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

north east greenland shelf north of 79 n results of a reflection seismic experiment in sea ice abstract the first multichannel seismic reflection measurements of the north east greenland shelf north of 79 n were made as part of the r v polarstern cruise ark v 1988 this area is perennially covered by sea ice the sparse seismic data show a very deep rift basin filled with up to 10 km of high velocity rocks interpreted as carboniferous and permian sediments conspicuous structures interpreted as marginal reefs are associated with the western flank of the rift basin

vegetation and climate during the last glacial high stand ca 28 22 ka bp of the sea of galilee northern israel abstract despite ongoing discussions on hydroclimatic conditions in the southern levant during the last glacial detailed knowledge about the levantine paleovegetation which is an important indicator for the paleoclimate is limited to investigate the paleovegetation in northern israel we analyzed the pollen assemblage of a sediment core that was drilled at the ohalo ii archaeological site on the southwestern shore of the sea of galilee lake kinneret we refined the lithology and the age depth model with the help of five new radiocarbon dates the core comprises a continuous sediment profile of mainly laminated authigenic calcites and detrital material that deposited between ca 28 000 and 22 500 years before present when the sea of galilee rose above the modern lake level stand and temporarily merged with lake lisan the precursor of the dead sea the well dated and high resolution pollen record suggests that steppe vegetation with grasses other herbs and dwarf shrubs predominated in northern israel during the investigated period in contrast to the holocene there was no continuous vegetation belt of the mediterranean biome in the vicinity of the sea of galilee mediterranean elements such as deciduous oaks only occurred in limited amounts and were probably patchily distributed these results disagree with previous pollen based hypotheses from the region that assumed the spread of mediterranean forest during glacial periods while the pollen data may indicate semiarid conditions in northern israel and give no evidence of increased effective moisture previous hydroclimatic studies suggested increased precipitation rates that are consistent with high lake levels sea of galilee lake lisan thus we discuss factors influencing the pollen assemblage and the plant cover

crustal structure off norway 62 to 70 north abstract extensive geophysical surveys have been undertaken on the volcanic passive continental margin offshore norway between 62 and 70 n during the last 25 years three main margin segments have been identified the lofoten vesteralen margin the voring margin and the more margin the main features of the margins are prominent marginal highs including seaward dipping reflector sequences and an up to 22 km thick volcanic and transitional crust prominent escarpments the voring plateau escarpment and the faeroe shetland escarpment and up to 12 km deep post jurassic sedimentary basins east of the escarpments velocity depth solutions from about 250 sonobuoys expanding spread profiles and refraction profiles have been compiled and contoured isovelocity horizon contour maps and velocity transects outline a crust which broadly thickens from an oceanic crust with a normal oceanic type velocity structure to a ca 35 km thick continental crust with a continental velocity structure beneath the norwegian coast anomalous features include local crustal thickening below the more and vering marginal highs and high velocity bodies in the lower crust in the extension of the precambrian lofoten vesteralen archipelago the free air seasat derived gravity anomalies show a good correlation with the high velocity bodies and show prominent ne trending highs from the rockall plateau porcupine plateau region over the more voring and lofoten vesteralen margins to the southwestern barents sea

breakup and early seafloor spreading between india and antarctica summary we present a tectonic interpretation of the breakup and early seafloor spreading between india and antarctica based on improved coverage of potential field and seismic data off the east antarctic margin between the gunnerus ridge and the bruce rise we have identified a series of ene trending mesozoic magnetic anomalies from chron m9o 130 2 ma to m2o 124 1 ma in the enderby basin and m9o to m4o 126 7 ma in the princess elizabeth trough and davis sea basin indicating that india antarctica and india australia breakups were roughly contemporaneous we present evidence for an abandoned spreading centre south of the elan bank microcontinent the estimated timing of its extinction corresponds to the early surface expression of the kerguelen plume at the southern kerguelen plateau around 120 ma we observe an increase in spreading rate from west to east between chron m9 and m4 38 54 mm yr 1 along the antarctic margin and suggest the tectono magmatic segmentation of oceanic crust has been influenced by inherited crustal structure the kinematics of gondwanaland breakup and the proximity to the kerguelen hotspot a high amplitude e w oriented magnetic lineation named the mac robertson coast anomaly mca coinciding with a landwards step down in basement observed in seismic reflection data is tentatively interpreted as the boundary between continental transitional zone and oceanic crust the exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust which typically occurs above anomalously hot mantle together with the existence of the mca zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring steady state magmatic production of the kerguelen lip an alternative model suggests that the northward ridge jump was limited to the elan bank region whereas seafloor spreading continued in the west enderby basin and its sri lankan conjugate margin in this case the mca magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 ma

late pleistocene paleomagnetic secular variation from the sea of galilee israel 1 stable magnetization of laminated lacustrine lisan formation show typical paleomagnetic secular variation and a possible geomagnetic field excursion 14c ages from the top and assumed 0 9 mm yr deposition rate constrain the recorded section between 29 28 ka and 22 23 ka samples were recovered from two trenches at the southwestern shore of the sea of galilee a graben within the dead sea transform system the angle between the two site mean directions is 21 but retilting the beds to horizontal reduces it to 7 indicating pre folding probably depositional magnetization aberrations of field directions at one locality where vgps deviate 40 from the geographic north and low nrm intensities between 29 28 ka and 25 24 ka are contemporaneous with the mono lake excursion if the correlation is correct the new data support the hypothesis that the excursion was global justifying its utilization as a chronological datum

