variations in magmatic processes along the east greenland volcanic margin summary seismic velocities and the associated thicknesses of rifted and igneous crust provide key constraints on the rifting history the differentiation between non volcanic and volcanic rifted margins the driving force of magmatism at volcanic margins that is active or passive upwelling and the temperature anomaly in the lithosphere this paper presents two new wide angle seismic transects of the east greenland margin and combines the velocity models with a compilation of 30 wide angle seismic velocity models from several publications along the entire east greenland margin compiled maps show the depth to basement depth to moho crustal thickness and thickness of high velocity lower crust hvlc with velocities above 7 0 km s 1 first we present two new wide angle seismic transects which contribute to the compilation at the northeast greenland margin and over the oceanic crust between shannon island and the greenland fracture zone velocity models produced by ray tracing result in total traveltime rms misfits of 100 120 milliseconds and 2 values of 3 7 and 2 3 for the northern and southern profiles with respect to the data quality and structural complexity 2 d gravity modelling is used to verify the structural and lithologic constraints the northernmost profile awi 20030200 reveals a magma starved break up and a rapidly thinning oceanic crust until magnetic anomaly c21 47 1 ma the southern seismic transect awi 20030300 exhibits a positive velocity anomaly associated with the shannon high and a basin of up to 15 km depth beneath flood basalts between shannon island and the continent ocean boundary break up is associated with minor crustal thickening and a rapidly decreasing thickness of oceanic crust out to anomaly c21 the continental region is proposed to be only sparsely penetrated by volcanism and not underplated by magmatic material at all compared to the vast amount of magmatism further south break up is proposed to have occurred at the seaward boundaries of the continent ocean transition zones at between 50 and 54 ma propagating from north to south based on a joint analysis incorporating transects from the kejser franz joseph fjord and godthab gulf secondly the variation of the hvlc along the east greenland margin from 60 to 77 n and from transects of its conjugate margin shows inverted emplacement of prominent landward and seaward hvlc thickness portions from north to south in a distribution chart the differences in the hvlc distribution are attributed to one or more of the following three models in the first model it is inferred that a transfer zone detachment acts as a barrier to northward magma flow in the second model underplating results in thicker and highly intruded lower crust with several small scale feeder dykes that locally increase the lower crustal velocities in the third model a second magmatic event associated with the separation of the jan mayen microcontinent is considered lithospheric scale inhomogeneities might be responsible for the heterogeneous melt generation the inversion of the hvlc distribution in continental and oceanic domains and differences in its velocities

the mid norwegian margin a discussion of crustal lineaments mafic intrusions and remnants of the caledonian root by 3d density modelling and structural interpretation the high density lower crustal body lcb on the mid norwegian margin is almost universally interpreted to represent magmatically underplated material added to the crust during early tertiary opening of the ne atlantic the thickness of the lcb is uneven and its distribution along the margin is sharply limited by margin perpendicular lineaments three dimensional density modelling constrained by petrophysical and seismic data was performed to investigate the expression of the various lineaments bivrost jan mayen gleipne etc in the crustal architecture with and without the lcb the bivrost lineament separating the voring and lofoten margin segments is clearly expressed and so is arguably a lineament in the sw voring margin notably most of the lineaments can be interpreted as offshore prolongations of major onshore detachments stemming from late caledonian orogenic collapse an alternative interpretation of the lcb is thus that it represents high grade metamorphic rocks remnant from the caledonian root determining the nature of the lcb has profound effects for the volume estimate of magmatic rocks in the north atlantic igneous province and consequently also for the degree of crustal thinning and heat flow

crustal lineaments distribution of lower crustal intrusives and structural evolution of the voring margin ne atlantic new insight from wide angle seismic models abstract five lineaments on the volcanic voring margin ne atlantic have been identified in crustal scale models derived from ocean bottom seismograph obs data it is suggested that the voring basin can be divided in four compartments bounded by the jan mayen fracture zone lineament a new lineament defined from this study the gleipne lineament the surt lineament and the bivrost lineament the nw se trending jan mayen gleipne and bivrost lineaments probably represent old zones of weakness controlling the onset of the early eocene seafloor spreading whereas the surt and new lineaments rotated ca 30 symmetrically from the azimuth of the gleipne lineament may represent adjustment features related to the early cretaceous early tertiary rifting the longest landward extent of a lower crustal high velocity body assumed to represent intrusions related to the last phase of rifting is found between the new lineament and the gleipne lineament where the body extends across the helland hansen arch northeastwards in the voring basin the landward limit of the body steps gradually seawards closely related to the interpreted lineaments northeast of the gleipne lineament the body terminates close to the fles fault complex north of the surt lineament it extends across the nyk high and northeast of the bivrost lineament the intrusions terminate around the voring escarpment evidence for an interplay between active and passive rifting components is found on regional and local scales on the margin the active component is evident through the decrease in magmatism with increased distance from the icelandic plume and the passive component is documented through the fact that all found crustal lineaments to a certain degree acted as barriers to magma emplacement the increased thickness of the continental crust on the seaward side of the voring escarpment the upwarping of moho and thinning of the lower crustal high velocity layer in the western part of the voring basin as well as a strong shallowing of the moho observed in parts of the area between the jan mayen fracture zone lineament and the new lineament can be explained by lithospheric delamination models

crustal structure of the northern nova scotia rifted continental margin eastern canada 1 the nova scotia continental margin off eastern canada marks a transition from a volcanic to a nonvolcanic style of rifting the northern nonvolcanic segment of the margin was studied by a 490 km long refraction seismic line with dense air gun shots coincident with previous deep reflection profiles a p wave velocity model was developed from forward and inverse modeling of the wide angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles the continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5 7 6 9 km s crustal thinning down to 3 km occurs in a 180 km wide zone and the sediment cover in this area is up to 15 km thick farther seaward a 150 km wide transition zone is observed with a 5 km thick lower layer 7 2 7 6 km s interpreted as partially serpentinized mantle at the landward end this layer is overlain by highly altered continental crust 5 4 km s extending up to the seaward limit of the jurassic salt province farther seaward the upper layer is interpreted as exhumed and highly serpentinized mantle 5 1 km s separated from the lower layer by subhorizontal reflectivity which probably represents a serpentinization front oceanic crustal thickness is 4 km with layer 2 velocities of 4 6 5 0 km s layer 3 velocities of 6 4 6 55 km s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile this reduced magma production might be related to the proximity of the newfoundland transform margin

deep crustal structure and magmatic processes the inception of seafloor spreading in the norwegian greenland sea summary an extensive investigation into the deep crustal structure of the conjugate norwegian and e greenland margins utilizing two ship expanded spread and wide aperture cdp profiling has led to the identification of a unique suite of crustal structures developed during the onset of seafloor spreading on both margins a region of low velocity crust was found proximal to marginal escarpments immediately seaward of the low velocity crustal block seaward dipping reflectors occur within thick crust that is interpreted to be wholly igneous a product of voluminous melt production enhanced by convective partial melting processes this earliest formed oceanic crust is actually several kilometres thicker than the low velocity crust or the adjacent continental crust normal oceanic crustal thicknesses are not produced until about 5 my after the initiation of spreading when convective partial melting has abated and the magma budget has reduced the extrusive unit reduces to normal thicknesses long before the intrusive and plutonic section indicating that the thickening or thinning of oceanic crust is accomplished primarily by variations in the latter

evolution of oceanic crust on the kolbeinsey ridge north of iceland over the past 22 myr the evolution of oceanic crust on the kolbeinsey ridge north of iceland is discussed on the basis of a crustal transect obtained by seismic experiment from the kolbeinsey ridge to the jan mayen basin the crustal model indicates a relatively uniform structure no significant lateral velocity variations are observed especially in the lower crust the uniform velocity structure suggests that the postulated extinct axis does not exist over the oceanic crust formed at the kolbeinsey ridge but supports a model of continuous spreading along the ridge after oceanic spreading started west of the jan mayen basin the oceanic crust formed at kolbeinsey ridge is 1 2 5 km thicker than normal oceanic crust due to hotter than normal mantle from the iceland mantle plume the observed generally uniform thickness throughout the transect might also indicate that the temperatures of the astheno spheric mantle ascending along the kolbeinsey ridge have not changed significantly since the age of magnetic anomaly 6b

