Precambrian Research 123 (2003) Received 1 April 2001; received in revised form 20 January 2002; accepted 12 April 2002

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1 Precambrian Research 123 (2003) Structural and tectonic evolution of the Neoproterozoic Feiran Solaf metamorphic belt, Sinai Peninsula: implications for the closure of the Mozambique Ocean M.K. El-Shafei a,b, T.M. Kusky a, a Department of Earth and Atmospheric Sciences, St. Louis University, St. Louis, MO 63103, USA b Department of Geology, Suez Canal University, Ismailia, Egypt Received 1 April 2001; received in revised form 20 January 2002; accepted 12 April 2002 Abstract The Feiran Solaf metamorphic belt of the southern Sinai Peninsula consists predominantly of amphibolite facies metapelitic, arenaceous, and minor calcareous rocks deformed during Neoproterozoic ( Ma) orogenesis and intruded by dioritic tonalitic granitic plutons. The belt is divided into the Solaf Zone in the southeast and the Feiran Zone in the northwest. Twenty-five detailed structural cross-strike transects reveal that intense polyphase deformation resulted in complex fold geometries and fabric relationships. On outcrop scale, the progressive D 1 deformation is recognized as early, NW-striking, intrafolial isoclinal folds (F 1 ), an associated steeply dipping gneissic (S 1 ) fabric, and the development of calc-mylonite zones along the NE margin of the belt. Tight, NW-striking, recumbent similar folds (F 2 ) characterize the late D 1 deformation, whereas NE-striking open (F 3 ) folds represent D 2. Macro-scale folds of the F 1 and F 2 account for the map-scale structural pattern shown by the lithologic units. F 3 is represented mostly by meso-scale folds. The early SE- and NW-plunging F 1 and F 2 fold axes may be related to accretion of intraoceanic sediments deposited between arcs and/or microcontinents, whereas the late NE-trending upright F 3 folds may be related to post-accretionary tectonic events. In particular, the F 3 folds and associated mineral stretching lineations, which trend NW SE, may be related to the Najd fault system Elsevier Science B.V. All rights reserved. Keywords: Arabian Nubian Shield; Sinai Peninsula; Feiran Solaf metamorphic belt; Deformational phases; Mesoscopic folds; Structural evolution 1. Introduction 1.1. Geologic setting and tectonic significance of the southern Sinai Peninsula The Arabian Nubian Shield is part of the East African Orogen formed in the late Proterozoic Corresponding author. Tel.: ; fax: address: kusky@eas.slu.edu (T.M. Kusky). ( Ma; Bentor, 1985; Kröner, 1985; Stern, 1994; Loizenbauer et al., 2001) by the accretion and amalgamation of oceanic and continental magmatic arcs and accretionary prisms during subduction and obduction of oceanic crust, and the closure of the Mozambique Ocean, suturing East and West Gondwana (e.g. Kröner et al., 1987). Late stages of this orogenic cycle are marked by abundant intraplate magmatism, intraplate rifting, and transcurrent faulting associated with the Najd fault system among others (Bentor, 1985; Sultan et al., 1992; Johnson, /03/$ see front matter 2003 Elsevier Science B.V. All rights reserved. doi: /s (03)00072-x

2 270 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) ; Kusky and Matsah, 1999, 2003; Blasband et al., 2000). The rugged mountains of the southern Sinai Peninsula form a critical link between the Arabian and Nubian Shields in the northern part of the East African Orogen. However, little is known about the structural sequence and chronology of events in this region, nor of the tectonic setting or correlation of major rock belts with other regions. Most structures related to accretionary events during closure of the Mozambique Ocean are oriented roughly NE to ENE in southern Sinai (Shimron, 1984; El-Shafei et al., 1992), with the exception of the Feiran Solaf metamorphic belt, which is dominated by NW strikes. The Arabian Nubian Shield is divided into a number of arc, microcontinent and other terranes, with southern Sinai sitting near the northern exposed limit of Neoproterozoic crust. The Midyan terrane to the east in Saudi Arabia has affinities to oceanic arc terranes, as do rocks of the Gerf terrane (also known as the Aswan and SE Desert terrane; see Abdelsalam et al., in press; Johnson, 2000; Kusky and Ramadan, 2002). The Feiran Solaf belt, and its possible correlatives in the Kid and Sa al-zaghra belts of southern Sinai, form a relatively thin elongate belt of highly deformed metasedimentary and metavolcanic rocks that separate the parts of the Midyan terrane from parts of the Gerf terrane. As such, the studies reported here have implications about correlations between the Gerf and Midyan terranes, and whether these represent separate or similar terranes. The entire Egyptian part of the Arabian Nubian Shield was subdivided by El-Gaby et al. (1988, 1990) into an older gneissic unit considered to be basement, upon which an upper structural level (including ophiolitic melange of Shackleton et al., 1980) was obducted during the late Pan-African orogeny. Stern and Hedge (1985) and Kröner et al. (1990, 1994) suggested that basement domes formed during the Pan-African magmatic evolution of island-arcs without the involvement of older continental crust. This interpretation is based on isotopic ages and geochemical constraints. The Feiran Solaf metamorphic belt forms a NW striking, 35-km long belt of migmatites, gneisses, and schists in the southern Sinai Peninsula (Fig. 1). Rock types include several varieties of quartzofeldspathic and hornblende-biotite gneiss, migmatites, and calc-silicate schists, intruded and surrounded by plutonic rocks, including non-deformed meladiorite, a pre- to syn-tectonic granodiorite tonalite quartz diorite association, and late-, syn-, and post-tectonic granites and dikes. The rocks are disposed in a regional dome, with higher-grade migmatites forming the core of the dome in the more deeply eroded Feiran Zone, and a rim of amphibolite-grade metapelites and metapsammites in the Feiran and Solaf Zones. Metacarbonates are rare except for in a narrow strip along the east side of the belt. A Phanerozoic cover sequence unconformably overlies the western part of the belt. U Pb zircon ages of some of the paragneissic units include an age of 632 ± 3Ma (Stern and Manton, 1987), and a granodiorite east of the belt have yielded U Pb ages as old as 782 ± 7Ma (Jarrar et al., 1983). Post-tectonic dikes have yielded Rb Sr whole-rock ages of 591 ± 9Ma (Ayalon et al., 1987). We report preliminary U Pb results that indicate a granodiorite that intrudes the eastern side of the belt is ± 4.7-Ma-old. During this study about 25 cross-strike transects were made throughout the belt, along which several thousand planar and linear structural elements were measured. Penetrative planar and linear fabrics display a close geometric relationship with outcrop- and map-scale folds in the Feiran Solaf belt. In this contribution, we describe the major rock units and metamorphism of the Feiran Solaf belt, present a detailed analysis of the structural geometry and evolution of the belt, and offer a preliminary tectonic model that explains our observations. 2. Description of major rock units The Feiran Solaf belt is a medium- to high-grade gneiss terrane. We subdivide the belt into two zones with different metamorphic characteristics: (1) the Solaf Zone to the southeast; and (2) the Feiran Zone to the northwest. A meladiorite intrusion roughly separates the two zones (Fig. 1). The Solaf Zone comprises predominantly amphibolite-grade metasediments (pelitic, semi-pelitic, quartzofeldspathic, calc-silicate rocks, quartzite and marble), whereas a migmatized paragneiss complex is the most abundant outcrop unit in the Feiran Zone. The belt was mapped at a scale of 1:10,000 so that complex relationships and data from small-scale

