petrogenesis and tectonic setting of late precambrian ensimatic volcanic rocks central eastern desert of egypt abstract early stages in the geologic evolution of the central eastern desert of egypt ced reflect an intense episode of ensimatic volcanic activity similar to modern magmatism of the ocean floors and island arcs this paper reports results from studies of the petrology and petrogenesis and interprets the significance of these late precambrian volcanic rocks a three fold stratigraphy is preserved in the basement of the ced a basal section of oceanic crust includes ultramafics gabbros and pillowed basalts these older metavolcanics omv are conformably succeeded by dominantly volcanogenic metasediments which are in turn succeeded by a dominantly andesitic calc alkaline sequence of younger metavolcanics ymv the omv and ymv are largely restricted to the ced in egypt but analogous terranes are found in northern arabia 40 400 ppm and ni 30 260 ppm they are poor in k 2 o 0 05 0 92 rb 0 3 5 0 ppm and ba 11 89 ppm on ti zr cr v ni p discriminant diagrams the omv plot in the field of modern abyssal tholeiites high k rb 450 1800 and light ree depletions support this inference although k ba 25 45 is lower than modern mid ocean ridge basalts morb the sum of omv geochemical characteristics requires that these magmas were derived by the fractional fusion of the mantle it is suggested that the omv were generated by 20 25 fractional melting of previously depleted mantle at depths of less than 60 km relatively little fractionation accompanied ascent to the surface where the omv were erupted in a primitive crustal environment either a small oceanic rift or a back arc basin metamorphism of the ymv resulted in little elemental redistribution these andesites have sub alkaline clinopyroxenes and major element geochemical characteristics indistinguishable from modern calc alkaline andesites ymv andesites in the central and western ced have k rb 400 600 k ba 20 40 and are light ree enriched and heavy ree depleted high concentrations of cr 50 150 ppm and ni 20 100 ppm and low initial 87 sr 86 sr ratios 0 7028 0 7030 indicate that these magmas were generated by melting in the mantle modelling studies and consideration of experimental data indicate that these andesites were formed by 2 10 fractional fusion of hydrous undepleted garnet therzolite at depths of 65 km or more in the mantle the data show that an intense episode of instability convection and widespread melting occurred in the mantle beneath afro arabia at the end of the precambrian

late proterozoic lavas of the central arabian shield evolution of an ancient volcanic arc system a petrological assessment of 11 volcano sedimentary belts in the central arabian shield cas using 118 new analyses is presented the same samples have been used in a rb sr dating programme also reported in this volume the oldest volcano sedimentary belts sequence c 900 ma are chemically immature bimodal suites of low k tholeiites and sodic dacite rhyolite depleted in lithophile elements these lavas have chemical characteristics similar to immature island arcs such as the tonga kermadee and lesser antilles arcs younger lavas of sequence b 900 700 ma are slightly more mature in composition being predominantly calc alkaline and low k arc tholeiite series with higher lithophile element contents they are comparable with pacific island are averages the youngest voluminous lavas sequence a 700 570 ma are generally calc alkaline or high k calc alkaline series lavas with moderately high lithophile element abundances they are comparable to volcanic arcs such as central america and indonesia which are transitional between island arcs and continental margin volcanic arcs the cratonization process in the cas thus involved the superimposition of three volcano sedimentary sequences formed in progressively maturing volcanic arcs early formed immature island arcs were succeeded by chemically more mature island arc deposits and then by deposits formed in volcanic arc s transitional between island arcs and continental margins the evolution ceased at this stage and probably did not advance to the equivalent of an andean continental margin

paleogene igneous underplating and subsidence anomalies in the rockall faeroe shetland area abstract crustal thickening caused by igneous underplating related to continental break up in the northeast atlantic has been invoked as the cause of the apparent lack of thermal subsidence seen in many mesozoic basins in the northwest european area in this study we examine a set of commercial wells in the rockall faeroe shetland region and calculate tectonic subsidence histories the rate of post rift basement subsidence during the cretaceous cenozoic determined from backstripping calculations always implies a higher degree of extension than the total basement subsidence assuming uniform extension furthermore the size of this discrepancy increases northward along the rockall trough and westwards in the northern north sea towards scotland we interpret this depth anomaly to reflect the effect of igneous underplating rethickening the crust following mesozoic and cenozoic extension late paleocene extension is identified in subsidence reconstructions throughout the west of britain area and is greatest in the rockall trough assuming that the underplating is gabbroic in composition the subsidence depth anomalies indicate a regional pattern of 2 3 km underplated between the erris trough and porcupine basin increasing to 4 5 km in the west lewis basin and hebrides shelf area in the faeroe shetland basin underplating is 1 0 1 5 km in the east and increases to 4 5 km in the west the data support the hypothesis of a major asthenospheric thermal anomaly centered under the faeroes in the paleocene dynamic uplift of the region is often difficult to identify but is locally seen to develop rapidly around 63 ma implying rapid emplacement of the mantle anomaly the intrusion of gabbroic bodies to the base of the crust in the late paleocene may have had an impact on the thermal maturation of petroleum source rocks in the region however thermal maturity indicators show smaller peak heatflows than predicted from simple thermal models suggesting that underplating was a gradual process achieved through the accretion of several small rapidly cooled bodies

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

age determinations in the precambrian basement of the wadi araba area southwest jordan abstract the precambrian basement of jordan belongs to the northern margin of the arabian nubian shield age determinations by u pb isotopic analyses on sized and magnetic zircon fractions a monazite and an apatite sample and by rb sr isotopic studies on whole rocks and minerals of metasedimentary rocks granodiorites granites and dykes have elucidated the following events 1 a major regional high grade metamorphism accompanied by migmatization and synkinematic plutonism occurred at about 800 ma according to u pb zircon ages of metasediments and granites 2 during a postkinematic plutonic event between 615 and 600 ma extensive masses of granodioritic to granitic composition and dykes were emplaced the u pb data of zircons of the rocks yielded upper intercept ages with the concordia consistent with rb sr biotite ages the rb sr mineral ages of the older metasedimentary rocks document the resetting of the rb sr system due to the thermal pulse at this time 3 a younger plutonic event produced diorites and dykes at about 570 ma the plutonic events are related to the pan african orogenic phase the low initial87sr 86sr ratios of the plutonic rocks 0 7032 0 7046 correspond to values reported from equivalent rocks throughout the arabian nubian shield and suggest that no significant portions of ancient sialic crustal material contributed to the generation of the granitic to granodioritic magmas

heat flow in rift basins above a hot asthenosphere when continental rifting takes place above a hot asthenosphere pressure release melting of adiabatically upwelling mantle may generate large volumes of basaltic melts which subsequently are emplaced at crustal levels and cool to correctly estimate the heat flow from tectonic subsidence and crustal thinning it is necessary to account for the melt volumes a simple physical model of heat flow that incorporates a crustal growth correction on lithospheric extension estimates as well as the heat in the emplaced magma has been developed the principal result is that heat flow may be substantially increased for several million years after rifting even for a moderately heated asthenosphere

subsidence analyses from the north sea triple junction when compared with theoretical subsidence curves calculated from the uniform stretching model water loaded subsidence data from the triple junction region of the north sea suggest that the jurassic cretaceous rifting event caused lithospheric thinning by a factor of c 2 0 although somewhat larger than the stretching factors found elsewhere in the north sea this amount of thinning is anticipated from the overall geometry of the three graben system and is consistent with the observed volume and elemental and isotopic composition of the jurassic forties volcanic province apart from in the southern north sea permo triassic extension is thought to have been relatively minor in comparison to the later jurassic cretaceous phase the anomalously small amount of late jurassic syn rift subsidence in those wells where local fault controlled effects are minimal supports the well known idea of localized relative uplift or doming in the triple junction area c 10 4 km 2 prior to and during the early stages of the jurassic cretaceous rift phase a time dependent differential stretching model in which the lithospheric mantle is initially stretched by a greater amount than the crust is stretched might provide an explanation such a model would require the total amount of stretching integrated over space and time to be the same for the lithospheric mantle and for the crust in order to avoid space problems alternatively the same data could be explained by invoking a small transient thermal anomaly in the asthenosphere

