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

continental oceanic crustal transition off southwest africa more than 1 426 mi 2 300 km of 48 trace 12 fold seismic reflection profiles were used to examine the nature of the continent ocean boundary off southwest africa south of the orange river faulted blocks which we interpret as rifted continental crust can be traced seaward to the 3 km isobath the transition to oceanic basement occurs in a zone 19 to 25 mi 30 to 40 km wide beneath the continental rise although structural details are obscured by a thick seaward dipping wedge of pre aii pre late early cretaceous overburden oceanic crust at the presumed contact is not older than magnetic anomaly m9 126 to 121 m y b p north of the orange river a pronounced hinge in continental crust correlates with magnetic anomaly g of rabinowitz 1976 seaward of the hinge a complex fault block terrane is evident oceanic basement can not be traced with any certainty landward of anomaly m4 intracontinental stretching and associated volcanism appear to have been important in the early history of the cape basin rifting and local dike intrusion may explain the presence of some lineated magnetic anomalies previously attributed to sea floor spreading related extrusives form at least part of the pre aii wedge just seaward of the hinge zone our interpretation of the seismic data suggests that the initiation of normal spreading in the cape basin postdates by 4 to 9 m y the valanginian age derived from prevailing plate tectonic reconstructions of the south atlantic

late paleozoic glacial episodes in gondwanaland reflected in transgressive regressive depositional sequences in euramerica the late paleozoic glaciation of gondwanaland comprised two short episodes in the famennian i and visean ii confined to brazil and adjacent northwest africa and a long episode that started in the namurian iiia of eastern australia and bolivia argentina expanded to cover much of gondwanaland in the stephanian asselian iiib and collapsed in the early sakmarian iiic dropstones in eastern australia indicate that small ice centers lingered to the kazanian across the belt of low latitudes north of gondwanaland short ranging fossils in widespread shallow marine and paralic deposits indicate synchronous deposition of transgressive regressive sequences in different parts of euramerica these sequences correlate with glacial events in gondwanaland at three levels a four major regressions in euramerica in the famennian 1 visean 2 namurian 3 stephanian 4 and the tastubian transgression that preceded the sterlitamakian regression 5 also recorded in gondwanaland correlate with glacial episodes i ii and iiia iiib and iiic b the time interval of cyclothemic deposition in euramerica brigantian or latest visean to sterlitamakian correlates with that of glacial episode iii and c the dominant period of the euramerican cyclothems as estimated from the middle and late pennsylvanian deposits of the mid continent of north america and of the thickest known gondwanaland glacigenic sediment the earliest permian lyons group of western australia is 0 4 ma equivalent in turn to the long orbital eccentricity period of the quaternary ice age and the dominant period of fluctuation of the late miocene antarctic ice cap the three levels of correlations confirm wanless and shepard9s 1936 hypothesis that the late paleozoic cyclothems are controlled largely by sea level fluctuations related to the gondwanaland glaciation

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

two cryogenian glacial successions compared aspects of the sturt and elatina sediment records of south australia abstract two cryogenian glacial sequences the sturt and elatina occur within the adelaide rift complex s australia and are part of an 8 km thick succession recording rodinia fragmentation and subsequent passive margin development we compare and contrast these glacial successions presenting new data including detailed logged sections for both glacial sequences facies analysis reveals a total of six facies associations recording a range of glacial and glacially related processes these are a diamictite facies association ice proximal debris flows a sheet sandstone facies association fluvial deposits a flaser and rhythmically bedded facies association tidal flat a hummocky cross stratified sandstone facies association shoreface to offshore transition zone deposits and an underflow facies association recording offshore deposition a sixth facies association a ferruginous lonestone bearing siltstone is restricted to the top of the elatina formation and records rapid environmental amelioration and emergence from glaciation in the central flinders ranges the sturt succession commences with a glacial erosion surface ges which in agreement with previous workers was probably enhanced by rifting this ges is overlain in ascending order by the pualco tillite holowilena ironstone and wilyerpa formation with the warcowie dolomite member at the base of this latter formation a second ges is preserved within this latter unit and is overlain by a stratigraphic motif that records an alternation of offshore transition zone and offshore deposits significant base level falls within the wilyerpa formation are identified by fluvial deposits sitting disconformably upon offshore deposits in contrast within the elatina formation a single ges is proposed which defines the base of the formation the elatina formation is split into two informal members on the basis of analysis of sediment stacking patterns and the recognition of a sharp stratigraphic surface that divides the formation apart from a locally preserved basal diamictite the lower member consists of fluvial deposits whereas the upper member is dominated by a mixture of tidal flat sediments and debris flow deposits the boundary between these units is therefore interpreted as a hybrid surface recording tidal ravinement in some areas and a glacial advance in others deglaciation left contrasting deposits the sturt deglaciation culminated in deposition of black shale of the tindelpina shale member of the tapley hill formation seemingly as a sheet like deposit over the adelaide rift complex and beyond into the amadeus basin of central australia the elatina deglaciation meanwhile began first with deposition of the sixth facies association ferruginous dropstone bearing siltstone then proceeded with deposition of cap dolostone of the nuccaleena formation

paragenetic relations in gedrite cordierite staurolite biotite sillimanite kyanite gneisses at ajitpura rajasthan india aluminous parageneses containing gedrite cordierite garnet staurolite biotite sillimanite kyanite quartz or spinel plus corundum are found as dark colored lenses in the polymetamorphic multideformed archean complex at ajitpura in northwest peninsular india staurolite like kyanite is a relict phase of earlier metamorphism and is excluded as a paragenetic mineral in view of its incompatibility with quartz and gedrite and its lower xmg values than for garnet of the assemblage its stability here is attributed to zinc content of up to 3 wt the xmg in other ferromagnesian minerals decreases in the order cordierite biotite gedrite garnet as found elsewhere in high grade rocks

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

holocene evolution of an estuarine coast and tidal wetlands modern facies distribution patterns extensive core data and chronostratigraphic cross sections provide a detailed history of holocene inundation within the delaware bay estuary and sedimentation in adjacent coastal environments flooding of the estuary occurred with rising sea level as the shoreline retreated northwest along a path determined by the pre transgression topography simultaneous migration of an estuarine turbidity maximum depocenter provided the bulk of fine sediments which form the coastal holocene section of the estuary prior to 10 ka the ancestral bay was predominantly a tidal river and the turbidity maximum depocenter was located southeast of the modern bay mouth by 10 ka lowlands adjacent to the ancestral channel of the delaware river were flooded forming localized tidal wetlands and the depocenter had initiated high rates of fine grained sedimentation near the present bay mouth at that time coastal holocene strata began to onlap the interfluve highlands by 8 ka the fine grained depocenter had migrated northwest along the main channel of the delaware river although the widened mouths of tributary valleys continued to be active sites of sediment accumulation following the passage of the fine grained depocenter coarse grained sediments accumulated along the coast in response to increased wind wave activity during the middle holocene portions of the estuarine coast began to resemble modern geomorphology and washover barrier sands and headland beach sandy gravels accumulated along the southwest shore the late holocene was characterized by erosional truncation and submergence of aggraded coastal lithofacies and by planation of remnant highland areas knowledge of the eroded holocene section is fragmentary at present continued sea level rise is accompanied by deposition of tidally transported muds in coastal environments and deposition of sandy sediments in some offshore regions an unconformity marks the base of the developing open estuarine sequence of coarse clastic lithofacies and denotes the end of coastal accumulations modeling of coastal lithofacies transitions identifies specific lithofacies complexes in the holocene stratigraphic section which were influential in the evolution of the coast development of the holocene section of the estuary coast involved both constructive or aggradational and destructive or erosional phases

