control of pre rift lithospheric structure on the architecture and evolution of continental rifts insights from the main ethiopian rift east africa we investigate the along axis variations in architecture segmentation and evolution of the main ethiopian rift mer east africa and relate these characteristics to the regional geology lithospheric structure and surface processes we first illustrate significant along axis variations in basin architecture through analysis of simplified geological cross sections in different rift sectors we then integrate this information with a new analysis of ethiopian topography and hydrography to illustrate how rift architecture basin symmetry asymmetry is reflected in the margin topography and has been likely amplified by a positive feedback between tectonics flexural uplift and surface processes fluvial erosion unloading this analysis shows that 70 of the 500 km long mer is asymmetric with most of the asymmetric rift sectors being characterized by a master fault system on the eastern margin we finally relate rift architecture and segmentation to the regional geology and geophysical constraints on the lithosphere we provide strong evidence that rift architecture is controlled by the contrasting nature of the lithosphere beneath the homogeneous strong somalian plateau and the weaker more heterogeneous ethiopian plateau differences originating from the presence of pre rift zones of weakness on the ethiopian plateau and likely amplified by surface processes the data provided by this integrated analysis suggest that asymmetric rifts may directly progress to focused axial tectonic magmatic activity without transitioning into a symmetric rifting stage these observations have important implications for the asymmetry of continental rifts and conjugate passive margins worldwide

2 d 3 d irregular shortest path ray tracing for multiple arrivals and its applications summary the purpose of this study is to introduce a multistage irregular shortest path method ispm for tracking multiple seismic arrivals including any combinations of transmissions reflections or refractions and mode conversions in complex 2 d 3 d layered media incorporating irregular interfaces or subsurface in 3 d and velocity discontinuities the basic principle is to first divide the model into several different layers using irregular cells near each interface discontinuity and the earth s undulating surface topography and then to apply the multistage technique to trace the multiple arrivals it is possible to realize the multiple arrival tracking with the multistage scheme because the multiple arrivals are just different combinations or conjugations of the simple incident transmitted reflected or refracted and mode converted waves via the velocity discontinuities and the interfaces benchmark tests against the popular multistage fast marching method fmm and the multistage mspm modified shortest path method are undertaken to assess the solution accuracy and the computational efficiency the results show that the multistage ispm is advantageous over both the multistage fmm and the multistage mspm in both solution accuracy and cpu time several examples including the marmousi model are used to demonstrate the viability and versatility of the multistage ispm in heterogeneous media even in the presence of high velocity contrasts involving interfaces of relatively high curvature applications to the seismological problems such as traveltime tomography and earthquake location indicate that it is possible to improve the spatial resolution in traveltime tomography and solution accuracy in earthquake location if later arrival information is combined with the first arrivals

potential links between surging ice sheets circulation changes and the dansgaard oeschger cycles in the irminger sea 60 18 kyr surface and deepwater paleoclimate records in irminger sea core so82 5 59on 31 ow and icelandic sea core ps2644 68on 22ow exhibit large fluctuations in thermohaline circulation thc from 60 to 18 calendar kyr b p with a dominant periodicity of 1460 years from 46 to 22 calendar kyr b p matching the dansgaard oeschger d o cycles in the greenland ice sheet project 2 gisp2 temperature record grootes and tuiver 1997 during interstadials summer sea surface temperatures sstsu in the irminger sea averaged to 8oc and sea surface salinities sss averaged to 36 5 recording a strong irminger current and atlantic thc during stadials sstsu dropped to 2o 4oc in phase with sss drops by 1 2 they reveal major meltwater injections along with the east greenland current which turned off the north atlantic deepwater convection and hence the heat advection to the north in harmony with various ocean circulation and ice models on the basis of the ird composition icebergs came from iceland east greenland and perhaps svalbard and other northern ice sheets however the southward drifting icebergs were initially jammed in the denmark strait reaching the irminger sea only with a lag of 155 195 years we also conclude that the abrupt stadial terminations the d o warming events were tied to iceberg melt via abundant seasonal sea ice and brine water formation in the meltwater covered northwestern north atlantic in the 1 1460 year frequency band benthic 15 80 brine water spikes led the temperature maxima above greenland and in the irminger sea by as little as 95 years thus abundant brine formation which was induced by seasonal freezing of large parts of the northwestern atlantic may have finally entrained a current of warm surface water from the subtropics and thereby triggered the sudden reactivation of the thc in summary the internal dynamics of the east greenland ice sheet may have formed the ultimate pacemaker of d o cycles

the role of magmatic loads and rift jumps in generating seaward dipping reflectors on volcanic rifted margins abstract the largest volcanic constructs on earth are the seismically imaged seaward dipping reflector sdr units found offshore of many rifted continental margins including most that border the atlantic ocean whether their formation requires large magnitude i e 10 s of km of normal fault slip or results from the deflection of the lithosphere by the weight of volcanic flows is controversial though there is evidence for faulting associated with some sdrs this paper considers the range of structures that can be produced by magmatic and volcanic loading alone to do this an idealized mechanical model for the construction of rift related volcanic flow structures is developed dikes open as plates move away from the center of a model rift and volcanic flows fill the depression produced by the load caused by dike solidification the thin elastic plate flexure approximation allows a closed form description of the shape of both the contacts between flows and between the flows and underlying dikes the model depends on two independent parameters the flexure parameter and the maximum isostatically supported extrusive layer thickness w 0 for reasonable values of these parameters the model reproduces the observed down dip thickening of flows and the range of reflector dip angles a numerical scheme using the analytic results allows simulation of the effect of temporal changes in the locus of magmatic spreading as well as changes in the amount of volcanic infill either jumps in the location of the center of diking or periods with no volcanism result in separate units or packages of model sdrs in which the flow dike contact dips landward consistent with observations previously attributed only to listric normal fault offset when jumps in the spreading center are small i e less than they result in thicker narrower volcanic units on one side of a rift compared to those on the other side this is similar to the asymmetric distributions of volcanic packages seen across some conjugate margins

conceptual model for millennial climate variability a possible combined solar thermohaline circulation origin for the 1 500 year cycle dansgaard oeschger and heinrich events are the most pronounced climatic changes over the last 120 000 years although many of their properties were derived from climate reconstructions the associated physical mechanisms are not yet fully understood these events are paced by a 1 500 year periodicity whose origin remains unclear in a conceptual model approach we show that this millennial variability can originate from rectification of an external solar forcing and suggest that the thermohaline circulation through a threshold response could be the rectifier we argue that internal threshold response of the thermohaline circulation thc to solar forcing is more likely to produce the observed do cycles than amplification of weak direct 1 500 year forcing of unknown origin by thc one consequence of our concept is that the millennial variability is viewed as a derived mode without physical processes on its characteristic time scale rather the mode results from the linear representation in the fourier space of nonlinearly transformed fundamental modes

on crustal corrections in surface wave tomography summary mantle models from surface waves rely on good crustal corrections we investigated how far ray theoretical and finite frequency approximations can predict crustal corrections for fundamental mode surface waves using a spectral element method we calculated synthetic seismograms in transversely isotropic prem and in the 3 d crustal model crust2 0 on top of prem and measured the corresponding time shifts as a function of period we then applied phase corrections to the prem seismograms using ray theory and finite frequency theory with exact local phase velocity perturbations from crust2 0 and looked at the residual time shifts after crustal corrections residuals fall within the uncertainty of measured phase velocities for periods longer than 60 and 80 s for rayleigh and love waves respectively rayleigh and love waves are affected in a highly non linear way by the crustal type oceanic crust affects love waves stronger while rayleigh waves change most in continental crust as a consequence we find that the imperfect crustal corrections could have a large impact on our inferences of radial anisotropy if we want to map anisotropy correctly we should invert simultaneously for mantle and crust the latter can only be achieved by using perturbation theory from a good 3 d starting model or implementing full non linearity from a 1 d starting model

dansgaard oeschger and heinrich event imprints in alboran sea paleotemperatures past sea surface temperature sst evolution in the alboran sea western mediterranean during the last 50 000 years has been inferred from the study of c37 alkenones in international marine global change studies md952043 core this record has a time resolution of 200 years allowing the study of millennial scale and even shorter climatic changes the observed sst curve displays characteristic sequences of extremely rapid warming and cooling events along the glacial period comparison of this alboran record with 18o from greenland ice greenland ice sheet project 2 core shows a strong parallelism between these sst oscillations and the dansgaard oeschger events five prominent cooling episodes standing out in the sst profile are accompanied by an anomalous high abundance of neogloboquadrina pachyderma sinistral which is confined to the duration of these cold intervals these features and the isotopic record reflect drastic changes in the surface hydrography of the alboran sea in association with heinrich events hl 5

abrupt climate changes from the past to the future a review a review of climatic variability is given with a focus on abrupt changes during the last glacial evidence from palaeoclimatic archives suggests that these were most likley due to reorganisations of the atmosphere ocean system the mechanisms responsible for these changes are presented their implication for future climate changes is discussed in light of recent climate model simulations

climate records covering the last deglaciation the oxygen 18 oxygen 16 ratio of molecular oxygen trapped in ice cores provides a time stratigraphic marker for transferring the absolute chronology for the greenland ice sheet project gisp ii ice core to the vostok and byrd ice cores in antarctica comparison of the climate records from these cores suggests that near the beginning of the last deglaciation warming in antarctica began approximately 3000 years before the onset of the warm bolling period in greenland atmospheric carbon dioxide and methane concentrations began to rise 2000 to 3000 years before the warming began in greenland and must have contributed to deglaciation and warming of temperate and boreal regions in the northern hemisphere

