magmatic history of red sea rifting perspective from the central saudi arabian coastal plain a newly recognized tertiary dike complex and comagmatic volcanic rocks exposed on the central saudi arabian coastal plain record early stages of magmatism related to red sea rifting intrusive and stratigraphic relationships and new potassium argon dating indicate episodic magmatism from about 30 ma to the present additional stratigraphic and radiometric evidence suggests that limited rift related magmatism began as early as about 50 ma an early phase of crustal extension in the region was accompanied by faulting and graben formation and by dike swarm intrusion the style of extension and intrusion changed at 20 ma localized volcanism and sheeted dike injection ceased and were replaced by the intrusion of thick gabbro dikes this change may mark the onset of sea floor spreading in the central red sea the dikes and volcanic rocks consist of a bimodal mafic felsic suite with transitional subalkaline to alkaline chemistry although no unique petrogenetic model can be developed for the suite the following observations and conclusions have been drawn from the available reconnaissance geochemistry 1 strontium isotopic initial ratios overlap in the range 0 7031 to 0 7047 and are not clearly related to alkalinity or silica content the bimodality and alkalinity of the suite are therefore not products of contamination by the radiogenic precambrian upper crustal granitic rocks exposed in the region 2 several of the evolved rocks rhyolites and comendite show large to extreme negative europium anomalies and heavy rare earth element hree enrichment features that cannot be explained by low pressure crystal fractionation of potential parent magmas an intriguing possibility is that hree enrichments and large negative eu anomalies of some peraluminous rhyolites and granites may result from the complete melting of garnet from middle or lower crustal rocks 3 most of the basalts are light rare earth element lree enriched consistent with derivation from fertile mantle in a continental rift setting however interaction of the mafic melts with lower continental crust cannot be ruled out a model of poly baric mantle melt derivation producing several alkaline subalkaline cycles best explains magmatism in the red sea region differences in the depths and dynamics of mantle melt extraction and transport brought about through changes in crust and mantle structure as the rift and paar developed may account for the transition from mixed alkaline subalkaline bimodal magmatism of the pre 20 ma rift basin to exclusively subalkaline tholeiitic magmatism at the red sea spreading axis and to predominantly alkali basalt volcanism inland within the arabian shield

magma systems related to the red sea opening abstract four distinct magma environments can be recognized in association with the red sea basin 1 1 mid ocean ridge basalts within the present actively spreading axial trough 2 2 basaltic dikes and layered gabbros within the extended precambrian crust on the arabian margin 3 3 granites and rhyolites developed in the extended precambrian crust and 4 4 plateau basalts asymmetrically distributed on the arabian flanks of the red sea these magmas developed as the result of the initial break up of africa and the establishment of an oceanic spreading center within the red sea original asymmetric continental extension occurred on east dipping detachment faults in the oligocene and caused morb like magmas to invade an extending precambrian crust contemporaneous small scale mantle convection initiated limited partial melting under the arabian plate to form the undersaturated alkali olivine plateau basalts slow extension through the miocene allowed an evaporite basin to develop in the red sea while small volumes of rift and plateau basalts formed on the arabian plate a small change in plate motion in the pliocene initiated the seafloor spreading in the axial trough where morb magmas formed oceanic crust rise of the asthenosphere produced small scale thermal perturbations under the arabian plate controlled by an east dipping master detachment fault small scale mantle convection extended partial melting 2 5 eastward under the arabian plate producing more undersaturated diverse alkali olivine plateau basalts distinct chemical and isotopic signatures of these four magma types lead to obvious actualistic correlations with the geodynamic setting

propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

upwelling asthenosphere beneath western arabia and its regional implications two distinct phases of continental magmatism are evident in western arabia the first from about 30 to 20 ma produced tholeiitic to transitional lavas emplaced along nw trends the second from about 12 ma to recent produced transitional to strongly alkalic lavas emplaced along n s trends the older phase is attributed to passive mantle upwelling during extension of the red sea basin whereas the younger phase is attributed to active mantle upwelling but was facilitated by minor continental extension perpendicular to plate collision the younger magmatic phase is largely contemporaneous with a major period of crustal uplift to produce the west arabian swell after about 14 ma a variety of evidence suggests that the west arabian swell is thermally supported by hot upwelling asthenosphere in contrast to the distinct asymmetry of uplift and magmatism on opposing sides of the red sea basin these processes were symmetric across a n s line marking the central axis of the west arabian swell this axis coincides with two fundamental features the ha il rutbah arch in the north and the makkah madinah nafud mmn volcanic line in the south the symmetry of magmatism is demonstrated by petrochemical evidence that the mmn harrats were derived by greater degrees of partial melting at shallower depths than those harrats lying to the west and east of the mmn line the potential temperature of the asthenosphere is estimated to be about 1436 c beneath the mmn line and about 1354 c beneath the flanking more undersaturated harrats the source of upwelling is either a mantle plume centrally located beneath the west arabian swell or an elongated and extended lobe of hot asthenosphere emanating from the ethiopian mantle plume convective flow may have been channelled along a preexisting regional flexure in the continental lithosphere which concentrated hot asthenosphere beneath the central axis of the afroarabian dome this crest of mantle upwelling underlies the mmn line in the north and the danakil depression and ethiopian rift system in the south it also passes through the red sea basin at the midpoint of an unusual doubly propagating rift system where axial seafloor spreading began 4 5 m y ago the nw trend of the red sea basin was well established by crustal attenuation during the older magmatic phase the pliocene invasion of this basin by a n s zone of mantle upwelling has resulted in seafloor spreading parallel to the preexisting structure along a rift system that has continued to propagate away from its eccentric mantle source in two opposing directions

reply to comment on timing of mantle upwelling evidence for a passive origin for the red sea rift by a v mcguire and r g bohannon

the arabian continental alkali basalt province part i evolution of harrat rahat kingdom of saudi arabia the harrat rahat lava field forms a major component of an extensive but poorly known continental alkali basalt province extending from yemen in the south through saudi arabia and jordan to syria in the north the continental intraplate volcanism which produced the arabian lava fields harrats was contemporaneous with opening of the red sea collision of the arabian and eurasian plates and uplift of the afro arabian dome harrat rabat has evolved over the past 10 m y it contains two prominent lateritic disconformities which separate three stratigraphic units the shawahit 10 2 5 ma hammah 2 5 1 7 ma and madinah 1 7 recent basalts comprising 68 19 and 13 of the total harrat volume respectively the shawahit basalt is composed of coarse grained dictytaxitic olivine transitional basalt otb and minor 10 alkali olivine basalt aob which erupted through scoria cones and shield volcanoes in contrast the hammah basalt is dominated by equal volumes of fine grained intergranular to intersertal aob and hawaiite which erupted mainly through scoria cones and relatively few shield volcanoes sparse domes and flows of mugearite and benmoreite first appear in the hammah basalt the madinah basalt spans the full compositional range of harrat rahat 103 analyzed samples comprise otb 8 aob 47 hawaiite 32 mugearite 4 benmoreite 8 and trachyte 2 here scoria cones dominate whereas shield volcanoes are sparse and occur only in the lower madinah basalt domes and associated pyroclastic deposits are common the final events of harrat volcanism include 11 post neolithic eruptions 4 500 yr old and two historical eruptions in 641 and 1256 a d arabian harrat magmatism may have been initiated by partial melting of garnet perido tite in the asthenosphere at a depth 100 km accumulation of this partial melt at the crust mantle boundary 37 44 km may have resulted in miocene uplift of the afro arabian dome harrat rahat volcanism began shortly after significant initial uplift with the voluminous extrusion of shawahit otb lava chemical and petrographic data suggest that these otb magmas ascended rapidly with little crustal residence time the data support a model of open system fractionation for aob and hawaiite lavas of the younger hammah and madinah basalts in this model some of the potential shawahit otb was trapped in crustal chambers during magma ascent crystal fractionation in these chambers was accompanied by their periodic replenishment by rising otb magma this resulted in the mixing of magma types and the development of aob and hawaiite magmas which are anomalously enriched in highly incompatible elements the mugearite benmoreite and trachyte lavas are the products of advanced fractionation in these crustal chambers with little or no magma replenishment

seismic gravity and magnetic surveys in the central part of the red sea their interpretation and implications for the structure and evolution of the red sea abstract while there is unanimous agreement that the axial part of the red sea has witnessed seafloor spreading since the early pliocene opinions diverge considerably with regard to the situation in the main trough and shelves new high precision data comprising palaeomagnetic seismic gravity magnetic and bathymetric measurements from the central and northern parts of the red sea have been examined to discriminate between the earlier proposed hypotheses and to formulate an evolutionary model for the red sea the results show that oceanic crust extends for about 80 km at 19 n on each side of the spreading axis symmetrical seafloor spreading magnetic anomalies are recognized out to anomaly 3a 6 m y b p between 18 30 and 19 45 n anomaly 2a between 19 45 and 20 n and near 21 n anomaly 2 near 22 n and only anomaly 1 near 23 30 n although anomaly 3a is the oldest recognizable magnetic anomaly the extent and configuration of the oceanic basement as determined from seismic measurements indicate that spreading has started and continued without a hiatus since 10 12 m y b p prior to the initiation of oceanic accretion an extension of 40 55 km i e 50 70 has taken place in the proterozoic continental lithosphere whereas a separation of 70 160 km has occurred during the seafloor spreading phase several lines of evidence indicate that the shelves and the marginal parts of the main trough are of continental nature

