Gravitational Collapse Origin of Shear Zones, Foliations and Linear

Home , Mylonite, Nappe, Pure shear, Shear zone

Journal of African Earth Sciences 38 (2004) 23–40 www.elsevier.com/locate/jafrearsci

Gravitational collapse origin of shear zones, foliations and linear structures in the Neoproterozoic cover nappes, Eastern Desert, Egypt Abdel-Rahman Fowler a,*, Baher El Kalioubi b a Department of Geology, Faculty of Science, United Arab Emirates University, P.O. Box17551, Al-Ain, United Arab Emirates b Department of Geology, Ain Shams University, Abbassiyya, Cairo, Egypt Received 3 December 2002; received in revised form 6 April 2003; accepted 12 September 2003

Abstract The Um Esh–Um Seleimat area lies to the west of the Meatiq Core Complex (MCC), in the Central Eastern Desert (CED), Egypt, which forms part of the Neoproterozoic Arabian–Nubian Shield in NE Africa and Western Arabia. The study area is a NW- trending zone of intensely foliated ophiolitic melange and molasse sedimentary rocks. There is a single regional foliation, S1, defined mainly by low- to very low-grade metamorphic phases, though grade increases to amphibolite facies in the areas bordering the MCC. S1 is associated with shearing and passes directly into the mylonites of the MCC sheared carapace. The foliations and mylonites together define an originally subhorizontal thick ductile shear zone of regional extent. The sense of shearing is top- to-the-NW, parallel to NW–SE trending stretching lineations, L1. S1 is folded by open rounded symmetrical mesoscopic F2 folds with NW–SE trending subhorizontal hinges and variably dipping axial planes. F2 folds are folded by coaxial (i.e. NW–SE trending) but non-coplanar close to tight macroscopic folds (F3). Subhorizontal S1 foliation formed continuously during F2 folding and perhaps also into the early stages of F3 folding. This reflects top-to-the-NW shearing under laterally confined conditions produced by the onset and gradual dominance of NE–SW shortening. SW-ward thrusts and NW–SE trending sinistral brittle faults are late stage structures. The NW-ward shear translation of the ophiolite and molasse cover nappes results from gravitational collapse following arc-collision and crustal thickening. A gliding–spreading nappe emplacement mechanism is most consistent with the field evidence. The steep metamorphic gradient from low-grade cover rocks downwards into gneissic rocks is interpreted as a result of vertical thinning of the ductile shear zone during collapse. Amphibolite facies conditions are found at the base of other top-to-the-NW low- angle major shear zones associated with gneissic complexes in the CED (e.g. El-Sibai, El-Shalul complexes) suggesting that the crustal level of the shear zone may be determined by thermally controlled rock rheological factors. 2003 Elsevier Ltd. All rights reserved.

Keywords: Shear foliations; Stretching lineations; Nappe transport; Neoproterozoic; Gravitational collapse; Egypt

1. Introduction Camp, 1985; Kroner€ et al., 1987; Vail, 1988; Kroner€ et al., 1991; Stern, 1994; Abdelsalam and Stern, 1996; The basement rocks of the Eastern Desert of Egypt Stern and Abdelsalam, 1998; Stern, 2002). form the western exposures of the Arabian–Nubian Recent tectonic models for the evolution of the Shield–a collage of intraoceanic island arc complexes Eastern Desert have concentrated on the origin, signi- and microcontinental blocks. They were assembled as a ﬁcance and mechanism of formation of several gneiss- result of the Neoproterozoic extension, and subsequent cored dome structures (Wadi Kid, Meatiq, Um Had, accretion and collision of East and West Gondwanaland Gebel El-Sibai, Gebel Um El-Shalul, Hafaﬁt), with the (900–700 Ma) to produce a southward narrowing zone characteristics of metamorphic core complexes (Sturchio of complexly deformed juvenile crust, referred to as the et al., 1983a,b; El Ramly et al., 1984; Habib et al., 1985; East African Orogen (Garson and Shalaby, 1976; Gass, Bennett and Mosley, 1987; El-Gaby et al., 1990; Greil- 1977; Engel et al., 1980; McWilliams, 1981; Stoeser and ing et al., 1993; Wallbrecher et al., 1993; Kroner€ et al., 1994; Fritz et al., 1996; Greiling, 1997; Neumayr et al., * Corresponding author. Tel.: +971-506935982; fax: +971-37671291. 1998; Blasband et al., 2000; Fowler and Osman, 2001; E-mail address: [email protected] (A.-R. Fowler). Loizenbauer et al., 2001; Fritz et al., 2002).

0899-5362/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2003.09.003 24 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 Shear zones play an important role in the evolution of 2. General geology these core complexes. The gneissic rocks of the core complexes are separated from low-grade metamor- 2.1. Location and setting phosed upper crustal rocks by low-angle mylonitic shear zones (Sturchio et al., 1983a,b; Ries et al., 1983; Habib The Um Esh–Um Seleimat area is a NW-trending et al., 1985; Blasband et al., 2000). Transcurrent ductile strip of exceptionally well-foliated rocks extending from shear zones and normal ductile shear zones framing the Wadi Um Esh in the north to Wadi Um Seleimat in the core complexes have also been included in models for south, and bordered by the El-Sid Metagabbro on the Eastern Desert core complex exhumation involving west, and the Meatiq Core Complex (MCC) on the east strain partitioning between Ôinternal’ and Ôexternal’ parts (Fig. 2a). Akaad (1996) has described this area as a of the orogen (Fritz et al., 1996; Fritz and Puhl, 1996; ‘‘formidable shear zone’’. It consists of foliated ophio- Neumayr et al., 1998; Fritz et al., 2000). litic melange interrupted by NW–SE trending belts of This contribution describes field relations of regional foliated clastic metasedimentary formations, generally foliations and associated linear structures in the Um accepted as parts of the Hammamat molasse units. In Esh–Um Seleimat area (Fig. 1), a zone of intensely the east, the sheared ophiolitic melange lies structurally foliated cover rocks along the Qift-Quseir road, adjacent above the MCC gneisses and is separated from them by to the Meatiq Core Complex (MCC) in the Central a thick westerly dipping schistose to mylonitic carapace Eastern Desert (CED) of Egypt. The aim of this study is (Fig. 2b). To the west of the study area, along Wadi to present new structural data from the cover rocks Atalla, the folded foliations of the melange are cut by adjacent to the Meatiq, and to develop a structural W- to SW-directed thrusts against the NE limb of an- model to explain their origin. Following this, the sig- other gneiss-cored antiformal structure of the Um Had– nificance of the structural model for the tectonics of the Um Effein area, described by Fowler and Osman (2001). CED is discussed, including consequences for existing The Um Esh–Um Seleimat area lies in Fritz et al.’s exhumation models of the MCC. (1996) external part of the orogen, which they described

