morphostructure of the galicia continental margin and adjacent deep ocean floor from hyperextended rifted to convergent margin styles abstract a joint study and mapping of the morphological features and architecture of the galicia margin based on high resolution multibeam bathymetry data and multichannel seismic profiles provides new insights of the margin morphostructure tectonic processes are the major control of the margin s morphology where the imprint of the north atlantic rifting and opening and later subduction of the bay of biscay and compressional events are still well preserved the west galicia margin which is up to 350 km wide is composed from east to the west of inner transitional and outer sectors the inner sector shows a graded upper slope strongly dissected by a system of submarine canyons and valleys that flow into the 170 km long and 40 60 km wide nw se valle inclan valley the transitional sector which is up to 75 km wide displays a smooth relief dominated by landslides and giant craters finally the outer sector is characterised by several banks n s sub parallel ridges and elongated valleys that project seaward over 200 km i e galicia bank half graben deep galicia margin and peridotite ridge domains the north galicia margin is characterised by a system of terraces up to 30 km wide that form a stepped slope followed by an abrupt lower slope affected by large scale rotational failures the galicia margin constitutes an exceptional area within the atlantic margins for identifying different morphostructure styles that characterised the main stages of the margin s evolution along the wilson cycle the galicia margin hosts the transition between two end members of continental margin styles a western hyperextended rifted margin being the conjugate with the newfoundland margin during the late jurassic and a northern convergent margin now fossilised associated with the partial subduction of the southern sector of the bay of biscay underneath the north iberian margin which occurred in paleocene eocene times thus the morphostructure of the west galicia margin still reflects the detached fault systems and tilted blocks that resulted from the mesozoic north atlantic rifting furthermore the coruna and jean charcot seamounts which are adjacent to the galicia margin correspond to spreading ridges formed by the simultaneous opening of north atlantic ocean and the bay of biscay in the upper cretaceous the rest of the terraced morphology of the north galicia margin is derived from extensional faults caused by an overriding of the iberian plate in relation to the subduction of the bay of biscay oceanic crust during cenozoic times since the late neogene compressional tectonics resulting from the reorganisation of the eurasia iberia and africa plates is also reflected on the present day morphology as the following i an elevation of the biscay abyssal plain up to 150 m over the iberia abyssal plain ii deep seated landslides associated with an uplift of the galicia bank and iii an orientation of submarine canyons along nw se strike slip faults

morphological feature analyses of the prestige half graben on the sw galicia bank abstract morphological analysis was carried out on the sw flank of the galicia bank as part of detailed geological studies of the half graben where the prestige oil tanker wreckage is located broad multibeam echosounder simrad em 12s 120 and seismic reflection airgun data were collected on board rv hesperides three groups of morphological features were identified as tectonic erosive and depositional the main tectonic morphological feature is a n s oriented fault scarp in the eastern part of the study area main erosive features are gullies slide scars e w oriented inter lobe channels and a nne ssw oriented main channel dominant depositional morphologies comprise thirteen e w oriented sedimentary wedges seven large e w to ne sw oriented sedimentary lobes and the relatively smooth sedimentary layer covering the eastern and western ridges that comprise the galicia bank s sw flank characterization classification and distribution of these features indicate that the study area constitutes a half graben structure comprising a n s oriented normal fault scarp abruptly connecting steep gradients in the eastern area with the gentle slope of the central area a thick but narrow wedge of sediment has accumulated at the foot of the scarp as a result of mass movement processes that govern the formation of a series of short lived wedges overlapping lobes and also channels this particular kind of slope apron system corresponds to a tectonic controlled drainage pattern from a sharply tilted hanging wall block supplied by sediment from the ridge finally we can conclude that the prestige stern and bow are located at the steep fault scarp close to a channel head and at the axis of inter lobe channels developed by mass movement processes thus morphologic tectonic and sedimentary factors can be considered as potential hazards

