Late Pan-African granite emplacement during regional deformation, evidence from magnetic fabric and structural studies in the Hammamat–Atalla area, Central Eastern Desert of Egypt

Abstract

Field investigations, microstructural observations, and magnetic fabric analyses revealed a polyphase, late Pan-African deformational evolution in the Um Sheqila–Um Had (595Ma) composite pluton and in the Hammamat and Atalla areas of the Central Eastern Desert of Egypt in Ediacaran times. Major stages are early shortening (NNW–SSE), subsequent strike-slip (NW–SE shear zones), and late shortening (NW–SE). Strain studies on pebbles and xenoliths together with AMS data show a predominance of shallow, NW–SE trending X axes or magnetic lineations, associated with steep, NW–SE striking magnetic foliations. Magnetic fabrics and microstructures indicate a tectonic fabric in the Um Sheqila–Um Had granitoid plutons, which is dominated by steep NW–SE striking foliations and shallow NW–SE trending lineations, similar to those in the high-angle Atalla Shear Zone. There is a change of lineation directions from ESE–WNW at Um Sheqila (oldest) to NW–SE to Um Had II (youngest). This pattern may indicate an influence of strike-slip and is also consistent with NE–SW compression. This holds also true for the asymmetry of the contact aureole, which is extended towards NW, parallel with the trend of the magnetic lineation. The character and orientation of the deformation pattern in the Um Sheqila–Um Had plutons and the Atalla Shear Zone is thus similar to the pattern of the late shortening phase. The intrusion of the Um Sheqila–Um Had granitoid rocks, therefore, took place before the late shortening stage, but postdates early deformation, which, according to published data, was associated with lithospheric thinning in the Central Eastern Desert. Therefore, these Pan-African plutons do not represent the earliest post-deformational intrusions but a late stage of syn-deformational magmatic activity. At a regional scale, this deformation with steep foliations and shallow lineations may also be related with lateral escape tectonics. The pluton emplacement, the importance of transcurrent shear zones, and the low lithospheric thickness in the area are not consistent with tectonic elements at the Pan-African orogenic margin but imply a more internal position for the Wadi Hammamat area.