extension crustal structure and magmatism at the outer voring basin norwegian margin regional analysis of new 2d and 3d multichannel seismic data has improved interpretation of the crustal configuration and structural style along the norwegian margin five domains with different structural styles and evolutions are defined along the outer voring basin 1 the nyk high naglfar dome 2 the north gjallar ridge 3 the gleipne saddle 4 the south gjallar ridge 5 the ran ridge early campanian early paleocene and early to mid cretaceous extensional events are evidenced timing of deformation and structural styles observed along each segment reflect a lateral variation of the rifted system probably affected by magma tectonic processes correlation with the deep structures of the outer voring basin shows that the shallow structure in that basin is directly controlled by a deep seated strong high amplitude reflection the t reflection marking the top of a high velocity body v p 7 km s 1 the relation between the lower crustal architecture and the subsurface basin structures has implications for the margin evolution and for the nature of the high velocity body

transformations of runoff chemistry in the arctic tundra northwest territories canada the transformation of snowmelt water chemical composition during melt elution and runoff in an arctic tundra basin is investigated the chemistry of the water flowing along pathways from the surface of melting snow to the 95 5h a basin outlet is related to relevant hydrological processes in so doing this paper offers physically based explanations for the transformation of major ion concentrations and loads of runoff water associated with snowmelt and rainfall along hydrological pathways to the stream outlet late lying snowdrifts were found to influence the ion chemistry in adjacent reaches of the stream channel greatly as the initial pulse of ion rich melt water drained from the snowdrift and was conveyed through hillslope flowpaths the concentrations of most ions increased and the duration of the peak

crustal structure of the lofoten continental margin off northern norway from ocean bottom seismographic studies summary in 1988 an extensive geophysical experiment was performed on the lofoten passive continental margin which was formed by the continental break up between greenland and northern norway at 58 ma the geological units of the outer lofoten margin are characterized by seaward dipping reflectors sdrs and landward flood basalt which extends up to 100 km landwards of sdrs in this study we obtain the p wave velocity structure beneath the lofoten basin the sdrs and the landward flood basalt by use of ocean bottom seismograph refraction profiling and we also discuss the formation of the northern norwegian passive continental margin in the lofoten basin the crust is of oceanic type consisting of sedimentary layers oceanic laver 2 4 9 5 5 km s 1 layer 3a 6 3 6 8 km s 1 and layer 3b 7 0 7 1 km s 1 beneath the sdrs the crustal layers are identical to those of the lofoten basin but the thickness of the lower crust which represents the same velocity as layer 3b increases to 5 km towards the continent side and a high velocity lower crustal layer 7 3 km s 1 is formed at the base of the crust the ocean continent transition zone is situated between the landward side of the sdrs and the northward continuation of the voring plateau escarpment in this region the velocity of the lower crust gradually decreases and approaches the lower crustal velocity beneath the lofoten islands 6 8 km s 1 the model also indicates that the high velocity layer disappears in this region comparing our model with the crustal structure on the voring margin it is clear that the lower crustal body 7 km s 1 thickens southwards along the northern norwegian continental margin recent results from petrological and geophysical studies of the generation of the oceanic crust have shown that increasing the temperature of the upwelling asthenospheric material increases the thickness of the oceanic crust we interpret this as that the oceanic crust in the southern area in the voring lofoten margin was generated by hotter material than that of the northern area

the ocean continent boundary off the western continental margin of iberia crustal structure west of galicia bank a seismic refraction transect across the galicia bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin parallel peridotite ridge pr immediately west of the pr oceanic crust is only 2 5 3 5 km thick but farther west oceanward it thickens to 7 km the pr caps a similar to 60 km wide lens shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust when superimposed on a reflection time version of the velocity model the s reflector is clearly intracrustal at its east end westward s cuts down to lower crustal levels eventually coinciding with the top of the serpentinized peridotite lens original crust mantle boundary these observations render almost impossible the seafloor exposure of the pr by s acting as a top to the west detachment fault numerical models of melting and borehole subsidence information constrain our rifting model the easternmost continental crust experienced a total stretching factor of 4 3 most likely in two stages it probably occurred over similar to 25 m y with the highest fate of stretching at the beginning of the main earlier rift phase valanginian 141 135 ma the 3 4 7 km thick continental crust depending on whether serpentinized peridotite is assigned to crust or mantle which may include melt products requires stretching factors 6 more than 11 7 and a rift duration of more than 25 13 m y the thin oceanic crust immediately west of the pr is explained by conductive cooling of the mantle during the long prebreakup stretching phase which temporarily caused reduced melting immediately after breakup

crust and uppermost mantle structure of greenland and the northwest atlantic from rayleigh wave group velocity tomography the greenland landmass preserves 4 billion years of tectonic history but much of the continent is inaccessible to geological study due to the extensive inland ice cap we map out for the first time the 3 d crustal structure of greenland and the nw atlantic ocean using rayleigh wave anisotropic group velocity tomography in the period range 10 80 s from regional earthquakes and the ongoing glatis glisn seismograph networks 1 d inversion gives a pseudo 3 d model of shear wave velocity structure to depths of 100 km with a horizontal resolution of 200 km crustal thickness across mainland greenland ranges from 25 km to over 50 km and the velocity structure shows considerable heterogeneity the large sedimentary basins on the continental shelf are clearly visible as low velocities in the upper 5 15 km within the upper continental basement velocities are systematically lower in northern greenland than in the south and exhibit a broadly nw se trend the thinning of the crust at the continental margins is also clearly imaged upper mantle velocities show a clear distinction between typical fast cratonic lithosphere vs 4 6 km s 1 beneath greenland and its ne margin and anomalously slow oceanic mantle vs 4 3 4 4 km s 1 beneath the nw atlantic we do not observe any sign of pervasive lithospheric modification across greenland in the regions associated with the presumed iceland hotspot track though the average crustal velocity in this region is higher than that of areas to the north and south crustal anisotropy beneath greenland is strong and complex likely reflecting numerous episodes of tectonic deformation beneath the north atlantic and baffin bay the dominant anisotropy directions are perpendicular to the active and extinct spreading centres anisotropy in the subcontinental lithosphere is weaker than that of the crust but still significant consistent with cratonic lithosphere worldwide

crustal structure of the kolbeinsey ridge north atlantic obtained by use of ocean bottom seismographs we present the results of a seismic survey performed around the kolbeinsey ridge north atlantic the seismic data were acquired along three profiles on the ridge axis ll 12 km off ridge l2 and perpendicular to the ridge axis l3 the models indicate lower velocity beneath the ridge axis than in the off ridge area along l3 we obtain velocities of 2 6 3 6 km s upper crust 4 8 6 0 km s middle crust and 6 6 6 9 km s lower crust beneath the ridge axis while at more than 12 km off ridge the model shows velocities of 3 6 4 6 km s upper crust 5 6 6 4 km s middle crust and 6 9 7 2 km s lower crust the model of l1 also shows significant lateral variation along the axis in the lower crust the velocity in the lower crust decreases to 6 3 km s in the southwestern part of ll where the moho depth decreases to 7 2 km the low velocity structure beneath the ridge axis is interpreted to be due to high porosity in the upper crust and to higher temperatures as well as a few percent porosity in the middle and lower crustal material according to this interpretation the crustal model of l3 implies that the cracks have been sealed and the temperatures have decreased to normal in 1 2 m y old crust the anomalous low velocity structure along ll can be interpreted as related to the most recent injection processes or a thermal influence of the spar fracture zone situated 50 km south of the profile

