الفهرس 
Experimental techniquesOnly 14 pages are availabe for public view
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Abstract
6.1 Introduction This work was committed for a palaeomagnetic and rock magnetic study of some Palaeozoic and Mesozoic rocks aiming t o make a contribution t o the Egyptian and t h e A f r i c a n Phanerozoic data base. The l a t t e r s t i l l comprises many uncertainties and discrepancies, p a r t i c u l g r l y i n i t s Palaeozoic part where data are sparse and poorly dated. The magnetic f a b r i c technique was also used t o enable the petrofabric patterns t o be i d e n t i f i e d . These patterns are sensitive i n recording d i f f e r e n t geologic events t h a t have a f f e c t e d t h e rock throughout i t s history. Therefore they could be very helpful i n i d e n t i f y i n g the o r i g i n and mode of formation which could also help i n tracing the tectonic h i s t o r y of the rock. Such information lead t o a better understanding o f the o r i g i n of the remanent magnetization carried by the rock. The rocks selected f o r t h i s study were collected from the type and/ or proper l o c a l i t i e s representing d i f f e r e n t geologic periods. A Palaeozoic section comprising rocks assigned ages o f Cambrian, Cambro- Ordovician, and Upper Carboniferous was sampled from the Abu Durba area (SW Sinai). Samples of calcareous sandstones (15 s i t e s ) , sandstones (12 s i t e s ) , and a l t e r n a t i n g sandstones & shales (13 s i t e s ) were collected respectively. A well represented Mesozoic section was also collected from north Sinai and north Eastern Desert. Triassic sandstones and carbonates were collected from the type l o c a l i t y a t the core of Gebel A r i f El Naga. Samples from a l t e r n a t i n g carbonate and sandstone beds representing the type section of the Jurassic were taken from the core of Gebel Maghara. Sandstones and carbonates o f Lower Cretaceous age were sampled from Gebel Shabrawet (north Eastern Desert). F i n a l l y an Upper Cretaceous section composed mainly o f carbonates and sandstones was taken from SW Sinai (Gebels Qabaliat and Nezzezat). 6.2 Techniques were used: 1. IRM acquisition and back-field c o e r c i v i t y , which i s a useful method i n distinguishing between haematite and magnetite. Magnetitebearing rocks acquire a saturation remanence a t f i e l d s much lower than those required f o r saturating haematite. Back-field c o e r c i v i t y experiments were also useful i n distinguishing between s o f t magnetic components, l i k e magnetite, usually destroyed a t f i e l d s much lower than the hard magnetic components l i k e haematite and goethite. 2. Thermal demagnetization o f IRM, which i s based on the f a c t t h a t minerals which have s i m i l a r maximum c o e r c i v i t y generally have d i f f e r e n t c h a r a c t e r i s t i c unblocking temperatures. This enabled d i f f e r e n t magnetic minerals t o be i d e n t i f i e d according t o t h e i r unblocking temperature spectrum. 3. NRM i n t e n s i t y decay during demagnetization, which gives some useful information on the magnetic mineral content although it i s not a satisfacory way. Magnetite can easily be demagnetized with AF than haemati te. I n t e n s i t y decay during thermal demagnetization i s a1 so diagnostic where consistent measurements above 600’~ indicate t h a t haematite i s present. 6.3 Discussion o f the Results 6.3.1 Palaeozoic Results from these rocks indicate t h a t they have a very stable . magnetization which helped i n determining a palaeomagnetic dating f o r these rocks and t h e i r dating controversy has been thus solved. The
obtained remanence directions from most o f what was previous1 y known as Cambrian and Cambo-Ordovician rocks were very close and found t o be comparable t o the overlying sandstone-shales o f the Upper Carboniferous age. When a l l the directions from the e n t i r e Palaeozoic section were compared t o the r e l i a b l y a v a i l a b l e A f r i c a n data they were close t o or w i t h i n the area of the Carboniferous palaeomagnetic poles. The sparsity of the African data, i n the present stage, did not permit a clear difference between the Lower and Upper Carboniferous poles. However and i n , conjunction with the recent geologic work by Klitzsch (1990), the results from both the Cambrian and Cambro-Ordovician rocks were combined together and an overall palaeomagnetic pole considered t o be of Lower Carboniferous age was then computed. The resultant pole from the overlying sandstone-shale section was considered separately and found i n agreement with Kora’s (1989) results that these rocks are of Upper Carboniferous age. 6.3.2 Mesozoic Unfortunately none of the results yielded from these rocks could be considered as a r e l i a b l e palaeomagnetic contribution t o the palaeomagnetic, data base. However they have been very useful i n developing magnetotectonic models f o r the areas under consideration. The obtained palaeomagnetic directions from the Syrian arcs of A r i f E l Naga (Triassic), Maghara (Jurassic) and Shabrawet (Lower Cretaceous) suggest magnetotectonic models which propose some l a t e r a l tectonic variations. I n order t o restore the obtained directions t o the expected ones, stepwise rotations of both the horizontal and v e r t i c a l palaeomagnetic components were used. I n the case of both the Triassic and Jurassic rocks, the thrusts bounding these folds at t h e i r southern borders were considered responsible f o r the p e c u l i a r i t y of the results. It i s proposed that these thrusts dragged these blocks during or soon a f t e r the f o l d i n g i n the Upper Cretaceous-Lower Tertiary times. These models suggest a 40•‹ angle of dragging t o the SE f o r the A r i f E l Naga a n t i c l i n e , while an angle of only 30•‹ was required f o r the restoration of the Maghara a n t i c l i n e . Another rotation of 25O about the v e r t i c a l axis was also required f o r a complete restoration of the Maghara a n t i c l i n e t o the expected predragging attitude.