ORIGINAL PAPER Synoblique convergent and extensional deformation and metamorphism in the Neoproterozoic rocks along Wadi Fatira shear zone, Northern Eastern Desert, Egypt Mohamed Abd El-Wahed & Mohamed Abu Anbar Received: 19 July 2008 / Accepted: 8 October 2008 / Published online: 16 December 2008 # Saudi Society for Geosciences 2008 Abstract 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 sinis- tral 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 tona- lites. 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 inter- mediate and acidic metatuffs. Contractional and transpres- sional 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. Arab J Geosci (2009) 2:29–52 DOI 10.1007/s12517-008-0016-y M. Abd El-Wahed (*) : M. Abu Anbar Geology Department, Faculty of Science, Tanta University, Tanta 31527, Egypt e-mail: [email protected]
Transcript
ORIGINAL PAPER
Synoblique convergent and extensional deformationand metamorphism in the Neoproterozoic rocksalong Wadi Fatira shear zone, Northern EasternDesert, Egypt
Mohamed Abd El-Wahed & Mohamed Abu Anbar
Received: 19 July 2008 /Accepted: 8 October 2008 / Published online: 16 December 2008# Saudi Society for Geosciences 2008
Abstract The Wadi Fatira area occurs at the southernmargin of the Northern Eastern Desert (NED) of Egypt andis occupied by highly sheared metavolcanics tectonicallyalternated with banded iron formations and intruded byBarud tonalite–granodiorite, post-tectonic gabbroic andgranitic intrusions. Detailed structural investigation showedthat the schists and migmatitic amphibolites are formed byshearing in metavolcanics and syntectonic Barud tonalite–granodiorite due to movement along the Wadi Fatira shearzone (WFSZ). This shear zone starts as a NW–SE strikingfault along Wadi Barud Al Azraq and the Eastern part ofWadi Fatira and turns to a E–W trending fault to the northof Wadi Fatira. Microstructural shear sense indicators suchas asymmetric geometry of porphyroclasts such as σ-typeand asymmetric folds deforming fine-grained bands whichare frequently found around porphyroclasts indicate sinis-tral sense of shearing along the WFSZ. This shear zone ischaracterized by transitions from local convergence to localextension along their E–W and NW–SE trending parts,respectively. The NW–SE part of the WFSZ is of about200 m in width and characterized by synmagmatic
extensional features such as intrusion of synkinematictonalite, creation of NE–SE trending normal faults, andformation of migmatitic amphibolites and schlieric tona-lites. This part of the shear zone is metamorphosed undersynthermal peak metamorphic conditions (725°C at 2–4 kbar). The E–W compressional part of the WFSZ is up to3 km in width and composed of hornblende, chlorite,actinolite, and biotite schists together with sheared inter-mediate and acidic metatuffs. Contractional and transpres-sional structures in this part of the WFSZ include E–Wtrending major asymmetrical anticline and syncline, nearlyvertical foliation and steeply pitching stretching lineations,NNE dipping minor thrusts, and minor intrafolial folds withtheir hinges parallel to the stretching lineation. P–Testimates using mineral analyses of plagioclase andhornblende from schists and foliated metavolcanics indicateprograde metamorphism under medium-grade amphibolitefacies (500–600°C at 3–7 kbar) retrogressed to low-gradegreenschist facies (227–317°C). The foliation in Barudtonalite–granodiorite close to the E–W part of the WFSZruns parallel to the plane of shearing and the tonalite shownumerous magmatic flow structures overprinted by foldingand ductile shearing. The WFSZ is similar to structuresresulted from combined simple shear and orthogonalshortening of oblique transpressive shear zones and theirsense of movement is comparable with the characteristics ofthe Najd Fault System.
Arab J Geosci (2009) 2:29–52DOI 10.1007/s12517-008-0016-y
M. Abd El-Wahed (*) :M. Abu AnbarGeology Department, Faculty of Science, Tanta University,Tanta 31527, Egypte-mail: [email protected]
Keywords Northern Eastern Desert . Fatira shear zone .
Oblique convergent . Transpression . Extension .
Najd Fault System
Introduction
The Eastern Desert of Egypt has been divided by Stern andHedge (1985) into three domains: the Northern EasternDesert (NED), the Central Eastern Desert (CED), and theSouthern Eastern Desert (SED). The CED/SED margin is alow-angle thrust (Greiling and El-Ramly 1985) or a low-angle normal ductile shear (detachment fault) formed duringthe Neoproterozoic extensional tectonic phase of the EasternDesert that began ∼600 Ma, and followed arc collision andNW-ward ejection of nappes (Fowler and Osman 2008). TheNED/CED boundary is described as a curved N60°E trendingthrust or dextral strike–slip fault (e.g., Stern and Hedge 1985;Stern and Gottfried 1986; Bennett and Mosley 1987; Greilinget al. 1988; El-Gaby et al. 1990; El-Gaby 1994; Greiling et al.1994). El-Gaby (1983) proposed the presence of an obliquethrust with NE trending dextral component along the NED/CED margin. This thrust uplifted the NED in relation to CEDand put the continental basement in the front of ensimaticcover rocks. Fowler et al. (2006) stated that the thrusting atthe southern margin of the NED is not responsible for the∼620–600 Ma uplift of the NED because the thrusts arerelated to the earlier event of arc accretion.
Themain characteristic features of the NED compared withthe CED are: (a) prevalence of late and post-tectonicgranitoids, (b) NE–SW and E–W steeply dipping structuraltrends compared with NW–SE trends in the CED, (c) absenceof ophiolites, core complexes, and Najd Fault System (NFS),(d) existence of NE trending dykes, and (e) prevalence of NE–SW trending normal faults (Stern 1985; Greiling et al. 1988).
The NFS is NW striking sinistral ductile shear zones thatcut through the Arabian Nubian Shield (Moore 1979; Stern1985, 1994; El-Rabaa et al. 2001; Johnson and Kattan2001). It is developed in the crust of central Arabia duringthe Proterozoic post-orogenic stage due to the convergenceof a continental fragment from the east. This was accompa-nied by E–W compression and N–S extension in the form ofan escaping block (Stern 1994). The NFS regarded as thelast significant structural event affected the Precambrianrocks in Egypt and Saudi Arabia (Moore 1979; Stern 1985,1994; El-Rabaa et al. 2001; Johnson and Kattan 2001; AbdEl-Wahed 2006, 2007, 2008; Abdeen et al. 2008). TheCentral part of the Eastern Desert constitutes the target areafor the activity of the NFS. It is already described from WadiHodein, some 20 km west of Shalatein at the Red Sea coastin the Southern Eastern Desert of Egypt (Abdeen et al.2008), but it is not recorded in the NED.