structural segmentation and the influence of basement structure on the namibian passive margin passive margins are segmented into discrete elements by segment boundaries exhibiting a variety of structural styles the namibian margin is bounded by a hinge line that is offset by segment boundaries the most important is the cape cross segment boundary here pre existing basement lineaments and karoo rift elements were reactivated during late mesozoic rifting it marks the northern limit of the earliest of three rift sequences and relates to a temporal change in break up and formation of the south atlantic a similar change occurs across other important segment boundaries but smaller examples lack this major segment boundaries with a temporal change in rifting and break up exhibit a strong connection to the basement province

epipalaeolithic settlement dynamics in southwest asia new radiocarbon evidence from the azraq basin a series of radiocarbon dates from two epipalaeolithic sites kharaneh iv and ayn qasiyya in the azraq basin of eastern jordan provide a new perspective on the chronology and settlement patterns of this part of southwest asia during the late pleistocene we discuss the implications to our understanding of the chronology of late pleistocene lithic industries particularly in regard to current hypotheses for the abandonment of eastern jordan s mega sites kharaneh iv and jilat 6 modelling a series of accelerator mass spectrometry dates from kharaneh iv indicates a much shorter span of occupation for the site than previously assumed by the size and density of its deposits given the high density of material accumulated over a relatively short time span we show that kharaneh iv was an aggregation site occupied intensively by a significant number of people providing new perspectives on the east jordanian phenomenon of epipalaeolithic mega sites copyright 2013 john wiley sons ltd

upper quaternary evolution of the dune field of the bolson de fiambala catamarca sand dispersal at the andes piedmonts abstract the south american arid diagonal extends in argentina from puna to patagonia regions several dune fields are scattered between the andes and pampas ranges covering plains and plateaus one of the northernmost dune fields is occupying the tectonic valley known as bolson de fiambala 27 s 67 w white transverse dunes extend from west to east climbing to the fiambala range some outcrops from the surrounding area allow to understand the evolution of this dune field at the chaschuil valley a profile composed of 4 m of stratified sands and gravels were sampled at las lozas site epiphytes and benthic diatom assemblages suggest a shallow lake located at 3700 m altitude many of these brackish freshwater aerophilous taxa tolerate desiccation periods and salt concentration the assemblages were dominated by nitzschia frustulum nitzschia linearis achnanthes thermalis var rumrichorum epithemia adnata pinnularia viridis y nitzschia capitellata limestones from this outcrop were dated on 32 000 520 and 29 380 410 14c years bp garleff et al 1993 suggesting the marine isotopic stage 3 the holocene interglacial is recorded either at the chaschuil and abaucan rivers valleys lake deposits at 3000 m altitude gave ages between 6175 70 and 2990 70 14c years bp garleff et al 1993 the sand of this dune field at the northern part of the bolson de fiambala was supplied by the fanglomerates located to the west at medanitos there is also a significant supply of pumicite derived from the cerro blanco volcanic dome an aridity increase have been previously recorded from archaeological data and confirmed by the discharge records of the abaucan river during the 1919 1957 interval although these sand dunes with a high content in volcanic glass clearly derived from the andes other sand dunes of central argentina derived from the deactivation of the desaguadero curaco system

dynamic models of continental rifting with melt generation abstract active or passive continental rifting is associated with thinning of the lithosphere ascent of the asthenosphere and decompressional melting this melt may percolate within the partially molten source region accumulate and be extracted two dimensional numerical models of extension of the continental lithosphere asthenosphere system are carried out using an eulerian visco plastic formulation the equations of conservation of mass momentum and energy are solved for a multi component crust mantle and two phase solid melt system temperature pressure and stress dependent rheologies based on laboratory data for granite pyroxenite and olivine are used for the upper and lower crust and mantle respectively rifting is modelled by externally prescribing a constant rate of widening with velocities between 2 5 and 40 mm yr a typical extension experiment is characterized by 3 phases 1 distributed extension with superimposed pinch and swell instability 2 lithospheric necking 3 continental break up followed by oceanization the timing of the transition from stages 1 to 2 depends on the presence and magnitude of a localized perturbation and occurs typically after 100 150 km of total extension for the lithospheric system studied here this necking phase is associated with a pronounced negative topography rift valley and a few 100 m of rift flanks the dynamic part of this topography amounts to about 1 km positive topography this means if rifting stops e g due to a drop of external forces immediate additional subsidence by this amount is predicted solidification of ascended melt beneath rift flanks leads to basaltic enrichment and underplating beneath the flanks often observed at volcanic margins after continental break up a second time dependent upwelling event off the rift axis beneath the continental margins is found producing further volcanics melting has almost no or only a small accelerating effect on the local extension value value for a constant external extension rate melting has an extremely strong effect on the upwelling velocity within asthenospheric wedge beneath the new rift this upwelling velocity is only weakly dependent on the rifting velocity the melt induced sublithospheric convection cell is characterized by downwelling flow beneath rift flanks melting increases the topography of the flanks by 100 200 m due to depletion buoyancy another effect of melting is a significant amplification of the central subsidence due to an increase in localized extension subsidence modelled magma amounts are smaller than observed for east african rift system increasing the mantle temperature as would be the case for a large scale plume head better fits the observed magma volumes if extension stops before a new ocean is formed melt remains present and convection remains active for 50 100 myr and further subsidence is significant

