formation and deformation of hyperextended rift systems insights from rift domain mapping in the bay of biscay pyrenees the bay of biscay and the pyrenees correspond to a lower cretaceous rift system including both oceanic and hyperextended rift domains the transition from preserved oceanic and rift domains in the west to their complete inversion in the east enables us to study the progressive reactivation of a hyperextended rift system we use seismic interpretation gravity inversion and field mapping to identify and map former rift domains and their subsequent reactivation we propose a new map and sections across the system illustrating the progressive integration of the rift domains into the orogen this study aims to provide insights on the formation of hyperextended rift systems and discuss their role during reactivation two spatially and temporally distinct rift systems can be distinguished the bay of biscay parentis and the pyrenean basque cantabrian rifts while the offshore bay of biscay represent a former mature oceanic domain the fossil remnants of hyperextended domains preserved onshore in the pyrenean cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins reactivation initiated in the exhumed mantle domain before it affected the hyperthinned domain both domains accommodated most of the shortening the final architecture of the orogen is acquired once the conjugate necking domains became involved in collisional processes the complex 3 d architecture of the initial rift system may partly explain the heterogeneous reactivation of the overall system these results have important implications for the formation and reactivation of hyperextended rift systems and for the restoration of the bay of biscay and pyrenean domains

rock quality designation rqd time to rest in peace rock quality designation rqd was introduced by don deere in the mid 1960s as a means of using diamond core to classify rock for engineering purposes subsequently it was incorporated into the ro

40ar 39ar ages of intrusions in east greenland rift to drift transition over the iceland hotspot abstract sixteen 40 ar 39 ar ages are presented for alkaline intrusions to appraise prolonged post breakup magmatism of the central east greenland rifted margin the chronology of rift to drift transition and the asymmetry of magmatic activity in the northeast atlantic igneous province the alkaline intrusions mainly crop out in tectonic and magmatic lineaments orthogonal to the rifted margin and occur up to 100 km inland the area south of the kangerlussuaq fjord includes at least four tectonic lineaments and the intrusions are confined to three time windows at 56 54 ma 50 47 ma and 37 35 ma in the kangerlussuaq fjord which coincides with a major tectonic lineament possibly the failed arm of a triple junction the alkaline plutons span from 56 to 40 ma to the north and within the continental flood basalt succession alkaline intrusions of the north south trending wiedemann fjord kronborg gletscher lineament range from 52 to 36 ma we show that post breakup magmatism of the east greenland rifted margin can be linked to reconfiguration of spreading ridges in the northeast atlantic northwards propagation of the proto kolbeinsey ridge rifted the jan mayen micro continent away from central east greenland and resulted in protracted rift to drift transition the intrusions of the wiedemann fjord kronborg gletscher lineament are interpreted as a failed continental rift system and the intrusions of the kangerlussuaq fjord as off axis magmatism the post breakup intrusions south of kangerlussuaq fjord occur landward of the greenland iceland rise and are explained by mantle melting caused first by the crossing of the central east greenland rifted margin over the axis of the iceland mantle plume 50 47 ma and later by uplift associated with regional plate tectonic reorganization 37 35 ma the iceland mantle plume was instrumental in causing protracted rift to drift transition and post breakup tholeiitic and alkaline magmatism on the east greenland rifted margin and asymmetry in the magmatic history of the conjugate margins of the central northeast atlantic

a recipe for microcontinent formation accreted slivers of continental margins are common in the geologic record but the processes that lead to their formation are poorly understood we observe an association of plume related microcontinent isolation and subsequent long term asymmetries in oceanic crustal accretion based on four recent examples the seychelles in the indian ocean jan mayen in the norwegian greenland sea and the east tasman plateau and the gilbert seamount complex in the tasman sea these microcontinents formed by rerifting of a young continental margin 25 m y old in the vicinity of a mantle plume stem followed by asymmetric seafloor spreading two dimensional numerical stochastic basin modeling suggests that a yield strength minimum along the landward edge of a rifted margin thermally enhanced by heating from a mantle plume may cause a spreading ridge to jump onto this zone of weakness this action isolates a passive margin segment the association of large igneous provinces and microcontinents should be useful for identifying similar events in the geological record

