petrogenesis and tectonic setting of late precambrian ensimatic volcanic rocks central eastern desert of egypt abstract early stages in the geologic evolution of the central eastern desert of egypt ced reflect an intense episode of ensimatic volcanic activity similar to modern magmatism of the ocean floors and island arcs this paper reports results from studies of the petrology and petrogenesis and interprets the significance of these late precambrian volcanic rocks a three fold stratigraphy is preserved in the basement of the ced a basal section of oceanic crust includes ultramafics gabbros and pillowed basalts these older metavolcanics omv are conformably succeeded by dominantly volcanogenic metasediments which are in turn succeeded by a dominantly andesitic calc alkaline sequence of younger metavolcanics ymv the omv and ymv are largely restricted to the ced in egypt but analogous terranes are found in northern arabia 40 400 ppm and ni 30 260 ppm they are poor in k 2 o 0 05 0 92 rb 0 3 5 0 ppm and ba 11 89 ppm on ti zr cr v ni p discriminant diagrams the omv plot in the field of modern abyssal tholeiites high k rb 450 1800 and light ree depletions support this inference although k ba 25 45 is lower than modern mid ocean ridge basalts morb the sum of omv geochemical characteristics requires that these magmas were derived by the fractional fusion of the mantle it is suggested that the omv were generated by 20 25 fractional melting of previously depleted mantle at depths of less than 60 km relatively little fractionation accompanied ascent to the surface where the omv were erupted in a primitive crustal environment either a small oceanic rift or a back arc basin metamorphism of the ymv resulted in little elemental redistribution these andesites have sub alkaline clinopyroxenes and major element geochemical characteristics indistinguishable from modern calc alkaline andesites ymv andesites in the central and western ced have k rb 400 600 k ba 20 40 and are light ree enriched and heavy ree depleted high concentrations of cr 50 150 ppm and ni 20 100 ppm and low initial 87 sr 86 sr ratios 0 7028 0 7030 indicate that these magmas were generated by melting in the mantle modelling studies and consideration of experimental data indicate that these andesites were formed by 2 10 fractional fusion of hydrous undepleted garnet therzolite at depths of 65 km or more in the mantle the data show that an intense episode of instability convection and widespread melting occurred in the mantle beneath afro arabia at the end of the precambrian

curie point depth beneath the barramiya red sea coast area estimated from spectral analysis of aeromagnetic data abstract the geothermal regime beneath the barramiya red sea coast area of the central eastern desert of egypt has been determined by using the curie point depth which is temperature dependent this study is based on the analysis of aeromagnetic data the depth to the tops and centroid of the magnetic anomalies are calculated by power spectrum method for the whole area the result of this investigation indicates two new maps of the curie point depth cpd and the surface heat flow q maps of the study area the coastal regions are characterized by high heat flow 83 6 mw m2 due to the geothermic nature of the region and shallow curie depth 22 5 km where cpd depends on the tectonic regime and morphology in the eastern part of the area the western portion of the studied area has a lower heat flow

age determinations in the precambrian basement of the wadi araba area southwest jordan abstract the precambrian basement of jordan belongs to the northern margin of the arabian nubian shield age determinations by u pb isotopic analyses on sized and magnetic zircon fractions a monazite and an apatite sample and by rb sr isotopic studies on whole rocks and minerals of metasedimentary rocks granodiorites granites and dykes have elucidated the following events 1 a major regional high grade metamorphism accompanied by migmatization and synkinematic plutonism occurred at about 800 ma according to u pb zircon ages of metasediments and granites 2 during a postkinematic plutonic event between 615 and 600 ma extensive masses of granodioritic to granitic composition and dykes were emplaced the u pb data of zircons of the rocks yielded upper intercept ages with the concordia consistent with rb sr biotite ages the rb sr mineral ages of the older metasedimentary rocks document the resetting of the rb sr system due to the thermal pulse at this time 3 a younger plutonic event produced diorites and dykes at about 570 ma the plutonic events are related to the pan african orogenic phase the low initial87sr 86sr ratios of the plutonic rocks 0 7032 0 7046 correspond to values reported from equivalent rocks throughout the arabian nubian shield and suggest that no significant portions of ancient sialic crustal material contributed to the generation of the granitic to granodioritic magmas

strain analysis of wrench faults and collision tectonics of the arabian nubian shield a conjugate set of regional scale wrench faults produced by collision of converging plate boundaries about 500 700 m y ago extended across the arabian shield five major wrench fault zones distributed across the precambrian continent were subjected to finite strain analysis all were found to have undergone ductile strains in addition to brittle fault displacements four of the zones together enabled a composite strain profile to be constructed east west across the exposed precambrian continental crust that yielded an estimate of a total sinistral displacement of 280 120 km across the najd fault system the strain fields of all the zones deviated from simple shear strain with systematic heterogeneous variations in extensional strain across the zones finite shortening strains increased in intensity eastwards across the continent and were accompanied by a change in deformation mechanism which indicated that the zones in the east formed at higher crustal levels than the zones in the west the conjugate