linking rift propagation barriers to excess magmatism at volcanic rifted margins break up related extrusive magmatism imaged in reflection seismic data as seaward dipping reflectors sdrs extends symmetrically along the volcanic margins of the atlantic ocean recent research found distinct along margin variations in the distribution of sdrs and abundance of volcanic material was found to be spatially linked to transfer fault systems these segmented the propagating rift that later developed into the ocean and are interpreted as rift propagation barriers based on these observations we develop a numerical model which shows that rift parallel mantle flow and locally enhanced rates of volcanism are the result of delays in rift propagation and segmented opening our model suggests that segmentation is one of the major factors in the distribution and localization of riftrelated extrusive magmatism we conclude that in addition to mantle temperature and inherited crustal structures e g weaknesses from previous rift episodes rift propagation delay plays an important role in the distribution of extrusive volcanism at volcanic passive margins by controlling the mantle flow beneath the rift axis

an improved tectonic model for the eocene opening of the norwegian greenland sea use of modern magnetic data abstract compilation of new and vintage aeromagnetric data from the norwegian and greenland seas of the ne atlantic provides evidence for a different interpretation of several tectonic elements the previously interpreted oceanic fracture zones gleipne surt bivrost jenegga and vesteralen do not exist these were artefacts of poor navigation and wide line spacing of the vintage dataset this reinterpretation impacts our understanding of the early spreading history of the north atlantic as the opening of the norwegian greenland sea between the jan mayen and senja greenland fracture zones occurred along a stable axis without offsets of the oceanic spreading anomalies or jumps in spreading axis these results contradict the hypothesis that a spatial relationship exists between transfer zones and fracture zones on the lofoten margin and on the ne greenland margin to which they have been projected simplified palaeogeographic reconstruction of the aeromagnetic map to anomaly 22 reveals that a c 50 km wide magnetic anomaly cuts across spreading anomalies 24a 24b and 23 from the voring marginal high on the norwegian margin to traill o on the east greenland coast the anomaly is interpreted to represent an igneous complex referred to as the traill voring igneous complex tvic the complex crosscuts anomaly 22 on the greenland margin suggesting that the igneous activity was active until c 50 ma and can be linked up with the nne trending initial magmatic lineament iml extending between traill o and kangerlussuaq the iml has been suggested to relate to a failed attempt of direct linkage between the reykjanes and mohns ridges the magnetic response of the tvic along the voring margin has previously been interpreted as representing anomaly 24a and 24b such an interpretation required the erroneous introduction of an abandoned spreading ridge

the use of acoustic imaging to reveal fossil fluvial systems a case study from the southwestern sea of galilee abstract the analysis of reflected high resolution seismic data shows a distinct separation of regions with good and poor seismic penetration additional analysis of core data revealed good correlation between grain size and seismic penetration as a case study a shallow geophysical survey using a chirp profiler was conducted in the southwestern part of the sea of galilee by correlating the seismic and core data we found that areas with good seismic penetration represent coarse clastics while poor seismic penetration is related to fine clays new detailed bathymetric mapping and bottom morphology images combined with the penetration characteristics of the chirp signal reveal a large alluvial fan consisting mainly of coarse material sand to pebbles a fine grained band of mostly clay size material associated with an asymmetric bathymetric channel continues the trend of the old entrance of the yavniel creek into the sea of galilee we interpret the fine clay stripe to be a low energy streambed of the yavniel creek the clear relations between the reflected chirp signal and the grain size of the water bottom sediments suggests that this type of survey can be used to characterize depositional environments

complex exposure histories of chert clasts in the late pleistocene shorelines of lake lisan southern israel activities of 26 al and 10 be in five chert clasts sampled from two beach ridges of late pleistocene lake lisan precursor of the dead sea in southern israel indicate low rates of chert bedrock erosion and complex exposure burial and by inference transport histories the chert clasts were derived from the senonian mishash formation a chert bearing chalk which is widely exposed in the nahal zin drainage basin the drainage system that supplied most of the material to the beach ridges simple exposure ages assuming only exposure at the beach ridge sampling sites range from 35 to 354 ky using the ratio 26 al 10 be total clast histories range from 0 46 to 4 3 my unrelated to the clasts current position and exposure period on the late pleistocene beach ridges 160 177 m below sea level optically stimulated luminescence dating of fine sediments from the same and nearby beach ridges yielded ages of 20 0 s 1 4 ka and 36 1 s 3 3 ka these ages are supported by the degree of soil development on the beach ridges and correspond well with previously determined ages of lake lisan which suggest that the lake reached its highest stand around 27 000 cal years bp if the clasts were exposed only once and than buried beyond the range of significant cosmogenic nuclide production then the minimum initial exposure and the total burial times before delivery to the beach ridge are in the ranges 50 1300 ky and 390 3130 ky respectively alternatively the initial cosmogenic dosing could have occurred during steady erosion of the source bedrock back calculating such rates of rock erosion suggests values between 0 4 and 12 m my 1 t he relatively long burial periods indicate extended sediment storage as colluvium on slopes and or as alluvial deposits in river terraces some clasts may have been stored for long periods in abandoned pliocene and early pleistocene routes of nahal zin to the mediterranean before being transported again back into the nahal zin drainage system and washed on to the shores of lake lisan during the late pleistocene copyright 2003 john wiley sons ltd