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

toward 1 mgal accuracy in global marine gravity from cryosat 2 envisat and jason 1 more than 60 of the earth s land and shallow marine areas are covered by 2 km of sediments and sedimentary rocks with the thickest accumulations on rifted continental margins figure 1 free air marine gravity anomalies derived from geosat and ers 1 satellite altimetry fairhead et al 2001 sandwell and smith 2009 andersen et al 2009 outline most of these major basins with remarkable precision moreover gravity and bathymetry data derived from altimetry are used to identify current and paleo submarine canyons faults and local recent uplifts these geomorphic features provide clues to where to look for large deposits of sediments while current altimeter data delineate large offshore basins and major structures they do not resolve some of the smaller geomorphic features and basins yale et al 1998 fairhead et al 2001 improved accuracy and resolution is desirable to facilitate comparisons between continental margins as an exploration tool and to permit extrapolation of known structu

repeat ridge jumps and microcontinent separation insights from ne arabian sea abstract microcontinents separate due to ridge jumps associate either asymmetric sea floor spreading or plume ridge interactions india separated from seychelles at 64 ma by asymmetric sea floor spreading initially when the spreading centre in the mascarene basin jumped towards the indian sub continent between magnetic chrons c29 and c28 the subsequent tectonics is difficult to comprehend since laxmi ridge another microcontinent formed during the later phase most of the studies considered the laxmi ridge as a sliver others considered it to be oceanic crust high resolution deep 25 km seismic data reveals that i the ridge possesses 15 km deep sea ward dipping reflector sdr packages ii normal faulted rift valleys devoid of syn rift sedimentary packages and iii axial magma chambers 5 7 km beneath the ridge top additionally from 2d forward gravity models we deduce that the ridge most possibly comprises of high density oceanic crust we conclude the laxmi ridge to be indeed composed of oceanic crust and a fossil spreading centre we thus identified the ridge jumps and their relation to the seychelles microcontinent separation previous numerical models suggest that the time required for a ridge jump is controlled by magmatic heating spreading rate at the ridge and plate ages for repeated ridge jumps the additional factor is the dynamic relation between the plume and lithosphere in terms of melt transfer and heating we find that the medium spreading rates and high magmatic heating due to the reunion plume and young plates favoured rapid and repeated ridge jumps towards the plume

origin and evolution of the kolbeinsey ridge and iceland plateau n atlantic variations in crustal structure along the 700 km long krise7 refraction reflection and gravity profile straddling 66 5 n across the iceland shelf iceland plateau and western norway basin confirm that extinct spreading centers coexisted with the now extinct aegir ridge prior to the initiation of the kolbeinsey ridge at 26 ma the western 300 km of the profile across the iceland shelf formed by rifting at the kolbeinsey ridge whereas the eastern 400 km across the iceland plateau and the western norway basin formed by earlier rifting possibly containing slivers of older oceanic or continental crust rifted off the central e greenland margin along with the jan mayen ridge crustal thickness increases gradually across the iceland shelf from 12 to 13 km near the kolbeinsey ridge to 24 28 km near the eastern shelf edge decreasing abruptly across the shelf edge to 12 13 km the iceland plateau has crustal thickness ranging from 12 to 15 km decreasing to 5 8 km across the western norway basin and 4 5 km at the aegir ridge we suggest that high velocity lower crustal domes and corresponding gravity highs across the iceland plateau mark the location of extinct rift axes that coexisted with the aegir ridge similar lower crustal domes are associated with the currently active rift segments within iceland and the kolbeinsey ridge

