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

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

mechanisms of uplift preceding rifting abstract mareschal j c 1983 mechanisms of uplift preceding rifting in p morgan and b h baker editors processes of continental rifting tectonophysics 94 51 66 the feasibility of three mechanisms that have been proposed to account for the uplifts and heat flow anomalies which might precede rifting is examined the conductive heating following an increase in the heat flux at the base of the lithosphere could result in large scale uplifts 1000 m for 15 mw m2 change in heat flux on a time scale of 108 years a pulse of additional heat flow could induce a more rapid uplift but not the surface heat flow anomaly which is always delayed the advection of heat by magma intrusion into the lithosphere could also produce the uplift and heat flow anomaly if magmas are injected at a steady rate an uplift of the order of 1000 m can be induced in about 50 106 years but it requires the rate of magma injection to be equivalent to 0 3 of the total volume of the lithosphere per 106 years a short intense episode of magma invasion could explain a rapid uplift and change in surface heat flow but it requires the replacement of about 30 of the total volume of the lithosphere the complete replacement of the lithosphere by the rise of a diapir of lighter and hotter asthenospheric material has been modelled numerically the model indicates that the rise of the asthenospheric diapir can take place in a very short time 3 106 years and produce the surface uplift and the crustal thinning observed in rift systems provided that following heating the lower lithosphere viscosity is reduced to 1021 pa s

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

new age constraints on the evolution of the mt carmel structure and its implications on a late miocene extensional phase of the levant continental margin abstract the mt carmel structure is one of the most prominent topographical elements along the levant continental margin and is the surface expression of a deep tectonic structure the use of 3d seismic data allows us to set new constraints on the uplift of the carmel structure and define the or akiva fault as its southern boundary the or akiva fault was created during a late miocene extensional phase that had also created the carmel fault at the northern edges of mt carmel the different topographic expression across these two boundary faults reflects a change in the post miocene activity where activity along the or akiva fault ceased and its southern side began subsiding whereas its northern side bordered by the carmel fault continued to rise we conclude that the late miocene north south extension previously recognized only along branches of the dead sea transform in northern israel and lebanon affected larger areas than previously thought and can be related to regional kinematic changes during the breakup of arabia from africa and its collision with eurasia

epeirogenic motions of africa as inferred from cretaceous shoreline deposits epeirogenic motions can be quantified if paleodatum planes can be recognized these motions have been determined for the time interval mid cretaceous cenomanian to recent in continental africa using the vertical displacement of ancient shoreline sediments and other nearshore or low elevation deposits that are inferred to represent a cenomanian sea level datum plane the cretaceous trans saharan seaway brought shoreline sediments deep into the african continental interior and additional data suggest that most of the remaining terrestrial regions were also low lying during the cenomanian the present elevations of the sea level sediments indicate that large portions of africa have undergone epeirogenic uplift since the cenomanian the character of the motions in north africa can be described as small scale high amplitude 3 km doming separated by relatively flat lying areas of deposition the largest single epeirogenic feature is the uplift associated with red sea and east african rift development the timing of post cenomanian epeirogenic motions can be further constrained using paleontologic k ar faulting paleodrainage and sedimentary data the most likely causal mechanism for african epeirogeny may be hot spot activity reflective of the mantle convection regime underlying the african lithosphere