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

gisp2 oxygen isotope ratios the gisp2 oxygen isotope record with its high resolution detail yields crucial information on past climate change the glacial 18o oscillations of the gisp2 core with their very fast onsets are templates of a prototype oscillation of variable duration with an amplitude of 3 9 the halfway mark of the cold warm transition is reached in 2 years the top is reached in 50 years the 18o time gradient of the leading front is about 7 8 per 100 yr after reaching the top 18o slowly declines by 0 14 per 100 yr the duration of 18o decline varies from a couple of centuries for fast oscillations to about 4000 yr for slower ones the subsequent 18o downturn during the warm cold transition has a 18o time gradient of 3 2 per 100 yr and lasts about 80 yr

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

weak oceanic heat transport as a cause of the instability of glacial climates the stability of the thermohaline circulation of modern and glacial climates is compared with the help of a two dimensional ocean atmosphere sea ice coupled model it turns out to be more unstable as less freshwater forcing is required to induce a polar halocline catastrophy in glacial climates the large insulation of the ocean by the extensive sea ice cover changes the temperature boundary condition and the deepwater formation regions moves much further south the nature of the instability is of oceanic origin identical to that found in ocean models under mixed boundary conditions with similar strengths of the oceanic circulation and rates of deep water formation for warm and cold climates the loss of stability of the cold climate is due to the weak thermal stratification caused by the cooling of surface waters the deep water temperatures being regulated by the temperature of freezing weaker stratification with similar overturning leads to a weakening of the meridional oceanic heat transport which is the major negative feedback stabilizing the oceanic circulation within the unstable regime periodic millennial oscillations occur spontaneously the climate oscillates between a strong convective thermally driven oceanic state and a weak one driven by large salinity gradients both states are unstable the atmosphere of low thermal inertia is carried along by the oceanic overturning while the variation of sea ice is out of phase with the oceanic heat content during the abrupt warming events that punctuate the course of a millennial oscillation sea ice variations are shown respectively to damp amplify the amplitude of the oceanic atmospheric response this sensitivity of the oceanic circulation to a reduced concentration of greenhouse gases and to freshwater forcing adds support to the hypothesis that the millennial oscillations of the last glacial period the so called dansgaard oeschger events may be internal instabilities of the climate system

seismic ray path variations in a 3d global velocity model abstract a three dimensional 3d ray tracing technique is used to investigate ray path variations of p pcp pp and pp phases in a global tomographic model with p wave velocity changing in three dimensions and with lateral depth variations of the moho 410 and 660 km discontinuities the results show that ray paths in the 3d velocity model deviate considerably from those in the average 1d model for a pcp wave in western pacific to east asia where the high velocity 1 2 pacific slab is subducting beneath the eurasian continent the ray path change amounts to 27 km for a pcp ray in south pacific where very slow 2 velocity anomalies the pacific superplume exist in the whole mantle the maximum ray path deviation amounts to 77 km ray paths of other phases p pp pp are also displaced by tens of kilometers changes in travel time are as large as 3 9 s these results suggest that although the maximal velocity anomalies of the global tomographic model are only 1 2 rays passing through regions with strong lateral heterogeneity in velocity and or discontinuity topography can have significant deviations from those in a 1d model because rays have very long trajectories in the global case if the blocks or grid nodes adopted for inversion are relatively large 3 5 and only a low resolution 3d model is estimated 1d ray tracing may be feasible but if fine blocks or grid nodes are used to determine a high resolution model 3d ray tracing becomes necessary and important for the global tomography

on the coupling between barotropic and baroclinic modes of extratropical atmospheric variability abstractthe baroclinic and barotropic components of atmospheric dynamics are usually viewed as interlinked through the baroclinic life cycle with baroclinic growth of eddies connected to heat fluxes barotropic decay connected to momentum fluxes and the two eddy fluxes connected through the eliassen palm wave activity however recent observational studies have suggested that these two components of the dynamics are largely decoupled in their variability with variations in the zonal mean flow associated mainly with the momentum fluxes variations in the baroclinic wave activity associated mainly with the heat fluxes and essentially no correlation between the two these relationships are examined in a dry dynamical core model under different configurations and in southern hemisphere observations considering different frequency bands to account for the different timescales of atmospheric variability it is shown that at intermediate periods longer than 10 days the decoupling of the baroclinic and barot

large igneous province on the us atlantic margin and implications for magmatism during continental breakup rifted continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust explanations for this voluminous magmatic accretion during rifting include plume models1 3 which require a deep seated thermal or chemical anomaly in upwelling mantle and non plume models4 7 which call on broad shallow thermal anomalies and or rapid upwelling of mantle through the melting zone new seismic models from two transects across the continent ocean transition on the us atlantic margin8 10 confirm the presence of a 20 25 km thick igneous section here we argue that the similarity of the crustal structure on these and two previous transects spanning 1 000 km of the margin and the association of thick igneous crust with the east coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3 2 106 km3 the distribution of volcanic and plutonic rocks details of the seismic structure and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non plume processes created the thick igneous crust

rift jump process in northern iceland since 10 ma from 40ar 39ar geochronology due to the westward american eurasian plate boundary migration the rift zone location in northern iceland imposed by the stable icelandic hotspot has jumped eastward the present day north volcanic zone of iceland is thus shifted about 100 km to the east with respect to the kolbeinsey ridge a miocene paleo rift location was proposed along the hunafloi skagi axis unconformities inside the lava pile on both sides of the present day rift as a result of the new rift formation have been used to date the rift jump but timing is still controversial whether extinction of the paleo rift was synchronous with initiation of the new rift or the two rifts were active during some time period is unknown using the 40ar 39ar radiochronology method 37 dykes were dated along a 350 km profile parallel to the divergent plate motion direction i e ese wnw crossing the present rift the unconformities and the inferred paleo rift our results reveal that the paleo rift axis was not located along the hunafloi skagi axis but 60 km to the east this previously unknown paleo rift is named the skagafjordur paleo rift it remained active until approximately 3 ma the activity of the present north volcanic zone started about 8 8 5 ma intruding 9 9 5 myr rocks located at least 10 km east of the skagafjordur paleo rift axis consequently the two rifts were simultaneously active for 5 5 5 myr during this time period their accretion rates were nearly equal but with asymmetrical opening the largest rates occurring on external flanks these results are coherent in the icelandic context and permit a first reconstruction of the evolution of northern iceland since 10 ma

a mechanism for construction of volcanic rifted margins during continental breakup research into the breakup of continents and formation of ocean basins has advanced in recent years with deep marine seismic data providing much of the impetus for new concepts attention has been focused on nonvolcanic magma poor margins meanwhile a satisfactory explanation for volcanic magmatic margins with their distinctive seaward dipping reflectors sdrs has been missing sdrs are composed of subaerial lava flows of tholeiitic basalt erupted during the transition from continental rifting to seafloor spreading when the crust is both stretched and constructed from asthenosphere derived magma here we present remarkable seismic images down to 40 km offshore east greenland showing that a newly described type of extensional fault plays an integral part in the ascent of magma and the building of transitional crust two such faults define an axial horst block supported by rising magma where the plates separate lava collects in seaward growing half graben on either side of the horst while new gabbroic crust is accreted along the flanks of the magma chamber in dilational shear zones beneath where the faults detach ultimately rising asthenosphere reaches the base of the magma chamber causing the horst to split thereby initiating seafloor spreading the model we describe shows how extension and magmatism interact at the boundaries of rifting plates

a new greenland deep ice core the polar ice sheets are rich sources of information on past atmospheric conditions including paleoclimates a new deep ice core has been drilled in south greenland comparison of the oxygen isotopic profile with that from camp century and with a deep sea foraminifera record indicates that the new core reaches back to about 90 000 years before present in a continuous sequence the details in the wisconsin part of the ice core records seem to be climatically significant and the general trends reveal all of the relevant emiliani stages recorded in deep sea cores the redated camp century record suggests a dramatic termination of the eem sangamon interglacial

