontal lineations. In the moderately deformed zones in WGSZ, folds have moderately plunging axes and a pervasive NVW-SSE striking axial planar slaty cleavage (S3), defined by an alignment of white mica, chlorite and elongate quartz; S2 dips 60–80° to either the SW or NE. In Ghadir shear belt, two generations of fabric (S1 and S2) were recognized around the major plunging folds. S2 is locally recognized in low strain areas, but is commonly transposed and overprinted by S2 in high strain zones, which is the dominant fabric throughout the WGSZ (Fig. 19b). S2 pervasive NNW-SSE striking axial planar slaty cleavage (S2), defined by an alignment of white mica, chlorite and elongate quartz; S2 dips 60–80° to either the SW or NE, and is associated with a subhorizontal to moderately stretching lineation (L2).

NNW-SSW striking S2 fabric in WGSZ have been progressively rotated into a NE-SW orientation (arcuate zone) in the Ambaut shear belt (ASB). S3 striking NE-SE and dipping moderately to steeply to SE and NW. Thrust faults are commonly associated with a number of large scale asymmetrical ENE-plunging F3 folds (Fig. 19c). Northwestward imbricate thrusting in the northwestern part of ASB and southeastern imbricate thrusting in southeastern part form positive flower-like pattern in ASB. These thrusts, considered coeval with a consistent down-dip mineral elongation lineation (L3), indicate a general top-to-the-northwest and southeast sense of reverse shear.

D2 structures formed during the NW-trending sinistral shearing along EGSB and WGSZ and the imbricate thrust in ASB thrust fan are strongly overprinted and dislocated by another imbricate fan (Abd El-Wahed et al., 2016) which formed during oblique NE-trending dextral shearing (D3) along the Sukari shear zone to the north of the study area (Fig. 19c). Abd El-Wahed et al. (2016) stated that the tightness and steepness of major folds and curvature of the thrust fan in ASB increases with increasing D3 strain. In narrower and discrete high-strain zones, steeply dipping mylonitic foliation (S3, 65°–85°) developed parallel to the axial planes of NE-plunging major folds (F3) in intensely foliated schists. The geometrical relationship between D2 major folds in the EGSB and WGSZ and the D3 NNE-dextral shearing along ASB indicates that the two events represent stages within a progressive deformation episode as indicated by the cessation of the D3 NNE-trending ASB along the inflection planes of D2 NNW-trending WGSZ (Fig. 15c). Sinistral transpression along the NW-trending shear zone in the CED (660–645 Ma) was accompanied and followed by NE-trending dextral transpression (645–540 Ma).

The latest structures in the Ghadir shear belt normal, strike-slip faults and major fractures in syn- to post tectonic granites which probably associated with intrusion of post tectonic granites.

7.2 New structural evidence for transpression

The data presented here display strong evidence for compressive and transpressive deformation affecting Ghadir shear belt. Mega scale structures are obviously visible on satellite scene (available at http://maps.google. com/). Lineament analysis reveals linear structures within the Ghadir shear belt such as kilometre-scale undulations and tens of kilometres wide S-shaped features. Field investigations confirm our interpretation of the mega scale structures presented in Figs. 5 and 7.

EGSB is characterized by an asymmetric semi flower-like shape caused by southwestward thrusting instead of the typical features of a pop-up structure. The EGSB is almost a deformation zone with a kinematics combining both southwest-directed thrusting and sinistral strike-slip shear zones with the development of a series of NNW-trending folds with southwestward vergence. Furthermore, in this belt many low-angle thrusts are developed in volcaniclastic metasediments. Since there are more number of thrusts and the folds are much tighter in the western part of EGSB as compared to those in the eastern part, the former region is considered to have experienced much stronger deformation. These thrusts and the asymmetric folds in the western part of EGSB jointly show the southwestward thrusting and display a semi-flower-like structure.

The most prominent outcrop-scale deformation features in Ghadir shear belt, including folds, metamorphic fabric, and associated lineations, are the result of regional, northwest-directed, sinistral transpression assigned to D2. On outcrops along Wadi Lawi, Zabara and Fegas, the core of WGSZ expresses as a near-vertical ductile deformation zone rather than an imbricate structure as in the east. Several kinematic indicators of oblique thrusting suggest that the WGSZ is a transpressional zone with both thrusting and strike-slip movements. The highly deformed zone is a vertical ductile deformation zone of about 10 km width and a strike direction of N17W. Kinematic indicators, such as a sinistral porphyroclast system, reclined and vertical folds with hinges plunging to NNW at an angle of 65 in the volcaniclastic metasediments also indicate the strike-slip movement of the WGSZ. However, a series of asymmetric folds parallel to the strike of the deformation zone resulted from the compression perpendicular to the deformation zone, indicating a thrust movement within the shear zone. We