One remarkable structural feature in the Eastern Desert ofEgypt is presence of gneiss-cored domes (e.g., Meatiq, Sibai,Hafafit, Shalul). Nowadays, there are twomain tectonic modelsproposed for the formation of these dome structures. The firstmodel considers the gneiss-cored domes as metamorphic corecomplexes formed during transpression combined with lateralextrusion along the NW trending zones bounded by sinistralstrike–slip shears of the NFS with the formation of synexten-sional plutonism (e.g., Fritz et al. 1996 and 2002; Bregar et al.2002; Abd El-Wahed 2008). This model highlights theimportance of the NFS during the tectonic evolution of theEastern Desert of Egypt (e.g., Fritz and Messner 1999; Unzogand Kurz 2000; Loizenbauer et al. 2001; Makroum 2001;Abdeen and Greiling 2005; Shalaby et al. 2005, 2006; AbdEl-Wahed 2006, 2007, 2008; Abdeen et al. 2008). The secondmodel argues against the presence of core complexes in theEastern Desert of Egypt and explains the development ofgneiss-cored domes by an overlap between NW–SE complexfolding and NW–SE extension parallel to the fold hinges(e.g., Fowler and Osman 2001; Fowler et al. 2007; Andresenet al. 2008). This model diminished and ignored the role ofthe NFS in the tectonic history of the Egyptian Eastern Desertand ascribed the evolution of gneisses, migmatites, and themajor ductile shear zones to extensional tectonics (e.g.,Greiling et al. 1988; Fowler et al. 2006; Fowler and Hassan2008; Fowler and Osman 2008).
The Wadi Fatira area occurs some 40 km NW of Safagacity on the Red Sea coast and about 25 km to the north ofQena-Safaga road (Fig. 1). The area around Wadi Fatiraconsists of highly sheared metavolcanics incorporated in aductile shear zone and intruded by syntectonic Barudgneissic tonalite, post-tectonic gabbroic, and graniticintrusions of Gebels Umm Inab, Abu Hamr, Tarbush AlMisri, Ras Barud, Abu Dalf, and Umm Kibash. The shearedmetavolcanics are composed of foliated basaltic metaande-site, dacitic metatuff, metadolarites, and schists. The laterinclude hornblende, chlorite, actinolite, and biotite schists.
This paper presents mesoscopic to microscopic scalestructural and petrological evidence across the Wadi Fatirashear zone (WFSZ). We document an outstandinglyexposed transpressional shear zone in the NED in whichstrike–slip and shortening displacements are partitioned onmeter to centimeter scales. This is an attempt to correlatethis shear zone with the characteristics of the NFS.
Geology of the Fatira area
Location and pervious works
The Wadi Fatira area is located to the north of Qena-Safagaroad and to the south of Gebel Shayib Al-Banat (Figs. 1and 2). It is drained mainly by Wadi Fatira and Wadi Barud
30 Arab J Geosci (2009) 2:29–52
Al Azraq. This area was originally investigated by Hume(1934), Sabet et al. (1972), and Akaad et al. (1973) andform a part of the Al Qusayr Quadrangle 1:250,000geological map (Masoud et al. 1992) and Gabal ShayibAl-Banat geological map (Masoud et al. 1999). Thesouthern part of the study area is occupied by the northernpart of Barud Belt. This belt constitutes a huge mass ofsyntectonic granites extending around Qena-Safaga roadand across the entire width of the NED. The Barud Beltconsists generally of granitic gneisses together withsubordinate amphibolites and migmatized sedimentaryrelics comprising biotite gneiss, hornblende gneiss, andbanded hornblende gneiss (Habib 1972; Abu El-Ela 1979;El-Gaby and Habib 1982; Kamal El-Din and Asran 1994;Masoud 1997). The southern part of the Barud belt, to thesouth of Qena-Safaga road, was structurally investigated byAbd El-Wahed and Abdeldayem (2002) and they reportedthree phases of deformation and two phases of metamor-
phism and migmatization. Fowler et al. (2006) concludedthat the amphibolites, schists, and migmatites of Barud area(to the south of Qena-Safaga road) were formed byextensional shearing of previous arc metavolcanics. Manygeochemical studies on the syntectonic and post-tectonicgranitoids have been reported by Abdel Aal (1995), Ahmed(1995), Abu El-Maaty et al. (1998), Ahmed et al. (1998),Hassan (1999), Mahmoud (2004), and Helmy et al. (2004).The descriptive migmatite terms used in this paper are afterMehnert (1968), Ashworth (1985), and Johannes (1988).
The Fatira sheared metavolcanics
These metavolcanics crop out as a narrow belt in the centralpart of the mapped area (Fig. 3). The highly schistose rocksof this belt were mapped as metasediments includingmigmatites and gneisses by Sabet et al. (1972). This beltconsists of two connected parts with two different direc-
Fig. 1 Simplified geological map of the northern part of the CED andthe southern part of the NED of Egypt. Compiled from the GeologicalMap of Egypt (El-Ramly 1972) and the geological map of Quseir(Klitzch et al. 1987). Major structures are after Fritz et al. (1996). UHUm Had granite pluton, MCC Meatiq core complex, GK G. Kafari, GS
G. Gasus, GD G. El-Dob, GF G. Abu Furad, GT G. Umm Taghir, GRG. Ras Barud, GM G. El Magal, GA G. Umm Inab, GY G. Samyuk,GN G. Shayib El Banat, GQ G. Qattar. The thick gray line is theQena-Safaga line of El-Gaby (1994) and represent the approximateboundary between CED and NED
Arab J Geosci (2009) 2:29–52 31
tions of strike (Fig. 4a). The western part is trending E–Wand extends 8 by 3 km width between Gebel Abu Hamr andGebel Umm Inab. It consists of foliated basaltic metaande-site, metadolarites, and dacitic metatuffs and hornblende,chlorite, actinolite, and biotite schists. There are someunmappable lenticular bodies and pods of serpentinites andrelated talc carbonate rocks at the northern slop of GebelAbu Hamr. These masses striking E–W parallel to theplanes of schistosity in the enveloping basaltic metaandesiteand dacitic metatuffs. The sheared metavolcanics aretectonically intercalated with bands of iron-rich horizonssuch as those reported by Bishara and Habib (1973) fromGabal Semna (30 km south of the Barud area at 26°26′ N,33°34′ E) and the banded quartz–magnetite rocks recordedin the Abu Furad amphibolites (20 km south of the Barudarea at Wadi El Bula; Fowler et al. 2006). They are intrudedin the east and west by post-tectonic granites of Abu Hamrand Umm Inab, respectively (Fig. 4b). The contact with theBarud tonalite–granodiorite is characterized by the devel-opment of migmatitic amphibolites and schlieric tonalites(Fig. 4c). The rocks exhibit moderately to steeply dippingfoliation and show increase of deformation intensity towardthe Barud tonalite–granodiorite. The rocks are highlyfolded and show alternating asymmetrical and E–Wtrending antiforms and synforms.