total tectonic subsidence a parameter for distinguishing crust type at the u s atlantic continental margin total tectonic subsidence tts is the difference between the prerifting continental crust elevation and the present sediment unloaded basement depth in a sedimentary basin tts is calculated using observations of bathymetry and basement depth and assumptions about sediment density and the loading response of the lithosphere i use the extensional model for sedimentary basin formation to interpret the tts of a passive continental margin that model predicts that the tts on an old margin should define the major crust type boundaries and indicate the lateral distribution of extension within the extended continental crust tts analysis of the u s atlantic continental margin indicates that the boundary between extended continental and oceanic crusts lies along the east coast magnetic anomaly ecma to the north of the blake spur fracture zone bsfz and along the blake escarpment to the south there is no tts anomaly associated with the blake spur magnetic anomaly which seems to be underlain by normal oceanic crust the ecma appears to be associated with a tts minimum but i attribute that to artificially shallow estimates of depth to magnetic basement under the ecma tts estimates of extended continental crust thickness agree well with gravity models except where the problem with magnetic basement depth estimates interferes the amount of crust extension prior to seafloor spreading varies along the strike of the margin the most abrupt change is at the bsfz across the bsfz the amount of extension varies by 160 km but the volume of continental material does not vary the extension variation may be caused by lateral changes in the gross rheology of the continental lithosphere in the case of the bsfz it would be the suture of more ductile pre late paleozoic african craton with more brittle north american craton that could cause the abrupt change

the crustal structure of central east greenland results from combined land sea seismic refraction experiments summary we present results from the first combined marine land seismic refraction survey in the area of the east greenland caledonides together with gravimetric measurements the seismic and gravimetric models show a consistent picture of the crustal structures in the western area which is part of the caledonian mountains of east greenland crystalline rocks with p wave velocities of about 5 5 km s 1 occur at the surface seismic velocities increase continuously with depth and reach values of 6 6 km s 1 at 12 km the total thickness of the crust as revealed by the seismic measurements reaches high values of up to 48 km in the southwestern part of the region 28 w gravity data which also cover the region west of the seismic lines indicate a possible further increase of the crustal thickness these high values raise questions about whether they represent a caledonide crustal root which would stand in strong contrast to the findings in the caledonian areas of europe or whether the crustal thickening represents a pre caledonian structure and is affine rather to the older regions west of the caledonides and adjacent to the greenland inland ice towards the east crustal thickness decreases rapidly it reaches a minimum of 22 km under the late palaeozoic mesozoic sedimentary basin of jameson land the crustal thinning is the result of the stretching of east greenland which was produced by a general extensional tectonic regime and the collapse of the caledonian mountain chain from devonian time onwards in the area of transition from thick to thin crust the seismic data indicate a layered structure at the moho this region is interpreted as a zone of extensive intrusion or underplating during the rifting in the tertiary

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

effect of thermal contrasts on gravity modeling at passive margins results from the western barents sea the western barents sea passive margin is a key locality to demonstrate the effect of the thermal structure of the lithosphere on forward gravity modeling this margin developed by shear motion between the eurasian and greenland plates during the early tertiary and it is a significant border zone between young hot oceanic lithosphere and cooler continental lithosphere we construct two dimensional gravity models of 125 km thick lithosphere based on expansion of mantle rocks determined from thermal modeling the approach has a substantial impact over traditional shallow gravity models here demonstrated on a previously published model on the basis of a 140 mgal free air anomaly the old model proposes an anomalous high density oceanic crust emplaced in a leaky transform adjacent to the continent during early margin development however the lithospheric models predict a homogeneous oceanic crust while preserving regional isostasy at base lithosphere from continent to ocean two further tests agree with this conclusion a map of bouguer corrected ers 1 satellite data reveals no residual anomalies originating from the oceanic crust at the margin admittance analysis shows a strong oceanic lithosphere and the high coherence between bathymetry and free air gravity discounts a significant subsurface load the high gravity anomalies at the margin are thus an edge effect enhanced by sedimentation onto the strong oceanic lithosphere and shaped by the effect of the lithospheric thermal field other results of this work include a new continent ocean boundary map and two crustal transects across the margin

structure of the jan mayen microcontinent and implications for its evolution an extensive seismic survey using ocean bottom seismographs obs was performed in the area across the jan mayen basin north atlantic from the jan mayen ridge to the iceland plateau the jan mayen ridge and surrounding area are considered to be a fragment of a continent which was separated from greenland just prior to magnetic anomaly 6 this study presents the crustal structure of the jan mayen microcontinent and the ocean continent transition to the west of the jan mayen ridge the crustal structures from the centre of the jan mayen ridge to the jan mayen basin are characterized by a deep sedimentary basin a thin basaltic layer within the sedimentary section and extreme thinning of the continental crust towards the iceland plateau the obs data indicate that a continental upper crust v p 5 8 6 1 km s 1 and lower crust v p 6 7 6 8 km s 1 underlie the deep sedimentary basin the thickness of the continental lower crust varies significantly from 12 km beneath the jan mayen ridge to almost zero thickness beneath the northwestern part of the jan mayen basin an ocean continent transition zone is found at the western edge of the jan mayen basin within the 10 km wide transition zone crustal velocities increase towards the iceland plateau and approach the velocities of the oceanic crust obtained at the iceland plateau that is 3 8 5 1 km s 1 oceanic layer 2a 5 9 6 5 km s 1 oceanic layer 2b and 6 8 7 3 km s 1 oceanic layer 3 the crustal model indicates very thin oceanic crust 5 km immediately oceanwards of the ocean continent transition zone beneath the iceland plateau the oceanic crust is thicker 9 km than the typical thickness of normal oceanic crust this might imply that the oceanic crust at the iceland plateau has been generated by asthenospheric material slightly hotter than normal from the crustal structure obtained by the present study it is proposed that the western part of the jan mayen ridge may be referred to as a non volcanic continental margin generated by a long duration of rifting even if the asthenospheric material upwelling along the margin were hotter than normal only small amounts of magmatic intrusions and extrusions would have been generated because of significant conductive cooling under the long duration of rifting

a review of the ne atlantic conjugate margins based on seismic refraction data the ne atlantic region evolved through several rift episodes leading to break up in the eocene that was associated with voluminous magmatism along the conjugate margins of east greenland and nw europe existing seismic refraction data provide good constraints on the overall tectonic development of the margins despite data gaps at the ne greenland shear margin and the southern jan mayen microcontinent the maximum thickness of the initial oceanic crust is 40 km at the greenland iceland faroe ridge but decreases with increasing distance to the iceland plume high velocity lower crust interpreted as magmatic underplating or sill intrusions is observed along most margins but disappears north of the east greenland ridge and the lofoten margin with the exception of the vestbakken volcanic province at the sw barents sea margin south of the narrow lofoten margin the european side is characterized by wide margins the opposite trend is seen in greenland with a wide margin in the ne and narrow margins elsewhere the thin crust beneath the basins is generally underlain by rocks with velocities of 7 km s 1 interpreted as serpentinized mantle in the porcupine and southern rockall basins while off norway alternative interpretations such as eclogite bodies and underplating are also discussed