transient regional uplift in the early tertiary of the northern north sea and the development of the iceland plume abstract forward and reverse 2d flexural modelling of syn rift and post rift stratigraphy has been used to investigate departures from mckenzie post rift subsidence for the cretaceous tertiary following late jurassic rifting in the northern north sea basin the analysis shows uplift in the palaeocene of 375 525 m relative to calibrated mckenzie subsidence curves followed by a rapid decrease in uplift in the early eocene of 160 310 m both palaeocene uplift and eocene subsidence increase northwards the timing distribution and magnitude of regional palaeocene uplift is consistent with an origin relating to the development of a mantle plume between 65 and 55 ma transient regional palaeocene uplift in the northern north sea basin was produced by long wavelength dynamic uplift generated by the flow field of a mantle plume heating of the asthenosphere or lithosphere beneath the north sea by hot plume material or magmatic underplating are not believed to have contributed to palaeocene uplift in the northern north sea basin rapid early eocene subsidence was generated by a decrease in dynamic uplift caused by a reduction in plume activity the plume responsible for north sea palaeocene uplift may have been the early iceland plume or a distinct british tertiary igneous province plume related to the same mantle convection event

geochemistry of pan african volcanic arc sequences in southeastern sinai peninsula and plate tectonic implications abstract major and trace elements of lavas dykes and plutonic rocks of the late proterozoic orogenic sequence in se sinai the kid group have been analysed the dykes and lavas of the southermost sequences the tarr complex and heib formation are calc alkaline whereas the lavas of the malhak formation and the sharira gabbro to the north of the heib formation show both calc alkaline and tholeiitic trends the trace element characteristics of the tarr complex and the heib and malhak formations despite between sequence variations are all comparable with ensialic island arc magmatism whereas the sharira gabbro shows some morb characteristics in addition to the island arc imprints the sharira gabbro and the lavas of the malhak formation possibly formed in a developing back arc basin behind a continental marginal ensialic island arc the tarr complex and heib formation during the pan african orogeny the constituent units of the kid group were mutually juxtaposed along major ductile shear zones of thrust fault character this plate convergence regime involved initial magmatic arc development following northward subduction and subsequent collision between the arc complex and the proterozoic continental margin

origin of anomalous tertiary subsidence adjacent to north atlantic continental margins abstract at the beginning of the tertiary the northern north atlantic continental margins were characterized by voluminous igneous activity caused by continental rifting in the presence of the iceland hot spot many of the mesozoic sedimentary basins close to these margins have been permanently uplifted by igneous underplating and remain close to or above sea level at the present day however a significant number of other basins underwent rapid subsidence at the same time sometimes by as much as 3 km of sediment loaded subsidence in 15 20 ma several hypotheses attempting to explain the cause of this tertiary subsidence have been proposed the observational data are summarized here and then each hypothesis is considered in turn it is argued that many of these are implausible one possible alternative explanation links early tertiary subsidence uplift and magmatism if basaltic melts are trapped within the lithosphere it is possible to produce either uplift or subsidence depending on the density of the trapped melt at depths greater than 50 km melts will solidify rapidly in the garnet stability field to form primary eclogite which is dense enough to cause subsidence while satisfying the constraints of long wavelength airy isostasy unfortunately such an hypothesis is difficult to test the simplest explanation for tertiary subsidence is still a minor episode of lithospheric stretching although to date there is only limited evidence for sufficient tertiary normal faulting further work must be carried out before this simple and testable hypothesis can be confidently rejected

cenozoic postrift domal uplift of north atlantic margins an asthenospheric diapirism model postrift domal uplifts are a distinct feature of the northern north atlantic rifted margins uplifted regions southern norway finnmark svalbard northern united kingdom occur with a typical spacing of 900 km the amount of domal uplift is substantial 1 2 km thermochronological geomorphological and seismic stratigraphic data suggest a coincident neogene onset of uplift postdating north atlantic breakup and predating onset of glaciation the norwegian domes are characterized by strongly negative gravity anomalies and reduced mantle p wave and s wave velocities previous models for uplift around the north atlantic e g glacial erosion intraplate stresses secondary convection fail to explain the above characteristics we present an asthenospheric diapir model based on a rayleigh taylor instability wherein a hot icelandic asthenosphere layer meets cold cratonic lithosphere the generated asthenospheric diapirs and associated partial melting offer a viable explanation for updoming application of this model to the south norwegian dome succesfully explains most of the currently available data

faunal turnover and changing oceanography late palaeozoic warm to cool water carbonates sverdrup basin canadian arctic archipelago abstract carbonate sedimentary rocks in the fosheim hamilton sub basin at the eastern end of the sverdrup basin contain a particularly sensitive record of changing palaeoceanography and faunal turnover during the northward movement of pangaea through the late palaeozoic carboniferous and earliest permian inner ramp carbonates are a photozoan association of colonial corals fusulinids algae and ooids with sparse heterozoan elements crinoids brachiopods and bryozoans based on modern analogues these biotic components reflect warm water oligotrophic marine environments contemporaneous mid ramp and basin biotas are dominated by a heterozoan fauna solitary corals crinoids bryozoans and brachiopods that grew in cooler deeper waters that may have been beneath a thermocline initial cooling began in the latest sakmarian with an influx of cool deep water and climatic deterioration allowing cool and warm waters to mix disrupting the thermocline and producing a sub tropical marine biota whose remains were reworked by storms and re deposited as tempestites faunas are heterozoan crinoids bryozoans brachiopods and sponges with photozoan elements colonial corals and symbiont bearing fusulinids prominent only in inner ramp facies by late artinskian these photozoan components are only present in shallowest basin margin environments artinskian subtropical waters contained somewhat elevated trophic resources that together with mixing of photozoan and heterozoan elements in the subtropical inner ramp produced the greatest faunal diversity seen in the sverdrup basin the biota was completely boreal in nature and warm shallow waters with a warm refuge biota were now supplemented by cool waters derived from the panthalassa ocean profound change took place in the kungurian coincident with localised uplift during the melville disturbance and increased siliciclastic input neritic storm dominated sediments are wholly heterozoan brachiopods bryozoans crinoids and sponges with no remaining photozoan elements nutrient levels are elevated but not eutrophic as indicated by phosphate and glauconite extensive bioerosion numerous infaunal foraminifers and an impressive and diverse shelly biota that during times of high siliciclastic input was replaced by a profuse zoophycos infauna brachiopod and bryozoan faunas are distinctly boreal continued cooling relatively high nutrients as indicated by abundant glauconite and phosphate and decreased siliciclastic input eventually resulted in an abundant heterozoan biota brachiopods bryozoans crinoids and sponges with local gigantism but reduced brachiopod and bryozoan diversity these middle permian rocks may be the coolest water deposits in the sverdrup basin the faunas are still part of a boreal arctic province with strong ties to cool water faunas of uralian basin the succession ends with late permian biosiliceous spiculitic cherts containing accessory heterozoan components brachiopods bryozoans and few crinoids these sediments in part coeval with evaporites and terrestrial red beds on the eastern european platform are interpreted to reflect a combination of cool waters elevated nutrients and mid latitude storminess

flexural response of passive margins to deep sea erosion and slope retreat implications for relative sea level change we suggest that the magnitude of the early late oligocene eustatic fall as predicted from sequence stratigraphy has been overestimated because of flexural rebound along margins that was induced by late eocene to early oligocene erosional unloading the unloading of sediments from a continental margin during periods of deep sea erosion and slope retreat will be accompanied by some form of flexural rebound the magnitude and form of this rebound are controlled by 1 the amount of sediment removed from the margin and 2 the strength of the lithosphere at the time of sediment removal along margins with narrow continental shelves this rebound will induce a relative sea level fall resulting in a basinward shift in the shoreline and the predicted systems tracts will be similar to those expected for a eustatic fall in contrast rebound along wide continental shelves will not alter the position of the shoreline even though a relative sea level fall may be recorded across the outer shelf deep sea drilling and seismic reflection data from many margins support the interpretation of a late eocene to early oligocene 38 34 ma deep sea erosional event best developed along the continental slope and rise seismic reflection and drilling data also document a prominent canyon cutting event across the shelf during the early late oligocene 32 29 ma this event has been interpreted in terms of a large eustatic fall 150 m nevertheless there appears to be a correlation between late eocene to early oligocene deep sea erosion and the development or at least enhancement of unconformities across the shelf during the early late oligocene

