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

study of heat flow in the sengan hachimantai region northeastern honshu japan abstract in the sengan hachimantai region of northeastern honshu more than 100 shallow heat flow holes with depths ranging from 80 to 250 m have penetrated volcanic lavas and tuffs of andesitic to rhyolitic compositions as well as older sediments it is generally difficult to determine the heat flow for those holes because their temperature depth curves seldom show linear conductive patterns due to the large irregularities in these curves it is not easy either to clearly determine the physical processes responsible for the data obtained at individual heat flow sites therefore the temperature as well as the thermal conductivity of these holes has been discussed collectively area by area the kowase gawa area shows higher than normal conductive heat flow whereas four other areas show scattered but extremely high heat flows differences in the areal extent of extremely high heat flow 400 mw m 2 among the five areas in the sengan hachimantai region may be explained by assuming different structures and sizes of the geothermal systems results of subsequent deep drilling to verify the above inference are presented the background heat flow value is probably about 150 mw m 2 which was estimated at a 2000 m deep well in the kowase gawa area this high regional heat flow value is consistent with other geophysical data anomalously low p wave velocity and high electrical conductivity below the region these lines of evidence indicate the existence of a partially molten zone in the crust

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

gradual encroachment of a rocky shoreline by an invasive sea during the mississippian at the southeastern margin of the maritimes basin nova scotia canada onlap of a rocky shoreline by marine beds of the mississippian windsor group occurred at the southeastern margin of the composite upper paleozoic maritimes basin in central nova scotia the sedimentology of thin basal rudaceous deposits suggests that although the transgression was rapid there were times of prolonged shoreline stabil ity boulder beach deposits indicate a high energy coastline and a large marine basin clastic delta deposits were proxi mal to thick biohermal banks in shallow areas near the southeast margin of the sea while sulphate deposition was occurring basinward according to their contained biota and stratigraphic relationships the marginal banks evolved in a less saline environment than banks forming away from river inputs on paleotopographic highs located farther offshore because diversity of the biota decreases with an increase in both water depth and distance from the paleoshore and because bank development in shallow or marginal areas was apparently synchronous with evaporitic deposition in deeper or less marginal areas we conclude that both a vertical and a lateral gradient of salinity existed due to the thrusting of fluvial fresh water above salt water and its infiltration along the paleoshoreline as salinity increased with time bank building eventually aborted and sulphate deposition gave way to salt deposition in the deepest parts of the basin lastly the onlap of evaporites above marginal clastic deposits indicates that evaporite deposition occurred in a transgressive context although a gradual thinning of the water column may have simultaneously occurred due to basin infilling

a method for determining terrestrial heat flow in oil basinal areas abstract a method for determining the terrestrial heat flow has been studied in the oil gas basinal areas in japan to estimate the heat flow an available zone was selected from a geologic section observed in a well the selected zone is covered by many bottom hole temperatures sufficient to determine a reliable geothermal gradient the bottom hole temperature was converted into an equilibrium formation temperature mainly by using the simple equation obtained from the relations between bottom hole and reservoir temperatures the thermal conductivity of the zone was primarily determined by both the measurements of well core samples and the experimental relations of thermal conductivity to porosity of some specific rocks then the conductivity was adjusted to subsurface temperature by using the empirical equation deduced from the relations of thermal conductivities of materials to temperatures

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

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

geochemistry and volcanic features of harrat el fahda a young volcanic field in northwest arabia jordan abstract the wadi fill basalt of harrat el fahda was derived from the youngest volcanic activity in northeast jordan and forms one of the most recent harrats of the arabia plate it is characterized by the presence of abundant well preserved pressure ridges and pahoehoe lava structures it was developed due to intermittent eruption from an isolated volcanic center in a form of 3 5 successive basaltic pahoehoe flows the associated pressure ridges are elliptical in shape up to 50 m high and 200 m long the orientation with regard to flow direction is sub parallel along the margins and perpendicular along the center of the flow these ridges were formed by inflation and heaving pressure from below petrographical data show that the basalt is hollocrystalline fine grained and golmeroporphyritic modally three petro types are recognized olivine plagioclase basalt 20 olivine pyroxene basalt 25 and olivine basalt 55 geochemically the three petro types can be classified as alkaline to sub alkaline and termed as alkali olivine basalt they have similar compositional ranges of major and trace elements the magmas may have been derived from low degrees i e 5 9 of partial melting of an enriched garnet peridotite mantle source with limited fractionation and crustal contamination

ordovician paleotopography as evidenced from original dips and differential compaction of dolostone and shale unconformably overlying precambrian basement on manitoulin island canada abstract the middle to basal upper ordovician lindsay formation unconformably overlies the paleoproterozoic bar river and lorrain formations on manitoulin island ontario canada representing a depositional hiatus of at least 1 7 billion years the paleoproterozoic inliers currently exposed as basement highs in the sheguiandah region comprised a 14 km2 area of quartz arenite archipelago during the middle to late ordovician inland sea transgression of north america the quartz arenite outcrop collectively covers an area of approximately 1 8 km2 high energy transport and reworking during wave and tropical storm activity along a high relief rocky shoreline is indicated by dolostone containing 2 5 m size rounded to subrounded quartz arenite boulders with percussion marks up to 4 5 m size subangular quartz arenite blocks consistent with mechanical breakage along joints fractures or bedding planes lenticular conglomerate beds and cracks in quartz arenite basement containing mixtures of dolostone quartz granules and fossiliferous debris with continued transgression organic rich carbonate muds were deposited under low energy conditions either on carbonate strata or directly on quartz arenite basement in restricted regions protected from the open sea clast size distribution in the dolostone clast rounding and the location of the shale quartz arenite unconformity relative to basement highs are all consistent with a longshore current running from the present northeast to southwest with the leeward sides of rocky islands protected from high wave and storm energy processes subsequent deposition of upper ordovician and silurian strata resulted in burial and differential compaction of the lindsay formation units the compaction may have been associated with fractures through which mg rich fluids migrated locally altering the lowermost lindsay formation to dolostone evidence for differential compaction is provided by bed thickening down dip of basement highs and boulder flanks and higher regional dips in shale beds proximal to basement highs compared with dolostone strata the relatively high angle strata of the lindsay formation are a result of the combined effects of paleotopography a high relief rocky shoreline and differential compaction