fission track analysis of basement apatites at the western margin of the gulf of suez rift egypt evidence for synchroneity of uplift and subsidence fifty six apatite fission track ages and 52 horizontal confined track length measurements are reported from precambrian crystalline rocks along the western margin of the gulf of suez egypt ages fall in the range of ca 11 385 m y and older ages often occur within very close geographic proximity to younger ones indicating non uniform uplift the wide range in ages is accompanied by a systematic variation in the distribution of horizontal confined fission track lengths on the basis of apatite fission track ages and their length distributions data fall into three distinct groups group i ages ranging from 43 to 385 m y length distributions are all positively skewed and with decreasing age become progressively broader with shorter mean track length group ii ages ranging from 23 to 31 m y length distributions are negatively skewed with either a distinct tail or a small peak of short tracks group iii ages ranging from 11 to 20 5 m y length distributions are al unimodal narrow negatively skewed and have the longest mean lengths among samples studied apatite ages from groups i and ii are interpreted as mixed ages as a result of cooling during uplift from different levels within the apatite partial track annealing zone ages from group iii are interpreted as cooling ages due to uplift from the apatite total track annealing zone with minor partial annealing correcting the ages of the two oldest samples in this group for track length reduction yields ages of21 2 2and23 1 5m y it is proposed that the onset of rift flank uplift in the gulf of suez northern red sea area occurred between 21 and 23 m y ago fission track analysis in combination with subsidence data from the gulf of suez basin indicate that commencement of basement uplift postdate the start of rifting and is interpreted as evidence for passive rifting at the gulf of suez furthermore this uplift is contemporaneous with and is directly related to the process of extension and subsidence at the gulf of suez

geothermal measurements in the northern red sea implications for lithospheric thermal structure and mode of extension during continental rifting the northern red sea is a continental rift in the process of transition from continental to oceanic rifting we present 191 new heat flow measurements from the northern red sea forming three traverses across the water covered portion of the rift the heat flow across the rift systematically increases from values of about 125 mw m2 seaward of the coasts to average values greater than 250 mw m2 in the axial depression the heat flow measurements are evaluated for environmental disturbances these are found to be generally small the largest estimated disturbance results from the relief of the seafloor and of the top of a subbottom evaporite layer the relief on these surfaces can account for the 20 point to point scatter typically observed in the heat flow measurements limits are placed on systematic disturbances to the heat flow pattern across the rift the estimated largest systematic disturbance results from sediment blanketing which may cause a reduction in the heat flow on the order of 10 the heat flow variation across the rift is then utilized to examine the lithospheric thermal structure and the geometry and mechanism of extension we employ a two dimensional time dependent numerical technique to follow the advection and diffusion of heat for a simple shear model and various pure shear models of lithospheric extension the model of simple shear lithospheric extension along a planar shallow dip detachment produces significantly lower than observed heat flow results in relatively small amounts of thermal thinning of the lithosphere compared to pure shear extension and does not result in conditions likely to generate melt the pure shear models with widening or constant width zones of extension also do not match the observed heat flow nor generate partial melt the modeling study indicates that the actively extending region must become narrower through time in order to match the heat flow high in the center of the rift generate some degree of partial melt and produce crustal subsidence within the observed width of the rift a narrowing of the zone of active extension is also consistent with the pattern of tectonic activity inferred from sedimentary structures observed on seismic reflection profiles which indicate an abatement of activity in the marginal areas and intense present day activity in the axial depression in all of the models an additional component of lithospheric heating other than that produced by extension appears necessary to match heat flow values in the marginal and near coastal areas as well as explain the observed magnitude of rift flank uplift several scenarios in which the zone of extension eventually becomes narrow are consistent with the heat flow data alone including those resulting in seafloor spreading for the past several million years however additional information provided by gravity magnetic and seismic reflection and refraction data appear to be most consistent with a history in which a period of relatively uniform extension across the rift is followed by a concentration of extension toward the center of the rift such a model retains a small thickness of continental crust in the center of the rift and does not necessitate active seafloor spreading in the northern red sea the three heat flow traverses indicate that maximum lithospheric thinning is occurring within the axial depression in the northern red sea which is very close to initiating seafloor spreading there is evidence that upper crustal extension is locally concentrated in specific areas along the strike of the axial depression forming deeps associated with large recent intrusions these deeps may constitute a precursory stage to seafloor spreading and appear to be nucleating at discrete and regularly spaced sites along the axis of the rift the deeps are located midway between regularly spaced cross trending fault zones or accommodation zones which in the marginal areas separate sets of fault blocks along strike crustal extension appears to be taken up more diffusely in the accommodation zones than in the intervening fault block domains which are characterized by fewer larger faults as a result initial crustal rupturing and emplacement of large intrusive bodies occurs halfway between the accommodation zones where more focused crustal extension forms the deeps comparison with other rifts suggests that the pattern of fault block domains separated by cross trending fault zones is a common feature that it originates early in the development of a rift and that it may continue to influence the development of the rift up to the establishment of a continuous seafloor spreading axis

