Geol Rundsch (1994) 83:484-501 © Springer-Verlag 1994

R. O. Greiling • M. M. Abdeen • A. A. Dardir H. E1 Akha l . M. F. E1 Ramly • G. M. Kamal E1 Din A. F. Osman • A. A. Rashwan • A. H. N. Rice M. E Sadek

A structural synthesis of the Proterozoic Arabian-Nubian Shield in Egypt

Received: 26 January 1994~Accepted: 24 May 1994

Abstract Detailed structural geological and related stu- dies were carried out in a number of critical areas in the Proterozoic basement of eastern Egypt to resolve the structural pattern at a regional scale and to assess the general characteristics of tectonic evolution, orogeny and terrane boundaries. Following a brief account of the tectonostratigraphy and timing of the orogenic evolution, the major structural characteristics of the critical areas are presented. Collisional deformation of the terranes ended about 6 1 5 - 6 0 0 Ma ago. Subsequent extensional collapse probably occurred within a relatively narrow time span of about 20 Ma ( 5 7 5 - 595 Ma ago) over the Eastern Desert and was followed by a further period of about 50 Ma of late to post-tectonic activity. The regional structures originated mainly during post-collisional events, starting with those related to extensional collapse (molasse basin formation, normal faulting, generation of metamorphic core complexes). Subsequent N N W - S S E shortening is documented by large-scale thrusting (to- wards the NNW) and folding, distributed over the Eastern Desert, although with variable intensity. Thrusts are overprinted by transpression, which was localized to particular shear zones. Early transpression produced, for example, the Allaqi shear zone and final transpression is documented in the Najd and Wadi Kharit-Wadi Hodein

R. O. Greiling (I~) - M. M. Abdeen - A. H. N. Rice Geologisch-Pal~iontologisches Institut, Ruprecht-Karls-Universitfit, INF 234, Fax: (0 62 21) 56 55 03 D-69120 Heidelberg, Germany

M. M. Abdeen - A. A. Dardir " M. F. Sadek Egyptian Geological Survey and Mining Authority, 3 Salah Salem Road, Abbassiya, Cairo, Egypt

H. E1 Akhal Department of Earth Science, University of Yarmouk, Irbid, Jordan

M. F. E1 Rarely • A. F. Osman Geology Department, Faculty of Science, Ain Shams University, Cairo, Egypt

G. M. Kamal E1 Din - A. A. Rashwan Geology Department, Quena Faculty of Science, Assiut University, Quena, Egypt

zones. Two terrane boundaries can be defined, the Allaqi and South Hafafit Sutures, which are apparently linked by the high angle sinistral strike-slip Wadi Kharit-Wadi Hodein shear zone with a tectonic transport of about 300 km towards the W/NW. In general, the tectonic evolution shows that extensional collapse is not necessa- rily the final stage of orogeny, but may be followed by further compressional and transpressional tectonism. The late Pan-African high angle faults were reactivated during Red Sea tectonics both as Riedel shears and normal faults, where they were oriented favourably with respect to the actual stress regime.

Key words A r a b i a n - N u b i a n Shield • Egypt • Pan- African orogeny • Proterozoic - Extensional collapse • Thrust tectonics • Transpression

Introduction

The structure of the Pan-African basement in Egypt, as it is exposed today, is the product of a complex, Proterozoic orogenic evolution following terrane col- lision and accretion onto a pre-Pan-African continent to the west of the Nile. As a consequence of the complex evolution, pre-collisional and early orogenic structures are severely overprinted and only locally preserved and the regional structure is dominated by a late orogenic structural grain. This regional structure, which is the major topic of this paper, can be attributed to phase 4 in the five phase tectonic evolution of mountain belts (Dewey, 1988; Table 1). However, a com- parison of Dewey's phases with the general evolution in the Egyptian basement reveals some particularities. Whereas late orogenic magmatism and the formation of extensional basins are well documented (e.g. Tables 2 - 4 ) , related structures, in particular those associated with extensional collapse, are only locally observed. Perhaps even more importantly, the formation of ex- tensional basins with Verrucano-type deposits is followed by intense compression and/or transpression (see below).

Table 1. Five phase tectonic evolution of mountain belts, simplified from Dewey (1988; see also Dewey et al. 1993) and separated into general and structural events. TBCL: thermal boundary condition layer of the continental lithosphere. Based on the Pan-African structural evolution, the phase 4 is divided into an early, extensional part (4 A) and a subsequent compressional part (4 B). Approximate ages for some tectonic episodes in the northwestern Arabian-Nubian Shield are given, for references see text.

5

4

4

Tectonic evolution of mountain belts (Dewey 1988)

Structure (and General

Pan-African timing) post extensional thermal recovery, thickening of TBCL, eventual retrograde metamorphism, marine transgression

(subsidence)

530 Ma

convective TBCL thinning, mantle partial melting, mafic magmatism, minimum-melting granite suites, HT mantle diapirs, irapid subsidence, ;xtensional basins, /errucano-type deposits

subsequent thrust stacking

575 Ma accelerating extensional collapse. (radial) extension, development of metamorphic core complexes, little relationship between displacement vectors and bounding plate slip vectors

595 Ma :atastrophic erosion of the TBCL, "apid uplift, ~ro-grade HT metamorphism, ~osttectonic granite suite

600 Ma

beginning extension

)ost or slow convergence, ~low uplift, ~0-few Ma thermal reequilibration, dow thinning of TBCL, ninor alkaline to silicic granites

thrust and strike-slip structures, strong direct relationship between shortening structures and plate slip vector

615 Ma ithospheridcrustal shortening and :hickening, -IP/LT blueschist and kyanite )earing metamorphic assemblages, :ontinental collision, "ift inversion, :ompressive continental margin arc

Such compression after extensional collapse may be due, for example, to continued plate convergence after collision (e.g. Dewey, 1988). For that reason, Dewey's phase 4 has been divided here into an early, extensional phase (4A) and a late, compressional phase (4B; see Table 1).

Owing to the complex deformational pattern and a lack of comprehensive, detailed structural information at a regional scale, contrasting and apparently mutually incompatible models for the structural evolution of the Arabian-Nubian Shield have been proposed. Such models include high angle block faulting and 'block tectonics' (e.g. Sabet, 1983; Sultan et al., 1988), large- scale low angle normal faulting (e.g. Sturchio et al., 1983; Kamal E1 Din et al., 1992), reverse faulting (e.g. Ries et al., 1983; E1 Ramly et al., 1984) and wrench tectonics

485

(e.g. Stern, 1985; Shimron, 1990), or a combination of some of these processes (e.g. Shimron, 1990; Abdeen et al., 1992). Therefore, the present working group stu- died a number of critical areas to assess whether the Shield as a whole is dominated by a single structural domain or whether there are various domains of contrast- ing deformational type and tectonic evolution. The investigated areas comprise the extreme south of the Eastern Desert of Egypt (Wadi Allaqi to Abu Swayel) and several areas in the central parts (Wadi Hafafit, Wadi Ghadir, Wadi Igla, Gabal E1 Shallul, Gabal E1 Sibai, Wadi Queih; see Figs 1, 2 and 6). These areas were surveyed by detailed structural geological and strain studies, supported by petrographic, metamorphic and geochemical work and remote sensing. Following a brief account of the tectonostratigraphy of the rock types involved and the tectonic evolution, the major structural characteristics of these areas are briefly presented and are used, together with published data, as a base for a regio- nal synthesis and a discussion of new implications with

I I I

MEDITERRANEAN SEA

~ ~ 0 ~ 200 31 ° I I

km

CAIR ] S I N A I

29°1 SAHARA

2 7 ° -

SAHARA ~ ~NUBI~i~

As.= l ,l,,lllllll,llk . 23 °- uweinat-Bir Safsaf-Aswan Uplift " '

2;0 311 o 3'30 3'5 o 37 °

Fig. 1. Distribution of Precambrian basement (vertically hatched) as exposed in the footwall of the northern Red Sea rift and associated rift systems towards the north (compiled from E1 Gaby et al., 1988; Schandelmeier et al., 1988; Stoeser and Stacey, 1988). The Precam- brian is divided into the Pan-African, A r a b i a n - Nubian Shield and a pre-Pan-African craton towards the west, the East Sahara Craton. A tentative boundary between these domains has been based on isotopic data (e.g. Harris et al., 1990). The present paper is concerned with the Nubian Shield in Egypt and in particular with the areas shown in detail on Figs 2 and 6 (boxes)

486

regard to tectonic evolution, terrane margins and poten- tial inversion and reactivation of the Pan-African struc- tures.