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

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

the timing of uplift volcanism and rifting peripheral to the red sea a case for passive rifting prior to the formation of the red sea the northeastern afro arabian continent had low relief and was largely below sea level from the late cretaceous to the early oligocene the events leading to the formation of the red sea followed the sequence 1 alkaline volcanism and rifting beginning about 30 32 ma affecting a narrow linear zone in the continent 2 rotational block faulting and detachment faulting well underway by 25 ma 3 gabbro and diorite magmatism andesite to rhyolite volcanism and fine grained nonmarine sedimentation in the rift between 20 and 25 ma 4 fine grained marine sedimentation in the rift as the early shelves started to subside in the middle miocene and 5 uplift of the adjacent continents about 3 km and subsidence of the shelves about 4 km between 13 8 and 5 ma the youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the proterozoic arabian shield that range in age from 13 8 to 568 ma the youngest of these ages coupled with the present high relief along the arabian escarpment and published heat flow measurements indicate that 2 5 4 km uplift has occurred in the last 13 8 m y the sequence volcanism rifting followed by uplift leads to our adoption of a passive mantle model for rift origin models that require uplift to create the rift are rejected because of the late uplift we advocate a model of lithospheric extension caused by two dimensional plate stress over those requiring tractional drag at the base of the lithosphere caused by vigorous flow in the asthenosphere it is acknowledged that traction models could explain the observed data but they imply a rigid static lithosphere and seem to require a link between the direction of flow in the asthenosphere and plate motions neither requirement is necessary in the extension model the rift starts with mechanical extension in a narrow zone of lithosphere between 25 32 ma in our model the thinned lithosphere is replaced by upwelling asthenosphere and by rocks from the adjacent deep continental lithosphere which flow into the rift ductile flow of the deep continental lithosphere is accelerated by partial melting as rocks flow upward toward the rift axis once partially melted rocks formerly part of the continental lithosphere join the upwelling asthenosphere resulting in a rapid erosion of the lithospheric mantle beneath the continent near the rift edge the resulting density decrease explains the uplift we think that the red sea began as a consequence of changing plate geometries resulting from the collision of india and eurasia after the collision the segment of the owens fracture zone north of the carlsberg ridge became locked forcing the northeast corner of afro arabia to rotate with the indian plate away from the rest of africa

seismic study of the crust of the northern red sea and gulf of suez we report the results of fifteen expanding spread profiles esp s and a seismic wide angle reflection refraction une performed during march april 1986 in the gulf of suez and the egyptian part of the northern red sea area north of 25 n four 16 4 air guns were used as a sound source on board r v le suroit and a 96 channel 2 4 km long streamer was towed by a supply vessel the whity tide most of the profiles show good crustal reflection and refraction arrivals and often good moho arrivals obtained for a distance of 80 to 100 km we present the results of x t and p analysis obtained by a velocity inversion performed in the p plane and by ray tracing modeling of both the p and the x t sections the velocity models are computed for planar and linear gradient velocity layers the northernmost part of the red sea appears to be characterized by a continental type crust extremely thinned 4 lying at a mean depth of 7 8 km whereas the moho is at a mean 14 15 km depth the southern part shows a seismic velocity structure of an oceanic type except in the 40 km closest to the coastline in both parts seismic waves progressively get more attenuated with distance from the shore to the axial zone which is presently tectonically active the difference between the northern continental and southern oceanic zone is related to the termination of the levant fault the northern continental area appears to represent the termination of the levant fault as a zone of distributed deformation

geometry of propagating continental rifts reconstruction of past plate configurations through palinspastic restoration of continental margins1 4 requires a detailed knowledge of the geometries of horizontal continental extension extension is often restricted to discrete linear zones termed rifts5 6 and rift models that invoke propagating vertical dykes and cracks7 10 are useful for assessing the relative significance and timing of uplift subsidence and extension in the tectonic evolution of rift basins however many models do not adequately account for the observed asymmetry of rifts i describe here the general three dimensional character of young and aborted continental rifts which can be used to derive a structural model for the propagation of rifts in continental lithosphere the rifts become asymmetric as a consequence of the role played by low angle normal faults in the overall rift geometry

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

surface effects and tectonic setting of the 13 december 1982 north yemen earthquake abstract the north yemen earthquake m b 6 0 of 13 december 1982 is the first earthquake in the southern arabian peninsula known to be accompanied by surface displacements the extensive destruction and loss of lives resulted entirely from widespread collapse of unreinforced masonry and mud brick structures maximum modified mercalli intensity was probably vii to viii the only surface manifestation of tectonic activity was the occurrence of earthquake related extensional ground cracks in the epicentral region the cracks occur mainly in four relatively continuous north to northwest trending linear zones that range from a few hundred meters to 15 km in length and in irregular areas of polygonal extension cracks the area within which the cracks occur is 22 5 km long by about 10 km wide continued dilation across some cracks was measured almost 1 month after the main shock seismicity and active volcanism in this region are inferred to reflect slow extension of the southwestern margin of the arabia plate perpendicular to the red sea spreading axis

sedimentology of the lowest windsor carbonate rocks base metal hosts in the maritimes basin of eastern canada the mississippian visean macumber and gays river formations of the windsor group in nova scotia are host to pb zn ba cu ag deposits transport and deposition of base metals were possible because of an efficient porosity permeability system developed in both units depositional facies is a key parameter controlling the potential openness of carbonate units the macumber facies were deposited after the collapse of the acadian orogen and ensuing tectonically driven sea level rise the macumber formation is divided into two regional lithosomes a lower thin unit 2 m of fine and coarse grained limestones deposited below the fairweather wave base positioned on an outer shelf with local chemosynthetic mounds and an upper thicker unit avg 10 m of planar bedded micritic limestones most likely representing upper slope microbial mats interbedded with deep marine sulfates the permeability system was created by various brecciation events affecting the upper part of the formation the gays river formation deposited upslope of the macumber facies is dominated by various organic mounds with intervening facies the boundstone facies are indicative of below fairweather wave base deposition roughly at the limit of the photic zone primary growth porosity of the mounds provided the plumbing system later used by mineralizing fluids these mounds also record the effects of a sea level rise culminating in regional deposition of deep marine sulfates outside the eastern margin of the laurentia craton a similar visean aged mixed limestone sulfate succession is known from the franco belgian basin in baltica distribution of shelf and slope facies within the visean mid european ocean separating laurentia baltica and gondwana suggests that restricted oceanic circulation in a rapidly closing sea was instrumental in the local occurrence of a salinity stratified water mass and hence responsible for the presence of below wave base marine limestone and sulfate facies on the laurentia and baltica continental margins

the mantle beneath the red sea margin xenoliths from western saudi arabia abstract xenoliths from alkali basalts in western saudi arabia provide the opportunity to study the composition and rheology of the mantle beneath the red sea rift margins characteristics of mantle xenolith suites from each of three localities in western saudi arabia can be related to locality position relative to the rift axis and to crustal thickness and heat flow at each locality mantle xenoliths from harrat al birk nearest the rift axis are dominantly websterites spinel plagioclase amphibole olivine garnet clinopyroxenite and two pyroxene gabbro olivine peridotite xenoliths are rare garnet clinopyroxenites contain zoned clinopyroxene with fe al rich rims and reaction rims on garnet formed by breakdown of garnet to orthopyroxene clinopyroxene spinel plagioclase zoning and reaction rims are interpreted as forming under conditions of increasing temperature thermobarometry on harrat al birk garnet bearing xenoliths yield high temperatures 1015 1040 c at about 12 kbar the abundance of plagioclase bearing assemblages may be related to a relatively shallow upper mantle which extends up into stability fields for plagioclase bearing pyroxenite and peridotite harrat al kishb and harrat hutaymah are farther from the red sea axis on the flanks of the rift the mantle xenolith suites of al kishb and hutaymah are similar consisting of abundant spinel peridotite and spinel pyroxenite xenoliths and minor garnet pyroxenites plagioclase bearing xenoliths are extremely rare the harrat hutaymah suite includes wehrlite and amphibole bearing peridotite lithologies not found at al kishb variation of peridotite composition may reflect varying degrees of partial melt extraction igneous textures of some pyroxenite xenoliths and structural relationships in composite peridotite pyroxenite nodules suggest that pyroxenites formed by crystallization of magmas within mantle veins abundant pyroxenites and fragments of amphibole veins reflect the activity of magmas and hydrous fluids within the mantle thermobarometry of al kishb and hutaymah garnet bearing nodules yield temperatures of 1000 1050 c at pressures of about 13 5 16 0 kbar mineral zoning and exsolution features indicate decreasing temperature conditions in the mantle at the flanks of the red sea rift