3 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) Fig. 1. Location and simplified geologic map of the Feiran Solaf metamorphic belt, modified after Ahmed (1981).

4 272 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) structures could be shown in adequate detail. Outcrop maps rather than contact maps were made so that all structural interpretations shown on the maps could be objectively evaluated. Lithologic units and distinctive lithologies (marker beds) rather than stratigraphic units (cf. El-Gaby and Ahmed, 1980; Hegazi, 1988) were mapped because they delineate map-scale structures best. For mapping, the Feiran Solaf belt was subdivided into broad lithological units, some of which are biotite-, hornblende- and biotite-, or quartz-rich. The present study reveals that the Feiran Solaf belt contains at least three-fold generations of alternating and superimposed antiforms and synforms, which play a great role in the spatial distributions of the different rock units. Based on the spatial distribution, contact relationships, and the structural pattern of the lithologic units, the sequence of main rock units exposed in the belt is arranged from oldest to youngest in Fig. 1, and discussed in that order here Gneissic and migmatitic units Amphibolite Amphibolite of the Feiran Solaf belt occurs as inclusions, bands, linear bodies of variable thickness, and irregular lenses in paragneisses (too small to be shown in Fig. 1). Previous investigators (e.g. El-Tokhi, 1992) suggested an igneous origin for the amphibolites, based on chemical evidence. They suggested that the amphibolites were derived from tholeiitic magmas, transitional in character between continental and island arc chemistry. Ortho-amphibolite enclaves are found in different metamorphic rock units and also in the syntectonic granitoids; some of them are massive, whereas others are foliated. They are fine- to medium-grained, grayish green to black in color. Mineralogically, the amphibolites consist of hornblende, plagioclase, biotite, iron oxides, and quartz. High contents of opaque minerals, titanite, and apatite indicate the basic nature of their parent rocks and support an igneous origin for the amphibolites (El-Tokhi, 1992) Migmatized gneissic complex A migmatized gneiss complex forms the core of the Feiran antiform outcropping along Wadi Feiran (Fig. 1). This complex is about 10 km long and 2 km wide, and includes dioritic, tonalitic, granodioritic, and granitic orthogneiss, intercalated with banded hornblende-biotite gneiss of probable metapelitic origin. The migmatitic paragneisses show compositional banding on the millimeter to centimeter scale, defined by alternation of coarse-grained leucosomes and medium- to fine-grained mesosomes (both in the sense of Ashworth, 1985). The gneiss belt is intruded along the eastern and the southern parts of the mapped area (Fig. 1) by granitoid rocks that contain xenoliths of the migmatites, amphibolites, and gneisses (Fig. 2a). The migmatites can be easily differentiated in the field and separated into two separate rock units, including granitic and trondhjemitic migmatites, and constitute the area between Wadi Aleiyat and Wadi Tarr (Fig. 4). Further west, the migmatites occur at the exposed base of the metamorphic sequence. No migmatized rocks are recorded in the Solaf Zone. The megascopic appearance of the migmatites in the study area is strongly variable. Stromatic, agmatic, ptygmatic, and folded migmatitic structures are present (sensu Sederholm, 1907, 1934; Mehnert, 1968; Ashworth, 1985; Johannes et al., 1995). The most common migmatitic structures are the stromatic type with leucosomes parallel to the foliation of the gneisses. Light color, coarse-grain size, and the absence of gneissosity and common fold interference patterns characterize the migmatite leucosomes, which typically consists of quartz and plagioclase as dominant components. Melanosomes, which occur adjacent to leucosomes, are present as layers of variable thickness (up to 2 cm), but typically are thinner than the leucosomes. The leucosomes and the melanosomes are mostly concordant with sharp contacts between them. Ptygmatic structures in the migmatites reflect the effect of partial melting (i.e. the deformation occurred while the leucosome was mostly liquid and the melanosome was recrystallized) (e.g. Mehnert and Bush, 1982). Two generations of leucosomes occur in the Feiran migmatite: granitic and tonalitic. The granitic leucosomes exist mostly as equigranular coarse- to medium-grained layers dissected by the tonalitic leucosomes. They range in color from pinkish white to reddish and are composed mainly of quartz, plagioclase, K-feldspar and biotite. Accessory minerals include titanite, epidote, zircon and iron oxides. Quartz, plagioclase, biotite, and/or hornblende dominate the tonalitic leucosomes, which are generally white to