Fig. 1. Location map for the Um Esh–Um Seleimat area in the Central Eastern Desert, Egypt, adapted from the ‘‘Geological Map of the Pre- cambrian of the Eastern Desert’’ (O’Connor et al., 1996). M ¼ Meatiq Core Complex; F ¼ Fawakhir Granite, UH ¼ Um Had Granite; WH ¼ Wadi Hammamat molasse deposits; WK ¼ Wadi Kareim molasse deposits; GS ¼ Gebel El-Sibai. Bold lines are faults, short thin lines are foliation trend lines, broken lines are bedding trend lines. A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 25

Fig. 2. (a) Geological map of the Um Esh–Um Seleimat area showing the ophiolitic melange and conglomerate formations. Macroscopic fold axial traces are shown. A–A0, B–B0 and C–C0 refer to cross-sections. (b) Cross-sections for (a). Fold axial planes are represented by dotted lines. Faults are represented by bold lines. Lithological symbols as for (a). Thicker lines in section B–B0 near the boundary between mica schist and metabasalt represent the mylonitic carapace separating the ophiolites from the Meatiq Core Complex.

as a zone of W- to SW-directed thrust imbrication, in Detailed mapping of the Um Esh–Um Seleimat area contrast to the zone of NW–SE extension in the internal was carried out by Noweir (1968), Akaad and Noweir part of the orogen (incorporating the MCC). (1969, 1980) and Akaad et al. (1996) from mainly 26 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 sedimentological, petrological and stratigraphic points Hammamat exposures of the areas west of Meatiq as of view. Various authors have included the Um Esh–Um foreland basin deposits affected by W- to SW-ward Seleimat area in their broader areas of study (El Ramly directed thrusts in the external part of the orogen. They and Akaad, 1960; Akaad and Shazly, 1972; Stern, 1979, were thus structurally distinguished from strike-slip 1981; Ries et al., 1983; El-Gaby et al., 1984; Habib, fault-controlled Hammamat basins (e.g. the Wadi 1987; Bennett and Mosley, 1987; Wallbrecher et al., Kareim basin) which reflected NW–SE extension and 1993; Ragab et al., 1993; Greiling et al., 1996; Messner, were associated with the rise of the CED core complexes 1996; Kamal El-Din et al., 1996; Ragab and El-Alfy, (Fritz and Messner, 1999). 1996; Fritz et al., 1996). High-grade metamorphic gneisses of the MCC were not examined in this study. The mica schists forming the 2.2. Stratigraphic units and lithology thick sheared carapace of the MCC lie at the eastern margin of the study area. These foliated rocks have been The ophiolitic melange of the Um Esh–Um Seleimat referred to as the Abu Fannani Schists by Akaad and area consists of greyish metagabbro, metadolerite and Noweir (1969, 1980). Lithologies include garnet mica metabasalt blocks, up to several kms in dimension, and schist, amphibolite and mica phyllonites. appear as elongate or lenslike bodies incorporated within foliated serpentine and dark grey pelite (Fig. 2a). 2.3. Petrography and metamorphism Associated minor lithologies are lenses of cherts, pelites, tuffs, greywackes and rare limestone. Previously this Petrographic data from the ophiolitic melange melange was referred to as the Abu Ziran Group, and it lithologies of the Um Esh–Um Seleimat area are sum- was subdivided into stratigraphic formations (Akaad marized in Table 1, and illustrated in Fig. 3. The and Noweir, 1969, 1980), until Ries et al. (1983) dis- ophiolitic melange of the CED, in general, is charac- covered that the ’’formations’’ are actually chaotic terized by mainly greenschist facies assemblages with blocks with no regular stratigraphic succession. The metamorphic temperatures (adjacent to Meatiq) >350 melange has been referred to as Older Metavolcanics C but <540 C (Fritz and Puhl, 1996; Neumayr et al., (Stern, 1981), Eastern Desert Ophiolitic Melange (Ries 1998) and pressure conditions <4 kbar. From their et al., 1983; Habib, 1987), or Umm Esh melange (Akaad transect along Wadi Um Esh, Ries et al. (1983) reported et al., 1996). that the metamorphic grade of the melange nappes in- The conglomeratic formations attain a thickness of creases downward towards the shear zone separating the 4000 m in the nearby Wadi Hammamat section (Akaad nappes from the Meatiq gneisses. They also noted the and Noweir, 1980) and are probably much thicker over presence of hornblende and garnet in the lower sections the entire Qena-Quseir section (Messner, 1996). The of the sheared melange. Both Ries et al. (1983) and El- sediments consist of alternating purplish red and Gaby (1994) drew attention to the steep thermal gra- greenish grey metaconglomerate, metagreywacke and dient from the melange nappes downwards into the metapelite. There are clasts of mudstone, felsic to mafic gneisses. Ries et al. (1983) also recognized that the volcanics and pink granite (Akaad and Noweir, 1969; metamorphic isograds dip more steeply than the tectonic Akaad et al., 1996; El Kalioubi, 1996). These late-oro- units suggesting that metamorphic heating continued genic molasse sediments (Grothaus et al., 1979; Akaad after folding. The zone of hornblende stability extends and Noweir, 1980; El-Gaby et al., 1984; El-Gaby, 1994) up to 2.5 km away from the MCC margin. accumulated in intermontane normal fault-bounded A brief description of the petrography of Hammamat basins as alluvial fan braided stream and lake deposits lithologies from the Um Esh–Um Seleimat area is pre- (Grothaus et al., 1979; Messner, 1996; Fritz and Mess- sented in Table 1 and illustrated in Fig. 3. The CED ner, 1999). The sediments were derived from a recycled Hammamat metasedimentary rocks in general show Pan-African orogen and dissected arc terrain (El Ka- anchizonal metamorphic grade indicated by the pres- lioubi, 1996; Osman, 1996; Messner, 1996). Originally ence of montmorillonite and pumpellyite (Soliman, most of the deformed conglomerates were identified as 1983; Osman et al., 1993; Osman, 1996). However, Atud Formation of the Abu Ziran Group by Akaad and chlorite zone greenschist metamorphic facies is also Noweir (1980), though later authors regarded them as found (El Kalioubi, 1996; Greiling et al., 1994; Neumayr higher strained stratigraphically lower sections of the et al., 1996; Fritz and Messner, 1999), with biotite zone Hammamat Formation (Stern, 1979, 1981; Ries et al., or higher grades at distances up to 5 km from the MCC 1983; El-Gaby et al., 1984; Hassan and Hashad, 1990; margin. Messner (1996) described somewhat higher Messner, 1996; Ragab and El-Alfy, 1996; Fritz et al., grades in the lower stratigraphic sections of the Ham- 1996). In a later revision (Akaad et al., 1996), the Atud mamat in the Wadi Hammamat–Wadi El-Qash area, equivalent formation (the Muweilih Conglomerate) was and correlated this with increased shearing effects. found to occupy a more restricted area only to the south Fowler and Osman (2001) also noted the increased of Wadi Um Seleimat. Fritz et al. (1996) identified the shearing effects in the lower stratigraphic section of the A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 27