morphological characteristics of the basque continental shelf bay of biscay northern spain their implications for integrated coastal zone management this contribution integrates and analyzes data from high resolution multibeam seismic profiles bathymetric lidar and surficial sediment data for the geomorphological seascapes characterisation and process description of the basque inner and middle continental shelf northern spain from the data obtained the basque shelf is characterised by a heterogeneous seafloor where on a small spatial scale different morphologies and sedimentary processes can be observed tectonic activity and sea level changes together with present processes of sediment supply and climatic conditions have a critical influence on the present configuration of the continental shelf and the distribution of seafloor types on the basis of all of the datasets seafloor classification bedform analysis long term sea level change induced seafloor features and anthropogenic features over the seafloor are described as a result three distinct zones have been identified for the basque shelf related to the main geomorphological features and seascapes as summarised below i a western part with a northwestern orientation which coincides with the prevailing wave direction this zone is characterised by a predominantly rocky substrate except within the mouth of the nervion estuary where sandy sediments appear to infill a paleo channel ii a central part which is oriented towards the northeast and receives less wave energy within this zone a sedimentary seabed is predominant sorted bedforms are well developed and can be detected in water depths in excess of 90 m iii an eastern zone which is characterised by sedimentary seafloor and shore terraces here a flat rocky seafloor is covered by a thin layer of sediments throughout the study area the bathymetry and sedimentary features of the shoreface and inner shelf are controlled mainly by climatic conditions coastal dynamics and the underlying geological framework the integration of these data sets together with their interpretation is a valuable source of information for integrated coastal zone management iczm and constitutes a useful tool for implementing various european directives

40ar39ar dating of amphiboles from zabargad island red sea is precluded by interaction with fluids abstract stepwise heating data on four amphiboles separated from various zabargad rocks a dolerite dike a peridotite a gneiss and a pyroxenite invariably show saddle shaped age spectra the age information in the spectrum is generally masked by 1 2 nl g excess 40ar the spectrum least affected by excess ar constrains the emplacement age of the dolerite dike z 2137 to be 20 ma the greatest concentration of excess ar is found in the peridotite implying that the ultramafic rocks were metasomatized by a very 40ar rich fluid a complex history of subsolidus reactions with hydrothermal fluids is suggested by the irregularities in the 37 ar 39 ar spectra

zabargad and the isotopic evolution of the sub red sea mantle and crust abstract zabargad island in the red sea exposes ultramafic bodies in contact with partially retrograded granulite facies lower crustal rocks the contact could represent the sub red sea moho prior to the opening of the red sea the ultramafic rocks are largely protogranular spinel lherzolites that grade locally to porphyroclastic to mylonitic amphibole peridotites some of the peridotites are infiltrated by high mg gabbros troctolites the peridotites and the lower crustal rocks are cut by diabase dikes the zabargad peridotites are isotopically heterogeneous on a scale of meters or less sr and nd isotopic ratios of clinopyroxenes cpx from spinel lherzolites lie in the depleted portion of the 143 nd 144 nd 87 sr 86 sr mantle array whereas cpx and amphiboles from amphibole peridotites fall off the array towards the low 143 nd 144 nd and sol 87 sr 86 sr field that may characterize portions of the sub continental mantle cpx from troctolites plot in the middle of the 143 nd 144 nd 87 sr 86 sr mantle array and have higher 87 sr 86 sr ratios and more depleted lree patterns than the intruded spinel lherzolites and amphibole peridotites all whole rock lherzolites and peridotites and most diabase dikes from zabargad as well as a variety of rocks from other parts of the red sea area morb glasses a diabase dike from the brothers island basanites and clinopyroxenes from associated nodules from the ataq diatreme in yemen define a linear array on a sm nd isochron diagram with a best fit age of ca 675 ma and an initial nd of 6 7 this array is probably a mixing line between depleted morb type mantle and a low 143 nd 144 nd 87 sr 86 sr metasomatic component the age defined by the linear array may date either an episode of pan african mantle homogenization or in our preferred model the pan african segregation of a previously homogeneous mantle into two or more reservoirs of identical 143 nd 144 nd but different sm nd ratios the subsequent partial remixing of these isotopic reservoirs during metasomatism associated with the opening of the red sea did not perturbed the slope of the array the lree enriched reservoir may have been the pan african lower crust since there is field and textural evidence that the metasomatic fluids were derived from or equilibrated with the adjacent crustal complex alternatively the lree enriched component may have been a mantle reservoir similar to that which generated basalts and nodules from the nearby ataq diatreme the troctolites are isotopically distinct from the host peridotites and retain igneous textures suggesting they intruded the peridotites during the opening of the red sea their isotopic patterns are similar to those of recent volcanics from afar portions of the ataq diatreme and some zabargad dikes indicating derivation from a similar mantle source their very lree depleted patterns suggest they are either re melted fractions of previously very depleted spinel lherzolites or early formed mineral aggregates that became enmeshed in the peridotites as fractionating melt ascended to higher levels our results indicate that at least two major processes metasomatism and intrusion have modified the isotopic pat terns of the spinel lherzolites of zabargad had the resultant chemically and isotopically heterogeneous bodies stayed in the mantle instead of being uplifted to their present positions they would have proven to be enigmatic sources for future basalts and xenoliths