influence of margin segmentation upon the break up of the hatton bank rifted margin ne atlantic the hatton bank margin flanking the iceland basin is an example of a volcanic rifted margin and has been studied to examine the along margin tectono magmatic variability integration of 5660 km of new seismic reflection profiles with 60 000 km2 of new multibeam bathymetry has allowed the margin to be divided into three segments each of which are flanked by oceanic crust the southernmost segment is characterised by a series of inner and outer seaward dipping reflector sdr packages which are separated by an outer high feature the outer sdrs are truncated by endymion spur a chain of steep sided volcanic cones connected by narrow septa or necks the central segment has no inner sdr package and is characterised by the presence of a continental block the hatton bank block hbb the northern segment is adjacent to lousy bank with a wider region of sdrs recognised than to the south and characterised by many volcanic cones the variations in the distribution of the sdrs along the margin the presence of the hbb and endymion spur all suggest that the break up process was not a uniform smooth process along strike structural segmentation controlled the variations along the margin with break up initiated in the south producing the sdr packages the hbb prompted the focus of break up to relocate outboard of the block the northern segment was closest to the iceland hot spot and regular seafloor spreading did not become established until chron 21 shortly after break up the eruption of endymion spur occurred and may have been triggered by the passage of a pulse of hot asthenospheric material along the margin the margin segmentation pattern we describe controlled the location of the enhanced volcanism along the endymion spur to the southern sector in addition the segmentation has influenced the break up style presence or absence of sdr and also the location and nature of post break up volcanism

role of subaerial volcanic rocks and mantle plumes in creation of south atlantic margins implications for salt tectonics and source rocks abstract seaward dipping reflectors sdrs represent flood basalts rapidly extruded during either rifting or initially subaerial sea floor spreading evaporites can form on this basaltic proto oceanic crust as in the afar triangle today evidence for sdrs in south atlantic deep water regions comes from proximity to the uniquely large parana etendeka volcanic province onshore the tristan and gough hot spots drilled volcanic rocks and seismic profiles showing sdr provinces more than 100 km wide as much as 7 km thick and thousands of kilometers long sdrs are clearest adjoining the aptian salt basins however we speculate that sdrs are also present but seismically obscured below the salt basins we argue that the conjugate aptian salt basins are post breakup not pre breakup they were separated from the start by a mid oceanic ridge distal salt accumulated on proto oceanic crust not rift basins this hypothesis is supported by seismic stratigraphy and structure magnetic anomalies plate reconstructions and hydrothermal potash evaporites an important implication for exploration is that thick basalts rather than rift age source rocks may underlie distal parts of the salt basins

composite projectiles and hafting technologies at ohalo ii 23 ka israel analyses of impact fractures morphometric characteristics and adhesive remains on microlithic tools abstract this study focuses on the correlation between the production of backed microliths in the levant during the late upper palaeolithic and their use as side elements of composite projectile weapons the investigation is based on assemblage sample of microliths from ohalo ii a 23 000 years old submerged campsite and involves analyses of diagnostic impact fractures location of adhesive remains and morpho metric characteristics of the tools two distinct adhesive materials have been also analyzed and the preliminary results indicate the use of both calcareous and organic substances the use of backed microliths as side elements of composite projectiles has been followed for scalene triangles a microlithic type unknown from the earlier upper palaeolithic in the levant the reconstruction of their use has been established based upon all the three evidence lines i e patterns of impact fractures as well as the location of adhesive remains and the morpho metric characteristics of the microliths the results of the study show an upper palaeolithic epipalaeolithic continuity in the approach to the design of hunting weapons and hafting technologies it is also suggested that the origins of composite projectiles in the levant may be found in the earlier phases of the upper palaeolithic

crustal structure of the lofoten vesteralen continental margin off norway abstract by compiling wide angle seismic velocity profiles along the 400 km long lofoten vesteralen continental margin off norway and integrating them with an extensive seismic reflection data set and crustal scale two dimensional gravity modelling we outline the crustal margin structure the structure is illustrated by across margin regional transects and by contour maps of depth to moho thickness of the crystalline crust and thickness of the 7 km s lower crustal body the data reveal a normal thickness oceanic crust seaward of anomaly 23 and an increase in thickness towards the continent ocean boundary associated with breakup magmatism the southern boundary of the lofoten vesteralen margin the bivrost fracture zone and its landward prolongation appears as a major across margin magmatic and structural crustal feature that governed the evolution of the margin in particular a steeply dipping and relatively narrow 10 40 km wide moho gradient zone exists within a continent ocean transition which decreases in width northward along the lofoten vesteralen margin to the south the zone continues along the voring margin however it is offset 70 80 km to the northwest along the bivrost fracture zone lineament here the moho gradient zone corresponds to a distinct 25 km wide zone of rapid landward increase in crustal thickness that defines the transition between the lofoten platform and the voring basin the continental crust on the lofoten vesteralen margin reaches a thickness of 26 km and appears to have experienced only moderate extension contrasting with the greatly extended crust in the voring basin farther south there are also distinct differences between the lofoten and vesteralen margin segments as revealed by changes in structural style and crustal thickness as well as in the extent of elongate potential field anomalies these changes may be related to transfer zones gravity modelling shows that the prominent belt of shelf edge gravity anomalies results from a shallow basement structural relief while the elongate lofoten islands belt requires increased lower crustal densities along the entire area of crustal thinning beneath the islands furthermore gravity modelling offers a robust diagnostic tool for the existence of the lower crustal body from modelling results and previous studies on and off shore mid norway we postulate that the development of a core complex in the middle to lower crust in the lofoten islands region which has been exhumed along detachments during large scale extension brought high grade lower crustal rocks possibly including accreted decompressional melts to shallower levels