structure of the se greenland margin from seismic reflection and refraction data implications for nascent spreading center subsidence and asymmetric crustal accretion during north atlantic opening 1 seismic reflection and refraction data from the se greenland margin provide a detailed view of a volcanic rifted margin from archean continental crust to near to average oceanic crust over a spatial scale of 400 km the sigma iii transect located 600 km south of the greenland iceland ridge and the presumed track of the iceland hot spot shows that the continent ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup initially 18 3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8 10 km thick crust changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system which was close to sea level for at least 1 m y of subaerial spreading despite a reduction in productivity from 17 to 13 5 km thick crust over this time interval this could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system the thermal anomaly must be exhaustible consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial to shallow marine and finally to deep marine extrusion in 2 m y is shown by the morphological changes in addition comparison to the conjugate hatton bank shows a clear asymmetry in the early accretion history of north atlantic oceanic crust nearly double the volume of material was emplaced on the greenland margin compared to hatton bank and may indicate east directed ridge migration during initial opening

plate bending stresses at axial highs and implications for faulting behavior abstract we develop flexural models of axial high topography which include the effects of plastic yielding and use the results to examine the contribution of plate bending stresses and strains to fault patterns at the east pacific rise epr the lithosphere responds to buoyancy near the ridge axis created by the presence of elevated temperatures and melt within the crust parameterizing these quantities we find topography at wide axial highs can be fit assuming the crust cools to lithosphere temperatures to 3 4 km depth within 2 km from the axis and the presence of 20 30 melt in the lower crust a trade off in buoyancy implies the greater the amount of crustal cooling the less melt is required significant crustal cooling leads to lithospheric strengthening but plastic bending effects allow the plate to deflect over the distance needed to fit the observed axial relief the plate develops at the axis with non zero curvature and unbends as it moves away from the axis placing a section of the upper surface or seafloor into extension seafloor stresses are extensional beginning a few kilometers from the axis and continuing up to distances of 35 km from the axis for models that fit wide axial high topography a compilation of previous faulting studies at the epr reveals that distance from the axis over which fault slip remains active appears to depend on the width of the axial high for narrow highs this region is confined to at most 15 km from the axis but at wider axial highs it reaches distances of 20 45 km comparable to regions of extension predicted by the flexural models the model strain can be as large as 1 9 amounting to half the observed strain assuming a fault slip angle of 45 but most strain assuming a slip angle of 60 these results strongly suggest that bending stresses contribute significantly to normal faulting on the flanks of fast spreading ridges and may have a strong influence on faulting patterns there

the rise and fall of axial highs during ridge jumps 1 we simulate jumps of ocean spreading centers with axial high topography using elastoplastic thin plate flexure models processes considered include ridge abandonment the breaking of a stressed plate on the ridge flank and renewed spreading at the site of this break we compare model results to topography at the east pacific rise between 15 25 n and 16 n where there is strong evidence of a recent ridge jump at an apparently abandoned ridge gravity data do not suggest buoyant support of topography model deflections during cooling and melt solidification stages of ridge abandonment are of small vertical amplitude because of plate strengthening resulting in the preservation of a frozen fossil high the present day high is bounded by slopes with up to a 40 grade a scenario very difficult to achieve flexurally given generally accepted constraints on lithospheric strength we model these slopes by assuming that the height at which magma is accreted increases rapidly after the ridge jumps this increase is attributed to high overburden pressure on melt that resided in an initially deep magma chamber followed by a rapid increase in temperature and melt supply to the region shortly after spreading began the high is widest at the segment center suggesting that magmatic activity began near the center of the segment propagated south and then north the mantle bouguer anomaly exhibits a bull s eye pattern centered at the widest part of the high but the depth of the axis is nearly constant along the length of the segment we reconcile these observations by assigning different cross axis widths to a low density zone within the crust

the jan mayen microcontinent s cenozoic stratigraphic succession and structural evolution within the ne atlantic abstract cenozoic seismic stratigraphic units of the jan mayen microcontinent jmmc are presented in context with kinematic modelling of plate reorganisation correlation with conjugate margins and their chrono stratigraphic correlations our model of development of the microcontinent from continental breakup to present day is based on the interpretation of new and legacy geological and geophysical data from these data we have established eleven cenozoic seismic stratigraphic units jm 70 to jm 01 separated by ten locally varying unconformities and disconformities six of which are regional and reflect discrete tectonostratigraphic phases in the evolution of the jan mayen region these six main phases are 1 pre breakup paleocene 55 ma extension represented by seismic unit jm 70 which comprises terrigenous and marine sedimentary rocks and a thick plateau basalt volcanic succession 2 early eocene 55 52 ma syn breakup rift to drift represented by seismic unit jm 60 and comprises a mix of intrusive igneous rocks subaerial lavas and hyaloclastites and non marine to shallow marine sedimentary rocks 3 early to mid eocene 52 43 ma syn breakup rift to drift and rift transfer across the iceland plateau rift represented by seismic unit jm 50 that comprises volcanic rocks and shelf margin deltaic sediment succession 4 late eocene early oligocene 43 30 ma ridge transfer and tectonic re arrangement in proximity of the iceland hotspot represented in the rock record by shallow to deep marine deposits preserved as units jm 40 jm 35 and jm 30 5 in the late oligocene 30 22 ma a second breakup occurred along the western jmmc igneous margin together with the formation of the south western jan mayen igneous province the proto kolbeinsey ridge and the initiation of proto iceland all of which is represented in the rock record by seismic units jm 20 and jm 15 and 6 miocene to present day separation of the jmmc from east greenland since 22 ma which is represented by seismic units jm 10 jm 05 and jm 01 that reflect a general deepening of the microcontinent

effect of plastic viscous layering and strain softening on mode selection during lithospheric extension 1 factors controlling the selection of deformation modes during continental extension are investigated using analytical and numerical methods we view the lithosphere as a laminate and examine a simple system with a uniform plastic layer overlying a uniform linear viscous layer the rate of energy dissipation is analyzed for pure shear ps symmetric plug sp and asymmetric plug ap extension modes and the analysis reveals that the primary control is the relative rate of dissipation in the two layers a basic difference is that the plastic layer yield strength is independent of the strain rate whereas the viscous stress depends on strain rate therefore dissipation scales linearly and quadratically with extension velocity for these respective layers when other parameters e g extension velocity and properties are held constant minimum dissipation predicts that the modes ap sp and ps will be selected in this order with increasing viscosity of the lower layer transition viscosities between modes t1 and t2 are 4 1021 pa s and 8 1022 pa s respectively for our parameters values numerical models confirm the analysis results inferred mode controls and order of mode selection when strain softening of the plastic layer occurs during extension implications for lithosphere that acts as a bonded plastic viscous laminate include the following 1 asymmetric extension ap mode is preferred when the extension rate and or effective viscosity is low or the viscous region temperature is high 2 symmetric sp mode extension is preferred for intermediate combinations of parameters and 3 overall pure shear ps mode may occur for opposite end member parameter combinations

using holo hilbert spectral analysis to quantify the modulation of dansgaard oeschger events by obliquity abstract astronomical forcing obliquity and precession has been thought to modulate dansgaard oeschger do events yet the detailed quantification of such modulations has not been examined in this study we apply the novel holo hilbert spectral analysis hhsa to five polar ice core records quantifying astronomical forcing s time varying amplitude modulation of do events and identifying the preferred obliquity phases for large amplitude modulations the unique advantages of hhsa over the widely used windowed fourier spectral analysis for quantifying astronomical forcing s nonlinear modulations of do events is first demonstrated with a synthetic data that closely resembles do events recorded in greenland ice cores ngrip grip and gisp2 cores on gicc05 modelext timescale the analysis of paleoclimatic proxies show that statistically significantly more frequent do events with larger amplitude modulation in the greenland region tend to occur in the decreasing phase of obliquity especially from its mean value to its minimum value in the eastern antarctic although statistically significantly more do events tend to occur in the decreasing obliquity phase in general the preferred phase of obliquity for large amplitude modulation on do events is a segment of the increasing phase near the maximum obliquity implying that the physical mechanisms of do events may be different for the two polar regions additionally by using cross spectrum and magnitude squared analyses greenland do mode at a timescale of about 1400 years leads the antarctic do mode at the same timescale by about 1000 years

