the evolution of early oceanic lithosphere in the southern red sea abstract it seems generally agreed that the red sea has formed by the rotation of arabia from africa about a pole of rotation at 36 5 n 18 0 e but the exact way in which the oceanic lithosphere has evolved between the separating plates remains unclear this paper is concerned with the evolution of the earliest oceanic lithosphere in the southern red sea a combined study is made of gravity magnetic and seismic refraction data for a region on the southeast margin of the red sea between 16 3 n 17 3 n and 41 6 e 43 1 e a model is evolved which satisfies all the geological and geophysical data a feature of this model is that the ocean continent boundary might be 35 km inland from the coast and only 8 km from the exposed precambrian shield in this region the coastal plain being underlain by early oceanic lithosphere this implies that the break in the continental lithosphere was fairly clean

sedimentary and structural evolution of the northwest arabian red sea margin abstract an investigation of the northwestern margin of the arabian peninsula midyan region provides new insights into the two stage development of the red sea graben system the early red sea suez stage and the later red sea aqaba stage the early stage is characterized by the passive subsidence of a continental graben since the early oligocene after a period of enhanced subsidence of the basin in the early miocene this stage ended at the langhian serra valian boundary 14 ma the later stage was initiated by a release of stresses by strike slip movement along the newly formed aqaba levant transform creation of the new plate boundary was accompanied by stagnation of extension in the gulf of suez branch of the early rift stage strike slip motion on the aqaba levant structure is younger than 14 ma and is recorded by the post langhian compressional structures in the area of the midyan peninsula which developed by transpression along the curved path of a left lateral strike slip fault

the tectonic development of the western margin of the gulf of elat aqaba rift abstract along the western coast of the gulf of elat a 30 km wide shear belt of subparallel faults trending n s to ne sw is developed this shear belt is observed mainly within the precambrian basement terrain sinistral movements on these faults have been recognized based on offsets of magmatic bodies and lithological contacts in rocks of precambrian age the cumulative displacement measured independently at several localities across the belt attains a total of 24 km early neogene volcanism in the form of long dikes trending nw se is known from the eastern desert of egypt of the gulf of suez and along the red sea coast of saudi arabia volcanic bodies of a similar pattern have been identified in eastern sinai these bodies mainly dolerites and basalts intruding mesozoic sediments are parallel to each other and to the gulf of suez they are tens to hundreds of kilometers long and spaced several tens of kilometers apart dating k ar of bodies in sinai saudi arabia and in the eastern desert of egypt suggest an age of 20 22 m y for this extensive volcanism all the dikes are sinistrally displaced by the individual faults of the eastern sinai shear belt the amount of movement recorded using the precambrian markers is identical to movement recorded by offsets of the early neogene dikes this proves that the total 24 km shear of this fault system postdates the 20 22 m y volcanic phase it is assumed that the recorded movements are part of the 105 110 km shear persumed to exist along the gulf of elat which is also younger than 20 22 m y preliminary results suggest that a similar amount of movement is observable along a shear zone developed on the eastern coast of the gulf of elat thus only some 60 km movement are taken up by faults in the gulf itself the evolving model suggests that 1 a the intrusion of very long basaltic dikes in a nw se direction on both sides of the red sea accompanied the initial stage of development of the red sea gulf of suez rift valley 2 b a basic change in the geotectonic regime occurs after the intrusion of this volcanic suite large scale horizontal movements are initiated being recorded as a 105 110 km sinistral movement along the arava jordan rift this movement implies a synchronous opening of the red sea

some comments on two layer extensional models for the evolution of sedimentary basins simple extensional models that involve listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various continental sedimentary basins and continental shelves in this paper we present structural evidence that extension in the continental lithosphere during rifting is not uniform we review evidence that the rifting process in major intracratonic rift basins is spasmodic rather than continuous that extension and subsidence within the rift during a tectonic phase is generally accompanied by uplift and erosion of the rift flanks that subcrustal heat input during rifting occurs over an area much broader than the rift proper and that there may be broad regional uplift before and or during rifting to explain the uplift of the flanks during rifting and their postrift subsidence we invoke two mechanisms subcrustal thinning by extension below the flanks and tilted major normal faults in the crust between graben and flanks the uplift and erosion of the flanks due to tilted faults probably do not significantly affect estimates of extension from subsidence analysis but uplift due to both sources can in certain cases be incorrectly attributed to regional uplift or doming we present a simplified analysis of the two layer extensional model for the elementary case in which extension is instantaneous and the crust is thinned by a different amount from the subcrustal lithosphere we provide a simple graphical method of data analysis to obtain the parameters that describe thinning of the crust and lithosphere during extension from the subsidence history application of this method to the central part of the pattani trough in the intracratonic gulf of thailand shows that extension and thinning of the lithosphere of a factor of 2 since late eocene can account for much of the subsidence history and the present day heat flow our analysis suggests that thinning of the crust in this area is 20 greater than thinning of the whole lithosphere assuming that none of the lithosphere is removed from the region of extension during rifting we show by simple geometric examples that a process of nonuniform extension implies that much greater thicknesses of sediment can be deposited in the center of a young basin than in the case of uniform extension of both crust and subcrustal lithosphere such an extensional process produces significant uplift of the flanks of the rift and as a result of erosion of the uplifted areas the effective cross sectional area of the basin can be increased by as much as 10 20 depending on the rate of erosion compared to the area that would have been created by a process of uniform extension finally we show that it may ultimately be possible to compute erosion rates from seismic cross sections of basins