5 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) Fig. 2. Outcrop field photographs: (a) different relics of migmatitic structures engulfed in the pre- to syn-tectonic granite, Feiran Zone; (b) pinch-and-swell boudinage structures, Feiran Zone; (c) fold flattened by ductile shearing, Feiran Zone; (d) F 1 isoclinal folds in hornblende biotite gneiss of Feiran Zone;(e) Parasitic folds in pegmatite veins in hornblende-biotite gneiss showing systematic change in symmetry between limbs of larger fold; (f) Z-shaped fold in calc-silicate rocks of Solaf Zone. Note the intensive development of the smallest wavelength at the hinge of the larger fold; (g) type-3 interference pattern: F 1 isoclinal fold refolded by F 2 dextral fold, migmatized gneiss of Feiran Zone; (h) F 3 open fold superimposed on both F 1 and F 2 structures, Solaf Zone calc-silicate rocks; (i) dextral simple-shear sense shown by rotated porphyroclast, Solaf Zone. light-gray in color, and coarse- to medium-grained. Mesosomes are medium-grained, equigranular to sub-equigranular, and show variable mineral composition from one mesosome layer to another. The dominant minerals are quartz, plagioclase, biotite, and hornblende. Melanosomes are fine-grained, strongly foliated, and display a granoblastic texture composed essentially of quartz, plagioclase, biotite, and hornblende; biotite is generally more abundant than hornblende Hornblende-biotite gneiss Hornblende-biotite gneissic rocks are widely distributed in the area. They are interlayered with the biotite and quartzofeldspathic gneisses and also occur as xenoliths in the quartzofeldspathic gneiss. They dominate the outcrop pattern of the western part of the belt (Feiran Zone, Fig. 1), and field relationships suggest that they represent the oldest exposed gneissic rock unit in the Solaf Zone. The boundary shown on the map is based on the relative abundance of these rocks with regard to other units. The hornblende-biotite gneiss is fine- to medium-grained, strongly foliated, and dark-colored. It shows banding, and recumbent folds in Feiran Zone and similar folds in Solaf Zone. The hornblende-biotite gneisses consist of hornblende, plagioclase, biotite, and quartz with subordinate amounts of sillimanite, chlorite, and

6 274 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) epidote (clinozoisite) (El-Shafei, 1998). Iron oxides, zircon, apatite and titanite are accessory minerals. The presence of titanite and apatite, suggests that these rocks could be metamorphosed mafic rocks, as opposed to metapelitic rocks (El-Tokhi, 1992). However, their intricate intercalation with other metasedimentary gneisses suggests that they may have had a sedimentary protolith Biotite gneiss Biotite gneisses are brownish gray in color, fine- to medium-grained, exhibit a pronounced foliation, and predominantly crop out in the Solaf Zone, where they are intercalated with hornblende-biotite gneiss and interlayered with quartzofeldspathic gneiss. These rocks are strongly foliated and altered, and display similar fold styles and common boudinage structures. Near the contact with the syntectonic granitoids, these rocks become rich in garnet, cordierite and spinel, while the hornblende-biotite gneisses become rich in diopside (El-Shafei, 1998) Biotite, hornblende-biotite, and local quartzofeldspathic gneiss Biotite, hornblende-biotite, and local quartzofeldspathic gneisses crop out at the eastern part of the Solaf Zone in contact with the granitoid rocks. These rocks form the uppermost unit in the succession and define the closure of a large antiform (Fig. 1). They are fine-grained and of a medium amphibolite facies. They have experienced thermal and dynamic metamorphism, giving rise to granoblastic textures, which are more massive than in the lower-grade varieties Quartzofeldspathic gneiss Quartzofeldspathic gneisses are characterized by light-whitish red and grayish to reddish color with variable grain size. On the meso-scale, they also show clear banding, faults and open folds. A foliation and overturned meso-scale open folds are observed in the Feiran Zone. In the southeast of the belt, the quartzofeldspathic gneisses are interlayered with the hornblende-biotite gneisses and biotite gneisses, and are generally cut by numerous pegmatite veins. The fine-grained quartzofeldspathic gneisses are characterized by granoblastic textures, with rare scattered biotite flakes oriented within the quartzofeldspathic mosaic Calc-silicates Calc-silicate rocks form a relatively minor component of the Feiran Solaf belt. However, the exposures are remarkable in the field, displaying a wide variety of mesoscopic structures and containing garnet crystals of relatively large size. They occur in the Solaf Zone as a relatively narrow, discontinuous, highly deformed belt (70 m wide), running parallel to and in contact with the granitoid rocks in the eastern part of the study area. Northwards, these rocks become pure marble 500 m before the junction of Wadi Feiran, Wadi El-Sheikh, and Wadi Solaf (Fig. 3), and show a slight difference in the attitude of the foliation and banding. Calc-silicate rocks are fine-grained, massive, light-green in color, and show distinctive banded structures defined by the alternation of white and dark green bands Quartzite Quartzite forms small exposures in the Solaf Zone. It is composed essentially of quartz, K-feldspars, and opaque minerals, and shows fine-grain size and granoblastic textures Metavolcanic rocks Metavolcanic rocks were recorded at only two localities in the Solaf Zone. The first is located to the SE in contact with the granitoid rocks and is characterized by a fine-grain size, light green color, and low metamorphic grade. The second location is close to the junction of Wadi Solaf and Wadi Rim (Fig. 3), where the rocks are leucocratic, fine-grained, hard, banded, and have a high specific gravity Plutonic rocks Pre- to syn-tectonic granitoids Pre- to syn-tectonic granitoid rocks include a gray granodiorite tonalite quartz diorite association that intrude along the south and east borders of the Solaf Zone (Fig. 1). They extend further east forming numerous low-relief hills characterized by intense shearing and they contain many xenoliths of amphibolite and hornblende-biotite gneiss, particularly near the southern and eastern contacts with the metamorphic belt. Kusky et al. (in preparation) report a U Pb zircon age for this pre- to syntectonic granodiorite of ± 4.7 Ma, providing