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

continent ocean transition and voluminous magmatic underplating derived from p wave velocity modelling of the east greenland continental margin summary deep seismic refraction data were gathered across the entire east greenland rifted margin north of the jan mayen fracture zone between 72 n and 75 n in 2003 investigations of the deep crustal structure of this continental margin provide constraints on the formation of the margin and its structural evolution during and after late cretaceous early tertiary rifting and continental break up we present here the results along two profiles located in the prolongation of the godthab gulf and the kejser franz joseph fjord regional p wave velocity models were derived from forward traveltime modelling of land stations and ocean bottom hydrophone obh recordings for the first time long deep seismic sounding transects off east greenland provide a full insight into the crustal architecture of the transition from continental to oceanic crust a mean result is the identification of voluminous magmatic underplating which is wider and thicker than previously thought p wave velocities of the underplated material range between 7 1 and 7 4 km s 1 and the horizontal extents on the profiles are 225 and 190 km the maximum thickness of the underplated material is 15 16 km furthermore the p wave velocity models reveal a 120 130 km wide continent ocean transition zone cot based on an interpretation of the extent of cretaceous syn rift sediments mixed with basaltic intrusions and the lateral increase of velocities in the crustal layers excess magmatism must have been present during a long term rifting process accompanying the extension of the continental crust and giving rise to the voluminous magmatic underplating a consequence of our interpretation of the seismic refraction data is a likely rift propagation in the greenland sea from north to south additionally a comparison of p wave velocity models of the east greenland margin and voring margin reveals significantly asymmetric crustal architectures the voluminous magmatic underplating and asymmetrical conjugate margins formations are considered as a mirror of complex pre and syn rift processes

crustal structure of the ocean continent transition at flemish cap seismic refraction results 1 we conducted a seismic refraction experiment across flemish cap and into the deep basin east of newfoundland canada and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots the continental crust at flemish cap is 30 km thick and is divided into three layers with p wave velocities of 6 0 6 7 km s across the southeast flemish cap margin the continental crust thins over a 90 km wide zone to only 1 2 km the ocean continent boundary is near the base of flemish cap and is marked by a fault between thinned continental crust and 3 km thick crust with velocities of 4 7 7 0 km s interpreted as crust from magma starved oceanic accretion this thin crust continues seaward for 55 km and thins locally to 1 5 km below a sediment cover 1 9 3 1 km s oceanic layer 2 4 7 4 9 km s is 1 5 km thick while layer 3 6 9 km s seems to disappear in the thinnest segment of the oceanic crust at the seawardmost end of the line the crust thickens to 6 km mantle with velocities of 7 6 8 0 km s underlies both the thin continental and thin oceanic crust in an 80 km wide zone a gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth normal mantle velocities of 8 0 km s are observed 6 km below basement there are major differences compared to the conjugate galicia bank margin which has a wide zone of extended continental crust more faulting and prominent detachment faults crust formed by seafloor spreading appears symmetric however with 30 km wide zones of oceanic crust accreted on both margins beginning about 4 5 m y before formation of magnetic anomaly m0 118 ma

subsurface drainage from hummock covered hillslopes in the arctic tundra in the arctic tundra subsurface drainage occurs predominantly through the saturated zone within the layer of peat that mantles the hillslopes in plan view the peat cover is fragmented into a network of channels due to the presence of mineral earth hummocks in cross section the physical and hydraulic properties of the peat vary with depth and the water transmission characteristics e g hydraulic conductivity of the upper profile differ distinctly from those of the lower water flow through the peat is laminar therefore the friction factor f and the reynolds number nr are inversely related average values for the coefficientc of the relation f a c nr vary from 300 near the surface to 14 500 at depth this large difference in c confirms that the larger diameter soil pores of the living vegetation and lightly decomposed peat near the surface offer much less resistance to water motion than the finer grained peat deeper in the profile also the variability suggests that subsurface drainage is strongly affected by the position and thickness of the saturated zone within the peat matrix a first approximation for a model or simulation of the flow regime may consider a peat profile with depth varying resistance properties in respect to subsurface flow q 2000 elsevier science b v all rights reserved

early tertiary magmatism in ne greenland abstract between 73 and 74 n in ne greenland a lower series of quartz tholeiite lavas is overlain by an upper series which varies from quartz tholeiitic to basanitic the lower series basalts are attributed to large melt fractions produced east of the present outcrop where lithospheric thinning permitted the iceland plume to ascend to shallow levels relocation of eruption sites up to 150 km further west preceeded upper series activity in this small degree melts generated principally from garnet lherzolite facies sources were followed by products of more extensive shallower melting which gave rise successively to the upper series lavas a post upper series basaltic dyke swarm and its associated caldera complex palaeomagnetic observations and 40 ar 39 ar dating suggest that the lower series and part of the upper series erupted in c24r at 58 57 ma rejuvenescent activity at c 32 ma marking the termination of volcanism may relate to decompressive mantle melting associated with late regional uplift

ne atlantic continental rifting and volcanic margin formation the ne atlantic rift system which has been under extensional deformation since the caledonian orogeny experienced a series of rift episodes in late paleozoic to early cenozoic times fig 1 late paleozoic rifting is poorly constrained particularly with respect to timing however rifted basin geometries inferred to be of this age are observed at depth in seismic data from the flanks of the younger rift structures

the crustal structure of the southern davie ridge offshore northern mozambique a wide angle seismic and potential field study abstract some of the oldest surviving oceanic basins in the world the mozambique and west somali basins were created during the breakup of gondwana starting around 180 ma between the two basins relative movements of west gondwana and east gondwana including madagascar created a shear zone the davie fracture zone dfz with a topographic elevation davie ridge dr marking its centre the crustal composition of the dfz and dr is a subject of speculation and debate in this study we present seismic refraction data across the prominent topography of the southern dr ray tracing of the wide angle data as well as additional seismic amplitude modelling and 2 5d density modelling constrain its crustal structure and architecture the data indicate that in the mozambique channel the dr consists of fragments of continental crust with a thickness of 10 to 12 km an oceanic crust indenter extends northward from the mozambique basin into the area between the dr and the east african margin at 16 5 s northeast of the dr at 41 8 w 14 5 s the somali basin is probably floored by 6 km thick oceanic crust hence the continental dr separates oceanic crust of the somali and mozambique basins the transitional crustal area at the central mozambican margin is underlain by high velocity lower crust hvlc the hvlc has velocities up to 7 3 km s and extents along the margin vanishing northward between 16 5 and 14 5 s at the madagascan side of the dr at 16 5 s the highly intruded stretched continental crust is 9 km thick and possibly underlain with a smaller hvlc of 2 9 km thickness and an e w extent of 120 km the oceanic crust at 14 5 s represents the oldest part the somali basin which formed after the initial nw se rifting between east and west gondwana

rates of continental breakup magmatism and seafloor spreading in the norway basin iceland plume interaction 1 in year 2000 an ocean bottom seismometer obs profile was acquired across the more margin to the aegir ridge an extinct seafloor spreading axis the margin is an early eocene volcanic passive margin located between the faeroe iceland ridge fir and the east jan mayen fracture zone ejmfz the p wave data were modeled by ray tracing to give a crustal transect showing a 10 11 km thick igneous crust created by breakup magmatism tapering off to magma starved seafloor spreading by c23 time 51 4 ma the location of the ejmfz was reinterpreted from a satellite derived gravity map and spreading direction in the norway basin reevaluated no other fracture zones were confirmed and both thin oceanic crust 4 5 km and lack of fracture zones resemble ultraslow spreading on the arctic gakkel ridge magnetic seafloor spreading anomalies were identified from the magnetic track recorded with the obs profile and half spreading rates were derived early seafloor spreading was slow 15 32 mm yr 1 approaching ultraslow 6 8 mm yr 1 by c20 time 42 7 ma a v shaped pattern seen in the gravity field located only around the northern part of the aegir ridge corresponds to increased crustal thickness in the seismic model recording northeast transport 3 6 mm yr 1 of more melt fertile asthenosphere zones the magma starved character of the norwegian basin seen also during slow seafloor spreading may be the result of depletion of the asthenosphere when the iceland plume constructed the fir to the south as the asthenosphere is subsequently transported into the norway basin