high resolution seismic stratigraphy of the galicia bank region and neighbouring abyssal plains nw iberian continental margin abstract the high resolution seismic stratigraphy of the galicia bank region and adjacent deposits of the neighbouring iberian and biscay abyssal plains was included as part of the geological studies conducted in the area where the oil tanker prestige wreck is located this seismic stratigraphy is characterized by five seismic units 5 to 1 from oldest to youngest lying above an irregular acoustic basement defined by a highly fractured system of horsts and grabens these faulted systems have controlled the local depositional architecture deforming fracturing relocating and distributing sediments since the valanginian three depositional models of facies can be recognized on the galicia bank region and surrounding abyssal plains bank escarpment and abyssal plain facies the bank facies reflects the depositional evolution from initial filling to obliterating stages of the horsts and grabens the escarpment facies has a local distribution and represents deposition associated with active slopes of the faulted escarpments the abyssal plain facies represents gravity flow deposits coming mostly from the bounded fault flanks of the galicia bank region the valanginian to quaternary sedimentary evolution of the galicia bank region reflects depositional responses to tectonic condition the occurrence of mass movements confirms that the syn rift period valanginian to aptian age along the galicia margin was diachronous beginning in the interior basin and shifting later toward the west of the galicia bank region deep galicia margin the albian to quaternary post rift evolution is characterized by a decrease in tectonic activity and sedimentary variations in the style of deposition valley and related features mass movement deposits contourites and drape deposits conditioned by changes in the sedimentary processes paleoenvironments and tectonic reactivation of the regional slopes the effects of tectonic movements persist as evidenced in areas where faulted escarpments and highs outcrop and in areas of near surface faulting the syn rift and post rift stages have also conditioned the depositional architecture of the prestige sinking area the evolution of the surrounding abyssal plains does not reflect the same pattern to tectonic condition

detrital and early diagenetic processes in late pleistocene and holocene sediments from the sw galicia bank inferred from high resolution enviromagnetic and geochemical records abstract u channel based high resolution magnetic and geochemical measurements supported by scanning electron microscopy x ray diffraction and fluorescence and grain size analyses of discrete samples were carried out on six sediment cores recovered from the sw flank of the galicia bank at the prestige s wreck area five magnetochemical facies reflecting the different sedimentary environments i e hemipelagic contouritic turbiditic sediment provenances and early diagenetic redoxomorphic processes i e mn peaks are defined on the basis of a cluster analysis the records showed climatically forced detrital signatures of regional importance associated with continental slope sedimentary processes the chronostratigraphy and correlation of the cores is based on 20 14 c ams dating recognition of heinrich like events h1 to h3 and well documented holocene climatic features from a geochemical and enviromagnetic perspective the late pleistocene to holocene evolution of the area shows two different sedimentation contexts mis 2 is dominated by down slope depositional processes the highest sedimentation rates 9 1 cm ky were achieved during this interval the major turbidite activity is recorded between 31 000 yr bp and 13 500 yr bp successive arrivals of ird h1 to h3 are also identified during this period in contrast holocene sedimentation was dominated by a mixture of pelagic hemipelagic and along slope depositional processes and presented considerable lower sedimentation rates 3 cm ky bottom currents intensification at 7000 yr bp and 4500 yr bp coinciding with late holocene warm events are also recognised during this stage