Tectonostratigraphy and time constraints

Two-tier subdivision

A coarse, two-tier subdivision has been applied through- out much of the Arab ian - Nubian Shield (e.g. E1 Rarely, 1972; Hermina et al., 1989; Shimron, 1990). In the central part of the Eastern Desert Bennet and Mosley (1987) established a subdivision with suites of gneisses at the bottom as tier 1 and structurally overlying low grade successions as tier 2. E1 Gaby et al. (1988; 1990) applied a two-tier subdivision for the whole Egyptian part of the Arab ian-Nubian Shield using the terms 'infrastructure' and 'suprastructure' for the lower and upper levels, respectively. The infrastructure is implied to consist mainly of pre-Pan-African (continental) crustal rocks, whereas the suprastructure is of Pan-African origin (e.g. E1 Gaby, in press). However, this interpretation ist not unanimously accepted (e.g. Kr6ner et al., 1988; Harris et al. 1990) and, therefore, the 'neutral' terminology of two tiers is used here. A comprehensive review of formations and lithology and relevant references was compiled by Hermina etal. (1989) and only a brief account is given here. The upper level, tier 2, is composed of essentially low (subgreenschist to biotite, with local high pressure or high temperature assemblages) grade metavolcanic, metasedimentary and plutonic rocks. The tier 2 rocks can all be related to igneous and sedimentary rocks of the oceanic lithosphere, of accretionary prisms and subduction zones and evolving island arcs of Neopro- terozoic, Pan-African age (e.g. Ries et al., 1983; Kr6ner et al., 1987). Tier 2 also contains late tectonic, molasse- type sediments (e.g. the Hammamat Group with the basal Igla Formation; Akaad and E1 Rarely, 1958), and bimodal, andesitic Dokhan volcanics and the Attala Felsites (e.g. Stern and Gottfried, 1986; Stern etal., 1988). Locally, tier 2 may include isolated slices of gneissic rocks (e.g. Shimron, 1984; E1 Bayoumi and Greiling, 1984; E1 Akhal 1993), but these rocks are not a major constituent. In contrast, gneisses prevail in the rock assemblage of the lower level, tier 1, which is generally exposed structurally beneath the higher units. Although dominated by granitoid gneisses, the tier 1 se- quences also contain ultramafic, mafic and intermediate igneous rocks and metasediments. Their metamorphic grade is characterized by relatively high temperature (migmatites) and pressure (e.g. garnet, locally kyanite; e.g. Stietzel, 1987; Rashwan, 1991; Abdel Khalek et al., 1992), indicating lower crustal conditions. Lithologically and geochemically these rock sequences relate to those of the oceanic lithosphere and island arcs (e.g. Rashwan, 1991; Kamal El Din Saber 1993). Therefore, there is no conclusive evidence that tier 1 is considerably different from tier 2 with regard to protolith and geochemistry (e.g.

Rashwan, 1991 ; see, however, E1 Gaby et al., 1988). The gneisses are distinct from the low grade rocks only by their metamorphic grade and fabric. This deformational fabric has no age implication (e.g. gneisses are not necessarily older than low grade rocks; Kr6ner et al., 1988) but is, apparently, an expression of the general increase in deformation intensity towards depth. Conse- quently, gneissic domains usually represent relatively low crustal levels and the surrounding low grade rocks can be taken as representing upper crustal rocks.

Timing of the orogenic evolution

Ages of early Pan-African igneous and metamorphic events vary widely between about 900 and 600 Ma and across the Arabian-Nubian Shield (e.g. Stern and Hedge, 1985, Kr6ner et al., 1987; 1988; 1992; Stoeser and Stacey, 1988; Miller and Dixon, 1992). At the end of this early Pan-African evolution, Beyth et al. (1994) recogniz- ed a transitional period between the early compressional tectonic phase with calc-alkaline, collisional, I-type grani- toid batholiths and a subsequent extensional phase with A-type alkali granites. This transitional episode can be followed across the northern part of the Arab ian-Nu- bian Shield and, in Egypt, lasted from about 615 to 600 Ma (Beyth et al., 1994). The ages of the igneous rocks and molasse sediments related to the extensional event fall in a similarly narrow time span between about 575 and 595 Ma ago (e.g. Stern and Hedge, 1985; Willis et al., 1988; Miller and Dixon, 1992). This time interval is assumed here to define the age of the extensional collapse. The end of Pan-African activity is marked by some post-tectonic granitoids (Younger Granite, etc.) and related rocks, which vary in age from about 580 to 530 Ma (e.g. Sturchio et al., 1984; Willis et al., 1988). Some of these rocks intrude and 'seal' the shear zones related with compression and transpression after the extensional collapse (see later).

Regional structure

Shear zones within and between the low grade (tier 2) and gneissic domains (tier 1) document substantial tectonic transport which eventually led to the present map pattern. This deformation also formed structural culminations, in the core of which the gneissic rocks are exposed. Results of detailed studies on major antiformal domains (e.g. Sturchio et al., 1983; Greiling et al., 1988 a; Kamal el Din et al., 1992; Abdel Khalek et al., 1992; Wallbrecher et al., 1993) reveal that such structures are dominated by either compressional, extensional or strike-slip linked faults. Detailed examples of these respective deformational types are treated in the following, together with an account of the structure of potentially extension-related molasse basins. Tables 2--4 provide a brief summary of the structural evolution of various domains which are representative of tiers 1 and 2, respectively. It is remar- kable in this context that all of the known late orogenic, molasse-type sediments rest unconformably over tier

487

Fig. 2. Structural sketch map of the central part of the Eastern Desert of Egypt, simplified from Bennet and Mosley (1987), Greiling et al. (1988 b), Sultan et al. (1988), Tehrani et al. (1988). For location, see Fig. 1 ; further explanations on the figure and in the text

metamorphic core complex and thrusting? Gabal Meatiq

basin formation, thrusting and transpression in Wadi Queih,Figure 7

Wadi Zeidun£

Figure 4 ..~

normal faulting/ shearing at Gabal El Shalul granite

- 25ON

0 L

km

~.,~ SAFAGA

Wadi Meesar

f ...¢,"

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Wadi Shait

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s°l ! ,

. 134°E

QUSEIR

PHANEROZOIC COVER T I E R 2

['--'-1 PAN-AFRICAN LOW GRADE SUCCESSIONS, INCL. LATE OROGENIC IGNEOUS AND

~ SEDIMENTARY ROCKS T I E R 1

[ - - ~ GNEISSIC UNITS,

STRIKE-SLIP FAULT

LOW-ANGLE SHEAR ZONE

,4'

÷ 5",

MARSA

extension and formation ol metamorphic core complex, Gabal El Sibai

reverse faulting in 'Wadi Umm Nar

• extension/basin formation in Wadi Igla, see Figure 3 thrust systems of the

• Wadi Hafafit Culmination,

• Wadi Ghadir etc.