scandinavia s north atlantic passive margin 1 the tectonics of the eastern passive margin of the north atlantic are reexamined the scandinavian north atlantic passive margin includes not only the offshore exploration and basin domain but also large portions of the onshore domains of the scandinavian caledonides combined information from structural geology potential field data regional geology basin development and geomorphology made it possible to propose a new definition of the passive margin the rift shoulder is formed by linked faults defining the present rift flank these faults are the extensional structures located furthest to the east onshore norway sweden that can be linked to the rifting that led to the formation of the north atlantic this innermost boundary fault system ibf is formed by a set of normal west dipping crustal faults it extends over a distance exceeding 2000 km from the north sea across the caledonian mountain belt to the barents sea the passive margin width between the continent ocean boundary and the ibf ranges from 550 km in the south to over 700 km in mid norway to 165 km north of lofoten rifting and faulting on the ibf started in permo carboniferous and a succession of rift phases eventually culminated in continental breakup and the formation of the north atlantic basin development between permian and cretaceous was toward the future breakaway fault but faulting was also active on land from mesozoic to present onshore offshore crustal scale cross sections show the geometry of the passive margin the dip orientation of the major faults and the changes in crustal thickness

rapid intra arc rifting in miocene northeast japan although life times of major continental rifts are tens of millions of years intra arc rifting in miocene ne japan lasted for only about 3 m y before back arc spreading began the rate of initial subsidence was over 1 km m y much faster than that in major continental rifts thus this intra arc rift evolved 10 times as fast as major intracontinental rifts the large temperature dependence of lithosphere rheology and the hot thermal regime of island arc lithosphere account for the rapid rifting less than 1 m y before the rifting spreading transition the inner ne japan arc subsided from shallow marine depths to middle or lower bathyal depths 1 0 2 5 km the average total basement subsidence was 2 3 km during the rifting the rapid submergence occurred during one third of the duration of rifting but accounts for two thirds of the total subsidence during rifting namely the initial subsidence rate accelerated toward the time of breakup of the arc crust at the same time the region along the present pacific coast remained only shallow marine or terrestrial throughout presumably influenced by global eustacy

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

geochemical dynamics of the atlantis ii deep red sea silica behavior abstract previous chemical analyses of atlantis ii deep sediments and waters have shown that silica concentrations are abnormally high in comparison with those from other marine hydrothermal sites new physical and chemical data have been obtained during recent cruises to the atlantis ii area seawater free and interstitial brines have been analyzed for chloride and silica the hydrographic measurements show that two additional brine layers have been created since 1977 these new observations allow us to re evaluate the hydrodynamics and the consequences for mass transfer silica concentration in the lower brine pool is 913 18 mol 1 within the upper brine layers silica concentrations are higher than those expected from simple mixing between the lower brine and seawater the chemical mass transfers through the brine interfaces are explained by double diffusion phenomena silica concentrations in the interstitial waters mol 1 are much lower than those in the lower brine the high silica concentration gradients observed at the brine sediment interface suggest that silica precipitates at this boundary layer thermodynamic calculation indicates that the interstitial brines are undersaturated with respect to quartz and amorphous silica these species act probably as major sources for silica whereas authigenic clay minerals could be the major sink