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

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

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

record of late mississippian tectonics in the new perce group visean of eastern gaspesie quebec visean clastic units and structures at the northwest margin of the upper paleozoic maritimes basin provide information on tectonic events that are only poorly recorded in more central parts of the basin these continental units are time equivalent to marine sediments of the windsor group of nova scotia they are herein assigned to the new perce group which includes the la coulee and bonaventure formations as well as a new unit the cap d espoir for mation the latter unit unconformably underlies the bonaventure formation in a small but thick sub basin of the ristigouche basin in eastern quebec it is characterized by a succession of sandstone and mudstone rhythmites that contrast with the coarse alluvial fan deposits of the overlying bonaventure formation the cap d espoir formation was sourced from a broad area of subdued topography occupied by the visean la coulee formation and underlying units erosional remnants of the la coulee formation are unconformably overlain by the bonaventure formation in marginal parts of the ristigouche basin whereas these units are separated by the cap d espoir formation in more central areas of the basin the la coulee and cap d espoir formations underwent tilting and erosion during a normal faulting event that preceded deposition of the fault controlled bonaventure formation this series of events is interpreted to represent different steps in the reactivation of a pre carboniferous dextral strike slip system in response to northwest southeast compression during the visean in gaspesie resume des unites clastiques viseennes et leurs structures intrinseques dans la marge nord ouest du bassin des mari times paleozoique superieur illustrent des evenements tectoniques qui ne sont que pauvrement enregistres dans les parties plus centrales du bassin ces unites continentales sont chronologiquement equivalentes aux sediments marins du groupe de windsor en nouvelle ecosse elles sont ici assignees au nouveau groupe de perce lequel inclut les forma tions de la coulee et de bonaventure ainsi qu une nouvelle unite la formation de cap d espoir cette derniere est re couverte en discordance par la formation de bonaventure a l interieur d un petit mais relativement epais sous bassin du bassin de ristigouche dans l est du quebec elle est caracterisee par une succession de depots rythmiques de gres et de mudstone qui contraste avec les depots grossiers de cones alluviaux de la formation de bonaventure sus jacente la formation de cap d espoir est le produit de l erosion d une region source de grande etendue et a faible gradient topo graphique laquelle etait occupee par la formation de la coulee et les unites qui lui sont sous jacentes les reliques d erosion de la formation de la coulee sont recouvertes de facon discordante par la formation de bonaventure dans les parties marginales du bassin de ristigouche alors que ces unites sont separees par la formation de cap d espoir dans les parties plus centrales du bassin les formations de la coulee et de cap d espoir ont ete basculees et partiel lement erodees a l interieur d un evenement de fracturation ayant precede le depot de la formation de bonaventure dont les facies ont ete controles par des escarpements de failles cette serie d evenements est consideree comme etant le resultat de differentes etapes dans la reactivation d un systeme de coulissage dextre pre carbonifere en reponse a des contraintes orientees nw se durant le viseen en gaspesie

sedimentation pattern and tectonic evolution of the proterozoic singhbhum basin in the eastern indian shield abstract the proterozoic singhbhum basin psb located on the flank of an archaean platform ca 3400 ma is a distinct lithotectonic unit of the singhbhum crustal province of the eastern indian shield the present contribution traces the evolution of this basin filled by volcanics dominated submarine supracrustals and sedimentary formations the proterozoic singhbhum basin has been identified to be a marginal basin evolved in a back arc setting an axial volcanic belt manifesting mor like basaltic eruptions divided the basin into two domains with parallel but distinct sedimentation patterns an asymmetry in sedimentation pattern and source polarity are documented in litho as semblages and ree distribution patterns of cover rocks in the basinal domain flanking the southern platform prevailing tectonics strongly controlled the mechanism of sedimentation which coupled with the volcanic history of the basin indicates its riftogenic character progressive lithospheric stretching successively produced picritic volcanics and mor type basalts along the axial zone of the basin being related to a stretching factor of 2 continental margin rifting ultimately resulted in an oceanic transition as supported also by available geophysical data the evolution of the proterozoic singhbhum basin in its broad perspective was controlled by subduction related processes

magnetic evidence for the nature of the crust beneath the southern red sea magnetic data over the southern red sea are used to examine various models for the evolution of the region the linear nature of the magnetic anomalies over the coastal margins and adjacent portions of the main trough together with their decrease in amplitude with elevation is shown to be compatible with deep thin sources that are strongly magnetized the general lack of correspondence between gravity anomalies and deskewed magnetic anomalies indicates that the anomalies are due to intrabasement bodies rather than basement relief associated with density contrasts across axis correlations of individual anomalies indicate a symmetrical disposition of magnetic sources about a line close to the present deep water axis the regular symmetrical pattern of linear anomalies strongly suggests a seafloor spreading origin the combined geologic and geophysical evidence therefore supports a model in which the margins are underlain by material more typically oceanic in character than continental the magnetic evidence further implies that this oceanic crust formed during an earlier phase of spreading in the red sea although the age of this crust is unknown anomaly correlations suggest spreading may have taken place during the late oligocene early miocene

conodont biostratigraphy of the codroy group lower carboniferous southwestern newfoundland canada four stratigraphically successive conodont assemblage zones have been recognized in the lower carboniferous codroy group of southwestern newfoundland the diplognathodus zone is confined to the basal ship cove limestone and to a highly fossiliferous correlative at aguathuna the overlying taphrognathus zone occurs in carbonates in the stratigraphic interval above the sequence of massive sulphates and thick clastics the taphrognathus zone as well as the successive cavusgnathus zone has been recognized on fischells brook as well as in the complex section south of codroy the highest conodont zone the gnathodus zone has been recognized in the crabbes jeffreys limestone of the st george s bay area and from south of codroy the discovery of conodonts of the diplognathodus zone in marine strata that are stratigraphie and lithologie correlatives of the macumber and the gays river formations of nova scotia now makes it possible to microfaunally characterize the a subzone a macrofaunal subzone established

dike induced faulting in rift zones of iceland and afar geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the earth9s surface elastic modeling of the vertical ground surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at depth a mechanical model that includes dikes and frictional faults shows that dike opening tends to compress and lock faults located to either side of the dike therefore slip extending into or beyond the dike cavity must occur either 1 on faults that intersect the dike near its top above the zone of dike induced compression or 2 on faults that slip ahead of the dike as it propagates laterally data from iceland indicate that slip occurred on deep faults that presumably slipped in advance of the laterally propagating dike