In thin section, the igneous textures in basaltic meta-andesite and dacitic metatuffs are well-preserved. Thebasaltic metaandesite consists of plagioclase and horn-blende phenocrysts set in a much finer foliated groundmassof plagioclase, actinolite, chlorite, iron oxides, and epidote(Fig. 4d). Plagioclase phenocrysts are usually zoned andpredominantly sericitized and replaced by clinozoisite.Mafic phenocrysts consist of hornblende replaced byactinolite and chlorite indicating retrograde greenschistfacies of metamorphism. Dacitic metatuffs are predominateto the north of the E–W trending part and completelyconverted to biotite schists close to the contact with Barudtonalite–granodiorite. Dacitic metatuffs consists of quartzphenocrysts set in a schistose groundmass of quartz,plagioclase, hornblende, and iron oxides. Four main typesof schists occur between the sheared metavolcanics andthe Barud tonalite–granodiorite. These include hornblendeand chlorite schists and minor amounts of biotite andactinolite schists. Hornblende schists constitutes the mainrock type and consists of augen-shaped hornblende porphy-roclasts set in highly schistose groundmass of hornblende,quartz, plagioclase, chlorite, and iron oxides (Fig. 4e).Sometimes, augen-shaped quartz porphyroclasts are pres-ent. Chlorite, actinolite, and biotite schists (Fig. 4e–g) arehighly schistose and consist principally of variable amounts
Fig. 2 Geological map of Wadi Fatira area. Lithology modified after Masoud et al. (1999). Structures are identified in the present study
32 Arab J Geosci (2009) 2:29–52
of chlorite, actinolite and biotite together with quartz,plagioclase, carbonates, and iron oxides.
The NW–SE part of the Fatira metavolcanics beltextends (about 8 km by 200 m width) from Gebel UmmInab southeastward to Gebel Ras Barud. It occurs as anarrow zone of highly sheared and migmatized rockscompletely surrounded by Barud tonalite–granodiorite. Itconsists mainly of migmatitic amphibolites and hornblendeschists. The foliations in these rocks are nearly vertical andstriking NW–SE. The Barud tonalite–granodiorite assimi-late a great part of these rocks and sends a network oftonalitic dykes running parallel to the foliation planes.
Migmatitic amphibolites are the most common migma-tization product from metavolcanics protoliths. They arestrongly foliated and consist of dark colored hornblende-rich bands of volcanic protoliths (paleosome) and sheet-likeleucocratic tonalitic rocks, but more strongly melteddiatexitic migmatites as well as better-preserved leucosome.Sporadically, large masses and lenticular bodies of thepaleosome are present within a thick sheet of the leucosomeand visa versa. The paleosomes are highly foliated comparedwith the tonalitic leucosomes that exhibit moderate to faint
foliation. The decrease of intensity of foliation is usuallyaccompanied by the revelation of granoblastic texture. Thepaleosomes are composed of hornblende, plagioclase, andquartz in a decreasing order of abundances (Fig. 4h–i),whereas the leucosomes are mainly schlieric tonaliteconsisting of plagioclase, quartz, hornblende, and minoramounts of biotites, sphene, and epidote.
Barud tonalite–granodiorite
Only the northern part of the Barud tonalite–granodiorite(Barud gneiss of Hume 1934) exists in the study area. Thisbatholith is a huge igneous mass traversed by Qena-Safagaroad. It was the subject of many contributions especiallyits central and southern parts (e.g., Habib 1972, 1987;Akaad et al. 1973; Abu El-Ela 1979; El-Gaby and Habib1982; Kamal El-Din and Asran 1994; Masoud 1997; AbdEl-Wahed and Abdeldayem 2002; Fowler et al. 2006). TheBarud batholith exhibit distinct intrusive features to theFatira sheared metavolcanics and also to metavolcanicsand amphibolites around Qena-Safaga road (Abd El-Wahed and Abdeldayem 2002; Fowler et al. 2006), and
Fig. 3 Geological map of WFSZ showing the structural domains and the location of the samples selected for microprobe analyses
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not an autochthonous igneous body separated from aremobilized granitized pre-Pan-African basement (Fowleret al. 2006), as proposed by Akaad et al. (1973), El-Gabyand Habib (1982), Habib (1987), El-Gaby et al. (1990),
El-Gaby (1994), and El-Shazley and El-Sayed (2000).Petrochemical studies of the Barud tonalite–granodioriteconsidered it as subduction-related volcanic arc melts ofprobable mantle origin (Hussein et al. 1982; Abdel-
Fig. 4 a Fatira sheared metavolcanics showing E–W and NW–SEstriking S1 foliations, b Gebel Umm Inab monzogranites and Barudtonalite intruded in sheared metavolcanics, c schlieric tonalite fromWadi Barud, d–i Photomicrographs from the Fatira study area (crossedpolars): d hornblende schist; e actinolite schist; f chlorite schist;
g biotite schist after banded tuffs still preserve the original bandingwith S1 parallel to S0, the light band is rich in chlorite retrogressedafter biotite; h migmatitic amphibolite from the eastern domainexhibiting weak foliation; and i migmatitic amphibolite showinggranoblastic texture
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Rahman 1995; Moghazi 1999; El-Shazley and El-Sayed2000; El-Sayed et al. 2003). The intrusion age of thegneissic syntectonic granitoids is 680–620 Ma (Stern andHedge 1985). The Barud tonalite–granodiorite is intrudedat about 630 Ma (Fowler et al. 2006).
In the study area, the Barud batholith consists of gneissictonalite with rare gneissic granodiorites. The intrusivemargin and the foliation in Barud tonalite–granodioritebatholith are markedly concordant to the foliation trends ofthe sheared metavolcanics. Texturally, the tonalitic rocksare mainly medium- to coarse-grained, hypidiomorphicgranular and are nonfoliated to strongly foliated. Gneissictonalite is medium-grained and consists of subhedraloscillatory-zoned plagioclase (An26), anhedral quartz, andacicular to prismatic olive green hornblende and biotitewith minor amounts of iron oxides, apatite, chlorite,sphene, and zircon. Some hornblende and plagioclasephenocrysts are observed in tonalite. The schlieric tonalitescrop out around the NW–SE trending part of the shearedmetavolcanics together with migmatitic amphibolites. Usu-ally, the schlieric tonalite contains isolated schollen andlarge meter- to tens of meter-scale mafic pods consisting ofmigmatitic amphibolites. The gneissic granodiorite ismedium- to coarse-grained and consists mainly of plagio-clase (An25), quartz, potash feldspar, biotite, hornblende,and variable amounts of epidote, sericite, chlorite, sphene,apatite, and iron oxides. The gneissic texture in boththe gneissic tonalite and granodiorite is identified bysubparallel arrangement of biotite plates and hornblendeprisms.