zabargad st john s island an uplifted fragment of sub red sea lithosphere zabargad is a small island located in the red sea about 50 km w of its axis between 23o and 24on the island is not volcanic but probably represents an uplifted block of upper mantle and crustal rocks and yields information on the nature of the underlying lithosphere peridotites are the main rock type consisting of exceptionally fresh spinel lherzolites which equilibrated last at a depth 30 km in the mantle other ultramafic facies some representing rocks which mixed with a basaltic melt fraction and some which underwent metasomatic exchange at depth are also present the peridotite bodies are in tectonic contact with a metamorphic formation consisting of gneissic and amphibolitic rocks similar to those outcropping in the pan african metamorphic basement of eastern egypt peridotites and metamorphic rocks are both crossed by several generations of basaltic dykes the sedimentary zabargad formation of probable cretaceous or palaeocene age consists of alternations of silicified limestones sandstones black shales and phosphoritic lenses and is overlain by evaporites probably of miocene age and by reef limestone units one probably of early pleistocene age the other probably deposited during the last interglacial various formations are affected by localized metasomatic mineralizations recrystallized large gem quality olivine cancrinite scapolite garnierite the uplift of upper mantle ultramafic bodies with fragments of crust which created the island might have occurred in connection with the early stages of development of the rift which preceded the formation of the red sea an alternative explanation that the zabargad peridotites are remnants of a pan african palaeozoic ophiolitic complex is possible but less likely

crustal and uppermost mantle structure beneath the afro arabian rift system abstract this contribution presents data and results from long range seismic refraction wide angle reflection lines in the afro arabian rift system from the derived crustal structure an evolutionary model is proposed in terms of the amount of crustal thinning in this model the gulf of aden and the axial trough of the southern red sea where the continental crust was thinned so much as to allow thin oceanic crust to form are representative of one end of the spectrum progressing through the northern part of the red sea main trough on the egyptian side with a crustal thickness of 11 14 km to the northern and southern parts of the red sea main trough on the arabian side and the northern afar depression with 15 20 km thick crusts followed by the southern afar depression with a crustal thickness of 22 26 km the members of the other end of the spectrum are reached these include the jordan dead sea and east african rifts which each with a crustal thickness of 30 36 km represent the least thinned closest to normal thickness continental crust in determining the amount and nature of opening of the red sea there remain however many problems argely due to the non unique correspondence of seismic velocity to rock type this may lead to uncertainty in moho depths especially in areas close to the mid ocean ridge system e g the southeastern part of the afar depression in the region around the gulf of tadjura where velocities as low as 6 8 6 9 km s have been interpreted as representirg hot anomalous mantle it also causes uncertainty as to what percentage of the present crust is stretched and thinned original continental crust and what percentage is new crust whether it be truly oceanic or basic intrusiors into continental crust thus it is felt that there is still an urgent need for geoscientific experiments in the afro arabian rift to resolve the details of continental rifting and subsequent oceanic crust emplacement

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

from rifting to passive margin the examples of the red sea central atlantic and alpine tethys abstract evolution of the red sea gulf of suez and the central atlantic rift systems shows that an initial transtensive rifting phase affecting a broad area around the future zone of crustal separation was followed by a pre oceanic rifting phase during which extensional strain was concentrated on the axial rift zone this caused lateral graben systems to become inactive and they evolved into rift rim basins the transtensive phase of diffuse crustal extension is recognized in many intra continental rifts if controlling stress systems relax these rifts abort and develop into palaeorifts if controlling stress systems persist transtensive rift systems can enter the pre oceanic rifting stage during which the rift zone narrows and becomes asymmetric as a consequence of simple shear deformation at shallow crustal levels and pure shear deformation at lower crustal and mantle lithospheric levels preceding crustal separation extensional denudation of the lithospheric mantle is possible progressive lithospheric attenuation entails updoming of the asthenosphere and thermal doming of the rift shoulders their uplift provides a major clastic source for the rift basins and the lateral rift rim basins their stratigraphic record provides a sensitive tool for dating the rift shoulder uplift asymmetric rifting leads to the formation of asymmetric continental margins corresponding in a simple shear model to an upper plate and a conjugate lower plate margin as seen in the central atlantic passive margins of the united states and morocco this rifting model can be successfully applied to the analysis of the alpine tethys palaeo margins such as rif and the western alps

the jabal tirf layered gabbro and associated rocks of the tihama asir complex sw saudi arabia initiation of sea floor spreading in the red sea 4 5 m y ago was preceded by an early tertiary period of continental extension accompanied by intrusive and extrusive magmatic activity during the miocene tholeiitic basaltic magma was intruded along faults and fractures in rifted continental crust to form extensive dike swarms and layered gabbroic plutons along with associated silicic intrusive bodies these intrusions best exposed sporadically in a 4 6 km wide zone at the edge of the red sea coastal plain in southwest saudi arabia are known collectively as the tihama asir complex one of these intrusions a layered gabbro at jabal tirf near jizan consists of clinopyroxene gabbro and clinopyroxene olivine gabbro plus minor troctolite and anorthosite displaying cumulate textures microprobe analyses performed on a sequence of samples from the bottom to the top of the intrusion reveal no systematic variation in mineral chemistry with stratigraphic height in the intrusion mineral compositions in the