controls on erosional retreat of the uplifted rift flanks at the gulf of suez and northern red sea the gulf of suez and red sea rifts are currently bordered by large asymmetric uplifts that are undergoing erosion we find that the amount and timing of erosion vary systematically along the strike of the margin and have been controlled by variations in the prerift stratigraphy the prerift strata are composed of cliff forming eocene cretaceous carbonates overlying the easily eroded cretacous cambrian nubian sandstone this lithologic succession promotes scarp retreat of the sedimentary section followed by dissection of the underlying basement the prerift section thins from over 2000 rn at the northern end of the rift to less than 400 rn at its junction with the red sea thus at the northern part of the gulf of suez the nubian sandstone is minimallyexposed and the carbonates form a scarp at the rift border fault farther south undercutting of the carbonates by erosion of the sandstone has resulted in scarp retreat the escarpment cuts diagonally away from the border fault and is over 100 km inland from the border fault at the southernmost gulf of suez the amount of retreat varies inversely with the sediment thickness exposure and erosion of basement are initiated by the retreat of the escarp ment and the depth of erosion as indicated by fission track ages increases with distance from the escarpment these observations are explained by a model in which erosion along the gulf of suez is initiated as rift flank uplift becomes sufficiently large to expose the friable sandstones undercutting of the escarpment and exhumation of basement has been propagating northward and westward for at least 20 m y the average rate of scarp retreat has been 6 km m y and the along strike propagation of the erosion has been 12 km m y the diachronous erosion of the the rift flanks at the gulf of suez highlights the importance of distinguishing between the timing of uplift and of erosion both thermochronometric and stratigraphic data primarily indicate the timing of erosion which may differ significantly from the timing of the uplift that initiates it they must be interpreted carefully to avoid erroneous conclusions about rift tectonics

age and geochemistry of late precambrian sediments of the hammamat series from the northeastern desert of egypt willis k m stern r j and clauer n 1988 age and geochemistry of late precambrian sediments of the hammamat series from the northeastern desert of egypt precambrian res 42 173 187 late precambrian crustal evolution in the northeastern desert of egypt culminated in a major episode of strong extension accompanied by bimodal igneous activity and deposition of clastic sediments in restricted terrigenous basins these sediments known as the hammamat series record the uncovering of the evolving crust of the region we have undertaken petrographic geochemical and rb sr and k ar geochronologic studies of hammamat samples collected along a single traverse the lower half of a 400 m thick section consists of coarse detritus shed from nearby ensimatic terranes while the upper half was derived from rapid reworking of bimodal igneous rocks similar to those of the dokhan volcanics and pink granites units that are preserved within the region rb sr whole rock analyses give an 8 point isochron age of 585 15 ma with an initial stsr sr of 0 70323 0 00013 interpreted to approximate closely the time of sedimentation this is very similar to the k ar ages of the coarser clay fractions 2 0 5 m 588 567 ma this result is consistent with stratigraphic considerations and other geochronological studies indicating that hammamat deposition occurred at 590 ma rb sr analyses of leached and unleached size fractions indicate thermal resetting at 524 17 ma accompanied or followed by fluxing of a radiogenic sr rich pore fluid stsr s sr 0 7124 this is further indicated by k ar ages of the 0 2 0 5 ttm size fraction of 542 532 ma

an integrated approach for identifying aquifers in transcurrent fault systems the najd shear system of the arabian nubian shield groundwater potentials were assessed for major transcurrent fault systems in basement complex terrains using the najd transcurrent shear system of the arabian nubian shield as a test case spatial analysis within a geographical information system environment of relevant data sets field remote sensing geophysical and digital elevation revealed a spatial correlation between existing well locations and distribution of najd transcurrent shear system structural elements i e faults shear zones melange dykes within highly fractured domains suggesting a causal relationship specifically at 1 intersection of najd brittle shear zones and fault systems 2 intersections of two or more fault systems 3 intersections of najd related dykes with the main streams within catchment areas and 4 within tectonic melange for each of the former aquifer types aquifer specific field and satellite based characteristics were identified and used in conjunction with complementary data e g watershed morphology precipitation soil moisture to target aquifers of similar settings postulated well locations were then successfully tested and refined using very low frequency technologies results could potentially highlight the important role of major transcurrent fault systems in providing settings conducive for hosting and transmitting groundwater and could potentially serve as a replicable model for regional assessment of groundwater potentials in similar tectonic and hydrologic settings

crustal accretion in the pan african nd and sr isotope evidence from the arabian shield abstract nd and sr isotope determinations on late precambrian to early palaeozoic igneous and sedimentary rocks from the arabian shield are used to investigate the proportion of reworked older crust and the rate at which new crust was generated during the pan african event eight rb sr whole rock isochrons on igneous suites yield ages in the range 770 590 ma and initial 87 sr 86 sr ratios of 0 7038 0 7023 these data confirm that magmatism in this area was largely restricted to the period 850 550 ma and the initial ratios are sufficiently low to preclude significant contributions from a long lived upper crustal source the initial 143 nd 144 nd ratios of a variety of lithologies including several samples of possible basement are all higher than the contemporaneous values for chur e nd 1 6 to 6 9 suggesting that many were derived directly from the upper mantle and that any inferred crustal source regions for the remainder could not have separated from likely lree depleted mantle reservoirs before 1200 ma the arabian shield therefore provides an example of rapid crustal growth during the late proterozoic and contrasts with the damara intracratonic belt of namibia where nd and sr isotopes provide strong evidence for extensive reworking of older continental crust during the same period