7 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) Fig. 3. Structural map showing the main planar and linear structural elements of the Solaf Zone; inset is equal-area projection of the linear elements throughout the entire belt. a minimum age for the Feiran Solaf metamorphic units Meladiorite A large non-deformed meladiorite body forms an elongated, NE-striking intrusion that contains inclusions of darker diorite, gabbro, and amphibolite. The elongation of the intrusion suggests that it may have intruded along a pre-existing crustal weakness located roughly to the north between the Solaf and Feiran Zones Um Takha white granite Um Takha white granite crops out along the upper part of Wadi Um-Takha (Solaf Zone). Red alkali granites of Gabel Serbal intrude it (Fig. 1). Small outcrops also occur in contact with calc-silicate rocks causing local contact metamorphism. They show gneissic

8 276 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) textures and contain xenoliths of hornblende-biotite gneiss and different metamorphic enclaves Late syn- and post-tectonic granites Late syn- and post-tectonic granites form large mountainous outcrops bordering the belt and the syntectonic granitoids. Reddish color and the absence of dike swarms that are recorded within the early plutonic phases characterize late syn- and post-tectonic granites (e.g. Ahmed, 1981) Dikes The belt and surrounding plutonic rocks are crosscut by a great number of Mesozoic Cenozoic and, less-common, Precambrian dikes of variable grain size and composition. The dikes trend mostly NE, but also NW, parallel to the Gulf of Aqaba and the Gulf of Suez. The Mesozoic Cenozoic dikes are related to the opening of the Red Sea (e.g. Ahmed and Youssef, 1976). Most of the dikes cut the rocks along fractures whereas some of them run parallel to foliation planes. 3. Metamorphism and protoliths of metamorphic rocks According to a detailed petrographic study (El-Shafei, 1998) and results of previous investigators (e.g. Akaad, 1959; Ahmed, 1970; El-Gaby and Ahmed, 1980; Hegazi, 1988; Belasy, 1991; El-Tokhi, 1992), it is concluded that the parent rocks of the gneisses and migmatites were sedimentary with minor volcanic intercalations. We suggest that the primary rock types included, in order of abundance, pelite, graywacke, sandstone, volcanic intercalations, and calcareous sandstone. This interpretation is based on the following observations: (1) preserved relict bedding revealing intercalated pelitic, semi-pelitic, and marly layers; (2) presence of well-rounded zircon grains marked by corroded outlines; and (3) abundance of biotite and the occurrence of cordierite and sillimanite, defining the argillaceous parent sedimentary rocks. The quartzofeldspathic gneiss, calc-silicate rocks, biotite gneiss, hornblende-biotite gneiss, and amphibolite are derived from arenaceous, arenaceous limey, pelitic, calcareous pelitic or marly, and limey sediments, respectively. The higher quartz contents in the quartzofeldspathic gneiss and calc-silicate rocks indicate their arenaceous nature. The argillaceous nature of the biotite gneiss is evidenced from the higher volume percentage of biotite and K-feldspar. The occurrence of iron oxides surrounded by thick clusters of titanite in some amphibolites suggests an igneous origin (e.g. Williams et al., 1982). Alternatively, these oxides may be simple reaction products as seen in many metamorphosed pelites. Also, it is concluded that the area experienced regional amphibolite facies metamorphic conditions. Akaad (1959) suggested that highly metasomatic metamorphism affecting the gneiss belt took place in two stages, namely, an early phase of medium-grade regional metamorphism, followed by a highly metasomatic phase. However, El-Gaby and Ahmed (1980) stated that these two stages were related to one cycle of metamorphism and that no tectonic events intervened between them. The micro-structural generations recognized by El-Shafei (1998) formed under the same metamorphic conditions and show evidence of gradual changes in the metamorphic conditions as the deformation progressed. The metamorphic units of the belt show the effects of two successive phases of regional amphibolite-facies metamorphism (M 1 and M 2 ), followed by a later thermal overprint. Specific mineral assemblages, fabric orientations and compositions characterize each metamorphic phase (El-Shafei, 1998). 4. Mesoscopic structures of the Feiran Solaf belt In this section, we describe, correlate, and analyze the field orientation data of the outcrop-scale structures in the rocks of the Feiran Solaf belt. Within the Feiran and Solaf Zones, 25 cross-strike transects were made, along which the available planar and linear structural elements were measured. The most prominent planar and linear elements in the Solaf and the Feiran Zones are shown in Figs. 3 and 4, respectively. The belt contains a continuous, albeit highly deformed, stratigraphic sequence repeated numerous times by complex folds. Thrusts and other early faults locally repeat the section, although these appear to have a minor influence on the structure of the belt with the important exception of the thrusts that form calc-mylonites along the NE margin of the belt. Based on the