Wadi Ranga

'{ Wadi K a s h a / ~ ( ~ Wadi Ghadir "accretionary prism/ fore-arc section

minor late-tectonic X restores to south , of Wadi Hafafit

extension 35OE ' Culmination

2 rock sequences and no primary sedimentary contact of molasse-type sediments over tier 1 gneisses has been observed (e.g. Egyptian Geological Survey, 1981; Bennet and Mosley, 1987). Together with structural data (Table 2 and below) this observation indicates that molasse-type sedimentation took place before or during the activity of regional shear zones, which brought the tier 1 gneisses up to their present, relatively high level in the crust. This relationship will be used to speculate on the absolute timing of the Neoproterozoic structural evolution in Eastern Egypt. Meanwhile, as a working hypothesis, all the molasse-type deposits discussed here are taken as contemporaneous, about 595-575 Ma old (e.g. Willis et al., 1988). Consequently, extension related to molasse basins is of about the same age; subsequent compression and wrench-faulting took place later but before the end of the intrusion of post-tectonic granitoids ca. 530 Ma ago (see previous section). According to this time sequence,

examples of structures related to the evolution of molasse basins are discussed first followed by examples of sub- sequent compressional deformation and wrench-faulting.

Structures related to the evolution of molasse basins

Wadi [gla basin

The Wadi Igla basin (Fig. 2, Rice et al., 1993), is the type locality of the Igla Formation, which is assumed to be the oldest part of the Pan-African molasse sequence (e.g. Akaad and E1 Rarely, 1958; Hermina et al., 1989). The basin occupies a half-graben structure with thick basal conglomerates along the south-eastern margin, associ- ated with several large Atalla Felsite intrusions, reflecting the major (extensional?) fault zone. Some Younger Granites were intruded before sedimentation as they are unconformably overlain by the basal conglomerate or form abundant clasts within the basal conglomerate while

488

A N B N

:e Veins nd ~-~(i3reccias. N=56 N=2~07

Vector Mean 072 ° Vector Mean 064 °

C N N

Normal Faults ,da N=

6 4 * Poles to Normal Faults.N=58.

• Poles to Fibrous Quartz Veins. Mean 330°-01 °, N=130

Fig, 3A--D. Orientation of extensional features from the Wadi Igla basin (see Fig. 2 for location). A Direction rose showing orientation of extensional quartz carbonate veins in Wadi Igla. B Direction rose showing trend of dyke margins. C Direction rose showing trend of normal faults in Wadi Igla. D Stereonet showing orientation of normal faults and (extensional) quartz veins

some minor intrusions along the northern margin cut the molasse sediments. Doleritic to andesitic dykes with a dominant NE - SW trend are particularly abundant in the north-east (Fig. 3 B); these cut both the Hammamat Group and Atalla Felsite. The Igla Basin has been affected by minor N N E - SSW dextral transtensional strike-slip defor- mation (Rice et al., 1993; Stanek et al., 1993). Although no major strike-slip fault has been found cutting the molasse sediments, subhorizontal slickensides are abundant on joints, dyke and quartz vein margins, indicating that deformation was post-sedimentation. Abundant fibrous quartz veins lie parallel to the mafic dykes (Fig. 3); the fibres indicate extension normal to the vein margins. Normal faults are also common, although difficult to interpret due to the presence of older Pan-African faults and those associated with the Red Sea rift. Late Pan-African compressional structures (folds and thrust faults) are rare and a weak subvertical cleavage is locally developed (Abdel Khalek and Hafez, 1986). However weak, the late compression may be of general importance as an indication of late Pan-African inversion of a molasse basin.

Minor basins near the Wadi Hafafit Culmination

To the south-east of Wadi Igla, Hammamat Group sediments in Wadi Ranga are exposed close to the coast, forming a thin N N W - S S E trending series of outcrops overlain by Miocene sediments (Fig. 2). They comprise a relatively thin basal conglomerate which, in turn, rests on volcanic rocks. Atalla Felsites are present both as large bodies close to, or underlying, the basal conglomerate and as thin sills within the molasse. Andesitic sills up to 7 m thick, which have extensively baked the marginal

sediments, are also present within the molasse and can be traced along strike for several kilometres. Apart from the regional tilting to the west and localized north-south to NE - SW faulting, both of which may be related to either molasse basin formation or Red Sea rifting, no deforma- tion has been observed (Akaad and E1 Ramly, 1958). A similar lack of small-scale deformation has been noted in the molasse sediments at Wadi Kashab and at Wadi Shait, immediately to the south and north of the Wadi Hafafit Culmination, respectively (Fig. 2). However, the latter occurrence is folded into a fault-bend fold (which originated during thrusting over a ramp) and is thus part of a thrust system that is related to the Wadi Hafafit antiformal stack (see below). From the regional context it can be inferred that the molasse deposits of Wadis Ranga and Kashab also form part of thrust systems (Tehrani et al., 1988; Greiling et al., 1993). Farther south of the Wadi Hafafit Culmination no major extensional defor- mation has been observed in the Eastern Desert (e.g. in the Gabal Muqsim ~Wadi Allaqi area). However, con- glomeratic sediments to the north of Abu Swayel may be candidates for molasse sediments (Fig. 6) and, therefore, also be' related to basin formation and extension (see also E1 Gaby et al., 1988).

Normal faults and rnolasse basins north of Gabal El Shalul

In the area to the north and north-east of Gabal E1 Shalul three molasse basins have been studied, Wadi Zeidoun East, Wadi Zeidoun West and Wadi Meesar (Osman, 1994; see Fig. 2). The first two are dominated by thick deposits of basal conglomerate. Dokhan volcanics are absent, although Atalla Felsite dykes were emplaced along eas t -wes t and N W - S E oriented faults and a Younger Granite has extensively contact metamorphosed the Hammamat Group in Wadi Zeidoun East (Osman et al., 1993). In Wadi Zeidoun no deformation has been recorded, possibly due to the massive nature of the rocks. In Wadi Zeidoun East and Wadi Meesar a weak pebble preferred orientation towards the west and north has been recorded (Fig. 4 A - C). Pebbles in the metavolcanic basement show a marked preferred orientation to the north-west, related to the extensional shear zone at the Gabal E1 Shalul granite margin (Fig. 4 D, E and below). In Wadi Zeidoun East a cleavage is developed, associated with large-scale upright folding (Osman et al., 1992), documenting subsequent (local) compression.

Low angle normal faulting and transtension

Gabal El Sibai Antiform

The Gabal E1 Sibai antiform consists of a metamorphic core of gneisses, garnetiferous migmatites and amphiboli- tes with granitoid intrusives (tier 1), exposed beneath a rim of low grade metavolcanic and metasedimentary rocks (tier 2; Kamal E1 Din, 1993; Fig. 2). A summary of the tectonic evolution is given in Table 2. The two tiers are separated by (early) low angle shear zones (D3 of Table 2) towards the north-east and south-east, a high angle shear zone with sinistral strike-slip towards the south and (late)

489

Table 2. The sequence of deformational, magmatic and metamorphic events in tier i gneisses as tentatively related to the four major phases of the tectonic evolution of mountain belts (compare Table 1), modified from Kamal E1 Din (1992), Rashwan (1991) and E1 Ramly et al. (1993). For location see Figure 2.

Gabal El Sibai area gneisses

Magmatic activity Deformation Metamorphism

b S regional folds with axial surface NW-SE

late tectonic intrusions, El Dabbah and El Sibai granites

D 4 imbrication of major shear-zone, extensional faults

D 3 regional low-angle shear- zone, mylonitic foliation

local retrogressior

Um Luseifa porphyritic granite

Et Mirifiya~ Um Shaddad intrusions, alkaline granite

D 2 foliation

calc-alkaline tonalite, El Shush

D I penetrative schistosity regional meta- morphism, green- schist and amphi- bolite grade, local migmatites

amphibolites, probably ophiolitic

B

A

3

2

I

Migif-Hafafit gneisses

~lagmatic activity Deformation Metamorphism

D 9 open folds, generally flat-lying axial surface, uplift of granitoid domes

/ounger granite D 8 thrusting/faulting, mylonite retrograde

formation, regional metamorphism, low-angle thrusts and metasomatic related fault-bend folds, various directions changes

D 7 variable open to tight drag- folds, NW-SE to WNW- ESE directions, crenulation /fracture cleav~e

not documented ?

