Structural and tectonic evolution of the Umm Gheig/ El-Shush region, central Eastern Desert of Egypt

The rocks of the Wadi Umm Gheig/El-Shush area in the central Eastern Desert of Egypt form part of the Nubian Shield, a component of the Neoproterozoic Pan-African Orogeny. The rocks have been divided into three units: (i) low-grade metamorphosed rocks, which consist of metavolcanic rocks interleaved with slices of ophiolitic melange; (ii) high-grade metamorphic rocks, which consist of syn-tectonic granitoids; and (iii) post-tectonic granites, which intrude into both the lowand high-grade rocks. Three distinct tectonic and two magmatic events have been deduced from the structural analysis of the area. These are listed in chronological order: (i) the formation of the major D, sinistral strikeslip El-Shush Shear Zone, which occurs within the granitoid rocks (six individual granitoid bodies, all now intensely sheared, are thought to have been intruded into the active El-Shush Shear Zone); (ii) the emplacement of the metavolcanic rocks over the granitoid rocks by major D,thrusting along a low angle dip-slip shear zone; (iii) upright open folding of the rocks during D,; and (iv) the intrusion of late stage granites, which are virtually undeformed. A model for the tectonic evolution of the study area is proposed. It is argued that the major strike-slip shear zone of the region is possibly related to island-arc accretion and that the various granitoid rocks were intruded along this active strike-slip zone. During the collision associated with island-arc accretion, ophiolite sheets were interleaved with the volcanic rocks and together were thrust over the granitoid basement. Field observations show that an important episode of folding occurred after thrusting and that the folds possess an axial planar fracture cleavage. They are, therefore, the result of a sub-horizontal tectonic compression rather than of diapirism as previously suggested. Thus, the domal structures of the Eastern Desert may be the result of folding rather than diapirism.