tectonic evolution of the northwestern red sea margin as related to basement structure abstract at the northwestern egyptian margin of the red sea a crystalline basement of pan african late proterozoic age is exposed beneath a phanerozoic sedimentary cover this tectonic situation provided an opportunity to study the younger red sea tectonic evolution in relation to features that are inherited from or reactivated in an underlying basement two structural maps compiled from published sources and our data covering the eastern egyptian basement from hurghada to the south show the most important structural elements the major central and southern parts of the basement in eastern egypt ced and sed are dominated by low angle thrusts other fractures n s e w nw se are distinctly less frequent and of local importance nw se oriented strike slip faults najd system overprinted the low angle thrusts of the northern ced these fault are related to ne sw trending normal faults which characterize the northern part of the basement ned before discussing general relationships between basement structure and red sea evolution a detailed example of the post pan african phanerozoic evolution of the low angle thrust dominated domains of the ced and the sed from a traverse south of 25 n is presented there detailed structural mapping showed phanerozoic normal faults close to the red sea trending nnw sse and at the margin of the phanerozoic cover east of the nile trending nnw sse and nw se leading to a horst like position of the basement within this horst deformation is restricted to a local sinistral strike slip fault trending n s the normal faults frequently used and reactivated fractures parallel or normal to the axial surface of earlier pan african folds igneous activity is confined to the western part of the basement where mafic dykes mafic to trachytic volcanics the alkaline gabal abu khrug ring complex and trachyte plugs in that time sequence from 104 to 80 ma are exposed mafic dykes are dependent on host rock properties distributed mainly in relatively brittle granitoid rocks trachyte intrusions are generally not related to particular host rocks but may follow pre existing structures crosscutting relationships with the igneous rocks imply a late cretaceous age for the faults in the west of the traverse however in the east at the red sea coast faults cut middle miocene sediments and are thus younger tectonic activity in the traverse area is therefore confined to two distinct episodes a late cretaceous event of alkaline magmatism and normal faulting at the western margin of the basement and a miocene or younger event of normal faulting at the present red sea coast in contrast to earlier views neither transform faults nor large scale riedel shears related to red sea tectonics have been met in the area this holds also true for the ced and sed as a whole where only local faults remain for potential reactivation during red sea tectonics except perhaps the duwi shear zone at quseir the prevailing n s e w or nw se directions of these faults preclude an interpretation as possible transform faults normal to the spreading axis of the red sea therefore it is no longer possible to assume a continuation of red sea transform faults into the continental crust

review of late proterozoic sutures ophiolitic melanges and tectonics of eastern egypt and north east sudan the distribution structural relations and ages of ophiolites and ophiolitic melanges in the eastern deserts of egypt and north east sudan and their relation to arc and post arc magmatism are used to infer the probable positions of sutures and the directions of obduction and subduction the ages of the successive tectonic stages in the terranes decrease northwards by about 100 ma in the region discussed obduction was north westwards the obducted sheets of ophiolite and ophiolitic melange extending about 500 km north from the onib sol flamed suture may all represent a single back arc basin the direction of subduction is still not clearly proved more geochemical evidence of arc magmatic polarity is needed the late tectonic north westward thrusting the nw se stretching lineation the transpressional north south hamisama shear zone and the nw se sinistral najd faults are all attributed to north westward movement present coordinates of terranes towards the older craton west of the nile the najd faults are interpreted as intracontinental transform faults

crustal structure of central syria the intracontinental palmyride mountain belt an edited version of this paper was published in tectonophysics by elsevier science copyright 1992 elsevier science see also http dx doi org 10 1016 0040 1951 92 90395 m http atlas geo cornell edu syria al saad et al 1992 html

geochronology of the arabian nubian shield in southern israel and eastern sinai the metamorphic and igneous orogenic basement rocks of southeastern israel and eastern sinai evolved as part of the arabian nubian shield during latest proterozoic time pelitic schist of the elat association from wadi nahal shlomo south of eilat israel is dated at about 800 m y but the geologic significance of this date is not clear in southeastern sinai metapelitic rocks of the kid group have a date of diagenesis or metamorphism of about 615 m y syn orogenic and post orogenic plutons which intrude the layered metamorphic complex s are dated at 600 m y near eilat and 590 m y at wadi kid other dated plutons include the timna granite at 592 7 m y in the area of nevi ot at 595 m y and in the vicinity of dahab at 588 2 m y and 595 0 3 m y the radiometric dates and low initial 87 sr 86 sr ratios 0 7034 imply that pre 1000 m y continental sialic crust was not involved in the formation of the shield in southeast israel and eastern sinai

granitoid associated gold mineralization in egypt a case study from the atalla mine gold bearing sulfide quartz veins cutting mainly through the atalla monzogranite intrusion in the eastern desert of egypt are controlled by subparallel ne trending brittle shear zones these veins are associated with pervasive sericite altered silicified and ferruginated rocks the hosting shear zones are presumed as high order structures of the najd style faults in the central eastern desert 615 585 ma ore minerals include an early pyrite arsenopyrite pyrrhotite mineralization partly replaced by a late pyrite galena sphalerite chalcopyrite gold electrum tetrahedrite hessite assemblage gold occurs as small inclusions in pyrite and arsenopyrite or more commonly as intergrowths with galena and sphalerite tetrahedrite in microfractures arsenopyrite geothermometry suggests formation of the early fe as sulfide mineralization at 380 340 c while conditions of deposition of the late base metal gold assemblage are assumed to be below 300 c rare hessite electrum and bi galena are associated with sphalerite and gold in the late assemblage the early and late sulfide minerals show consistently a narrow range of 34s 3 4 6 5 that overlaps with sulfur isotopic values in ophiolitic rocks