9 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) Fig. 4. Structural map showing the main planar and linear structural elements of the Feiran Zone. overprinting relations between the different fold systems, three successive phases of folding are observed Foliations Schistosity and gneissic foliations are among the continuous foliations observed in the Feiran Solaf belt. This type is given the symbol S 1. Intersection and crenulation lineation define foliations and are given the symbol S 2. Most foliations in the Feiran Solaf belt are axial-planar to folds. Quartzofeldspathic gneisses of the Solaf Zone are weakly foliated, whereas the same rock types in the Feiran Zone are strongly foliated. The change in the spacing of the foliation within the entire belt may be related to strain variation, however, it may also be a consequence of original lithologic variations. According to the styles of the mesoscopic folds and the overprinting relationships, the map-scale folds of the belt occur as three-fold generations designated F 1,

10 278 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) F 2, and F 3. Crosscutting relationships between foliations, folding and veins that cut them are used to derive the relative ages of different generations of structures. However, some veins are cut by the S 2 foliation and are also folded (Fig. 5b), indicating their formation after F 1 and prior to or during the second F 2 folding phase. Gneissic foliations in the area are generally defined by medium- to coarse-grained minerals that form compositional banding with a preferred planar orientation of platy, tabular, or prismatic minerals, and by subparallel lenticular mineral grains and grain aggregates. The compositional banding occurs at all scales from thick, continuous bands and layers that can be mapped across the entire field area, to discontinuous laminae that pinch out within individual outcrops, hand specimens, or thin sections (El-Shafei, 1998). Some mesoscopic structures also are associated with thin layers and pods of granitic material, forming migmatites and agmatites in the Feiran Zone. Foliation planes strike mainly NW SE, roughly parallel to the elongation of the belt, with varying dip directions. In the eastern portion of the Solaf Zone, foliations have shallow dips (average 25 NE), whereas in the central portion of the Solaf Zone foliations show moderate dips (45 SE), but along the western part of the Solaf Zone, they show steep dips (81 NW). In the Feiran Zone, foliations show moderate to steep dips (Fig. 4). The variation in strike and the dip of foliations throughout the belt reflects the effect of fold overprinting. The presence of inclined axial planar foliations of the Solaf Zone, upright folds, and sub-horizontal foliation of the Feiran Zone recumbent folds suggest that these two folds may be related to two different fold systems. Crenulation foliations are recorded only in the biotite-gneisses of the Feiran Solaf belt as crenulations of S 1 cleavage Lineations Linear structures are well developed throughout the area. They have been divided into the following types: intersection lineations, fold hinge lineations, boudin lineations, and mineral lineations. They are found in nearly all rock types, but are generally best developed in migmatites, especially in the crests and troughs of the macroscopic folds. The mean geometric orientation of intersection lineations is parallel to that of the major fold axes (F 2 ). The preferred orientations of mesoscopic folds define fold hinge lineations. The measured mesoscopic fold axes are treated geometrically as a regionally penetrative lineation. They show the presence of three clusters on the stereonets that are related to the three observed macroscopic fold axes, as explained later. Boudins display a wide variety of shapes in the area. Leucocratic plagioclase-rich layers are commonly segmented forming boudins, while the surrounding melanocratic material flowed in between them. In this type of boudinage the necks between boudins are smoothly curved. The existence of pinch-and-swell structures (Fig. 2b) is strong evidence for non-linear flow in the stiff layer (e.g. Hudleston and Lan, 1993). Some boudins are rotated about their long axes showing right-lateral sense of shearing (Fig. 2d; Wilson, 1982). The long axes of most boudins are aligned parallel to the F 2 macroscopic fold axes (recumbent folds). Some other boudins are oriented normal to the F 2 axes, and these groups are related to post-f 2 folding (i.e. parallel to F 3 macroscopic folds). Comparisons of the orientation of layers that have been shortened versus those that were lengthened and boudinaged were used to infer the sense of shear during boudin formation (Davis and Reynolds, 1996). Other boudinage structures show no evidence of shearing (i.e. they were formed during pure shear deformation). Mineral lineations in the area are developed in the plane of the foliation and are typically marked by a streaky, fiber-like lineation, composed of aligned crystals of quartz, feldspar and hornblende. This mineral fabric (stretching lineation) is parallel to the geometric axes of the macroscopic F 1 (SE plunges) and F 2 (NW plunges) folds Fold style, orientation, and overprinting relationships Mesoscopic folds with wavelengths of tens of centimeters to a few meters and variable styles and orientations are numerous in the Feiran Solaf belt (Figs. 3 and 5). Most folds in the Feiran Solaf belts are parallel Class 1B types (e.g. Fleuty, 1964; Ramsay, 1967; Hudleston, 1973; Twiss, 1988; Hudleston and Lan, 1993), although more complex types are also present.

11 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) Fig. 5. Nature, style, and orientation of some mesoscopic folds recognized within Feiran Solaf belt. All are traced from photographs. (a) Asymmetry sense of the later F 2 mesofolds superimposed the F 1 fold; (b) Post F 1 and syn-f 2 folded quartz vein; (c) type-1 interference pattern (dome and basin); (d) isoclinal F 1 fold developed in the hornblende-biotite gneiss of the Feiran Zone; (e) S-shaped fold in migmatites of Feiran Zone; (f) Z-shaped fold developed in the calc-silicate rocks of Solaf Zone; (g) zigzag pattern type of interference showing F 2 fold superimposed the F 1 ; (h) F 3 fold superimposed both F 1 and F 2 folds; (i) dextral shearing indicated from the rotated sigma porphyroclast, Solaf Zone; (j) sketch showing style of minor F 1 and F 2 folds from the Feiran Zone; (k) F 3 folds superimposed on both F 1 and F 2 folds from the Feiran Zone.