recent changes in the north atlantic it has long been recognized that the atlantic meridional overturning circulation moc is potentially sensitive to greenhouse gas and other climate forcing and that changes in the moc have the potential to cause abrupt climate change however the mechanisms remain poorly understood and our ability to detect these changes remains incomplete four main interrelated types of ocean change in particular are associated in the literature with greenhouse gas forcing these are a slowing of moc overturning rate changes in northern seas which might effect a change in atlantic overturning including changes in the freshwater flux from the arctic and changes in the transport and or hydrographic character of the northern overflows which ventilate the deep atlantic a change in the trans ocean gradients of steric height both zonal and meridional which might accompany a change in the moc and an intensification of the global water cycle though as yet we have no direct measure of the freshwater flux passing from the arctic to the atlantic either via the canadian arctic archipelago or along the east greenland shelf and no direct measure yet of the atlantic overturning rate we examine a wide range of time series from the existing hydrographic record for oceanic evidence of the other anticipated responses large amplitude and sustained changes are found or indicated by proxy over the past three to four decades in the southward transport of fresh waters along the labrador shelf and slope in the hydrography of the deep dense overflows from nordic seas in the transport of the eastern overflow through faroe bank channel and in the global hydrologic cycle though the type and scale of changes in ocean salinity are consistent with an amplification of the water cycle we find no convincing evidence of any significant concerted slowdown in the atlantic overturning circulation

comparison of oxygen isotope records from the gisp2 and grip greenland ice cores recent results1 2 from the greenland ice core project grip summit ice core suggest that the climate in greenland has been remarkably stable during the holocene but was extremely unstable for the time period represented by the rest of the core spanning the last two glaciations and the intervening eemian inter glacial here we present the complete oxygen isotope record for the greenland ice sheet project 2 gisp2 core drilled 28 km west of the grip core we observe large rapid climate fluctuations throughout the last glacial period which closely match those reported for the grip core however in the bottom 10 of the cores spanning the eemian interglacial and the previous glacia tion there are significant differences between the two records it is possible that ice flow may have altered the chronological sequences of the stratigraphy for the bottom part of one or both of the cores considerable further work will be necessary to evaluate the likelihood of this and the extent to which it will still be possible to extract meaningful climate information from the lowest sections of the cores

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

paleoclimatic variability inferred from the spectral analysis of greenland and antarctic ice core data paleoclimate variations occur at various time scales between a few centuries for the heinrich events and several hundreds of millenia for the glacial to interglacial variations the recent ice cores from greenland greenland ice core project and greenland ice sheet project 2 and antarctica vostok span at least one glacial oscillation and provide many opportunities to investigate climate variations with a very fine resolution the joint study of cores from both hemispheres allows us to distinguish between the sources of variability and helps to propose mechanisms of variations for the different time scales involved the climate proxies we analyze are inferred from 18o and d for temperature and chemical species such as calcium for the joint behavior of the major ions in the atmosphere which yield an estimate of the polar circulation index those data provide time series of climatic variables from which we extract the information on the dynamics of the underlying system we used several independent spectral analysis techniques to reduce the possibility of spurious results those methods encompass the multitaper spectral analysis singular spectrum analysis maximum entropy method principal component analysis minimum bias spectral estimates and digital filter reconstructions our results show some differences between the two hemispheres in the slow variability associated with the astronomical forcing common features found in the three ice core records occur on shorter periods between 1 and 7 kyr the holocene also shows recurrent common patterns between greenland and antarctica we propose and discuss mechanisms to explain such behavior

new constraints on the age of the opening of the south atlantic basin abstract detailed high quality marine total field magnetic data recently acquired over parts of the south atlantic ocean off the southwestern margin of south africa display a pattern of well defined nw se striking linear magnetic anomalies that can be traced with confidence over distances 150 km the magnetic anomalies are interpreted to be m series seafloor spreading anomalies m9 to m11 which are consistent with the initiation of seafloor spreading at approximately 135 ma late valanginian early hauterivian seafloor spreading models indicate a more rapid 44 mm yr initial spreading phase between m11 and m4 m5 followed by slower 29 mm yr spreading from m4 m5 to m0 this two rate spreading model also matches m series anomalies previously reported over the conjugate south american margin offshore argentina where the rates are slightly breaks in the continuity of the linear anomaly pattern observed in map view are oriented approximately ne sw and are considered sites of possible fracture zones one such discontinuity which we have termed the cape lineament cl marks a significant change in crustal character and cretaceous depositional history as revealed by gravity data and seismic reflection data respectively crust nw of the cl in the orange basin is characterized by greater thickness and the presence of seismically imaged seaward dipping reflectors sdrs whereas se of the cl the crust has a more normal oceanic thickness and sdrs are either absent or more limited in areal extent magnetic gravity and seismic data were combined to develop a crustal model of the south african margin north of the cl that includes a a region of rifted attenuated continental crust landward of magnetic anomaly g b a 60 80 km wide zone of substantially intruded underplated thin continental crust overlain by sdrs between anomalies g and m11 and c a 90 km wide zone of thick oceanic crust associated with a wide intrusion zone and smoother seafloor spreading magnetic anomalies that thins progressively to normal oceanic crustal thickness at the seaward edge of the overlying sdrs south of the cl the change in crustal character from attenuated continental crust to normal thickness oceanic crust occurs over a much shorter distance of approximately 35 40 km possibly indicating the diminished influence of magmatic material although linear magnetic anomalies are observed both nw and se of cl anomalies to the se display a better correlation with those predicted by our seafloor spreading model we have successfully reconstructed the positions of the african and south american margins south of 32 s at m11 time i e 135 ma using a new rotation pole at 38 86 n 31 46 w this pole applies only to the southernmost margins of the austral segment where m11 is observed further north at this time the margins are undergoing non rigid deformation that includes both crustal extension and magmatic underplating

fault controlled lithospheric detachment of the volcanic southern south atlantic rift we present structural models of two exemplary conjugate seismic lines of the southernmost south atlantic margins to examine their initial evolution especially the seaward dipping reflectors sdrs modeling illustrates the different structure and inclination angles of the sdrs which therefore require different subsidence histories since typical symmetrical subsidence models are not applicable we suggest a model with a westward dipping detachment fault that offsets the sdrs on the south american margin and we speculate on passively subsided sdrs on the south african margin we propose a simple shear rifting mechanism to explain the initial break up of the south atlantic

a compilation of igneous rock volumes at volcanic passive continental margins from interpreted seismic profiles abstract the rifting and breakup of continents and subsequent seafloor spreading give rise to passive continental margins many of which are underlain by enormous volumes of igneous rocks and termed volcanic passive margins the relationships between the igneous rocks rifting and mantle plumes remain unresolved despite extensive studies and proposed alternative models to support such studies we use published previously interpreted seismic reflection and refraction data to compile a dataset of igneous rock volumes and geometries at volcanic passive continental margins the volmir volcanic passive margin igneous rocks dataset is based on margin crossing profiles on which the volumes and geometries of both shallow seaward dipping reflector sdr and deeper high velocity lower crustal hvlc units can be measured it includes information about the ages of continental breakup and profile distances from the associated euler pole and from hotspots that may have played roles in the breakup process despite local variations the dataset shows general patterns the average ratio of sdr to hvlc volumes 0 3 0 4 is relatively consistent between profiles implying that formation of these units is related during continental breakup the volumes of igneous rocks display a moderate positive correlation with distances from the euler pole but only a weak negative correlation with distances from the nearest hotspot at the time of margin formation suggesting that in continental breakup lithospheric processes passive rifting have greater effects than hotspots active rifting these results suggest that this dataset will be useful for exploring aspects of the rifting process relationships between volcanic rifted margins and potential explanations for their similarities and differences

temporal geographical meltwater influences on the north atlantic conveyor implications for the younger dryas the temporal and geographical roles of meltwater discharge from the laurentide ice sheet on north atlantic deep water nadw production are investigated utilizing a global realistic geometry coupled climate model which does not require the use of flux adjustments model results suggest that preconditioning by meltwater discharge to the mississippi prior to the younger dryas yd is capable of pushing nadw beyond the limit of its sustainability the diversion of meltwater to the st lawrence then merely serves to completely inhibit nadw production the modeled change in surface air temperature generally agrees with the global pattern and magnitude of temperature change seen in paleoclimatic reconstructions of the yd and is intimately linked to changes in nadw formation the global thermohaline circulation provides an interhemispheric teleconnection with the southern oceans while changes in the atmospheric heat transport reacting to a global redistribution of oceanic heat transport provide a mechanism for interbasin teleconnection although the primary thermodynamic and hydrological cycle feedback processes are included within the atmospheric model in the absence of additional feedbacks an equilibrium without the presence of nadw is possible the inclusion of the wind stress speed feedback is found to significantly contribute to the resumption of nadw production as suggested by previous studies contrary to these same studies however the coupled model indicates an advective spin up timescale is required for resumption of nadw production and hence the termination of the modeled yd like climate event as opposed to a decadal century timescale the reason for the discrepancy is unclear but may be associated with the use of fixed salt flux fields applied in previous studies or the duration strength and geographical location of the imposed meltwater applied