lithospheric controls on melt production during continental breakup at slow rates of extension application to the north atlantic rifted margins form from extension and breakup of the continental lithosphere if this extension is coeval with a region of hotter lithosphere then it is generally assumed that a volcanic margin would follow here we present the results of numerical simulations of rift margin evolution by extending continental lithosphere above a thermal anomaly we find that unless the lithosphere is thinned prior to the arrival of the thermal anomaly or half spreading rates are more than 50mmyr 1 the lithosphere acts as a lid to the hot material the thermal anomaly cools significantly by conduction before having an effect on decompression melt production if the lithosphere is thinned by the formation of extensional basins then the thermal anomaly advects into the thinned region and leads to enhanced decompression melting in the north atlantic a series of extensional basins off the coast of northwest europe and greenland provide the required thinning this observation suggests that volcanic margins that show slow rates of extension only occur where there is the combination of a thermal anomaly and previous regional thinning of the lithosphere

magma induced strain localization in centrifuge models of transfer zones abstract scaled centrifuge experiments have been used to investigate the dynamic relations between deformation and magma distribution in rift related transfer zones the physical models were built using suitable analogue materials such as sand to represent the brittle upper crust various kinds of silicone mixtures to simulate the lower crust and upper mantle and glycerol to reproduce magma models simulated the development of transfer zones across pre existing glycerol reservoirs placed at the base of the analogue continental crust in plan view different geometries dimensions and positions of subcrustal reservoirs were reproduced in three different sets of experiments to compare results models were also performed without magma simulating glycerol set 1 experiments incorporating a narrow rectangular glycerol reservoir show that the low viscosity material is able to localise deformation into the overlying crust giving rise to discrete transfer zones this concentrated surface deformation corresponds at depth to major magma accumulation set 2 experiments with an initial wide squared glycerol reservoir show instead that deformation is distributed across the whole model surface corresponding at depth to relatively minor magma accumulation set 3 experiments explored various positions of a small squared reservoir that invariably localised faulting in the overlying analogue brittle crust at the onset of model deformation the overall model behaviour suggests that magma distribution at depth can effectively control the strain distribution in the overlying crust and the deformative pattern of transfer zones strain distribution in turn may control magma emplacement as localized deformation would favour major accumulation of magma at transfer zones coupled to a strong thermal weakening of the country rocks this process may ultimately lead to a positive feedback interaction between magma and deformation

episodic volcanism during break up of the north atlantic evidence from the east greenland plateau basalts abstract the lower tertiary upper paleocene lower eocene plateau basalts in the scoresby sund region of east greenland formed during three distinct magmatic episodes the activity of the first episode was centred in the present inland areas to the northwest and produced a voluminous regional basalt sequence lower sequence the activity of the second episode was centred to the southeast offshore the present atlantic coast and this also produced a voluminous regional basalt sequence upper sequence the third episode was centred on the atlantic coast and gave rise to a dense coastal dyke swarm and local lavas which are now only locally preserved by downfaulting the lavas are all tholeiitic basalts after an initial phase the two regional lava sequences of the first and second episodes show a similar vertical compositional zonation pattern a thin lower formation with variable composition is followed by a thick uniform formation which shows systematic compositional change with height from tio2 rich feti basalts to tio2 poor basalts and ending with a reversal to tio2 rich basalts each of these cyclic patterns apparently resulted from a temporary rifting episode and much of the compositional variation found can be explained by magma development in open magma chambers the dykes and lavas of the third episode of activity have chemical affinities to ocean floor basalts and their tio2 p2o5 ratios indicate formation at a spreading ridge centre the pattern of two failed rifting episodes and one event that produced oceanic crust can be correlated with stages in the complicated initiation of seafloor spreading in the region and the volcanism itself is closely related to this process