timing of mantle upwelling evidence for a passive origin for the red sea rift mantle xenoliths from western saudi arabia some 220 km east of the red sea margin indicate temperatures as high as 900 c in the mantle at about 40 km depth beneath an area of the arabian shield where surface heat flow is lower than the world average this suggests that there has been a large increase in the mantle temperature that has not had time to equilibrate at the surface the amount of uplift and the difference between the observed xenolith temperature and that obtained by projecting the surface heat flow to depth suggests that the lithosphere has been thinned by 30 to 100 km to its present calculated thickness of about 80 km calculations of the time necessary for thermal reequilibration indicate that the lithospheric thinning probably occurred less than 20 m y ago so it postdates rifting which started about 30 ma if there was a prolonged upwelling of deep mantle material resulting in a thinner lithosphere prior to rifting there should have been enough time for some reequilibration at the surface and the observed heat flow should be higher over the broad area of uplift adjacent to the red sea our calculations are consistent with a postrift age of uplift adjacent to the red sea as determined from regional geologic evidence and fission track ages on apatites from shield rocks the red sea appears to have begun with alkaline volcanism on the arabian shield followed by rifting uplift and lithospheric thinning adjacent to the red sea rift began 5 10 m y after rifting this suggests that the red sea is a passive rift that formed because of two dimensional plate motions rather than as a consequence of mantle upwelling a possible cause for the stress in the plates might have been the change of plate motions in the indian ocean that resulted from the collision of india with eurasia

tertiary anorogenic granites of the western border of the yemen plateau north yemen y a r was affected from 30 to 20 ma ago by intense magmatic activity trap series contemporary with the main tensional tectonic phases of the afro arabian plate in the trap activity two main phases have been recognized the former mainly basaltic between 30 and 26 ma and the second mainly acidic between 23 and 20 ma during the latter period of activity a basaltic dyke swarm and stock like bodies and dykes granophyres were emplaced along the escarpment and coastal plain some granitic bodies intruded the trap series towards the end of the first phase and during the second one on the basis of geochemical and petrological data the tertiary granites of yemen belong to alkaline or peralkaline associations of a type the textural characteristics indicate that the granites are either hypersolvus or subsolvus the mafic mineral assemblages allow three main types of granites to be distinguished by their different environments of crystallization as regards the origin of these kinds of granites several hypotheses account for the peculiarities observed the lack of crustal xenoliths the preliminary data on the sr isotopic ratio the data on major and trace elements point to a direct origin by fractional crystallization without substantial crustal interaction this agrees well with the strong tensile tectonics linked with the uplift of the afro arabian dome and with the lithosphere thinning which provide a minimal opportunity for interaction of the magmas with the crust

structure and evolution of the northern red sea combined analyses of new geophysical data in the northern red sea with data from the gulf of suez and the gulf of aqaba dead sea rift strongly suggest that the development of the red sea took place in three stages 1 a gulf of suez stage in the oligocene accompanied by thinning of the continental lithosphere and the formation of an early red sea gulf of suez graben 2 an early dead sea stage with slow seafloor spreading from 25 to 15 myr accompanied by 62km of shear along the aqaba dead sea transform and 3 a recent dead sea stage with slow seafloor spreading from 4 5 to 0 myr accompanied by 45km of shear along the aqaba dead sea transform