repeat ridge jumps associated with plume ridge interaction melt transport and ridge migration 1 repeated shifts or jumps of mid ocean ridge segments toward nearby hot spots can produce large long term changes to the geometry and location of the tectonic plate boundaries ridge jumps associated with hot spot ridge interaction are likely caused by several processes including shear on the base of the plate due to expanding plume material as well as reheating of lithosphere as magma passes through it to feed off axis volcanism to study how these processes influence ridge jumps we use numerical models to simulate 2 d in cross section viscous flow of the mantle viscoplastic deformation of the lithosphere and melt migration upward from the asthenospheric melting zone laterally along the base of the lithosphere and vertically through the lithosphere the locations and rates that magma penetrates and heats the lithosphere are controlled by the time varying accumulation of melt beneath the plate and the depth averaged lithospheric porosity we examine the effect of four key parameters magmatic heating rate of the lithosphere plate spreading rate age of the seafloor overlying the plume and the plume ridge migration rate results indicate that the minimum value of the magmatic heating rate needed to initiate a ridge jump increases with plate age and spreading rate the time required to complete a ridge jump decreases with larger values of magmatic heating rate younger plate age and faster spreading rate for cases with migrating ridges models predict a range of behaviors including repeating ridge jumps much like those exhibited on earth repeating ridge jumps occur at moderate magmatic heating rates and are the result of changes in the hot spot magma flux in response to magma migration along the base of an evolving lithosphere the tendency of slow spreading to promote ridge jumps could help explain the observed clustering of hot spots near the mid atlantic ridge model results also suggest that magmatic heating may significantly thin the lithosphere as has been suggested at hawaii and other hot spots

intraplate lithosphere deformation and the strength of the lithosphere summary the intraplate deformation of continental lithosphere in response to applied stress has been investigated using a mathematical model which incorporates the elastic ductile and brittle response of lithosphere material ductile deformation is assumed to be controlled in the crust by dislocation creep in quartz and in the mantle by dislocation creep and plasticity in olivine brittle failure is predicted using modified griffith theory a fundamental feature of the model is the redistribution of stress within the lithosphere following stress release by both ductile and brittle deformation this redistribution produces high levels of stress in the middle or lower crust immediately above the elastic ductile or brittle ductile transition lithosphere deformation is shown to be critically dependent on the temperature structure of which surface heat flow is a convenient indicator for higher geothermal gradients the release of stress in the lower lithosphere by ductile deformation is more rapid and complete and results in large stress levels in the upper lithosphere for sufficiently large applied stresses or steep geothermal gradients the stress levels in the upper and middle crust will cause complete fracture of the upper lithosphere whole lithosphere failure wlf then results by continued brittle and ductile deformation causing geologically significant strains the critical value of applied stress required to give wlf has been calculated for both tensional and compressional deformation as a function of surface heat flow the predicted lithosphere bulk strength is then compared with expected levels of intraplate stress arising from plate boundary forces and isostatically compensated loads which are thought to give net stress levels in the continental lithosphere in the range 0 25 to 0 25 kb using these expected maximum stress levels the model predicts significant extensional deformation in regions of moderate heat flow with q c 60mwm e g central europe as well as for areas of high heat flow like the basinand range province significant compressional deformation is predicted for areas of high heat flow with q c 75 mwm 2 but only for restricted conditions of stress combination these results are in good agreement with

the deglaciation of the northern hemisphere a global perspective orbitally induced increase in northern summer insolation after growth of a large ice sheet triggered deglaciation and associated global warming ice albedo sealevel and greenhouse gas feedbacks together with tropical warming from weakening winds in response to polar amplification of warming caused regionalto global near synchronization of deglaciation effects were larger at orbital rather than millennial frequencies because ice sheets and carbon dioxide vary slowly ice sheet linked changes in freshwater delivery to the north atlantic and possibly free oscillations in the climate system forced millennial climate oscillations associated with changes in north atlantic deep water nadw flow the north atlantic typically operates in one of three modes modern glacial and heinrich deglaciation occurred from a glacial mode ocean that in comparison to modern had shallower depth of penetration of nadw formed further south causing strong northern cooling and the widespread cold dry and windy conditions associated with the glacial maximum and the cold phases of the millennial dansgaard oeschger oscillations the glacial mode was punctuated by meltwater forced heinrich conditions that caused only small additional cooling at high northern latitudes but greatly reduced the formation of nadw and triggered an oceanic seesaw that warmed some high latitude southern regions centered in the south atlantic

the role of sea ice and other fresh water in the arctic circulation salinity stratification is critical to the vertical circulation of the high latitude ocean we here examine the control of the vertical circulation in the northern seas and the potential for altering it by considering the budgets and storage of fresh water in the arctic ocean and in the convective regions to the south we find that the present day greenland and iceland seas and probably also the labrador sea are rather delicately poised with respect to their ability to sustain convection small variations in the fresh water supplied to the convective gyres from the arctic ocean via the east greenland current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past the north atlantic salinity anomaly of the 1960s and 1970s is a recent example it must have had its origin in an increased fresh water discharge from the arctic ocean similarly the freshening and cooling of the deep north atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the arctic ocean into the convective gyres finally we note that because of the temperature dependence of compressibility a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean

millennial variability in an idealized ocean model predicting the amoc regime shifts abstracta salient feature of paleorecords of the last glacial interval in the north atlantic is pronounced millennial variability commonly known as dansgaard oeschger events it is believed that these events are related to variations in the atlantic meridional overturning circulation and heat transport here the authors formulate a new low order model based on the howard malkus loop representation of ocean circulation capable of reproducing millennial variability and its chaotic dynamics realistically it is shown that even in this chaotic model changes in the state of the meridional overturning circulation are predictable accordingly the authors define two predictive indices which give accurate predictions for the time the circulation should remain in the on phase and then stay in the subsequent off phase these indices depend mainly on ocean stratification and describe the linear growth of small perturbations in the system thus monitoring particular indices of the ocean state could help predict

laterally confined volcanic successions lcvs recording rift jumps during the formation of magma rich margins abstract seaward dipping reflectors sdrs are a characteristic feature of magma rich margins and represent the generation of large volumes of flood basalts at the point of continental breakup a number of recent studies provide new insights into the emplacement and tilting of sdrs and conclude that the majority of sdrs are contained within new magmatic crust that has a close affinity to oceanic crust however the process by which these initial magmatic systems evolve into a fully established spreading centre remains poorly understood several characteristic features of magma rich margins may be explained by the occurrence of rift jumps during sdr emplacement yet the cause and prevalence of such rift jumps remain unknown here we constrain the 3d geometry of the continent ocean transition in the orange basin offshore south africa this allows us to test if where and why such rift jumps occur our results demonstrate an order of along strike segmentation previously unobserved in these settings we demonstrate that the sdr belt is disrupted by the occurrence of a volcanic stratigraphic package defined as the laterally confined volcanic succession lcvs not previously identified on a rifted margin we interpret this as a magmatic spreading centre that was abandoned by a subsequent rift jump identification of lcvss is important for two reasons first we argue that the lcvs formed via the same process as sdrs and hence provides a unique example of sdr geometry prior to their separation onto conjugate plates second as we can map out the 3d geometry of the lcvs and sdrs we propose that rift jumps during magma rich margin formation may be fundamental to the establishment of a laterally continuous incipient spreading centre

origin composition and relative timing of seaward dipping reflectors on the pelotas rifted margin abstract the mechanism by which seaward dipping reflectors sdrs are formed is a topic of debate two contrasting models exist for their formation the volcanic faulting model and the volcanic loading model each of these models has important implications for the processes which control the structure and formation of magma rich rifted continental margins we have examined high quality deep seismic reflection data across the pelotas basin offshore brazil these data reveal a remarkable set of sdrs for which we have investigated the likely nature of their formation the total package of sdrs has an across strike width of 200 km and a variable vertical thickness of 10 17 km previously interpreted as volcanic flows detailed observations however show changes in seismic character and geometry within the sdr package which suggest a complex and varied evolution we have used gravity anomaly inversion and seismic observations together to investigate the likely composition of the sdrs by determining the proportion of basaltic material to sedimentary volcaniclastic material basalt fraction within the sdrs this has been achieved by minimising the difference between the depth of the gravity moho and seismic moho in order to quantify the lateral variation in basalt fraction taken to be proportional to the bulk density of the package the density of the sdr package together with seismic interpretation is then used to infer the composition depositional environment source and time of formation relative to breakup our analysis suggests that the overall sdr basalt fraction and bulk density decrease oceanwards possibly due a change in the type of volcanic deposits from predominantly subaerial to volcaniclastics possibly deposited subaqueously the sdrs can be split into three sub packages the two inner sdr packages are interpreted to consist of lava flows sourced from syn tectonic subaerial eruptions associated with the onshore parana large igneous province flowing eastwards into an extensional basin the outer sdr package shows reflector geometries that progressively offlap oceanwards interpreted as extrusives sourced from an eastwards migrating newly formed ocean ridge our analysis suggests that both the volcanic faulting and volcanic loading models for sdr formation are applicable to the pelotas rifted margin recording distinct syn rift and syn breakup magmatic events we show that both sdr formation models can be recognised in a naturally occurring example and can coexist on the same margin