tectonic configuration of the western arabian continental margin southern red sea the young continental margin of the western arabian peninsula is uplifted 3 5 to 4 km and is well exposed rift related extensional deformation is confined to a zone 150 km wide inland of the present coastline at 17 to 18 n and its intensity increases gradually from east to west extension is negligible near the crest of the arabian escarpment but it reaches a value of 8 to 10 in the western asir a highly dissected mountainous region west of the escarpment there is an abrupt increase in extensional deformation in the foothills and pediment west of the asir about 40 km inland of the shoreline where rocks in the upper plate of a system of low angle normal faults with west dips are extended by 60 to 110 the faults were active 23 to 29 ma ago and the uplift occurred after 25 ma ago tertiary mafic dike swarms and plutons of gabbro and granophyre 20 to 23 ma old are concentrated in the foothills and pediment as well the chemistry of the dikes suggests 1 fractionation at 10 to 20 kbar 2 a rapid rise through the upper mantle and lower crust and 3 differentiation and cooling at 1 atm to 5 kbar structural relations between dikes faults and dipping beds indicate that the mechanical extension and intrusional expansion were partly coeval but that most of the extension preceded the expansion a tectonic reconstruction of pre red sea afro arabia suggests that the early rift was narrow with intense extension confined to an axial belt 20 to 40 km wide steep moho slopes probably developed during rift formation as indicated by published gravity data two published seismic interpretations and the surface geology

fracture zone tectonics at zabargad island red sea egypt abstract zabargad island which lies along the western margin of the red sea rift is a remarkable place because it provides fresh exposures of undepleted mantle peridotite how this peridotite came to be exposed on zabargad remains unclear our field mapping indicates that most of the contacts between peridotite and the adjacent bodies of pan african gneiss and cretaceous zabargad formation on the island are now high angle brittle faults zabargad formation strata have been complexly folded partly in response to this faulting overall the array of high angle faults and associated folds on the island resembles those found in cross rift transfer zones we suggest therefore that the zabargad fracture zone a band of submarine escarpments on the floor of the red sea north of the island crosses zabargad island and has actively resolved differential movement between the central red sea rift and the northern red sea rift the final stage of uplift that brought the unusual peridotite to the earth s surface is related to shallow crustal transpression which may have inverted an earlier transtensional regime

a model for iron deposition to euxinic black sea sediments sulfur isotope and c s fe elemental data from the lower devonian siegenian emsian hunsruck slate of germany allow the conditions that led to the pyritization of soft bodied fossils to be determined the slates preserving pyritized soft tissues generally contain low concentrations of organic matter 0 3 0 4 percent c and pyrite sulfur 0 0 2 percent s respectively but are unusually rich in total and hcl extractable iron concentrations of iron are significantly lower in samples of the hunsruck slate from areas where fossils lack pyritized soft tissues the fossil pyrite is very enriched in s 34 compared to fine grained disseminated pyrite in the adjacent slates delta fossil sediment 10 to 50 permil suggesting that pyritization of the fossils persisted into later stages of authigenic mineralization pyrite formation in the sediments was limited by the low concentrations of metabolisable organic matter leaving residual sulfate and iron available for continued fossil pyritization in organic rich microenvironments a diffusion with precipitation model indicates that a critical control on the initial preservation of readily metabolisable soft tissue in this way is the presence of high concentrations of sediment iron capable of being solubilized during shallow burial diagenesis in association with low concentrations of metabolisable organic matter in the matrix pore waters rich in dissolved iron allow the sulfide generated by the decay of readily metabolisable soft tissue to be trapped efficiently within the carcass

conrad deep a new northern red sea deep origin and implications for continental rifting a previously unknown deep here called conrad deep was discovered during an extensive geophysical survey of the northern red sea in june 1984 conrad deep is located at 27 03 n 34 43 e only 90 km south of the gulf of suez and is the most northern deep yet discovered in the red sea it is located within a well developed axial depression which also contains charcot deep 100 km to the south the axial depression is associated with abundant recent deformation and is situated at the peak of a regional heat flow high extending across the rift conrad deep is typical of the small northern type red sea deep it is 10 km long 2 km wide and has a maximum depth of 1460 m it is associated with high and variable heat flow values and large magnetic anomalies there is no evidence of a dense brine layer detailed analysis of the geophysical data implies that the deep probably results from a very recent

punctiform initiation of seafloor spreading in the red sea during transition from a continental to an oceanic rift transition from a continental to an oceanic rift occurs in the red sea by initial emplacement of oceanic crust in regularly spaced hot points which serve as nuclei for axial propagation into segments of oceanic crust accretion and for initiation of seafloor spreading hot points develop above upwelling mantle diapirs caused by density viscosity inversion beneath continental and oceanic rifts