a laboratory simulation of pyroclastic flows down slopes abstract laboratory experiments are described which explore the dynamical consequences of buoyant convective upflow observed above hot pyroclastic flows in nature the convection is produced by the hot ash particles exchanging heat with air mixed into the front and top of the pyroclastic flow this effect on the buoyancy due to the mixing of air and ash has been modelled in the laboratory using mixtures of methanol and ethylene glycol meg which have a nonlinear density behaviour when mixed with water intermediate mixtures of these fluids can be denser than either initial component and so the laboratory experiments were inverted models of the natural situation we studied meg flowing up under a sloping roof in a tank filled with water the experiments were performed both in a narrow channel and on a laterally unconfined slope the flow patterns were also compared with those of conventional gravity currents formed using fresh and salt water the presence of the region of reversed buoyancy outside the layer flowing along the slope had two significant effects first it periodically protected the flow from direct mixing with the environment resulting in pulses of relatively undiluted fluid moving out intermittently ahead of the main flow second it produced a lateral inflow towards the axis of the current which kept the current confined to a narrow tongue even on a wide slope in pyroclastic flows the basal avalanche portion has a much larger density contrast with its surroundings than the laboratory flows calculations show that mixing of air into the dense part of a pyroclastic flow cannot generate a mixture that is buoyant in the atmosphere however the overlying dilute ash cloud can behave as a gravity current comparable in density contrast to the laboratory flows and can become buoyant depending on the temperature and ash content in the august 7th pyroclastic flow of mount st helens hoblitt 1986 describes pulsations in the flow front which are reminiscent of those observed in the experiments as proposed by hoblitt the pulsations are caused by the ash cloud accelerating away from the front of the dense avalanche as a density current the ash cloud then mixes with more air becomes buoyant and lifts off the ground allowing the avalanche to catch up with and move ahead of the cloud the pulsing behaviour at the fronts of pyroclastic flows could account for the occurrence of cross bedded layer 1 deposits which occur beneath layer 2 deposits in many sequences

island arcs and their role in the evolution of the western arabian shield abstract two northeast trending late precambrian ensimatic island arc systems in western saudi arabia are described each displays lithotec tonic provinces that conform to those found in modern examples of plate convergence the younger hijaz arc system 800 700 ma contains a thick fore arc sequence of al ays group turbidites overlying an accretionary prism of metamorphosed and fragmented volcanosedimentary rocks and associated ophiolites of the farri group and the al hinu formation the older asir arc system 900 700 ma is exposed at a much deeper erosional level in which only remnant arc and fore arc deposits are present it is identi fied in part by a frontal arc assemblage of abundant dioritic intrusions and high grade metamorphic rocks that are interpreted as a high t low p belt a mafic volcanic assem blage in th back are c area appears to deline ate an interarc basin situated between the at ta if frontal arc and a lithologically similar remnant arc west of bishah about 700 ma ago the hijaz island arc collided with the asir arc system along a southeast dipping subduction zone now rec ognized as the bi r umq port sudan suture epeirogeny during this period led to the crea tion of an intramassif basin in the hijaz arc system and an accretionary basin in the asir arc system these were filled by volcanic and shallow water to continental molasse depos its of the hadiyah and fatimah groups re spectively continued convergence between 700 and 675 ma ago resulted in development of the northeast trending samran fold belt the collision event marks the cessation of an episode of southeast convergence that is in terpreted as the initial phase in the progres sive eastward evolution of the arabian nubian shield introduction

u pb zircon geochronology and geological evolution of the halaban al amar region of the eastern arabian shield kingdom of saudi arabia u pb zircon model ages for eleven major units from the halaban al amar region of the eastern arabian shield indicate three stages of evolution 1 plate convergence 2 plate collision and 3 post orogenic intracratonic activity convergence occurred between the western afif and eastern ar rayn plates that were separated by oceanic crust remnants of oceanic crust now comprise the ophiolitic complexes of the urd group the oldest plutonic unit in the study is from one of these complexes and gave an age of 694 8 ma detrital zircons from the sedimentary abt formation of the urd group which is intercalated with the ophiolitic rocks were derived from source rocks with a mean age of 710 ma the abt formation may be an accretionary wedge on the western margin of the ar rayn plate plate convergence was terminated by collision of the afif and ar rayn plates during the al amar orogeny which began about 670 ma during collision the urd group rocks were deformed and in part obducted on to one or both plates synorogenic leucogranitoid rocks were intruded from 670 to 640 ma from about 640 to 630 ma widespread unfoliated dioritic plutons were emplaced in the ar rayn block and represent the end of orogenesis related to collision there is no definitive evidence for a significantly older basement beneath the study region