angle dependent reflectivity by means of prestack migration most present day seismic migration schemes determine only the zero offset reflection coefficient for each grid point depth point in the subsurface in matrix notation the zero offset reflection coefficient is found on the diagonal of a reflectivity matrix operator that transforms the illuminating source wave field into a reflected wave field however angle dependent reflectivity information is contained in the full reflectivity matrix our objective is to obtain angle dependent reflection coefficients from seismic data by means of prestack migration multisource multioffset after downward extrapolation of source and reflected wave fields to one depth level the rows of the reflectivity matrix representing angle dependent reflectivity information for each grid point at that depth level are recovered by deconvolving the reflected wave fields with the related source wave fields this process is carried out in the space frequency domain in order to preserve the angle dependent reflectivity in the im

the yemen trap series genesis and evolution of a continental flood basalt province abstract the yemen basaltic plateau trap series was built up by fissure eruptions related to tensional tectonics in the afro arabian plate two main phases of activity have been identified and dated between 30 and 26 and 23 and 19 ma respectively the first phase was dominated by eruptions of alkaline to transitional basalts with a few rhyolitic ignimbrites during the second phase rhyolites with minor transitional basalts were erupted in the southern and southwestern part of the area geochemical and isotopic data indicate that the basalts are the result of combined processes of crystal fractionation and crustal contamination on mantle derived parental liquids the degree of crustal contamination generally decreases from south to north

polarization based wave equation migration velocity analysis in acoustic media abstractthe source extrapolation step in wave equation prestack reverse time migration gives wavefield polarization information which can be used to generate angle domain common image gathers adcigs from seismic reflection data from acoustic media concatenation of p wave polarization segments gives wavefield propagation paths wavepaths which are similar to the raypaths in ray based velocity tomography the adcigs provide residual depth moveout rmo information from which a system of linear equations is constructed for tomography to solve for the velocity ratio used for velocity updating an empirical relation between the rmo data and the velocity ratio updates reduces the amount of computation and is stabilized by the feedback provided by the iterative loop through prestack migration to rmo to velocity update to prestack migration correcting the rmos to flatten the adgigs is the convergence condition synthetic data for a layered model with a fault successfully illustrates the method

geological evolution of the afro arabian dome abstract the afro arabian dome includes the elevated continental regions enclosing the red sea gulf of aden and the ethiopian rift system and extends northwards as far as jordan it is more than an order of magnitude larger than other african uplifts both the structures and the igneous rocks of the dome appear to be products of the superimposition of two perhaps three semi independent generating systems initiated at different times but all still active a strain pattern dominated by nw trending basins and rifts first became established early in the cretaceous by the end of the oligocene much of the extensional strain had been taken up along the red sea and gulf of aden axes which subsequently developed into an ocean palaeogene trap volcanism of mildly alkaline to transitional character was related to this horizontal extension rather than to doming further west the east sahara swell has a history of intermittent alkaline volcanicity which began in the mesozoic and was independent of magmatism in the afro arabian dome volcanicity specifically related to doming began in the miocene along a n s zone of uplift extending from ethiopia to syria this elongated swell forms the northern termination of the east african system of domes and rifts characterized by episodic vertical uplift but very little extension superimposition of epeirogenic uplift upon structures formed by horizontal extension took place in the neogene volcanicity related to vertical tectonics is mildly alkaline in character whereas transitional and tholeiitic magmas are found along the spreading axes

facies and sequence stratigraphy of eocene palaeovalley fills in the eastern eucla basin south australia abstract the eocene succession filling palaeovalleys in the northeastern eucla basin south australia is interpreted using facies and sequence stratigraphic models based on relative sea level changes the dominantly fluvial sediments were deposited in incised valleys which graded basinwards to an estuarine coastal plain under warm and humid palaeoclimatic conditions sedimentological examination suggests a tidal influence in this fluvial succession fluvial estuarine shoreline facies associations can be recognised in these eocene sequences each of which comprises a diverse assemblage of lithofacies that can be grouped into lowstand and or transgressive and highstand system tracts since the palaeorivers had hydrological connection with the sea deposition was dominantly controlled by sea level changes results of the study indicate that two third order eocene eustatic cycles have largely controlled sedimentation the resulting key surfaces unconformity and transgressive tidal wave ravinement and maximum flooding surfaces bound depositional sequences which extend over significant areas and may be used in basin wide correlations of stratal packages