The host tonalite also contains hornblende-rich schlie-ren, which grade into mafic enclaves, indicating mechan-ical mingling between volcanic and tonalitic phasesduring deformation in the magmatic state. Centimeter- tometer-scale mafic enclaves are most common in the Barudtonalite around the NW–SE trending part of the shearedmetavolcanics and dominantly comprise hornblende,plagioclase, and quartz with minor chlorite and carbo-nates. These enclaves are sometimes hosted by tonaliteenriched in hornblende but usually have diffuse contactsindicating interaction between the enclave and the hosttonalite.
Post-tectonic granitoids
The sheared metavolcanics and the Barud tonalite–granodiorite are intruded by a number of post-tectonicgranitic plutons. The Umm Inab pluton consists mainlyof monzogranite, monzodiorite, and granodiorite (Helmyet al. 2004). These rocks exhibit hypidiomorphic andallotriomorphic granular textures and uncommonly theporphyritic texture. They are mainly composed of albite,perthitic microcline, plagioclase, quartz, biotite, and green
hornblende as well as iron oxides and epidote as accessoryconstituents. Umm Inab pluton contains amphiboliticenclaves of different dimensions especially close to thecontacts with the highly sheared metavolcanics. Theseenclaves are highly foliated and very rich in hornblende.
Gebel Abu Hamr pluton consists predominantly ofsyenogranites and alkali feldspar granites (Mahmoud2004). Gebel Ras Barud and Gebel Tarbush Al Misri aremainly composed of syenogranites.
Post-tectonic dykes
The Barud tonalite–granodiorite and the sheared metavol-canics are dissected by a network of dykes composedmainly of rhyolite, rhyodacite, andesite, dacite, aplite, andlamprophyre. The acidic dykes are more resistant toweathering compared to the host rocks. Therefore, theyform conspicuous ridges and spines. These dyke strikingoccur mainly at the NE–SW and E–W.
Structure and constitution of the Wadi Fatira shearzone
The WFSZ is a ductile shear zone that extends E–W fromGebel Abu Hamr to Gebel Umm Inab and turns to NW–SEdirection to the north of Gebel Ras Barud and along WadiBarud Al Azrag to the east of Gebel Ras Barud. The high-strain part of the WFSZ is about 25 km long and consists ofhighly sheared rocks mainly of schists and mylonites. Thisshear zone can be classified into two domains: the westernand the eastern domain.
The western structural domain
The western and the eastern parts of this domain arecompletely obliterated due to the intrusion of Abu Hamrand Umm Inab plutons, respectively. It is intruded from thesouth and north by the Barud tonalite–granodiorite. In thelow strain metavolcanics where the rocks present unde-formed igneous textures, some shear zones are found in thenorthern exposures close to their contacts with thesyntectonic tonalite. These shear zones are 5–10 m thickand characterized by the development of S1 schistosityparallel to S0 in dacitic banded metatuffs. Some talccarbonate pods were observed with their axes runningparallel to S1 schistosity. Along these shear zones, themetavolcanics and the serpentine–talc carbonate rocksshow strongly inhomogeneous strain indicated by thevariation in the intensity of S1 schistosity in these rocks.There is a gradient in intensity of deformation across theentire mylonite zone where it increases from north to southfrom moderately deformed basaltic metaandesite, dacitic
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metatuffs, and metadolarites to strongly sheared schists andmylonites at the contacts with the Barud tonalite.
In the high-strain part, the mylonites include hornblende,biotite, and chlorite schists and consist of σ-shapedhornblende and plagioclase porphyroblasts set in foliatedfine-grained groundmass of hornblende, plagioclase, quartz,chlorite, and dark volcanic material. S1 in the hornblendeschist is defined by parallel-aligned green hornblendeprisms, plagioclase, quartz, and iron oxides. S1 protomylo-nitic to mylonitic foliation (Fig. 5a) strikes WNW–ESE(E–W) and dips between 45° and 85° generally to N and S(Fig. 6a). The subvertical mylonitic foliation contains aWNW–ESE to NW–SE trending subvertical stretchinglineation (Fig. 5b) with a plunge to the NW that isgenerally more than 70° (Fig. 6b).
The major structures include several asymmetrical F1antiforms and synforms with E–W to WNW–ESE axes anda NE vergence (Fig. 5c). Folds range in wavelength fromseveral centimeters to 20 m or more. They are typicallyWNW–ESE trending tight antiform–synform pairs that maybe traced for over 7 km in map view. Mean minor fold axesplunge steeply WNW and the fold hinges are markedlycurvilinear on centimeter- to kilometer-scales. These foldshave long subvertical limbs which sometimes slightlyreversed developing overturned folds. Sporadically, thereversed limbs are separated from the normal limbs alonghigh-angle reverse faults. A few minor F1 folds wereobserved in the western domain especially in the high-strain part (Fig. 5d,e). These are asymmetrical openfolds and their fold axes steeply plunging to the WNW(Fig. 6c).
At the contact with the high-strain part of the shear zone,the S1 magmatic foliation (Fig. 6d,e) in the Barud tonalitestriking parallel to mylonitic foliation within the shear zoneand subparallel to the axial–planar foliation of F1 folds.Parallelism of magmatic foliation in tonalite and myloniticfoliation and axial plane cleavage in schist and mylonitesuggest that the WFSZ is a synmagmatic ductile shearzone created during the intrusion of the Barud tonalite–granodiorite batholith. This also confirms that the Barudbatholith represents an intruded igneous body and notremobilized pre-Pan-African granitoids. Foliation in Barudtonalite–granodiorite to the south of Wadi Fatira and to thewest of Gebel Ras Barud striking mainly NE–SW anddipping 50–65° NW and SE.
Shear sense indicators suggest a sinistral sense ofoblique shearing along the WFSZ in the western domain.These include asymmetry of mesoscopic S–C fabrics(Fig. 7a), asymmetric geometry of porphyroclasts such asσ-type (Fig. 7b), shape-preferred orientation of plagioclase(Fig. 7c), augen-shaped recrystallized quartz porphyroclasts(Fig. 7d), broken plagioclase crystals in planes oblique tothe main foliation (Fig. 7e), and asymmetric folds deform-
ing fine-grained bands which are frequently found aroundporphyroclasts (Fig. 7b,c).
