The Precambrian basement of the Um El Tuyor area represents the central part of the ca. 750–720 Ma Allaqi–Heiani suture in the extreme
south of the Egyptian Eastern Desert. It is composed of allochtonous ophiolitic thrust slices and detached sheets, island arc metavolcanic–
volcanoclastic and metasedimentary assemblages, and syn- to post-orogenic granitoid intrusions. These rocks are collectively referred to as a foldand-
thrust belt in view of their deformational styles, and as ophiolites and their lithologic association are concerned. Structural analysis reveals
three major deformation events, namely: (a) D1: an early thrust-and-fold system, through which overturned to recumbent folds and major thrusts
were developed and later overprinted by a sinistral transpression, (b) D2: a NE–SW-compressional regime that deformed the D1 structures by
major folds (F2) and penetrative axial planar NW–SE foliations (S2) and associated major NW–SE sinistral faults and shear zone, and (c) D3: a
late ENE–WSW compressional strain increment expressed by a series of NNW–SSE striking folds and crenulations (F3). Related right-lateral
NNW–SSE strike slip faults and discrete brittle–ductile shear zones indicate a later transcurrent shear which is best developed in the
metasedimentary successions.
Mineral associations and porphyroblast-matrix fabrics in the pelitic metasediments indicate three phases of regional metamorphism
(M1→M3), syn-kinematic with D1, D2 and D3 respectively. Based on the petrographic and microprobe data, the peak metamorphic conditions
were attained at 534°–561 °C/5.26–6.20 kbar, under which garnet, staurolite and sillimanite coexisted during D2. Although associated with the
most significant metamorphic event, D2 corresponds roughly to a late collisional stage (650–620 Ma) during which the accreted Pan-African
terrains attached to the East Saharan Craton. This is further confirmed by the D2-related sinistral strike slip faults that affiliate to the ca. 630–
540 Ma Najd strike slip fault system in the Arabian–Nubian Shield.
Criteria including an anticipated fold–thrust vergence generally to northwesterly directions, discrete mesoscopic duplexes, shear bands, and
minor folds and thrust zones all indicate that the ophiolites were emplaced roughly from northwest to southeast. However, deformation of the
ophiolite basal contacts and associated fabrics argue that the northwestward rooting of the ophiolitic massif in the study area cannot be considered
as a criterion for the original direction of tectonic transport, which was likely from north to south if the entire Allaqi–Heiani belt is considered.
Such deviation is best attributed to rotation through deformation by the N–S Hamisana shear zone. This principally agrees with the tectonic
evolution of the Arabian–Nubian Shield in south Egypt proposed previously, which includes an early N–S arc–arc collision followed by a stage of
E–W crustal shor tening. |