lead isotope systematics of some igneous rocks from the egyptian shield lead isotope data on late precambrian igneous rocks from the eastern desert of egypt are presented previous work has indicated that this igneous suite is characterized by uniformly low initial ratios of 37sr86sr 0 7035 the pb data define three groups loosely corresponding to age an older tonalite to granodiorite otg suite with ages in the range 610 710 ma has pb isotope characteristics similar to modern mantle derived oceanic mafic rocks the age corrected initial pb isotope ratios of the otg group lie near the ocean regression line in correlation diagrams of 206pb204pb vs 207pb204pb and 203pb204pb the isotope data imply an origin for the otg group analogous to calc alkalic igneous rocks in modern intra oceanic island arcs a younger suite of k rich plutonic rocks 570 595 ma has similar 206pb204pb and 208pb204pb initial ratios relative to the otg suite but have higher 207pb204pb initial ratios when calculated from measured whole rock u and pb contents however k feldspar and galena associated with this suite indicate initial pb isotope ratios close to the otg suite we interpret this to indicate that the younger granites could have originated by anatexis of otg type material but they probably experienced an episode of metasomatism after emplacement a suite of volcanic rocks the dokhan volcanics from the central eastern desert are intermediate between the younger granites and otg in both age 610 ma and pb isotope characteristics limited data on initial 208pb204pb ratios suggest that all the egyptian samples originated from a source with anomalously low thpb ratios although the relative abundance of granitic rocks in the egyptian shield decreases to the south no evidence of north south heterogeneity in the pb isotope composition of the respective source regions is apparent in the data however the westernmost sample studied from the aswan area contains distinctly more radiogenic pb relative to the mean this location may mark the boundary between the late precambrian ensimatic orogen now outcropping in the egyptian and saudi arabian shields and an older sialic craton to the west

timing of tectonic events in the menderes massif western turkey implications for tectonic evolution and evidence for pan african basement in turkey the menderes massif forms the western of the two large metamorphic culminations within the turkish alpide orogen it has three major lithologic units with a gneissic core at the base and a schist and a marble envelope overlying it successively although relations between them have been largely obscured by the polymetamorphic and structurally complex history of the massif we present a review of the available stratigraphic evidence and combine it with new isotopic ages from the central and southern parts of the massif to constrain the timing of major tectonic events that shaped the massif since the late proterozoic pt3 in the southern part of the massif sensu stricto three episodes of deformation and metamorphism are distinguished whereas in its northern part there are four the first episode occurred at 500 10 ma with intense deformation and metamorphism at high grade amphibolite facies with local anatexis in the central part of the massif the 470 9 ma old now highly deformed tonalitic and granitic intrusions mark the end of this episode on a cambrian reconstruction of continents around the eastern mediterranean the menderes massif forms the southern end of the pan african orogenic collage of northeastern africa and arabia the area of earliest palaeozoic deformation in the menderes may connect with the northwest african orogenic collage along the strike via the bozburun and saricicek diabases arkoses and schists in the center of the karacahisar dome interpreted herein as fragments of a pan african suture the earliest palaeozoic deformations to affect the rocks were probably related to the last pan african collisions and associated postcollisional convergence the southern part of the massif was undeformed from early ordovician to the early later eocene whereas the northern part was deformed metamorphosed and intruded possibly during the late triassic related to the closure of the karakaya marginal basin of palaeo tethys the next major event affecting the whole of the massif was the intense deformation and widespread metamorphism that reached high amphibolite grade in the structurally lower parts and only greenschist grade in the outermost envelope this metamorphism here called the main menderes metamorphism mmm is biostratigraphically constrained between early eocene and early oligocene time rb sr isotopic data show a spread of ages between 60 ma and 25 ma with the greatest number of determinations around 35 5 ma this number is in excellent agreement with the stratigraphic evidence and shows that mmm took place during latest eocene time or at the transition from eocene to oligocene time along the northern border of the massif deformation and metamorphism had already taken place during the late cretaceous in a high pressure low temperature hp lt metamorphic belt northernmost part of menderes massif sensu lato which was then covered by palaeocene molasse the evolution of the hp lt belt was probably related to the obduction of the bozkir ophiolites from the izmir ankara branch of the neo tethys ocean and preceded terminal collision in the evolution of the massif the mmm was a product of the latest palaeocene collision across neo tethys and the consequent internal imbrication of the menderes taurus block that resulted in the burial of the menderes massif area beneath the lycian nappe complex a low grade metamorphic event dated at 10 to 5 ma is interpreted to be related to the extensional deformation in the aegean and western turkey which has been dated stratigraphically to have begun during the tortonian 11 5 ma

pan african structures ophiolites and melange in the eastem desert of egypt a traverse at 26 n a detailed structural traverse across the basement rocks of the eastern desert of egypt shows that they consist apart from intrusions of four broadly recumbent tectonic units the lowest of arkosic metasediments of continental shelf facies is exposed in a dome this unit is overlain by an allochthonous ophiolitic melange containing complete and dismembered ophiolitic masses in a matrix of deep oceanic graphitic pelites and turbidites a near horizontal schistose to mylonitic fabric most intense near and below the base of the melange indicates extensive lateral tectonic transport of the ophiolitic melange over the shelf metasediments but the melange originated as an olistostrome before being tectonically transported the melange extends over an area of at least 10 000 km 2 it is locally overlain by calc alkaline volcanics in which deformation is less intense but increases and schistosity flattens downwards indicating some translation over the melange unconformable on the melange and calc alkaline volcanics is a molasse facies series itself also locally strongly deformed late tectonic granites preceded and locally post date the molasse facies sediments still later diapiric peralkaline riebeckite granites locally up domed the recumbent structures the tectonic evolution is related to a late proterozoic subduction zone to the se of the region