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

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

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

sequence stratigraphy and controls on reservoir sandstone distribution in an eocene marginal marine coastal plain fairway taranaki basin new zealand abstract middle to upper eocene strata in taranaki basin new zealand consist of terrestrial marginal marine shallow marine and offshore sediments deposited in a series of transgressive and regressive cycles within a passive margin setting coastal plain facies are largely confined to eastern and southern sides of the basin and marine mudstones to western and northern parts shoreface to fluvial sandstones within a ne sw oriented coastal facies belt form the pre eminent petroleum reservoir fairway in taranaki basin and provide the bulk of new zealand production the terrestrial coastal plain part of the succession also contains coal rich proven source rocks we have interpreted twelve depositional sequences 3rd and 4th order within the middle to upper eocene strata the base of our study interval marks a relative transgression within the middle eocene and is overlain by composite sequences and parasequences represented by cyclically stacked packages overall regression occurs up to the top of the middle eocene which is overprinted by intermittent shorter term transgressions the overlying upper eocene strata are characterised by overall shoreline retreat with final transgression over the entire taranaki basin in the latest eocene early oligocene stratigraphic cyclicity within the terrestrial to shallow marine facies is noted from paleoenvironmental interpretations determined from well log core biostratigraphic and coal sulphur data whilst eustasy may have played a role in development of the taranaki stratigraphy tectonically induced subsidence was probably the over riding factor especially in the upper eocene petroleum producing reservoirs occur at several distinct horizons within the middle to upper eocene succession interpreted here to represent significant events during sequence development most reservoir sandstones were deposited as channelised facies following 3rd order lowstands sequence boundaries are interpreted at the base of many of these sandstone intervals some of which may represent type 1 boundaries developed as incised valley systems this regional study has used an integrated approach to show how sequence stratigraphy can be applied to a non marine to shallow marine succession to assist with exploration and reservoir development through our recent and multi disciplinary work we have been able to discriminate subtle base level fluctuations and greater marine control of depositional architecture than has previously been recognised with implications for source rock quality fairway exploration reservoir charge and production effectiveness

hydrodynamics and sediment dynamics of the wash embayment eastern england water and sediment movement in the wash embayment has been determined from an extensive set of data consisting of tidal current readings and suspended sediment concentration measurements instantaneous and residual currents in the embayment show a distinct lateral inhomogeneity whereas vertically the water column is almost homogenous the central deep water area 30 40 m of the embayment is dominated by a residual landward water movement whilst on the margins the residual movement is seaward sediment is supplied predominantly in suspension from the north through the northern extremity of boston deep suspended sediment pathways are coincident with the spring tide water movements and the subtidal channels act as the main conduits approximately 6 8 106 tonnes yr 1 of suspended sediments are supplied to the embayment from offshore areas bedload sediment supply is of lesser importance 1 4 104 tonnes yr 1 whereas suspended sediment movement appears to be the dominant mode of transport throughout the embayment bedload transport is important in reforming the sea bed into a variety of bedforms which are particularly well developed on the margins of channels and shoals

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

tidal modeling of an ancient tide dominated seaway part 2 the aptian lower greensand seaway of northwest europe abstract the cretaceous lower greensand deposits of southeastern england and northern france contain abundant evidence of diurnal once daily paleotides with tidal influence increasing into the late aptian the imperial college ocean model icom is used herein to determine the amplitude origin astronomical or co oscillating and characteristics diurnal or semidiurnal of the tides in the shallow marine lower greensand seaway where these sediments were deposited a global ocean paleotidal model of the aptian predicts very low tidal ranges microtidal in the proto central atlantic ocean and boreal ocean with higher tidal amplitudes especially of the diurnal constituents predicted in the northwestern neotethys ocean this suggests that the neotethys ocean was the only significant source of oceanic tides to the lower greensand seaway herein the results of the global ocean paleotidal model are used to supply boundary conditions to a higher resolution regional paleotidal model of the lower greensand seaway the regional paleotidal model shows that the tidal range of the lower greensand seaway is largely controlled by the width of the connections to the open ocean as sea levels rose during the aptian the oceanic connections widened resulting in reduced frictional damping of the oceanic tidal wave coupled with funneling shoaling and coriolis effects this led to tidal amplification sensitivity tests using a variety of paleogeography and paleobathymetry scenarios suggest that the model predictions are robust the global to regional scale paleotidal modeling presented here strongly suggests that there was a connection through france in the aube region of the paris basin from the neotethys ocean to southeastern england however there are no time equivalent deposits in this region to validate this suggestion it is herein argued that that this is due either to postdepositional erosion or to low sediment supply coupled with strong tidal currents sweeping the seafloor to exposed bedrock

spatial and temporal evolution of coastal depositional systems and regional depositional process regimes campanian western interior seaway u s a abstract this paper provides a critical review and regional synthesis of late cretaceous shallow marine deposits along part of the western margin of the western interior seaway of north america which contains the most extensively documented outcrop based studies of siliciclastic coastal depositional systems in the world the results of this synthesis are presented in the form of paleogeographic maps covering present day new mexico utah colorado and wyoming usa for five timeslices in the campanian these maps are used to evaluate the spatial and temporal evolution of regional depositional process regimes along a large 1000 m stretch of coastline the evolution of regional depositional process regimes is linked to tectonic and paleoceanographic controls on the western interior seaway which enables the results of this synthesis to be applied to prediction of depositional process regimes in other less intensively studied basins six gross depositional environments have been mapped for each timeslice 1 alluvial to coastal plain sandstones 2 coastal plain coals mudstones and sandstones 3 shoreline sandstones 4 marine mudstones 5 gravity flow siltstones and sandstones and 6 marine marls and chalk shoreline sandstones in each timeslice are interpreted in further detail using documented evidence for the three principal classes of depositional process wave tidal and fluvial and published reconstructions of coastal morphology which is widely considered to reflect depositional process regime based on these interpretations shoreline sandstones are assigned to five categories of depositional process regime 1 regressive wave dominated shorefaces and delta fronts 2 regressive river dominated delta fronts 3 regressive mixed tide and wave influenced delta fronts 4 regressive tide dominated delta fronts and 5 transgressive barrier islands back barrier lagoons and estuaries the accuracy of and uncertainty in classification of depositional process regime are critically evaluated additionally stratal thickness and sediment routing pathways have been interpreted in order to assess the impact of tectonic and paleoceanographic controls on spatial and temporal changes in all of the evaluated shallow marine successions thin tide influenced intervals were deposited in back barrier and lagoon systems associated with net transgressive shorefaces and estuaries however it is notable that all preserved tide dominated and tide influenced regressive deltaic systems are located along the northern margin of the utah bight a tectonically related embayment that had a pronounced expression during the middle campanian the southern margin of the embayment and coastlines to the north are conspicuously more wave dominated which supports their exposure to a larger wave fetch in contrast the northern margin of the embayment was relatively wave protected it is concluded that tidal range was amplified due to resonance of the principal semidiurnal tide in the strongly embayed geometry of the middle campanian utah bight

