Gravitational Collapse Origin of Shear Zones, Foliations and Linear
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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. Ó 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 Ó 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.