;ranite/pegmatite

D 6 open folds, NE-SW to E-W directions, crenulation cleavage

D 5 local faults, mylonites

D 4 penetrative foliation, iso- clinal folds, irregular folds

:alc-alkaline :rondhjemite

:alc-alkaline tonalite

:alc-alkaline gabbro

D 3 penetrative foliation, isoclinal folds

D 2 pentetrative foliation, metamorphic banding

D I planar fabric

contact metamorphism

regional metamor- phism (staurolite- kyanite), local migmatization

netagabbros, ultra- nafic complexes, )phiolitic

normal faults towards the west and north (D4). Shear criteria such as S - C fabrics and asymmetrical porphy- roclasts document a transport (top) to the south-east (120°), consistent with sinistral strike-slip at the southern margin of the antiform (Greiling et al., 1993). Further- more, the western part of the antiform is intruded by at least two major, post-deformational granite plutons (Kamal E1 Din et al., 1992). Similar to the Wadi Igla area (see above) the extensional deformation is followed by (weak) compression (D5 on Table 2). To the west of the Gabal E1 Sibai area further extensional structures are exposed, related in particular to the Gabal E1 Shalul granite and a number of molasse basins.

Gabal El Shalul culmination

Apparently similar to the Gabal E1 Sibai antiform, the Gabal El Shalul granite (gneiss) forms an open antiform, the western and perhaps major part of which is covered by Phanerozoic sediments. However, at the north-eastern and eastern margins of the Gabal E1 Shalul granite a major low angle shear zone is exposed (Osman et al., 1992). Transport-parallel stretching lineations and shear criteria document a transport of overlying low grade metasediments towards the south-east (see Fig. 4 D, E). The structural situation is, therefore, comparable to that of the Gabal E1 Sibai area and it is proposed that the

490

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1°'~" 2!0 3:0 4.'o Y/Z Fig. 4. Flinn plots and stereonets showing shape and long (X) axis orientation of deformed pebbles, respectively. Diagrams A to C refer to Hammamat (molasse) sediments, diagrams D and E to underlying volcanogenic metasediments. A Wadi E1 Gurd (northern part of Wadi Zeidun - west basin); B Wadi Zeidun - west; C Wadi Meesar; D at the contact with the E1 Shalul granite; E Wadi Me•sat; see Fig. 2 for location

Gabal E1 Shalul culmination is a similar metamorphic core complex.

Gabal Meatiq metamorphic core complex

The Gabal Meatiq area is characterized by a domal structure, which has often been studied during the past decade. There is, however, as yet no generally accepted model for the origin of the structure. Interpretations range from fold interference patterns (e.g. E1 Gaby et al., 1988) to culminations in thrust systems (e.g. Ries et al., 1983; Sturchio et al., 1984; Wallbrecher et al., 1993) and to metamorphic core complexes (e.g. Sturchio et al., 1983). In the light of the new recognition of metamorphic core complexes in the Gabal E1 Sibai and Gabal E1 Shalul areas and the overprinting relationships in the Wadi Queih area (see later), it is speculated that the Gabal Meatiq complex originated as an extensional structure, which was overprinted by compression.

Low angle reverse faulting and transpression

Wadi Hafafit Culmination

The Wadi Hafafit Culmination (WHC) is the most conspicuous part of a larger scale antiformal domain, where various gneisses are exposed beneath low grade successions (Fig. 2). The gneisses represent suites of igneous rocks of ocean floor and volcanic arc characteris- tics and related metasediments, together with a metasedi- mentary unit derived from a pre-Pan-African source area (Rashwan, 1991). Table 2 gives a summary of the complex structural evolution. The structure of the WHC is domi- nated by low angle thrusts (e.g. E1 Rarely et al., 1984; D8 on Table 2) and sections across the WHC show the thrusts to be linked to floor and roof thrusts, respectively (e.g. Greiling etal. , 1988a; 1993; Greiling and E1 Rarely, 1990). Therefore, the gneissic units form an antiformal stack beneath overlying low grade successions. Various shear criteria show a tectonic transport towards the north-west (e.g. Greiling et al., 1993; Greiling and Rash- wan, in press) and, consequently, an overall compression in a N W - S E direction. A steeply dipping S E - N W trending lateral ramp, misidentified earlier as a strike- slip fault (Sultan et al., 1988), bounds the WHC anti- formal stack towards the north-east, where it is overlain by thrust units of the low grade succession (El Bayoumi and Greiling, 1984). These low grade units include the well known and well documented ophiolite fragments of Wadi Ghadir.

Wadi Ghadir area

Detailed structural work in the Wadi Ghadir area (El Akhal, 1993) revealed a general compression of the area through low angle thrusts. These thrusts are part of the regional thrust system that extends as far west as the Wadi Hafafit Culmination (El Bayoumi and Greiling, 1984). However, in contrast with the Wadi Hafafit Culmination gneisses of tier 1 and, presumably because of their higher crustal level, the pre-thrust metamorphic and structural

491

Table 3. Sequence of tectonic events in the tier t gneisses of the Gabal Muqsim area, modified from Sadek (1994). For location see Figure 5, tectonic phases 1 - 4 as in Table 2.

_~ Muqs im area gneisses

I Magmatic activity Deformation Metam,

4 B

4 A

late granitoid intrusions

D 4 weak foliation

D 3 regional folds, crenulation and kink bands

Abu Fas-Um contact Dorni-EI Deiga metamorphism abbroic intrusions

D 2 shear fabric, rny lon i t i c foliation & stretching lineation, regional thrus- ting and transpression

~rly tonalite ntrusion

D I open-tight folds, regional penetrative foliation, metamorphism metamorphic banding rnigmatites

sland arc andesites md related tufts; ,~eneratJon of an 3ceanic CrUSt iophiolitic ~erpentinites, lrnphibolites and netagabbro)

evolution in the tier 2 rocks of the Wadi Ghadir area, as shown in Table 4, is less complex than in the gneisses. This may be why the early, pre-orogenic structures, as related to the island arc evolution, are relatively well preserved. For example, the fabric of a deep-sea trench tectonic m61ange can be clearly distinguished from later collisional or post-collisional structures (El Bayoumi and Greiling, 1984; E1 Akhal, 1993). These observations support the fundamental results from E1 Sharkawy and E1 Bayoumi (1979), Shackleton et al. (1980), E1 Bayoumi et al. (1983) and E1 Bayoumi (1984) about the existence of a tectonic m61ange in the area and rule out contrasting views (Sultan et al., 1988). Therefore, it can be concluded that the Wadi Ghadir area represents a section across a fossil deep-sea trench/accretionary prism and forearc (e.g. E1 Akhal and Greiling, 1987; Greiling et al., 1988a; E1 Akhal, 1993). Both structural criteria and the magmatic succession of arc-related intrusives from basic in the south-west to acidic in the north-east point to an island arc facing south-west, i.e. oceanic lithosphere in the south-west was subducted north-eastwards. Restoration of the Wadi Hafafit antiformal stack restores the Wadi Ghadir forearc domain to the south-east of the Wadi Hafafit island arc domain (Greiling etal., 1993; see Fig. 2) and it is suggested that both domains are part of the same island arc terrane.

Table 4. Sequence of tectonic events in tier 2 sequences of the Wadi Ghadir and Wadi Queih areas, respectively. Modified from E1 Akhal (1993) and Abdeen et al. (1992). For location see Figure 2, tectonic phases 1 - 4 as in Table 2.