seismic architecture stratigraphy and evolution of a subtropical marine embayment maputo bay mozambique abstract the seismic stratigraphy architecture and evolution of a large subtropical embayment maputo bay southern mozambique is described nine seismic units are documented and show repeated phases of incised valley formation and transgressive infilling trangressive infilling alternates between the classic wave dominated transgressive succession of fluvial and central estuarine bay deposits and the filling by intertidal flat and tidal bedforms developed during stages of inlet restriction and shallowing associated with barrier growth at stillstand intervals imprinted on the succession are several key facets of subtropical stratigraphy hurricane generated erosional surfaces and beachrock horizons the upper stratigraphy comprises the highstand bayhead delta a prograding feature constructed by several flood events in comparison with literature from other similar systems such as lagoons coastal lakes and estuaries a generic three stage stratigraphic model of large marine embayments is proposed that includes incision during glaciations transgressive infilling during deglaciations and shallowing during the stillstand episodes of the recent deglaciation

magnesium staurolite and green chromian staurolite from fiordland new zealand staurolite with up to 6 5 mgo and magnesium as the dominant divalent cation is recorded from an incompletely reconstituted metatroctolite from fiordland new zealand it occurs in complex assemblages including pyrope hornblende spinel and corundum and is estimated to have crystallized at about 12 kbar and 750 degrees c the a cell edge of this staurolite 7 891 a o is markedly larger than that of other natural staurolites which may be attributed to substantial substitution of mg for al in the kyanite layer of the staurolite structure a green staurolite with 2 cr 2 o 3 is also recorded from a mafic metatuff from fiordland

zincian staurolite in the dry river south volcanic hosted massive sulfide deposit northern queensland australia an assessment of its usefulness in exploration abstract staurolite from the dry river south volcanic hosted massive sulfide deposit in northern queensland australia is enriched in zno 2 5 6 8 only within the massive sulfide lens or in highly pyritic biotite chlorite schist just below the massive sulfide lens in the footwall alteration zone staurolite zno levels vary between 0 4 and 1 3 whereas in hanging wall metagraywackes and metapelites the zno content is mainly below 1 0 as staurolite from metapelitec rocks contains up to 7 9 zno high zno levels in staurolite do not necessarily indicate a relationship to zn rich massive sulfide staurolite grains from the dry river south amd other massive sulfide lenses have low tio2 concentrations mainly 0 4 the low concentration of tio2 in staurolite from the massive sulfide lens results from the low initial ti concentration in exhalative ores high zno and low tio2 values are indicative of staurolite associated with zn rich massive sulfide zincian staurolite is a potential exploration indicator at both prospect and reconnaissance scales at the prospect scale zinc levels have potential in distinguishing true from false gossans distinguishing low grade portions of massive sulfide lenses from barren massive pyrite bodies and characterizing alteration zones at the reconnaissance scale staurolite can be collected in the heavy mineral fraction of stream sediments and multiple grains can be analyzed rapidly using modern electron microprobes

extensional deformation along the late precambrian cambrian baltoscandian passive margin the sarektjakka nappe swedish caledonides the structural evolution of a part of the late precambrian baltoscandian passive margin just before the inception of seafloor spreading is described recording the change from deformation by faulting to dominantly magmatic extension of the crust the allochthon of the scandinavian caledonides contains the imbricated passive margin of continental baltica overlain by various exotic terranes the sarektjakka nappe in the seve nappe complex which contains the outer parts of baltica s passive margin consists of sedimentary rocks occurring as screens between vendian 573 74 ma diabase dykes these dykes constitute 70 80 of the nappe and locally form sheeted dyke complexes the sarektjakka nappe largely escaped penetrative caledonian deformation and preserves igneous metamorphic and structural elements that are linked to the evolution of a pre caledonian rift to a passive continental margin extensional deformation before dyke emplacement is recorded by normal faults pull apart structures and folds unconformities dykes affected by brittle deformation and fluidization of sediments during dyke emplacement indicate close relations between the deposition of sediments extensional deformation and dyke emplacement the sarektjakka nappe is compared with other parts of the baltica s passive margin and its tectonic evolution is discussed

fission track evidence on the initial rifting of the red sea two pulses no propagation fission track analyses indicate that the red sea initially opened simultaneously along its entire length two distinct pulses of uplift and erosion characterized the early stages of rifting in the red sea throughout egypt and in southwestern saudi arabia the first pulse began at 34 million years ago ma the second pulse began in the early miocene 21 to 25 ma and marked the start of the main phase of extension these data support a rigid plate model for continental extension these results also indicate that the initiation of rift flank uplift and therefore rifting and volcanism occurred nearly simultaneously this conflicts with classical models of active and passive extension that predict sequential development of these features

sedimentology and depositional architecture of tidal compound dunes on a carbonate ramp the lower miocene deposits of the central apennine latium italy abstract the sedimentology and sequence stratigraphy of the central apennine lower miocene carbonate deposits guadagnolo formation are the goal of this paper the guadagnolo carbonate ramp deposits consist of a thick succession of three main lithofacies marls marly limestones and cross bedded limestones the lateral and vertical facies distribution as well as the biota assemblages suggests a deposition of these sediments along the middle outer ramp sector of the latium abruzzi carbonate platform all the data suggest sedimentation under the influence of tidal currents that were responsible of bedforms generation as simple and compound dunes these bodies are developed on metric and decametric scale and are stacked one to other to form complex sedimentary bodies extending both in strike and dip section for several tens to hundred meters the dune system developed in a semiclosed basin the paleoadriatic sea open in the southern and closed in the northern sector respectively within this basin a probably amphidromic system developed the flow sediment transport was dominantly westward and was conditioned by the ramp paleotopography from a sequence stratigraphic point of view several high and low rank depositional sequences that were differentiated basing on their relative physical scale thickness of each unit and on the lateral extension of the unconformities and the correlative conformities bounding them were recognized the hierarchy of recognized sequence stratigraphic units include from the smallest to the largest simple depositional sequences low rank composite depositional sequences and high rank composite depositional sequences in the guadagnolo succession four high rank composite depositional sequences having a duration variable from 0 9 to 1 6 ma and named guadagnolo 1 2 3 and 4 were recognized these high rank composite sequences are internally constituted by a stacking of simple and low rank composite depositional sequences having a duration ranging from 40 ka to 200 ka all these units constitute part of a higher rank composite sequence developing between 21 and 14 80 ma that we name the guadagnolo depositional sequence the wedge shaped geometry the thickness variation and the stacking pattern of the guadagnolo succession are the response to eustasy and tectonic subsidence the glacial eustasy mostly controlled the formation of the high frequency depositional sequences tectonic subsidence related to the roll back of the hinge west directed subduction in turn connected to the advancement of the apennine thrust modulated the accommodation space