moissanite sic with metal silicide and silicon inclusions from tuff of israel raman spectroscopy and electron microscope studies abstract here we present studies of natural sic that occurs in situ in tuff related to the miocene alkaline basalt formation deposited in northern part of israel raman spectroscopy sem and fib assisted tem studies revealed that sic is primarily hexagonal polytypes 4h sic and 6h sic and that the 4h sic polytype is the predominant phase both sic polytypes contain crystalline inclusions of silicon sio and inclusions of metal silicide with varying compositions e g si58v25ti12cr3fe2 si41fe24ti20ni7v5zr3 and si43fe40ni17 the silicides crystal structure parameters match si2tiv5 pm 3m space group cubic fesi2ti pbam space group orthorhombic and fesi2 cmca space group orthorhombic respectively we hypothesize that sic was formed in a local ultra reduced environment at respectively shallow depths 60 100 km through a reaction of sio2 with highly reducing fluids h2o ch4 h2 c2h6 arisen from the mantle hot spot and passing through alkaline basalt magma reservoir sio2 interacting with the fluids may originate from the walls of the crustal rocks surrounding this magmatic reservoir this process led to the formation of sic and accompanied by the reducing of metal oxides to native metals alloys and silicides the latter were trapped by sic during its growth hence interplate hot spot alkali basalt volcanism can now be included as a geological environment where sic silicon and silicides can be found

stratigraphic significance of upper devonian and lower carboniferous miospores from the type area of the horton group nova scotia late devonian late famennian miospores have been found in the lowermost 7 3 m of the horton group on harding brook in the type area windsor subbasin nova scotia below tournaisian miospores of the emphanisporites rotatus indotriradites explanatus zone extending the age of the horton group in the type area down into the latest devonian shows that the lowest beds of the 1000 m thick group are coeval with latest devonian rocks elsewhere in the maritimes basin that have been excluded from the horton group by some authors evidence presented here favours the argument that late devonian rocks lithologically similar to the horton group deposited on the acadian unconformity should be included in that group miospore evidence indicates a minimum age of about 355 ma for exhumation of the northeastern part of the south mountain batholith

stratigraphy and depositional environments of the upper mississippian codroy group port au port peninsula western newfoundland upper mississippian middle visean biohermal limestones siliciclastics evaporites and red beds of the lower codroy group correlative with the lower windsor group in nova scotia bevel a rugged paleokarst of late devonian early mississippian age on the port au port peninsula western newfoundland laminated limestones ship cove formation and overlying evaporites codroy road formation are similar to the sequence in the bay st george subbasin southwestern newfoundland in the port au port region this sequence is laterally equivalent to biohermal limestones and plant bearing siliciclastics of the proposed big cove formation and red beds of the lower cove formation lithostratigraphic correlation is supported by miospore and conodont biostratigraphies use of macrofaunal subzones is cautioned because there is evidence for the need to reevaluate the ranges of fossils that have previously been considered key indicators e g martinia galataea bell for parts of the codroy and windsor groups the na

composition of basaltic liquids generated from a partially depleted lherzolite at 9 kbar pressure two contrasting views exist regarding the nature and origin of mid ocean ridge basalts morb first that these olivine tholeiites with mg mg fe 0 70 0 72 are primary and are generated by partial fusion of plagioclase spinel lherzolite at around 30 km 9 kbar pressure in the mantle1 others2 8 suggest that even the most primitive morbs are not primary but are derived by fractionation of olivine pyroxene from picritic parental magmas that are generated at depths of 45 km or more duncan and green2 3 argued for the existence of a second primary magma a high mgo quartz normative or olivine poor tholeiite which is produced at depths 25 km beneath the ocean ridges this problem concerning the composition and depth of origin of primary morbs has been approached here by experimentally melting a plagioclase spinel lherzolite xenolith from hawaii at 9 kbar pressure the experimental melts are very similar to some mid ocean ridge basalts which suggests that the latter are probably primary and are generated at shallow depths in the mantle

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

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

continuous sea floor spreading in red sea an alternative interpretation of magnetic anomaly pattern the magnetic anomaly pattern over the red sea can be modeled as a continuous system of sea floor spreading from the early miocene to the present by using a time varying process filter the half spreading rate is approximately 1 cm yr 0 4 in yr since initial rifting the parameters that determine the process filter and development of the transition zone are the intrusion parameter a measure of the dispersion of feeder dikes or horizontal strain about the rift axis a flow parameter a measure of the average flow width and the effusion parameter a measure of the volcanic effusion and thickness of layer 2 application of a time varying process filter appears to be a powerful tool in analyzing magnetic anomalies over passive continental margins we estimate the flow parameter to be 2 7 km 1 7 mi and the intrusion parameter to be 7 5 km 4 7 mi at early rifting these values suggest that a wide distribution of axial dikes or horizontal strain is the dominant factor in forming the magnetic anomaly pattern reduction in the width of the intrusion parameter and the effusion rate as rifting proceeded resulted in focusing of the strain thinning of layer 2 and formation of the red sea deeps the interpretation of the anomaly pattern implies that basement within the main trough of the red sea is composed of a thick accumulation of volcanic sills and flows deposited during early rifting extrusion of the lavas occurred over a wide up to 60 km or 37 mi zone of strain distributed about the rift axis the development of this thickened zone of igneous crust is the second of three successive stages of extension for the red sea basin these stages are as follows 1 an initial stage of crustal stretching or listric faulting in the oligocene accompanied by probable regional uplift 2 a period of high volcanic effusion accompanied by the emplacement of stratoid basalts the stratoid volcanics form overlapping flows and sills with intercalated sediments within the main trough the spreading center narrowed from the early miocene to the present 3 steady state sea floor spreading from pliocene to present south of a transitional zone near 22 s the process filter width derived from the magnetic anomaly pattern at earliest rifting can be used to estimate the lateral variation from igneous to continental type crust in a statistical sense our data suggest that the transition from 100 continental crust to 100 igneous or stratoid type crust should occur within an interval of approximately 32 km 20 mi our modeling suggests that phase 2 or the stratoid phase began about the time of anomaly 5c or chron c5c approximately 16 ma this age is compatible with geologic estimates of the initial rifting at the late oligocene to early miocene coleman 1974 gass 1977 the opening rate for africa arabia plate motion has remained relatively constant since early rifting although the african margin appears to be accreting faster than the arabian plate