the effect of ignoring earth curvature on near regional traveltime tomography and earthquake hypocentral determination earthquake hypocenter determination and traveltime tomography with local earthquake data are normally conducted using a cartesian coordinate system and assuming a flat earth model but for regional and teleseismic data earth curvature is incorporated and a spherical coordinate system employed however when the study region is from the local to near regional scale 1 4 it is unclear what coordinate system to use and what kind of incorrect anomalies or location errors might arise when using the cartesian coordinate frame in this paper we investigate in a quantitative sense through two near regional crustal models and five different inversion methods the hypocenter errors reflector perturbation and incorrect velocity anomalies that can arise due to the selection of the wrong coordinate system and inversion method the simulated inversion results show that the computed traveltime errors are larger than 0 1 s when the epicentral distance exceeds 150 km and increases linearly with increasing epicentral distance such predicted traveltime errors will result in different patterns of incorrect velocity anomalous structures a perturbed moho interface for traveltime tomography and source position which deviate for earthquake locations the maximum magnitude of a velocity image artifact is larger than 1 0 for an epicentral distance of less than 500 km and is up to 0 9 for epicentral distances of less than 300 km the earthquake source location error is more than 2 0 km for epicentral distances less than 500 km and is up to 1 5 km for epicentral distances less than 300 km the moho depth can be in error by up 1 0 km for epicentral distances of less than 500 km but is less than 0 5 km at distances below 300 km we suggest that spherical coordinate geometry or time correction be used whenever there are ray paths at epicentral distances in excess of 150 km

evaluation of morphological indicators of magma supply and segmentation from a seismic reflection study of the east pacific rise 15 30 17 n hydrosweep bathymetry and multichannel seismic reflection data from two contrasting segments of the east pacific rise 15 30 17 n are used to assess the relationship between crustal structure morphological indicators of magma supply and ridge segmentation from stacked and migrated seismic profiles we evaluate the width and depth of the axial magma lens and the geometry of the seismically inferred extrusive crust layer 2a in contrast to other studies correlation between axial morphology and magma lens characteristics is found with a lens that on average is twice as wide and slightly shallower 100 200 m beneath the unusually shallow and broad southern segment and that shoals and broadens beneath the shallowest part of the northern segment however large local scale variations in lens depth and width are also observed that are not related to morphology we conclude that magma supply contributes to the regional characteristics of the magma lens beneath a ridge segment with large variations within a segment due to local processes of magma eruption and delivery a negative correlation between zero age extrusive layer thickness and morphology is found with a thinner extrusive layer along the southern segment in this innermost axial region above the magma lens factors such as magma pressure not magma volume in the lens may govern the thickness of extrusives that accumulate the width of the zone over which the extrusive layer is built is positively correlated with morphology with a wide zone where extrusives approximately triple in thickness characterizing the southern segment 5 8 km and a narrower zone 2 5 km of minor thickening less than double along the northern segment and toward ridge segment ends this relationship could reflect changes in lava flow characteristics in these areas of contrasting magma supply we find a close correspondence between discontinuities in the narrow axial summit trough and changes in magma lens presence and geometry indicating a genetic link between the finest scale tectonic segmentation of the ridge and segmentation of the magma lens in several locations discrete magma lenses at different levels within the crust offset by several hundred meters are imaged beneath adjacent fourth order ridge segments

hydrostatic vertical velocity and incompressibility in the northern hemisphere the vertical velocity w is evaluated for the northern hemisphere from reanalysis data and two forms of the richardson equation this equation is based on the hydrostatic assumption and the thermodynamic energy equation the standard form of the richardson equation allows one to quantify the contributions to the vertical velocity of the horizontal divergence

the deccan traps an interpretation in terms of progressive lithospheric flexure in response to a migrating load abstract structural and geochemical data from the western ghats india have been used to construct an accurate north south section of the volcanic stratigraphy of the southwest part of the deccan province the characteristic style of the section is one of southerly dip reversing in the extreme south southward overstep of higher units onto pre deccan rocks and down dip thinning of individual formations in contrast an east west section shows continuity of essentially horizontal stratigraphic units without overstepping relationships previous authors have interpreted these data in terms of the progressive southward migration of the volcanic source as india drifted northwards over a hot spot now centered beneath reunion we have compared the observed stratigraphy to the predictions of a geophysical model based on the flexural response of the continental lithosphere to a migrating volcanic load the best fit to the observed section is for the structurally lowest part of the modelled sections with an elastic thickness of the lithosphere of 100 km this value explains the present day width of the traps in peninsula india but requires that up to 5 5 km of the upper part of the flank section has been removed by erosion the origin of the erosion is not clear but it may be related to flexural rebound following removal of the cone loads that formed along the trace of the plume the model cannot explain the subsidence of the traps beneath the shelf off bombay or the rim uplift of the traps in the western ghats we attribute these modifications of the stratigraphy to vertical movements uplift and subsidence that followed rifting of india from the mascarene plateau during the eocene

abrupt plate accelerations shape rifted continental margins rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved it is well known that strain rate dependent processes control rift evolution yet quantified extension histories of earth s major passive margins have become available only recently here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions we find that rifted margins feature an initial slow rift phase less than ten millimetres per year full rate and that an abrupt increase of plate divergence introduces a fast rift phase plate acceleration takes place before continental rupture and considerable margin area is created during each phase we reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions the extension models suggest that the two phase velocity behaviour is caused by a rift intrinsic strength velocity feedback which can be robustly inferred for diverse lithosphere configurations and rheologies our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force this mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes offering new insights into the balance of plate driving forces through time

crustal drift in iceland summary there are indications that tertiary and post tertiary volcanism in iceland has been confined to the present main belt of active volcanism the geological structure of the tertiary lava piles in the east and west of iceland and results of geophysical studies may be explained by invoking an appreciable amount of crustal drift since the beginning of the tertiary the rocks in the extreme east and west of the island may have been carried apart by 400 km or more the crustal drift is believed to result mainly from crustal extension through the injection of dykes the structure of iceland and possibly that of other flood basalt areas is believed to be closely related to the world wide rift system

deep probe imaging the roots of western north america analysis of the lithoprobe deep probe and southern alberta refraction experiment data sets focusing on the region between deep probe shots 43 and 55 has resulted in a continental scale velocity structural model of the lithosphere of platformal western laurentia reaching depths of 150 km three major lithospheric blocks were investigated i the hearne province a typical continental archean cratonic province lying beneath the western canada sedimentary basin ii the wyoming province an even older block of phanerozoic modified archean crust with an enigmatic lower lithosphere and iii the yavapai mazatzal province proterozoic terranes underlying the colorado plateau and southern rocky mountains in this study the northern two of these regions are investigated with a modified ray theoretical traveltime inversion routine that respects the spherical geometry of the earth the resulting crustal velocity structure combined with supporting geological and geophysical data reveals that the medicine hat block mhb lying between the hearne and wyoming provinces is a third independent archean crustal block the subcrustal lithosphere along the profile is homogeneous in velocity structure but two significant northward dipping reflectors are apparent and interpreted as relic subduction zones associated with sutures between the three archean blocks the hearne crust is typical of an archean shield or platform both in its thickness of 34 50 km and its seismic velocity structure the crust of the archean mhb and wyoming province which ranges in thickness from 49 to 60 km includes a 10 30 km thick high velocity layer interpreted to be proterozoic in age such a feature is unexpected beneath archean crustal provinces but if the region is considered to be the remanent marginal portion of a larger archean continent then the interpreted proterozoic underplating and lack of an archean lithospheric root can be explained the variable topography along the reflective upper and lower boundaries of this layer especially within the mhb suggests considerable variability in its emplacement and subsequent tectonic history