correlation of upper carboniferous pennsylvanian and lower permian cisuralian marine deposits of the peri tethys abstract using available published data and our own unpublished materials a correlation is proposed of marine pennsylvanian middle and upper carboniferous and cisuralian lower permian deposits of some important regions where new data have been collected and or a new interpretation of existing data that were previously not included in the peri tethys stratigraphic correlation program has been given special emphasis was laid on the correlation within the pre caspian region most of the suggested correlations are based on fusulinid biostratigraphy magnetostratigraphic data are used to provide a precise correlation between marine and continental strata in southern urals donets basin central asia and northern caucasus

diagenetic cycling of nutrients in seafloor sediments and the carbonate silica balance in a paleozoic cool water carbonate system sverdrup basin canadian arctic archipelago abstract seafloor processes are often destructive of biogenic remains while at the same time promoting growth of authigenic minerals such as hematite fluorapatite glauconite and chalcedony middle to late permian sedimentary rocks in the sverdrup basin are cool water marine sandstones shales limestones and cherts rich in carbonate and siliceous biotic components the authigenic minerals hematite fluorapatite glauconite and chalcedony are abundant from the interactions of nutrient elements silica phosphorus and iron with carbonate and siliceous biotas in ramp environments of variable energy and sediment accumulation rate these authigenic phases represent in part faunas that are no longer preserved and should not be viewed simply as diagenetic products early middle permian siliciclastic rocks the assistance formation contain carbonate heterozoan biotas with widespread siderite and hematite and local void filling phosphate deposited under moderate sediment accumulation rates sponge spicules were dissolved unless encased in hematite or siderite and incorporated in silica cement in contrast more slowly deposited younger middle permian inboard cherty limestones the lower trold fiord formation underwent frequent physical and biological reworking resulting in dissolution of biogenic silica and precipitation of glauconite and carbonate fluorapatite sediment reworking also excavated and produced rounded glauconite and phosphate clasts much of the glauconite was then oxidized to hematite on the seafloor again unless encased in phosphate sponge spicules were dissolved and the silica precipitated as glauconite or chalcedony coeval deeper water phosphatic cherty limestones the degerbols formation do however contain sponge spiculites reduced sediment reworking led to retention of silica in pore fluids and slowed dissolution of sponge spicules resulting in local preservation of sponge spiculite facies late permian organic rich cherts the lindstrom formation with prolific siliceous sponges and a less plentiful carbonate biota accumulated relatively slowly sponge spicules were preserved and carbonate biota partially dissolved in these organic rich deposits implying that the biota had to be overwhelmingly dominated by siliceous components to overcome ambient silica undersaturation in pore fluids each succession has a distinct composition depending upon the rate of sediment accumulation the original benthic biota and the nature and abundance of silica phosphate and iron in seawater at any given time these characteristics controlled the formation of authigenic minerals and the preservation or dissolution of carbonate and silica biotas

sedimentary basin inversion caused by igneous underplating northwest european continental shelf a considerable body of evidence indicates that many of the extensional sedimentary basins in the vicinity of the british isles underwent permanent exhumation during the tertiary the most dramatic indicator of this process is the present day absence of as much as 4 km of anticipated postrift thermal subsidence in basins just north and west of scotland any explanation of this observation must take into account the fact that the entire region has very small long wavelength free air gravity anomalies this important constraint implies either that the crust has been thickened or that low density material has been added to or formed from the lithosphere and rules out models that invoke flexural effects arising from the opening of the north atlantic tertiary epeirogeny is often attributed to compression that is assumed to be related in a general sense to alpine mountain building however to remove 3 km of sedi mentary rock from a basin 100 km wide requires 15 km of shortening minor tertiary compression is observed all over the continental shelf but nowhere is it sufficient to account for the required amount of uplift and erosion in addition exhumation dramatically increases from south to north whereas the observed compression decreases markedly in the same direction at the beginning of the tertiary rifting associated with the initiation of the iceland plume generated substantial volumes of melt inversion of rare earth element concentrations of mgorich igneous rocks suggests that a minimum of 5 km of melt was produced beneath at least part of the continental shelf we infer that much of this melt remains trapped within the lithosphere presumably close to the moho which acted as a density filter such underplating will generate rapid uplift

continental rifting and the implications for plate tectonic reconstructions previous plate tectonic reconstructions have tried to recreate the pre rifting pangea configuration of the continents by matching contours or lineaments that are thought to represent the continental boundaries such reconstructions have the inherent assumptions that no extension occurs within the continent during rifting that the continental boundaries are isochrons and that the continents rift without distortion this paper proposes a model for continental breakup with distortion where ocean basins are formed by propagating rifts this model challenges the assumptions made by previous reconstructions and presents a new method for representing the prerift geometry of the continents as rifts propagate through a continent the region in front of the rift extends by continental faulting and crustal thinning while the region behind the rift expands by seafloor spreading the propagating rift model implies that large amounts of continental extension up to 150 km occur and that the continental boundaries are not isochrons this extension due to rifting results in apparent overlap when the continents are returned to their original configuration because of the gradational nature of this extension the best representation of the pre rift configuration is obtained by matching the initial rifting point and having overlap increase in the direction of rift propagation the overlap is equivalent to the amount of extension that has taken place on both continental edges reconstructions based on this model are presented for the gulf of california the gulf of aden the norwegian greenland sea and the south atlantic ocean they are able to resolve the discrepancies that occur when rigid plate tectonics are applied this model has implications that can be used to predict the age and structure of the continental shelves and the location and orientation of the oldest magnetic isochrons

faunal migration into the late permian zechstein basin evidence from bryozoan palaeobiogeography abstract late permian bryozoans from the wegener halvo ravnefjeld and schuchert formations in east greenland have been investigated 14 genera are recognised integration of the new bryozoan data from the upper permian of east greenland with data on the distribution of permian bryozoans along the northern margin of pangea is used to test hypotheses concerning late palaeozoic evolution of the north atlantic region during the permian the atlantic rift system formed a seaway between norway and greenland from the boreal barents shelf to the warm and arid zechstein basin this seaway is considered to be the only marine connection to the zechstein basin and therefore the only possible migration route for bryozoans to enter the basin the distribution of permian bryozoans is largely in keeping with such a connection from the cool barents shelf past the east greenland basin to the warm zechstein basin and also corroborates the change in temperature through this connection

plate reorganization a cause of rapid late neogene subsidence and sedimentation around the north atlantic subsidence analysis of wells in the central north sea and labrador grand banks and oft the west greenland scotian shelf and united states atlantic margin shows distinct quantitative stratigraphic correlation patterns of circum north atlantic sites a significant departure from the overall decrease in subsidence for the pliocene occurs in many wells when the rate is found to have increased one or more orders of magnitude from oligocence miocene rates wells were selected along transects from shore to basin to find if relative basin position is influenced by differential basin subsidence although stratigraphic resolution is not detailed more basinward sites experienced up to four times larger subsidence rates in the late neogene than in the oligocene miocene with a peak in the pliocene wells at the basin edge experienced much less subsidence or showed uplift the observations are consistent with a rapid change of intraplate stress at the cause for this observed transition in neogene subsidence major reorganizations of spreading direction and rate occurred during the pliocene along the entire atlantic spreading system possibly in conjunction with more global changes in plate motions we propose that the associated changes in intraplate stress caused the excess margin subsidence relative uplift along basin edges is consistent with this mechanism of relative movement and may explain apparent eustatic changes in sea level

cold climate in the eastern australian mid to late permian may reflect cold upwelling waters a suite of ice rafted dropstones and glendonites throughout the permian succession of eastern australia indicates the cold climate associated with the late paleozoic ice age persisted longest in this part of gondwana paradoxically these cold climate indicators are preserved in transgressive and highstand facies and formed at mid to high latitudes at a time when paleofloral and sedimentological data suggest equable onshore environments during the intervening lowstands and temperate conditions at the pole these apparent inconsistencies suggest that eastern australia was anomalously cold in the context of post sakmarian gondwanan climates and the distribution of sedimentary indicators could indicate localized cooling by oceanographic processes modern upwelling of cold abyssal waters produces the specific physiochemical conditions necessary for ikaite for mation and is likely to have contributed to the development of glendonites in the eastern australian permian system such upwelling would have locally lowered surface water temperatures such that seasonal sea or river ice could form which rafted debris across the marine shelf and a cold water fauna could develop this hypothesis is supported by coupled atmosphere ocean models for the permian which suggest that wind systems may have driven upwelling along this section of the gondwanan coast colder conditions dissipated with the onset of the hunter bowen contractional event and the transformation of the basin system from an open marine shelf to a foreland basin this new hypothesis reconciles the prolonged deposition of cold climate indicators in the eastern australian permian with an apparent post sakmarian warming elsewhere in gondwana