Table 1 Brief petrographic description of the main lithologies of the Um Esh–Um Seleimat area Meatiq gneisses Quartz-, feldspar-rich garnet muscovite, red–brown biotite schists. There are signs of retrogression of garnet to chlorite. There are also mica schists with quartz and plagioclase porphyroclasts, which are probably sheared syn-kinematic intrusions into the carapace of the Meatiq dome Ophiolitic melange Ultramafics Include original pyroxenite and peridotite, now represented by serpentinite, talc- tremolite schist and rocks composed entirely of actinolitic amphibole Mafic rocks Originally gabbros, dolerites, basalts. These lithologies are represented by greenschist to amphibolite Amphibolites consist of blue-green hornblende, brown biotite, plagioclase, quartz and minor epidote, sphene and chlorite. Blue–green hornblende mantles and replaces an earlier brownish hornblende phase Greenschist facies rocks are dominated by actinolite, chlorite, epidote, plagioclase, quartz and sphene, and may contain minor calcite and biotite or stilpnomelane. In some examples blue–green hornblende (itself containing traces of brownish hornblende) remains as relicts partly replaced by actinolite Metagabbros have preserved igneous fabrics under low strain conditions and may even appear hornfelsic (Fig. 3(c)). Progressive development of foliation in metagabbros begins with crude foliation defined by parallel alignment of hornblende + biotite or actinolite + chlorite outlining lenses and boudins of recrystallized and strained mafic and plagioclase grains (Fig. 3d). As foliation enhances, these lenses, augen and boudins are progressively reduced and recrystallization decreases their grainsize. Ultimately mafic mylonites are produced. There are isoclinally folded, boudinaged and sheared quartz veins in the highly strained metamafic rocks (Fig. 3f)

Sedimentary rocks Include black cherts, grey pelites, rare limestone blocks. The grey pelites are now composed of metamorphically grown sodic plagioclase and chlorite, and minor polygonal quartz. They contain signiﬁcant quantities of hematite and calcite

Conglomerate and Greywacke and Composed mainly of sand-sized angular clasts of plagioclase, quartz and silicic and associated rocks conglomerate matrix intermediate volcanic groundmass. The sand grains are set in a ﬁner matrix of plagioclase, quartz, white mica, epidote, chlorite, magnetite and sphene (Figs. 3 a and b) Traces of tourmaline and zircon are usually present. Assemblages of white mica + greenish-brown biotite, or biotite + chlorite are found at higher metamorphic grade. The mica and chlorite phases deﬁne weak to extremely strong foliations enclosing strained and recrystallized clastic grains (Figs. 3a and b). Pebble lithologies are mainly felsic volcanics as described below. There are also pebbles of alkali granite Felsic volcanics Contain quartz and plagioclase phenocrysts in a groundmass of felsitic, spherulitic and microgranular textures. Groundmass phases include quartz, plagioclase, white mica, chlorite, epidote and sphene. Others have olive green biotite. Foliations may be weak or intense. Some examples look mylonitized and have mainly white mica foliations

Hammamat along Wadi Muweih. Based on different layering of black cherts, in dark coloured pelites, and as strain intensities, El Ghawaby (1973) divided the Ham- felsic tuffaceous or pebbly bands in the pelites. Bedding mamat into a lower and upper unit in the Wadi Zeidun planes trend generally NW–SE and have variable dips area. The lower unit contains flattened and stretched (Fig. 2a). On equal area plots they form a girdle of poles pebbles that are absent in the upper unit. Andrew (1939) defining a gently SE-plunging p-axis (Fig. 5). and Ries et al. (1983) described biotite zone grades, correlating with higher strains in Hammamat metasedimentary units along Wadi Um Esh. 3.2. Tectonic planar fabrics (S1 and Sm)

The study area is dominated by a regionally devel- 3. Structure oped, commonly penetrative phyllitic cleavage to schis- tosity, termed S1 (Figs. 3a and 4a–e), which is best 3.1. Bedding (S0) developed in the pelites, tuﬀs and serpentinites, and usually poorly developed in the metabasalts and meta- Bedding ðS0Þ is excellently preserved in the con- gabbros. S1 is locally parallel to the bedding (Fig. 4a) glomeratic formations where it is represented by pebbly but more commonly subtends an angle of 45 or less to bands within the greywacke (Fig. 4a and b). It is rarely beds (Fig. 4b). S1 orientations vary from gently to observed in the melange itself, though it occurs as thin steeply dipping (Fig. 2a). Where stretching lineations 28 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40