12 280 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) Mesoscopic folds Mesoscopic folds were analyzed in the field and grouped according to style, orientation, and overprinting relationships. NW- and SE-trending folds characterize the Solaf Zone. NW-trending folds generally are more steeply plunging than those with a SE trend. NE-trending folds are rare in the Solaf Zone. In the Feiran Zone, the number of SE-trending folds increases at the expense of the NW-trending folds, both of which have moderate to steep axial plunges. The earliest folds (F 1 ) exhibit variability in attitude from region to region throughout the belt, but show considerable similarity of style (i.e. they are isoclinal) (Figs. 2d and 5g), intensity of folding, and associated planar and linear mineral growth. The axial planes of the isoclinal folds are parallel to the regional foliation, which in turn is roughly parallel to the trend of the belt. This suggests a strong NE SW flattening normal to the trend of the gneiss belt at this stage of deformation. Some folds show broad rounded hinges, suggesting a low ductility contrast during folding (Model A of Ramsay, 1967; see also Ramsay and Huber, 1987), where the thickness ratio of the competent to incompetent layers is low. Elsewhere, both competent and incompetent layers are thickened in the fold hinge zone and are thinned in the fold limbs (Model B of Ramsay, 1967; Fig. 5j). Some folds in the Feiran Zone show highly attenuated limbs (Fig. 5i) as a result of progressive strain. Ptygmatic folds were also encountered in most of the metamorphic units. However, some mesoscopic folds display the effect of a simple shear mechanism as indicated by the presence of sigmoidal drag folds. Other mesoscopic folds show the effects of flattening by ductile shearing (Fig. 2c). The younger fold sets show a progressively more uniform orientation through the metamorphic rock sequence, and they are nearly identical in terms of style and orientation. F 2 fold sets are characterized by tight to recumbent folds, and are encountered predominantly in the hornblende-biotite gneiss of the Feiran Zone. F 2 fold sets have a strong and consistent sense of asymmetry; virtually all verge towards the SE (Figs. 2e and 5f) and are S-shaped when viewed down-plunge. F 2 folds refolded the F 1 structures and the variability of their geometry is ascribed to progressive development of folds and the original orientation of F 1 folds (Fig. 2d). F 3 open folds are only locally developed at outcrop scale (Fig. 5d). Z-shaped folds are recognized in many parts of the study area (Fig. 2f) and are used together with S-shaped folds to locate the large scale ones Geometry and mechanics of superimposed mesoscopic folds Fold overprinting relationships are clearly recognized within the metamorphic rock units of the Feiran Solaf belt. The resulting outcrop pattern of two successive folding events depends entirely on: (a) the style, orientation, and scale of the individual fold sets, including the shapes of the earlier folds and the inclination of their axial planes; (b) the orientation and intensity of the F 2 fold formation; and (c) the amount of flattening which accompanies the formation of F 2 folds and the orientation of the outcrop surface (e.g. Ramsay, 1962; Hudleston and Lan, 1993). The metamorphic rocks of the Feiran Solaf belt show interference patterns between F 1 and F 2 folds (Fig. 2f), and among F 1,F 2, and F 3 folds (Fig. 2g). F 1 folds are characterized by overturned isoclinal folds at small scales and resulted in the development of a strong schistosity (S 1 )(Fig. 5e), which is defined by mica in metapelites. Broadly spaced cleavage is developed in more competent lithologies such as quartzofeldspathic gneisses. The orientation of F 1 planar and L 1 linear fabrics varies according to their positions with relation to F 2 major folds. The second set of folding (F 2 ) is the main fold set of tight, recumbent and local similar fold styles. The mesoscopic folds of these sets show different geometries. The axial trends of these folds consistently plunge between 20 and 70 towards the NW and the associated planar fabric with this phase of deformation consists mainly of axial crenulations and spaced cleavage, which strikes NW and dips towards the NE. L 2 lineations within the belt are mainly boudins and intersection lineations, and plunge NW. F 3 is a local mesoscopic fold set, characterized by broad, open folds. In the Feiran Solaf belt, the axial surfaces of similar type folds show slight rotation or rearrangement of the inferred principal shortening direction. The geometry of structures indicates that flattening played a major role in the deformational history of the belt. Flattening probably dominated in the early stages (Fig. 2c) and gave way to local simple shear in the later stages

13 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) (Fig. 2i). The planar and linear fabrics in the metamorphic rock units display a close geometric relationship with the coeval larger-scale folds (i.e. outcrop-scale lineations are parallel to map-scale fold axes; Figs. 9 and 10) Early thrust faults and calc-mylonites A folded early thrust fault is recognized near the contact with the pre- to syn-tectonic granitoids in the eastern part of the gneiss belt. This early thrust fault is preserved as a NW-striking belt of calc-mylonites up to 50 m thick. The calc-mylonites are composed of diopside, plagioclase, quartz, and garnet. The garnets form large, spectacularly rotated porphyroblasts with deflected and rotated tails, indicating the sense of shear. The kinematic indicators from this mylonite zone show an early sense of thrusting from the NE toward the SW (Kusky and El-Shafei, in preparation). 5. Geometrical analysis of the structural elements In this section, we analyze the structural geometry and sequence of folding in the study area by describing the orientation of folds, fold shape and tightness, and graphically restoring folds to find the initial orientation of the early structural elements. Some modifications have been made to improve the appearance of the contour diagrams, including the removal of some intermediate contour lines. The maps of the Solaf and Feiran Zones (Figs. 3 and 4) show the distribution of the main planar and linear structural elements throughout the belt. The structural measurements on these maps represent the mean orientation of the structural elements and some data are eliminated due to the small scale of the map. Foliations, intersection lineations, mineral lineations, boudins, and mesoscopic fold hinge lineations are the main structural elements that are treated geometrically and statistically using stereographic projection techniques to calculate their original orientations Foliations Field studies of the Feiran Solaf belt indicate that the belt experienced multiple folding events. It is therefore necessary to subdivide the polyphase fold belt into smaller, homogenous subdivisions or domains each of which contains structures that are statistically homogeneous (e.g. domains characterized by cylindrical folds). The crude form of macroscopic folds was determined by drawing extrapolated strike lines on the map parallel to the measured attitudes of foliations in the Solaf (Fig. 6) and Feiran (Fig. 7) Zones. These maps show the general forms of the macrofolds within the belt, which cannot be deduced by standard field mapping methods because the folds are too large to be detected by direct observation in the field. The crude fold forms have been subdivided into seven Domains, three in the Solaf and four in Feiran Zones. This subdivision is based essentially on the configuration of the extrapolated strike lines, the unique orientation of both planar and linear elements, and the attitude of the included mesoscopic folds. For each domain, the available measurements were treated separately in order to construct the whole complex, noncylindrical structure. The axis and the axial plane of each fold dominating each domain were defined. The subdivisions displayed on Figs. 6 and 7 are based on the recognition of the rectilinear nature of the apparent trace of the hinge surfaces and other structural evidence (e.g. the attitude of mesoscopic folds). A plot of the data from each domain yields the orientation of the folds in each homogeneous part of the large heterogeneous structures; the results of the analyses are described below Solaf Zone The attitudes of 235 foliation planes were measured in Domain 1 in the eastern part of Solaf Zone (Fig. 6a). The stereogram in Fig. 6a shows one distinct maximum of mean orientation plunging 49 to S29 W. The contour lines are elongated along a great circle, which strikes S12 W and dips 80 W. The pole to the girdle (i.e. the fold axis) plunges 10 to S77 E. The axial plane (the great circle passing through the fold axis and the axial trace) strikes N30 W and dips 30 to the NE (Fig. 8a). The attitudes of 127 foliation planes were measured in Domain 2 in the central portion of the Solaf Zone. The stereogram in Fig. 6b shows a general concentration of points in the southwest quadrant of the projection, signifying that the foliations are moderately