continental rift architecture and patterns of magma migration a dynamic analysis based on centrifuge models 1 small scale centrifuge models were used to investigate the role of continental rift structure in controlling patterns of magma migration and emplacement experiments considered the reactivation of weakness zones in the lower crust and the presence of magma at moho depths results suggest that surface deformation which reflects the weakness zone geometry exerts a major control on patterns of magma migration in the case of a single rift segment the experimental lower crust and magma were both transferred in an extension parallel direction toward the rift flanks this lateral migration reflected the dominance of far field stresses over extension induced buoyancy forces local pressure gradients favored the raise of experimental magma in correspondence of marginal grabens the lateral migration gave rise to major accumulations below the footwall of major boundary faults providing the magma source able to feed off axis volcanoes in nature as inferred for the main ethiopian rift in the case of two offset rift segments a major transfer zone developed this transfer zone was characterized by prominent experimental lower crust doming and strong magma accumulation dynamic analysis showed that the transfer zone development caused a strong pressure difference in a rift parallel direction which dominated over the far field thinning owing to this pressure gradient almost all the underplated experimental magma collected below the lower crust dome suggesting a rift parallel extension orthogonal migration this process has a direct relevance for the localization of magmatic activity at transfer zones in natural continental rifts such as in the western branch of the east african rift system

segmentation and volcano tectonic characteristics along the sw african continental margin south atlantic as derived from multichannel seismic and potential field data abstract regional seismic reflection and potential field data document the south atlantic s break up history between 39 s and 19 s from the early cretaceous onwards previous maps of distribution of volcanics along the margin showed volcanics along the whole african margin based on extrapolation of data based on previously unpublished marine geophysical data we found the southernmost 460 km long margin segment to be lacking huge volumes of break up related volcanic effusives northwards break up was accompanied by the emplacement of huge volumes of volcanic material prominently featured in seismic sections as huge wedge shaped seaward dipping reflectors sdrs detailed mapping of offsets left and right stepping and variations in structural character of the volcanics reveal the segmentation along and the break up history of the margin several superimposed sdr sequences suggesting episodicity of volcanic emplacement divided by periods of erosion and sedimentation are distinct along southerly lines losing prominence northwards a main outcome of our study is that this passive margin is not continuously of the volcanic type and that the change from a non volcanic to a volcanic margin occurs abruptly we define four distinct first order segments along the 2400 km section of the southwestern african margin covered by our seismic data from south to north these first order segments are magma poor segment i segment ii with enormous sdrs volumes decreasing sdrs volumes in segment iii segment iv again with enormous volcanic output likely influenced by walvis ridge volcanism most important is that there is no systematic increase in the volumes of the effusives towards the tristan da cunha hot spot rather there is an alternating pattern in the sdrs volumes and widths the boundary between the volcanic and magma poor margin segments in the southernmost study area is sharp 10s of km which we propose is reflected in magnetic anomaly data as well we suggest that this variability along the margin is mainly due to a change in stretching rifting character from oblique during the early stages of breakup to conventional seafloor spreading from chron m4 130 ma onwards

margin segmentation and volcano tectonic architecture along the volcanic margin off argentina uruguay south atlantic abstract some 25 000 kilometers of regional multichannel seismic data acquired by bgr along the continental margins off argentina and uruguay document that the early cretaceous south atlantic continental breakup and initial sea floor spreading were accompanied by large scale transient volcanism emplacing voluminous extrusives manifested in the seismic data by huge wedges of seaward dipping reflectors sdrs the emplacement of the deeply buried 60 120 km wide sdrs was probably episodic as documented by at least three superimposed sdrs units distinct along margin variations in architecture volume and width of the sdrs wedges are probably related to margin segmentation it is suggested that the margin can be divided at least in four compartments bounded by the falkland fracture zone falkland transfer the colorado transfer the ventana transfer and the salado transfer the individual margin segments reflect in the distribution and thickness of the post rift sediments the individual transfer zones may have acted as rift propagation barriers selectively directing rift segments in left stepping patterns along the western south atlantic margin although we found extensive variations in the architecture style and extent of the seaward dipping reflector sequences a general trend is that the largest volumes are emplaced close to the proposed transfer zones and the width of the sdrs wedges decreases northward within the individual margin segments we suggest that mainly adiabatic decompression and melt generation from shallow sources can explain distinct along margin variations in the volcano tectonic architecture and volumes of extruded magmas

the norway basin revisited from continental breakup to spreading ridge extinction abstract we re evaluate the structure and spreading evolution of the southeastern norway basin nb based on a new high resolution aeromagnetic survey nb 07 the survey covers a complete oceanic spreading segment from the continental oceanic transition of the more margin off norway to the aborted aegir ridge the new survey documents a transform margin an orthogonally rifted segment and an oblique sheared volcanic margin formed during the onset of breakup observed from the east jan mayen fracture zone to the faroe platform the detailed fabric of the nb revealed by the new data indicates that two distinct tectonic phases have reshaped the basin before the cessation of seafloor spreading and abortion of the aegir ridge in the late oligocene after continental breakup phase i from c24 to c21r 52 to 49 ma marks the earliest phase of spreading probably initiated in the central and outer part of the more basin during this period competing oceanic segments led to the formation of overlapping systems and pseudo fault development we observe a significant change in the nb s oceanic spreading system in the late early eocene and based on observations from surrounding areas we suggest that this is a record of a major tectonic event in the norwegian greenland sea around c21r 49 47 9 ma during phase ii from c21r to c10 48 to 28 ma of nb s development spreading rates decreased spreading direction changed and the number of faulting with large displacement increased leading to the formation of unexpected n s oriented oceanic fracture zones the fan shaped development of the spreading system initiated around c21r 49 47 9 ma instead of c18 c17 40 38 ma or c24 53 3 52 3 ma as previously proposed these new observations were used to re evaluate the tectonic evolution of the norwegian greenland sea and discuss some implications on the syn and post breakup development of the surrounding continental margins and the evolution of the jan mayen microcontinent