Fig. 3. Thin section photomicrographs of the lithologies of the Um Esh–Um Seleimat area. (The short edge of each photograph represents a length of 3 mm, except for photograph e, with a short edge of 0.75 mm, and photograph f , with a short edge of 2.45 mm). (a) Foliated metagreywacke with quartz and plagioclase clastics and white mica and biotite foliation. (b) Mylonitized metagreywacke with foliation defined by biotite and epidote grains. (c) Isotropic polygonal granoblastic fabric of recrystallized metagabbro containing plagioclase, quartz and hornblende. (d) Foliation in metagabbro is defined by stretched plagioclase grains (white) and actinolite (pale green). (e) Metabasalt showing foliation defined by hornblende. A shear structure offset the larger amphibole grain. (f) Foliated metabasalt showing abundant slender hornblende grains defining a foliation, and a sheared quartz vein with quartz subgrains oblique to the vein margins. Both (e) and (f) give a shear sense of top-to-the-right.

ðL1Þ are evident, L1 invariably lies within S1. Poles to S1 foliation planes have developed into striated slip planes define a complete girdle with gently SE-plunging p-axis with the striations approximately parallel to the (Fig. 5). Flattened pebbles have long and intermediate stretching lineation; (d) the foliation forms the S element axes parallel to S1 foliation. Rare isoclinal rootless F1 of S–C structures; and (e) there is evidence for local slip fold axial planes and boudinaged layers are also parallel along the foliations (Fig. 3e). The mylonitic foliation to the foliation. S1 is parallel to the principal plane of (Sm) forms numerous thin shear zones in the ophiolitic flattening in the rocks, and is not itself a shear plane melange near the MCC carapace. Fine-grained horn- structure, however, it has a spatial, temporal and kine- blende defines the mylonite foliation Sm (Fig. 3f) and matic association with shearing in that: (a) it is better lineations Lm indicating that hornblende crystallization developed in areas bordering sheared contacts, e.g. be- temperatures existed during ductile shearing. Our results tween metabasalts and conglomerate units, and locally agree with those of Ries et al. (1983), Habib et al. (1985) merges with mylonitic foliations ðSmÞ at higher strains; and Neumayr et al. (1996) in finding that amphibolite (b) the stretching lineations on the foliation are parallel facies conditions existed in the melange adjacent to the to nearby mylonitic lineations ðLmÞ (Fig. 6); (c) some MCC. A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 29

Fig. 4. (a)–(b) Field photographs of bedding ðS0Þ and foliation ðS1Þ relations in the Um Esh–Um Seleimat area. (a) Bedding deﬁned by conglomerate layers in greywacke. S1 foliation is parallel to the bedding. (b) Conglomerate bed within greywacke. The S1 foliation lies 15 clockwise from the bedding. Direction of observation is NW, along the line of intersection of the bedding and foliation. (c)–(e) Mesoscopic shear sense indicators from the Um Esh–Um Seleimat area. (c) Foliation ’ﬁsh’, top-to-right shear sense (looking W ). (d) Shear boudins, top-to-right shear sense (looking W ). (e) Imbricate extensional small scale shears inclined to the foliation, top-to-left shear sense (looking NE). All indicators are consistent with top-to- the-NW shear sense.

Fig. 5. Schmidt net stereograms of poles to planar structures from the Um Esh–Um Seleimat area. Bedding ðS0Þ: 59 measurements (density contours 2%, 4%, and 8%; p-axis is 7 towards 153). Foliations ðS1): 271 measurements (density contours 1%, 2%, 4%, 8%; p-axis is 5 towards 144). Mylonitic foliations ðSmÞ: 25 measurements (p-axis is 6 towards 145). Mesoscopic F2 fold axial planes: 32 measurements (p-axis is 3 towards 148). Approximate great-circle girdles of best ﬁt (and their corresponding p-axes) to the data are shown. 30 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40

Fig. 6. Schmidt net stereograms of linear structural data from the Um Esh–Um Seleimat area. F2 fold hinge lines: 47 measurements (density contours 0 3%, 6%, 12%, 24%). Bedding–cleavage intersection lineations ðL1Þ: 23 measurements, ﬁlled and unﬁlled square symbols represent clockwise and anticlockwise orientation of beds relative to foliations, with direction of observation along the lineation looking approximately NW. Mylonitic lineations ðLmÞ: 20 measurements. Pebble long-axis lineations ðL1Þ: 37 measurements (density contours 3%, 6%, 12%, 24%, 48%). Pencil axes: 81 measurements (density contours 2%, 4%, 8%, 16%, 32%). Stretching lineations (other L1 than pebble long axes): 104 measurements (density contours 1%, 2%, 4%, 8%, 16%, 32%).

3.3. Shear sense indicators not with SW-ward thrusting as in current structural models for the area to the west of the MCC (Wallbre- There are a number of kinematic indicators within cher et al., 1993; Fritz et al., 1996). the S1 foliation in the study area. Rare S–C structures and foliation ’fish’ indicate top-to-the-NW shear sense (Fig. 4c). Another shear sense criterion is shear-seg- 3.4. Stretched particle lineations (L1) mented veins (Fig. 4d) which may superficially appear as boudins. Asymmetric intrafolial F1 folds are also con- Lineations defined by the long axes of particles, are sistent with top-to-the-NW shear sense. All of these common throughout the study area. The most impres- shear sense indicators have been rotated along with the sive are those defined by pebbles in the conglomeratic S1 foliation about later NW–SE trending fold axes, as formations. The pebbles have ’’beards’’ produced by S1 explained by Fowler and Osman (2001) for the Um Had foliation anastomosing around them. Most examples of area to the west, and detailed below. As a result of this elongate pebble fabrics show features consistent with later folding the F1 asymmetric folds now appear to extension parallel to the pebble long axis (e.g small scale indicate dextral or sinistral shear sense which reverses normal faulting; extension fractures with fibres parallel across the later fold hinges. Microscopic top-to-the-NW to the pebble long axis; and rare examples of boudi- shear sense indicators include mica ’fish’; asymmetric naged pebbles). On this evidence L1 is regarded as a pressure fringes (beards) on porphyroclasts; S–C fabrics; stretching lineation. Since L1 always lies within S1 it grain elongation fabrics inclined to mylonitic foliations appears that the elongation producing these lineations is (Fig. 3f); and microshear offsets (Fig. 3e). These kine- related to the same events that produced the foliations, matic indicators are consistent with the S1 foliations and therefore that extension and shearing are contem- being associated with NW-ward shear translation, and porary deformation effects. A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 31