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

cenozoic thermal evolution and tectonics of the coast mountains of british columbia 1 fission track dating apparent uplift rates and patterns of uplift the high relief of the coast mountains of british columbia has been produced in part by rapid late neogene uplift the relationship of this uplift to the presently diverse and complex plate tectonic regime off the west coast of british columbia however is not clear cut to study this problem the areal patterns rates and total amounts of cenozoic uplift from 50 to 55 n have been determined by using fission track dating of zircon and apatite separated from rocks collected at high and low altitudes along several traverses through the mountains accepting closure temperatures of 105 c 10 c and 175 c 20 c for apatite and zircon respectively and a likely closure temperature difference between zircon and apatite of 70 15 c paleogeothermal gradients have been measured at kemano c km 35 ma ago and near ocean falls king island c km 20 ma ago subcrustal heat flow is suggested to have significantly increased in the late neogene near king island uplift rates in the axial part of the northern 52 55 n coast mountains were 0 1 0 2 km ma from 25 to 15 ma ago this uplift was temporally and tectonically related to part of the subsidence in the adjacent queen charlotte basin uplift rates increased somewhat in the late miocene to about 0 4 km ma probably due to passage of the anahim hot spot beneath the area late miocene recent erosion in the north 52 55 n has reduced summit altitudes and been more extensive than farther south despite this relicts of miocene river valleys and topography are still locally preserved partly beneath 7 10 ma basalts the area of the present southern 50 to 52 n coast mountains was near sea level and experienced very low uplift rates 0 5 km ma of the southern coast mountains the profile of this uplift was broadly plateaulike and this uplift led to the inversion of the miocene coastal topography middle cenozoic uplift from 52 to 55 n was probably the result of denudation and diminishing uplift after the terminal paleocene eocene orogenic episode which grossly thickened the crust this middle cenozoic erosion resulted in the restoration of a normal crustal thickness late neogene uplift in the coast mountains was the result of thermal expansion in the mantle caused by a mantle hot spot near 53 n and changes in the geometry of the subducted slab farther south

tectono stratigraphic development of the qamar basin eastern yemen abstract the qamar basin is a polyphase rift basin located in eastern yemen adjacent to the indian ocean it probably originated as part of an extensive rift system which developed during the late jurassic to early cretaceous and led to the break up of gondwanaland rift reactivation during the mid cretaceous affected limited areas including the qamar basin a third episode of rifting occurred during the oligo miocene prior to sea floor spreading in the gulf of aden since spreading in the gulf began in the middle miocene the offshore qamar basin has been thermally subsiding in contrast the onshore part of the basin has been elevated by over 1 km the nature of the pre mid cretaceous basin fill is unknown during santonian to early campanian times thick fluvio deltaic clastics accumulated in a rapidly subsiding depocentre marked deepening during the early campanian led to deposition of deep water limestones during the late campanian prograding deep marine clastics infilled the basin minor transgression during the maastrichtian caused a resumption of marine carbonate deposition sedimentation was interrupted during the early palaeocene following which shallow marine carbonates were deposited over a large area of the arabian peninsula after a hiatus spanning the late eocene to late oligocene carbonate sedimentation resumed offshore in an active rift basin deep water carbonates and carbonate turbidites accumulated in lows whilst reefs developed on footwall highs from the late miocene onwards the reefs have been gradually drowned by rapidly accumulating clastics onshore the tertiary rift episode was associated with major uplift and only a thin veneer of recent wadi deposits overlies the rift topography the superposition of jurassic cretaceous and tertiary rifts which has not been described elsewhere in the region is tentatively related to the evolution of the indian ocean

possible hydration anomaly in the upper mantle prior to red sea rifting evidence from petrologic modeling of the wadi natash alkali basalt sequence of eastern egypt basalt flows of 90 m y age occur in the wadi natash area of the eastern desert of egypt and consist of an assemblage of alkali olivine basalt aob hawaiite mugearite hm and benmoreite the wadi natash occurrence is one of several upper mesozoic to lower cenozoic volcanic suites that surround the red sea and may be related to early mantle activity in the area prior to later rifting the most mafic rock type at wadi natash is aob which could not have equilibrated with the mantle because of low ni contents 50 ppm the aob9s presumably formed by extensive olivine fractionation from a primary picrite the hm9s could have fractionated from the aob9s by separation of a low ti amphibole alternatively both suites may have formed by melting of an eclogitic mantle with greater melting of jadeitic pyroxene contributing to the formation of the hm9s than to that of the aob9s the benmoreites apparently formed by equilibration of high ti kaersutitic amphibole with aob the wadi natash alkali basalt suite was probably generated from a mantle that contained high and possibly varying amounts of water mantle hydration is consistent with observations of the development of small extensional basins in the red sea area during the cretaceous and may be an expected precursor to rifting of the type that now characterizes the red sea