upper mantle beneath a young oceanic rift peridotites from the island of zabargad red sea information on the nature of the upper mantle in a young rift zone with lithosphere transitional from continental to oceanic was obtained from the island of zabargad probably an uplifted fragment of red sea lithosphere the island exposes exceptionally fresh mantle derived peridotite spinel iherzolite bodies the mineralogy and major element rare earth element and mineral chemistry of the spinel iherzolites suggest that they equilibrated last at depth 30 km in the mantle and that they have oceanic rather than continental affinity it is unlikely that the zabargad ultramafics are part of a late precambrian early paleozoic ophiolite as found elsewhere in circum red sea regions rather they were uplifted probably in connection with the development of the red sea rift the zabargad peridotites are in tectonic contact with continental metamorphic basement rocks it appears therefore that regions of an embryonic rift such as the red sea can be covered with continental type crust but be underlain by an upper mantle having already oceanic affinities

crustal underplating and evolution in the red sea rift uplifted gabbro gneiss crustal complexes on zabargad and brothers islands the nature of the crust of the red sea outside the oceanic axial trough segments can be inferred from uplifted crustal sections exposed on the islands of zabargad and the brothers in the central and northern red sea zabargad exposes in addition to mantle derived peridotites a crustal section consisting of metagabbros and metapyroxenites associated with silicic gneisses and cut by doleritic dikes seismic velocities of samples from this section are compatible with crustal velocities obtained by seismic refraction experiments in the central and northern red sea the zabargad peridotitesilicic gneiss gabbroic association provides a sample of upper mantle lower crust in a continent before the inception of a major rifting event the petrology and mineral chemistry of the zabargad metagabbros suggest that they were originally part of a basic layered complex which crystallized at relatively high pressure 900 1000 mpa or 9 10 kbar i e over 30 km deep the gneisses have tonalitic trondhjemitic affinity and were originally equilibrated in the garnet granulite facies implying also high pressure and derivation from the lower continental crust this suggests that after the subcrustal continental lithosphere along the proto red sea rift had been consumed possibly by convective thinning but before any significant thinning of the crust basaltic melts intruded the base of the continental crust underplating it with gabbroic complexes thus crustal rifting was probably preceded by a thermal anomaly in the mantle which supports an active rifting hypothesis the gabbro pyroxenite complex and the gneisses underwent subsequently retrometamorphic events under decreasing pt conditions from granulite to amphibolite to greenschist facies implying decompression and uplift probably related to the gradual thinning of the continental crust during rifting during this stage additional gabbroic intrusions and basaltic dikes exemplified by the gabbros and dolerites from the brothers islands were emplaced at decreasing depths beneath the thinned continental crust the northern red sea crust is presently in this stage it is neither oceanic nor continental it lacks a continuous thick basaltic upper crustal layer and is made prevalently of gabbroic and doleritic intrusives with relicts of sialic continental crust subsequently basaltic injections became progressively focused in a narrow axial zone and seafloor spreading initiated at discrete equidistant hot points as observed presently in the central red sea which by axial propagation evolved into nearly continuous zones of oceanic crust accretion as observed presently in the southern red sea processes similar to those proposed here for the evolution of the red sea rift can be documented in passive margins of the atlantic and in some alpine ophiolite complexes

the afar depression interpretation of the 1960 2000 earthquakes summary we studied the seismic activity of the afar depression ad and adjacent regions during the period 1960 2000 we define seven distinct seismogenic regions using geological tectonic and seismological data based on the frequency magnitude relationships we obtain b values of about 1 for the different regions the pattern of the distribution of the location of epicentres fits with the known active fault zone in the ad and the axial volcanic ridges the bab el mandab area and the danakil aysha a blocks are less active for 125 intermediate to strong earthquakes the seismic moment and source parameters were calculated the results of the fault plane solutions for the afar depression indicate mainly strike slip and normal sense of movement originating from fault planes striking nw se these results indicate a clockwise block rotation described previously as a bookshelf model in central ad there are a few rightlateral faults east of massawa with e w striking fault planes at the southern red sea north of the danakil block the mixed focal mechanisms with axial plane striking nw se comprise several reverse faulting strike slip motion and normal faulting right lateral movement was also calculated for a cluster of seismic events between the manda hararo and alyata volcanic ridges along nw se striking faults along the n s striking faults in the escarpment at the western afar margins there are two distinct clusters of epicentres the strong earthquakes at the southern cluster exhibit normal or strike slip motions the intermediate to small earthquakes in the northern cluster exhibit reverse and strike slip motions mainly normal faults were calculated along ne sw striking faults of the ethiopian east african rift estimates of the seismic efficiency suggest that the maximal values are about 50 per cent or less implying that most of the motion is taken aseismically