offset and angle domain common image point gathers for shot profile migration prestack depth migration of shot profiles by downward continuation is a practical imaging algorithm that is especially cost effective for sparse shot wide azimuth geometries the interpretation of offset as the displacement between the downward propagating shot wavefield and upward propagating receiver wavefield enables us to extract offset domain common image point cip gathers during shot profile migration the offset domain gathers can then be transformed to the angle domain with a radial trace mapping originally introduced for shot geophone migration the computational implications of this procedure include both the additional cost of multioffset imaging and an implicit transformation from shot geophone to midpoint offset coordinates although this algorithm provides a mechanism for imaging angle dependent reflectivity via shot profile migration for sparse shot geometries the fundamental problem of shot aliasing may severely impact the quality of cip gathers

angle domain common image gathers for migration velocity analysis by wavefield continuation imaging we analyze the kinematic properties of offset domain common image gathers cigs and angle domain cigs adcigs computed by wavefield continuation migration our results are valid regardless of whether the cigs were obtained by using the correct migration velocity they thus can be used as a theoretical basis for developing migration velocity analysis mva methods that exploit the velocity information contained in adcigs we demonstrate that in an adcig cube the image point lies on the normal to the apparent reflector dip that passes through the point where the source ray intersects the receiver ray the image point position on the normal depends on the velocity error when the velocity is correct the image point coincides with the point where the source ray intersects the receiver ray starting from this geometric result we derive an analytical expression for the expected movements of the image points in adcigs as functions of the traveltime perturbation caused by velocity errors by applying this ana

thermal origin of the hawaiian swell heat flow evidence and thermal models the lithospheric reheating hypothesis for the origin of the hawaiian swell has been tested by heat flow measurements along the swell between hawaii and midway ninety five measurements at eight carefully selected sites show that the heat flux on the swell increases systematically from near normal values 52 2 mw m 2 near hawaii to values near midway 59 2 mw m 2 about 20 25 higher than normal for lithosphere of that age 110 m y this heat flow pattern is qualitatively consistent with a deep seated thermal anomaly created in a lithospheric plate moving across a fixed heat source modeling of the heat flow and bathymetry data on the hawaiian swell strongly suggest that the swell is caused by a rapid heating within 40 to 50 km depth of normal 90 m y old lithosphere over a relatively broad 500 1000 km region near hawaii this model is also consistent with gravity and geoid anomalies associated with the swell which require a low density region in the lower lithosphere and with flexure studies of seamounts in the hawaiian chain which indicate that the rigidity of the lithosphere is not significantly reduced by the reheating appendices a c are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 005 1 00 payment must accompany order

age and petrology of the tertiary as sarat volcanic field southwestern saudi arabia abstract harrat as sarat forms the second smallest and southernmost of the basalt fields of western saudi arabia and is part of a voluminous red sea rift related continental alkali basalt province the rocks of the as sarat were emplaced during the first stage of red sea rifting and represent the northernmost extension of the tertiary trap series volcanics that occur mainly in the yemen arab republic and ethiopia the field consists of up to 580 m of basalt flows that are intruded by basaltic plugs necks minor dikes and highly evolved peralkaline trachyte intrusions k ar ages indicate that the as sarat field formed between 31 and 22 ma and contains an eruption hiatus of one million years that began about 25 ma ago pre hiatus flows are primarily hypersthene normative intersertal subalkaline basalt whereas the majority of post hiatus flows are nepheline normative alkali basalt and hawaiite with trachytic textures normative compositions of the basalts are consistent with their genesis by partial melting at varying depths trace element abundances in the basalt indicate that varying degrees of partial melting and fractional crystallization or crystal accumulation had major and minor roles respectively in development of compositional variation in these rocks modeling indicates that the pre hiatus subalkaline basalts represent 8 10 percent mantle melting at depths of about 70 km and the post hiatus alkali basalts represent 4 9 percent mantle melting at depths greater than 70 km

true amplitude angle domain common image gathers from one way wave equation migrations true amplitude wave equation migration provides a quality migrated image of the earth s interior in addition the amplitude of the output provides an estimate of the angular dependent reflection coefficient similar to the output of kirchhoff inversion recently true amplitude wave equation migration for common shot data has been proposed to generate amplitude reliable shot domain common image gathers in heterogeneous media we present a method to directly produce angle domain common image gathers from both common shot and shot receiver wave equation migration generating true amplitude shot domain common image gathers requires a deconvolution type imaging condition using the ratio of the upgoing and downgoing wavefield each downward projected to the image point producing true amplitude angle domain common image gathers requires instead the product of the upgoing wavefield and the complexconjugate of the downgoing wavefield in the imaging condition since multiplication is a more stable comput