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

the edoras bank margin continental break up in the presence of a mantle plume abstract edoras bank lies on the eastern continental margin of the north atlantic about 900 km south of the present day iceland plume we studied the continent ocean transition of the rifted margin using normal incidence and wide angle seismic reflection data seaward dipping reflectors are well developed over much of the margin and are underlain by a high velocity 7 3 7 5 km s 1 lower crust this is interpreted as either igneous underplating or as existing crust heavily intruded at the time of break up by melts with high mgo contents and suggests that the thermal anomaly associated with the iceland plume was well developed in this area at the time of break up

crustal structure of the central part of the voring basin mid norway margin from ocean bottom seismographs abstract regional ocean bottom seismograph obs data acquired in the central and northern part of the voring basin mid norway margin have been modelled by use of two dimensional 2 d ray tracing the regional dataset comprises thirty obss deployed along seven 100 170 km long profiles the deeper part of the voring basin is difficult to map using multichannel reflection data due to the presence of sills at intermediate sedimentary levels 2 5 km below sea floor but the modelling of the obs data reveals that this technique provides a reliable estimate of structures and seismic velocities from the sea floor to the upper mantle the shallow and intermediate sediments to 5 km below sea floor are characterized by a vertical increase in velocity due to increased confining pressure there is also considerable lateral variation in the velocities within sedimentary layers at all levels the obs data confirm that intrusions of sills are important at intermediate and deep sedimentary levels 2 10 km below sea floor in most parts of the area the sills seem to vary in lateral extent from about 20 km to more than 100 km and their thickness is generally inferred to be about 200 m the low velocity in the upper crystalline crust 6 2 km s confirms that the crust in the voring basin is of continental origin in most parts of the area the velocity of the lower crust is as high as 7 3 7 6 km s this high velocity layer is interpreted as a magmatic underplated body with strong lateral variations in thickness the base of the 7 3 km s layer is interpreted as the moho and the upper mantle velocity is estimated to 8 2 km s

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

seismic refraction investigations on the crustal structure of the western jameson land basin east greenland in 1990 a combined land sea experiment in the hall bredning scoresby sund east greenland revealed new information about the crustal structure of the jameson land basin the sediments and the upper and lower crust have been imaged by a 710 km long network of seismic refraction lines our data indicate that the crust mantle boundary is at a depth of 24 km close to the western coast of jameson land and deepens to 31 km toward the western termination of hall bredning the sediments thin from 8 km thickness in the east to almost 0 km close to milne land our cross sections indicate that jameson land is an asymmetrical half graben shaped sedimentary basin with a n s striking axis

crustal structure of the northern part of the voring basin mid norway margin from wide angle seismic and gravity data abstract three regional ocean bottom seismograph obs profiles with a total of fifteen recovered obss were acquired in the northern part of the voring basin mid norway margin in 1996 the data have been modelled by inversion and forward modelling of the obs vertical components p waves the velocity is found to increase downwards within the sedimentary layers due to increasing depth of burial confining pressure within the two deepest sedimentary layers there seems to be an increase of 0 5 1 km s in velocity northwestwards this increase is most likely caused by high velocity sill intrusions in the sedimentary rocks emplaced during the rift episode leading to early eocene opening of the ne atlantic the presence of significant amounts of sills seems to terminate close to the nyk high along profile 5 dip profile and close to a dome northwest of the shelf edge along profile 6 dip profile the velocity in the top of this dome is anomalously high 3 4 km s at only 1 km depth beneath the seafloor suggesting that the high consists of volcanic rocks or heavily intruded sedimentary rocks the relatively low velocities derived from within the upper crystalline crust 5 9 6 2 km s confirms that the crust is of continental origin within the lower crust there seems to be a clear increase in velocity northwestwards from about 7 0 km s to about 7 4 km s suggesting that the amount of high velocity intrusions underplating in the lower crust decreases landwards the modelling of the strike profile profile 7 suggests that the transition from a lower crust dominated by magmatic intrusions to a more undisturbed continental lower crust is located approximately beneath this profile the model for profile 7 ties closely southwestwards to the models derived from obs data acquired in 1992 and northeastwards to lofoten obs data acquired in 1988 the models indicate that the thick magmatic intrusions in the sedimentary rocks and lower crust extend further landwards in the area of the 1992 survey and that the transition zone is related to the surt lineament the distribution of magmatic rocks thus seems to correlate strongly with pre breakup structures in addition to the p wave modelling gravity modelling has been performed along all profiles the gravity modelling confirmed all main aspects of the obs models and provided constraints on the crustal structures towards the ends of the profiles

deep structures and breakup along volcanic rifted margins insights from integrated studies along the outer voring basin norway three dimensional multichannel seismic survey and well data allows a deeper insight into rifting mechanisms and crustal structure of the norwegian volcanic rifted margin a surprising high velocity lower crustal dome marked by a strong amplitude reflection t reflection is described below the north gjallar ridge ngr near the volcanic plateau formed during the late paleocene early eocene breakup faulting along the ngr occurs during early campanian paleocene and is controlled by the crustal dome the t reflection was previously related to either the top of mafic underplated crust or soft lower crust updomed during the continental breakup the influence and timing of the crustal dome on the basin structure shows clearly that the dome predates the continental breakup based on the structural relationship and the results of the modelling it is therefore concluded that the t reflection does not necessarily originate from an anomalous tertiary magmatic event related to the breakup underplated material but could be also partly attributed to pre breakup mafic material or and inherited high pressure granulite eclogite rocks in the continental domain q 2004 elsevier ltd all rights reserved

symmetric conjugation of continent ocean boundary structures along the norwegian and east greenland margins abstract a characteristic structural feature of the norwegian continental margin that has been described from multichannel seismic data as well as dsdp and odp drilling is a major buildup of layered extrusive basaltic rocks forming wedges that dip seaward along the entire margin models for their origin describe them as either products of a late stage intra continental rift phenomenon with the volcanic rocks overlying attenuated continental crust or as an integral component of the oceanic crust formed during the first few million years of seafloor spreading in the norwegian greenland sea from studies of the norwegian margin alone it was not possible to unequivocally distinguish between the two proposed models so an investigation of the conjugate portions of the east greenland margin was undertaken which used two ship multichannel seismic acquisition as its focus we found that the wedges were not only present on the east greenland margin but that details of their along strike variability are mirrored on both margins together with their spatial relationship to the oldest magnetic lineation pattern the wedges are relatively small in the northern part of the margin where they occur almost entirely landward of lineation 24 moving south we see that the wedges broaden encroach into the pattern of lineation 24 then develop into two separate wedges the seaward of which is associated with lineation 23 lineations 24 and 23 on both margins are distorted when associated with wedges we believe that these data lend strength to a tectonic association of the wedges with nascent plate boundary processes their observation in many margins throughout the world suggest that a prolific volcanic phase may be commonly associated with the inception of a divergent plate boundary and we believe that the most likely mechanism to generate the excess volcanism is the convective partial melting scheme recently proposed by mutter and buck 1986 and mutter et al in press

melt generation at volcanic continental margins no need for a mantle plume melt generation in a rifting environment is studied using a dynamic 2 d finite element model the lithosphere is extended to large realistic thinning factors assuming a mantle temperature of 1333 c the focussing of deformation results in a distribution of thinning factors along the margin at breakup time consistent with observations the timing of melt production late synrift and the amounts of melt are consistent with observations at volcanic margins the dynamical processes related to lithospheric rifting enhance the produced melt volumes sufficiently to explain the sometimes enigmatic melt volumes found at volcanic margins

the crustal structure of central east greenland i from the caledonian orogen to the tertiary igneous province summary we present a regional crustal model of the east greenland fjord region between 69 n and 74 nw hich spans the caledonian fold belt and the adjoining devonian and mesozoic basins the model is a compilation of existing seismic models that were partly reinterpreted and newly derived results from different modelling approaches remodelling of 33 stations on three deep seismic lines in the southern area yielded consistent p wave velocity models for the entire fjord region seismic velocities between 5 5 km s 1 near the surface and 6 9 km s 1 in the lower crust are typical for regions of palaeozoic age moho depths up to 48 km in the seismic models suggest the existence of a crustal root beneath the caledonian orogen shear wave modelling of 51 stations on six refraction seismic profiles resulted in s wave velocities between 3 2 km s 1 in the upper crust 4 0 km s 1 at the crust mantle boundary and 4 2 km s 1 in the partial magmatic underplating of the lower crust in the northern area calculation of poisson s ratio portrays a fairly homogeneous crust with only slight variations in poisson s ratio of 0 26 0 30 for the uppermost crystalline crust and 0 22 0 24 for the middle crust these values cannot be linked to lithological variations because they are either small scale or span several tectonic provinces finite difference modelling and amplitude analysis confirm models without magmatic underplating in the lower crust of the southern hall bredning scoresby sund area