radiocarbon dating of ohalo ii archaeological and methodological implications abstract charcoal samples from the early epipalaeolithic submerged fisher hunter gatherers site of ohalo ii have been dated by three laboratories the samples derive from huts hearths a grave and a stone installation twenty five date range between 17 500 21 050 bp and average c 19 400 bp the thickness of deposits the perishable building material of the huts the spatial organization of the camp and its rapid cover by sand and water suggest several occupational episodes lasting not more than tens of years together the discrepancy between the range of 14 c dates and the real length of occupation are due to the statistical limitations of the dating method

transient groundwater lake interactions in a continental rift sea of galilee israel the sea of galilee located in the northern part of the dead sea rift is currently an intermediate fresh water lake it is postulated that during a short highstand phase of former lake lisan in the late pleistocene saline water percolated into the subsurface since its recession from the kinarot basin and the instantaneous formation of the fresh water lake the sea of galilee the previously intruded brine has been flushed backward toward the lake numerical simulations solving the coupled equations of fluid flow and of solute and heat transport are applied to examine the feasibility of this hypothesis a sensitivity analysis shows that the major parameters controlling basin hydrodynamics are lake water salinity aquifer permeability and aquifer anisotropy results show that a highstand period of 3000 yr in lake lisan was sufficient for saline water to percolate deep into the subsurface because of different aquifer permeabilities on both sides of the rift brine percolated into aquifers on the western margin whereas percolation was negligible on the eastern side in the simulation after the occupation of the basin by the sea of galilee the invading saline water was leached backward by a topographydriven flow it is suggested that the percolating brine on the western side reacted with limestone at depth to form epigenetic dolomite at elevated temperatures therefore groundwater discharging along the western shores of the sea of galilee has a higher calcium to magnesium ratio than groundwater on the eastern side

the late quaternary limnological history of lake kinneret sea of galilee israel the freshwater lake kinneret sea of galilee and the hypersaline dead sea are remnant lakes evolved from ancient water bodies that filled the tectonic depressions along the dead sea transform dst during the neogene quartenary periods we reconstructed the limnological history level and composition of lake kinneret during the past 40 000 years and compared it with the history of the contemporaneous lake lisan from the aspect of the regional and global climate history the lake level reconstruction was achieved through a chronological and sedimentological investigation of exposed sedimentary sections in the kinnarot basin trenches and cores drilled at the ohalo ii archeological site shoreline chronology was established by radiocarbon dating of organic remains and of melanopsis shells the major changes in lake kinneret level were synchronous with those of the southern lake lisan both lakes dropped significantly 42 000 30 000 23 800 and 13 000 yr ago and rose 39 000 26 000 5000 and 1600 yr ago between 26 000 and 24 000 yr ago the lakes merged into a unified water body and lake level achieved its maximum stand of 170 m below mean sea level m bsl nevertheless the fresh and saline water properties of lake kinneret and lake lisan respectively have been preserved throughout the 40 000 years studied calcium carbonate was always deposited as calcite in lake kinneret and as aragonite in lake lisan dead sea indicating that the dead sea brine which supports aragonite production never reached or affected lake kinneret even during the period of lake high stand and convergence the synchronous level fluctuation of lakes kinneret lisan and the holocene dead sea is consistent with the dominance of the atlantic mediterranean rain system on the catchment of the basin and the regional hydrology the major drops in lake kinneret lisan levels coincide with the timing of cold spells in the north atlantic that caused a shut down of rains in the east mediterranean and the lakes drainage area d 2004 university of washington all rights reserved

the late rifting phase and continental break up of the southern south atlantic the mode and timing of volcanic rifting and formation of earliest oceanic crust

the importance of rift history for volcanic margin formation rifting and magmatism are fundamental geological processes that shape the surface of our planet a relationship between the two is widely acknowledged but its precise nature has eluded geoscientists and remained controversial largely on the basis of detailed observations from the north atlantic ocean mantle temperature was identified as the primary factor controlling magmatic production with most authors seeking to explain observed variations in volcanic activity at rifted margins in terms of the mantle temperature at the time of break up however as more detailed observations have been made at other rifted margins worldwide the validity of this interpretation and the importance of other factors in controlling break up style have been much debated one such observation is from the northwest indian ocean where despite an unequivocal link between an onshore flood basalt province continental break up and a hot spot track leading to an active ocean island volcano the associated continental margins show little magmatism here we reconcile these observations by applying a numerical model that accounts explicitly for the effects of earlier episodes of extension our approach allows us to directly compare break up magmatism generated at different locations and so isolate the key controlling factors we show that the volume of rift related magmatism generated both in the northwest indian ocean and at the better known north atlantic margins depends not only on the mantle temperature but to a similar degree on the rift history the inherited extensional history can either suppress or enhance melt generation which can explain previously enigmatic observations