oxygen 18 16 variability in greenland snow and ice with 10 3 to 105 year time resolution the 3 km long greenland ice sheet project 2 gisp2 ice core presents a 100 000 year detailed oxygen isotope profile covering almost a full glacial interglacial cycle measuranents of isotopic fluctuations in snow frost and atmospheric water vapor samples collected during summer field seasons up to 20 0 are compatible with the large and abrupt 80 160 changes observed in accumulated tim snow pit 15 80 profiles from the gisp2 summit area however show rapid smoothing of the 180 160 signal near the surface beyond about 2 m depth the smoothedi5180 signal is fairly well preserved and can be interpreted in terms of average local weather conditions and climate the longer climate fluctuations also have regional and often global significance in the older part of the record corresponding to marine isotope stages mis 5a to 5d the effect of orbital climate forcing via the 19 and 23 kyr precession cycles and the 41 kyr obliquity cycle is obvious from the end ofmis 5a at about 75 000 years b p till the end of the glacial at the younger dryas preboreal transition at 11 650 years b p the o180 160 record shows frequent rapid switches between intermediate interstadial and low stadial values fourier spectra of the oscillations that are superimposed on the orbitally induced changes contain a strong periodicity at 1 5 kyr a broad peak at 4 0 kyr and additional shorter periods detailed comparison of the gisp2 180 160 record with the vostok antarctica 15d record pacific ocean foraminiferal 180 160 grande pile france tree pollen and insolation indicates that a counterpart to many of the rapid 180 160 fluctuations of gisp2 can be found in the other records and that the gisp2 isotopic changes clearly are the local expression of climate changes of worldwide extent correlation of events on the independent gisp2 and specmap time scales for the interval 10 000 50 000 years b p shows excellent chronometric agreement except possibly for the event labeled 3 1 the glacial to interglacial transition evidently started simultaneously in the arctic and the antarctic but its development and its expression in greenland isotopes was later suppressed by the influence of meltwater especially from the barents sea ice sheet on deep water formation and ocean circulation meltwaters from different ice sheets bordering the north atlantic also influenced ocean circulation during the bolling allerod interstadial complex and the younger dryas and led to a distinct development of european climate and greenland 180 160 values the holocene interval with long term stable mean isotopic values contains several fluauations with periods from years to millennia dominant is a 6 3 year oscillation with amplitude up to 3 to 4 0 periodicities of 11 and 210 years also found in the solar modulated records of the cosmogenic isotopes 1obe and 14c suggest solar processes as the cause of these cycles depression of180 160 values cooling by volcanic eruptions is observed in stacked gisp215 80 records but the effect is small and not likely to trigger major climate changes

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

influence of rift jump and excess loading on the structural evolution of northern iceland 1 new structural data combined with published structural and geochronological data allow reconstruction of the structural evolution that followed the last rift jump across northern iceland tertiary lava flows erupted along the skagafjordur paleo rift have been down bent under the weight of and in the direction of plio pleistocene lava flows emitted from the northern volcanic zone and the central part of iceland this down bending process involved development of local flexure zones and a flexural extension along the resulting monoclines this structural reorganization explains the existence of the hunafloi skagi synform without need for a paleo rift axis along it in agreement with previous radiometric dating the large amount of plio pleistocene lava flows erupted in central iceland may have been enhanced by ice cap loading

the protracted development of the continent ocean transition in afar continental breakup and the transition to seafloor spreading is characterized by extensional faulting thinning of the lithosphere and at magmatic margins voluminous intrusive and extrusive magmatism1 2 3 4 it is difficult to discriminate between different mechanisms of extension and magmatism at ancient continental margins because the continent ocean transition is buried beneath thick layers of volcanic and sedimentary rocks5 6 and the tectonic activity that characterized breakup has ceased instead the timing of these mechanisms is inferred from theoretical models or from the geological record preserved at the fully developed ancient rifted margins1 5 7 8 ongoing rifting in ethiopia offers a unique opportunity to address these problems because it exposes subaerially the transition between continental rifting towards the south and seafloor spreading further northward here we synthesize constraints on the spatial and temporal evolution of magmatism and extension in ethiopia we show that although intrusion of magma maintains crustal thickness during the early stages of the continent ocean transition subsidence of the margin below sea level and eruption of voluminous basalt flows is initiated by late stage thinning of the heavily intruded weakened plate just before the onset of seafloor spreading we thus conclude that faulting stretching and magma intrusion are each important but at different times during breakup

how much magma is required to rift a continent article i nfo article history many continental rifts are associated with and preceded by the formation of large igneous provinces partial melting of a large region of anomalously hot upwelling mantle isrequired to explain these huge volumes of pre and syn rift basaltic magma the uplift associated with a shallow region of hot mantle is often invoked as a cause of the rift push force that may drive the observed extension the rift push force is likely to be an order of magnitude toosmall todrive tectonicstretching of normal continentallithosphere but may besufficient to drive opening of lithosphere cutting dikes intrusion of a sequence of such dikes along a narrow zone may provide enough heat to significantly weaken the lithosphere here we use a newly developed 2d numerical approach in which magma filled dikes open in continental lithosphere in response to an evolving model stress field and the advection and diffusion of heat is tracked to investigate the relation between the volume of magma and the amount of lithospheric weakening the available rifting force lithospheric structure and parameters controlling magma flux are varied in the numerical simulations for continental lithosphere 60 km thick hl h1200 c heat diffusion is unimportant and only a small amount of magma is required 10 20 m y similar to magmatic rifts like the red sea

a zonally averaged ocean model for the thermohaline circulation part ii interocean circulation in the pacific atlantic basin system abstract the zonally averaged latitude depth ocean model developed in part i is extended to a two basin system representing the atlantic and pacific steady states are calculated under two different surface boundary conditions to study a possible global thermohaline circulation linking the pacific and the atlantic if the surface temperature and salinity profiles are identical functions of latitude in both the pacific and the atlantic the steady state under restoring boundary conditions consists in spite of the different basin extensions of a two cell structure in each basin upon switching to mixed boundary conditions the state is unstable and a one cell circulation with downwelling at high northern latitudes develops in both basins for a realistic surface salinity profile that is fresher in the pacific the steady state under restoring boundary conditions is completely different it exhibits a global thermohaline circulation with strong interocean man exchange deep water is formed primarily in

the last glacial maximum climate over europe and western siberia a pmip comparison between models and data under the framework of the palaeoclimate modelling intercomparison project pmip 17 climate models 16 of which are atmospheric general circulation models have been run to simulate the climate of the last glacial maximum 21 000 years ago using the same set of boundary conditions parallel to these numerical experiments new consistent data bases have been developed on a continental scale the present work compares the range of the model responses to the large perturbation corresponding to the conditions of the last glacial maximum with consistently derived climate reconstructions from pollen records over europe and western siberia it accounts for the differences in the model results due to the models themselves and directly compares this error bar due to the models to the uncertainties in the climate reconstructions from the pollen records overall the last glacial maximum climate simulated by the models over western europe is warmer especially in winter and wetter than the one depicted by the reconstructions this is the region where the reconstructed increase in temperature precipitation and moisture index from the last glacial maximum to the present conditions is largest the same disagreement but of smaller amplitude is found over central europe and the eastern mediterranean basin while models and data are in broad agreement over western siberia the numerous modelling results allow a study of the link between the changes in atmospheric circulation and those in temperature and an interpretation of the discrepancies in precipitation in terms of those in temperature

seismic volcanostratigraphy of large volume basaltic extrusive complexes on rifted margins large volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments the presence and amount of water is of main importance while local rift basin configuration erosion and resedimentation determine the overall geometry of the volcanic constructions we have developed the concept of seismic volcanostratigraphy a subset of seismic stratigraphy to analyze volcanic deposits imaged on seismic reflection data the method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units interpretation of seismic reflection data along the atlantic and western australia rifted margins reveals six characteristic volcanic seismic facies units named 1 landward flows 2 lava delta 3 inner flows 4 inner seaward dipping reflectors inner sdr 5 outer high and 6 outer sdr these units are interpreted in terms of a five stage tectonomagmatic volcanic margin evolution model comprising 1 explosive volcanism in a wet sediment broad basin setting 2 subaerial effusive volcanism forming gilbert type lava deltas along paleoshorelines 3 subaerial effusive volcanism infilling a fairly narrow rift basin 4 shallow marine explosive volcanism as the injection axis is submerged below sea level and finally 5 deep marine sheet flow or pillow basalt volcanism further erosion and resedimentation processes are particularly important during the shallow marine stages seismic volcanostratigraphy provides important constraints on rifted margin development in particular on the prevolcanic basin configuration relative timing of tectonomagmatic events total amount of volcanic rocks location of paleoshorelines and margin subsidence history these parameters give key boundary conditions for understanding the processes forming volcanic margins and other large volume basaltic provinces

simulation of an abrupt change in saharan vegetation in the mid holocene climate variability during the present inter glacial the holocene has been rather smooth in compar ison with the last glacial nevertheless there were some rather abrupt climate changes one of these changes the desertication of the saharan and arabian region some 4 6 thousand years ago was presumably quite important for human society it could have been the stimulus leading to the foundation of civilizations along the nile euphrat and tigris rivers here we argue that saharan and arabian de sertication was triggered by subtle variations in the earth s orbit which were strongly amplied by atmosphere vegeta tion feedbacks in the subtropics the timing of this tran sition however was mainly governed by a global interplay between atmosphere ocean sea ice and vegetation