effects of basement topography on subsidence history analysis abstract tectonic subsidence history analyses performed without consideration of fault controlled or other basement topography and interpreted using the depth dependent extension model may yield very inaccurate estimates of crustal and subcrustal extension for sedimentary basins analyses of data from wells over local basement highs can lead to predictions of incorrectly low crustal extension and high subcrustal extension predictions based on data from wells over basement lows may be incorrect in the opposite sense calculating the sediment unloading correction assuming local isostatic compensation rather than flexural compensation reduces this problem this is not meant to indicate that depth dependent extension does not occur but that it should not be invoked simply to explain variations in basement depth due to rough topography

northern scotland as an atlantic north sea divide president9s anniversary address 1984 from the end of the caledonian orogeny until the present northern scotland has acted as a structural high separating the north sea and atlantic basins or their forerunners the geological history of this positive massif is discussed in relation to that of the adjacent cratonic basins with the objective of determining the extent to which it adapted to regional tectonic developments and the manner in which its responses were modified by the pre existing basement structure from devonian to early palaeocene times the massif was generally low lying and its behaviour with respect to vertical movements was essentially neutral the land surface carved in the crystalline basement during the devonian period was preserved with relatively small modifications and little metamorphic detritus was supplied to the adjacent basins tectonic events of major importance in the basins such as the formation of the north sea and hebridean rift systems produced little change of conditions in the intervening massif it is inferred that the underlying keel of lithospheric mantle was protected from the effects of lithospheric stretching the uneventful history of the massif was interrupted by two phases of mafic igneous activity in late carboniferous and palaeocene times which record responses to regional plate motions the formation of the palaeocene igneous province was followed by a unique phase of uplift as a result of which the ancient sub devonian land surface was elevated and dissected and abundant crystalline detritus was supplied to the north sea the style of these events and the anomalous character of the continental margin off nw britain may have been determined by the refractory nature of the precambrian basement which inhibited stretching prior to sea floor spreading

permian gondwana boreal antitropicality with special reference to brachiopod faunas in a reconstructed global permian marine biogeographic framework antitropicality is defined as a biogeographic phenomenon whereby a taxon occurs only in the boreal and gondwanan realms and is lacking completely or nearly so in the intervening palaeoequatorial realm we recognise three types of antitropical distributions bipolar high latitude generally 50 latitude in each hemisphere bitemperate approximately between 30 and 50 latitudes in each hemisphere and a combination of both evidence of permian antitropicality has been reported from almost all major marine invertebrate groups both pelagic or benthic forms and is best known at the generic level although an increasing number of antitropical species have also been recognised antitropicality appears to have occurred throughout most of the permian but its strength varied from stage to stage judging by the number of brachiopod genera shared between the two hemispheres the phenomenon manifests itself most strongly from the sakmarian to the kazanian wordian suggesting a prolonged interval of enhanced biotic interchanges between the gondwanan and boreal realms possible migration pathways and mechanisms are reviewed and discussed among a number of existing scenarios invoked to explain the origin of both modern and geological antitropicality we here present data particularly supporting 1 a south to north stepping stone migratory mechanism via island terranes scattered in the eastern palaeotethys 2 a vicariance model exemplified by the spatio temporal distribution of tomiopsis and 3 a shelf north to south migration along the western coast of the palaeotethys as suggested by sowerbina in addition we also consider dispersal along the western coast of pangea facilitated at times by upwelling systems as an additional migration pathway for intercontinental biotic exchanges during the permian although corroborating data are required at present

geochronology of the arabian nubian shield in southern israel and eastern sinai the metamorphic and igneous orogenic basement rocks of southeastern israel and eastern sinai evolved as part of the arabian nubian shield during latest proterozoic time pelitic schist of the elat association from wadi nahal shlomo south of eilat israel is dated at about 800 m y but the geologic significance of this date is not clear in southeastern sinai metapelitic rocks of the kid group have a date of diagenesis or metamorphism of about 615 m y syn orogenic and post orogenic plutons which intrude the layered metamorphic complex s are dated at 600 m y near eilat and 590 m y at wadi kid other dated plutons include the timna granite at 592 7 m y in the area of nevi ot at 595 m y and in the vicinity of dahab at 588 2 m y and 595 0 3 m y the radiometric dates and low initial 87 sr 86 sr ratios 0 7034 imply that pre 1000 m y continental sialic crust was not involved in the formation of the shield in southeast israel and eastern sinai

thermotectonic evolution of a rifted continental margin fission track evidence from the kangerlussuaq area se greenland fission track analyses of apatites from sediments precambrian gneisses and caledonian to tertiary intrusive rocks from the kangerlussuaq region reveal its post caledonian thermotectonic history inland the history involves cooling to temperatures within the high temperature part of the apatite annealing interval and slow cooling or reheating continued in cretaceous times through this interval apatites from coastal areas between kangerlussuaq and tasiilaq reveal only tertiary cooling in tertiary times cooling accelerated after the main intrusive phase in the tertiary the evolution is taken as evidence for a general uplift erosion since caledonian times probably disturbed by basin formation and sedimentation and reheating due to magmatic activity thermal subsidence of the rift shoulders following the opening of the adjacent oceanic basin is not indicated annealing patterns inland of the plume centre in the kangerlussuaq provide no evidence for the earlier movement of the plume from a westerly direction

carbonate sedimentation in a permian high latitude subpolar depositional realm queensland australia high latitude carbonate rocks that formed in subpolar paleozoic oceans contain critical information about past climate but they are neither well documented nor fully understood lower and middle permian limestones that were deposited in such paleoenvironments at 45 s to 55 s along the southeastern margin of gondwana are now exposed in eastern australia such carbonates constitute a small but important and well preserved part of the siliciclastic glacigene succession in the bowen basin from southern and central queensland cleanest limestones accumulated on topographic highs in the eastern seaward parts of the basin that were isolated from siliciclastic sediment input from the western craton shallow water deposits are composed of large eurydesmid bivalves spiriferid and productid brachiopods bryozoans infaunal and epifaunal benthic foraminifers calcareous sponge spicules and abundant crinoids in the form of locally cross bedded rudstones and grainstones deeper water muddy carbonates are dominated by bryozoans crinoids and productids shells and spines deepest water sediments are argillaceous floatstones or shales containing fenestrate bryozoans thin walled productids and siliceous sponge spicules when integrated with recent studies of coeval sedimentary rocks along the 2000 km long paleocontinental margin a clear pattern of sedimentation emerges wherein deposition took place in similar settings during three discrete periods of deglaciation late sakmarian late artinskian and wordian all of the carbonates are distinguished by a low diversity high abundance heterozoan biota with no phototrophs and express a recurring deepening upward stratigraphic motif that is interpreted to reflect rising sea level accompanying glacial meltdown they also record a strong poleward gradient of increasing ice rafted debris increasing skeleton size decreasing invertebrate diversity decreasing epifaunal calcareous benthic foraminifers and reduction in crinoids glendonites are confined to siliciclastic sediments deposited during glacial periods comparison with coeval northern hemisphere carbonates indicates a strong parallelism of facies and depositional systems this study helps to establish these deposits as a template for a global paleozoic high latitude carbonate facies model