A noncoaxial component of the D1 deformation isexpressed locally by formation of NNE dipping obliquethrusts (Fig. 5f,g), parallel to the axial–planar foliation of F1folds. Changes in dip of S1 axial planes cleavage andmylonitic foliation from 85° to 45° (Fig. 6f,g) and thewedging-out of biotite schist within the high-strain part ofthe shear zone suggest a mid-angle to high-angle thrustfaults. The thrust zone consists mainly of chlorite schist andmylonites. Inside the thrust zones, subvertical S1 cleavageand mylonitic foliation are converted to WNW–ESE trendingS2 mylonitic foliation that dip 40° NNE (Fig. 6f,g).Development of drag folds (F2) and kinked foliation planes(Fig. 6h) in the thrusted sheets indicate top-to SSW sense ofmovement along these oblique thrusts (Fig. 5h). F2 foldshave no axial plane foliation and their axes are nearlyhorizontal and trending E–W. The axes of kinked foliationare usually parallel to the axes of F2 folds. The low-anglepitch of slickensides measured in ductile–brittle to brittlefault surfaces suggests an oblique sense of movement duringthrusting.
The eastern structural domain
The WFSZ in this domain extends southeastward fromGebel Umm Inab to Gebel Ras Barud. The high-strain zoneconsists of hornblende schists and migmatitic amphibolites.The term amphibolite is applied in this study for themetavolcanics that have the medium-grade metamorphicassemblage plagioclase plus hornblende and foliated fabric.S1 mylonitic foliation is subvertical and contains NW–SEtrending subhorizontal stretching lineation with a plunge tothe NW at less 20° (Fig. 6i,j). S1 strikes NW–SE close toGebel Ras Barud and turns to WNW–ESE close to GebelUmm Inab with the plunging of lineation increases from20° to 60° along the foliation planes. S1 magmatic foliationin Barud gneissic tonalite striking parallel to those in themigmatitic amphibolites and has subhorizontal stretchinglineation (Fig. 6k,l).
The WFSZ in the eastern domain have no folds and thrustsand characterized by some extensional features such as (a)boudins of hornblende-rich enclaves in gneissic tonalite, (b)boudinaged calcite veins in hornblende schist (Fig. 7f) (c)microscopic fractures filled with actinolite and epidote(Fig. 7g), (c) synextensional pull-apart between plagioclasecrystals (Fig. 7h), and (e) NE–SW and ENE–WSW brittlenormal faults and major fractures. Hornblende-rich boudinslocally developed in the plane of the S1 foliation and showcomplex brittle effects accompanied by tonalite injections.These hornblende-rich boudins were probably produced byinstantaneous extension at right angles to the L1 stretchinglineation. The asymmetric shape and an en echelon
36 Arab J Geosci (2009) 2:29–52
arrangement of this boudins are attributed to obliqueextension combined with spinning of the boudins arrange-ment (e.g., Price and Cosgrove 1990; Fowler and Hassan2008).
The Barud tonalite–granodiorite is intruded synkinemati-cally along theWFSZ, this gave rise to an internal subverticalNW–SE trending magmatic to solid state foliation in thegneissic tonalite and the migmatitic amphibolites. This
Fig. 5 a E–W trending subvertical mylonitic foliation, westerndomain; b steeply plunging stretching lineation marked by stretchedhornblende, western domain; c very tight WNW–ESE trendingantiform, the dark bands are hornblende schists, whereas the lightbands are biotite schists, western domain; d minor F1 open fold in
chlorite schist with steeply plunging axes; e minor F1 fold inhornblende schist; f NNE dipping oblique thrust; g SSW-directedoblique thrust showing development of drag folds and kinking in theoverthrusted sheet; and h drag fold with horizontal WNW–ESEtrending axis along the oblique thrust in g
Arab J Geosci (2009) 2:29–52 37
foliation runs parallel to the S1 mylonitic foliation in WFSZ.To the east of Gebel Ras Barud, foliation in Barud tonalite–granodiorite striking mainly NW–SE parallel to myloniticfoliation in the eastern domain of WFSZ. This may suggeststhe existence of WFSZ along Wadi Barud Azraq and theirextension was disrupted by the intrusion of Gebel RasBarud granites and by the dextral displacement along theNE–SW strike–slip faults. The absence of migmatiticamphibolites along Wadi Barud Azraq indicates that therocks of the high-strain zone of the WFSZ are completelyassimilated by the tonalitic magma.
The stereonets of subvertical shear bands and subhor-izontal stretching lineation orientations suggest a predom-inantly strike–slip movement. The asymmetry of S–C
fabrics and tails of plagioclase and hornblende porphyr-oclasts in the WFSZ in the eastern domain generallyindicate a sinistral sense of shearing parallel to thesubhorizontal stretching lineation.
Normal and strike–slip faults
WFSZ is dissected by a series of ENE–WSW and NE–SWdextral strike–slip faults and NNW–SSE sinistral strike–slipfaults and NE–SW normal faults. Three NNW–SSEsinistral strike–slip faults displace the western domain inan en echelon arrangement. NE–SW dextral strike–slipfaults may represent the second-order shear of the NFS andprobably rejuvenated during the Red Sea rifting. The main
Fig. 6 Stereograms of mesoscopic structural data for Wadi Fatiraarea. All stereograms are equal area lower hemisphere Schmidt net:a–h data from the western domain, j–l data from the eastern domain,density contours 0%, 5%, 10%, 15%, and 20%. a Poles of S1schistosity and mylonitic foliation in sheared metavolcanics β definethe hinge of WNW–ESE trending folds, b L1 stretching lineation onS1, c F1 folds hinges, d poles to S1 magmatic foliation in Barud
gneissic tonalite, e L1 magmatic linear fabric in Barud gneissictonalite, f pole to S2 foliation along the thrust planes, g L2 lineation onS2, h F2 fold hinges, i poles to S1 foliation in sheared metavolcanics, jL1 stretching lineation on S1, k S1 magmatic foliation in Barudgneissic tonalite of the eastern domain, and i L1 magmatic linearstructures in Barud gneissic tonalite
38 Arab J Geosci (2009) 2:29–52
Fig. 7 Photomicrographs showing sinistral shear sense indicatorsfrom WFSZ (crossed polars): a hornblende schist showing S–C fabric,b protomylonites showing rotated plagioclase porphyroclast withwinged trails, c shape-preferred orientation of plagioclase oblique tothe main foliation, d augen-shaped recrystallized quartz porphyroclast
in actinolite schist, e broken plagioclase crystal in planes oblique tothe main foliation. Photomicrographs showing some extensionalfeatures from WFSZ: f boudinaged talc carbonate in chlorite schist,g microscopic fracture, h synextensional pull-apart between plagio-clase crystals
Arab J Geosci (2009) 2:29–52 39
E–W fault is running across Wadi Fatira displacing WFSZand the NNW–SSE sinistral faults. The NE–SW normalfaults are one of the main extensional features characteriz-ing the NED of Egypt.