http://maps.google/

thus infer that the WGSZ was formed in a transpressive stress field rather than a compressive one. The transpressive stress field is expressed most clearly along Wadi Um Abid. Here, the shear zone is steep plane with a dip of 85 to N20W. Two sets of striations are developed with northwestward pitch angles of 15 and 42, respectively, implying that the WGSZ underwent at least two stages of sinistral strike-slip oblique thrusting. Top-up-to-the-NNW and less common -SSE sense-of-shear indicators are consistent with interpretations of WGSZ as a ductile sinistral strike-slip shear zone with scarce dextral sense of shear.

Controlled by the thrust and back-thrust on the southeastern and northwestern boundaries, the ASB form pop-up structure constituting the northeastern bidirectional thrust system and positive flower-like structure in Ghadir area. The ASB is characterized by two opposite thrusting directions, in the northwestern part, for example, their dip directions are to the southeast, indicating a northwestward thrusting, whereas in the southeastern part they inclined to the northwest, suggesting a southeastward thrusting (Fig. 19c), and finally, in the middle part, they are nearly upright. These structural characteristics in this section form the positive flowerlike pattern as a whole. The southeastward-thrust portion takes two-thirds of the belt, whereas the northwestward-thrust part accounts for the remaining one third. The deformational structures in the southeastern part are dominated by linear asymmetric folds with NE-SW-trending axial planes and there are more thrusts in the southeastern part than the northwestern part. From the structural style, the tightness of the folds in the southeastern part indicate that the deformational intensity of the southeastern part is inferred to be a little stronger than that of the northwestern part.

The main structural feature in Ghadir shear belt is the change from thrusting in EGSB to strike-slip shearing within WGSZ, i.e. towards the major sinistral shear zone separating Ghadir shear belt from Hafafit core complex (HCC). Although the effects of strike-slip shearing, in the form of sub-horizontal mineral lineation, shear bands, boudinage, and other shear criteria are most clear within the WGSZ, we consider that the whole shear belt was subject to prolonged transpression, with increasing shear components towards the west.

Our field observations and structural study in the WGSZ shows a strong shallowly plunging stretching lineation on almost all subvertical foliation planes associated with a shearing component, consistent with major strike-slip deformation. This in agreement with the currently proposed models for the Nugrus Shear Zone (NSZ) as a major sinistral strike-slip shear zone in the Central Eastern Desert (CED) of Egypt. Structures indicating subvertical movement such as steeply plunging stretching lineations have been recorded in shear zones at sheared contacts between metavolcanic and metagabbroic rocks. Furthermore, pervasive fabrics displaying strike-slip movements that show dominant sinistral and subordinate dextral kinematics seem to correlate more with a complex transpression zone with a dominant horizontal shearing component, similar to that suggested for the NFS.

This study favor a transpressional model with subhorizontal general shear flow instead of a simple shear model for following reasons: (i) Simple shear flow requires a shear zone of constant thickness and has a straight parallel boundaries and. The WGSZ has no parallel boundaries and show variable thickness, (ii) The existence of shear zones with dominant sinistral and subordinate dextral components is typical for shear belt that deform in a bulk non-coaxial system (e.g., Gagnon et al., 2016). (iii) WGSZ shows a relationship to shear zones in which the length of the shear zone boundaries increases and the thickness of the shear zone decreases with ongoing shear deformation (e.g. Ramsay and Graham, 1970). (iv) The deformation intensity in WGSZ is highly irregular and varies from weak to very strong. This variable distribution of deformation is typical for shear belt with strain partitioning between low-strain and shear zones, where the low-strain shear belt reveals the non-coaxial component of the deformation (Pennacchionia and Mancktelow, 2007).

Structural analysis in Ghadir shear belt revealed a series of ~NW-SE kilometer-scale open reclined folds, which are bounded by ~NW–SE sinistral mylonite zones with steeply dipping foliation and shallowly SE- to SSE-plunging stretching lineations. The combined sinistral strike-slip shear zones, reverse shearing and folding structures have been interpreted to result from a phase of transpressional deformation in response to oblique squeezing of Arabian Nubian Shield between East and West Gondwana. The parallelism between striking direction of mylonitic foliation and axial plane of major folds indicate that transpressional deformation has been partitioned into a simple shear component in the mylonite zones and a pure shear component in the fold zones (e.g. Schulmann et al., 2003).

8 Concluding Remarks

1. Wadi Ghadir area represents a widely distribuated stretch of Neoproterozoic ophiolitic mélange

constituted mainly of allocthonous ophiolitic fragments mixed firmly and incorporated in a sheared matrix, as well as, other different mappable units. The most extensive and widely disseminated unit of the area is the mélange assemblages.