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

upper mantle structure beneath the arabian peninsula and northern red sea from teleseismic body wave tomography implications for the origin of cenozoic uplift and volcanism in the arabian shield upper mantle structure between 150 and 400 km depth is imaged beneath the arabian shield and northern red sea by modeling p and s traveltime residuals from teleseismic events recorded on the saudi arabia national digital seismic network the 1995 1997 saudi arabian passcal experiment and three permanent stations rayn eil and mrni relative traveltime residuals were obtained using a multichannel cross correlation method and inverted for upper mantle structure using vandecar s inversion method the resulting images reveal a low velocity region 1 5 for the p model and 3 for the s model trending nw se along the western side of the arabian shield and broadening to the northeast beneath the makkah madinah nafud volcanic line we attribute the low velocities to a mantle thermal anomaly that could be as large as 330 k and that is associated with the cenozoic uplift of and volcanic centers on the shield our tomographic images are not consistent with models invoking separate mantle upwellings beneath the northern and southern regions of the shield and instead favor single plume or superplume models we also find little evidence for low velocities beneath the northern red sea suggesting that there might not be a geodynamic link between rifting in the red sea and plateau uplift and volcanism in the shield

fine structure beneath the tohoku district northeastern japan arc as derived by an inversion of p wave arrival times from local earthquakes abstract three dimensional p wave velocity structure beneath the tohoku district northeastern japan arc is investigated by an inversion of arrival times from local earthquakes using the method originally due to aki and lee 1976 in the crust 0 32 km depth a low velocity region is found along the volcanic front and its vicinity velocities at depths of 32 65 km are low beneath the regions where many quaternary volcanoes and geothermal areas are distributed in the region deeper than 65 km the subduction of the pacific plate is clearly revealed and the mantle structure above the descending plate is rather uniform these features suggest that volcanic activities have relation to the upper mantle structure the results obtained in this study will be helpful in investigating the mechanism of magma generation in a subduction zone

miocene magmatism of sinai related to the opening of the red sea extension associated with the initial early miocene stages of opening of the red sea resulted in intrusion of a widespread system of dikes and smaller bodies primarily along the northeastern margin of the red sea gulf of suez axis dikes some up to more than 800 km in length were emplaced dominantly parallel to the red sea gulf of suez from the sinai peninsula to yemen intrusive rocks are mainly basaltic in composition but minor granophyres are also present in the sinai this magmatic event is about 20 m y old dikes of the southern and southeastern sinai crop out in precambrian terrane in the central and northern sinai they occur in phanerozoic terrane where they may be associated with linear grabens and cryptovolcanic features basaltic dikes of the sinai underwent variable degrees of alteration alteration phases include seriate quartz smectite chlorite and serpentine x ray diffraction and petrographic analysis indicate that in spite of extensive alteration primary minerals still control the major element compositions of the rocks basalts are tholeiitic in composition with sio2 48 5 to 50 and na2o k2o 3 to 3 5 rb and sr concentrations are probably dominated by alteration and by introduction of secondary material primarily carbonate zr tio2 which is relatively unchanged during alteration substantiates that the basaltic rocks are subalkaline initial 87sr 86sr determined after leaching of samples is approximately 0 7043 0 7063 much higher than red sea axial trough basalts 0 703 we infer that these sinai basalts equilibrated at depths of 35 50 km possibly within an enriched mantle whether the main trough of the red sea is underlain by extended and modified continental crust or by oceanic crust is controversial since miocene dikes were intruded along both coasts but primarily the saudi arabian coast of the red sea gulf of suez axis in a zone up to 150 km wide they may also underlie the main trough of the red sea and result at least partially in the discontinuous magnetic and gravity anomalies observed there possibly the abundance of dikes beneath the main trough is much greater than adjacent to the red sea giving rise to properties intermediate between those of continental and oceanic crust downwarping and widespread intrusion of dikes along the red sea 18 26 m y ago probably signified crustal dilation and the initiation of a continental rift because tholeiitic basalts of the sinai are higher in 87sr 86sr than red sea axial trough basalts we infer that the conditions of their origin must also be very different we suggest that the sinai basalts may have been generated within but near the base of subcrustal continental lithosphere perhaps indicating thermal thinning of lithospheric mantle above upwelled asthenosphere associated with continental rifting this inference if correct would indicate that the lithosphere was at least 35 50 km thick during the early miocene phase of rifting but possibly not much thicker since dikes farthest from the red sea are apparently identical in composition to those closest asthenospheric upwarping must have been broad enough that all magmas were derived from approximately the same depth in the yemen ethiopia areas volcanism began earlier about 30 m y ago and was much more voluminous than elsewhere along the red sea gulf of suez axis suggesting the presence of underlying mantle upwelling accompanying rifting of the red sea in spite of differences in age and volume the presence of large volumes of tholeiites among the miocene basalts of the yemen ethiopia areas suggests that the lithosphere was similarly thinned along the entire length of the red sea

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

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

late carboniferous and early permian drainage patterns in atlantic canada paleoflow data have been compiled for late carboniferous late westphalian a to early permian alluvial deposits over a large area of atlantic canada the data which include more than 36 000 measurements of large scale trough cross strata indicate a predominantly northeasterly paleoflow and suggest that a major source area lay to the southwest of the region throughout the 30 ma period represented uplands within the basin deflected paleoflow and probably formed important local drainage and sediment sources tectonostratigraphic analysis suggests that the drainage originated in the fold and thrust belt of the central appalachians and parts of the northern appalachians rivers probably followed northeast oriented structural lineaments through the older acadian mountains of the northern appalachians a considerable proportion of the rising orogen s drainage and probably detritus may have traversed basins along the strike of the mountain belt a situation analogous to that of the modern himalayas

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

early carboniferous low temperature hydrothermal vent communities from newfoundland living vent communities dependent on microbial chemosynthesis rather than photosynthesis were discovered only in the 1970s1 since then six fossil vent communities have been identified they range in age from early carboniferous to early tertiary2 7 here we describe fossil tubes an abundant low diversity fauna and sulphide mineralization in 340 myr old carboniferous carbonate mounds in newfoundland fig 1a these features together with evidence for microbial activity point to the existence of a seventh chemosynthetic community clustered around low temperature hydrothermal vents the remarkable preservation of this newfoundland community allows a direct comparison to be made with the composition and structure of modern vent communities

phases of activity and geochemistry of basaltic dike systems in northeast jordan parallel to the red sea abstract k ar dates petrographic and chemical data were obtained from representative samples from three nw se trending basaltic dikes in the harrat ash shaam volcanic field northeast jordan compared with other ne sw trending dikes in jordan three phases of activity were determined for the studied dikes starting from the oldest around 23 ma to the youngest at around 1 7 ma three dominant petrotypes of basalt were distinguished and related to the three phases during the early miocene there was emplacement of a pyroxene iddingsite basalt 23 which is characterized by a tholeiitic composition later in the middle to late miocene the composition was that of pyroxene olivine basalt 34 with an increase in the alkaline character of the magma chamber in the final stage the dikes comprised olivine basalt 22 with a basanite composition it is reasonable to assume that the emplacement of the dikes was part of the same volcanic phase and regional tectonic regime associated with the phases of volcanic activity of the red sea system