Mineral composition and geothermobarometry
Mineral analyses were carried out using a Jeol ScanningElectron Microscope (JMS 840 A) equipped with the LINKanalytical energy-dispersive system at the Laboratory ofScanning Electron Microscopy and Microanalysis of theInstitute of Geological Sciences at the Polish Academy ofSciences, Warsaw, Poland. Operating conditions were15 kV accelerating voltage, 50 nA beam current, and 60 slifetime. The raw data were recalculated through MinpetSoftware after Richard (1995). Microprobe analyses werecarried out on amphibole, plagioclase, chlorite, biotite, andepidote from hornblende schist, chlorite schist in proto-mylonite of the western domain, and from migmatiticamphibolites of the eastern domain in order to constrain themetamorphic conditions by geothermobarometric estima-tion. Also, some analyses were carried from Barud tonalite–granodiorite and Umm Inab monzogranite.
Plagioclases (Table 1) from protomylonite are mainly ofalbite compositions, those from hornblende schist areoligoclase, and andesine in chlorite schist (Fig. 8a).Plagioclases in migmatitic amphibolites with foliated fabricare oligoclase, whereas those in rocks with granoblastictexture are labradorite in composition (Fig. 8b). Plagio-clases in tonalite are oligoclase whereas those in monzog-ranite are albite in composition. K-feldspar in monzograniteare anorthoclase in composition.
The analyzed amphiboles (Table 2) from the highlysheared metavolcanics and tonalites range in compositionfrom magnesiohornblende to tschermakitic hornblende(Fig. 9) following the nomenclature of Leake (1978). Lairdand Albee (1981) introduced a set of diagrams by which thezones and grades of metamorphism can be estimated fromthe chemical composition of amphiboles (Fig. 10). Themajority of amphibole analyses from the eastern domainfall mainly in low-pressure field, whereas those fromwestern domain fall in medium-pressure field (Fig. 10a).Most of the amphiboles from the eastern domain plot in thebiotite zone whereas amphiboles from the western domainshow gradient from biotite to garnet zones (Fig. 10b).Plotting of investigated amphiboles on NaM4–AlIV diagram(Fig. 11) shows a wide range of pressure (2–5 kbar). The
pressures estimated from the eastern domain range between2.5 and 4 kbar, whereas those from western domain rangebetween 3.5 and 5 kbar. The analyzed chlorites (Table 3)from the examined rock samples are mainly ripidolite.Sample 3E has higher MgO and Al2O3 and lower FeOcontents compared with the other samples. Epidotes(Table 4) from hornblende schist and migmatitic amphib-olites have higher content of Al2O3 and CaO comparedwith the other rock types.
Amphibole–plagioclase geothermometer
The logarithmic plot of XAn/Aab in plagioclases versusCaM4/NaM4 in amphibole on the amphibole–plagioclasegeothermometer after Spear (1980), the amphibole–plagioclase pairs from the eastern domain range between600°C and 725°C (Fig. 12) whereas those from the westerndomain cluster between 500°C and 530°C. The sameresults are obtained from the amphibole–plagioclase geo-thermometer after Plyusnina (1982) where the P–T obtainedusing the amphibole–plagioclase pairs from the easterndomain range between 600°C and >650°C at <2–4 kbarwhereas those from the western domain show 490–530°Cat 3–7 kbar (Fig. 13).
Chlorite geothermometer
Cathelineau (1988) presented the following chlorite geo-thermometer: T (°C)=106 (AlIV)cor +18 where AlIVCor=AlIV±0.7(Fe/Fe+Mg). On the basis of this equation, thecalculated temperatures are 227–294°C from chlorites ofthe western domain and 280–300°C from those of theeastern domain. Using the chlorite geothermometer of Zangand Fyfe (1995): T (°C)=106.2×AlIV+17.5 where AlIV=AlIV measured−0.88×[(Fe/(Fe+Mg)−0.34], the tempera-tures ranges between 233°C and 317°C from chlorites ofthe western domain and 281°C and 300°C from those of theeastern domain.
Metamorphism
The S1 foliation developed in metavolcanics and Barudgneissic tonalite-granodiorite within the WFSZ includethree types of syntectonic fabrics developed during contin-uous metamorphic event. These fabrics include thoseassociated with high-temperature peak (high temperature–low pressure) and development of migmatitic amphibolites,those formed with the medium grade of metamorphism, and
those developed during an early stage of retrogressionunder greenschist facies metamorphic conditions.
Synthermal peak metamorphism
Our observations indicate that the metamorphism reachedgrades higher than upper amphibolites facies where partialmelting led to digestion of a great of part of themetavolcanics and development of migmatitic amphibo-lites and schlieric tonalite. Migmatitic amphibolites consistof leucosome (quartz and feldspar-rich layers) andmelanosome (hornblende-rich layers) surrounded by tona-lites with no migmatization (mesosome). In some parts ofthe high-strain zone, small volumes of schlieric tonalitesare seen, often in close association with tonalitic andgranodioritic bodies which may indicate that morecomplete melting has taken place (Sawyer 1996) althoughon a local scale.
S1 is defined by parallel orientation of mafic and felsicminerals. L1 mineral lineation is defined by the orientationof hornblende nematoblasts in the melanosome and theelongated quartzofeldspathic aggregate in the leucosome.An increase in width of leucosome and development ofmesosome is accompanied by decrease in intensity offoliation and development of granoblastic and hypidiomor-
phic fabrics. The calculated P–T is about 725°C at 2–4 kbarin the varieties with no foliation and lineation and thegranoblastic aggregates consists of hornblende, plagioclase,and quartz with minor sphene, apatite, and iron oxides.
Post-thermal peak metamorphism
A major part of WFSZ was developed under P–T conditionscomparable with medium-grade amphibolites facies in therange of 500–600°C at 3–7 kbar. This was accompanied bydevelopment of hornblende and biotite schists and proto-mylonites within the high-strain part of the shear zone. Relicttextures of quartz, plagioclase, and hornblende phenocrystssuch as those in the low-grade metavolcanics to the south ofthe western domain indicate that the hornblende schists are ahigher temperature basaltic metaandesite or probably meta-basalts and the biotite schists are higher temperature dacite ordacitic metatuffs. Scarcity of biotite schists confirms that theacidic verities of the metavolcanics are much lesser than thebasic varieties. The grade of metamorphism increases fromgreenschist facies in basaltic metaandesite and daciticmetatuffs to amphibolite facies within the high-strain partof the WFSZ and close to the Barud tonalite. Increasingdeformation and metamorphism from north to south acrossthe western domain reflect the role of Barud tonalite as an
intrusive igneous body during the development of WFSZ asa hot ductile shear zone. The presence of schistose texture,hornblende, epidote, biotite, and chlorite imply metamor-phism under upper greenschist transitional to epidote–amphibolite facies conditions. Within the high-strain part ofWFSZ, the penetrative fabrics are characterized by horn-blende and plagioclase porphyroclasts that exhibit a variabledegree of dynamic recrystallization with their long axesparallel to L1 stretching lineation. Presence of theseporphyroclasts together with extensive recrystallization ofplagioclase indicates that the development of the ductileshear zone during D1 deformation took place under mediumamphibolite facies conditions and can be considered torecord the beginning of cooling during D1. Ductile deforma-tion produced S1 protomylonitic and mylonitic S–C fabricsand shear bands. Lobate and interpenetrating grain bound-aries in polycrystalline quartz aggregates in protomylonite,and comparable features in the nearby sheared metavolcanicswith no mylonitic foliation are indicative of high-temperatureconditions during shearing. The fabrics related to extensioninclude intergrowth of quartz, plagioclase, and actinolite inthe pressure shadows around plagioclase porphyroclasts andin synextension pull-apart between plagioclase crystals.