14 282 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) Fig. 6. Structural orientation data from the Solaf Zone: (a c) equal area, lower-hemisphere projections of poles to foliations and simplified map showing boundaries of structural domain; (d and e) show point diagram and its contour equivalent to the Solaf Zone mesoscopic fold hinges, respectively.

15 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) Fig. 7. Structural orientation data from the Feiran Zone: (a d) equal area, lower-hemisphere projections of poles to foliations and simplified map showing boundaries of structural domains; (e) contoured diagram of poles to foliation along transect A B in Wadi Feiran; and (f and g) point and contour diagrams of mesoscopic fold hinges in the Feiran Zone, respectively.

16 284 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) Fig. 8. Attitudes of dominant axial plane in each structural domain in the Feiran Solaf belt. dipping towards the east and the center of the contoured pattern corresponds to the preferred orientation of the foliation. The contour lines are elongated along a great circle striking S41 W and dipping 77 W. The fold axis plunges 13 to S49 E, and the axial plane strikes N43 W and dips 70 to the NE (Fig. 6b). The attitudes of 234 foliation planes were measured in Domain 3 in the western part of the Solaf Zone

17 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) Fig. 9. Structural cross-section A B along Wadi Dehiset Abu-Talb and Wadi Um Takha, Solaf Zone.

18 286 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) Fig. 10. Structural cross-section C D through the Feiran Zone.

19 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) and the eastern part of the Feiran Zone (Fig. 6c). The stereogram shows one distinct maximum (Figs. 6c and 8c) situated in the southwest quadrant of the net that strikes S59 W and dips 22 to the SE, signifying that the foliations are steeply dipping towards the northeast. The contour lines are elongated along one great circle. The axial surface of the of the best-fit great circle strikes S15 E and dips 22 to the SW and the pole to this great circle plunges 68 to N75 E. The axial plane strikes N27 W and dips 68 to the NE (Fig. 8c). A point diagram and its contour equivalent of the mesoscopic fold hinges measured across the entire Solaf Zone are shown in (Fig. 6d), which indicates that these mesoscopic folds are parallel to the larger folds deduced by stereographic projection Feiran Zone The attitudes of 380 foliation planes measured in Domain 4 from the area around the Feiran Oasis are plotted in Fig. 7a. The diagram shows the elongation of the contour lines along a great circle striking S54 W and dipping 69 NW. The pole (fold axis) to this great circle plunges 21 to S36 E. The attitude of the axial plane strikes N22 W and dips 55 to the NE (Fig. 8d). The attitudes of 413 foliation planes were measured in Domain 5 and are plotted in Fig. 7b. The stereogram shows the elongation of the contour lines defining a fold girdle. The attitude of the best-fit great circle strikes N32 E and dips 64 to the NW and the fold axis plunges 26 to S58 E. The axial plane strikes N34 E and dips 26 to the SE (Fig. 8e). The fold axis plunges directly down the dip of the axial surface. Field measurements and the stereographic projection reveal that this reflects the presence of a cylindrical fold system in which the folds have roughly the same axial trends and axial surfaces. The attitudes of 87 foliation planes were measured in Domain 6. The stereogram of these data (Fig. 7c) shows the presence of two-point maxima, one of which is situated at the SW quadrant of the projection, signifying that this maximum is moderately dipping towards the NE, whereas the second maximum is located at the NE quadrant, signifying a foliation that is moderately dipping towards the SW. The two maxima are aligned together along a great circle that strikes S19 W and dips 85 W, and whose pole (the fold axis) plunges 5 to S71 E. The axial plane strikes N70 W and dips 82 to the NE (Fig. 8f). Domain 7 is located in the western portion of the Feiran Zone. The attitudes of 185 foliation planes were measured within this domain (Fig. 7d). The stereogram shows one distinct maximum, which is situated in the southwest quadrant of the net and steeply dipping towards the northeast. The contour lines are elongated along a best-fit great circle striking S21 W and dipping 82 W, whose pole (fold axis) plunges 8 to S69 E. The axial surface strikes N66 E and dips 67 to the NE (Fig. 8g). Table 1 exhibits the net result of analyses of all domains and the description of the resultant folds on the basis of the geometric relationship between axial surfaces and fold axes. Domains 1, 6, and 7 contain subhorizontal to gently plunging folds that are gently inclined (Domain 1), steeply inclined (Domain 7) or upright (Domain 6) while Domains 2, 3, 4, and 5 show gently plunging folds that are steeply inclined (Domains 2 and 3), moderately inclined (Domain 4), or gently inclined, reclined folds (Domain 5). Mesoscopic fold hinges encountered in the Feiran Zone are plotted in Fig. 7e. The diagram shows that these mesoscopic folds are parallel to the larger folds deduced by domain analysis. Fig. 7f is a contour diagram of 146 foliation planes measured along profiles A B (Fig. 7) in Wadi Feiran. The fold axis plunges 23 to S49 E, which nearly represents the mean axial Table 1 Attitudes of dominant macroscopic folds in each structural domain of the Feiran Solaf belt D Poles no. Fold axis Axial plane Fold description S77 E/10 N30 W/30 E Sub-horizontal, gently inclined fold S49 E/13 N45 W/69 E Gently plunging, steeply inclined fold N75 E/68 N27 W/68 E Steely plunging, steeply inclined fold S36 E/21 N18 W/54 E Gently plunging, moderately inclined fold S58 E/26 N31 E/26 E Gently plunging, gently inclined, reclined fold 6 87 S71 E/05 N70 W/81 E Sub-horizontal upright fold S69 E/08 N67 W/67 N Sub-horizontal steeply inclined fold