rare earth element evidence concerning the origin of voluminous mid tertiary rhyolitic ignimbrites and related volcanic rocks sierra madre occidental chihuahua mexico abstract the mid tertiary volcanic sequence of the central sierra madre occidental in chihuahua mexico is about one kilometer thick and is composed predominantly of rhyolitic ignimbrites basaltic andesite to dacitic lavas are interbedded with the rhyolites but they are of minor volumetric importance rare earth element ree data are used to constrain a crustal anatexis model for the origin of the voluminous ignimbrites and to test a fractional crystallization model the ree patterns indicate that if the rhyolites were formed by direct crustal anatexis the residue from partial melting could contain no more than a few percent garnet or about 20 hornblende this eliminates residues with the mineralogy of amphibolite eclogite or garnet granulite but melting of a garnet free granulite source is permitted the crustal anatexis model is difficult to evaluate critically because of a lack of knowledge concerning the mid tertiary geothermal gradient and the composition of the crust beneath the sierra madre occidental in contrast the fractional crystallization model can be tested rigorously rayleigh fractionation calculations are used to closely model ree patterns in the basaltic andesite to rhyolite series the minerals involved are those occurring as phenocryst phases and the mineral proportions were generated by leastsquares major element calculations the results of the calculations are consistent with the hypothesis that the voluminous rhyolites originated by plagioclase dominated crystal fractionation

development of rift basins illustrated by the structural evolution of the oseberg feature block 30 6 offshore norway the mesozoic tilted fault block in norwegian north sea block 30 6 containing the giant oseberg oil and gas field is used to illustrate basin development during the jurassic and cretaceous there is seismic evidence for an early rifting phase in the northern viking graben on which the jurassic cretaceous rifting phase early bathonian to berriasian is overprinted the oseberg structure is representative of other fault blocks bordering the northern viking graben axis planar normal faults and listric faults which were restricted to the rifting phase dominate both fault types exhibit syn fault sedimentation listric faulting is recognized on seismic sections by the differential tilting reverse drag antithetic faulting and a wedge shaped syn rift reflector package the initiation of rifting is dated by baselap onto pre rift reflectors and cessation is dated by an unconformity beneath the generally onlapping post rift sediments steeply dipping planar normal faults often active along previously established fault zones played a significant part in the post rift development of the basin the last movement on planar normal faults in the oseberg fault scarp becomes younger away from the rift axis and this produced the complex fault scarp visible on many seismic lines the topography defined by the post rift unconformity is a composite surface which formed in response to subsidence processes it is not the result of orogenic events such as the kimmerian 9orogeny9 footwall uplift associated with planar normal faults caused local unconformities in complex fault scarps and on fault block crests

permian higher latitude bryozoan biogeography abstract bryozoans from gondwana and northern eurasia are within the higher latitude gondwana and boreal realms respectively and display a clear antitropical faunal distribution with lower diversity than tropical realms within these realms however faunas at highest latitudes are the least diverse and dominated by cosmopolitan genera further separate geological and oceanographic developmental histories of the boreal and gondwanan regions have resulted in the boreal realm showing much higher diversity than its southern counterpart gondwana was in a high latitude position during the permian experiencing the polar climatic conditions of the late palaeozoic ice age and the distribution of bryozoans show a distinct diversity decrease towards cold water high latitudes where the eurytopic fenestrates and trepostomes were the dominant groups the eastern australian margin with the least diverse faunas of the gondwanan realm is oriented north south as a narrow shelf hindering the migration of taxa into the southern coldest environments in contrast northern hemisphere mid to high latitude basins experienced cooling through the permian as pangaea moved northward and tectonics altered oceanographic circulation however glaciomarine deposits were not widespread even at highest latitudes the marine shelf of the northern eurasian margin was broad and oriented west east allowing establishment of widespread stable faunas in waters that became increasingly nutrient fed through the permian from cool water upwelling from the panthalassa ocean in the west this cool water mid latitude setting west east shelf and nutrient enrichment resulted in a boreal realm bryozoan fauna of over 100 genera more diverse than the gondwanan realm with only 61 genera antitropical taxa restricted to boreal and gondwanan realms only include diploporaria lyropora and metelipora however a number of other genera are more abundant outside the tropics even if they occur in all realms these genera include alternifenestella rectifenestella polypora polyporella shulgapora dyscritella stenopora streblascopora pinegopora maychella maychellina ramipora tabulipora and hexagonella the wide nutrient fed mid latitude shelf of the boreal realm also allowed development of a number of endemic taxa or taxa that are rare elsewhere including permoheloclema timanodictya permofenestella lyrocladia wjatkella parafenestralia triznella and gilmouropora in southern gondwana endemic taxa are limited to crockfordia and paramaychella however there is an abundance of evactinostella etherella and liguloclema that are distinctive of western australia

deposition of polar carbonates during interglacial highstands on an early permian shelf tasmania abstract the early permian lower parmeener supergroup contains two bioclastic limestones that accumulated during the period of prolonged deglaciation of southern gondwana as the region moved equatorward from 80 s to 70 s the darlington and berriedale limestones formed in neritic environments in areas where abundant ice rafted debris testifies to numerous icebergs and glendonites indicating near freezing seawater these limestones consist of argillaceous and clean bioclastic floatstone rudstone and grainstone that contain a high abundance low diversity heterozoan assemblage of calcareous invertebrates the components are dominated by large robust brachiopods bryozoans and eurydesma bivalves sponge spicules and crinoids are also common whereas coralline algae and conodonts are conspicuously absent carbonates were deposited on the middle shelf during sea level highstands below the iceberg grounding line where the faunal communities remained undisturbed in this setting the organisms were outboard of significant terrigenous sediment influx and brackish water which were trapped on the inner shelf by bathymetry and icebergs strong bottom currents also prevented the deposition of fine grained siliciclastics by continuous winnowing of the pure limestone facies upwelling of nutrient rich water inferred from the distribution of phosphate promoted high primary productivity which fueled this carbonate factory and inhibited dissolution of the biogenic carbonate

the origin of reversed geochemical zoning in the northern new hebrides volcanic arc integration of petrographic and geochemical data on each island in the banks group northern vanuatu new hebrides has revealed a decrease in k2o and related incompatible elements across the islands away from the new hebrides trench the correlation with depth to the benioff zone is therefore the reverse of that typically found in island arcs ree evidence and source modelling indicates that this variation represents a progressive depletion in lree hree in the upper mantle laterally away from the trench and a progressive increase in partial melting of the source in the same direction these variations in chemistry are attributed to an earlier west dipping miocene subduction system and the variation in degree of partial melting to the location of the islands with respect to the active back arc basin an enrichment of the upper mantle in the miocene is ascribed to the ascent of hydrous fluids enriched in incompatible elements from the west dipping benioff zone and subsequent reaction with the overlying upper mantle the model thus envisages approximately contemporaneous development of the central chain volcanoes and the backarc basin in the late pliocene with partial melting of hydrous laterally zoned upper mantle resulting from the convective thermal regime associated with the rifting apart of the back arc basin the role of the present subduction regime in magma generation is considered to be very limited

pan african structures ophiolites and melange in the eastem desert of egypt a traverse at 26 n a detailed structural traverse across the basement rocks of the eastern desert of egypt shows that they consist apart from intrusions of four broadly recumbent tectonic units the lowest of arkosic metasediments of continental shelf facies is exposed in a dome this unit is overlain by an allochthonous ophiolitic melange containing complete and dismembered ophiolitic masses in a matrix of deep oceanic graphitic pelites and turbidites a near horizontal schistose to mylonitic fabric most intense near and below the base of the melange indicates extensive lateral tectonic transport of the ophiolitic melange over the shelf metasediments but the melange originated as an olistostrome before being tectonically transported the melange extends over an area of at least 10 000 km 2 it is locally overlain by calc alkaline volcanics in which deformation is less intense but increases and schistosity flattens downwards indicating some translation over the melange unconformable on the melange and calc alkaline volcanics is a molasse facies series itself also locally strongly deformed late tectonic granites preceded and locally post date the molasse facies sediments still later diapiric peralkaline riebeckite granites locally up domed the recumbent structures the tectonic evolution is related to a late proterozoic subduction zone to the se of the region

sedimentology and palaeoecology of lonestone bearing mixed clastic rocks and cold water carbonates of the lower permian basal beds at fossil cliffs maria island tasmania australia insight into the initial decline of the late palaeozoic ice age