experimental study of abrasion and its role in producing downstream fining in gravel bed rivers abstract previous experimental studies on abrasion of gravel showed diminution rates much lower than those typical of gravel bed rivers this might be one reason many studies of downstream fining in rivers stress hydraulic sorting as the dominant fining mechanism previous experiments however did not replicate well the particle to particle impact between the bed load gravels and bed gravels in rivers on alluvial fans during floods the purpose of this study was to reexamine the effect of abrasion on downstream fining to determine if abrasion is responsible for longitudinal changes in gravel size in rivers on alluvial fans a rotating drum with three interior vanes was used to produce repeated collisions among test gravel particles in water the drum was only partially filled with water in order to simulate the impact velocity of particle collisions during a flood the test particles were slightly weathered andesite and chert taken from the bed of the watarase river japan experiments were conducted using three unisize grains l 7 0 128 mm to 6 5 91 mm m 5 5 45 mm to 5 0 32 mm s 4 0 16 mm to 3 5 11 mm and mixtures of two different sizes from the three size classes there are six principal results from the experiments 1 test gravel particles break frequently and decrease in weight rapidly 2 abrasion of chert produces mostly gravel while andesite produces mostly sand and silt 3 l size chert cobbles decrease in weight rapidly as a result of being broken into smaller pieces while andesite cobbles break so rarely that their weight decreases very slowly 4 s size andesite pebbles decrease in weight more rapidly than chert 5 abrasion depends strongly on the presence of different sizes because smaller fragments are crushed by larger gravel particles 6 diminution coefficients of andesite and chert obtained from the experiment are in the range of 10 3 10 1 km 1 diminution coefficients from this experiment are one to two orders of magnitude larger than those from previous experiments on abrasion this is mainly because of the larger impact velocities in this experiment diminution coefficients from the experiment are also consistent with those obtained from many japanese rivers on alluvial fans 10 2 10 1 km 1 this result suggests that downstream fining of river bed gravel on alluvial fans can be explained by abrasion

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

chemical and isotopic 87sr 86sr δ18o δd constraints to the formation processes of red sea brines abstract about twenty deeps filled with hot brines and or metalliferous sediments are located along the red sea axis these brines present a well suited framework to study the hydrothermal activity in such a young ocean the present study outlines the results of a geochemical approach combining major trace element and isotopic oxygen hydrogen strontium analyses of brines in six of the deeps to evaluate different processes of brine formation and to compare the evolution of each deep important heterogeneities in temperature salinity hydrographic structure and chemistry are recorded each brine having its own characteristics the intensity of hydrothermal circulation varies among the deeps and ranges from being strong atlantis ii and nereus to weak port soudan and even to negligible valdivia and suakin and it varies along the entire red sea axis these observations do not favour a unique formational model for all of the brines for example the brines of the suakin deep appear to have been formed by an old sea water which dissolved evaporite beds without significant fluid circulation and hydrothermal input while others such as atlantis ii or nereus deeps appear to be dominated by hydrothermal influences a striking feature is the absence of a relationship between the position of the deeps along the axis and their evolutionary maturity

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

the miocene sommieres basin se france bioclastic carbonates in a tide dominated depositional system abstract the miocene sommieres basin in se france is a semi enclosed depression that was connected to the mediterranean sea by a flooded paleo incised valley and then filled by a suite of sediments comprising carbonate grains coming from temperate factories that were largely deposited in tidal dominated paleoenvironments the strata are partitioned into two sequences that reflect repeated flooding of the incised valley system one of several similar situations in this region of france the carbonate grains are mostly bioclasts namely from barnacles bryozoans coralline algae encrusting branching and rhodoliths echinoids and benthic foraminifers large and small with ostracods sponge spicules and planktic foraminifers prominent in muddy facies particles were produced by shallow water carbonate factories on hard substrates valley walls in particular associated with subaqueous dunes and in deeper water basinal settings each depositional sequence is underlain by an eroded and bored hard surface that is progressively overlain by tst subaqueous tidal dunes or storm deposits that grade up in one case into hst marls the hst of the upper sequence has been removed by erosion the lower sequence is ebb tide dominated whereas the upper sequence is flood tide dominated the succession is interpreted to represent a tst whose tidal currents were focused by the narrow valley and a hst that reflected flooding of the overbanks this stratigraphic and depositional motif is comparable to that in other spatially separated neogene paleovalleys that are filled with tide dominated clastic carbonates in the region together with other recently documented similar systems these limestones constitute an important new group of carbonate sand bodies in the carbonate depositional realm

tectonic development of passive continental margins of the southern and central red sea with a comparison to wilkes land antarctica abstract the continental margins of the southern and central red sea and most of wilkes land antarctica have bulk crustal configurations and detailed structures that are best explained by a prolonged history of magmatic expansion that followed a brief but intense period of mechanical extension extension on the red sea margins was spatially confined to a rift that was 20 30 km in width the rifting phase along the arabian margin of the central and southern red sea occurred 25 32 ma ago primarily by detachment faulting at upper crustal levels and ductile uniform stretching at depth rifting was followed by an early magmatic phase during which the margin was invaded by dikes and plutons primarily of gabbro and diorite at 20 24 ma after the crust was mechanically thinned from 40 km to 20 km we infer continued spreading after that in which broad shelves were formed by a process of magmatic expansion because the offshore crust is only 8 15 km thick including sediment and seismic reflection data do not depict horst and graben or half graben structures from which mechanical extension might be inferred the wilkes land margin is similar to the arabian example the margin is about 150 km in width the amount of upper crustal extension is too low to explain the change in sub sediment crustal thickness from 35 km on the mainland to

the quaternary history of the subtidal central wash eastern england the history of quaternary sedimentation in the subtidal wash is described using high resolution seismic profiles the pleistocene sequence is divided into three depositional units comprising anglian till overlain by possible late devensian subglacial scour fill and lacustrine sediments these latter sediments may provide further evidence for a lake in the wash impounded by ice along the lincolnshire norfolk coast the holocene sequence is divided into six depositional units each truncated by the one above estuarine sediment resting on a marine flooding surface forms the earliest unit this sediment was partially eroded by migration of the shoreface as the marine flooding progressed landward the following four units comprise sand and gravel banks deposited on the erosion surface bank deposition was followed by an episode of tidal scour caused either by increased tidal current velocities following reclamation of the fenland or by breakdown of postulated former offshore barriers the youngest and most extensive holocene unit rests on the scoured surface and comprises several types of deposit these are large sand banks around the periphery of the subtidal area with sediment extending seawards into two ne sw aligned troughs low sand banks on a central ridge dividing the troughs and partially covering the sediments in the troughs thick gravels towards the mouth of the wash muddy sediments forming drapes over the sand in the centre of the wash the data provide information on the variety of processes related to the advance and retreat of pleistocene ice sheets in eastern england and the subsequent holocene marine flooding of the wash fenland embayment the holocene sequence reveals periods of widespread sedimentation separated by periods of both local and regional erosion with possible implications for climatic and hydrodynamic change 1997 john wiley sons ltd