Low-grade retrograde metamorphism
Retrograde greenschist facies deformation occurred underlow temperature conditions in the range of 227–317°C. Thisis indicated by the widespread retrogression of biotite andhornblende to actinolite and formation of calcite, epidote,albite, and chlorite. In the western domain, the more intenseretrograde effects are located along the thrust planes and thelow-grade mylonitic fabrics. In the eastern domain, retro-grade metamorphism occurred during the extensional phaseof WFSZ. Evidences of retrograde metamorphism includepartial replacement of feldspars by epidote and quartz
aggregates on extensional fractures, internal deformation,and chloritization of hornblende porphyroclasts and partiallyrecrystallized quartz aggregates.
Discussion
Subhorizontal versus subvertical stretching lineationsin the WFSZ
The pitch of the stretching lineation along WFSZ changesfrom subhorizontal in the eastern domain to subvertical inthe western domain while the oblique lineation donates inthe curved part between the two domains. Changingattitude of stretching lineation from nearly horizontal tonearly vertical is described from many tectonic orogenicbelts. Different orientations of lineations can be ascribed topolycyclic deformation (e.g., Collins et al. 1991; Goscombeet al. 1994), polyphase deformation in which successivecontractional and extensional strain regimes alternateduring a single orogenic cycle (e.g., Malavieille 1987;Dewey 1988; Faure 1995; Gardien et al. 1997), twothrusting episodes separated by an extensional phase (Roigand Faure 2000), and two extensional events separated by acontractional one (Tubía 1994) and progressive deformationduring monocyclic tectonic evolution and progressiverotation of the stress field (e.g., Sawyer and Benn 1993;Connors et al. 2002; Reddy et al. 2003). In zones ofdominant strike–slip and dominant pure shear, the occur-rence of two (or more) directions of lineations results fromstrain partitioning during transpressional deformation wherehorizontal and vertical lineations can coexist, respectively(e.g., Tikoff and Greene 1997; Goodwin and Tikoff 2002;Neves et al. 2005).
The directions of the stretching lineation may exhibitdifferent geometrical relations with respect to the transport
Fig. 8 Composition of plagioclases in sheared metavolcanics, tonalite, and monzograniteFig. 9 Composition of the investigated amphiboles (after Leake 1978, adopted by Richard 1995)
Arab J Geosci (2009) 2:29–52 43
direction. In many ductile shear zones, the stretchinglineation is oriented parallel to the direction of tectonictransport (Sengupta and Ghosh 2004). The stretchinglineations normal to the transport direction have beenrecorded from some subvertical strike–slip shear zones(e.g., Hudleston et al. 1988; Robin and Cruden 1994;Greene and Schweickert 1995; Sengupta and Ghosh 2004).Tikoff and Greene (1997) have described the occurrence ofboth transport-parallel and transport-normal stretchinglineations from the same transpressional shear zone.
Presence of oblique lineations in the WFSZ is resultedfrom a progressive deformation where displacement occursexactly along the same slip direction during the successivetectonic events. In such cases, the oblique lineationsdeveloped from progressive rotation of the stress field(changes in the direction of convergence) or from strainpartitioning. Strain partitioning during transpression occurswhere oblique convergence leads to contemporaneouswrenching and thrusting motions (e.g., Merle and Gapais1997; Holdsworth et al. 1998) or to adjacent regions
undergoing simple shear and pure shear (e.g., Tikoff andGreene 1997; Goodwin and Tikoff 2002).
Oblique convergence and thrusting
Based on the evidence of a NW–SE to WNW–ESE regionaltectonic trend represented by S1 schistosity in shearedmetavolcanics and gneissosity in Barud tonalite, a steepaxial–planar foliation, and shear sense indicators, theWFSZ could be described as a sinistral shear zonedeveloped during oblique convergent deformation andevolved during a regional NE–SW (to ENE–WSW)directed compression and attendant homogeneous shorten-ing (Fig. 14). The protomylonite has a prominent cleavagetype foliation (C-surface of Berthe et al. 1979 andBhattacharya 2004) and a stretching lineation. The stretch-ing lineation is subhorizontal in the eastern domain andturned to subvertical in the western domain. The F1 foldshave moderately plunging to subvertical axes. Moreover,the stretching lineations, which are best developed within
the zones of highest strain, are steeply plunging and areassociated with the steeply dipping foliations. The steeplyplunging F1 folds become tighter southwards toward thesezones, until they are transposed by the thrust zones. In thecurved part of the shear zone where the striking of foliationis turned from NW–SE to WNW–ESE, the plunging ofmineral stretching lineation from subhorizontal in the NW–SE part to moderately plunging (20–40°) in the curved partto subvertical in the E–W striking part. Subhorizontallineation in the eastern domain is consistent with subhor-izontal sinistral movement along this part of the shearzone.
The commonly oblique stretching lineation in the curvedpart of the shear zone indicates oblique-slip and isconsistent with oblique collisional deformation. Subverticalstretching lineation in the E–W trending part of the WFSZindicates changing direction of stress from NE–SW to N–Sand confirms the oblique convergence character of theWFSZ. This was accompanied by development of F2 foldsand kink bands and SSW-facing thrust plans and subvert-
ical reverse fault in the western domain. Changing regimeof deformation of subhorizontal sinistral movement alongthe WFSZ in the eastern domain to compression andthrusting in the western domain is accompanied by anextensional deformation in the eastern domain. The absenceof folds in the eastern domain may be due to assimilation ofmylonites in tonalitic magma and development of migma-titic amphibolites and schlieric tonalite. The folds may beformed earlier and then vanished during the migmatiticprocesses.
Presence of extensional features in the WFSZ ductileshear zone and the other shear zones in the southern part ofBarud area (e.g., Fowler et al. 2006) encouraged the authorsto explain such shear zones as extensional normal shearzones and neglected the transpressional shearing and minorthrusts along these shear zones. Small-scale asymmetricstructures (e.g., deformed talc carbonate pods and inclusionpatterns in porphyroblasts), which are better developed onL1-parallel steeper faces, show a prevalence of sinistralshear throughout the high-strain zone of the WFSZ.