the role of the deep ocean in north atlantic climate change between 70 and 130 kyr ago the suggestion1 that changes in north atlantic deep water nadw production are linked through surface heat flux to the atmospheric temperature over greenland is supported by earlier indications2 3 that nadw production decreased during glacial times and by the subsequent finding4 6 that it declined during the younger dryas cool period at the end of the last glaciation changes in north atlantic surface temperatures have been found7 to mirror high frequency temperature changes recorded in greenland ice cores over the past 80 kyr but the connection to abyssal circulation has yet to be established except for one or two isolated oscillations8 9 here we present carbon and oxygen isotope analyses of benthic foraminifera in a high resolution north atlantic deep sea sediment core for the period 70 130 kyr ago these data allow us to reconstruct the history of nadw production which shows a close correlation with greenland climate variability for much of this time interval suggesting that the climate influence of nadw variability was widespread we see no evidence however for changes in nadw production during substage 5e the eemian interglacial period in contrast with recent ice core data10 which suggest severe climate instability in greenland during this time period our results may support suggestions based on data from a second ice core that this apparent instability is an artefact caused by ice flow11 alternatively the eemian climate instability may have had a different origin from the subsequent climate events

stochastic resonance in the north atlantic large abrupt changes between warm and cold modes of north atlantic climate exhibit spectral power at 1500 years yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic we hypothesize that a weak periodic forcing has combined with noise from ice sheet related events to cause the observed mode switches this stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models the ice isotopic data from central greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested we thus support arguments for the existence of a periodicity of 1500 years in the north atlantic climate system and for the importance of ice sheet events in forcing north atlantic changes

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

magmatic ocean continent transitions abstract continental extension may or may not be coeval with significant mantle melting leading to the formation of distinct types of passive margins respectively magma poor or volcanic with distinct crustal structures especially in inter cratonic mobile areas magmatic breakup and development of volcanic passive margins vpms may postdate the early development of a non magmatic continental rift system the time span between the amagmatic and the magmatic systems may be relatively short or conversely long tens of millions years such evolution is often associated with a significant apparent asymmetry in the wideness of conjugate vpms in this paper we attempt to re interpret the structure of three paired vpms which developed close to but separately from a previous amagmatic aborted rift system due to opposite dips in major crustal detachment faults accommodating extension from sedimentary to volcanic stages those composite margins tend to individualize a fault dissected continental block here designed as l block along one of the conjugate margins in addition to the amount and distribution of amagmatic extension the time span between amagmatic and magmatic extension exerts a major structural and rheological control on the final structure of the ocean continent transitions the finite margin geometry may be erroneously interpreted as resulting from a continuous process from hyperextension to the final magmatic breakup however the early syn sedimentary extension appears in many cases to be low rate or episodic consequently the vpm final breakup of the lithosphere may shift away from the original amagmatic stretched area to the rifted margin

accretional curvature of lithosphere at magmatic spreading centers and the flexural support of axial highs fluid magma should tend to rise up to the level of local isostatic equilibrium at an axis of plate spreading the magma and underlying partial melt accretes to the side of the separating plates and gets denser as it cools and freezes this load bends down the lithosphere and additional accretion forms a plate with concave upward curvature the topography resulting from a curved plate passively moving off axis can be described analytically for accretion on the vertical side of uniform lithosphere when its flexural response is that of a thin elastic plate model predictions are consistent with the shape of the axial highs at fast spreading centers typically 400m high and 20 km wide as well as related gravity anomalies this model requires at least several kilometers of lithospheric thickening within a few kilometers of the ridge axis seismic data appear consistent with this lithospheric structure the magnitude and lateral distribution of faulting on the flanks of axial highs is crudely consistent with formation during plate unbending as lithosphere moves away from the axis according to this model abandoned fast spreading centers should retain an elastically supported axial high while for previous models abandoned highs should collapse

seismic velocity structure of seaward dipping reflectors on the south american continental margin abstract seaward dipping reflectors sdrs are a key feature within the continent to ocean transition zone of volcanic passive margins here we conduct an automated pre stack depth migration imaging analysis of commercial seismic data from the volcanic margins of south america the method used an isotropic ray based approach of iterative velocity model building based on the travel time inversion of residual pre stack depth migration move out we find two distinct seismic velocity patterns within the sdrs while both types show a general increase in velocity with depth consistent with expected compaction and alteration metamorphic trends those sdrs that lie within faulted half grabens also have high velocity zones at their down dip ends the velocity anomalies are generally concordant with the reflectivity and so we attribute them to the presence of dolerite sills that were injected into the lava pile the sills therefore result from late stage melt delivery along the large landward dipping faults that bound them in contrast the more outboard sdrs show no velocity anomalies are more uniform spatially and have unfaulted basal contacts our observations imply that the sdrs document a major change in rift architecture with magmatism linked with early extension and faulting of the upper brittle crust transitioning into more organised dike fed eruptions similar to seafloor spreading

changes in east atlantic deepwater circulation over the last 30 000 years eight time slice reconstructions using 95 epibenthic 13c records eight time slices were reconstructed to trace the distribution of east atlantic deepwater and intermediate water masses over the last 30 000 years our results show that there have been three distinct modes of deepwater circulation near the stage 3 2 boundary the origin of north atlantic deep water nadw was similar to today mode 1 however after late stage 3 the source region of the nadw end member shifted from the norwegian greenland sea to areas south of iceland mode 2 a reduced nadw flow persisted during the last glacial maximum with constant preformed 13c values the nutrient content of nadw increased markedly near the azores fracture zone from north to south probably because of the mixing of upwelled antarctic bottom water aabw from below which then advected with much higher flux rates into the northeast atlantic later the spread of glacial meltwater over the north atlantic led to a marked short term ventilation minimum below 1800 m about 13 500 14c years ago mode 3 the formation of nadw recommenced abruptly north of iceland 12 800 12 500 years ago and reached a volume approaching that of the holocene in the younger dryas 10 800 10 350 years b p another short term shutdown of deepwater formation followed between 10 200 and 9 600 years b p linked to a further major meltwater pulse into the atlantic each renewal of deepwater formation led to a marked release of fossil co2 from the ocean the likely cause of the contemporaneous 14c plateaus over the last 9000 years deepwater circulation varied little from today apart from a slight increase in aabw about 7000 14c years ago it is also shown that the oxygenated mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30 000 years

hydrological impact of heinrich events in the subtropical northeast atlantic reconstructing the impact of heinrich events outside the main belt of ice rafting is crucial to understanding the underlying causes of these abrupt climatic events a high resolution study of a marine sediment core from the iberian margin demonstrates that this midlatitude area was strongly affected both by cooling and advection of low salinity arctic water masses during the last three heinrich events these paleoclimatic time series reveal the internal complexity of each of the last three heinrich events and illustrate the value of parallel studies of the organic and inorganic fractions of the sediments

evidence for warm wet heinrich events in florida analyses of pollen and plant macrofossils from a new core spanning the past 60 000 years from lake tulane florida show a strong antiphase relationship in temperature between florida and the north atlantic region during the pleistocene oak scrub and prairie phases were coeval with long intense dansgaard oeschger interstadials warm periods that initiated bond cycles pine phases were coeval with the north atlantic long stadials cold periods that ended bond cycles and were terminated by heinrich events lake levels were higher during pine phases and climate was wetter however climate in florida was also warmer during these phases which were cold periods in the north atlantic perhaps diminution of thermohaline circulation before and during heinrich events reduced northward heat transport and retained warmth in the subtropical atlantic and gulf of mexico

new constraints on the formation of the non volcanic continental galicia flemish cap conjugate margins numerous multichannel seismic data acquired on the starved continental margin off galicia show that the prominent intra crustal s reflector is observed only below the oceanward portion of the continental margin close to the ocean continent boundary superficial extension has been computed from the geometry of the well imaged tilted fault blocks of the entire margin by using a series of parallel seismic lines specifically designed for this purpose the pure shear model clearly accounts for the lithospheric extension of the continentward portion of the continental margin but not for the oceanward portion of the continental margin where the s reflector is present the s reflector is a detachment zone which presents a decreasing offset in the direction of the continent and this detachment zone has been active since the beginning of rifting recent kinematic reconstructions show that the conjugate margin to galicia is the southeastern flemish cap margin from the few seismic profiles available on this canadian margin a major master fault is observed beneath the continental slope and is interpreted as the outcropping portion of a main detachment zone identified on the galicia margin as the s reflector the proposed mechanism of extension for these two conjugate margins is pure shear which affects the whole lithosphere but in the upper part of the crust the rifting mechanism is either pure shear for the continentward portion of the galicia margin or simple shear for the oceanward portion of the galicia and the whole southeastern flemish cap margins where the detachment zone is active i consider that pure shear is the basic lithospheric mechanism and that simple shear is a mechanism which affects only the upper crust