crustal structure in southeastern egypt symmetric thinning of the northern red sea rifted margins crustal structure in southeastern egypt has been investigated to elucidate the nature of crustal thinning across the northern red sea p wave receiver function modeling for 7 stations in southeastern egypt yields typical proterozoic crustal thicknesses of 35 38 km around lake aswan and thinner crust 25 26 km within 50 km of the red sea coast the vp vs ratios are on average 1 78 and indicate an intermediate composition crust these results when combined with other estimates of crustal thickness in the region reveal a symmetric pattern of crustal thickness beneath the conjugate margins of the northern red sea such a pattern is consistent with a pure shear model of extension and suggests that the greater amounts of uplift and volcanism on the eastern side of the red sea compared to the western side may be the result of deeper flow in the mantle associated with the african superplume and not a direct consequence of the rifting process

low angle detachment origin for the red sea rift system abstract the tectonic and magmatic history of the jizan coastal plain tihama asir southwest arabia suggests a two stage evolution a first stage of extension began during the oligocene and ended with uplift of the arabian graben shoulder which began about 14 ma ago it was followed by a period of approximately 10 ma characterized by magmatic and tectonic quiescence a second stage of extension began roughly contemporaneously with the onset of seafloor spreading in the southern red sea some 4 5 ma ago and is still active today the geometry of faulting in the jizan area supports a wernicke model of simple shear for the development of the southern red sea regional asymmetries of the red sea area such as the distribution of volcanism the marginal topography and asymmetries in the geophysical signatures are consistent with such a model available seismic profiles allow a rough estimate for values of the arabian red sea margin and were used to simulate subsidence history and heat flow of the red sea for classical two layer stretching models neither finite uniform nor finite non uniform stretching models can account for observed subsidence and heat flow data thus two model scenarios of whole lithosphere normal simple shear are presented for the geological history of the southwestern arabian margin of the red sea these models are limited because of the serravallian rearrangement in the kinematics of the red sea

ray path interpretation of the crustal structure beneath saudi arabia abstract interpretation of a long range seismic refraction line in saudi arabia has shown that beneath the arabian shield velocity generally increases with depth from about 6 km s 1 at the surface to about 7 km s 1 at the top of the crust mantle transition zone the base of this transition zone moho occurs at 37 44 km in depth intracrustal discontinuities can also be recognized the most important being in the 10 20 km depth range and separating the upper from the lower crust laterally the variations in the intracrustal discontinuities and the total crustal thickness can be correlated with previously defined tectonic regions beneath the red sea shelf and coastal plain the crust including 4 km of sediments is only 15 17 5 km thick with the aid of both seismic and gravity data an abrupt steeply dipping transition from the crust of the red sea shelf and coastal plain to that of the arabian shield has been derived with a jump of more than 20 km in moho depth this appears to be the major discontinuity between the red sea depression and the arabian continental shield

saudi arabian seismic refraction profile a traveltime interpretation of crustal and upper mantle structure abstract the crustal and upper mantle compressional wave velocity structure across the southwestern arabian shield has been investigated by a 1000 km long seismic refraction profile the profile begins in mesozoic cover rocks near riyadh on the arabian platform trends southwesterly across three major precambrian tectonic provinces traverses cenozoic rocks of the coastal plain near jizan and terminates at the outer edge of the farasan bank in the southern red sea more than 500 surveyed recording sites were occupied and six shot points were used including one in the red sea two dimensional ray tracing techniques used to analyze amplitude normalized record sections indicate that the arabian shield is composed to first order of two layers each about 20 km thick with average velocities of about 6 3 km s and 7 0 km s respectively west of the shield red sea margin the crust thins to a total thickness of less than 20 km beyond which the red sea shelf and coastal plain are interpreted to be underlain by oceanic crust a major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition elsewhere along the profile several high velocity anomalies in the upper crust correlate with mapped gneiss domes the most prominent of which is the khamis mushayt gneiss based on their velocities these domes may constitute areas where lower crustal rocks have been raised some 20 km two intracrustal reflectors in the center of the shield at 13 km depth probably represent the tops of mafic intrusives the mohorovicic discontinuity beneath the shield varies from a depth of 43 km and mantle velocity of 8 2 km s in the northeast to a depth of 38 km and mantle velocity of 8 0 km s depth in the southwest near the shield red sea transition two velocity discontinuities occur in the upper mantle at 59 and 70 km depth the crustal and upper mantle velocity structure of the arabian shield is interpreted as revealing a complex crust derived from the suturing of island arcs in the precarnbrian the shield is currently flanked by the active spreading boundary in the red sea