3.5. Mesoscopic folds (F2) and b), which are coaxial with the F2 folds (ie also NW– SE trending and gently plunging). The macrofolds, The beds and S1 foliations are folded about uniformly however, are moderate to tight, and have consistently NW–SE trending, gently plunging mesoscopic folds. upright to steeply inclined axial planes (Fig. 2b). F3 folds These folds are usually rounded, with one to tens of have no axial plane foliations. There are no asymmetry metres wavelengths. Almost all of these folds are gentle variations in the F2 mesoscopic folds systematic with to open, and they may be found in upright, inclined and their position on the limbs of the F3 folds. This is be- recumbent orientations (Figs. 7a–c). Some rare tight F2 cause the axial planes of the F2 folds are almost always folds have an axial planar crenulation cleavage ðS2Þ. nearly normal to the average orientation of the layering. More commonly the F2 folds have pencil structure in The broadly variable orientation of the F2 fold axial their cores (Fig. 7d). The relationship between the planes (Fig. 5) is a result of coaxial non-coplanar mesoscopic F2 folds and the macroscopic folds shown on refolding of the F2 folds by F3 folds. the cross-sections is discussed in Section 3.6. Combining the cross-section of Fig. 2b with the results of Fowler and Osman (2001) for the Um Had area to the 3.6. Macroscopic folds (F3) west, and the profile shown by Habib et al. (1985) for the MCC to the east, allows the construction of the profile The map and cross-sections of the study area dem- shown in Fig. 8. This interpreted cross-section shows a onstrate the existence of macroscopic folds ðF3Þ (Fig. 2a single thick shear zone defining the MCC carapace and

Fig. 7. (a)–d) Photographs of folds in the Um Esh–Um Seleimat area. (a) Upright open low amplitude F2 folded foliations from the central part of the area. (b) Recumbent open low amplitude F2 folded foliations from the eastern part of the area. (c) Open inclined low amplitude F2 symmetrically folded metaconglomerate and metagreywacke beds from the northern part of the area. (d) Pencil structure parallel to the hinge of an F2 fold. (e) Pencil structure in metagreywacke. 32 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40

Wadi Atalla Wadi Abu Diwan Wadi Muweih SW-ward thrusting

SW Um Had gneisses Meatiq gneisses NE

? ?

Um Esh - Um Seleimat area

Fig. 8. A schematic NE–SW projected cross-section through the Um Esh–Um Seleimat area extending from Wadi Muweih (where basement rocks first appear from beneath the Nubia Sandstone) to the NE flank of the Meatiq Core Complex (MCC) at Wadi Abu Diwan. The figure interprets the continuity of the originally subhorizontal S1 foliations and mylonites associated with top-to-the-NW regional shear translation. Data for the section from Wadi Muweih to Wadi Atalla are derived from Fowler and Osman (2001). The Um Esh–Um Seleimat section is based on Fig. 2a (Section A–A0) and shows the upright F3 macroscopic folds. Meatiq cross-section is modified from Habib et al. (1985). Close-spaced lines represent sheared ophiolitic and conglomeratic cover rocks. Thick grey line represents mainly mica schists of the sheared carapace of the MCC. No examples of these latter schists have been found in the more tightly folded synformal structures between the Um Had and Meatiq antiforms. Line of the cross-section represents very approximate level of present surface exposure. SW-ward thrusts in Wadi Atalla and NE-ward thrusts near wadi Muweih are also shown, and interpreted to be of similar age. Transcurrent faults are also shown with circle symbols indicating sense of slip (block with circled X symbol moves forwards, relative to the observer). sheared overlying nappes continuing into the Um Esh– shown in Fig. 10c, where gentle folds are transected by Um Seleimat area, and from there wrapping over the Um subhorizontal foliations. In the study area this repre- Had gneisses and being folded in the Wadi Muweih area, sents continued formation of S1 foliations into the stage west of Um Had. The implication is that this thick shear of F2 or even F3 folding. For simplicity, we have called zone was subhorizontal before folding and core complex the foliation S1 although it is not confined to the first rise, and of regional extent. A similar, but orogen-par- deformation event. This model also explains the gene- allel, continuity of the same shear zone from Hafafit rally low angle between beds and foliations (75% of the through Sibai to Meatiq was suggested in interpretations measurements lie between 0 and 40) by supposing that by Greiling and El Ramly (1985). the subhorizontal S1 foliation continued to form only up to the stage of F2 folding or as far as the early stages of 0 F3 folding, when limb dips did not exceed 40. This 3.7. Bedding–cleavage intersection lineations (L1) and pencil structures model supposes a stage of overlap of the top-to-the-NW shear displacement event and the NW–SE trending folding event. Tectonic aspects of this overlapping Bedding–cleavage intersection lineations ðL0Þ are 1 shearing and folding model are briefly discussed below. subparallel to the F2 and F3 fold hinges in the study area (Figs. 6 and 9). Three possible interpretations for bed- The rocks of the study area show remarkably good ding–cleavage intersections parallel to fold hinges are development of pencil structures in most lithologies shown in Fig. 10. The simplest interpretation (Fig. 10a), (Figs. 7e and 9), particularly the metabasalts and metagabbros. There is a strong spatial association be- with the S1 foliation as axial plane to the F2 folds, must tween pencil structure and F2 fold hinges (Fig. 7d), and obviously be rejected because S1 is folded by F2 and F3 folds, and is not axial plane to them (Fig. 7a and b). fold hinges are rarely more than 10 different in orien- Another interpretation is shown in Fig. 10b, where tation from pencils (Fig. 9). Pencil structure is normally bedding and an inclined foliation are coaxially folded. A the result of a weak flattening strain (here related to F2 model of this kind was inferred by El-Gaby et al. (1984) or F3 folding) superimposed on a weak pre-existing when they considered the NW–SE folds deforming planar fabric. imbricate SW-vergent thrusts and associated foliations. However, the data on angular relations between bedding 3.8. SW-ward thrusting and strike-slip faulting and S1 foliations do not support this model. According to Fig. 10b, the bedding should lie consistently anti- West of the study area, along Wadi Atalla, there are clockwise of the foliation, looking NW. The data from E- and NE-dipping thrusts which slice through (i.e. the study area are roughly evenly divided between post-date) the folded S1 foliations. These structures have clockwise and anticlockwise angular relations between been described by Stern (1979, 1985), El-Gaby et al. S1 foliation and bedding, looking NW (Fig. 6). The best (1984), Kamal El-Din et al. (1996) and Fowler and model to explain the bedding–cleavage relations is Osman (2001). West of Wadi Um Seleimat there are A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 33