W a d i Ghadir area

Magmatic activity Deformation Metamorphism

late orogenic, red granite, dark dykes

D 6 regional open folds

D 5 nappe transport

granite, grano- diorite, fine-grained granite aplite

D 4 kink folds, almost horizontal axial surface

D 3 congruent folds. crenulation foliation

diorite, granite

calc-alkaline dark dykes, aplite, tona- lite, granite dikes

dark dykes

D 2 isoclinal/tight folds. penetrative foliation

D I metamorphic banding. foliation

greenschist grade metamorphism

m~lange-formation

generation of an oceanic crust (ophiolithic serpentinite, gabbro. sheeted dykes, pillow lavas)

early shear-zones and faults

413

4 ~

Wad i Queih area

Magmatic activity Deformation Metamorphism

D 3 NW-SE trending left-lateral strike-slip faults

D 2 NW-trending folds with thrusts towards NE, along the folds' axial surface

D I thrusting towards N/NW and folding along E-W axes

Atalla Felsites extension 7.

basin formation

not documented 7.

greenscNst grade m e t a m o r hisph~m ~

island-arc magmatism

W.ALLAQI

33o'55 ,

o 1 [ I

3km I

%

LEGEND Younger granitoids

~ ' ] Layered gabbro, minor peridotite

Meta-a n desite, volcanoclaslic lulls and pyroclastic rocks

Actinolite chlorite epldote schists

[ IT[ ] 8erpentinite,talc carbonate rocks and amphiboliles

Gneissic tonafite

Quartzo-feldspathlc qneisses, micaschists, meta-greywackes, amphibolite bands

Massive meta-andes[les and metagabbros

f~" Layering trend in gabbro

--)(-- Axial trace of syntorm

4)- Axial trace of antlform

IJneaiion

/ Layering strike and dip

.-~ Foliation strike and dip

Shear zone

Thrust contact

i Faulls

~7

I 34 ° E

W ALLAQI

~V~VVV~VVV ~

492

Fig. 6. Structural compilation map based on a satellite image interpretation and including information from Fig. 5, Taylor et al. (1993) and our own field data. See Fig. 1 for location. The structure is dominated by the Allaqi shear zone as defined by Taylor et al. (1993) and is part of the Wadi Allaqi-Gabal Heiani belt as identified towards the east by Kr6ner et al. (1987). This shear zone changes its character gradually from sinistral strike- slip in the south-east to reverse fault/thrust at Wadi Allaqi and towards the north and changes back to sinistral strike- slip in the north-west. Related transport lineations are shown in the inset stereonets, divided into the area of Fig. 5 in the south-east (GM, below) and the area around Abu Swayel (AS, above). The location of Fig. 5 is marked in the south-east corner. For the sake of clarity, lithology and structure are simplified. Mylonites partly from Taylor et aL (1993)

. • Lake Nasser

N

[ ~ Late tectonic granite

[ - ~ gabbro intrusion Tier 2 ophiolite remnants

[ ~ low grade sequence

Tier 1 [ ~ gneisses,

migrnatites

N=44~'~.,., GM

\ - - - \L?"-\

strike and dip \ x :r-l~.\ \ ' ~ _'~J x ~ ~ ?

fault, shear zone ~ -- ~ t ~ # ' x x x X x X x L ~ " ~ ~ - . \ ~ ~ , .- ,

my,onite \ \ , - - T '---" % , - ~ ' ~ ~ - ( " ~ I !l I:< I I I IH iMuq~mj

~1 Fig. 5. Geological map of the area between Wadi Ungat and Gabal Muqsim south of Wadi Allaqi in south-eastern Egypt. The structure is dominated by a shear zone, extending from the south- east corner of the map towards the west and curving towards the north into the north-west corner. This shear zone separates low grade, calc-alkaline metavolcanics in the south-east (tier 2) from gneisses and associated rocks (tier 1) towards the north-east and north. Towards the east and north-east these gneisses are overlain along low angle shear zones by (i.a.) ophiolitic fragments and associated low grade sequences of tier 2. For location, see Fig. 6, which also contains a stereonet showing lineations related to tectonic transport along the shear zone

FiB. 7. Structural geological map of the Wadi Queih area, north- west of Quseir (see Fig. 2 for location). Major structural elements are NNW directed early thrusts and associated east--west trending folds (D1), followed by north-west trending folds and associated north-east directed late thrusts (D 2). These latter structures are accompanied and followed by major N W - S E trending left-lateral transcurrent faults, which are related to the regional Najd wrench fault system (e.g. Stern, 1985). Normal faults are related to late Mesozoic-Cenozoic Red Sea rifting

493

The Gabal Muqsim, Wadi Allaqi and Abu Swayel areas

Field studies in the Gabal Muqsim area revealed the presence of a tier 1 succession of gneisses and schists (Sadek, 1994; Fig. 5). A summary of the structural evolution is shown in Table 3. Towards the east, the tier 1 gneisses are overlain tectonically by the Gabal Muq- s im-Gabal Um Domi ophiolitic assemblage, towards the north by low grade metasediments (tier 2). The northern boundary of the gneisses is an east -west trending high angle shear zone (north of Wadi Allaqi; Fig. 5), characterized by steeply plunging stretching linea- tions and associated stretched pebbles (orientation shown in stereonet in Fig. 6). The steep attitude of the shear zone and related lineations is due to folding along east -west trending folds (Fig. 5), which are related to north-west directed transpression (D2 on Table 3) and, if restored, imply a shearing/transport in about a north-south direction. This shear zone is only schematically shown in Fig. 5, as it has not been mapped in detail. Both the gneisses and the ophiolitic assemblage were juxtaposed

~ ' ' ~ x ~ x x ...< > c > < x x x x x x ~

x x x x x ~ . ~ . ~ x x x

• < x ~<x X x x x x x x x > Z x X x ,~ . X x X x X x >

x x x x ~ : x X x X x > < } x X x X x X x : " : ~ x x x

x x x x x x x > < . X x X

> < X x X x ~ r x x " x x x x X x X x x x x x

x x ~. x x )< x

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x x ~ x ~ x X x x x x x

• x x ¸ x .

x > ~ x X x ~ < x > ~ x X x ~ x >< x ~< x x x X x x . x X x X ~

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x ~ x x x ~,",< x x x , , .<Xx~,__ ~.. > . . . . . . x x x x x

x x x x x X x x , ~,~, ××××××××~.×××2× x : ~ x x

:::.::.:..:::....:...:..:..::.::...~::....`~..~:..:....~....~..::..:..:..:...~.`:...~..::..:

~ ¢ x X ~ x

x x x

:::::::

• Atalla Felsite

Hammat-(molasse) clastic sediments:

~ Sand/siltstone, conglomerate

[ - ~ Basal conglomerate

F ~ pre-Hammamat "basement"

early thrust

late thrust

'~ . normal fault

".. Direction and \ plunge of

stretching • lineation

.~'x~ ~l~n°~es2f° ws ,.~ slickensides - ~ on fault , ~i-~ surfaces

1 I

km

x x

X x X x >~

x X x X x x X x X x X x X x X x >

• x X x Z ~ x X x >'z- : < X x X x X x X x > < x x x ;~< x ~< x x x x x x x x x x

X x X x X x X x X x X x x x ~< ~-c ~ x x x x x x x x x - x >< x x x X x ~ x x x x x

x x x x x >c~ >~ x ~<" x X 50.< x x x x x x x x x x x x x x x x x > C x X x X x > ~ x > < x X x X x X x X x X . . . . ~ X ~ X X X X x x . . . . ~ x x