red sea detachment and basement core complexes in eritrea eritrea s war of independence precluded checking a 12 yr old prediction that a low angle detachment along which the red sea opened might be exposed on the western shore following eritrea s success we have found a suite of regional scale detachments along which northeast southwest extension began in rocks that were ductile at depth but became increasingly brittle as they cooled adiabatically on the way to exhumation the only signs of red sea detachment in the rift shoulder are steep and low angle faults offsetting oligocene dikes and lavas by contrast low angle ductile mylonites with a consistent top to the basin sense of shear were superimposed on steep pan african fabrics in the coastal lowlands these mylonites define core complexes beneath hanging walls of embrittled mylonites and synextensional dikes in which panels of domino fault blocks face both toward and away from the basin over low angle semi brittle detachments both the early ductile and later semi brittle low angle detachments were extended by a later set of grabens that probably sole out to an even younger detachment beneath the coastal lowlands

the gulf of aden structure and evolution of a young ocean basin and continental margin new marine geophysical data are used to describe the structure and history of the gulf of aden magnetic anomaly data shows seafloor spreading magnetic anomalies of sheba ridge from the axial anomaly to anomaly 5 10 m y b p between the owen fracture zone and 45 sup 0 e and to anomaly 2 3 m y b p or anomaly 3 4 m y b p west of 45 sup 0 e the data does not support the two episodes of seafloor spreading recently proposed landward of the seafloor spreading magnetic anomalies is a magnetic quiet zone of uncorrelatable anomalies the magnetic quiet zone boundary is also a structural boundary effectively marking the edge of sheba ridge with deeper basement lacking a significant topographic gradient found on the landward side a magnetic quiet zone is found not only where sheba ridge splits continental lithosphere but also on east sheba ridge where the ridge splits the old oceanic lithosphre of the owen and somali basins there the position occupied by the continental margin within the gulf is marked by nonmagnetic ridge complexes that stretch from the continents to the owen fracture zone the magnetic quiet zone boundary is not an isochron in either the gulf of aden ormore the red sea suggesting that significant horizontal motions can occur prior to the initiation of seafloor spreading the offset on the dead sea rift is used to estimate that from 80 to 160 km of opening amounting to between 65 and 200 extension of the initial rift valley occurred in the gulf of aden and red sea prior to the establishment of a mid ocean ridge it is suggested that the development of a new ocean basin occurs in two stages the first involves diffuse extension over an area perhaps 10 km wide in a rift valley environment without an organized spreading center this is followed by concentration of the extension at a single axis and the beginning of true seafloor spreading less

thermal history of the eastern gulf of suez ii reconstruction from apatite fission track and k feldspar measurements abstract apatite fission track aft measurements from miocene graben fill sediments of the eastern gulf of suez in three deep boreholes belayim 113 m 2 6 ras garra m 1 and alma 2 yield a wide range of ages from 125 to 320 ma and mean track lengths 10 1 12 7 m this range is similar to that recorded from precambrian crystalline basement flanking the graben which is the major source for the rift fill since the aft ages exceed the age of the host strata which are presently at their highest post depositional temperatures the rift related thermal regime is one of only moderate heating downhole aft data in the belayim 113 m 2 6 borehole attest to a higher rift temperature than in the ras garra m 1 borehole some 85 km to the south where little or no thermal overprinting is evident these findings are consistent with previously reported bottom hole temperatures and vitrinite reflectance data in the study area despite the higher syn rift thermal regime indicated for belayim 113 m 2 6 apatite provenance ages in ras garra m 1 are considerably younger thus the aft data in the ras garra m 1 area do not record significant rift related thermal effects but rather they mainly retain a pre rift provenance signature which reflects the order and depth of erosion at the uplifted flanks the younger aft ages in ras garra m 1 despite the weaker rift related thermal effect suggest a deeper level proportional to an additional 5 10 c of exhumation of the uplifted crystalline basement southwards along the eastern rift flank by mid miocene time this result is consistent with earlier findings which indicate both increased extension and heat flow southwards in the gulf of suez and earlier exhumation of the rift flank immediately preceding extension and opening of the gulf of suez the most deeply exhumed basement rocks presently exposed on the rift flanks were heated to temperatures 110 c total annealing of apatite but 40 ar 39 ar data and non resetting of zircon ft clocks in sinai 40 ar 39 ar data from granite penetrated in graben drilling at 3 89 km further corroborates pre rift palaeotemperatures

the gneiss of zabargad island deep crust of a rift abstract the gneisses of the zabargad island are in contact with lherzolites that were emplaced by mantle diapirism during the early rifting of the red sea the gneisses and the peridotites at their contact develop similar structures kinematics and common granulite metamorphism away from the contact the shear strain and associated recrystallizations decrease rapidly thus the zabargad gneisses are metamorphosed and deformed by the mantle diapiric intrusion this is confirmed by preliminary isotopic dates the gneisses have a tripartite polygenic composition of felsic granulites metagabbros and amphibolites the metagabbros and amphibolites are metamorphosed only by the mantle diapir intrusion which also induces a partial anatexis of the gneisses the felsic granulites record a proceeding high p granulite facies metamorphism they are assumed to represent screens of ancient continental crust