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

pleistocene macrotidal tide dominated estuary delta succession along the dong nai river southern vietnam abstract this paper focuses on the facies and sequence stratigraphic interpretation of a tide dominated incised valley system that is rare among geological records of the world for its completeness the subject is the middle upper pleistocene ba mieu and thu duc formations exposed along the dong nai river southern vietnam both the ba mieu and the thu duc formations are interpreted as a transgressive tide dominated estuary succession overlain by a highstand tide dominated delta succession the tide dominated estuary succession is generalized as follows in ascending order fluvial facies in landward portions tidal flat upward coarsening and thickening succession of marine sands with a tidal ravinement surface trs created by tidal scouring during transgression at the base and subtidal lag in seaward portions this tide dominated estuary is distinguished from wave dominated and wave and tide dominated estuaries by 1 the lack of a central basin lagoon tidal delta or bayhead delta facies 2 the widespread presence of marine sands 3 the presence of a subtidal lag and 4 the presence of an upper flow regime sand flat facies the tide dominated delta succession is generalized as follows in ascending order upward convex unit of marine sand and onlapping tidal flat deposits a thick upward fining succession of tidal flat salt marsh deposits and coarse fluvial deposits this delta is distinguished from an estuary as follows 1 it shows thick regional upward fining successions indicating shallowing whereas upward coarsening successions indicating deepening are recognized in an estuary a maximum flooding surface mxfs is recognized in the coarsest part of the succession in marine sand and tidal flat deposits 2 fluvial processes dominate the sediment supply in contrast to an estuary in which a thick marine sand body fed from marine sources is dominant 3 ebb tidal currents are dominant whereas flood tidal currents are dominant in an estuary key features of the incised valley system are 1 amplification of the incised valley geometry by tidal ravinement erosion 2 development of tidal channels filled with mud clasts in the estuary 3 a thick marine sand body in the estuary and 4 upward convex unit of marine sand and a thick tidal flat fluvial succession in the delta the ba mieu formation is recognized as depositional sequence sq 1 between sequence boundary sb 1 below and sb2 above sq1 consists of transgressive systems tract tst 1 and highstand systems tract hst 1 sq1 was formed by a transgression regression cycle during marine isotope stage mis 7 sb2 was formed by fluvial incision during the mis 6 lowstand and is amalgamated with the trs of mis 5 the thu duc formation is recognized as sq2 between sb2 below and sb3 above and it consists of tst2 and hst2 sq2 was formed by a major transgression regression cycle on which a lower rank cyclicity was superposed during mis 5 tst2 thins seaward as a result of tidal ravinement erosion the landward extent of marine environment in mis 5 is inferred to have been of greater scale than that in mis 7 from the distribution of the facies and the trs

how much divergence has occurred between africa and arabia as a result of the opening of the red sea large coastline gaps in red sea palinspastic reconstructions are not tenable if interpretations of seismic refraction data from the continental margin of southwest saudi arabia in the southern red sea typify the crustal configuration of most of the basin the composition of the crust beneath 4 6 km of shelf and coastal plain sediment is not certain ely assuming 1 that it is extended continental crust that could be reorganized in a kinematically reconstructed cross section of the prerift continent the minimum possible divergence between the stable parts of africa and arabia can be determined the maximum divergence is derived by assuming 2 that the crust formed from a mantle source by rift related igneous processes i e oceanic crust such crust would disappear during the reconstruction assumption 1 suggests that the widest possible shoreline gap was 30 km at lat 17 n corresponding to a minimum divergence of about 320 km the arabian and african shorelines would overlap by at least 25 40 km if assumption 2 is used this suggests a maximum divergence of about 380 km the buried rocks are probably a mixture of continental and oceanic types thus some amount of coastline overlap is desirable in tectonic reconstructions of the early afro arabian continent

a geophysical transect of the adelie margin east antarctica abstract in 1982 a geophysical survey of the antarctic margin including multichannel seismic gravity magnetic and bathymetric surveying was carried out off adelie coast wilkes land and in the eastern ross sea of the 5000 km of lines recorded 3000 km were in the adelie coast area lines atc 101 102 approximately following meridian 138 e show the first complete transect of the adelie coast margin from the southeast indian abyssal plain at dsdp site 269 to the continental shelf these lines reveal a thick sedimentary series divided into three main acoustic units by two major unconformities considered to be upper eocene 42 ma and late oligocene 25 ma oceanic or continental basement can be traced under the whole area and the ocean continent boundary clearly lies beneath the lower continental slope a deep high amplitude low frequency horizon extending under the oceanic basement for over 300 km of line is considered to be the moho our interpretation supports a recent revision be cande and mutter proposing an early upper cretaceous opening between australia and east antarctica

results from three refraction profiles in the northern red sea above 25 n recorded with an ocean bottom vertical seismic array abstract we present here results of three refraction profiles obtained during the minos cruise march april 1986 in the northern red sea above 25 n with our new marine seismic observation system the ocean bottom vertical seismic array obvsa in the first part of the paper we briefly describe the instrument and give some of its main characteristics in the second part we present a velocity depth model for each one of the three non reversed profiles the model is first inferred from interpretation of the hodochron using the slope intercept method and assuming plane interfaces and iteratively modified by application of a ray tracing program our results support other data collected durng the cruise with expanding spread profiles and reported elsewhere in this issue by gaulier et al 1988 the two northern profiles are about 50 km long and are oriented n15 e the recording instrument was located at the southern extremity of the profiles at 26 35 n 34 46 e and 27 00 n 34 26 e respectively the basement velocity was found to be in the 5 2 5 8 km s 1 range the moho is located at about 14 km below the sea level the upper mantle apparent velocity is about 7 3 km s 1 the southern profile is 40 km long oriented s45 w with the obvsa located at its eastern extremity at 26 06 n 35 23 e the moho depth is about 11 5 km the crustal structure under the eastern part of the profile is interpreted as being of the oceanic type

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

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

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

titanium and the color of staurolite the visually assessed colour intensity of staurolite appears to be directly proportional to the titanium content it is inferred that the colour is caused by absorption due to fe 2 or ti super 4 charge transfer from this and the pleochroic scheme it is further inferred that the titanium in staurolite is located in the tetrahedrally coordinated fe site

seismic reflection signature of cretaceous continental breakup on the wilkes land margin antarctica the passive rifted continental margin of wilkes land antarctica is characterized on seismic reflection records by i in the south a block faulted sequence of highly stratified continental beds overlain by two distinct unconformities ii a transitional greatly thinned continental crust overlain by material interpreted to be flood basalt and iii in the north oceanic crust with a boundary ridge at its edge the mohorovicic discontinuity can be followed across the continent ocean boundary and shows a progressive thinning of continental crust to a minimum of 2 5 kilometers at its northern edge

rift basins of the long island platform four buried early mesozoic rift basins on the long island platform region of the united states atlantic continental margin were mapped using seismic reflection magnetic and gravity data low angle normal border faults and high angle cross faults control the structure of three of the basins and some of these faults can be traced deep into the crust faulted basement blocks characterize the fourth basin adjacent basins form pairs in which the border faults dip toward each other and may have acted to localize crustal thinning two phases of rifting took place an early stage characterized by widespread faulting leading to basin formation and a late stage characterized by uplift and erosion near the hinge zone and subsidence seaward of the hinge zone the new york bight basin at the western end of the platform may be an offshore extension of the hartford basin paleozoic tectonic and structural trends are inferred to be the primary controls on the location of these basins although our data are not specific enough to address local structural details

the red sea 50 years of geological and geophysical research abstract the red sea displays lithosphere that is in transition from rifting to drifting and therefore provides enormous opportunity to understand incipient seafloor spreading development despite more than 50 years of extensive geological and geophysical research there remains significant conjecture concerning the timing of sea floor spreading initiation the extent of spreading along the axis of the red sea and the geodynamic processes responsible for the onset of crustal extension red sea tectonic models based on geological data are dominated by single stage rift models involving protracted stretching of continental crust followed by sea floor spreading at ca 5 ma and include both asymmetric and symmetrical extension models geophysical data suggest that an oceanic crust occurs beneath the red sea shelves although exactly how this crust has evolved remains undecided equally arguments have been made in favor of the sedimentary shelves forming on extended continental crust and that oceanic crust is restricted to the present day red sea axial valley however our synthesis shows that none of the models proposed so far is applicable to the entire red sea basin the distribution and timing of the red sea crustal extension and sea floor spreading may have been influenced by subduction related far field forces due to complex plate interactions along the northern edge of the arabian plate during early miocene our reassessment suggests that following the arrival of the afar plume the southern red sea underwent rapid extension culminating in the transition from rifting to seafloor spreading which stalled during the early miocene in the central red sea due to either slowing of the convergence between arabia and eurasia or the onset of the arabian plate passive margin collision with eurasia off axis magmatism is focused on the arabian margin and afar depression and has been episodically active since the miocene we consider a formation of the red sea as a hybrid process involving both active and passive rifting

