الفهرس 
ACKNOWLEDGMENTOnly 14 pages are availabe for public view
 |  | from | 98 |  |  |
| | | from | 98 | | |
Abstract
The Mudargag area, ” ” " ~ " ” ” 70 km southwest of Marsa Alam, is covered by a tectono-stratigraphic sequence comprising two structural units separated by a northwest dipping high angle normal fault dissecting northwest gently dipping old thrust fault related to the growing of the Egyptian basement during the accretion of island arcs.
The lower infrastructural unit is represented by the dome ”A” which covers the southern part of the map area and represents the prime dome of the five separated gneissic domes of WHC (El-Ramly et al., 1988; Shalaby, 2010). It is differentiated into four structural subunits separated by normal fault contacts as follows: the lower first subunit consists of a high-grade metamorphic interlayered sequence of migmatic granite gneisses and subordinate proportion of banded migmatic amphibolites, followed upwards by subunit 2 which builds up a medium to high-grade layered sequence of interbedded fine-grained amphibolites and subordinate granite gneisses, the third subunit is composed of medium-grade repetitive alternated psammitic gneisses and amphibolite layers, whereas, the upper fourth subunit is mainly composed of medium-grade interlayered sequence of coarse-grained psammitic gneisses and minor amphibolite bands. Both gneisses and amphibolites building these subunits were found to be derived mainly from igneous and sedimentary parentages, and also underwent regional metamorphism (M1) under medium to high P-T conditions as indicated by the development of almandine, feldspars and the wide-scale development of anatexites particularly through the lower subunit. The anatexites are subdivided into diatexites characterized by nearly homogeneous nebulitic migmatite structure and metatexites which obviously exhibit both melano- and leucosome phases having different migmatitic structural forms (e.g. stromatic form etc.) later on, during the growth of the Egyptian basement complex during the late Precambrian, it is manifested by the accretion of island arcs onto the East Sahara Craton. During this tectonic event, the whole metamorphosed association was variously diaphthorized forming protomylonites, mylonites, quartzfeldspathic schists, augen schists and augen gneisses. Structural data from the mylonitized zone of the ”A” dome over which the Shait and Nugrus mélange nappes were thrusted indicate that the direction of their transport was from the SE toward the NW as deduced from the general NW direction of the stretching lineation analysis of the measured planar and linear structural data and also from the observed NW vergence of the overturned folds of the metatextic migmatites.
The upper suprastructural unit covering the northern part of the map area includes two structural domains referred to as : (1) Shait ophiolitic mélange domain and (2) Nugrus ophiolitic mélange domain. Each domain consists of low-grade imbricated island arc slices intervened by layered slices of metasiltstone and metaconglomerates. The Shait ophiolitic mélange is emplaced by Mudargag El-Ahmar composite calcalkaline-alkaline granite whereas Nugrus ophiolitic mélange domain is intruded by the two feldspar Nugrus granites.
Detailed field mapping and petrographic study of the mapped structural units of the Mudargag area reveal that they show evidence of polyphase deformation in addition to two metamorphic events (M1&M2) affecting the lower dome ”A” structural unit. Three deformational phases (D1-D3) were recorded. Their sequential development can be summarized as follows:
1- D1 deformation phase characterizes impure graywackes, calcareous graywackes and disintegrated granites were petrographically found to form the main protoliths of the medium to high-grade gneisses while the banded amphibolites were mostly derived from calcareous graywackes, calcareous sandstone or intermediate tuffs, though basic lava flows origin cannot be excluded. These lithologies were deposited and subsequently underwent medium to high-grade metamorphism (M1) prior to the accretion event of the East African ophiolites and island arc cover nappes toward the western East Sahara craton.
2- D2 deformation phase is related to the large scale thrusting of the imbricated late-Precambrian ophiolite and island arc nappes as indicated above, during the East African-Orogeny. Consequently, the metamorphosed lithologic sequence underwent diaphthorized metamorphism (M2a) ranging between greenschist facies and epidote amphibolite facies. On the otherhand, the rock assemblage of the overthrusted suprastructural nappes were also affected by greenschist metamorphism contemporaneously prior to the episode of thrusting event.
3- D3 phase deformation is represented by the intersection of the Egyptian basement and the studied area by two subparallel NW trending sinistral strike-slip faults. In this episode the studied A dome and the remaing domes of the Wadi Hafafit Culmination were exhumed within a crustal-scale wrench corridor of these strike-slip faults belonging to the major NW trending Najd fault system (Stern, 1985). In some details the exhumation of the dome ”A” was happened under an E-W bulk compression joined by contraction in the eastern part (eastern limb) and SW extension in the western one (western limb) besides another extension developing at the northern margin. The contraction is illustrated by formation of close folding, and steep inclination of the structural subunits of the northeastern limb of the dome compared with those of the southwestern limb (Fig. 5-1). The SW extinction is registered by the dissection of the limb into several southwestward dipping low-angle normal faults. On the other hand the northern extension occurring at the NW margin of the dome ”A” is manifested by development of NW to NE deeply dipping Shait normal fault.
Furthermore, field observations reveal the influence of the Wadi Shait ophiolitic mélange domain by a NW trending large-scale crustal extension illustrated by its transformation into a curved array of elongate subparallel fault-bounded ridges (e.g. Mudargag- Ras Shait ridge) intervened by descendant fault wadis or ravines. This extensional fault system have a general attitude similar to that of Shait low-angle normal fault, therefore they appear to be coeval with the extensional phase influencing the northern zone of the dome ”A”. The location of the composite calc-alkaline elongate granite mass of the Mudargag El-Ahmar along the western extension of one of these noted faults indicates its intrusion within the same episode of the crustal extensional event of the Wadi Shait ophiolitic mélange domain. Again, the formation of these transtensile structures can be interpreted by the termination of the Nugrus strike-slip fault at its NW end portion into a series of splays with left-sense of movement similar to the main Nugrus fault.