: . >< 5< X X X ~<

< \X X X Xl0>< 5 x x x X x x ~< > < x x x

~ X x X ~ X x

X

494

against low grade meta-andesites and their pyroclastic equivalents towards the south and west along a major shear zone (Fig. 5). This shear zone is part of the so-called Allaqi-Heiani Belt (Kr6ner et al., 1987), which can be followed for over 200 km in the southern Eastern Desert of Egypt (Fig. 6 and below). Along the main shear zone the meta-andesites are highly sheared, mylonitized and transformed into actinolite-chlorite-epidote schists. They show a pencil-like structure and exhibit mineral lineations marked by the alignment of epidotized feld- spars. Several lenticular slices of serpentinite and talc-carbonate rock are incorporated within these schists. They are mostly remnants of (thrust) slabs from the main ophiolitic ultramafic body, their trend is gene- rally N W - S E , concordant with the general trend of the shear zone. Most of the recorded stretching lineations (low angle lineations in stereonet in Fig. 6) and the kinematic criteria indicate a sense of movement with a general orientation of the top towards the north-west. In these meta-andesites of Wadi Abu Fas-Wadi Ungat (south-west corner of Fig. 5) and in the schists to the north, the foliation essentially strikes W N W - E S E and locally N W - S E , generally with a dip towards the north-east (Fig. 5). North-westwards this trend gradually swings towards the north-west and cuts through Wadi Allaqi, Wadi Murra and Wadi Umm Rilan (Taylor et al., 1993). It continues to reach the Abu Swayel area, where it curves back into a WNW direction. Figure 6 shows a structural compilation map based on a satellite image interpretation, which includes both the Gabal Muqsim and the Abu Swayel areas. From this map it becomes clear that the fault systems documented around Wadi Allaqi (Taylor et al., 1993), in the Gabal Muqsim area towards the east (Sadek, 1994) and in the Abu Swayel area towards the north-west (e.g. E1 Shazly et al., 1977; our data) form a linked system of (mainly) thrusts in the central part with transitions towards transpressional and sinistral strike-slip systems towards the north-west and south-east, respectively.

Wadi Queih basin

A complex structural evolution after (molasse) basin formation has also been documented in the northern Eastern Desert, in the Wadi Queih area (Fig. 2). The Wadi Queih basin (Fig. 7) lies between two major Red Sea rifting faults. Basal conglomerate mainly crops out at the northern margin, overlying Dokhan Volcanics, and the asymmetry of these conglomeratic deposits within the basin suggest an eas t - west oriented half-graben geomet- ry. After emplacement of the Atalla Felsites (penecon- temporaneous with molasse sedimentation), three main phases of deformation affected the basin (Abdeen et al., 1992; Table 4). In the first event, compression resulted in the development of essentially east-west oriented thrusts and folds, locally overturned, along the northern margin. Along the southern margin, thrusts define most of the basin margin, with the pre-Hammamat basement thrust

N

4-

O

O

A N=23

0

B N=I/.

N

Fig. 8. Stereonets showing lineation orientation in the Wadi Queih area. A Stretching lineations and orientation of long axes of stretched pebbles. B Slickensides related to north-west directed (early) low angle thrust surfaces. C Slickensides related to left-lateral high angle wrench faults. Contours at 2 - 4 - 6 - 8-10-12%

both over the Atalla Felsite and the Hammamat Group. These thrusts moved towards the NNW and are associat- ed with the development of a marked preferred orienta- tion in conglomeratic pebbles (Fig. 8A and 8 B). The parallelism of the early structures with the east-west orientation of the basal conglomerates in the north suggests that their orientation may have been controlled by inversion of the extensional structures; a similar model is less easy to sustain in the south, due to the lack of primary data on the basin margin orientation. These early structural elements are overprinted by north-west trend- ing folds and some associated north-east directed thrusts. These latter were observed by Ries et al. (1983) and

495

interpreted as backthrusts synchronous with (north) west directed thrusts. However, the observed geometric and overprinting relationships imply a two-phase origin for these two sets of thrusts. In the last phase of deformation sinistral strike-slip faulting occurred along a number of discrete N W - S E to W N W - E S E oriented curvilinear and branching subvertical faults (Figs 7 and 8 C). These cut both the Hammamat Group, with its related volcanic suites, and the metavolcanic sequences to the south. These faults form a large positive flower structure, which is cutting the earlier thrust with an associated footwall syncline. Farther away from the fault, other N W - S E trending folds, probably tip folds over blind imbricate thrusts of the flower structure, are developed.

Discussion

Regional structure

Figure 9 shows a compilation map of the structural grain of the Proterozoic basement in the Eastern Desert of Egypt. It shows a number of domains which can be distinguished by their contrasting structural grain. Based on the orientation of structures, the basement can be divided, from north to south, into a domain of N W - SE trending grain (Najd trend, e.g. Stern 1985), followed by one of broadly eas t -wes t grain, including the Wadi Hafafit Culmination. South of the Wadi Hafafit Culmi- nation another domain of N W - S E grain is delimited southward by the equally N W - S E trending zone of Wadi Khar i t -Wadi Hodein. South of this zone and towards the Allaqi-Heiani Belt is an area with variable orientations, mainly eas t -wes t and nor th-south . Whereas the N W - SE grain domains can be easily related to late stage Najd (transpressional) faulting (e.g. Stern, 1985; Shimron, 1990; Stern et al., 1990; Hussein et al., 1992), the origin of the structural grain in the other domains may be more complex (Tables 2 -4 ) . The southern area is characterized by essentially compressio- nal/transpressional deformation and a major extensional collapse structure has yet to be documented, apart from minor extensional structures at the southern margin of the Wadi Hafafit Culmination (Rice et al., 1992; Fig. 2). Possibly, north o fAbu Swayel (see Fig. 6), this compres- sional regime has overprinted minor molasse basins, which can be interpreted as extensional features. How- ever, there is no direct structural evidence of extension in this domain. In contrast, abundant extensional structures characterize the domain in the north (see section on molasse basins and, e.g. Stanek et al., 1993). In the area marked as 'extension-dominated domain' in Fig. 9, exten- sional basins and metamorphic core complexes are only weakly affected by later deformation, whereas extensio- nal features farther north are overprint both by reverse faulting and (transpressional) strike-slip faulting (e.g. Wadi Queih basin; see Fig. 7 and 8).

Tectonic evolution

Early evolution

Early collision in the south of the Eastern Desert was probably relatively gentle (e.g. Shackleton, 1986; Miller and Dixon, 1992) and led to an accretion of a number of terranes with only weak deformation. Alternatively, traces of a more violent collision may have been severely overprinted during the subsequent structural evolution and are no longer readily recognizable at a regional scale, as has been found, for example, in the Wadi Hafafit Culmination (e.g. E1 Rarely et al., 1993). Tables 2 and 3 show local examples of pre-collisional structures, which originated in an oceanic island arc stage as they are intimately related to island arc igneous activity (e.g. E1 Akhal, 1993; E1 Rarely et al., 1984; Rashwan, 1991; see also Miller and Dixon, 1992). Although collision may have been rather gentle, at least in the north it was succeeded at about 600 Ma ago by crustal extension (Table 1), documented both by molasse basin formation and extensive magmatism, which was at least partly derived from mantle sources (e.g. Stern and Gottfried, 1986; Stern et al., 1988) and thus implies mantle uplift. This situation is comparable to that of late orogenic extensional collapse (Dewey, 1988) and of convective removal of upper mantle lithosphere and consequent uplift of asthenosphere (e.g. Molnar et al., 1993). Both these scenarios are the consequence of earlier lithospheric thickening due to plate collisions. By inference, such a lithospheric thickening must have also preceded the crustal thinning in the Arabian-Nubian Shield.