the structure and subsidence of rockall trough from two ship seismic experiments coincident multichannel seismic reflection and refraction profiles acquired northwest of britain during a two ship seismic experiment in the southern rockall trough provide evidence for a thin 6 km syn rift crust below rockall trough the variation of seismic velocity with depth is consistent with either oceanic crust similar to that found in other parts of the north atlantic or with thinned continental crust heavily intruded by syn rift igneous rocks the syn rift crust beneath rockall trough is now buried by nearly 5 km of sediments that are intruded by tertiary volcanics varying in extent from isolated sills to a large sill complex extending 100 km laterally the basement imaged on seismic profiles exhibits a band of subhorizontal reflectors that extend laterally over distances of up to 40 km which they interpret as sediments intercalated with submarine lava flow generated during the opening of the trough estimates of stretching from crustal thinning and from subsidence indicate that rockall trough has undergone extensive rifting beta 6 upwelling asthenosphere beneath the thinned lithosphere generated at least 1 3 km thickness of melt as it decompressed the molten rock rose upward from the mantle until it was in part extruded as lava flows and in partmore intruded into the crust less

the common origin and alteration history of the hypabyssal and volcanic phases of the wadi tarr albitite complex southern sinai egypt new data and interpretations are presented for the igneous albitites of the wadi tarr area southern sinai egypt the albitite masses are isolated in outcrop from any granitic intrusions and have intrusive contacts against the country rocks without any structural control they have marginal zones of breccias with jigsaw fit angular clasts suggesting explosive in situ formation the albitites are of two types the western medium grained hypabyssal albitite and the eastern fine grained porphyritic albitite the field relations suggest emplacement at different levels in a magmatic cupola the hypabyssal texture and steeply dipping slope of the upper contact of the western albitite imply deeper emplacement whereas the gently dipping contacts and porphyritic texture of the eastern albitite masses indicate that they define the probable location of the cupola apex both types of albitites consist of albite 92 97 with minor amounts of quartz k feldspar and biotite the accessory minerals include fe oxides augite sulphides zircon rutile xenotime titanite allanite and monazite the whole rock compositions of the hypabyssal and porphyritic albitites are closely related but the porphyritic type has lower abundances of sr ba y nb th and zr we show that the hypabyssal and porphyritic albitites have a common petrogenetic origin most likely as late stage cumulates from a fractionating strongly alkaline a type magma consistent with the compositions of the mafic minerals the source magma was probably a tephritic liquid we use melts models to show that only a sufficiently alkaline magma follows a differentiation path that both avoids quartz saturation and encounters the alkali feldspar solvus reaching a residual liquid in equilibrium with highly sodic feldspar although the melts results show a chemically consistent means of forming igneous albitite they are incomplete in that physical segregation mechanisms are still required to isolate the albite from mafic minerals and or a low temperature aqueous alteration stage is needed to leach k from the feldspar alteration surrounding the wadi tarr albitites is extensive and dominated by alkali metasomatism similar to fenitization alteration in the marginal breccia zone of the albitite is dominated by precipitation of amphibole and carbonate in veins and in the breccia matrix whereas the volcanic country rocks show replacement of feldspars by sericite carbonate and epidote as well as vein carbonate the altered volcanic country rocks show lower concentrations of fe 2o 3 sr cu pb ba and ce accompanied by higher concentrations of na2o and mgo compared to unaltered equivalent samples

major cretaceous volcanic province in southern rockall trough abstract a group of three large curvilinear ridges called the barra volcanic ridge system has been mapped in the acoustic basement of southern rockall trough typically each ridge is about 2 km high and 20 km wide at its base a crudely layered acoustic character moderate density but high strength of magnetisation point to a volcanic sedimentary ash composition for the ridges seismic continuity with the acoustic basement of the rest of rockall trough suggests that the trough basement is of similar composition an age for the ridges of lower cretaceous is indicated by well ties and consideration of regional geology volumetrically the ridges are on the scale of hot spot features such as the wyville thomson ridge

chalk porosity and tertiary uplift western approaches trough sw uk and nw french continental shelves porosities have been derived from sonic logs for the cenomanian maastrichtian chalk of 22 hydrocarbon exploration wells in the western approaches trough although the burial depth of the chalk studied ranges from 40 1500 m below sea bed porosities are all within the range 0 45 the most deeply buried chalk sequences exhibit close to normal porosity depth relations for offshore chalks abnormally low porosities in the remainder of the wells result from burial at greater depth than that presently observed low porosities were preserved during tertiary alpine related uplift of the area for a given well the magnitude of uplift not reversed by subsequent subsidence apparent uplift equals the displacement of its porosity depth relation from the normal relation along the depth axis apparent uplift in the western approaches trough ranges from 0 1 3 km in the western approaches trough chalk porosity data provide evidence of more widespread tertiary uplift than do seismic reflection records alpine related compression and uplift fault bounded basin inversion is apparent only on seismic records in the southern part of the trough brittany and south west channel basins whereas chalk porosity data suggest regional uplift occurred throughout the trough and adjacent highs chalk porosity data were also used to quantify tertiary uplift in the cleethorpes l well of the east midlands shelf the resultant value of 1 1km is considerably greater than that previously derived from shale sonic velocities approximately 0 3 km and slightly less than that based on apatite fission track analysis 1 3 1 7 km

geochemical characterization of pan african dyke swarms in southern sinai from continental margin to intraplate magmatism abstract the chemical and mineralogical features of three pan african dyke swarms occurring in the wadi feiran area southern sinai have been examined two swarms striking approx 35 exhibit calcalkaline chemical properties the oldest s1 can be related to calcalkaline rocks from active continental margins by means of incompatible element plots displaying negative nb ta anomalies they dykes were emplaced synchronously with post orogenic alkaline granites to which they cannot be assigned chemically a second dyke generation l1 with the same strike shows apparent similarities with continental flood basalts e g columbia river basalts indicating a growing contribution of continental components to melt chemistry subduction related features are displayed by negative nb ta anomalies the youngest dyke swarm s2 striking approx 135 and crosscutting s1 displays transitional to mildly alkaline properties without negative nb ta anomalies its chemistry is fully consistent with intraplate basalts of continental areas the source regions of the three swarms cannot be related via assimilation of crustal material because decreasing lile percentages coupled with increasing amounts of nb and ta cannot be explained by this means the overlap of m values of rocks to be found with the three swarms indicates the unlikeliness of a development of the three magmas via fractional crystallization the emplacement of two totally different types of dykes in the wadi el sheikh area requires a source region which is either chemically inhomogeneous in a more or less vertical scale or changing its composition and melting conditions with time although the dykes were emplaced in an extensional tectonic setting the subcrustal lithosphere still produced calcalkaline melts documenting its subduction related magmatic history

evolution of the kid group southeast sinai peninsula thrusts melanges and implications for accretionary tectonics during the late proterozoic of the arabian nubian shield the late proterozoic age kid group in the southeast sinai peninsula is a volcano sedimentary sequence of island arc affinities the group deformed in four main phases with a thermal peak after d1 followed by a regional greenschist facies overprint during d2 recumbent folding followed by thrusting began during d2 with the folds being propagated southward toward the arc foreland until d4 the last folding event the thrusts are responsible for the juxtaposition of four megatectonic units above a substrate of unknown composition these units probably allochthonous comprise a basal complex of infracrustal gneiss and three arc related units including inferred vestiges of a forearc accretionary prism and a marginal basin narrow slices of ophiolitic rocks were emplaced tectonically along the boundaries of some of these units melanges attributed to gravitational collapse of the forearc edifice and thrust tectonics formed in particular along the arc forearc transition zone the evolution of the kid group coupled with data from the rest of the arabian nubian shield can be well accommodated within the concept of oceanic crust subduction magmatic arc development and ultimate collision between the arc complex and a continental margin