treitel ridge a unique inside corner hogback on the west flank of extinct aegir spreading ridge norway basin abstract detailed shipborne mapping sidescan sonar swath bathymetry seismic reflection 3 5 khz profiling gravity and magnetics of the extinct aegir spreading axis ca 55 25 ma in the norway basin revealed a highly unusual in a mor setting narrow 130 km long asymmetrical basement hogback treitel ridge tr on the southern west flank of the extinct rift close to its junction with the iceland faeroe aseismic ridge and the putative paleo iceland hotspot center the ridge seems to continue north another 130 km in the form of a wider more irregular basement rise for a combined length of ca 260 km the southern narrow part of tr approximately following or slightly younger than chron 18n ca 39 ma attains a basement relief of ca 1000 m in the south its summit descending gradually 1 7 100 towards the ne although an east flank conjugate appears generally lacking along both northern and southern parts of tr asymmetry of the southern portion steeper slope facing towards the extinct rift valley suggests volcanic formation and axially dipping normal faulting at or near the active plate boundary the possibility of continental crust extending close to the southern end of tr may also be important because some of this crust may have been remelted at depth contributing magma to feed tr eruptions southern tr may record a brief 0 1 1 m y inside corner tectonic event probably a flexurally compensated normal faulted offset in effect a mega abyssal hill we suggest tr was probably not an expression of change in iceland hotspot activity or somehow related to nearby continental crust but rather represents the volcano tectonic response of the aegir accretion axis at the time it became the eastern edge of the new jan mayen micro plate to extinction of the mid labrador sea spreading center as proposed by nunns 1983 the annexation of the greenland plate by the north american plate forced a plate boundary reorganization east of greenland with northward propagation of the mid atlantic ridge rift along the greenland margin to form the jan mayen micro plate simultaneously two ca 107 trending transforms developed from the previously 146 trending faeroes fracture zone both tr formation and fracture zone trend changes may record aegir ridge response to this major plate kinematic reorganization because tr was formed during a short interval spreading along the mid labrador sea ridge may have ceased abruptly as well

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

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

asymmetric sea floor spreading caused by ridge plume interactions crustal accretion at mid ocean ridges is generally modelled as a symmetric process regional analyses however often show either small scale asymmetries which vary rapidly between individual spreading corridors or large scale asymmetries represented by consistent excess accretion on one of the two separating plates over geological time spans1 2 3 4 5 6 in neither case is the origin of the asymmetry well understood here we present a comprehensive analysis of the asymmetry of crustal accretion over the past 83 myr based on a set of self consistent digital isochrons7 and models of absolute plate motion8 9 we find that deficits in crustal accretion occur mainly on ridge flanks overlying one or several hotspots we therefore propose that asymmetric accretion is caused by ridge propagation towards mantle plumes or minor ridge jumps sustained by asthenospheric flow10 11 between ridges and plumes quantifying the asymmetry of crustal accretion provides a complementary approach to that based on geochemical12 and other geophysical data13 14 in helping to unravel how mantle plumes and mid ocean ridges are linked through mantle convection processes

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

early cenozoic crust at the norwegian continental margin and the conjugate jan mayen ridge multichannel seismic profiles at the voring plateau margin off norway and the northern jan mayen ridge have provided a framework for the early tertiary evolutionary history of these areas which were adjacent prior to the opening of the norwegian sea we propose that their paleogene evolution is quite similar and that the two acoustic basement reflectors ee and jo were formed in the early eocene by extrusion of flow basalts from the time of opening to about anomaly 23 time oceanic seafloor was generated by icelandic type spreading during that time the central jan mayen fracture was the main transform the seaward dipping reflector sequences are flow basalts generated by the icelandic spreading the existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation during the early eocene subaerial spreading episode which is related to the north atlantic volcanic province basalt flows also covered the adjacent thinned and heavily intruded continental crust it is proposed that the continent ocean boundary is located just landward of anomaly 24b a short distance seaward of the voring plateau escarpment reflector k is only recognized landward of this boundary in a region of transitional crust the escarpment experienced syndepositional faulting and the most elevated parts of the voring marginal high did not subside below shallow water depths before oligocene miocene time we suggest that the voring margin and the jan mayen ridge represent volcanic passive margins the evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust