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

pattern and tempo of great escarpment erosion extensive geological geophysical structural and geochronological data suggest that the locations of great escarpments bordering passive margins are exceptionally stable and are probably determined by crustal structure together the data do not support the established paradigm of ongoing significant and parallel escarpment retreat rather thermochronologic data and sedimentary sequences in ocean basins suggest that initial tectonically controlled rift escarpments undergo rapid and significant erosion only during the earliest stages of seafloor spreading development of stable passive margin escarpments follows this period of intense erosion escarpments increase in sinuosity as embayments retreat more rapidly than interfluves measurements of 24 escarpments suggest that sinuosity and the rate at which it increases depends upon the location of maximum uplift the geometry of the preescarpment drainage system and margin age all data suggest that the location of passive margin escarpments does not change significantly over time

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

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

lithospheric strength variations as a control on new plate boundaries examples from the northern red sea region abstract the complex plate boundary between arabia and africa at the northern end of the red sea includes the gulf of suez rift and the gulf of aqaba dead sea transform geologic evidence indicates that during the earliest phase of rifting the red sea propagated nnw towards the mediterranean sea creating the gulf of suez subsequently the majority of the relative movement between the plates shifted eastward to the dead sea transform we propose that an increase in the strength of the lithosphere across the mediterranean continental margin acted as a barrier to the propagation of the rift a new plate boundary the dead sea transform formed along a zone of minimum strength we present an analysis of lithospheric strength variations across the mediterranean continental margin the main factors controlling these variations are the geotherm crustal thickness and composition and sediment thickness the analysis predicts a characteristic strength profile at continental margins which consists of a marked increase in strength seaward of the hinge zone and a strength minimum landward of the hinge zone this strength profile also favors the creation of thin continental slivers such as the levant west of the dead sea transform and the continental promontory containing socotra island at the mouth of the gulf of aden calculations of strength variations based on changes of crustal thickness geotherm and sediment thickness can be extended to other geologic settings as well they can explain the location of rerifting events at intracratonic basins of backarc basins and of major continental strike slip zones

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

two phases of cretaceous tertiary magmatism in the eastern desert of egypt paleomagnetic chemical and k ar evidence abstract paleomagnetic and radiometric data confirm the existence of two phases of igneous activity in the late mesozoic and cenozoic history of eastern egypt in the mid to late cretaceous the earlier and more extensive episode produced the wadi natash alkaline basalts and numerous small trachytic intrusions the ring complexes of abu khrug and gebel el kahfa were also emplaced at this time the mean paleopole position for the intrusive rocks is at 65 3 n 111 2 w a 95 11 4 31 sites chemical analyses of the trachytes show that they are enriched in nb zr and y and depleted in sr ba cr and ni suggesting either prolonged fractionation or limited partial fusion the second phase of activity during oligo miocene time is represented by small tholeiitic basalt flows on the red sea coast and along the nile valley the mean paleopole position for the flows is at 68 2 n 101 5 e a 95 27 8 3 sites the wide scatter is interpreted as reflecting secular variation the chronological and chemical data are used with studies of sedimentary formations coeval with the igneous rocks to apply a general model of continental rifting to the northern red sea the earlier alkaline phase of igneous activity is related to regional uplift while the second tholeiitic phase is associated with widening of the rift valley

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

the cooling history of silurian to cretaceous alkaline ring complexes south eastern desert egypt as revealed by fission track analysis abstract forty fission track ages of apatite zircon and sphene and nine horizontal confined track length distribution patterns in apatite have been used to establish the cooling history of nine silurian to late cretaceous alkaline ring complexes which intrude precambrian basement in the southern eastern desert of egypt zircon or sphene fission track ages were determined from three complexes for whichk ar andrb sr ages on the same samples were also available these ages are concordant and are interpreted as emplacement ages resulting from rapid cooling following high level crustal intrusion into relatively thick volcanic piles average apatite ages for each of the eight ring complexes range from 33 to 167 m y track length distribution patterns for apatites taken together with their ages invite subdivision into two groupings those complexes yielding early oligocene apparent apatite ages suggest cooling from the total track annealing zone followed by a relatively lengthy residence near the base of the partial annealing zone whereas those with late cretaceous ages indicate cooling from a shallower level in the partial annealing zone variations in cooling history resulted from differential uplift between fault bound blocks one block that containing the late cretaceous abu khruq complex was relatively stable and the different degrees of partial resetting recorded in apatites of this complex are attributed to the thermal effect of localised tertiary dyke intrusion fission track analysis in combination with geologic data indicates that in the south eastern desert of egypt a phase of uplift commenced in late oligocene time and was accompanied by paleogeothermal gradients of ca 40 50 c km uplift was more pronounced at least 2 2 5 km in areas within about 100 km from the present red sea coast this uplift which is viewed as part of a broader regional tectonism related to the opening of the red sea occurred along a northwest fracture pattern and was controlled by pre existing lines of weakness in the basement complex