Fig. 9. Map showing the relatively uniform NW–SE trending orientations of mesoscopic F2 fold hinges, pencil axes and L1 extension lineations (including pebble long axes) in the Um Esh–Um Seleimat area.

0 Fig. 10. Sketches showing three simple models for the signiﬁcance of bedding–cleavage intersection lineations ðL1Þ being approximately parallel to F2 and F3 fold hinges in the Um Esh–Um Seleimat area. See text for discussion. (a) The foliations have axial plane relations to the folds. (b) The beds and inclined foliations are folded coaxially. (c) The subhorizontal foliations transect the folds.

N–S trending sinistral strike-slip faults at the western associated with much kinking of this foliation. Abrupt margin of the study area (Fig. 2a). Also, NW to WNW anti-clockwise deflections in S1 and L1 trends in the SW trending brittle sinistral strike-slip faults interrupt the part of the study area (Fig. 9) are consistent with fault folded S1 foliations throughout the study area and are block rotation associated with the sinistral movements. 34 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 4. Discussion models NW-ward thrusting is pictured as resulting from the same compressional events as those that formed the 4.1. Structural model for the Um Esh–Um Seleimat area arc collision-related sutures. From such models one would expect the thrusting and the arc collision events A summary of structural events is given below, and is to be approximately synchronous. followed by a discussion of key aspects relevant for re- Blasband et al. (2000) has dated arc collision in the gional tectonics in the CED. Sinai to 750–650 Ma, and this is comparable with the Stage 1: Top-to-the-NW shear translation occurred 800–700 Ma range for arc collisions in the Arabian– on a thick regionally extensive subhorizontal ductile Nubian Shield reported by other authors (Bentor, 1985; shear zone generating initially subhorizontal S1 folia- Stern and Hedge, 1985; Stoeser and Camp, 1985; tions and Sm mylonitic foliations. These foliations con- Kroner€ et al., 1992; Stern, 1993; Greiling et al., 1994; tain F1 isoclinal rootless intrafolial folds, and NW–SE Abdelsalam and Stern, 1996). Blasband et al. (2000) trending mylonite lineations ðLmÞ and stretching linea- also dated the activity on subhorizontal mylonitic foli- tions ðL1Þ in the direction of transport (Fig. 11a). We ations with top-to-the-NW shear sense in the Sinai to prefer the terms ‘‘top-to-the-NW translation’’ or ‘‘NW- 620–580 Ma, i.e. coeval with other NW–SE directed ward nappe translation’’ over Ries et al. (1983) ‘‘NW- extensional phenomena in that area (dyke swarms, NE– ward thrusting’’ since the latter requires that the SW striking graben formation, post-orogenic A-type shear surfaces dip consistently SE. Our conclusions are granites). They also presented field evidence that these that the thick ductile shear zone is essentially subhori- subhorizontal shear foliations cut across the fold struc- zontal. tures associated with arc accretion and crustal thicken- Stage 2: Before the end of the top-to-the-NW ing, and are therefore distinctly younger than the shearing, low amplitude rounded open F2 mesoscopic arc accretion event. Elsewhere in the CED, structures folds began to form with axes parallel to L1 (Fig. 11b). belonging to the top-to-the-NW nappe translation The onset of F2 folding reflects a progressive NE–SW event commonly affect Hammamat and Dokhan litho- shortening, which eventually came to dominate the logies, which are generally accepted as having been deformation. The transection of the F2 (and probably deposited in a post-collision extensional tectonic setting also early stages of F3) folds by continuing formation (Abdeen et al., 1992; Rice et al., 1993; Greiling et al., of S1 foliations produced bedding–cleavage lineations 1994; Stern, 1994; Naim et al., 1996; Fowler and Osman, 0 L1 (Fig. 11c). Pencil structures formed during stages 2 2001). Greiling et al. (1994) opted to have another and 3. NNW–SSE compressional event following post-collision Stage 3: Top-to-the-NW shearing ceased as NW–SE extension in order to explain the NNW-ward thrust- trending tight upright macroscopic F3 folds deformed ing in Hammamat basins, but simultaneous NNW–SSE the earlier structures (Fig. 11d). These fold were compression and NNW–SSE stretching are incom- accompanied or followed by W- to SW-ward thrusts at patible. the western margin of the area. The above facts argue strongly that the top-to-the- Stage 4: Sinistral shearing along N to NW trending NW shearing displacement is post-arc collision and transcurrent brittle faults generated local kink zones associated with an extensional tectonic event. Blas- and produced some fault block rotations (Figs. 2a and band et al. (2000) have argued that the evidence in 9). the Wadi Kid area, Sinai, favours an extensional col- The simultaneous top-to-the-NW shear translation lapse NW-ward nappe transport mechanism, and and NW–SE trending folds (stage 2) has been treated in that this led to crustal thinning and isostatic rise of Fowler and Osman (2001). In the following discussion the CED core complexes, in a similar fashion to the we will concentrate on the implications of the simulta- north American core complexes. Fritz et al. (2002) neous NW-ward nappe translation and NW–SE exten- and Bregar et al. (2002) argue against Blasband et al.’s sion (stage 1). model on the grounds that there is insufficient evidence for collision-related crustal thickening in the CED. They preferred a magmatic assisted core com- 4.2. Simultaneous top-to-the-NW nappe translation and plex rise leading to a magmatic core complex, in the NW–SE extension event case of the Gebel El Sibai complex. Fritz et al. (2002) proposed that crustal thickening during arc Many of the tectonic models for the Pan-African accretion is compensated by the tendency for lateral basement which attempt to explain the orogen-parallel flow away from the collision zone of magmatically NW-ward nappe translation are essentially rear com- softened crust in an oblique collision zone. We will re- pression thrusting mechanisms (Ries et al., 1983; Greil- turn to this problem in Section 4.4. after discussing the ing et al., 1994; Shackleton, 1994; Fritz et al., 1996; possible mechanisms for NW-ward nappe translation in Neumayr et al., 1998; Loizenbauer et al., 2001). In these the CED. A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 35