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

growth and demise of permian biogenic chert along northwest pangea evidence for end permian collapse of thermohaline circulation abstract the permian chert event pce was a 30 ma long episode of unusual chert accumulation along the northwest margin of pangea and possibly worldwide the onset of the pce occurred at about the sakmarian artinskian boundary in the sverdrup basin canadian arctic where it coincides with a maximum flooding event the ending of high frequency high amplitude shelf cyclicity the onset of massive biogenic chert deposition in deep water distal areas and a long term shift from warm to cool water carbonate sedimentation in shallow water proximal areas a similar and coeval shift is observed from the barents sea to the northwestern usa a landward and southward expansion of silica factories occurred during the middle and late permian at which time warm water carbonate producers disappeared completely from the northwest margin of pangea biotically impoverished and increasingly narrow cold water carbonate factories characterised by non cemented bioclasts of sponges bryozoans echinoderms and brachiopods were then progressively replaced by silica factories by late permian time little carbonate sediments accumulated in the barents sea and in the sverdrup basin where the deep to shallow water sedimentary spectrum was occupied by siliceous sponge spicules by that time biogenic silica sedimentation was common throughout the world silica factories collapsed in the late permian abruptly bringing the pce to an end in northwest pangea the end permian collapse of the pce was associated with a major transgression and with a return to much warmer oceanic and continental climatic conditions chert deposition resumed in the distal oceanic areas during the early middle triassic anisian after a 8 10 ma interruption early triassic chert gap the conditions necessary for the onset expansion and zenith of the pce were provided by the thermohaline circulation of nutrient rich cold waters along the northwestern and western margin of pangea and possibly throughout the world oceans these conditions provided an efficient transportation mechanism that constantly replenished the supply of silica in the area created a nutrient and oxygen rich environment favouring siliceous biogenic productivity established cold sea floor conditions hindering silica dissolution while increasing calcium carbonate solubility and provided conditions adverse to organic and inorganic carbonate production the northwest margin of pangea was for nearly 30 ma bathed by cold waters presumably derived from the seasonal melting of northern sea ice the assumed engine for thermohaline circulation this process started near the sakmarian artinskian boundary intensified throughout middle and late permian time and ceased suddenly in latest permian time it led to oceanic conditions much colder than normally expected from the palaeolatitudes and the influence of cold northerly derived water was felt as far south southern nevada the demise of silica factories was caused by the rapid breakdown of these conditions and the establishment of a much warmer marine environment accompanied by sluggish circulation and perhaps a reduced input of dissolved silica to the ocean complete thawing of northern sea ice would have ended thermohaline circulation and led to warm and sluggish oceanic conditions inimical to the production accumulation and preservation of biogenic silica

constraints on the evolution of the east greenland margin evidence from detrital apatite in offshore sediments we test a new approach to understanding the tectonic evolution of passive margins by using fission track analysis on detrital apatites from sediments deposited offshore east greenland these apatites have not undergone postdepositional track annealing and therefore reflect provenance the apatites preserve a component of the source rocks thermal history that otherwise may not be retainedwithinthepresent dayoutcrop fission track deriveddenudational histories from samples at ocean drilling program drill sites offshore east greenland at lat 63 n are compared with data fromtheonshoresingertatcomplex previousapatitefission track studies and geomorphic mapping of the east greenland coast have shown that locally up to 6 km of denudation may have occurred implying significant tectonic or magmatic activity starting as much as30m y afterbreakupat56ma incontrast apatitefission track datapresentedhererecord 2kmofcenozoicdenudationinsoutheast greenland probably driven by magmatic underplating at the time of breakup large magnitude postrift denudation of east greenland is restricted to the area around kangerdlugssuaq 68 n thetiming 40 50ma andmagnitudeareinaccordwith revised plume track models suggesting that the iceland plume crossed the margin here during the late eocene

late palaeozoic global changes affecting high latitude environments and biotas an introduction abstract the late palaeozoic ice age lpia spanning approximately from 320 ma serpukhovian late mississippian to 290 ma mid sakmarian early permian represents the vegetated earth s largest and most long lasting regime of severe and multiple glaciations involving processes and patterns probably comparable to those of the last ice age accompanying the lpia occurred a number of broadly synchronous global environmental and biotic changes these global changes as briefly reviewed and summarized in this introductory paper comprised but are not limited to the following massive continental reorganization in the lead up to the final assembly of pangea resulting in profound changes in global palaeogeography palaeoceanography and palaeobiogeogarphy substantially lowered global atmospheric carbon dioxide concentrations p co 2 coupled with an unprecedented increase in atmospheric oxygen concentrations reaching earth s all time high in its last 600 million year history sharp global temperature and sea level drops albeit with considerable spatial and temporal variability throughout the ice age and apparently a prolonged period of global sluggish macro evolution with both low extinction and origination rates compared to other times in the aftermath of the lpia the world s climate entered into a transitional climate phase through the late early to middle permian before its transformation into a greenhouse state towards the end permian in recent years considerable amount of data and interpretations have been published concerning the physical evidence in support of the lpia its broad timeframe and eustatic and ecosystem responses from the lower latitudes but relatively less attention has been drawn to the impact of the ice age on late palaeozoic high latitude environments and biotas it is with this mission in mind that we have organized this special issue with the central focus on late palaeozoic high latitude regions of both hemispheres that is gondwana and northern eurasia our aim is to gather a set of papers that not only document the physical environmental changes that had occurred in the polar regions of gondwana and northern eurasia during the lpia but also review on the biotic responses at different taxonomic ecological and spatial scales to these physical changes in a refined chronological timeframe this introductory paper is designed to provide a global context for the special issue with a brief review of key late palaeozoic global environmental changes including changes in global land sea configurations atmospheric chemistry global climate regimes global ocean circulation patterns and sea levels and large scale biotic biogeographic and evolutionary responses followed by a summary of what we see as unresolved scientific issues and various working hypotheses concerning late palaeozoic global changes and in particular the lpia as a possible reference to future research

geochemical and sr nd pb isotopic data bearing on the origin of pan africangranitoids in the kid area southeast sinai egypt the kid plutonic suite in southeast sinai egypt is composed of 1 a gabbro diorite complex 2 quartz monzodiorite 3 granodiorite monzogranite association 4 syenogranite major and traceelement analyses and radiogenic isotope data rb sr sm nd and pb pb are presented for these rocks rb sr isochrons indicate ages of 581 11 ma 576 11 ma and 570 4 ma for the quartz monzodiorite granodiorite monzogranite association and syenogranite respectively the gabbro diorite complex is a layered intrusion varying in sio2 from 42 to 59 wt it shows a calc alkaline trend and the trace element characteristics of arc lavas subsequently this magma fractionated to give quartz monzodiorite andsyenogranite magmas the quartz monzodiorite has low initial 87 sr 86 sr ratios 0 703 and high nd values 4 4 0 3 with trace element characteristics of volcanic arc granites the granodiorite monzograniteassociation has higher initial sr ratio 0 7044 lower nd 1 5 2 7 and relatively high 207 pb 204 pb and 208 pb 204 pb significantly different from those of the older rocks isotopic data and ree modelling suggestthat it formed by partial melting of an early pan african amphibolitic lower crust the syenogranite isa typical highly fractionated post orogenic granite with a deep negative eu anomaly hfse and reeenrichment a 87 sr 86 sr initial ratio of 0 7033 nd of 4 5 0 5 and distinct variations in the 206 pb 204 pbratio the isotopic compositions of sr nd and pb indicate that the syenogranite and the quartzmonzodiorite are comagmatic increasing fluorine concentration from the quartz monzodiorite to thesyenogranite suggests that fluid fractionation involving fluorine complexing played an important role inthe genesis of the syenogranite this process led to the enrichment of hfse and hree to extensivefractional crystallization to changes in the u pb ratios and a distinct variation in 206 pb 204 pb

the crustal structure of the rockall trough differential stretching without underplating the crustal structure along the axis of the rockall trough in the north atlantic has been studied along a 600 km refraction wide angle reflection transect containing three lines each 200 to 250 km long using explosives and ocean bottom seismometers one dimensional inversions of each section were made using the p method and forward modeling of the observed travel times in the next stage travel times and amplitudes were modeled using ray tracing techniques through two dimensional heterogeneous structures the results indicate that there are three sedimentary layers with velocities ranging from 2 km s to 4 5 km s the whole sedimentary section is up to 6 km thick and interpreted as late paleozoic to tertiary in age a two layer continental crust 5 to 7 km thick occurs along the length of the profile the upper crust 6 0 6 3 km s is circa 2 km thick and the lower crust 6 6 6 9 km s is circa 3 km thick a moho transition zone approximately 1 km thick lies at the base of the crust velocities in this transition zone increase from 6 9 km s up to 7 8 km s along the profile the underlying upper mantle has a laterally variable velocity between 7 6 and 7 8 km s unstretched crust onshore in ireland comprises a three layered crust with each layer approximately 10 km thick and a moho transition zone which is about 3 km thick the two upper layers in the onshore region are interpreted as corresponding to the upper crust in the rockall trough and indicate a stretching factor of 8 10 the velocity pattern in the lower crust in the rockall trough and under ireland are similar suggesting significantly less stretching 2 3 the differential stretching model is supported by the presence of the moho transition zone which is stretched by a similar factor to the lower crust the bulk stretching factor for the crust as a whole is in the range of 4 6 if this represents the lithospheric stretching factor significant underplating would be expected however if the stretching factor for the lower crust in the differential stretching model is more representative of overall lithospheric stretching little or no underplating is predicted the velocity patterns observed in the rockall trough indicate the absence of any significant underplating at the base of the crust such as that observed at the continental margin west of the hatton basin