western extension of the iberian european plate boundary during the early cenozoic pyrenean convergence a new model abstract a dense seismic net work allows of drawing the western prolongation of the northern boundary of the iberian plate this early cenozoic deformed boundary is disrupted by two prominent nw se transform faults a kinematic reconstruction with a nw late cretaceous lower eocene motion of the iberian plate is proposed the azores biscay rise is interpreted as a short lived subduction zone whereas king s trough could be a transform fault

a pan african age for the hp ht granulite gneisses of zabargad island implications for the early stages of the red sea rifting abstract up to now the age of granulite gneisses intruded by the zabargad mantle diapir has been an unsolved problem these gneisses may represent either a part of the adjacent continental crust primarily differentiated during the pan african orogeny or new crust composed of miocene clastic sediments deposited in a developing rift crosscut by a diabase dike swarm and gabbroic intrusions and finally metamorphosed and deformed by the mantle diapir previous geochronological results obtained on zabargad island and al lith and tihama asir complexes saudi arabia suggest an early miocene age of emplacement for the zabargad mantle diapir during the early opening of the red sea rift in contrast sm nd and rb sr internal isochrons yield pan african dates for felsic and basic granulites collected 500 600 m from the contact zone with the peridotites devoid of evidence for retrograde metamorphic minerals from a felsic granulite provide well defined rb sr and sm nd dates of 655 8 and 699 34 ma for the hp ht metamorphic event 10 kbar 850 c the thermal event related to the diapir emplacement is not recorded in the sm nd and rb sr systems of the studied gneisses in contrast the development of a retrograde amphibolite metamorphic paragenesis strongly disturbed the rb sr isotopic system of the mafic granulite the initial 143 nd 144 nd ratio of the felsic granulite is higher than the contemporaneous value for chur and is in agreement with other nd isotopic data for samples of upper crust from the arabian shield this result suggests that source rocks of the felsic granulite were derived at 1 0 to 1 2 ga from either an average morb type mantle or a local 2 2 ga lree depleted mantle zabargad gneisses represent a part of the disrupted lower continental crust of the pan african afro arabian shield during early stages of the red sea rifting in the miocene these precambrian granulites were intruded and dragged upwards by a rising peridotite diapir

fracture zone tectonics at zabargad island red sea egypt abstract zabargad island which lies along the western margin of the red sea rift is a remarkable place because it provides fresh exposures of undepleted mantle peridotite how this peridotite came to be exposed on zabargad remains unclear our field mapping indicates that most of the contacts between peridotite and the adjacent bodies of pan african gneiss and cretaceous zabargad formation on the island are now high angle brittle faults zabargad formation strata have been complexly folded partly in response to this faulting overall the array of high angle faults and associated folds on the island resembles those found in cross rift transfer zones we suggest therefore that the zabargad fracture zone a band of submarine escarpments on the floor of the red sea north of the island crosses zabargad island and has actively resolved differential movement between the central red sea rift and the northern red sea rift the final stage of uplift that brought the unusual peridotite to the earth s surface is related to shallow crustal transpression which may have inverted an earlier transtensional regime