20 288 M.K. El-Shafei, T.M. Kusky / Precambrian Research 123 (2003) trend of the F 1 fold set. The diagram shows the effect of the F 2 folds, which deform the F 1 trends along a girdle indicated by the elongation of the contour lines Lineations The most common lineations encountered in the Feiran Solaf belt include intersection lineations, mineral lineations, boudins, and fold hinge lineations. The attitudes of 156 intersection lineations, mineral lineations, and the long axes of boudins are plotted in Fig. 3 (inset). The diagram shows the presence of three distinct clusters. The first cluster is situated in the southeastern quadrant of the net, which is parallel to the direction of the geometric F 1 axes of the macro folds. The second cluster is located in the northwestern quadrant parallel to the axes of major F 2 folds (Fig. 6b). The third cluster is located in the northeastern quadrant and coincides with F 3 axes, whose trend is N75 E and plunges 68 (Fig. 6c). The attitudes of 29 mesoscopic fold hinges within the Solaf Zone are plotted in Fig. 6c and d. Fig. 8d shows that the hinges are distributed along a great circle striking NW SE and dipping NE, coinciding with the axial plane of the geometric folds (see Figs. 6 and 8). Fig. 6e shows the presence of two maxima. The first is located in the southeastern quadrant and it coincides with the major F 1 axis. The second maximum is situated in the northwestern quadrant and is parallel to the F 2 axis. Fig. 6d also shows the best-fit great circle controlling the projected hinges. The pole to this circle plunges 13 to S42 W, which is the mean pole of the foliation planes in the Solaf Zone. A third group of mesoscopic fold hinges also appear in the northeastern quadrant and have the same orientation as the geometric F 3 fold axis (Fig. 6c). The attitudes of 59 fold hinges measured in the Feiran Zone are shown in the point diagram in Fig. 7f and contour diagram in Fig. 7g. The hinges are configured along a great circle and the pole to that circle plunges 26 to S55 W, i.e. consistent with that in the Solaf Zone (Fig. 6d). Fig. 7f also shows the density of hinges in the southeastern quadrant of the net. These hinges coincide with the geometric F 1 axis. The hinges in the northwestern quadrant are parallel to the F 2 axis, whereas those in the northeastern quadrant represent the F 3 axis. 6. Discussion 6.1. Structural interpretation The analyses of the planar and linear structural elements in the Feiran Solaf metamorphic belt show clearly the presence of three phases of folding (see Figs. 2f, g and 5a, h, i and k). Using field-overprinting data together with the equal-area projections, the mean axis of F 1 plunges 13 to S49 E. The F 2 axis plunges 36 to N12 W, while the F 3 axis plunges 68 to N75 E. F 1 and F 2 account for the overall structural pattern shown by the lithologic units, while the F 3 occurs mostly as mesoscopic folds. In order to clearly display and simplify the major structures that affected the Feiran Solaf belt, two structural cross-sections are constructed. The first one extends E-W for about 13 km along Wadi Um Takha and Wadi Dehiset Abou Talb in the Solaf Zone (Fig. 9), while the second extends NW SE for about 18 km through the Feiran Zone (Fig. 10). From these structural cross-sections, the larger folding geometry may be described as a series of superimposed and alternating anticlines and synclines (Fig. 10), which control the map distribution of the metamorphic units. Following ductile folding, the metamorphic units were dissected by later brittle structures. The results of the structural cross-sections are consistent with the geometrical and field analyses. Hegazi (1988) proposed similar results on his study of the area around the Feiran Oasis. Finally, the mesoscopic structural elements, described above, expressed by fold overprinting relationships are in harmony with the conclusion reached on the macroscopic scale (Figs. 9 and 10) Evolution of the Feiran Solaf belt We suggest that the parental sedimentary rocks of the Feiran Solaf belt formed a thick sedimentary sequence deposited before the ± 4.7 Ma, based on the age of pre- to syn-tectonic granitoids that intrude the sequence. The sequence is composed of metamorphosed mudstones, graywacke, and minor calcareous sediments, with minor amounts of arenaceous sediments and minor bands of volcanic rocks. The characteristic of these sediments strongly suggests that the sediments represent a deep water