tectonic magmatic stages of shield evolution the pan african belt in northeastern egypt abstract the arabian nubian shield illustrates an example of plate tectonics during the pan african orogenic event ca 950 550 ma it is considered one of the most remarkable proterozoic shield areas yet known although there is now a general agreement that the shield has evolved by a series of magmatic arcs and terrain accretion the evolution of its northernmost part exposed in northeastern egypt remains controversial two contrasting tectonic models have been proposed for the evolution of this region regional crustal extension and magmatic arc regimes there is no convincing evidence of older archean sialic materials within the region although the region is flanked by archean crust the neoproterozoic crustal components exposed in northeastern egypt preserve a record for its evolution a large gabbro diorite tonalite complex gdt 881 ma emplaced during the early stage of the pan african orogeny is geochemically primitive exhibits a low initial 87 sr 86 sr ratio 0 704 and shows trace element characteristics of oceanic arc related lavas the gdt complex was produced by fractionation from a mantle derived tholeiitic magma formed within an island arc tectonic environment a synorogenic stage characterized by extensive volcanism that produced the dokhan volcanic suite dvs 620 ma forms a continuous basalt to rhyolitic ignimbrite calc alkaline magma series exhibiting features of arc related volcanism with ree profiles similar overall to andean andesites and ignimbrites the dokhan suite was produced in a continental arc setting a granodiorite adamellite leucogranite composite batholith 552 ma emplaced during the late stage of the pan african orogeny exhibits typical features of i type complexes trace element traits of volcanic arc granites and is also interpreted to have been formed in an andean type setting a trondhjemite pluton was also formed at this late orogenic stage by partial melting of gdt host rocks at depth cooling and relaxation of the newly formed pan african crust caused extensive fracturing which was followed by intrusion of ne sw and nnw sse trending dyke swarms at 493 ma this episode marks a fundamental inversion from orogenic compressional to anorogenic extensional processes around the neoproterozoic paleozoic boundary anorogenic magmatism was locally associated with such extension induced structures the mount gharib peralkaline granite 476 ma for example exhibits trace element traits of a type within plate granites and a high initial 87 sr 86 sr ratio 0 711 and was formed in a rift environment thus the region is characterized by lengthy episodes of discontinuous subduction 880 550 ma during which it evolved to form a primitive crust by early pan african island arcs and late pan african continental arcs this was followed by an even longer period of crustal extension 550 90 ma that produced localized anorogenic magmatism

petrogenesis of oceanic andesites the andesites and cogenetic rocks of the intraoceanic island arc systems vary in silica content from 49 50 sio2 to about 76 sio2 the most primitive compositions having contents of about 50 sio2 are the basaltic andesites which are primary compositions generated by partial melting of the uppermost section of the subducted oceanic crust the oceanic andesites cannot have assimilated material from the continental crusts but their petrogenesis bears on the origin of the continental crust the fractionation of andesitic magmas occurs at relatively low pressures within the stability range of plagioclase basaltic andesites fractionate mainly orthopyroxene plagioclase while andesites fractionate clinopyroxene orthopyroxene plagioclase the petrogenesis of the continental andesites is more complex as the primary andesites contain sedimentary material from the continental crusts

plume tectonics as a cause of mass wasting on the southeast greenland continental margin deposition of sediments on the southeast greenland shelf is limited by its narrow width low degrees of tectonic subsidence and lack of a block faulted basement topography a marked hiatus on the continental slope removing the eocene and oligocene together with a late eocene early oligocene hiatus on the continental rise can be related to mass wasting along the continental slope erosion correlates with a phase of rapid tectonic subsidence caused by relative retreat of the iceland plume from the area affecting the oceanic crust more than the continental shelf relative deepening of 1 km over 10 my steepened the continental slope and triggered erosion following the cessation of rapid subsidence a submarine fan progressively onlapped the continental slope the lack of a major mid oligocene erosion surface on the shelf despite shallow water depths indicates that the magnitude of the mid oligocene eustatic sea level fall is much less than the 200m predicted by the exxon sea level curve

postglacial early permian late sakmarian early artinskian shallow marine carbonate deposition along a 2000 km transect from timor to west australia abstract late sakmarian to early artinskian early permian carbonate deposition was widespread in the marine intracratonic rift basins that extended into the interior of eastern gondwana from timor in the north to the northern perth basin in the south these basins spanned about 20 of paleolatitude approximately 35 s to 55 s this study describes the type section of the maubisse limestone in timor leste and compares this unit with carbonate sections in the canning basin nura nura member of the poole sandstone the southern carnarvon basin callytharra formation and the northern perth basin fossil cliff member of the holmwood shale the carbonate units have no glacial influence and formed part of a major depositional cycle that in the southern basins overlies glacially influenced strata and lies a short distance below mudstone containing marine fossils and scattered dropstones perhaps indicative of sea ice in the south marine conditions became more restricted and were replaced by coal measures at the top of the depositional sequence in the north the carbonate deposits are possibly bryozoan crinoidal mounds whereas in the southern basins they form laterally continuous relatively thin beds deposited on a very low gradient seafloor at the tops of shale limestone parasequences that thicken upward in parasequence sets all marine deposition within the sequence took place under very shallow inner neritic conditions and the limestones have similar grain composition bryozoan and crinoidal debris dominate the grain assemblages and brachiopod shell fragments foraminifera and ostracod valves are usually common tubiphytes ranged as far south as the southern carnarvon basin albeit rarely but is more common to the north gastropod and bivalve shell debris echinoid spines solitary rugose corals and trilobite carapace elements are rare the uniformity of the grain assemblage and the lack of tropical elements such as larger fusulinid foraminifera colonial corals or dasycladacean algae indicate temperate marine conditions with only a small increase in temperature to the north the depositional cycle containing the studied carbonate deposits represents a warmer phase than the preceding glacially influenced asselian to early sakmarian interval and the subsequent cool phase of the mid artinskian that is followed by significant warming during the late artinskian early kungurian the timing of cooler and warmer intervals in the west australian basins seems out of phase with the eastern australian succession but this may be a problem of chronostratigraphic miscorrelation due to endemic faunas and palynofloras

carbonate deposition during a time of mid latitude ocean cooling early permian subtropical sedimentation in the sverdrup basin arctic canada abstract carbonate rocks of the lower permian sakmarian to kungurian raanes and great bear cape formations accumulated in the sverdrup basin during a critical transition period when ocean waters cooled dramatically these rocks have many attributes similar to sediments deposited in modern subtropical zones the biota is dominated by a heterozoan assemblage of pelmatozoans bryozoans and brachiopods with the addition of photozoan elements such as fusulinids and colonial corals associated with the change in the sverdrup basin from a tropical to subtropical environment is the first occurrence of glauconite iron oxides and storm deposits the shift in depositional conditions is interpreted to reflect decreased seawater temperature that was brought about by a northward migration to a mid latitude position frequented by storms oceanographic change however was not unidirectional but was complicated by concomitant variations in sea level and oceanography recorded as three distinct phases determined by water depth and temperature 1 rocks of the raanes and lower great bear cape are deep water and heterozoan 2 the middle great bear cape records shallow water subtropical conditions with fusulinids and large colonial corals 3 the upper great bear cape is entirely heterozoan in all locations and by implication all depths all post raanes and great bear cape rocks of the sverdrup basin are entirely heterozoan taking caution for possible evolutionary changes subtropical carbonates can be recognized in rocks much older than previously believed the most important components for identifying subtropical carbonates of the past are large benthic foraminifers in an otherwise heterozoan assemblage and as such the ability to delineate this carbonate realm in the past is currently restricted to times when large benthic foraminifers grew in shallow marine environments