new evidence on the accurate displacement along the arava araba segment of the dead sea transform the sinistral displacement along the dead sea transform dst the plate boundary between the african and the arabian plates south of the dead sea basin was previously attributed to two main fault zones the arava araba or dead sea fault and the feinan or al quwayra fault zone this was based on similarities of features on either side of the araba valley in particular the timna and the feinan copper mines located north of the themed and dana faults and the onlap of the cambrian formations southward onto the amram rhyolite and ahyamir volcanics to these we add a more accurate offset indicator in the form of an offset early cambrian 532 ma dolerite dyke previously mapped in mount amram israel on the african plate and recently discovered across the araba valley in jabal sumr al tayyiba southwest jordan on the arabian plate this dolerite dyke is 20 m thick strikes n50 e and is the only dyke intruding the jabal sumr al tayyiba pink rhyolite flows of the ahyamir volcanics geochemical and geochronological correlations between the jabal sumr al tayyiba dolerite dyke and the mount amram dolerite dyke demonstrate 85 km of sinistral offset across the arava araba fault our results also suggest approximately 109 km of combined sinistral displacement across the arava araba and feinan faults based on petrological correlations between the timna and jabal hanna igneous complexes on the african and arabian plates respectively this constrains the total sinistral displacement of the feinan fault and its accessory faults to be 24 km

alleghanian deformation in the eastern gaspe peninsula of quebec canada juxtaposition of the mississippian risti gouche and cannes de roches basins which are subbasins of the composite late paleozoic maritimes basin occurred through dextral movement along the north west striking percefault system in the east ern gaspepeninsula of quebec the north west striking faults are truncated by small north northwest striking dextral strike slip faults which probably developed as region al stress gradually rotated clockwise from north northwest south southeast to north east southwest this study provides the first evidence in eastern quebec for significant post acadian block displacement other than normal faulting and indicates that al leghanian deformation extended much far ther north than previously thought iden tification of these structures formed during the alleghanian orogeny but more than 1000 km away from areas of peak allegh anian metamorphism in the southeastern united states underlines the far reaching effects of continental collisions it also casts doubt on the age attribution of brittle strike slip faults elsewhere in the gaspe peninsula away from mississippian expo sures such brittle faults were previously associated with late stages of the acadian orogeny but could in fact be considerably younger

paleomagnetic and geological investigation into southern sinai volcanic rocks and the rifting of the gulf of suez two volcanic occurrences from southern sinai were sampled for paleomagnetic petrographic and geochemical investigations 29 oriented samples were taken from six sites of the middle jurassic basaltic intrusion at wadi budra southwest sinai 28 55aen 33 20aee after alternating field af demagnetization and tilt correction all samples exhibit normal magnetization with a mean paleomagnetic direction d 63 3 i 8 0 and a 95 10 2 this yields a paleopole position at lat 25 n long 133 e and a 95 8 6 also six oriented blocks 34 core samples were taken from the sant catherine doleritic dike at wadi sahl el raha south sinai 28 33aen 33 55aee after af demagnetization all sites showed normal polarity with a mean direction of magnetization d 3 i 39 and a 95 15 3 and a paleopole position at lat 83 e long 179 e and a 95 16 5 the paleomagnetic pole of w budra is found to be incompatible with poles of similar age from africa the cause of this cannot be uniquely determined but most probably could be referred to local tectonic rotations and or geomagnetic secular variations avecting the paleomagnetic directions of these samples in contrast the tertiary pole of wadi sahl el raha is consistent with the part of the apparent polar wander path apwp of africa for the time period 20 ma oligo miocene age petrographical and geochemical investigations revealed that the volcanic rocks of both localities are continental within plate rift related alkali olivine basalt w budra and olivine dolerite sahl el raha the primary magnetite is the main opaque mineral present in the samples which represents the main carrier of magnetization in these rocks 2000 elsevier science b v all rights reserved

link between long lasting evaporitic basins and the development of thick and massive phreatic calcrete hardpans in the mississippian windsor and perce groups of eastern canada part of the visean upper mississippian succession in the upper paleozoic maritimes basin of eastern canada is overprinted by massive phreatic calcrete hardpans that can exceed 10 m in thickness and that are characterized by the thorough mineral replacement of most of their host sediment by calcite similar to those that are currently forming around salt lakes in quaternary sediments of central australia the precise timing and paleogeography of this carboniferous event was until now poorly known but lateral correlations over a large study area indicate that phreatic calcretization occurred in the vicinity of large evaporitic basins following a marine transgression and regression cycle in chadian to holkerian times this relation confirms that the previously proposed model for modern analogs in central australia which states a genetic link between the salt lakes and the thick and massive phreatic calcrete hardpans can be applied to ancient environments and that such occurrences may be used to infer the former presence of an evaporitic basin in their vicinity finally our relatively large dataset indicates that the stable isotopes of carbon and oxygen are successful in differentiating ancient marine carbonates from ancient phreatic calcretes 2007 elsevier b v all rights reserved

deposition of a saline giant in the mississippian windsor group nova scotia and the nascent late paleozoic ice age abstract saline giants are vast marine evaporite deposits that currently have no modern analogues and remain one of the most enigmatic of chemical sedimentary rocks the mississippian windsor group ca 345 ma maritimes basin atlantic canada is a saline giant that consists of two evaporite rich sedimentary sequences that are subdivided into five subzones sequence 1 is composed almost entirely of thick halite belonging to subzone a osagean sequence 2 is in unconformable contact and comprised of stacked carbonate evaporite peritidal cycles of subzones b through e meramecian subzone b the focus of research herein documents the transition from wholly evaporitic to open marine conditions and thus preserves an exceptional window into the processes forming saline giants lithofacies stacking patterns in subzone b reveal that higher order fluctuations in relative sea level produced nine stacked parasequences interpreted to reflect high frequency glacioeustatic oscillations during the onset of the late paleozoic ice age each parasequence reflects progradation of intertidal and sabkha sediments over subtidal carbonate and evaporite deposits dissimilarities in cycle composition between sub basins imply the development of contrasting brine chemistries from differing recharge rates with the open ocean what the windsor group shows is that evaporite type is ostensibly linked to the amplitude and frequency of sea level rise and fall during deposition true saline giants like the basinwide evaporites of sequence 1 apparently require low amplitude long frequency changes in sea level to promote the development of stable brine pools that are only periodically recharged with seawater by contrast the high amplitude short frequency glacioeustatic variability in sea level that controlled the accumulation of peritidal evaporites in subzone b produce smaller subeconomic deposits with more complex facies relationships