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

maturation anomalies in sediments underlying thick volcanic strata oregon evidence for a thermal event vitrinite reflectance profiles from three drill holes in oregon where sediments are mantled by thick sequences of volcanic rocks suggest that the sediments were subjected to an unusual diagenetic history anomalous near constant maturation profiles in sedimentary sequences more than 1300 m thick indicate that the levels of thermal maturity of organic matter in the sediments below the volcanic cover of the pacific northwest were sufficiently high to have generated hydrocarbons a hydrothermal mechanism appears to be the most effective form of thermal input and supporting evidence can be found in the authigenic mineralization of the sediments the occurrence of vertical maturation profiles in east central oregon may be associated with moderate temperature geothermal systems resulting from a late oligocene to early miocene thermal event related to the incipient stages of basaltic volcanism of the columbia river group the uniform maturation of the entire sedimentary column under the volcanic cover has broad implications for the exploration for oil and gas in the pacific northwest because the presence of mature source rocks in these volcanically lidded basins provides some insight into the petroleum potential of the large volcanic covered areas in oregon washington and california

thermochronological data from sudan in the frame of the denudational history of the nubian red sea margin low temperature thermochronology provides information on the timing of rifting and denudation of passive margins and the red sea with its well exposed young rift margins is a suitable setting for its application here we present new apatite fission track aft data from sudan northern hinterland and red sea coastal areas from the former region we obtained ages between 270 2 ma ad 253 53 ma and from the coastal belt between 83 8 ma and 39 7 ma these data prompted a review and comparison with low temperature thermochronological data from the whole nubian red sea margin and a discussion on their implication in assessing the margin evolutionary style aft data are available for egypt and eritrea as well as apatite u th he ahe ages for two transects transversal to the margin in eritrea both in egypt and eritrea aft data record a cooling event at about 20 25 ma early miocene and an earlier more local cooling event in egypt at about 34 ma early oligocene the thermal modeling of the sudan samples provides an indication of a rapid cooling in miocene times but does not support nor rules out an early oligocene cooling phase the re assessment of new and existing thermochronological data within the known geological framework of the nubian and conjugate arabian margins favours the hypothesis that early rifting stages were affecting the whole gulf of suez red sea gulf of aden system since the oligocene these precocious more attenuated phases were followed by major extension in miocene times as to the mode of margin evolution aft age patterns both in egypt and eritrea are incompatible with a downwarp model the distribution of ahe ages across the eritrean coastal plain suggests that there the escarpment was evolving predominantly by plateau degradation copyright 2009 john wiley sons ltd

a comment on geomorphic development of the escarpment of the eritrean margin southern red sea from combined apatite fission track and u th he thermochronometry by balestrieri m l et al earth planet sci lett 231 2005 97 110 published without an abstract from the introduction the western red sea margin in eritrea comprises 25 km of coastal lowlands backed by a deeply eroded escarpment that rises 2 2 km in 40 km the plateau atop this zone of neogene uplift is capped by 30 ma continental flood basalts cfb the cfbs rest conformably on thin palaeogene 40 ma lateritic palaeosols 1 beneath which is a neoproterozoic metamorphic complex in the southern part the laterites are separated from basement by discontinuous patches of jurassic sandstones the laterites observed in the field and on satellite images occur in this stratigraphic position throughout the hinterland of the western red sea margin from eastern sudan 2 through eritrea and northern ethiopia 3 and 4 to the blue nile gorge 5 a n s range of 800 km and further to the south 6 because they formed on a low relief surface close to sea level 7 across most of ne africa and western arabia 4 the laterites are an important datum for assessing the later effects of regional tectonics 3 and 8 post 30 ma tectonics has displaced occurrences of this datum on the plateau and coastal lowlands by 2 2 2 8 km vertically the opportunities presented by the laterite datum clearly encouraged balestrieri et al 9 and abbate et al 10 to analyse geomorphic processes at the western red sea margin thermochronologically so too did previous work on the initiation of rifting in the red sea and gulf of aden areas 7 11 12 and 13 which converged on initiation of uplift and erosion from 25 to 20 ma on the basis of 13 apatite u th he cooling ages apatite he ages from the eritrean escarpment balestrieri et al 9 conclude a 10 ma younger age for the start of rifting in the southern red sea