2. The structural belt of the area will be divided into three main shear belts named, East Ghadir Shear Belt (EGSB), West Ghadir Shear Zone (WGSZ), and Ambaut Shear Belt (ASB). The main structural feature in Ghadir shear belt is the change from thrusting in EGSB to strike-slip shearing within WGSZ. In the EGSB, foliations associated thrusts are deflected, folded and transposed into the foliation within the high strain zone of the NW-striking shear zones. Foliation is steeply SE- to E-dipping parallel to the axial plane of NNW- and NE-trending folds folds. The thrusts in the EGSB show a typical sequence of propagation forming piggy-back thrust


system. Pop-up structures formed where the block of deformed rocks moved upward antithetically between a pair of oppositely moving conjugate thrusts.

3. The WGSZ comprises an association of metavolcanics, serpentinites, talc-carbonate and metagabbros-diorites embedded in a matrix of volcaniclastic metasediments and schists. The West Ghadir shear zone (WGSZ) is the frontal part of the Nugrus shear zone separating the Hafafit core complex in southwest from Ghadir ophiolitic mélange to the northeast. WGSZ is a NNW-trending transpressional shear zone where reverse oblique slip branching arrays propagate from the WGSZ forming the NE-trending Ambaut imbricate thrust fan (ASB). The thrusts in the WGSZ show a typical sequence of propagation leading to the growth of flower-like and pop-up structures. Map-scale (up to 10 km long) reclined folds mark the boundary between EGSB and WGSZ. Thrusting dominates over strike-slip shearing in WGSZ where both steeply dipping and sub-horizontal stretching lineation are well developed. The main ductile deformation in the WGSZ was partitioned into a sub-horizontal sinistral strike-slip and a top-up-to-the-NNW and less common -SSE sense of shear. Strongly plunging down-dip slip lineations along thrust planes indicate top-to SW reverse movements.

4. The Ambaut Shear belt (ASB) is elongated parallel to the axial planes of NE-plunging major folds. Shallowly south-westerly plunging mineral lineation are confined to steeply dipping strike-slip shear zones. The ASB is characterized by bidirectional thrust system forming pop-up structures that marked by southeast and northwest-side-up reverse movements along fore- and back-thrusts respectively.

5. The D2-D3 deformational phases, of ductile nature, is associated with an average E–W shortening and sinistral movements which caused a main sinistral strike-slip displacement with a strongest thrust component toward the SW in the WGSZ and a dextral strike-slip displacement in the ASB. This deformational phase associated with sinistral movement in the WGSZ is represented by a N20°W-trending shear zone, with intermediate to strong dips to SW and two sets of stretching lineations. The first one plunging to E to SE thus giving a thrust component toward the SW. The second one plunging moderately to SSE is compatible with a sinistral movements of the shear zones.

6. The combined sinistral strike-slip shear zones, reverse shearing and folding structures have been interpreted to result from a phase of transpressional where East and West-Gondwana collided and the Arabian Nubian Shield was obliquely squeezed.

Acknowledgments

The authors would like to express their deep thanks to Dr. Ismail Thabet, Geology Department, Tanta

University, Tanta, Egypt, for his kind support in the field. We are grateful to Dr. Degan Shu (Editor-in-Chief) and the anonymous reviewers for their constructive comments, corrections and helpful suggestions which greatly improved the manuscript

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About the first author and the corresponding author

MOHAMED Abd El-Wahed is a full professor of structural geology and geotectonics at the Geology Department, Faculty of Science, Tanta University. He has graduated from the same department 1988, with honor degree, and got the M.Sc. and Ph.D. degrees in 1995 and 2005, respectively. He joined the Geology Department, Faculty of Science, Omar Al Mukhtar University, Libya, during the period 15/10/ 2005 to 30/8/2012. He is judge member of the Egyptian Promotion Committee. He has worked in many field-related sub-disciplines of Earth Sciences including structural and microstructural analysis, geologic mapping, strain analysis, tectonic geomorphology, crustal deformation and remote sensing. He used all these fields to study key areas in the Egyptian Nubian Shield, and to decipher their deformation history. MOHAMED Abd El-Wahed has co-published 53 research articles in national and international indexed and refereed journals and authored three Arabic books.

Email: [email protected], [email protected]; Phone:

+201273960983


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Transpressive Structures in the Ghadir Shear Belt, Eastern ......2019/05/16 · MOHAMED Abd El-Wahed*, SAMIR Kamh, MAHMOUD Ashmawy and ALY Shebl Geology Department, Faculty of Science,