Development of regional structures

The post-collisional extensional stage, lasting from about 595-575 Ma ago (Table 1), was followed by shortening, mainly in a N N W - S S E direction, which led to folding and thrusting towards NNW (e.g. Wadi Hafafit, Wadi Queih, Tables 2 and 4; Greiling, 1987). Although of varying intensity, traces of this shortening have been found in all the areas studied. Subsequently, these compressional structures were overprinted by transpression, which is localized to particular zones, for example the Allaqi-Heiani shear zone and the Wadi Khari t -Wadi Hodein, the Hamisana and the Najd fault zones (Fig. 10). As is clear particularly from the Hamisana shear zone this transpression in itself is polyphase (e.g. Miller and Dixon, i992). This is consis- tent with the geometric relationship between the eastern extension of the Allaqi shear zone and the Hamisana shear zone. Figure 10 shows that the Allaqi-Heiani shear zone is apparently cut by the Hamisana shear zone. No Pan-African structures younger than those related to this wrench-faulting event have been ob- served.

If this evolution is interpreted in terms of orogenic processes, the regional compression after extension in- dicates a continuation of plate convergence and con- sequent compression/transpression after an episode of

496

J

- 26°N ~ QENA

IDFU

KOM OMI30" 6

. . . . . . . . ° . . . .

ASWAN

. " .

LAKE NASSER

• %

o ~ SAFAGA

Q U S E I R

MARSA ALAM

(2' J O J m

PHANEROZOIC COVER TIER 2

--'-]PAN-AFRICAN LOW GRADE SUCCESSIONS,INCL. LATE OROGENIC IGNEOUS AND SEDIMENTARY ROCKS TIER 1 GNEISSIC UNITS

EXTENSION- DOMINATED DOMAIN

~ STRIKE-SLIP FAULT , ~ LOW-ANGLE

SHEAR ZONE

;f~t FOLIATION, BANDING

, ABU , GHUSUN

RAS BANAS

% %

. f

33°E . • " ~\ 'k , \ \ ~34."E,

Fig. 9. Structural compilation map of the Pan-African basement in the Eastern Desert of Egypt, compiled from Figs 2 and 6, Dixon et al. (1987), E1 Ramly (unpublished map of gneisses between 23 ° and 25 ° N), E1 Ramly and Salloum (1974), Egyptian Geological Survey (1981), Greiling et al. (1988 b), Hassan and Hashad (1990), Hussein et al. (1992) and Tehrani et al. (1988). Most of the tier 1 gneissic units

in ~che northern part of the map area form structural culminations, whereas a number of gneissic units in the south form fault-bounded slices or horsts, imbricated into tier 2 low grade successions. For the sake of clarity the tier 1 gneisses are shown without their internal structural grain, which is usually discontinuous across the margins and more complex than that of the surrounding tier 2.

- 26ON

"Barramiya

22°N ' ~Uture

0 50 100 I r /

km

33OE 34OE I I

r ,

)/ 35~'E 36~E

Fig. 10. Tectonic sketch map of the Arab ian -Nub ian Shield in eastern Egypt, simplified from Fig. 9 and extended southward to include the Hamisana shear zone (from Dixon et al., 1987; Kr6ner et al., 1987). The sinistral Wadi K h a r i t - Wadi Hodein shear zone may have connected the - perhaps once continuous - Allaqi and South Hafafit sutures (before being overprinted by the Hamisana shear zone?). Possible correlatives across the Wadi Khar i t -Wad i Hodein Zone may be the tier 1 gneisses to the north of the sutures (dark pattern), structurally overlying ophiolitic nappes (cross-hatched) and the late tectonic granites (G, crosses). Restoration of about 300 km of sinistral displacement along the Wadi Khar i t -Wadi Hodein Zone may bring together the ophiolite remnants of the Gabal Gerfand Barramiya nappes and the late tectonic granites near Aswan and in the north-eastern Desert and link the sutures in a general W N W - E S E trend

extensional collapse of the Pan-African orogen. In ge- neral terms, this example shows that extensional collapse (Dewey, 1988) is not necessarily the final stage of orogeny, but is one episode in the complex evolution of mountain belts and the lithosphere as a whole.

497

Potential sutures and terrane margins

As is clear from the structural evolution outlined above, potential 'sutures' and terrane margins were overprinted intensely by post-collisional deformation. Consequently, the most obvious, late orogenic shear zones cannot a priori be correlated with terrane margins (see discussion in Church, 1988; Miller and Dixon, 1992)• A more reliable means of identifying terrane margins is the com- bination of a number of criteria that document terrane margins, for example a destructive plate margin with traces of accretionary prism and forearc basins, subduc- tion-related high pressure-low temperature metamor- phic assemblages and a diagnostic sequence of island arc related igneous rocks in the hangingwall of the (former) subduction zone. At present at least two such terrane margins can be identified in the Eastern Desert of Egypt with reasonably reliability: the Wadi Ghadir assemblage related to a 'South Hafafit suture' to the south of the Wadi Hafafit island are terrane and the 'Allaqui suture' to the south-east of lake Nasser (see Fig. 10): The Wadi Ghadir assemblage combines sedimentological and structural features of an accretionary prism and an igneous sequen- ce of calc-alkaline mafic to felsic intrusives, which is typical of island arcs (e.g. E1 Sharkawy and E1 Bayoumi, 1979; E1 Bayoumi et al., 1983; E1 Bayoumi, 1984; E1 Bayoumi and Greiling, 1984; E1 Alkhal and Greiling, 1987; El Akhal, 1993) and thus documents its palaeotec- tonic situation at and/or above a destructive plate margin. The Allaqi suture may be the first where high pressure- low temperature metamorphic assemblages have been observed in the Eastern Desert of Egypt (Taylor et al., 1993). These assemblages occur in the footwall of the (east to north-east dipping) Allaqi shear zone (Fig. 6), together with island arc volcanics and marbles• In the hangingwall of this shear zone, relatively high grade gneisses represent the uplifted and/or thrusted parts of lower (island arc) crust. This situation may imply the (simplistic) interpreta- tion that the gneisses were obducted towards the WSW during the final stage of collision and overriding the subduction-related high pressure rock assemblages. However, due to intense post-collision/accretion defor- mation (see Table 3) it is not yet clear whether the Allaqi shear zone represents the original suture or a more or less unrelated later structure. As defined earlier, the terrane to the south of the Allaqi suture is called the Gabgaba terrane (Kr6ner et al., 1987) and composed essentially of Pan-African island arc associations• The Gabgaba terra- ne is distinct from the 'Aswan terrane' to the north of the Allaqi suture by the occurrence of marbles, which prob- ably represent shallow water carbonates fringing island arcs. The Aswan terrane as defined in Fig. 10 represents the western part of the Gerf terrane of Kr6ner et al. (1987) and is composed of ensimatic island arcs, ophiolite nappes (e.g.G. Muqsim, Fig. 5; GerfNappe, Fig. 10) and late tectonic granitoids. The Aswan terrane is bounded towards the north-east by a N W - S E trending sinistral shear zone which was identified earlier (e.g. E1 Gaby et al., 1988; Shimron, 1990; Stern et. al., 1990) and is called here

498

the Wadi Kha r i t - Wadi Hodein zone (Fig. 10). The Wadi Kharit-Wadi Hodein zone is interpreted as an equivalent of the Najd Fault zone farther north, in the northern part of the Central Eastern Desert (e.g. E1 Gaby et al., 1988; Shimron, 1990; Stern et al., 1990) and as the youngest major structural element in the Eastern Desert (e.g. Stern, 1985). Such a relationship is supported by structural evidence in the Wadi Queih area (Table 4) of the Najd fault zone but there is as yet no conclusive evidence from the Wadi Khar i t -Wadi Hodein zone.