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

crustal structure volcanism and opening mode of the ulleung basin east sea sea of japan abstract a study of the ulleung basin in the southwestern east sea sea of japan based on single and multi channel seismic reflection profiles and ocean bottom seismometer obs data suggests that the basin formed largely by pull apart opening and the deep northern basin is underlain by thicker than normal oceanic crust 10 km thick probably formed during the earliest stage of seafloor spreading volcanic activity in association with basin opening occurred over much of the basin but it ceased first in the south while it continued in the north until recently the principal evidence for pull apart opening includes 1 an overall rhomboidal shape of the basin consisting of two basement lows and a median high known to be characteristic of pull apart basins 2 the location and orientation of the major volcanic sources suggesting more or less n s opening and 3 the strike slip fault system along the western basin margin which appears to have guided the pull apart opening the lack of extensional deformation along the western basin margin which would have been prevalent if sw japan had rotated away from the korean peninsula as suggested by paleomagnetic data is also not in favor of the alternative rotational opening of the basin the absence of graben and rift topography in the acoustic basement in the deep northern ulleung basin is indicative of the non brittle or ductile nature of the crust tau p analysis of the obs data reveals that the igneous crust consists of upper and lower layers with velocities typical of oceanic layers the velocity depth profiles with a two gradient velocity structure is also characteristic of oceanic crust however the lower layer is much thicker than oceanic layer 3 and the change of velocity gradient which occurs at around 6 5 km s in typical oceanic crust occurs at about 6 km s suggesting that the crust is not a typical oceanic crust the meager basin size and short opening period may suggest that the basin opening did not lead to the formation of full fledged oceanic crust but instead resulted in embryonic or incipient oceanic crust thickened by upper mantle thermal perturbations evoked by rifting

a seismic refraction and reflexion study of the continent ocean transition beneath the north biscay margin the structure of the northern margin of the bay of biscay consists of a series of tilted and rotated blocks bounded by prominent listric faults whose polarity is consistently down toward the continent ocean boundary these blocks formed by rifting in late jurassic early cretaceous time and are now thinly covered by post rift sediments of aptian to recent age seismic refraction profiles were occupied on the shelf on either side of and across the continent ocean transition to the shelf using p ubs and o bs with explosives and a 4 x 1000 in 3 4 x 16400 cm 3 airgun array two ship expanding spread multichannel 48 trace seismic reflexion profiles and 30 km fixed offset reflexion profiles were located along the seismic refraction profiles on either side of the transition a two ship 30 km fixed offset multichannel profile was located across the transition as well as a 5 km fixed offset multichannel profile extending from the ocean crust to the shelf conventional 48 trace single ship multichannel profiles were located along all the refraction and two ship reflexion lines interpretation of the refraction profiles has been made by using ray tracing as well as synthetic seismograms conventional seismic processing techniques have been used to prepare the two ship multichannel seismic data for interpretation the survey is believed to be the first attempt to apply two ship multichannel seismic data to the study of the change in crustal structure of a rifted passive margin from the shelf to the ocean crust the results from the experiment led to the identification of a zone of transition between continental and oceanic crust about 8 km wide the seismic refraction data show progressive thinning of the continental crust from 33 km to about 5 km close to the transition zone however extension values calculated in the upper crust from the rotation of fault blocks are much less 1 1 1 4 and suggest that the majority of the thinning is achieved by extensive attenuation of the lower crust

glaciation and deglaciation of the libyan desert the late ordovician record detailed outcrop studies at the flanks of al kufrah basin libya reveal the nature of glacially related sedimentation and post depositional deformation styles produced in association with the late ordovician glaciation during which ice sheets expanded northward over north africa to deposit the mamuniyat formation at the se basin flank jabal azbah the mamuniyat formation is sand dominated and incises interfingering braidplain and shallow marine deposits of the hawaz formation the glacially related sediments include intercalations of mud chip bearing tabular sandstones and intraformational conglomerates which are interpreted as turbidite and debrite facies respectively these record aggradation of an extensive sediment wedge in front of a stable former ice margin an increase in mudstone content northward is accompanied by the occurrence of more evolved turbidites a widespread surface bearing streamlined nw se striking ridges and grooves punctuates this succession the structures on the surface are interpreted to have formed during a regional north westward ice advance above siltstones bearing arthrophycus burrows and orthocone bearing sandstones beneath tidal bars testify to glaciomarine conditions for deposition of the underflow deposits beneath by contrast the northern basin margin jabal az zalmah is appreciably different in recording shallower water paralic sedimentation styles and major glaciotectonic deformation features although facies analysis also reveals northward deepening here a siltstone wedging from 8 to 50 m toward the north was deposited lower delta plain succeeded by climbing ripple cross laminated sandstones up to 60 m in thickness distal through proximal delta mouth bar deposits with occasional diamictite interbeds these rocks are deformed by thrusts and 50 m amplitude fault propagation folds the deformation locally sealed by a diamictite then overlain by conglomeratic lag during ultimate deglaciation integrating observations from both basin margins a model of fluvial dominated delta systems feeding a pulsed debrite and turbidite fan system in a shallow proglacial shelf is proposed

morphotectonics of rifted continental margins in embryo east africa youth africa arabia and maturity australia rifted margin s develop through embryonic youthful and mature stages and some diversity is attributable to the along strike position of the embryonic rift valley stage between the extremes of the crest of a rifted arch and the saddle between arches in east africa arabia an embryonic stage of arches and saddles cleft by rift valleys is connected with the youthful 20 m y red sea gulf of aden in australia the 53 m y southern margin represents early maturity and the 125 to 160 m y western margin late maturity the dominant morp ho tectonic feature of the embryo is the arch 2000 km long and 1000 km across cleft by rift valleys and separated along strike from its neighbour by a saddle with the emplacement of oceanic litho sphere the arch splits into half arches or rims along the newly formed margins the rims subside about a hinge situated 500 km landward of the continent ocean boundary the rim along the saddle becomes submerged soon after breakup whereas the rim along the crest initially s