Consequently, the WFSZ is interpreted as a transpressiveshear zone involving vertical extension during high-temperature ductile shearing.
Tectonic evolution of Wadi Fatira shear zone
The general perspective of Wadi Fatira area is a few meterswide, steeply inclined transpressional shear zone character-ized by a sinistral sense of movement and subhorizontal tosteep stretching lineation. This shear zone began to developduring a monocyclic D1 synmetamorphic progressivedeformation during oblique arc collision and produced astrain gradient across the sheared metavolcanics and theBarud tonalite–granodiorite (Fig. 14). The high-temperaturemetamorphic peak (725°C at 2–4 kbar) is manifested bypartial melting and development of migmatitic amphibolitesand schlieric tonalites that were produced by incompletemelting of metavolcanics. These rocks are restricted to thehighest metamorphic grade/highest strain zones (the mig-matite complex) of the WFSZ.
In the western part of the shear zone, the high-strain partoccurs between the protolith metavolcanics in the north andthe Barud gneissic tonalities in the south. There is anincrease in the intensity of deformation and grade ofmetamorphism from north to south with the highly
deformed zone occurring at the contact with the Barudgneissic tonalite. The grade of metamorphism increasesfrom low-grade greenschist facies in the metavolcanics tomedium-grade amphibolites facies (500–600°C at 3–7 kbar)in the high-strain part of the shear zone. Progressivelocalization of deformation led to the development ofprotomylonite and protomylonitic foliation. S1 axial planefabrics related to the folding are well-developed in the high-strain part of the shear zone where the schists andprotomylonites are metamorphosed under amphibolitefacies conditions.
Changing orientation of major stress from ENE–ESWto N–S (Fig. 14) was accompanied by: (a) turning inorientation of foliation from NW–SE in the eastern part toWNW–ESE in the western part of the WFSZ, (b) changingattitude of stretching lineation from subhorizontal in theeastern part to subvertical in the western part, (c)development of E–W steeply plunging major and minorF1 folds, and (d) development of SSW-facing thrust andE–W nonplunging drag folds and kink bands under low-grade greenschist facies metamorphic conditions (277–317°C).
In WFSZ, there is a remarkable relation betweenincreasing strain across the zone and the progressive natureof the metamorphic, melting, and magmatic effects. The
Fig. 10 The studied amphiboles compared with the formula proportion diagrams (after Laird and Albee 1981)
Fig. 11 Plot of AlIV against NaM4 of the studied amphiboles (after Brown 1977)
tonalite melt probably intruded as subvertical lensoidbodies, both parallel and oblique to S1 fabrics which actedas conduits or feeder for migrating melts. The monocyclicD1 deformation event is attributed to sinistral transpres-sional shearing and is synchronous with crustal emplace-
ment of the Barud tonalite–granodiorite batholith. Thesteeply plunging stretching direction (i.e., transport direc-tion) may have facilitated melt ascent (e.g., Wolf andWyllie 1995; Tikoff and Saint-Blanquat 1997; Druguet andHutto 1998).
Fig. 14 Sketch showing tectonic evolution of WFSZ and changing stress from ENW–WSW compression to N–S compression and developmentof SSW-directed thrusts and ENE–WSW trending folds
Fig. 12 Empirical model for plagioclase–amphibole exchange equilibria (after Spear 1980)Fig. 13 Plagioclase–hornblende geothermobarometer after Plyusnina (1982)
48 Arab J Geosci (2009) 2:29–52
The main conclusion in the Wadi Fatira transpressionalshear zone is that we can observe a strong relation betweenincreasing strain, increasing metamorphic grade, increasingmelting of the country rocks, and the occurrence of Barudtonalite as an intrusive magmatic igneous body. Weinterpreted WFSZ as a hot ductile transpressional shearzone developed as the result of oblique collision.
The deformation may also be relatively homogeneouswhere strike–slip and compressional displacements occur inthe same place. Compression deformations is characterizedby SSW-directed thrust and reverse faulting, folding, andcleavage development with a local subvertical stretchinglineation, and this deformation is associated with agreenschist to amphibolite facies regional metamorphicevent. Analog and mathematical models predict that achange from strike–slip-dominated to thrust-dominatedtranspression occurs at a convergence angle of 20° (e.g.,Tikoff and Teyssier 1994; Marcotte et al. 2005).
Relation between WFSZ and the Najd Fault System
It is widely accepted that the NW trending sinistral shearzones in the CED of Egypt is related to the NFS. Theseshear zones mark the external boundaries of the corecomplexes (e.g., Sibai, Meatiq, Hafafit) and control themain deformation events in the CED (Fritz et al. 1996,2002; Shalaby et al. 2005, 2006; Abd El-Wahed 2006,2007, 2008). Sinistral shearing along the NFS was active inthe period 650–540 Ma. This was accompanied byintrusion of syntectonic older granite around 650–615 Ma(Loizenbauer et al. 2001; Bregar et al. 2002; Fritz et al.2002; Shalaby et al. 2006). The shear zones pertaining tothe NFS are concentrated mainly in the CED andcompletely disregarded or treated with lack of concern inthe NED.
The WFSZ is interpreted to be the continuation of theNajd Shear System in the NED for the following reasons:(a) the WFSZ is NW–SE to WNW–ESE trending shearzone showing both transpressional and extensional features,(b) it exhibits sinistral sense of movement, (c) it ischaracterized by subvertical foliation and the switch ofstretching lineation and plunge of fold axes from subvert-ical to subhorizontal, (d) it is synmagmatic hot ductile shearzone developed due to the high temperature associated withthe intrusion of Barud tonalite–granodiorite igneous body,(e) the presence of extensional features, strain variationacross shear zone, and sinistral horizontal simple shear, and(f) the WFSZ is similar to structures resulted fromcombined simple shear and orthogonal shortening ofoblique transpressive shear zones (e.g., Robin and Cruden1994; Jones et al. 1997, 2004; Dewey et al. 1998; Johnsonand Kattan 2001; Tavarnelli et al. 2004; Sullivan and Law2007; Sarkarinejad and Azizi 2008).
Acknowledgements The authors acknowledge the fruitful discus-sions with Prof. Mahmoud Ashmawy, Geology Department, TantaUniverstity. Dr. Samir Kamh and Hossam Abd El-Monem, GeologyDepartment, Tanta University are thanked for their sincere assistancein the field. Dr. Ryszard Orlowski is acknowledged for his technicalassistance with the electron microprobe at the Laboratory of ScanningElectron Microscopy and Microanalysis, Institute of GeologicalSciences, Polish Academy of Sciences, Warsaw, Poland.
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