sedimentary and structural patterns on the iberian continental margin an alternative view of continental margin sedimentation abstract a long range side scan sonar gloria survey of the entire west iberian slope and rise has provided the first overview of the interrelationship between structure and sedimentation patterns on a continental margin the results emphasize the importance of slope following contour currents as a depositional mechanism in fashioning this continental rise terrigenous sediments transported down canyon by pass the rise which does not consist of a series of coalescing fans the sedimentation patterns identified on the sonographs can be interpreted in terms of facies models and caution must be exercised against over emphasis of downslope processes in models for the construction of lower slopes and rises

galicia bank sediment transport activity in response to continuous sedimentary instability dynamics a geotechnical perspective the sedimentary instability dynamics occurring over time throughout the isolated galicia bank atlantic ocean nw iberian peninsula have been studied using a sedimentological and geotechnical approach featuring a quantitative assessment of slope stability under different scenarios including earthquake activity the erosion of the scarps in the galicia bank includes the continuous development of small instabilities evacuated along gullies and channels or deposited on slopes these deposits may be subsequently mobilized evolving into new sedimentary gravity flows e g turbidity flow the studied sediments consist mostly of poorly to very poorly sorted sands and silts transported by turbidity currents and occasionally by debris flow processes the sediments in the study area identified as normally consolidated and located on gently sloping areas gradients less than 5 may become unstable if low magnitude seismic events occur pga 10 without any trigger other than oversteepening in contrast overconsolidated sediments may remain stable under static conditions and may become unstable on slope gradients 10 when earthquakes occur with the maximum peak ground acceleration pga 0 34 the sedimentological and geotechnical models presented herein are complementary approaches that can be utilized to understand the long term sedimentary instability dynamics observed within the study area such results are critical for better understanding sedimentary models of the dismantling processes of deep seamounts located far away from continental sedimentary inputs

lead isotope systematics of some igneous rocks from the egyptian shield lead isotope data on late precambrian igneous rocks from the eastern desert of egypt are presented previous work has indicated that this igneous suite is characterized by uniformly low initial ratios of 37sr86sr 0 7035 the pb data define three groups loosely corresponding to age an older tonalite to granodiorite otg suite with ages in the range 610 710 ma has pb isotope characteristics similar to modern mantle derived oceanic mafic rocks the age corrected initial pb isotope ratios of the otg group lie near the ocean regression line in correlation diagrams of 206pb204pb vs 207pb204pb and 203pb204pb the isotope data imply an origin for the otg group analogous to calc alkalic igneous rocks in modern intra oceanic island arcs a younger suite of k rich plutonic rocks 570 595 ma has similar 206pb204pb and 208pb204pb initial ratios relative to the otg suite but have higher 207pb204pb initial ratios when calculated from measured whole rock u and pb contents however k feldspar and galena associated with this suite indicate initial pb isotope ratios close to the otg suite we interpret this to indicate that the younger granites could have originated by anatexis of otg type material but they probably experienced an episode of metasomatism after emplacement a suite of volcanic rocks the dokhan volcanics from the central eastern desert are intermediate between the younger granites and otg in both age 610 ma and pb isotope characteristics limited data on initial 208pb204pb ratios suggest that all the egyptian samples originated from a source with anomalously low thpb ratios although the relative abundance of granitic rocks in the egyptian shield decreases to the south no evidence of north south heterogeneity in the pb isotope composition of the respective source regions is apparent in the data however the westernmost sample studied from the aswan area contains distinctly more radiogenic pb relative to the mean this location may mark the boundary between the late precambrian ensimatic orogen now outcropping in the egyptian and saudi arabian shields and an older sialic craton to the west