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

the goban spur transect geologic evolution of a sediment starved passive continental margin leg 80 of the dsdp ipod program drilled a transect of four core holes 548 551 across the continent ocean boundary at goban spur a prominent southwest trending structural high on the irish continental slope multichannel seismic reflection profiles show that during rifting continental basement rocks of goban spur were broken up by northwest trending listric normal faults to form a series of half graben basins two of these half grabens were sampled during leg 80 sites 548 and 549 site 550 was located on the adjacent oceanic crust of porcupine abyssal plain site 551 was located on transitional crust at the foot of goban spur the objectives were lo analyze the structural the depositional and the paleoenvironmental development of this sediment starved passive continental margin at sites 548 and 549 basement comprises continental hercynian metasediments of devonian age at sites 550 and 551 the basement is tholeiitic basalt the oldest syn rift sediments barremian age or perhaps late hauterivian were penetrated at site 549 lying uuconformably below aptian strata seismic sequence analysis reveals that aptian strata also overlie this unconformity farther northeastward in the basin an unconformity above the aptian section marks the end of rifting and the beginning of sea floor spreading an albian age for the initiation of sea floor spreading was corroborated at site 550 where abyssal late albian chalks rest upon and are interbedded with oceanic basalts indicating an initial water depth of 2 000 m as sea floor spreading progressed goban spur subsided rapidly so that by cenomanian time bathyal to abyssal chalks were accumulating at deeper sites after two periods of partial stagnation in the aptian albian and in the turonian chalk deposition in well oxygenated environments took place at all sites modified chiefly by shifts in deep circulation patterns and the calcite compensation depth ccd by periodic influx of terrigenous detritus during low stands of sea level especially in the cenozoic and by frequent displacement of older carbonates from the slope to abyssal sites a number of major unconformities correspond to those most often reported from other widespread locations in the north atlantic basin and on surrounding continental shelves and coastal plains several unconformities are preserved undisturbed in our cores and can be correlated with sea level fluctuations with paleoceanographic events and with tectonic movements a thick quaternary section at site 548 records prominent fluctuations of glacial inter glacial paleoclimates an even thicker paleogene section at site 549 provides unusually well preserved and uninterrupted sequences suitable for detailed sedimentological and stratigraphic studies

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

an interdisciplinary approach to reservoir characterisation an example from the early to middle eocene kaimiro formation taranaki basin new zealand abstract the kaimiro formation is an early to middle eocene ne sw trending reservoir fairway in taranaki basin and comprises a range of coastal plain through to shallow marine facies a time of regional transgression is observed across the paleocene eocene transition which is linked to a general global warming trend and to regional thermal relaxation related subsidence in new zealand the earliest eocene transgressive deposits pass upwards into a series of cyclically stacked packages interpreted as 3rd and 4th order sequences maximum regression occurred within the early eocene and was followed by punctuated retrogradational stacking patterns associated with shoreline retreat and subsequent regional transgression in the middle eocene the kaimiro formation is considered a good reservoir target along most of the reservoir fairway which can largely be attributed to a consistently quartz rich lithic poor composition and reasonably coarse sand grain size correlations demonstrate that within the early eocene the main reservoir facies are channel fill sandstones overlying candidate sequence boundaries in paleoenvironmentally landward proximal settings and upper shoreface shoreline sandstones in relatively basinward distal settings middle eocene reservoir facies are not represented in distal wells due to overall transgression at this time yet they form a significant target in more proximal well locations particularly on the taranaki peninsula depositional facies is one of the principal controls on sandstone reservoir quality however while reservoir facies have been proven along the length of the reservoir fairway it is evident that diagenesis has significantly impacted sandstone quality relatively poor reservoir properties are predicted for deeply buried parts of the basin maximum burial 4 5 km due to severe compaction and relatively abundant authigenic quartz and illite in contrast good reservoir properties are locally represented in reservoir facies where present day burial depths are these results illustrate how an interdisciplinary approach to regional reservoir characterisation are used to help reduce risk during prospect evaluation assessment of both reservoir distribution and quality is necessary and can be undertaken through integrated studies of facies sequence stratigraphy burial modelling and petrography

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

the influence of tectonics on early cretaceous sedimentation in bedfordshire england the lower cretaceous sediments of the bedfordshire area are aptian and albian in age during this time the area was situated within an extensional tectonic regime and the pattern of sedimentation in terms of lateral variation both in sediment type and thickness reflects the influence of normal faulting jurassic topography created by block faulting and erosion controlled the distribution of later facies largely by continued tectonic processes especially the reactivation of the jurassic fault block system which produced syn sedimentary faulting of the overlying sequence thus the traditional view of the infill of an erosional basin by the lower greensand is challenged

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

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

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

models of the development of the west iberia rifted continental margin at 40 30 n deduced from surface and deep tow magnetic anomalies the ocean continent transition oct on nonvolcanic rifted continental margins is important in that it contains evidence concerning the breakup of the continents and the onset of seafloor spreading the nature of the oct off western lberia has recently been attracting attention following seismic and other geophysical studies there and drilling of acoustic basement by leg 149 of the ocean drilling program here we concentrate on the interpretation of a new digital magnetic anomaly chart for the iberia abyssal plain between 39 5 o and 42 2 o n the most striking anomaly is a trough which exists immediately east of and locally parallel to anomaly j and is closely coincident with a basement peridotite ridge we conclude from modeling surface magnetic anomalies and a deep tow magnetometer profile that seafloor spreading began about 129 9 m y ago barremian at a rate of 10 0 mm yr consistent with drilling and seismostratigraphic results from the southern lberia abyssal plain a second feature is the existence of linear low amplitude isochron parallel magnetic anomalies east of the peridotite ridge trough and a clear change in the trend of these anomalies east of about 11 o 15 w it is possible to model these anomalies by a series of 7 to 30 km wide weakly magnetized crustal blocks of about the same thickness as the oceanic crust the west to east anticlockwise change in trend at 11 o 15 w suggests either a major change in stress regime and or in the type of magnetic anomaly source we speculate that the region between the peridotite ridge and 11 o 15 w represents a transitional crust whose formation was dominated by the same stress regime which accompanied eventual seafloor spreading we explain the change in trend at 11 o 15 w by a model featuring a rift which propagated through continental crust from south to north we consider three scenarios for the formation of the transitional zone crust first continental crust is surrounded and impregnated by intrusive and extrusive material formed from passive upwelling of magma created by the limited adiabatic decompression partial melting which occurs as the two continental lithospheric plates first begin to separate second such crust is surrounded by unroofed upper mantle material or third the zone is the product of ultraslow seafloor spreading present evidence tends to support the first scenario in this scenario the top of the asthenosphere eventually becomes shallow enough to enable magma to penetrate the thinnest continental crust fill the space caused by additional extension and form the transitional crust already described over a broad zone at least several tens of kilometers wide at breakup upper mantle rocks become exposed on the seafloor and undergo extensive serpentinization in a final stage we envisage focusing of the igneous activity leading to the onset of seafloor spreading initially producing thin oceanic crust because of a restricted melt supply the width of continuous tectonically extended continental crust in the southern iberia abyssal plain is less than was previously supposed

aluminous reaction textures in orthoamphibole bearing rocks the pressure temperature evolution of the high grade proterozoic of the bamble sector south norway aluminous reaction textures in orthoamphibole bearing rocks from the froland area bamble south norway record the prograde pressure temperature path of the high grade kongsbergian orogeny c 1600 1500 ma and the low mid amphibolite facies overprint during the sveconorwegian orogeny c 1100 1000 ma the rocks contain anthophyllite gedrite garnet cordierite biotite quartz andalusite kyanite cr rich staurolite tourmaline ilmenite rutile and corundum in a variety of parageneses the p t path is deduced from petrographic observations mineral chemistry and zoning geothermometry and n fmash equilibria the results indicate the sequence of metamorphic stages outlined below a an m1 phase characterized by the presence of strongly deformed andalusite gedrite and tourmaline b an m2 phase with the development of kyanite after andalusite and the growth of staurolite associated with strong na al mg zoning in orthoamphibole indicating an increase in pressure 4 8 kbar and temperature 500 650 c c pressure decrease at high p 6 7 kbar and high t 600 700 c during m3a with the production of cordierite corundum between kyanite staurolite and orthoamphibole and cordierite growth between corundum and orthoamphibole d temperature increase to 740 60 c and 7 kbar static growth of garnet m3b at the metamorphic climax peak t the heat supply necessary to explain the temperature increase between the m3a and m3b phases is correlated with synkinematic enderbitic charnockitic and basic intrusions in the arendal granulite facies terrain e m3b metamorphic conditions were followed by an initial isobaric cooling path early m4 and late stage pressure decrease late m4 early m4 conditions of 6 7 kbar and 550 600 c assuming ph2o ptotal are indicated by a retrograde talc kyanite quartz assemblage in late quartz cordierite veins late m4 conditions of 3 4 kbar and 420 530 c are inferred from a kyanite andalusite chlorite quartz assemblage in vein cordierite the m1 m3 stages are interpreted as being the result of the same metamorphic p t path which was caused by both tectonic and magmatic thickening a prolonged crustal residence time is proposed for the bamble sector before uplift during the later stages of m4 occurred