crustal structure of the northern harrat rahat volcanic field saudi arabia from gravity and aeromagnetic data abstract new gravity data reveal a prominent negative anomaly along the main vent axis of the northern harrat rahat volcanic field in saudi arabia the low continues north of the volcanic field onto exposures of proterozoic rocks indicating that the low is caused not only by the volcanic field and possibly also underlying cenozoic sediments but also the underlying proterozoic basement an inversion of the gravity field guided by analysis of aeromagnetic data indicates 1 a broad depression of the basement surface that is deeper along the main vent axis in the eastern part of the harrat and in the southwest part of the volcanic field and 2 less dense basement beneath the vent axis low densities within the basement most likely arise from lithologic variations in the basement predating cenozoic volcanism although our analysis does not rule out small volumes of partial melt and higher temperatures or extensive fracturing at depth

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

thermochronologic evidence of major tectonic denudation associated with detachment faulting northern ruby mountains east humboldt range nevada fission track ft thermochronology studies on igneous and metamorphic rocks from the ruby mountains east humboldt range metamorphic core complex provide important constraints on the timing and nature of major mid tertiary extension in northeast nevada rocks from within the ruby east humboldt detachment brittle ductile normal sense shear zone were analyzed the dominant lithology studied was variably mylonitic mafic orthogneiss nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the oligocene circa 36 ma harrison pass pluton were also studied ft ages on apatite zircon and sphene except one 18 4 ma apatite from the basal portion of the detachment zone are concordant and range in age from 26 5 ma to 23 6 ma all ages overlap at 1 between 25 4 ma and 23 4 ma these data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above 285 c sphene closure temperature to below 70 c lower temperature limit of track stability in apatite near the beginning of the miocene minimum cooling rate of 40 c ma the length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly fish canyon ash flow tuff the lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval but apparently only cooled to a temperature within the zone of partial annealing of apatite 70 130 c residence of these rocks in this zone led to a partial retention of fission tracks thus resulting in a reduced age confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history remained hotter longer than structurally higher levels of the zone ft data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23 4 ma rapid cooling of the region is considered to reflect large scale tectonic denudation intracrustal thinning the vertical complement to crustal extension rocks originating in the middle crust 10 15 km were quickly brought near the surface along the ruby east humboldt detachment fault brittle ductile simple shear zone and juxtaposed against brittlely extended rocks deformed under upper crustal conditions

the tectonic significance of pgm bearing chromitites at the ranomena mine toamasina chromite district madagascar this paper presents new data on platinum group mineral pgm bearing chromitites from the ranomena mine the only past producing chromite mine in the north toamasina chromite district of eastern madagascar such data can provide an insight on tectonic setting that led to emplacement of their host ultramafic body and can help to highlight the differences with other chromite deposits of madagascar chromitite host rocks are harzburgite and pyroxenite and gangue minerals are orthopyroxene and minor olivine chromite mineral chemistry shows a strong affinity with chromitites from layered intrusions and pge patterns are similar to those from the stratigraphically lowest chromitites in the bushveld complex pgms are dominated by laurite and are free of alloys pge mineralogy is consistent with early crystallization of laurite erlichmanite at low fs2 previous studies have suggested that the north toamasina ultramafic bodies are fragments of dismembered ophiolites and that their presence within the so called suture zone supports the suggestion that they mark the closure of an ocean between east and west gondwana even though these ultramafic bodies have been interpreted as ophiolitic our data better support an origin as small layered intrusions rather than as fragments of oceanic crust