multibeam backscatter as a tool for sea floor characterization and identification of oil spills in the galicia bank this study is focused on the potential of using the multibeam acoustic backscatter both for sea floor characterization of the galicia bank and for identification of sea bottom oil spills an extended multibeam data set was acquired during the rg 1 and rg 2 surveys aboard rv hesperides in 2003 for the geological and geophysical characterization of the galicia bank where the oil tanker prestige wreck is located 3 d images have been made by integrating bathymetry and backscatter data multibeam data have been additionally supported by gravity core information in order to correlate sea floor acoustic backscatter with the surficial sediments and their physical properties the major factors that seem to control the zonation of the backscatter intensity images are i the type of the sedimentary processes gravitational turbiditic hemipelagic which control at the same time the grain size distribution and ii the sea bottom morphologies such as basement outcrops however there are some backscatter halos that cannot be explained by these processes in the backscatter maps the bow and stern of the prestige wreck are clearly marked by a high backscatter intensity anomaly whose existence is discussed

types of submarine fan lobes models and implications different lobe models imply significantly different reservoir geometries thus one must apply the proper lobe model to ancient fan sequences braided suprafan lobes are characterized by stacked channel sand bodies with good lateral and vertical communication and they constitute excellent reservoir facies depositional lobes composed of sheet like sand bodies with good lateral and moderate vertical communication exhibit properties of good reservoir facies fanlobes which refer to meandering channels and associated levee facies of large mud rich submarine fans such as the mississippi fan are characterized by offset stacked sand bodies with poor lateral and vertical communication these lenticular sands have the potential to be moderately good reservoir facies ponded obes which represent mud rich slump facies of slope environments comprise poor reservoir facies because of low sand content and poor sand body connectivity furthermore the presence of contorted mud layers in ponded lobes is expected to hinder fluid flow external mounded reflections in seismic profiles often are interpreted as lobes however there are no definite seismic criteria to delineate mud rich lobes from sand rich lobes

submarine fan systems i characterization and stratigraphic prediction submarine fan and related deep marine clastic systems such as submarine ramps and slope aprons are important hydrocarbon reservoirs in many parts of the world the origin and character of these systems reflect a complex interplay between a range of autocyclic and allocyclic controls including sea level fluctuations basinal tectonics and the rate type and nature of sediment supply these controls are commonly interdependent this paper reviews the characteristics which distinguish different submarine fans and their subsurface prediction then proposes a classification framework for fans which discriminates between different systems on the basis of their fundamental controls the proposed classification is discussed in terms of its value as a predictive tool in hydrocarbon exploration development and production a modus operandi is developed which uses the classification for reservoir description and prediction and involves three stages of investigation basin screening fan delineation and fan characterization each stage demands the integration of a range of tools and techniques including sequence stratigraphy seismic facies analysis seismic redisplay and geophysical modeling these stages of analysis are linked in turn with core and wireline log data to constrain the range of possible fan types within a basin fill and assess the implications for reservoir seal and trap development this structured approach to stratigraphic analysis is also valuable in constraining the range of reservoir architectural options in appraisal development and production

late ordovician glaciation in southern turkey abstract we present a new survey of several palaeozoic sections in both the taurus range and the border folds that documents typical glacial features including a glacial pavement and striated dropstones halevikdere formation and demonstrates the former presence of an ice sheet in southern and south eastern turkey evidence for the late ashgill hirnantian age of this episode is provided by macro and microfossils found within the glacial formation the extension of ice related deposits into the northernmost part of the arabian platform mardin region implies a much wider distribution of the ordovician ice sheet than was previously believed and strongly suggests that southern turkey lay close to egypt during the lower palaeozoic