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

interpretation of a seismic refraction survey across the arabian shield in western saudi arabia abstract in february 1978 seismic refraction profiles were recorded by the u s geological survey along a 1000 km line across the arabian shield in western saudi arabia this report presents a traveltime and relative amplitude study in the form of velocity depth functions for each individual profile assuming horizontally flat layering the corresponding cross section of the lithosphere showing lines of equal velocity reaches to a depth of 60 80 km the crust thickens abruptly from 15 km beneath the red sea rift to about 40 km beneath the arabian shield the upper crust of the western arabian shield yields relatively high velocity material at about 10 km depth underlain by velocity inversions while the upper crust of the eastern shield is relatively uniform the lower crust with a velocity of about 7 km s is underlain by a transitional crust mantle boundary for the lower lithosphere beneath 40 km depth the data indicate the existence of a laterally discontinuous lamellar structure where high velocity zones are intermixed with zones of lower velocities beneath the crust mantle boundary of the red sea rift most probably strong velocity inversions exist here the data do not allow a detailed modelling velocities as low as 6 0 km s seem to be encountered between 25 and 44 km depth

modeling heterogeneous stretching during episodic or steady rifting of the continental lithosphere the geometry of brittle ductile extension and modes of normal faulting during steady or episodic rifting of the continental lithosphere are investigated by analogue modeling using the centrifuge technique we use a frictional material to simulate the brittle behavior and various nonlinear bouncing putties to represent the ductile rheology of the stratified lithosphere steady stretching of the continental lithosphere supports rapid differential subsidence and flexure of the brittle crust with formation of marginal grabens and a panel of domino style fault blocks in contrast episodic stretching of the lithosphere progressing in a series of pulses separated by longer periods of quiescence supports formation of spaced grabens and horsts with less subsidence this is due to a counterflow of ductile material from the margins to the central rift zone we show here that the isostatic adjustment in the lithosphere can be related to the steady or episodic mode of heterogeneous stretching we apply the model results to the southern red sea for which the inferred opening history involved continuous or stepwise extension

saudi arabian refraction profile crustal structure of the red sea arabian shield transition abstract an interpretation of deep seismic sounding measurements across the ocean continent transition of the red sea saudi arabian shield is presented using synthetic seismograms based on ray tracing we achieve a good fit to observed traveltimes and some of the characteristic amplitudes of the record sections crustal thickness varies along the profile from 15 km in the red sea shelf to 40 45 km beneath the asir mountains and the saudi arabian shield based on the computation of synthetic seismograms our model requires a velocity inversion in the red sea arabian shield transition high velocity oceanic mantle material is observed above continental crust and mantle thereby forming a double layered moho our results indicate a thick sedimentary basin in the shelf area and zone of high velocities within the asir mountains probably uplifted lower crust prominent secondary low frequency arrivals are interpreted as multiples

red sea related history of extension and magmatism in the jizan area southwest saudi arabia indication for simple shear during early red sea rifting the tectonic and magmatic history of the jizan coastal plain of the red sea tihama asir in southwest saudi arabia is dominated by sw ne extension this extension manifests itself by a basic dike swarm and several generations of normal faults that trend roughly nw se the oldest synrift deposits are non marine clastics and bimodal volcanics of the jizan group which were intruded by basic dikes gabbros and granophyres of the tihama asir magmatic complex tamc radiometric ages for the tamc rocks range from 26 to 18 ma rifting in the southern part of the red sea system therefore is certainly older than 26 ma between 26 and 20 ma the southwestern arabian margin was affected by a monoclinal downwarp flexure the oldest extensional features in the jizan area are n s to nnw sse striking dikes and normal faults which were overprinted by other nw se striking extension structures an array of en echelon extensionzones is linked by complex patterns of transfer faults mainly wnw ese striking dextral strike slip or oblique slip faults the observed superposition of dikes and normal faults of different orientations was not necessarily caused by a counterclockwise rotation of the regional stress field but more likely reflects the clockwise rotation of individual faultbounded blocks between the two phases of extension uplift of the graben shoulder east of the jizan coastal plain is probably younger than 14 ma and its development coincides with a major rearrangement of the kinematics along the red sea this new kinematic pattern probably led to the onset of strike slip movement along the aqaba levant transform at the same time extension and volcanism ceased in the jizan area or shifted further to the west after a long hiatus of about 10 ma renewed extension facilitated extrusion of alkali olivine basalts since 2 ma ago