mesozoic break up of sw gondwana implications for regional hydrocarbon potential of the southern south atlantic abstract this work provides new palinspastic palaeofacies reconstructions of sw gondwana incorporating rotation of a falkland malvinas microplate we discuss the implications of this for the tectonic evolution of the southern south atlantic and hence for the regional hydrocarbon potential existing gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents in most continental reconstructions the falkland malvinas plateau was assumed to be a rigid fragment of pre permian south american crust however it has been suggested on the basis of palaeomagnetic data that the falkland malvinas islands were rotated by 180 after 190 ma this rotation hypothesis has been successfully tested on the basis of devonian stratigraphy and palaeontology permian stratigraphy and sedimentology and late palaeozoic and early mesozoic structure making it unlikely that the plateau behaved as a rigid structure during breakup we have explored the consequences of accepting this hypothesis for the tectonic evolution of sw gondwana by compiling new palaeogeographic maps for the permian cretaceous of the southern atlantic area to achieve a realistic close fit we have devised a pre rift proxy for the ocean continent boundary for the south atlantic in order to produce the best fit it is necessary to subdivide south america into four plates the consequences of this are far reaching our work suggests that although sedimentary basins were initiated at different times three major tectonic phases can be recognised in regional terms these can be thought of as pre syn and post rift during the pre rift time until the late triassic the area was dominated by compressional tectonism and formed part of the gondwana foreland the falkland malvinas islands lay east of africa the falkland malvinas plateau was 33 shorter and patagonia was displaced east with respect to the rest of south america in part along the line of the gastre fault system potential source facies are dominantly post glacial black shales of late permian age deposited in lacustrine or hyposaline marine environments these rocks would also be an effective regional seal sandstones deposited in the late permian would be dominantly volcaniclastic with poor reservoir qualities triassic sandstones tend to be more mature there was significant extension from about 210 ma end triassic until the south atlantic opened at about 130 ma early cretaceous in the early syn rift phase extension was accompanied by strike slip faulting and block rotation later extension was accompanied by extrusion of large volumes of lava early opening of the south atlantic was oblique which created basins at high angle to the trend of the ocean on the argentine margin and resulted in microplate rotation in ne brazil intermittent physical barriers controlled deposition of upper jurassic cretaceous anoxic sediments during breakup some of these mudrock units are effective seals with likely regional extent during crustal reorganisation clastic sediments changed from a uniform volcaniclastic provenance to local derivation with variable reservoir quality in the late rift and early post rift phase continental extension changed from oblique to normal and basins developed parallel to the continental margins of the south atlantic this change coincides with the main rifting in the equatorial basins of brazil and the early impact of the santa helena plume it resulted in widespread development of unconformities the abandonment of the reconcavo tucano jatoba rift and the end of ne brazil plate rotation which remained attached to south america there was extensive deposition of evaporites concentrated in but not restricted to the area north of the rio grande rise walvis ridge widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements our main conclusion is that the regional hydrocarbon potential of the southern south atlantic has been constrained by the tectonic evolution

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

timing and duration of volcanism in the north atlantic igneous province implications for geodynamics and links to the iceland hotspot abstract we combine new and published 40 ar 39 ar age determinations from incremental heating experiments on whole rocks and mineral separates to assess the timing duration and distribution of volcanic activity during construction of the north atlantic igneous province we use these ages together with volume estimates of erupted magmas and their cumulates to calculate melt production rates for the early tertiary flood basalts of east greenland and the faeroes islands the lavas lie at opposite ends of the greenland iceland faeroes ridge the postulated iceland hotspot track and record volcanic activity leading up to during and following continental breakup between greenland and europe dominantly basaltic but also alkalic and picritic magmas were erupted from beneath thick continental lithosphere simultaneously at 61 ma from baffin island to the western and eastern margins of greenland to the faeroe islands and the western british isles a roughly circular area 2000 km in diameter volcanic activity was increasingly intermittent by 57 56 ma but at 56 1 0 5 ma the average melt production rate increased by more than an order of magnitude over previous levels from 3000 km 3 km of rift my coinciding with continental rifting and shallow decompressional melting of the mantle that fed subaerial volcanic activity along most of the eastern margin of greenland and its complement on the northwestern european margin offshore abnormally thick oceanic crust was produced along the line of plate separation the upper part of this crust comprises seismically imaged seaward dipping subaerially erupted lavas by 50 ma eruption rates had diminished drastically and volcanic activity had narrowed to a much restricted portion of the east greenland margin at the western end of the greenland iceland ridge that subsequently connected in an age progressive trend to present hotspot activity in southeastern iceland the startup mantle plume head and tail model with moderate excess temperature t 100 c and active upwelling best explains the time space and compositional aspects of volcanism in this province

quantifying the thermo mechanical impact of plume arrival on continental break up abstract the arrival of a plume head at earth s continental lithosphere is often considered to be an important factor for continental break up however the impact of plume impingement on strength and duration of a rift remains unclear in this study we quantify the mechanical and thermal influence of a plume i e lithosphere erosion on continental break up to do that we apply the three dimensional numerical code slim3d that features realistic elasto visco plastic rheology we model the thermo mechanical response of a segment of earth s lithosphere that is affected both by extension as well as plume related lithosphere erosion in order to evaluate the influence on the overall force budget we find that lithosphere erosion leads to a moderate lithospheric strength reduction of several tn m in a force limited environment however this strength reduction may have strong influence on the timing of continental break up or it may even control whether continental break up takes place at all additional reduction of the lithospheric strength is likely due to the massive emplacement of dikes that follows intensive melting within the plume head