rifts and swells geophysical constraints on causality abstract analyses are developed to examine whether rifting can generate broad topographic swells and whether swells formed by lithospheric thinning can generate rifts although swells can be supported by deep lithospheric cracks filled with low density intrusions observed topography and gravity constrain how this might occur single deep dikes apparently cannot explain the width of swells the low gravity gradients on swell flanks or the lack of a large negative free air anomaly over swell crests if swells are supported by deep cracks there must be numerous narrow dikes distributed over the full breadth of each swell comparing the hydrostatic pressure of various columns of lithosphere to that of an ocean ridge crest suggests the conditions under which a swell might spread apart oceanic basins are under deviatoric horizontal compression due to ridge push the uplift caused by lithospheric thinning will reduce this compression but because swells never rise higher than ridge crests oceanic swells should not develop deviatoric tensile stresses continental platforms are under less compression so formation of a continental swell can generate significant rifting forces

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

constraints on arabian plate motion and extensional history of the red sea analysis of the geologic and geophysical data from the western and northern margin of the arabian plate combined with data from the red sea gulf of aden leads to an interpretation of arabian plate motion that supports an episodic extensional history in the red sea and gulf of aden phase 1 extension began in medial to late eocene and was caused by the nw propagation of the reoriented central indian ridge spreading center it stretched and thinned continental crust along numerous wrench and normal faults that reactivated preexisting structures first phase extension stopped when the arabian plate terminally sutured to eurasia in the medial miocene and halted northward movement of arabia relative to africa the combined african arabian plates moved north more slowly constricting and thickening continental crust in the bitlis zagros sutures while the red sea remained technically quiescent and accumulated salt by the early pliocene continued northward movement was accommodated by lateral extrusion of large continental wedges north of the bitlis zagros sutures this allowed arabia to move northward faster than africa and reopen the red sea where phase 2 extension was expressed by seafloor spreading extension within the red sea gulf of aden rift system has been controlled by the interactions and effects of two factors 1 convergent processes between the north margin of the arabian plate and the eurasian plate to the north and 2 northwest propagation of extension associated with the medial eocene reorganization of the central indian spreading center

the rotation of arabia and the levant fault system abstract we establish the kinematics of the rotation of arabia with respect to africa and show that the rotation rate of africa increased by a factor of about four in late serravallian time about 12 13 ma ago we discuss evidence that oceanic accretion started simultaneously in the gulf of aden and in the red sea at this time it is also at this time that the levant and anatolian fault systems were initiated our initial reconstruction of the red sea rift in late oligocene early miocene time approximately superposes the present shore lines which coincide with the mid point of the zones of rapid crustal thinning the future continental slopes the amount of extension during the early phase of continental rifting is obtained by assuming that the total volume of continental crust is conserved during extension and by checking these estimates through closure around the sinai triple junction in the late miocene the southern evant fault zone was a zone of distributed shear deformation through counterclockwise block rotation extending soul h ward within the northernmost red sea since the early pliocene the shear motion has been localized within the gulf of aqaba changing the tectonic pattern in the northern red sea

angle domain common image gathers from anisotropic migration i present a general methodology for computing angle domain common image gathers adcigs in conjunction with anisotropic wavefield continuation migration the method is based on transforming the prestack image from the subsurface offset domain to the angle domain using slant stacks the processing sequence is the same as that for computing adcigs for the isotropic case though the interpretation of the relationship between the slopes measured in the prestack image and the aperture angles is more complex i demonstrate that the slopes measured by performing slant stacks along the subsurface offset axis of the prestack image provide a good approximation of the phase aperture angles and they are exactly equal to the phase aperture angles for flat reflectors in vertical transversly isotropic vti media in the general case of dipping reflectors the angles computed using slant stacks can be easily corrected by applying the relationships that i present in this paper and the accurate aperture angles can be determined as a function of the reflector dip and anisotropic slowness at the reflector i derive these relationships from both plane wave and ray viewpoints this theoretical development links the kinematics in adcigs with migration velocity errors i apply the proposed method to compute adcigs from the prestack image obtained by anisotropic migration of a 2d line recorded in the gulf of mexico i analyze the error introduced by neglecting the difference between the true phase aperture angle and the angle computed through slant stacks showing that at least for this data set these errors are negligible and can be safely ignored in contrast group aperture angles can be quite different from phase aperture angles thus ignoring the distinction between these two angles can be detrimental to practical applications