morphotectonics inferred from the analysis of topographic lineaments auto detected from dems application and validation for the sinai peninsula egypt abstract morphotectonic lineaments observed on the sinai peninsula in egypt were auto detected from shuttle radar topography mission 90 m digital elevation model dem and gravity grid data and then analyzed to characterize the tectonic trends that dominated the geologic evolution of this area the approach employed consists of dem shading segment tracing grouping statistical analysis of the distribution and orientation of the lineaments fault plane characterization and smooth representation techniques statistical quantification of counts mean lengths densities and orientations was used to infer the relative severity of the tectonic regimes to unravel the prominent structural trends and to demarcate the contribution of various faulting styles that prevailed through time restored to the present day geographic position prominent n50 60 w n20 40 w n50 60 e and n20 30 e and less prominent n s e w and ene trends were common the prominence of these trends varied through time the nw and ne trends showed relatively equal abundances in the precambrian and the cambrian whereas the prominence of the nw trends prevailed from the carboniferous to the holocene lineaments in all formations were near vertical and on average about 65 showed as strike slip 22 as reverse and 13 as normal faulting styles statistics from the detected linear features and the reference geological data reveal the relative severity of five dominant tectonic regimes precambrian compression followed by extension at its end cretaceous compression eocene compression miocene extension and finally holocene compression auto detected lineaments and the severity of the characterized tectonic periods correspond well with reference data on the geologic structure geodynamic framework and the gravity anomaly furthermore the recent significance of the broad structural zones was confirmed by the foci of the earthquake epicenters along and at the intra plate intersections of the broad lineament zones and at the plate boundaries the spatial distribution of trends with varying styles of faulting distinguished four main tectonic provinces of marked geodynamics variances

problems concerning the evolution of oceanic lithosphere in the northern red sea abstract the pole of rotation for the opening of the red sea at 36 5 n 18 0 e arabia from africa implies the fastest spreading rates are in the south and are about 50 less in the north various studies show the spreading rate in the south to be about 1 cm yr over the last 4 5 ma problems arise as to the nature of the lithosphere beneath the northernmost red sea the problem is further complicated by the fact that the red sea may have evolved in three phases the first is the gulf of suez phase mainly oligocene the second corresponds to the first 62 km movement along the aqaba dead sea shear latest oligocene early miocene and the third to the second 45 km movement along the aqaba dead sea shear plio pleistocene these motions and their implications for the northernmost red sea are assessed the northern red sea differs from the central and southern red sea in having no clear magnetic lineations only a few isolated circular anomalies are observed there are two possible explanations 1 1 there is extremely thinned continental lithosphere occasionally pierced by strongly magnetised volcanic centres or 2 2 the oceanic lithosphere is present but without magnetic anomalies it is suggested that the first explanation applies to the first gulf of suez phase and the second explanation to the two later aqaba dead sea phases it is further suggested that the lack of magnetic anomalies may be due to a combination of large thicknesses of unstable sediments salt high temperatures and low sea floor spreading rates these lead to the evolution of oceanic lithosphere with slow cooling inhibiting the acquisition of strong magnetisation and giving subdued magnetic anomalies

newly identified carboniferous units the pointe sawyer and chemin des pecheurs formations in the gaspe peninsula quebec implications regarding the evolution of the northwestern sector of the maritimes basin the upper member of the cannes de roches formation was recently recognized overlying the bonaventure formation in the new carlisle area over 100 km southwest of the previously documented exposures of this unit moreover remnants of the la coulee formation which are unconformably overlain by the bonaventure formation have also been recognized underlying with a similar type of contact the lower member of the cannes de roches for mation from this and from facies similarities the lower and middle members of the cannes de roches formation are now considered to be equivalent to the bonaventure formation it is proposed to abandon these two member designa tions and to only keep the bonaventure formation the remaining upper member of the cannes de roches formation is renamed the pointe sawyer formation a late visean to early namurian age is attributed to this grey clastic forma tion from spore analysis dark reddish brown sandstone conformably overlie the pointe sawyer formation in the mal bay area they correspond to the beginning of sedimentation from more distant sources within the regional carbonifer ous stratigraphic succession which was until then characterized by sedimentation from proximal sources this previ ously unidentified unit is here referred to as the chemin des pecheurs formation

magnetic character of a large continental transform an aeromagnetic survey of the dead sea fault new high resolution airborne magnetic hram data along a 120 km long section of the dead sea transform in southern jordan and israel shed light on the shallow structure of the fault zone and on the kinematics of the plate boundary despite infrequent seismic activity and only intermittent surface exposure the fault is delineated clearly on a map of the first vertical derivative of the magnetic intensity indicating that the source of the magnetic anomaly is shallow the fault is manifested by a 10 20 nt negative anomaly in areas where the fault cuts through magnetic basement and by a 5 nt positive anomaly in other areas modeling suggests that the shallow fault is several hundred meters wide in agreement with other geophysical and geological observations a magnetic expression is observed only along the active trace of the fault and may reflect alteration of magnetic minerals due to fault zone processes or groundwater flow the general lack of surface expression of the fault may reflect the absence of surface rupture during earthquakes the magnetic data also indicate that unlike the san andreas fault the location of this part of the plate boundary was stable throughout its history magnetic anomalies also support a total left lateral offset of 105 110 km along the plate boundary as suggested by others finally despite previous suggestions of transtensional motion along the dead sea transform we did not identify any igneous intrusions related to the activity of this fault segment

relation between continental rifting and uplifting evidence from the suez rift and northern red sea abstract the suez rift and the northern red sea are flanked by high standing shoulders with elevations often exceeding 2 km and with a structural relief reaching 4 5 km in this area pre rifting erosion was small not more than a few hundred meters there is no evidence that a precursory uplift formed along the site of future rifting uplifting and erosion began essentially at the same time as rift faulting and magmatism in late oligocene and early miocene limes in the gebel serbal area sinai where the uplifting history is resolved best the tectonic relief reached about 800 m by the end of the early miocene and a similar amount of tectonic uplifting developed in later times other parts of the rift shoulders probably had a similar uplifting history but the total uplifting was often less considerable erosion in places more than 3 km occurred mainly since middle miocene times and caused considerable isostatic rebound this magnified the uplifting and caused the structural and topographic relief to be considerably greater than the tectonic uplifting initial uplifting and contemporaneous magmatic activity were probably caused by rising of deep hot material which thinned the lithosphere and heated it such deep seated processes probably continued to operate also during the later stages of tectonic uplifting though igneous activity no longer occurred on the surface stretching of the lower lithosphere under the rift shoulders could have also contributed to their uplifting the features of uplifting history of the studied region may well be typical of other active rifts

magmatic occurrences in the central arava southern israel based on geology and magnetometry abstract the eshet ridge is located in the central arava near wadi paran geological data were collected using ground magnetic surveys and petrophysical measurements magnetic susceptibility and density the goal was to reveal the structure and nature of a concealed magmatic body under the ridge integrated gravity and magnetic interpretation together with seismic reflection data including 2 modeling indicated the presence of a deep seated basic magmatic intrusion occurrence of fe mineralized rocks along the ridge supports subsurface data the magmatic body was intruded in the hard turonian rocks of the eshet ridge basic magmatic exposures dating from the early miocene were evident along wadi ashosh in the eastern negev and its margins 18 km to the north of the ridge a new outcrop of basic magmatic intrusion was found southwest of the tzukim settlement magnetic measurements indicated a similarity between their magnetic pattern and the ashosh basic magmatics which were dated to 20 4 0 7 ma volcanoclastic tuff pebbles magnetic susceptibility of around 2 7 10 3 si surrounded by conglomerate were discovered at two sites in wadi demma and wadi menuha streams that drain near the eshet ridge and 2 km south of it respectively the conglomerates contained mostly limestone chalk and chert fragments not more than 5 of which were volcanic pebbles two pebbles were dated to 24 4 0 7and 21 5 0 5 ma the magmatic outcrops in wadi ashosh the magmatic dyke near tzukim and the volcanic purple and black pebbles near wadi menuha were all dated to the very late oligocene early miocene the magmatic body identified in this study under the eshet ridge was termed the eshet intrusion and is connected to and even the source of all these phenomena