the lower palaeozoic of oman and its context in the evolution of a gondwanan continental margin the lower palaeozoic haima group of the sultanate of oman comprises a sequence dominated by siliciclastic rocks up to several kilometres thick and broadly divisible into two groups the sequence is known in outline from subsurface exploration but recent field work on outcrops in the isolated huqf area in east central oman has allowed a more detailed environmental interpretation of the succession at the base coarse continental deposits of the lower haima group rest unconformably on precambrian lower cambrian huqf group sediments mainly carbonates and evaporites the overlying upper haima group comprises from base to top the aeolian dominated amin formation the non marine coastal plain to shallow marine intertidal subtidal sediments of the andam formation the non marine to marginally marine ghudun formation and the complex cyclical deep to shallow water deposits of the safiq formation correlation of the haima group of oman with the lower palaeozoic of the arabian peninsula north africa and iran allows the evolution of the gondwanan margin to be modelled a widespread lower cambrian sequence of coarse alluvium resting on peneplained upper precambrian infracambrian huqf group strata represents the final stage of uplift and basin fill associated with the late proterozoic suturing of arabia and adjacent plates which resulted in the formation of the gondwanan continent the overlying thick sequence of continental fluvial and aeolian sediments was deposited in a series of stable intracratonic basins across the gondwanan landmass in the mid and late cambrian an overall sea level rise led to the repeated development of shallow marine shelf carbonates across the margin the subsequent replacement of this marginal carbonate sequence with a thick marginal to non marine sand dominated sequence may reflect the interplay of several events the drift of gondwana towards more southerly latitudes a phase of strong tectonic rejuvenation during the latest cambrian early ordovician and or a eustatic fall in sea level organic rich marine mudrocks were deposited over much of the gondwanan platform during a series of marked eustatic sea level rises during the mid late ordovician and early silurian in oman no evidence has yet been found for the significant late ordovician glacial event recorded from north africa and saudi arabia

comprehensive source rock evaluation of the kazhdumi formation in the iranian zagros foldbelt and adjacent offshore abstract the kazhdumi formation is regarded as one of the most prolific hydrocarbon sources in the zagros foldbelt and adjacent offshore rock eval pyrolysis data were collected from 2300 rock samples of the kazhdumi formation in 64 wells core and cutting including 37 oil and gas fields and five outcrops 200 vitrinite reflectance measurements were carried out to calibrate and map thermal maturity over the study area palynofacies analyses and thermal history modeling were applied on the wells located in the study area based on the total organic carbon toc map the kazhdumi formation toc values range from 0 to 11 in the central part of dezful embayment its average value is about 5 while average toc value of the kazhdumi formation is lower than 0 5 in qatar gawbandi paleo high hydrogen index hi values represented types ii and iii kerogen for central part of the dezful embayment and qatar gawbandi paleo high respectively t max ranges from 370 to 455 c in the study area the maximum and minimum values for this parameter are recorded in the central dezful embayment and central persian gulf anticlines respectively vitrinite reflectance vr r map also confirms the concluded results of t max data reconstructed burial history and thermal modeling of three wells are also in full agreement with the results of other maturity evidence t max and vr r palynofacies analyses revealed that this formation in the central persian gulf south pars field clearly represents the dominance of terrigenous elements which were resulted from a close proximity to a fluvio deltaic source however towards the northwest amorphous organic matter aom dominates in central dezful embayment indicating a distal dysoxic anoxic environmental condition

distribution of oceanic and continental leads in the arabian nubian shield new common lead data for feldspar whole rock and galena samples from the arabian nubian shield together with data from previous work can be divided into two main groups group i leads have oceanic mantle characteristics whereas group ii leads have incorporated a continental crustal component of at least early proterozoic age the group i leads are found in rocks from the red sea hills of egypt and the western and southern parts of the arabian shield group ii leads are found in rocks from the northeastern and eastern parts of the arabian shield as well as from the southeastern shield near najran they are also found in rocks to the south in yemen to the east in oman and to the west at aswan egypt this distribution of data suggests that the arabian nubian shield has an oceanic core flanked by rocks that have developed at least in part from older continental material two mechanisms are suggested by which this older lead component could have been incorporated into the late proterozoic rocks and each may have operated in different parts of the shield the older lead component either was derived directly from an underlying early proterozoic basement or was incorporated from subducted pelagic sediments or sediments derived from an adjacent continent

chlorine isotopes as tracers of solute origin and age of groundwaters from the eastern desert of egypt abstract chlorine 36 abundances and stable chlorine isotope ratios 37 cl 35 cl were analyzed to decipher sources of chloride and to estimate subsurface residence times of groundwaters from the nubian sandstone aquifer system nsas alluvial aquifers and fractured basement aquifers in the eastern desert of egypt chlorine 36 isotopic abundances 36 cl cl ranged from 5 10 15 to 343 10 15 the range in 37 cl values was 0 61 to 0 90 groundwater solute compositions were dominated by na ca cl and so 4 with total dissolved solids ranging from 445 to 13 700 mg l chemical and isotopic data indicate that dry fallout of marine aerosols and evaporated vadose zone residues from precipitation that recharged the aquifers may have been the main sources of groundwater solutes apparent groundwater 36 cl model ages range from 0 40 to 1300 300 180 kyr groundwater from the fractured basement aquifers in the southwestern part of the study area had the youngest apparent ages while that from deep wells tapping the nubian aquifer had the oldest apparent ages shallow alluvial aquifers having anomalously old apparent 36 cl model ages indicate mixing of old nubian aquifer water with modern recharge in alluvial aquifers stable isotope ratios of hydrogen and oxygen support this inferred mixing relationship and allowed quantitative estimation of old young water mixing fractions

late precambrian rifting and crustal evolution in the northeastern desert of egypt basement exposures along the northwestern flanks of the gulf of suez record the rapid formation of continental crust during the interval 670 550 ma a variety of field petrographic petrologic and isotopic considerations indicate that this episode of crust formation took place in an extensional tectonic setting analogous to that of the late paleozoic oslo rift of norway crustal evolution in this region thus contrasts with other regions of the afro arabian shield where the crust appears to have been formed by convergent margin accretionary processes and collisional tectonics this imposes new constraints on our understanding of late precambrian crustal evolution