circular geological structures outcropping in the sedimentary basins of saudi arabia abstract a number of circular geological structures outcrop in the sedimentary basins of saudi arabia several are reviewed here to assess their mode of origin they are unrelated to each other are easily seen on aerial imagery and their origins are assessed here on the basis of new fieldwork and reflection seismic data the structures range in size from hundreds of meters to several kilometers in diameter jabal rayah 28 39 n 37 11 30 e is 5 km in diameter with ring faults and anomalously steep dips in siluro devonian strata reflection seismic data demonstrate that underlying strata are structureless and jabal rayah is interpreted as a probable impact structure ash shutbah 21 37 08 n 45 39 21 e is 2 3 km in diameter and consists of a zone of concentric sub horizontal folds in mesozoic carbonates surrounding a central area of disharmonic steeply plunging folds and a mass of stratigraphically anomalous sandstone there are dissolution structures in the vicinity and although this is a viable explanation the uniquely large and complex nature of ash shutbah suggests an alternative origin by impact reflection seismic data show that underlying strata in the vicinity of ash shutbah are structureless a province of at least 100 circular structures occurs in northeast saudi arabia aerial imagery and reflection seismic data shows that these structures are a mixture of bioherms and dissolution collapse features the kidan crater 22 06 53 n 53 26 38 e is a subcircular feature approximately 100 m in diameter exposed in quaternary sabkha between large sand dunes in the rub al khali field investigation reveals an unfaulted bowl with a slightly fractured floor in sabkha facies 3d reflection seismic data shows a small fault underlying the crater leading to a fluid escape interpretation for the depression

crystal rotation by mechanical interaction between plastically anisotropic crystals abstract in polycrystalline material minerals with pronounced plastic anisotropy may experience inhomogeneous deformation and significant mechanical interaction between neighboring crystals to study the effect of crystal interaction on preferred orientation pure shear of a polycrystalline material with identical orthogonal slip systems was modeled two dimensionally by the finite element method such a material is non ductile in the formal von mises sense because it lacks five independent slip systems as a result each crystal has highly anisotropic strength because a homogeneous irrotational strain history in a two dimensional crystal with identical orthogonal slip systems does not cause lattice rotation rotation caused by crystal interaction is readily observed in the finite element models slip planes rotate toward a preferred orientation parallel to maximum shear stress 45 to shortening and extension directions mechanical interaction between neighboring crystals produces this preferred orientation by rigid body rotation of crystals or parts of crystals not favorably oriented for slip

a diffuse plate boundary model for indian ocean tectonics it is suggested that motion along the virtually aseismic owen fracture zone is negligible so that arabia and india are contained within a single indo arabian plate divided from the australian plate by a diffuse boundary the boundary is a zone of concentrated seismicity and deformation commonly characterized as intraplate the rotation vector of australia relative to indo arabia is consistent with the seismologically observed 2 cm yr of left lateral strike slip along the ninetyeast ridge north south compression in the central indian ocean and the north south extension near chagos

lithospheric structure of the arabian shield from the joint inversion of receiver functions and surface wave group velocities abstract we estimate lithospheric velocity structure for the arabian shield by jointly modeling receiver functions and fundamental mode group velocities from events recorded by the 1995 1997 saudi arabian portable broadband deployment receiver functions are primarily sensitive to shear wave velocity contrasts and vertical travel times and surface wave dispersion measurements are sensitive to vertical shear wave velocity averages so that their combination bridge resolution gaps associated with each individual data set our resulting models correlate well with the observed surface geology the asir terrane to the west consists of a 10 km thick upper crust of 3 3 km s overlying a lower crust of 3 7 3 8 km s in the afif terrane to the east the upper crust is 20 km thick and has an average velocity of 3 6 km s and the lower crust is about 3 8 km s separating the terranes the nabitah mobile belt is made of a gradational upper crust up to 3 6 km s at 15 km overlying an also gradational lower crust up to 4 0 km s the crust mantle transition is found to be sharp in terranes of continental affinity east and gradual in terranes of oceanic affinity west the upper mantle shear velocities range from 4 3 to 4 6 km s temperatures around 1000 c are obtained from our velocity models for a thin upper mantle lid observed beneath station taif and suggest that the lithosphere could be as thin as 50 60 km under this station