genesis of collision volcanism in eastern anatolia turkey late cenozoic volcanism in eastern anatolia extends in a broad sw ne trending belt across the arabia eurasia collision zone from the arabian foreland basin in the southwest and to the kars plateau and lesser caucasus in the northeast foreland volcanism is dominated by basaltic shield and fissure eruptions of transitional tholeiitic alkaline composition volcanism on the thickened crust north of the bitlis thrust zone varies from the mildly alkaline volcano nemrut and older mus volcanics in the south through the transitional calc alkaline alkaline volcanoes bingol and suphan and the alkaline volcano tendurek to the calc alkaline volcano ararat and older kars plateau volcanics in the north isotope sr nd and trace element systematics indicate that the lavas from the foreland were derived from the mantle lithosphere of the arabian continent which had been enriched by small volumes of asthenospheric melts over a period of time and that lavas from the alkaline volcanic area around mus and the volcanoes nemrut and tendurek north of the bitlis thrust zone were derived from a lithospheric source of similar composition either from the same underthrust arabian continent or from the bitlis massif microcontinent by contrast the transitional lavas from bingol and suphan and the calc alkaline lavas from ararat and kars were derived from lithosphere carrying a distinct subduction signature inherited from pre collision subduction events positive correlations between 87sr 86sr and sio2 and rb nb and sio2 in the alkaline and transitional lavas suggest that combined assimilation and fractional crystallization was an important process within at least part of the thickened crust of the collision zone trace element covariation diagrams such as y rb indicate the importance of hornblende crystallization at depth and orthopyroxene at shallow levels within the calc alkaline provinces in contrast to the consistently anhydrous crystallization sequences of the alkaline lavas trace element diagrams based on the covariation of compatible and incompatible elements point to moderate low degrees of partial melting with residual clinopyroxene throughout and residual garnet in the foreland province consideration of mineral stabilities mantle solidi and geothermal gradients before and after collision suggest that lithospheric thickening should both increase the thickness of metasomatized lithosphere and depress the metasomatized zone to greater depths probably beneath the amphibole and dolomite breakdown curves perturbation of the thickened lithosphere by delamination of the thermal boundary layer perhaps coupled with local stretching associated with pull apart basins on strike slip fault systems is then sufficient to generate melt the composition of that melt being largely dependent on the enrichment history of the lithosphere in question in eastern anatolia volcanism appears to have started between about 8 and 6 ma ago some 5 ma after the start of rapid uplift of the east anatolian plateau

crustal structure and the mechanism of extension in the northern red sea constraints from gravity anomalies gravity data from detailed marine surveys were used to investigate the density structure of the crust and upper mantle beneath the northern red sea a continental rift considered by many to be in the process of establishing a mid ocean ridge type spreading center a gravity and bathymetry profile across the sea near 27 n was analyzed in detail model constraints include single channel and multichannel seismic reflection lines coincident with the gravity profile which provide detailed information on sedimentary layers a series of esp seismic experiments centered nearly on the gravity line which constrain the sediment and crustal thickness and a series of 58 closely spaced heat flow measurements nearly coincident with the gravity line which closely constrain the mantle thermal structure and thus density variations within the mantle the calculated gravity anomalies based on the known crustal and mantle structure fail to match the observed data specifically the model anomalies become steadily more negative from the margins toward the center of the sea with the result that the calculated gravity is approximately 25 mgal more negative near the axis relative to the marginal areas than are the observed anomalies two possible solutions were investigated one is to introduce a lower crustal layer resulting from crustal underplating within what was originally considered to be the upper mantle beneath the marginal areas the other is to introduce an area of slightly denser crust resulting from intrusion of new mafic crustal material within the axial depression both solutions can reproduce the observed gravity anomalies within the red sea however the complete set of geophysical data and in particular the heat flow in the red sea and gravity and seismic refraction data on land suggests that the most satisfactory model is that the crust of the axial depression is denser than that of the marginal areas this implies that crustal extension in the northern red sea is presently occurring in large part by intrusion of new mafic material

wave equation angle domain common image gathers for converted waves wavefield extrapolation methods can produce angle domain common image gathers adcigs to obtainadcigs for converted wave seismic data information about the image dip and the p to s velocity ratio must be included in the computation of angle gathers theseadcigs are a function of the half aperture angle i e the average between the incidence angle and the reflection angle we have developed a method that exploits the robustness of computing 2d isotropic single modeadcigs and incorporates both the converted wave velocity ratio and the local image dipfield it also maps the final converted wave adcigs into two adcigs one a function of the p incidence angle and the other a functionofthes reflectionangle resultswithbothsyntheticand real data show the practical application for converted wave adcigs the proposed approach is valid in any situation as long as the migration algorithm is based on wavefield downward continuation and the final prestack image is a function ofthehorizontalsubsurfaceoffset