geochemical characterization of pan african dyke swarms in southern sinai from continental margin to intraplate magmatism abstract the chemical and mineralogical features of three pan african dyke swarms occurring in the wadi feiran area southern sinai have been examined two swarms striking approx 35 exhibit calcalkaline chemical properties the oldest s1 can be related to calcalkaline rocks from active continental margins by means of incompatible element plots displaying negative nb ta anomalies they dykes were emplaced synchronously with post orogenic alkaline granites to which they cannot be assigned chemically a second dyke generation l1 with the same strike shows apparent similarities with continental flood basalts e g columbia river basalts indicating a growing contribution of continental components to melt chemistry subduction related features are displayed by negative nb ta anomalies the youngest dyke swarm s2 striking approx 135 and crosscutting s1 displays transitional to mildly alkaline properties without negative nb ta anomalies its chemistry is fully consistent with intraplate basalts of continental areas the source regions of the three swarms cannot be related via assimilation of crustal material because decreasing lile percentages coupled with increasing amounts of nb and ta cannot be explained by this means the overlap of m values of rocks to be found with the three swarms indicates the unlikeliness of a development of the three magmas via fractional crystallization the emplacement of two totally different types of dykes in the wadi el sheikh area requires a source region which is either chemically inhomogeneous in a more or less vertical scale or changing its composition and melting conditions with time although the dykes were emplaced in an extensional tectonic setting the subcrustal lithosphere still produced calcalkaline melts documenting its subduction related magmatic history

evolution of the kid group southeast sinai peninsula thrusts melanges and implications for accretionary tectonics during the late proterozoic of the arabian nubian shield the late proterozoic age kid group in the southeast sinai peninsula is a volcano sedimentary sequence of island arc affinities the group deformed in four main phases with a thermal peak after d1 followed by a regional greenschist facies overprint during d2 recumbent folding followed by thrusting began during d2 with the folds being propagated southward toward the arc foreland until d4 the last folding event the thrusts are responsible for the juxtaposition of four megatectonic units above a substrate of unknown composition these units probably allochthonous comprise a basal complex of infracrustal gneiss and three arc related units including inferred vestiges of a forearc accretionary prism and a marginal basin narrow slices of ophiolitic rocks were emplaced tectonically along the boundaries of some of these units melanges attributed to gravitational collapse of the forearc edifice and thrust tectonics formed in particular along the arc forearc transition zone the evolution of the kid group coupled with data from the rest of the arabian nubian shield can be well accommodated within the concept of oceanic crust subduction magmatic arc development and ultimate collision between the arc complex and a continental margin

fluid evolution in the el sid gold deposit eastern desert egypt auriferous quartz carbonate veins in the el sid mine cut through the western margin of the fawakhir granitic intrusion and the immediate country ophiolites gold mineraliz ation is spatially and temporally associated with ene wsw fault shear zones developed late in the deformational history of the area field and microscopic studies suggest two distinct ore stages namely an early gold fe as sulphide and a late gold base metal mineralization new microthermometric and raman data suggest gold deposition as a result of a complex history of fluid immiscibly dilution of low salinity aqueous carbonic fluids in the early mineralization stage while wall rock alteration and pressure loss precipitated au from intermediate salinity aqueous ore fluids during the late stage fluid inclusion isochoric reconstructions combined with oxygen and sulphur isotope data indicate conditions of 320 20 8c and 1 3 0 2 kbar for the early gold fe as sulphide mineralization and c 200 15 and 0 6 0 9 kbar for the late gold zn pb cu sulphide stage the clockwise evolution path in pressure temperature space likely documents gold mineralization under post peak metamorphic conditions the calcu lated sulphide d 34 sh2s equilibrium values 29 04 to 4 75 may refer to a variable redox state of sulphur in the ore fluids from the early to late mineralization stages the stable isotope signature of the vein quartz and calcite suggest mixed magmatic and metamorphic fluid sources d 18 o h2o 4 9 to 7 4 unusually low d 13 c values of calcite in the late mineralization 213 9 to 214 7 may reflect input of magmatic co2 and or oxidized carbonaceous mate rial in the infiltrating fluid gold bearing quartz veins associated with variably tectonized ophiolitic and island arc rocks or adja cent to their interferences are common in the southern and central sectors of the eastern desert of egypt whereas gold occurrences confined to granitic intrusions that are in some cases associ ated with felsic subvolcanic porphyries is a com mon feature in the northern eastern desert e g moharram et al 1970 sabet bordonosov 1984 klemm et al 2001 botros 2004 based on sev eral geological and mineralogical studies a gold granitoid association in the eastern desert has been suggested by many authors e g garson

the volcanic massif of biq at hayareah sinai negev a case of potassium metasomatism on biq at hayareah an area 80 km 2 in size on the border between sinai egypt and the negev israel a sequence of late precambrian alkali rhyolites lavas tuffs and ignimbrites and their hypa byssal equivalents rhyolite porphyries are exposed these rocks form part of the alkaline katharina province representatives of which are found from the indian ocean to west africa about half the rocks of biq at hayareah have undergone a mainly metasomatic alteration not recognizable in hand specimens chemically the alteration is expressed by the almost total exchange of na by k and by the addition of about 3 silica and of a small amount of alumina the trace element composition remained almost unaffected mineralogically albite has been replaced by k feldspar and secondary quartz has crystallized while some primary quartz has been replaced by k feldspar it is concluded that the potash metasomatism is the result of an interaction between the originally partly glassy rhyolites and hyd