the e greenland continental margin a transition between oceanic and continental magmatism the e greenland continental margin differs from typical margins in having a superimposed regional uplift which has exposed the early rift products for direct examination most of the igneous rocks in this region find counterparts in the oceanic setting with the proviso that the area is part of an aseismic ridge thus picrites basalts sheeted dyke swarms and layered gabbros are present and these can be used to shed light on oceanic magmatic events nephelinitic rocks are products of off axis volcanism and find no counterparts in the present oceanic rift system although they occur in marginal regions of the ocean basins and were generated in distal regions at the initiation of rifting in icelandic anomaly voluminous salic rocks in the area owe their generation largely to continental crust which has undergone extensive melting they find no direct counterparts in oceanic areas the outstanding degree of exposure in this area combined with its transitional nature and wide spectrum of rock types make it a unique laboratory for the elucidation of a variety of problems regarding both oceanic and continental volcanism together with the nature of rifted continental margins in general

paleoweathered surfaces on granitoids of southern nova scotia paleoenvironmental implications of saprolites in addition to minor postglacial weathering effects a complex history of three distinct weathering events has been identified within the granitoids of the south mountain batholith of southwestern

evidence from the northern red sea on the transition from continental to oceanic rifting abstract the northern red sea is an active rift in the last stages of continental rifting which is beginning to undergo the transition to oceanic seafloor spreading the rift can be divided into two distinct regions the marginal areas consist of a series of bathymetric terraces and as indicated by geophysical data are underlain by a series of rotated fault blocks the axial depression which forms the axis of deep water is a continuous region 10 to 25 km wide near the center of the sea which is characterized by high heat flow a large gravity minimum and very deformed sediments both regions are divided along strike into segments roughly 60 km long by cross trending transfer zones the presently active tectonics are characterized by the concentration of extension and deformation in the axial depression and by the emplacement of a number of large intrusions the intrusions occur at distinct sites located at the intersection of transfer zones with the faults bounding the rotated blocks in the marginal areas and at deeps located in the axial depression halfway between transfer zones although extension is becoming concentrated in the axial depression geophysical data imply that at least the upper few kilometers of the crust in that region consists of non magnetic continental type material and that this ca apace has been ruptured by large intrusions only at the deeps a wide range of crustal seismic velocities has been observed in the northern red sea including velocities normally associated with oceanic crust the velocity variations appear spatially related to corresponding large variations in basement rock type observed on shore in the eastern desert of egypt the northern red sea is in an intermediate stage between a continental rift valley and a young ocean basin and delineation of its structure and tectonics allows a model to be developed for the evolution of a rifted continental margin rifting starts in a series of linked half grabens 50 60 km long and formed over detachment surfaces which probably penetrate no deeper than the mid or lower crust this style of rifting can accommodate at least the amcunt of extension represented by the gulf of suez 35 km at some greater amount of extension the rift acquires a more symmetric shape probably through the development of antithetically faulted blocks in the hanging wall extension in this stage is still distributed throughout the rift and is accommodated in the upper crust by rotation of fault blocks at the stage represented by the northern red sea extension becomes concentrated in an axial depression and begins to be partially accommodated by large intrusions which form deeps the position of the deeps is controlled by the transfer zones established in the earliest stages of rifting with further extension the deeps serve as nucleation points that develop into small discrete cells of seafloor spreading these cells grow and coalesce to form a continuous mid ocean ridge spreading center such as is found in the southern red sea

cenozoic volcanism in the middle east petrogenesis of alkali basalts from northern lebanon large discontinuous exposures of basaltic lava flows ranging in age mostly from miocene to recent are present in several localities extending from sinai jordan palestine israel to lebanon and syria dubertret 1955 baldridge et al 1991 mouty et al 1992 heimann et al 1996 shaw et al 2003 these form the cenozoic volcanic province of the middle east occurring mostly along or near the transform faulted boundary the dead sea ghab transform fault system between the arabian and african plates and the levantine subplate fig 1 further to the south several other extensive cenozoic volcanic provinces occur in arabia and east africa mostly ethiopia figure 1 regional geological map showing the distribution of the various cenozoic volcanic provinces in east africa arabia and the middle east the cenozoic continental flood basalts in ethiopia yemen western arabia and jordan have been extensively studied camp roobol 1989 1992 altherr henjes kunst baumann 1990 stein hofmann 1992 baker thirlwall menzies 1996 baker et al 1997 shaw et al 2003 the cenozoic volcanic province of ethiopia is clearly related to the east african rift system barberi et al 1975 mohr 1983 the quaternary intraplate basaltic volcanic field in yemen appears to be the result of melting shallow mantle perhaps in response to small amounts of lithospheric extension that were metasomatized and hydrated by the afar plume during or shortly after oligocene flood volcanism baker et al 1997 the lavas of this basaltic field of western yemen were subjected to variable degrees of contamination 0 20 of an early proterozoic to late archaean silicic lower crustal component baker thirlwall menzies 1996 baker et al 1997 the cenozoic continental flood basalt provinces of

petrology and geochemistry of hawaiite lavas from crater flat nevada hawaiite type lavas were erupted in three cycles 3 7 1 2 and 0 3 m y at crater flat nevada the compositions of all three cycles considered together form a straddling alkalic series as defined by miyashiro in which the less evolved basalts plot near the normative olivine diopside divide and the more evolved basalts project into hypersthene or nepheline fields fractionation modeling based on the oldest cycle allows the removal of olivine clinopyroxene and amphibole to arrive at the more evolved hawaiite compositions in general fractionation of phlogo pite or feldspar is limited by the fractionation modeling and by eu ree relations in detail all hawaiites within one cycle 3 7 m y need not be derived from a single parent magma varied parentage is more evident between cycles although all cycles are consistently of hawaiite composition basalts of the youngest two cycles are generally enriched in trace elements superimposed on this enrichment is a lack of rb variation leading to rb sr ratios far lower than required to generate the high 87sr 86sr ratio 0 707 typical of basalts in this region the very low rb sr ratios limit processes that may lead to trace element enrichment during magma evolution cyclic recharge of a fractionating magma chamber decreased fractions of mantle melting leaving phlogopite in the residuum or an earlier event of metasomatic transport from phlogopite bearing mantle rocks into a phlogopite absent mantle assemblage might explain the observed trace element enrichment with low rb sr

novel type of crust produced during continental rifting a normal oceanic crust cannot be generated after continental crust splitting of rifts filled by a thick pile of young sediments the magma released by the rising mantle is trapped mainly in these sediments as basalts sills and plugs the sediments are themselves metamorphosed by the heat flux from the underlying mantle a metasedimentary crust is formed in direct contact with an abnormal mantle vp 7 5 km s l produced by partial serpentinization

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