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

crustal growth in south eastern australia evidence from lower crustal eclogitic and granulitic xenoliths mafic and ultramafic xenoliths in a basaltic cone at the anakies in south eastern australia are geochemically equivalent to continental basaltic magmas and cumulates the xenolith microstructures range from recognizably meta igneous for intrusive rocks to granoblastic for garnet pyroxenites contact relationships between different rock types within some xenoliths suggest a complex petrogenesis of multiple intrusive metamorphic and metasomatic events at the crust mantle boundary during the evolution of south eastern australia unaltered spinel lher zolite typical of the uppermost eastern australian mantle is interleaved with or veined by the metamorphosed intrusive rocks of basaltic composition geothermobarometry calculations by a variety of methods show a concordance of equilibration temperatures ranging from 880 c to 980 c and pressures of 12 to 18 kbar 1200 1800 mpa these physical conditions span the gabbro to granulite to eclogite transition boundaries the water vapour pressure during equilibration is estimated to be about 0 5 of the load pressure using amphibole breakdown data large fluid inclusions of pure co2 are abundant in the mineral phases in the xenoliths and it is suggested that flux of co2 from the mantle has been an important heat source and fluid medium during metamorphism of the mafic and ultramafic protoliths at the lower crust upper mantle boundary the calculated pressures and temperatures suggest that the south eastern australian crust has sustained a high geothermal gradient in addition the nature of the mineral assemblages and the contact relationships of granulitic rock with spinel lherzolite characteristic of mantle material suggest that the moho is not a discrete feature in this region but is represented by a transition zone approximately 20 km thick these inferences are in agreement with geophysical data including seismic heat flow and electrical resistivity data determined for south eastern australia underplating at the crust mantle boundary by continental basaltic magmas may be an important alternative or additional mechanism to the conventional andesite model for crustal accretion

transgressive changes from tidal estuarine to marine embayment depositional systems the lower cretaceous woburn sands of southern england and comparison with holocene analogs the lower cretaceous woburn sands lower greensand group in southern england constitutes one of the most intensively studied tidal sandstone outcrops for sedimentological and reservoir analog studies most recent workers have interpreted the whole 30 60 m 100 200 ft thick succession around leighton buzzard as representing an ancient tide dominated estuary however unequivocal estuary characteristics are limited to the lowermost part about 15 20 m 50 66 ft we suggest that a significant portion of the woburn sands and most of the middle part was formed in a tide dominated marine embayment hence the vertical facies change from the lower to middle part of the woburn sands is interpreted as a change from 1 a narrow estuary to 2 a broad marine embayment the wash embayment in eastern england is a striking modern analog it receives most of its sediments and waters from marine sources and is largely filled with nondiluted seawater moreover the holocene transgressive history of the wash is remarkably similar to the transgressive evolution of the woburn sands early estuarine sequence models predict landward translation of facies zones along the valley thalweg during transgression but eventual facies translation in the strike direction has not been fully documented or discussed an embayment facies that is not commonly confined in the early incised valley can occur vertically between the estuarine valley fill and marine shelf deposits and is probably underrepresented in current models however this segment of the transgression comprising along strike bay expansion and the development of a broad marine embayment may be more important than previously thought

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

evidence for prograde metamorphic evolution of sri lankan pelitic granulites and implications for the development of continental crust petrological evidence for prograde metamorphic evolution is obtained for pelitic granulites of the newly defined highland complex in sri lanka it includes the occurrences of relict kyanite quartz staurolite corundum kyanite hercynite kyanite and sapphirine kyanite spinel within garnet in the sillimanite quartz rich rocks of special interest is the mineralogy of khondalites graphitic garnet sillimanite k feldspar quartz gneisses the matrix of which is free of hydrous minerals indicating that dehydration reactions have been completed in this kind of rocks however relict hydrous minerals such as al ti rich more than 58 wt al2o3 and more than 1 wt tio2 staurolite and extremely ti rich up to 9 8 wt tio2 biotine occur as inclusions within garnet the local relict mineral assemblages within garnet in khondalites indicate the following continuous staurolite breakdown reactions which may have progressed as temperature increased and pressure decreased staurolite quartz kyanite garnet h2o staurolite corundum kyanite garnet h2o staurolite kyanite hercynite garnet h2o staurolite sillimanite hercynite garnet h2o some of these dehydration reactions are inferred to have also taken place in other widespread pelitic granulites of the highland complex moreover garnet in the highland complex pelitic granulites occasionally preserves growth zoning though modified slightly to extensively by post growth volume diffusion based on the evidence a clockwise prograde p t path of evolution is inferredfor meta sedimentary granulites belonging to the highland complex on the other hand no evidence for prograde metamorphism has so far been obtained in lower grade rocks of the wanni complex to the northwest of the highland complex such a difference between these complexes may be the first petrological feature which supports the new geotectonic division of the sri lankan basement rocks based virtually on the distinctly different neodymium model ages it suggests that these two complexes were amalgamated during the very last part of prograde metamorphism of the highland complex rocks that is during decompression and heating probably by the lower crustal lateral flow after tectonic overthickening and delamination the clockwise prograde p t t path followed by the meta sedimentary granulites of the highland complex is in marked contrast with the near isobaric cooling path for meta igneous granulites suggested by some authors it is proposed that both types of p t t paths albeit for different rock types are possible and resulted from diverse deep crustal processes such as underplating of continental crust by sediments and magmas or near convergent plate boundaries

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

episodic sea floor spreading in the southern red sea abstract the red sea represents the most spectacular example of a juvenile ocean basin on the modern earth synthesis of regional aeromagnetic data gravity data seismic refraction data coupled with structural mapping from the farasan islands suggest that the opening of the red sea is complex and episodic modeling of magnetic and gravity data constrained by seismic refraction data reveals the arabian shelf is underlain by oceanic and transitional crust and that mafic diking and intrusions are focused at the continental transitional crust boundary this relationship is interpreted to indicate that early miocene diking along the arabian escarpment heralded termination of oceanic basin formation and a shift in the locus of extension focused from a central mid ocean ridge spreading center to the continental transitional crust zone uplift along the arabian escarpment caused erosion and middle to late miocene sedimentation of the farasan bank onto existing oceanic crust suggesting that the extensive sedimentary banks of the southern red sea are not passive margins re initiation of spreading occurred at ca 5 ma pliocene to pleistocene shelf reef systems farasan islands developed on the flanks of the spreading ridge are extensively overprinted by normal faults suggesting that not all crustal extension is accommodated by active spreading