gneiss cored interference dome associated with two phases of late pan african thrusting in the central eastern desert egypt abstract structural analysis of the wadi um had area in the central eastern desert ced of egypt reveals the presence of another nw trending elliptical gneissic and migmatitic domal structure west of gabal meatiq outlined by a thick schistose and mylonitized carapace the sheared carapace formed during a nw nnw ward thrusting event which emplaced thrust slices of ophiolitic melange dokhan volcanics and hammamat group sediments over the higher grade gneisses the gneisses are subdivided by retrograde shear zones of this event the sheared carapace is interpreted as the roof thrust of an imbricate stacked antiformal duplex a second thrusting and folding event associated with ne sw shortening deformed the earlier structures producing the dome and re thrusting low grade units against it as sw and ne dipping thrusts the features of this gneiss dome are inconsistent with it being a metamorphic core complex caution must be exercised in the recognition of transcurrent shears in the eastern desert since folded low angle thrust related mylonite zones may be steepened by folding producing steep shear zones with low pitching slip lineations extension bi directional thrusting and najd faulting in the ced may be explained in an escape tectonic deformation regime involving extrusion of material away from the pivotal collision zone in the mozambique belt to the south

thick skinned tectonics and basement involved fold thrust belts insights from selected cenozoic orogens defining the structural style of fold thrust belts and understanding the controlling factors are necessary steps towards prediction of their long term and short term dynamics including seismic hazard and to assess their potential in terms of hydrocarbon exploration while the thin skinned structural style has long been a fashionable view for outer parts of orogens worldwide a wealth of new geological and geophysical studies has pointed out that a description in terms of thick skinned deformation is in many cases more appropriate this paper aims at providing a review of what we know about basement involved shortening in foreland fold thrust belts on the basis of the examination of selected cenozoic orogens after describing how structural interpretations of fold thrust belts have evolved through time this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics and emphasizes the key control exerted by lithosphere rheology including structural and thermal inheritance and local regional boundary conditions on the occurrence of thick skinned tectonics in the outer parts of orogens

the najd fault system saudi arabia and egypt a late precambrian rift related transform system the najd fault system is a complex set of left lateral strike slip faults and ductile shear zones that strike nw se across the precambrian of arabia and egypt this system was developed during the interval 540 620 ma it is up to 400 km wide with an exposed length of 1100 km inferred buried extensions of the najd give it a total length of 2000 km it is the best exposed and may be the largest pre mesozoic zone of transcurrent faulting on earth previous models for the najd fault system suggest it formed as a result of a major late precambrian continent continent collision this model is not preferred here because 1 the lack of evidence for a pre late precambrian continent to the east of the najd fault system 2 the difference between the orientation of the najd fault system and that predicted by slip line theory 3 the younger age of najd movements compared with that of collisional sutures in the arabian shield and 4 lack of evidence for wide spread crustal uplift that would be expected to accompany collision a new model for the origin of the najd fault system accounts for each of these objections the najd fault system formed in response to a broad zone of nw se directed crustal extension that accompanied juvenile continental crustal formation in northernmost afro arabia this model also accounts for the following observations 1 strands of the najd parallel the direction of extension in the north eastern desert of egypt and sinai 2 the timing of the principal rifting movements ca 575 600 ma overlap with those of the najd ca 560 620 ma 3 in spite of observation 2 the najd fault system is not recognized in northernmost afro arabia instead the najd deformation becomes increasingly ductile and these zones are more commonly intruded by sheared and foliated granites as the principal zone of extension is approached the najd fault system thus represents a set of continental transforms developed in response to a major episode of late precambrian extensional continental crust formation in northernmost afro arabia

morphology genesis and classification of surface crusts in loamy and sandy soils abstract in an attempt to organize the knowledge of soil crusting processes and to group soil crusts on the basis of common morphological features and physical properties we synthesized and reinterpreted information from previous publications as a result we propose a classification system for surface crusts formed by rainfall in loamy and sandy soils although this system does not aim to be comprehensive it was designed from the study of a great number of crust samples collected in the temperate and tropical zones thus corresponding to a wide range of environmental and land use conditions three main classes of crusts structural erosion and depositional were distinguished each with subclasses where possible correspondence has been made with former systems this system helps predict infiltrability in accounting not only for total porosity of surface crusts but also for pore shape and continuity since crust classes are genetically related this classification system also supplies some insight into predicting soil degradation and selecting the most suitable control techniques

geology and origin of meatiq dome egypt a precambrian metamorphic core complex meatiq dome a metamorphic complex in the precambrian basement of the eastern desert of egypt exhibits many of the essential features of cordilleran metamorphic core complexes it is an antiformal structure with low dipping foliation and unidirectional mineral slickenside lineation the core consists of granite gneiss and is conformably overlain by a heterogeneous isoclinally folded mylonitic carapace the carapace grades upward into a nonmylonitic cover of low grade ophiolitic rocks the mylonitic rocks are interpreted to have formed in a low angle ductile shear zone during a compressional tectonic event accompanied by syntectonic intrusion of tonalite doming postdated mylonitization the absence of a brittle detachment surface at meatiq dome is its primary distinction from the otherwise similar cordilleran complexes