crustal structure of the outer voring plateau offshore norway from ocean bottom seismic and gravity data four ocean bottom seismograph obs profiles acquired across the continent ocean transition zone of the outer voring plateau have been modeled by inversion and forward modeling of the obs vertical component traces p waves the thickness of the sedimentary layers deposited since the early eocene continental breakup varies from up to 2 2 km above the basaltic inner flows if to a minimum of 0 2 km about 20 km seaward of the voring escarpment ve with a velocity of 1 6 2 55 km s the thickness of the uppermost layer of basalt varies from 0 5 to 4 5 km and its velocity ranges from 3 4 to 5 km s the modeling of the two dip profiles suggests that strongly intruded preopening sedimentary rocks extend 40 km seaward of the ve farther seaward another layer of extrusives intrusives is found beneath the uppermost layer of basalt the lateral variations in velocity in the main crustal layer along the dip profiles indicate that the continent ocean transition extends over a 30 50 km wide zone in between strike profile 3 oceanic 6 6 7 35 km s and strike profile 4 continental 6 1 7 0 km s an up to 7 km thick lower crustal high velocity body 7 1 7 4 km s is interpreted in terms of intrusions in the lower crust beneath and landward of the continent ocean transition the total thickness of the normal oceanic crust is estimated to be 7 9 km the anomalously thick oceanic crust reaches a maximum thickness of about 25 km and the crustal thickness is estimated to be 17 22 km beneath the if gravity modeling of the profiles confirmed all main aspects of the velocity models and provided constraints on the crustal structures toward the ends of the profiles where the ray coverage was low

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

lateral flow of hot plume material ponded at sublithospheric depths local buoyancy forces drive the flow of ponded plume material along the base of the lithosphere this processes complicates the geological effects of plumes beyond those of simple passage of plates over point heat sources variations in the depth to the base of the lithosphere significantly affect lateral flow the flow rate is modified to a moderate extent by locally thin lithosphere beneath midplate hotspots such as hawaii the effect is relevant when numerical models are rescaled to better fit the observed uplift and the observed distance from the hotspot to the nose of the swell variations in the depth to the base of the lithosphere have a much stronger effect on flow near ridge axes effectively forming an upside down drainage pattern plume material flows upslope toward ridge axes from off axis plumes and along ridge axes from on axis plumes at sufficiently low spreading rates thinner lithosphere exists above plume material than above normal asthenosphere the edges of normal lithosphere adjacent to plume material act to some extent as levees which limit the lateral spreading of plume material pressure release melt is associated with the upslope flow of plume material it is expected to cause volcanism away from the plume and to sometimes supply melt depleted plume material to ridge axes that are upslope from the region of voluminous melting

continent ocean transition on the voring plateau ne atlantic derived from densely sampled ocean bottom seismometer data 1 a 180 km long seismic wide angle profile was acquired across the voring plateau ne atlantic using a tuned air gun array and three component ocean bottom seismometers deployed with 5 km spacing the seismic p wave data have been modeled by ray tracing inversion and the model has been constrained by s wave and gravity modeling the data and modeling have allowed us to depict the crustal structure and nature of the continent ocean transition cot in a classical volcanic margin case the p wave velocity near the top of the main crustal layer is estimated to 6 0 km s landward of the 25 km wide cot the seaward increase to 6 5 km s in the cot is conformable with heavily intruded continental crust within this zone farther seaward the velocity increase to 6 9 km s in the same layer suggests the presence of oceanic crust the abundant magmatism landward of and within the cot is primarily observed as extrusives forming wedges of seaward dipping reflectors and mafic lower crustal intrusions underplating the maximum thickness of the oceanic crust is measured to 23 5 km at the seaward termination of the cot

mantle mixing and continental breakup magmatism abstract the frequent formation of large igneous provinces during the opening of the atlantic ocean is a surface manifestation of the thermal and chemical state of convecting mantle beneath the supercontinent pangea recent geochemical and geophysical findings from the north atlantic igneous province all point to the significant role of incomplete mantle mixing in igneous petrogenesis on the basis of a whole mantle convection model with chemical tracers i demonstrate that sublithospheric convection driven by surface cooling can bring up dense fertile mantle without a thermal anomaly when small scale convection in the upper mantle breaks down into the lower mantle strong counter upwelling takes place entraining a large amount of dense crustal fragments accumulated at the base of the mantle transition zone this multi scale mantle mixing could potentially explain a variety of hotspot phenomenology as well as the formation of both volcanic and non volcanic rifted margins with a spatially and temporally varying distribution of fertile mantle

the crustal structure of central east greenland ii from the precambrian shield to the recent mid oceanic ridges we present a 3 d crustal model of the east greenland fjord region between 69 n and 74 n the model covers the precambrian shield and the caledonian orogenic belt the adjoining devonian and mesozoic basins the continent ocean transition and the cenozoic oceanic areas as far as the kolbeinsey and the mohns mid oceanic ridges existing seismic models of the crustal structure are extrapolated into adjacent areas using 3 d gravity modelling for this purpose we compile a new regional scale bouguer anomaly map the precambrian shield west of the caledonian orogen approximately west of 32 w shows a mean thickness of 35 km with only small scale undulations this thickness is at the lower limit of the global range in shield thickness the caledonian orogen exhibits a pronounced mountain root with overall crustal thicknesses up to 51 km beside the urals the east greenland caledonides are one of the two palaeozoic mountain belts where a crustal root has preserved to the present day continuation of the crustal model to the east beyond the continent ocean transition yielded thicknesses of the crystalline oceanic crust from 9 km near the kolbeinsey ridge to 3 km west of the mohns ridge differences in the thermal structures of the old continental and the young oceanic lithosphere are responsible for the low density mantle beneath the oceanic crust which is also demonstrated by 3 d gravity modelling

dynamic support by the icelandic plume and vertical tectonics of the northeast atlantic continental margins late paleocene early eocene continental rifting in the northeast atlantic differs significantly from earlier episodes of margin formation in the central atlantic at a nonvolcanic margin rifting occurs over a wide area with little associated magmatism postrift subsidence decreases in a predictable exponential pattern with time in contrast subsidence analysis of ocean drilling program and deep sea drilling project drill sites from the voring plateau hatton bank and east greenland margin show that in these areas the continent ocean transition cot is very sharp with s increasing over a horizontal distance of 30 50 km from values of 1 1 1 50 on the continent side to at the cot drilling penetrated the thick seaward dipping basaltic sequences that typify the east greenland margin at site 917 fluvial sandstones underlying the basalts show that the area was subaerially exposed prior to continental breakup but the amount and timing of any uplift are presently unconstrained sediment backstripping techniques allow a comparison between the reconstructed and predicted subsidence histories and thus an estimate of the thermal anomaly through time anomalous slow subsidence in the early postrift period at 63 n offshore east greenland is attributed to support by the icelandic plume dynamic support is weaker on the hatton bank and on the voring plateau variations in the strength and duration of the support suggest that the plume was a large 1000 km radius structure that lay under the greenland craton at the time of breakup current data suggest that it may have crossed the east greenland coast at 40 ma discrepancies between uniform stretching models and the reconstructed subsidence for sites on the east greenland shelf and the voring plateau allow the amount of igneous underplating at the time of breakup to be estimated calculations suggest a maximum of around 8 8 km of gabbroic underplating occurred at the time of breakup under the east greenland shelf and 2 4 km under the voring plateau although both these bodies thin rapidly away from the cot