recent sedimentary processes in the prestige site area galicia bank nw iberian margin evidenced by high resolution marine geophysical methods an echo character analysis of the oil tanker prestige wreck area was undertaken using high resolution marine geophysical methods topas and airgun seismic reflection profiles multibeam echosounder and tobi sidescan sonar integration and comparison of the results using all these methods is presented given some practical applications for indirect near surface and seafloor interpretations ten different echo types were identified and grouped into four main classes i distinct ii indistinct iii irregular and iv hyperbolic echo character distribution enabled us to determine recent sedimentary processes in the area two major depositional systems can be found through the identification of these sedimentary processes a a slope depositional system sds located in the eastern and central area where mass movement processes are dominant and b a turbiditic main channel depositional system tds located in the western area where channel related processes are dominant both of these interact over the half graben structure of the southwestern sector of the galicia bank where the prestige wreck is located within the sds erosive and depositional mass movement processes characterised a complex depositional system erosive processes occur on the fault scarp channels head inter lobe channels and distal part of the sedimentary lobes moreover depositional processes take place on the top of the fault scarp sedimentary wedges sedimentary lobes and on the west flank of the main channel both depositional systems interact but the sds should be more active during fault scarp reactivation periods through relief rejuvenation and new exposed deposits microearthquake activity would favour the available materials being weathered eroded and transported by mass movements in such a situation the tds acts as the main collector of eroded sediment derived from the fault scarp throughout the sds being responsible for its evacuation into the iberian abyssal plain however outside of fault reactivation periods the sds is less active such as during the present situation

crustal underplating and evolution in the red sea rift uplifted gabbro gneiss crustal complexes on zabargad and brothers islands the nature of the crust of the red sea outside the oceanic axial trough segments can be inferred from uplifted crustal sections exposed on the islands of zabargad and the brothers in the central and northern red sea zabargad exposes in addition to mantle derived peridotites a crustal section consisting of metagabbros and metapyroxenites associated with silicic gneisses and cut by doleritic dikes seismic velocities of samples from this section are compatible with crustal velocities obtained by seismic refraction experiments in the central and northern red sea the zabargad peridotitesilicic gneiss gabbroic association provides a sample of upper mantle lower crust in a continent before the inception of a major rifting event the petrology and mineral chemistry of the zabargad metagabbros suggest that they were originally part of a basic layered complex which crystallized at relatively high pressure 900 1000 mpa or 9 10 kbar i e over 30 km deep the gneisses have tonalitic trondhjemitic affinity and were originally equilibrated in the garnet granulite facies implying also high pressure and derivation from the lower continental crust this suggests that after the subcrustal continental lithosphere along the proto red sea rift had been consumed possibly by convective thinning but before any significant thinning of the crust basaltic melts intruded the base of the continental crust underplating it with gabbroic complexes thus crustal rifting was probably preceded by a thermal anomaly in the mantle which supports an active rifting hypothesis the gabbro pyroxenite complex and the gneisses underwent subsequently retrometamorphic events under decreasing pt conditions from granulite to amphibolite to greenschist facies implying decompression and uplift probably related to the gradual thinning of the continental crust during rifting during this stage additional gabbroic intrusions and basaltic dikes exemplified by the gabbros and dolerites from the brothers islands were emplaced at decreasing depths beneath the thinned continental crust the northern red sea crust is presently in this stage it is neither oceanic nor continental it lacks a continuous thick basaltic upper crustal layer and is made prevalently of gabbroic and doleritic intrusives with relicts of sialic continental crust subsequently basaltic injections became progressively focused in a narrow axial zone and seafloor spreading initiated at discrete equidistant hot points as observed presently in the central red sea which by axial propagation evolved into nearly continuous zones of oceanic crust accretion as observed presently in the southern red sea processes similar to those proposed here for the evolution of the red sea rift can be documented in passive margins of the atlantic and in some alpine ophiolite complexes

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

late pleistocene and holocene sedimentary facies on the sw galicia bank atlantic nw iberian peninsula abstract five main pleistocene holocene lithofacies are defined in three different sedimentary environments fault scarp sedimentary lobe and inter lobe channel of the sw galicia bank 1 turbidites biogenous and terrigenous 2 hemipelagites 3 pelagites 4 debrites and 5 heinrich sediments in the sedimentary lobe and inter lobe channel the stratigraphic record consists mainly of turbidites interbedded with debrites and heinrich sediments and hemipelagites that are covered by hemipelagites or pelagites in the fault scarp the stratigraphy comprises hemipelagites and turbidites covered by pelagites frequency of turbidite events has varied between 1 1 2 ka and 1 3 1 ka since 31 3 ka bp at least four turbidite events 1 to 4 between 9 1 and 31 3 ka bp have been correlated between the different sedimentary environments the downslope of turbidity flows prevailed until 9 1 ka after which vertical settling and slow lateral advection have controlled sedimentation the source area of turbidites and debrites is the fault scarp erosion of the slope near surface pelagites hemipelagites and ancient outcropping deposits could explain the presence of biogenous turbidites and terrigenous turbidites respectively the rhythmic development of turbidites interrupted by hemipelagites could represent the manifestation of different pulses of sedimentary instability induced by a combination of oversteepening up to 29 and occurrence of earthquakes