asymmetric geophysical signatures in the greenland norwegian and southern labrador seas and the eurasia basin a thorough examination of geophysical data from the greenland norwegian sea eurasia basin and southern labrador sea shows significant asymmetry of several parameters basement topography adjusted for sediment loading free air gravity anomaly spreading half rate and seismicity with respect to crustal age 1 1 average zero age depth 0 57 m y b p depth of highest rift mountain summits and depth to magnetic basement 10 30 km from axis of mohns and knipovich ridges is less on the north american plate flanks the zero age depth asymmetry is 400 500 m for the eurasia basin 0 57 m y b p and for mohns ridge 57 22 m y b p and 150 200 m for younger mohns ridge crust 22 0 m y b p and for the extinct aegir ridge 57 27 m y b p there is little or no asymmetry in the labrador sea except near the extinct rift valley where the east flank is 150 300 m shallower magnetic depth to source computations provide an independent confirmation of basement asymmetry the belts 10 30 km from the axis of mohns and knipovich ridges are 100 150 m shallower on the west flank of these ridges the shallower ridge flank is topographically rougher so that average rift mountain summits are 300 m shallower on the west flanks of the mohns knipovich ridges a larger asymmetry than for average zero age depth the amount of topographic asymmetry is greatest near the mohns knipovich bend asymmetry appears to be greatest for ridges oriented normal to the spreading direction and less for oblique spreading 2 2 free air gravity anomaly asymmetries of 5 to 20 mgal sign indicates west flank is more positive are associated with topographic asymmetry at least within 10 15 m y of the axis of mohns and knipovich ridges gravity is reduced on the older flanks west of the extinct mid labrador ridge and east of mohns ridge asymmetric crustal layer thicknesses or densities provide one possible explanation although deep seated sources e g mantle convection unrelated to the crust cannot be excluded 3 3 spreading half rate was about 5 15 lower on the north american plate flanks of mohns ridge 57 35 m y and in the eurasia basin 0 57 m y thus the fast spreading flank tends to produce deeper smoother crust however topographic asymmetry cannot relate only to spreading rate asymmetry since for the young mohns ridge crust 9 m y b p faster spreading and higher topography are both associated with the west flank 4 4 mid plate seismicity is higher on the eurasia eastern flank of mohns and knipovich ridge but this effect may be unrelated to the other three the fluid dynamical model of stein et al correctly explains the sense of spreading rate asymmetry the north american plate moving faster over mantle is growing more slowly however the other asymmetries and their causal relationships remain theoretically unexplained

resolution of direction of oceanic magnetic lineations by the sixth generation lithospheric magnetic field model from champ satellite magnetic measurements the champ satellite continues to provide highly accurate magnetic field measurements from decreasing orbital altitudes 350 km at solar minimum conditions using the latest 4 years 2004 2007 of readings from the champ fluxgate magnetometer including an improved scalar data product we have estimated the lithospheric magnetic field to spherical harmonic degree 120 corresponding to 333 km wavelength resolution the data were found to be sensitive to crustal field variations up to degree 150 down to 266 km wavelength but a clean separation of the lithospheric signal from ionospheric and magnetospheric noise sources was achieved only to degree 120 this new mf6 model is the first satellite based magnetic model to resolve the direction of oceanic magnetic lineations revealing the age structure of oceanic crust

the rotations opening the central and northern atlantic ocean compilation drift lines and flow lines we provide an up to date compilation of euler rotations that model the evolution of the central and northern atlantic ocean table 1 the data basis forms seafloor spreading magnetic anomalies of the atlantic we checked the published rotations and selected those that form a consistent model the increments of the euler rotations going back in time from magnetic anomaly to magnetic anomaly can be illustrated by chains of points on drift lines that are paths of motions from continent to continent along these paths the continents bordering the atlantic ocean can be moved back to their mesozoic position within pangea other figures exhibit the early rifting of the north atlantic the drift of iberia and the evolution of the greenland ellesmere region the points on the drift lines do not correspond directly to the lines of magnetic anomalies or their picks displayed today symmetrically in the atlantic ocean to acquire correspondence symmetric flow lines are constructed analogous to the spreading procedure but points on the flow lines constructed by half of the increments partially also deviate from the expected symmetric position and in this way quantify displacements or jumps of the axis of rifting or spreading most of the selected rotations are from the excellent analyses of previous work essential deviations from published rotations are the m 0 rotations of eurasia and of the porcupine unit with respect to north america eur nam and por nam they lead to a better coincidence between the back rotated m 0 magnetic anomalies in the bay of biscay on the one side and a change of the first transform motions between greenland and svalbard on the other side through this explanation an overlap in pangea sw of svalbard is avoided and transform motions instead of strong extension are predicted some additional data are needed to complete the model the earliest part of the path of iberia to north america iba nam up to m 4 is calculated assuming that iberia moved parallel to the african plate though with slower spreading rates the evolution of the central and north atlantic ocean system is described in short this model of the central and north atlantic was produced with the primary intention of clearing and fixing the positions of africa iberia and eurasia as a framework for an improved reconstruction of the western tethys evolution