contrasting rifted margin styles south of greenland implications for mantle plume dynamics abstract we present new and reprocessed seismic reflection data from the area where the southeast and southwest greenland margins intersected to form a triple junction south of greenland in the early tertiary during breakup at 56 ma thick igneous crust was accreted along the entire 1300 km long southeast greenland margin from the greenland iceland ridge to and possibly 100 km beyond the triple junction into the labrador sea however highly extended and thin crust 250 km to the west of the triple junction suggests that magmatically starved crustal formation occurred on the southwest greenland margin at the same time thus a transition from a volcanic to a non volcanic margin over only 100 200 km is observed magmatism related to the impact of the iceland plume below the north atlantic around 61 ma is known from central west and southeast greenland the new seismic data also suggest the presence of a small volcanic plateau of similar age close to the triple junction the extent of initial plume related volcanism inferred from these observations is explained by a model of lateral flow of plume material that is guided by relief at the base of the lithosphere plume mantle is channelled to great distances provided that significant melting does not take place melting causes cooling and dehydration of the plume mantle the associated viscosity increase acts against lateral flow and restricts plume material to its point of entry into an actively spreading rift we further suggest that thick archaean lithosphere blocked direct flow of plume material into the magma starved southwest greenland margin while the plume was free to flow into the central west and east greenland margins the model is consistent with a plume layer that is only moderately hotter 100 200 c than ambient mantle temperature and has a thickness comparable to lithospheric thickness variations 50 100 km lithospheric architecture the timing of continental rifting and viscosity changes due to melting of the plume material are therefore critical parameters for understanding the distribution of magmatism

models of the development of the west iberia rifted continental margin at 40 30 n deduced from surface and deep tow magnetic anomalies the ocean continent transition oct on nonvolcanic rifted continental margins is important in that it contains evidence concerning the breakup of the continents and the onset of seafloor spreading the nature of the oct off western lberia has recently been attracting attention following seismic and other geophysical studies there and drilling of acoustic basement by leg 149 of the ocean drilling program here we concentrate on the interpretation of a new digital magnetic anomaly chart for the iberia abyssal plain between 39 5 o and 42 2 o n the most striking anomaly is a trough which exists immediately east of and locally parallel to anomaly j and is closely coincident with a basement peridotite ridge we conclude from modeling surface magnetic anomalies and a deep tow magnetometer profile that seafloor spreading began about 129 9 m y ago barremian at a rate of 10 0 mm yr consistent with drilling and seismostratigraphic results from the southern lberia abyssal plain a second feature is the existence of linear low amplitude isochron parallel magnetic anomalies east of the peridotite ridge trough and a clear change in the trend of these anomalies east of about 11 o 15 w it is possible to model these anomalies by a series of 7 to 30 km wide weakly magnetized crustal blocks of about the same thickness as the oceanic crust the west to east anticlockwise change in trend at 11 o 15 w suggests either a major change in stress regime and or in the type of magnetic anomaly source we speculate that the region between the peridotite ridge and 11 o 15 w represents a transitional crust whose formation was dominated by the same stress regime which accompanied eventual seafloor spreading we explain the change in trend at 11 o 15 w by a model featuring a rift which propagated through continental crust from south to north we consider three scenarios for the formation of the transitional zone crust first continental crust is surrounded and impregnated by intrusive and extrusive material formed from passive upwelling of magma created by the limited adiabatic decompression partial melting which occurs as the two continental lithospheric plates first begin to separate second such crust is surrounded by unroofed upper mantle material or third the zone is the product of ultraslow seafloor spreading present evidence tends to support the first scenario in this scenario the top of the asthenosphere eventually becomes shallow enough to enable magma to penetrate the thinnest continental crust fill the space caused by additional extension and form the transitional crust already described over a broad zone at least several tens of kilometers wide at breakup upper mantle rocks become exposed on the seafloor and undergo extensive serpentinization in a final stage we envisage focusing of the igneous activity leading to the onset of seafloor spreading initially producing thin oceanic crust because of a restricted melt supply the width of continuous tectonically extended continental crust in the southern iberia abyssal plain is less than was previously supposed

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

a seismic study along the east greenland margin from 72 n to 77 n summary around 4370 km of new seismic reflection data collected along the east greenland margin between 71 30 n and 77 n in 2003 provide a first detailed view of the sediment distribution and tectonic features along the east greenland margin after processing and converting the data to depth we correlated odp site 913 stratigraphy into the new seismic network unit gb 2 shows the greatest glacial sediment deposits beneath the east greenland continental shelf this unit is characterized by the beginning of prograding sequences and has according to our stratigraphic correlation a middle miocene age it might have been caused by rapid changes in sea level and or glacial erosion by an early ice sheet or glaciers along the coast a basement high presumably a 360 km long basement structure at 77 n 74 54 n prevents continuous sediment transport from the shelf into the deep sea area in times before 15 myr the origin of this prominent structure remains speculative since no rock sample from this structure is available seaward dipping reflectors at the eastern flank of this structure strongly support that it is a volcanic construction and is most likely emplaced on continental or transitional crust the compilation of sediment thickness provide an insight into the regional sediment distribution in the greenland basin an average sediment thickness of 1 km is observed the north bordering boreas basin has a sediment thickness of 1 8 km close to the greenland fracture zone gfz

redox dynamics in the active layer of an arctic headwater catchment examining the potential for transfer of dissolved methane from soils to stream water the linkages between methane production transport and release from terrestrial and aquatic systems are not well understood complicating the task of predicting methane emissions we present novel data examining the potential for the saturated zone of active layer soils to act as a source of dissolved methane to the aquatic system via soil water discharge within a headwater catchment of the continuous permafrost zone in northern canada we monitored redox conditions and soil methane concentrations across a transect of soil profiles from midstream to hillslope and compare temporal patterns in methane concentrations in soils to those in the stream we show that redox conditions in active layer soils become more negative as the thaw season progresses providing conditions suitable for net methanogenesis and that redox conditions are sensitive to increased precipitation during a storm event but only in shallower surface soil layers while we demonstrate that methane concentrations at depth in the hillslope soils increase over the course of the growing season as reducing conditions develop we find no evidence that this has an influence on stream water methane concentrations sediments directly beneath the stream bed however remain strongly reducing at depth throughout the thaw season and contain methane at concentrations 5 orders of magnitude greater than those in hillslope soils the extent of substreambed methane sources and the rates of methane transport from these zones may therefore be important factors determining headwater stream methane concentrations under changing arctic hydrologic regimes

development of volcanic passive margins two dimensional laboratory models 1 continental breakup above an anomalously hot mantle may lead to the development of volcanic margins volcanic margins are characterized by 1 thick seaward dipping lava flow sequences 2 central intrusive complexes associated with dyke swarms parallel to the coast and 3 high seismic velocity bodies in the lower crust attributable to magma underplating a conceptual model for volcanic margins development has recently been proposed based on onshore studies of the greenland margins and the british tertiary igneous province it is proposed that the long lived central intrusions are genetically linked to underlying persistent zones of mantle fusion these localized melting domains or soft spots equivalent to small mantle diapirs may locally soften the extending continental lithosphere the low viscosity diapirs would 1 localize tectonic strain and 2 feed the volcanic margin with magma thus such soft spots can control the along strike magmatic and tectonic segmentation of volcanic margins recent geophysical investigations appear to show that the along strike structure of volcanic passive margins is compatible with such a segmentation process here we present a set of scaled experiments designed to study how such localized rheological heterogeneities in the sub moho mantle may have a mechanical effect on continental breakup four layer models were constructed using sand and silicone putties to represent the brittle and ductile layers of both crust and mantle the soft spots are simulated by low viscosity silicone putty emplaced within the brittle material at the scale of the entire breakup zone the soft spots display an oceanic type strength profile defining low strength zones where continental breakup is initiated the rift orientation and segmentation are strongly controlled by the distribution of the low viscosity heterogeneities rather than by the direction of regional extension the experiments are compared with the geometry and segmentation of the onshore part of the greenland volcanic margins

