Structural evolution of the Neoproterozoic Western Allaqi–Heiani suture, southeastern Egypt

The Neoproterozic Allaqi–Heiani suture in southeastern Egypt is the western extension of the Allaqi–Heiani–Onib–Sol Hamed–Yanbu suture that represents one of arc–arc sutures in the Arabian–Nubian Shield. It extends for more than 250 km from the N-trending Hamisana Shear Zone in the east to Lake Nasser in the west. It separates the 750-Ma-old Southeastern Desert terrane in the north from the 830–720-Ma-old Gabgaba terrane in the south. Structural studies supported by remote sensing investigations including Landsat Thematic Mapper (TM) images show that the western part of Allaqi–Heiani suture zone constitutes three S- to SW-verging nappes in the north overriding an autochthonous block to the southwest. SW-verging, low-angle thrust sheets and folds, forming a 10-km wide imbrication fan, dominate the northern upper nappe (northern allochthon). These folds and thrusts deform shelf metasedimentary rocks including psammitic metasediments, marble and subordinate conglomerate. Volcanic rocks including rhyolites and felsic tuffs dominate the upper part of the northern allochthon. The contacts between the metasedimentary rocks on the one hand and the rhyolites and felsic tuffs on the other hand are extrusive. This allochthon overrides an internally deformed nappe (central allochthon) dominated by arc and ophiolitic assemblages now preserved as felsic and mafic schist, talc schist, serpentinites, and metagabbros. This allochthon is characterized by NW-trending, upright folds, which deform the earlier sub-horizontal structures. The structurally lower nappe (southern allochthon) is dominated by NNE-trending folds which deform amphibolite facies schistose metavolcanic and metavolcanoclastic rocks. The NNE-trending folds deform earlier NW-trending folds to produce crescentic dome interference pattern with well-developed NE-trending axial planar cleavage, consistent with ESE–WNW bulk shortening. The southernmost structural unit is an autochthonous block dominated by arc-related volcanic and volcanoclastic rocks. This block has suffered only minor deformation compared to the nappes to its north. The consistent SW-vergence of the structures indicates tectonic transport from northeast to southwest, followed by ESE–WNW shortening similar to that found in the Hamisana Shear Zone, further east. Collision between the Gabgaba–Gebeit terrane and the Southeastern Desert terrane along the Allaqi–Heiani suture, after the consumption of a marginal basin probably over an N-dipping subduction zone, led to the formation of EW- to NW-trending folds and thrusts. This was followed by ESE–WNW tectonic shortening to form NNE-trending folds, which are found to be overprinting the earlier structures. This latest shortening

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