Fig. 11. Sketch showing a structural model for the progressive development of the main structures in the Um Esh–Um Seleimat area, clarifying the time relations between shearing and folding. Elliptical ornaments represent pebble stretching lineations. Stages 1–4 are discussed in the text. Arrows indicating top-to-the-NW shearing decrease in size from (a) to (c) representing a gradual phasing-out of this deformation. Arrows indicating NE–SW shortening increase in size from (b) to (d) representing a gradual phasing-in of this deformation. (a) Subhorizontal S1 and Sm mylonitic foliations with top-to-the-NW shear sense and L1 and Lm lineations. (b) The subhorizontal foliations are gently folded about NW–SE trending symmetrical upright mesoscopic F2 folds. (c) Continued top-to-the-NW shearing during F2 and probably early F3 folding produces transection of these folds by progressive formation of subhorizontal S1 foliations. (d) NW–SE F3 macroscopic folding continues after cessation of top-to-the-NW shearing. Mesoscopic folds are passively rotated to variable axial plane orientations but remain symmetrical. Some pencil structures are formed by continuous cross-cutting of S1 folded foliations by other S1 foliations, some are formed by F2 fold-related shortening. SW-ward thrusts form during stage 4.

4.3. Nappe emplacement mechanisms the top of the nappe mass); spreading–gliding (combining spreading with gliding induced by a slope of the base Nappe emplacement mechanisms are detailed in of the nappes); and extruding–spreading (combining rear Merle (1989, 1998). Merle classified three main (non- compression and spreading). The nappe emplacement- brittle) nappe transport mechanisms: ductile gliding, related deformation is a combination of pure shear spreading mechanisms, and rear compression. The (vertical thickening or thinning, and a ‘‘pull-from-the- spreading mechanisms were subdivided into gravita- front’’ effect) and simple shear (related to nappe trans- tional spreading (relieving the steepened topography at lation). Identifying the operating nappe emplacement 36 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 mechanism requires recognition of patterns of foliation 4.4. Effects of collapse on arc collision-related structures trajectories (especially in vertical sections parallel to the displacement direction); finite strain gradients; strain Blasband et al. (2000) identified early upright isocli- regime (coaxial/non-coaxial); displacement gradients; nal NE–SW trending folds as evidence for crustal and relations between stretching lineations and dis- thickening associated with arc collision in the Wadi Kid placement directions. area of the Sinai. If similarly oriented upright isoclinal Some structural characteristics of the Um Esh–Um folds have existed elsewhere in the CED they may have Seleimat and surrounding areas in the CED which assist been deformed by later gravitational collapse as in constraining the identity of the NW-ward nappe explained by Aerden and Malavieille (1999) for the transport mechanism in the CED are represented in Variscan Orogeny. The latter authors discovered that Table 2, along with essential features of the ideal nappe collision-related upright isoclinal folds associated with transport mechanisms identified by Merle (1998). The crustal thickening had been rotated to recumbent structural evidence is presently insufficient to identify orientations in the deeper parts of a zone of subhori- the CED nappe mechanism but field evidence supports a zontal foliation, which formed a decollement for cover gravitational collapse mechanism (probably gliding– nappes transported during gravitational collapse. These spreading) (Table 3). recumbent folds were then extended in the direction

Table 2 Comparison of characteristics of the various nappe emplacement mechanisms with ﬁeld observations from the central Eastern Desert Nappe emplacement Summary of main structural characteristics of the Field structural observations from the central Eastern mechanism emplacement mechanism Desert Ductile gliding • Downwards increasing simple shear strain (1) Downwards increasing strain. This is shown by the