a pan african origin and uplift for the gneisses and peridotites of zabargad island red sea a nd sr pb and os isotope study a sr nd pb and os isotopic study of peridotites and granulite facies gneisses from zabargad island in the red sea suggests that the tectonothermal petrogenetic and geochemical evolution of these rocks occurred largely during the pan african orogeny rather than the recent opening of the red sea sm nd model ages and whole rock errorchrons indicate that spinel iherzolites and gneisses differentiated from a common depleted mantle source about 700 ma the iherzolites were mylonitized metasomatized and amphibolitized during a structural event that juxtaposed the peridotites with the gneiss complex and uplifted the gneiss peridotite complex to relatively shallow crustal levels most radiometric dating schemes suggest a pan african age for this event the gneisses generally have lower 143nd 144nd 87sr 86sr 208pb 204pb 207pb 204pb and 206pb 204pb ratios than the peridotites they extend linear trends defined by the spinel and amphibole peridotites on sr nd sm nd and pb pb diagrams suggesting the gneisses were either the source or buffering medium for the pan african metasomatism only one post pan african event had a significant effect on the geochemistry of the gneiss peridotite complex shallow level metasomatism by ultrahot 750 900 c hypersaline solutions with high 87sr 86sr 0 710 ratio led to the development of gem quality olivine crystals as well as low pressure mineral assemblages in the peridotites gneisses and younger rocks plagioclase rich assemblages with apparent igneous textures troctolites that are most common in the southern peridotite body may have formed by interaction of these fluids with peridotite i e are pseudo troctolites metasomatism changed the 87sr 86sr sm nd and re os ratios of the plagioclase peridotites making them unsuitable representatives of the pan african mantle

hydrothermal remobilization of rare earth elements and its effect on nd isotopes in rhyolite and granite abstract although several studies have shown that the sm nd radiometric system is not remobilized during the hydrothermal alteration of felsic rocks it is doubtful that the rare earth element ree mobility noticed under certain hydrothermal conditions always leaves the sm nd systematics unperturbed this contribution describes two cases of such isotopic disturbances of hydrothermalised felsic rocks from two different geological contexts in rhyolites from the esterel southeastern france and in the red granite of porto northwest corsica southeastern france although these two igneous bodies were initially very homogeneous as regards their nd isotopes some samples display very contrasting nd isotopic signatures i e comparatively low initial143nd 144nd ratios that are anomalous from a magmatic point of view moreover these isotopically perturbed rocks are invariably associated with peculiar ree patterns that are also difficult to explain in terms of differentiation or contamination at the magmatic stage it is thus inferred that the sm nd system in these samples was perturbed by hydrothermal alteration in the two examples studied the perturbation of the sm nd radiometric system cannot be solely explained by a modification of thesm nd ratio significantly after the emplacement of the igneous bodies although the net result of the hydrothermal alteration is leaching of the rees from the rocks i e mainly the middle and heavy ree for the granite and the light ree for the rhyolites the isotopic compositions of the samples were modified by a component introduced from a fluid having contrasting143nd 144nd ratios this inference together with structural observations indicates that rees were transported over distances exceeding several tens to hundreds of metres even in the case of the granite studied in this work where mineralised fractures are absent this is far greater than previous estimates and this conclusion is of great importance for environmental and nuclear waste disposal site studies where light ree are often taken as natural analogues for artificial radionuclides