the hatton bank continental margin ii deep structure from two ship expanding spread seismic profiles summary the continent ocean transition adjacent to hatton bank was studied using a dense grid of single ship and two ship multichannel seismic profiles the interpretation of the explosive expanding spread profiles esps which were shot as part of this survey are discussed here in detail extensive seaward dipping reflectors are developed in the upper crust across the entire margin these seaward dipping reflectors continue northwards on the faeroes and voring margins where they have been shown to be caused by basaltic lavas as well as on the conjugate margin of east greenland the dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension the esps show clear seismic arrivals out to ranges of 100 km wide angle moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals the determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms the results from the esps show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7 3 7 4 km s this lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the moho the lower crustal lens of high velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas now seen as dipping reflectors on the margin

tropical midlatitude interactions in the indian and pacific sectors of the southern hemisphere abstract observations and three general circulation model gcw simulations are analyzed to study interannual processes and interactions involved in the tropical indian and pacific sectors and higher southern latitudes major features involving the observed eastward progression of the tropical convective maximum from the indian monsoon in northern summer to the australian monsoon and pacific in southern summer are represented in all three model simulations this suggests that the distribution of land and sea accounts for the observed seasonal position of the convective maximum in these regions however the south pacific convergence zone spcz is not present at any time of year in the slab ocean models due to the anomalously warm sea surface temperatures simulated in the equatorial pacific analyses of observed zonal mean 500 mb temperatures and sea level pressure from 10 to 80 s for warm event composites minus cold event composites representing two extremes of interannual variation show the period of

the hatton bank continental margin iii structure from wide angle obs and multichannel seismic refraction profiles summary in two recent papers normal incidence and expanding spread profiles esps shot across the hatton bank passive continental margin showed seaward dipping reflectors in the upper crust and elevated velocities in the lower crust the reflectors were interpreted as subaerial basalt flows and the high velocities as underplated or intruded igneous material in the lower crust a recent gravity model shows high densities in the lower crust supporting the velocity model in this study we present the forward modelling of two wide angle seismic refraction datasets shot perpendicular to strike and through the esp midpoints the final velocity model across the margin is consistent with all the seismic datasets but differs in some important respects from the esp based model a high velocity volcanic pile is outlined mid way up the continental slope and the inferred velocities in the lower cnst are slightly lower the differences between the two models can be explained by the three dimensional nature of the margin the esps were modelled assuming that the sub sediment structure was invariant along strike the final velocity model is used to obtain an estimate of the total thickness of melt accreted across the margin which is calculated to thicken from zero at hatton bank to about 12 km mid way down the continental slope at the bottom of the continental slope the adjacent igneous section forming the oceanic crust is about 18 km thick and thins rapidly northwestwards to 11 km as the oceanic crust thins its seismic velocity decreases this suggests that it is less mafic and that there is a post rift decrease in the percentage of mantle melting this decrease may be explained by a decrease in asthenospheric temperature and or by a reduction in volatile content

measuring thaw depth beneath peat lined arctic streams using ground penetrating radar in arctic streams depth of thaw beneath the stream channel is likely a significant parameter controlling hyporheic zone hydrology and biogeochemical cycling as part of an interdisciplinary study of this system we conducted a field investigation to test the effectiveness of imaging substream permafrost using ground penetrating radar gpr we investigated three sites characterized by low energy water flow organic material lining the streambeds and water depths ranging from 0 2 to 2 m we acquired data using a 200 mhz pulsed radar system with the antennas mounted in the bottom of a small rubber boat that was pulled across the stream while triggering the radar at a constant rate we achieved excellent results at all three sites with a clear continuous image of the permafrost boundary both peripheral to and beneath the stream our results demonstrate that gpr can be an effective tool for measuring substream thaw depth copyright 2005 john wiley sons ltd

north atlantic sea floor spreading rates implications for the tertiary development of inversion structures of the norwegian greenland sea the tertiary development of the norwegian continental margin was dominated by the opening of the arctic north atlantic ocean the correct identification of magnetic anomalies and their ages and the analysis of spreading rates during the formation of this ocean are important in understanding the development of the region and specifically the history of its passive margins three ocean domains the aegir reykjanes and mohns regions were investigated in an effort to understand the lateral changes in structural development of the passive margin after continental break up spreading rates generally slowed down from 2 cm a 1 after early eocene initiation of sea floor spreading to values around 0 5 cm a 1 in oligocene time an increase in spreading rates to around 1 cm a 1 coincided with the positioning of the iceland hotspot under the north atlantic mid ocean ridge at the same time the european plate changed its absolute plate motion from a north directed drift to a motion more towards the east the location of inversion structures in the voring and faeroes basin rather than in the more basin is related to differences in spreading rates the mohns and the reykjanes ridges produced more ocean floor than the aegir kolbeinsey ridges asymmetric ocean floor formation in the aegir ridge led to differential stress at the base of the lithosphere which probably explains the absence of inversion features in the more basin less mantle drag furthermore upper plate margins such as the voring basin and possibly the faeroe basin have a lower compressional strength than lower plate margins such as the more basin and therefore preferentially developed inversion structures along the transform boundaries separating the domains additional stress probably built up along extension of the transform zones into the extended continental crust this additional stress probably also assisted initiation of the inversion structures in the voring basin and the faeroes area the amplification of the inversion structures in the voring basin and the faeroes basin was subsequently caused by a variety of processes related to sedimentation and uplift erosion

mantle thermal structure and active upwelling during continental breakup in the north atlantic seismic reflection and refraction data acquired on four transects spanning the southeast greenland rifted margin and greenland iceland ridge gir provide new constraints on mantle thermal structure and melting processes during continental breakup in the north atlantic maximum igneous crustal thickness varies along the margin from s 30 km in the near hotspot zone 6 500 km from the hotspot track to v18 km in the distal zone 500 1100 km magmatic productivity on summed conjugate margins of the north atlantic decreases through time from 1800 300 to 600 50 km 3 km ma in the near hotspot zone and from 700 200 to 300 50 km 3 km ma in the distal zone comparison of our data with the british faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist joint consideration of crustal thickness and mean crustal seismic velocity suggests that along margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature the thermal anomaly vt at breakup was modest v100 125 c varied little along the margin and transient data along the gir indicate that the potential temperature anomaly 125 50 c and upwelling ratio v4 times passive of the iceland hotspot have remained roughly constant since 56 ma our results are consistent with a plume impact model in which 1 a plume of radius v300 km and vt of v125 c impacted the margin around 61 ma and delivered warm material to distal portions of the margin 2 at breakup 56 ma the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin and 3 by 45 ma both the remaining plume head and the distal warm layer were exhausted with excess magmatism thereafter largely confined to a narrow 6 200 km radius zone immediately above the iceland plume stem alternatively the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre existing thermal anomaly unrelated to the plume fl 2001 elsevier science b v all rights reserved

lateral variations in tectono magmatic style along the lofoten vesteralen volcanic margin off norway abstract the 400 km long passive continental margin west of the lofoten vesteralen archipelago off northern norway links the volcanic rifted voring margin and the sheared w barents sea margin multi channel seismic reflection profiles supplemented with crustal velocity gravity and magnetic anomaly data are used to outline the regional setting and main tectono magmatic features a well defined along strike margin segmentation comprising three segments characterized by distinct crustal properties structural and magmatic styles sediment thickness and post opening history of vertical motion is revealed the margin segments are governed by changes in fault polarity on late jurassic early cretaceous border faults and are separated by coeval cross margin transfer zones which acted as persistent barriers to rupture propagation and reflect the trend and character of older structural heterogeneities the transfer zones spatially correlate to small offset early opening oceanic fracture zones implying a structural inheritance from one rift episode to another culminating with lithospheric breakup at the paleocene eocene transition the pre seafloor spreading margin structural evolution is governed by the older predominantly late jurassic early cretaceous structural framework however the margin also provides evidence for mid and late cretaceous extension events that are poorly understood elsewhere off norway furthermore the lofoten vesteralen post breakup subsidence history contrasts with the adjacent margins reflecting breakup in thicker crust and a diminishing volume of high velocity lower crust emplaced during breakup