• No vertical shortening increase in degree of development in S1 foliation down- • Concave foliation trajectories with foliation dips not wards in both the ophiolitic melange and conglomeratic exceeding 40 formations. Increasing simple shear strain with depth is • Parallel extension lineation and nappe translation supported by the predominance of mylonite zones in the directions lower parts of the shear zone Gravitational spreading • Downwards increasing simple shear strain (3D model) • Vertical shortening (2) Vertical shortening. As discussed in text Section 4.5. • Concave foliation trajectories with foliations approach- the steep metamorphic gradient from greenschist or ing vertical orientations in the upper parts of the nappe lower temperature facies nappe cover to gneissic rocks of • Nappe translation direction at 90 to extension lineations amphibolite or higher temperature facies (Ries et al., near the top of the nappe but parallel to the lineations in 1983), is interpreted as being due to vertical shortening the lower parts of metamorphic zones within the foliated zone separat- Gliding–spreading • Shear strain increases downwards and towards the front ing these rocks. This vertical shortening is responsible for • Vertical shortening the subhorizontal S1 foliations and is associated with • Convex (hindmost), sigmoid and concave (foremost) NW–SE extension foliations trajectories that are progressively modiﬁed to (3) Concave foliation trajectories. Equivalent foliations in concave as deformation proceeds. Foliations generally the stratigraphically higher parts of the Hammamat less than 45 dip sequence in the Um Had area west of the study area were • Parallel extension lineation and nappe translation direc- originally gently to moderately dipping to the S and SE tions (Fowler and Osman, 2001). The deeper parts of the same Extruding–spreading • Shear strain increases downwards and towards the rear sequence have foliations approaching parallelism with • Vertical thickening at rear passes forwards into vertical the bedding. Together these are consistent with upward shortening concave foliation trajectory though further work is • Concave foliation trajectories with upright orientations necessary to determine if there is a gradation in foliation near the top of the nappe dip values between these two levels • nappe translation direction at 90 to extension lineations near the top of the nappe but parallel to the lineations in (4) Foliation dips less than 45. The steepest foliations the lower parts which can be related to the top-to-the-NW nappe Rear compression • Shear strain increases downwards and towards the rear translation do not have dips much greater than 40 • Vertical thickening at the rear, decreasing towards the (Fowler and Osman, 2001) front • Concave foliation trajectories with increase in dip of (5) Extension occurs in the direction of nappe transport. foliations to vertical near the top of the nappe. Foliations Nappe transport direction indicators include striations, are never horizontal and increase in dip towards the front duplexes etc. showing thrust nappe transport to the NW, of the nappe i.e. in the same direction as the stretching lineations • Extension lineations are best developed at the rear and are approximately vertical A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 37

Table 3 Consistency of field observations (1)–(5) in Table 2 with characteristics of nappe emplacement mechanisms Ductile gliding Gravitational Gliding– Extruding– Rear compression spreading spreading spreading (1) Downwards increasing strain Yes Yes Yes Yes Yes (2) Vertical shortening No Yes Yes Yes? No (3) Concave foliation trajectories Yes Yes Yes? Yes Yes (4) Foliations dipping less than 45 Yes No? Yes No? No (5) Extension in direction of translation Yes No? Yes No? No of nappe transport. Fig. 12 shows a similar model 4.5. Thermal control on the level of the decollement for the formation of the nappes and Hammamat basins underlying the cover nappes in the CED, and offers another possible mechanism for the formation of recumbent sheath folds in the The foliated and mylonitic shear zone accommodat- gneissic rocks at Hafafit described by Fowler and El ing top-to-the-NW nappe transport in the study area Kalioubi (2002). If this is so, then some key structural straddles the greenschist to amphibolite transition. The evidence for collision-related crustal thickening in the gneissic complexes at Wadi Kid (Sinai) and Wadi Um Egyptian Eastern Desert may have been incorrectly Had, Gebel Meatiq, Gebel El Sibai and Gebel Um El- included in the category of structures associated with Shalul (Central Eastern Desert) also have mylonitic the NW-ward nappe translation event. Fig. 12 also carapaces, with NW–SE trending stretching lineations, shows how subhorizontal shearing could produce inter- some with top-to-the-NW kinematic indicators, with leaving of slices of ophiolitic melange and Hammamat amphibolite facies sheared rocks at the base of the shear metasediments seen in the cross-sections of the study zone and greenschist facies rocks in the upper parts and area (Fig. 2b). above. Metamorphism is roughly coeval with shearing

Fig. 12. (a) Idealized longitudinal section (oriented NW–SE along the direction of nappe transport) for the Eastern Desert basement rocks. Grey stippled areas represent Hammamat molasse in extensional basins. Areas with v-symbol ornament represent ophiolitic melange and Pan-African arc volcanic formations. Unornamented areas of folding beneath the melange and volcanics are gneissic rocks. Dashed lines represent subhorizontal S1 and Sm foliations produced during extensional collapse of the orogen. Crust is shown thinning to the NW. Hammamat molasse sequences show deformation in the stratigraphically lower parts depending on the degree of tectonic thinning of the crust beneath them (compare the basin on the left with the basin on the right). In the gneissic rocks, originally upright folds related to arc collision and crustal thickening have been rotated to recumbent orientations as a result of the gravitational collapse. Figures (b) and (c) provide a possible explanation for the interleaving of slices of Hammamat units and ophiolitic melange, seen on the Um Esh–Um Seleimat cross-sections (Fig. 2b). In (b) and (c) an irregular basin floor of the Hamammat basin is shown composed of ophiolitic melange fault blocks that were juxtaposed during basin opening. Progressive shear slicing (shear planes labelled ‘‘s’’ in (b)) at the lower levels of the Hammamat basins juxtaposes and sandwiches slices of sheared Hammamat metasediments and ophiolitic melange. 38 A.-R. Fowler, B. El Kalioubi / Journal of African Earth Sciences 38 (2004) 23–40 at the base of the cover nappes in the study area on the Akaad, M.K., 1996. Rock Successions of the Basement. An Autobio- evidence of greenschist and amphibolite facies phases graphy and Assessment. The Geological Survey of Egypt, Paper defining the mylonitic foliations (S ) and lineations (L ). no. 71. m m Akaad, M.K., Noweir, A.M., 1969. Lithostratigraphy of the Hamma- One interpretation of these facts is that after collision- mat-Um Seleimat District, Eastern Desert, Egypt. Nature 223, related crustal thickening, thermal re-equilibration of 284–285. the thickened Pan-African crust, involving upward Akaad, M.K., Noweir, A.M., 1980. Geology and lithostratigraphy of transfer of heat (perhaps magmatically as described by the Arabian Desert orogenic belt of Egypt between Lat. 25 350 and 0 Fritz et al., 2002), led to decreased shear strength of the 26 30 N. In: Al Shanti, A.M.S. (Ed.), Evolution and Mineraliza- tion of the Arabian–Nubian Shield, 4. Pergamon Press, Oxford, pp. rocks via the effect of temperature on rock rheology. At 127–135. a temperature-controlled depth (evidently roughly cor- Akaad, M.K., Shazly, A.G., 1972. Description and petrography of the responding to amphibolite facies conditions) a subhor- Meatiq Group, Eastern Desert. 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