The importance of this shear zone, however, and its sinistral displacement sense are clear from the map by Hussein et al. (1992). Consequently, the domain to the north-east of the Wadi Khar i t -Hodein shear zone restores towards the south-east and probably to the south of the (eastward continuation of the) Allaqi suture. Therefore, this domain is probably not part of the Aswan terrane. (This distinction was not known when the Gerf terrane was defined as covering the whole of south- eastern Egypt (Kr6ner et al., 1987) and to avoid confu- sion, the term Gerf terrane is not used here.) As a further consequence of the sinistral sense of the Wadi Kha r i t - Wadi Hodein shear zone, the South Hafafit suture restores to the south-east, perhaps as far as to form the eastward continuation of the Allaqi suture. In that case, the Aswan terrane is the equivalent and westward conti- nuation of the 'Hafafit terrane' and the terrane to the south of the South Hafafit suture, perhaps extending as far south as the Onib-Sol Hamed Belt, is an equivalent of the Gabgaba terrane (Fig. 10). Such a restoration implies a lateral transport distance of at least 300 km, a figure that is comparable with those of late tectonic strike-slip deformation in Phanerozoic fold belts (e.g. Ratschbacher et al., 1991 ; Soper et al., 1992). The restoration implies the huge Gabal Gerf ophiolite nappe to be an equivalent of the equally extensive Barramyia 'nappe' of ophiolitic fragments in the central Eastern Desert. Following the results of Hussein et al. (1992), the extensive ultrama- fic/mafic associations to the south-west of Baranis and north-east of the Wadi Khar i t -Wadi Hodein zone are post-ophiolitic intrusives and not ophiolite fragments as suggested by Shackleton et al. (1980), amongst others. Consequently, these intrusives may not be the northward extension of the Gabal Gerf ophiolite nappe. Finally, the vast domains of late tectonic granites in the Aswan are/t may be the equivalents of the late tectonic granites which dominate the northern part of the Eastern Desert (Fig. 10). Lithological similarities between bot of these granites (Klitzsch et al., 1987) support such a model. However, further structural information is required, for example on the relationships between the Wadi Kha- rit-- Wadi Hodein Zone and the Hamisana shear zone, to test the proposed restoration.

Correlations towards the east, across the Red Sea, are extremely difficult (see Kr6ner et al., 1987; Church, 1988; Vail, 1988; Shimron, 1990; Stern et al., 1990). Potential candidates for the eastward continuation of the South Hafafit suture may be the Jabal Ess ophiolite towards the north-east in Arabia (Church, 1988), or ophiolites in Sinai

and north-westernmost Saudi Arabia, if latest Protero- zoic transcurrent faulting along the Red Sea is assumed (Shimron, 1990). Correlations towards the Sudan are similary problematic, due to intense overprinting of the sutures (e.g. Miller and Dixon, 1992; Abdelsa!am and Stern, 1993). As the Allaqi suture at the northern bound- ary of the Gabgaba terrane trends towards the north-west at its western limit of exposure, it is unlikely to join up with the newly discovered Atmur-Delgo suture in the south-west (Schandelmeier et al., 1993).

Phanerozoic reactivation of Proterozoic structures

The extent and distribution of the Phanerozoic cover as shown in Figs 9 and 10 gives some indication of the potential reactivation of Pan-African structures. Most obvious and discussed earlier are the reactivated portions of the Najd Fault zone and the related Wadi Kha- r i t -Wadi Hodein zone (e.g. Stern, 1985; El Gaby et al., 1988; Greiling et al., 1988b). There, high angle Pan- African fault zones were reactived during late Mesozoic and Cenozic Red Sea tectonics, probably due to their favourable orientation during transtension between Afri- ca and Arabia in a position of Riedel shears. High angle normal faulting parallel to the present Red Sea coast and associated reactivation of fault surfaces of appropriate orientation may be related to footwall uplift of Red Sea marginal faults and may have led to a horst-like structure of the Pan-African basement in eastern Egypt (e.g. Egyptian Geological Survey, 1981; Klitzsch et al., 1987; Greiling et al., 1988 b). In contrast with the reactivation of high angle faults, low angle fault zones appear to be less affected by Phanerozoic tectonics, apart from small scale reactivation near the Red Sea coast (e.g. Greiling et al., 1988 b).

Conclusions

A comprehensive review of structures in the Pan-African basement of the Eastern Desert of Egypt revealed the regional structure to be due to post-collisional structural events, which affected the area to various degrees. Features of extensional collapse dominate some domains in the north, whereas other domains acquired their dominant structural grain during subsequent phases of compression and (late) transpression, respectively. The latter are characterized by a NW-- SE trend, related to Najd faulting. The post-collisional tectonic evolution started with extensional collapse (at least in the north), which was followed by N N W - S S E shortening and related large-scale thrusting (towards the NNW) and folding, distibuted all over the Eastern Desert, although with variable intensity. Subsequent transpression was localized to particular shear zones. Early transpression produced, for example, the Allaqi shear zone (Allaqi- Heinai Belt) and final transpression is documented in the Najd and Wadi Khar i t -Wadi Hodein zones. The

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ex tens iona l co l lapse p r o b a b l y occur red wi th in a re la t ively n a r r o w t ime span o f a b o u t 20 M a ( 5 7 5 - 5 9 5 M a ago) all over the Eas te rn Dese r t and was fo l lowed by a fur ther pe r iod o f a b o u t 50 M a o f late to pos t - t ec ton ic act ivi ty. F igu re 10 shows a tec tonic i n t e rp re t a t i on o f the N u b i a n Shield in Eas te rn E g y p t wi th po ten t i a l sutures and t e r rane margins . Based on a n u m b e r o f cr i ter ia , such as deep-sea t rench, fore arc and arc sed imenta ry , igneous and m e t a m o r p h i c sequences, two sutures can be defined, the Al l aq i and Sou th Hafa f i t Sutures , respectively. Bo th these sutures are a p p a r e n t l y l inked by a h igh angle s inis tral s t r ike-s l ip shear zone (Wadi K h a r i t - W a d i Ho- dein) wi th a tec tonic t r a n s p o r t o f a b o u t 300 k m tow a rds the W / N W (Fig. 10). As a consequence , the G a b a l G e r f N a p p e o f u l t r amaf i c rocks and ophio l i te r emnan t s m a y l ink with the B a r r a m y i a ' n a p p e ' complex and the late tec tonic grani tes a t A s w a n m a y l ink wi th those in n o r t h e r n Egypt .

In general , the tec tonic evo lu t ion shows tha t extensio- nal co l lapse is no t necessar i ly the f inal s tage o f o rogeny b u t m a y be fo l lowed by fur ther compres s iona l and t r anspress iona l tec tonism. D u r i n g the Phane rozo i c evo- lut ion, the late P a n - A f r i c a n h igh angle faul ts were reac t iva ted dur ing R e d Sea tectonics b o t h as Riede l shears and n o r m a l faults , a lbei t only where they were or ien ted f avou rab ly wi th respect to the ac tua l stress regime.

Acknowledgements This paper presents some of the results of a co-operation project between EGSMA and Geologisch-Palfiontolo- gisches Institut, Ruprecht-Karls-University Heidelberg, which is funded by BMFT, through Forschungszentrum J/ilich, International Bureau. Satellite images used for this work were also funded by the project and processed at Institut ffir Geologie, Geophysik und Geoinformatik, FU Berlin; Professor F. K. List. M. M. Abdeen was also funded by a scholarship from Graduiertenf6rderung, University of Heidelberg, G. M. Kamal E1 Din by a scholarship from the Egyptian Government (postgraduate mission student of Assiut University), Assiut and A. F. Osman by the German Academic Exchange Service (DAAD). We thank all these institutions for their support. This work also benefited from discussions with numerous colleagues in Egypt and Germany, in particular G. M. Naim and A. A. Hussein (EGSMA), S. E1 Gaby (Assiut University), M. E. Hilmy, A. I. Ragab (Ain Shams University, Cairo), participants of Marsa'Alam Workshop 1993, E K. List (FU Berlin), G. Matheis (TU Berlin), K.-R Stanek (TU Freiberg). Extensive reviews and helpful suggestions for the improvement of the paper by A. Kr6ner (Mainz), A. E. Shimron (Jerusalem) and W. E. G. Taylor (Luton) are gratefully acknowledged.

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