variations in amount and direction of seafloor spreading along the northeast atlantic ocean and resulting deformation of the continental margin of northwest europe the ne atlantic ocean opened progressively between greenland and nw europe during the cenozoic seafloor spreading occurred along three ridge systems the reykjanes ridge south of iceland the mohns ridge north of the jan mayen fracture zone jmfz and the aegir and kolbeinsey ridges between iceland and the jmfz at the same time compressional structures developed along the continental margin of nw europe we investigate how these compressional structures may have resulted from variations in the amount and direction of seafloor spreading along the ridge system assuming that greenland is rigid and stationary we have used a least squares method of palinspastic restoration to calculate differences in direction and rate of spreading along the reykjanes kolbeinsey aegir and mohns ridges the restoration generates relative rotations and displacements between the oceanic segments and predicts two main periods of left lateral strike slip along the main oceanic fracture zones 1 early eocene to late oligocene along the faeroe fracture zone and 2 late eocene to early oligocene and during the miocene along the jmfz such left lateral motion and relative rotation between the oceanic segments are compatible with the development of inversion structures on the faeroe rockall plateau and norwegian margin at those times and probably with the initiation of the fugloy ridge in the faeroe shetland basin during the eocene and oligocene the iceland mantle plume appears to have been in a position to generate differential seafloor spreading along the ne atlantic and resulting deformation of the european margin

crust generated above the iceland mantle plume from continental rift to oceanic spreading center since the north atlantic continental breakup in the early tertiary the process of rifting above a mantle plume has produced large thicknesses of igneous crust we report results of an integrated offshore onshore seismic study of the crust and upper mantle along a transect of the aseismic faroe iceland ridge between the continental fragment beneath the faroe islands and the present day spreading center in northeast iceland normal incidence seismic data provide an image of the uppermost crust which is complemented by a velocity model from streamer refraction analyses these data together image four sedimentary basins up to 400 m deep along the crest of the ridge the streamer refraction velocity model in turn forms the uppermost section of a full crustal velocity model derived from travel time modeling of air gun and explosive data the compressional to shear wave velocity ratio in the crust of the faroe iceland ridge is 1 77 0 02 similar to that of eastern iceland the moho beneath the faroe iceland ridge lies at a depth of 25 30 km crust of 25 30 km thickness requires an upper mantle potential temperature elevated 200 250 c above normal if formed by passive adiabatic decompression melting of the mantle or a somewhat less elevated temperature if as is likely there is a component of active convection in the underlying mantle plume core

quantification and restoration of extensional deformation along the western iberia and newfoundland rifted margins 1 many recent papers describe the structure of the iberia and newfoundland rifted margins however none of them propose kinematic restorations of the complete rift system to quantify the amount of extension necessary to exhume mantle and initiate seafloor spreading in our study we use two pairs of cross sections considered as conjugate lines one across the galicia bank flemish cap and the other across the southern iberia abyssal plain flemish pass both transects have been imaged by reflection and refraction seismic methods and have been drilled during ocean drilling program legs 103 149 173 and 210 drilling penetrated parts of the rift stratigraphy and the underlying basement the cross sections can therefore be considered as the best documented conjugate transects across present day hyperextended magma poor rifted margins the aim of this paper is threefold 1 provide a detailed description of the crustal architecture of the two conjugate sections 2 define the extensional structures and their ages and 3 quantify the amount of strain and strain rate accommodated along these lines this paper proposes a quantitative description of extension along the iberia newfoundland rift system and discusses the limitations and problems in quantifying extensional deformation along hyperextended rifted margins

conjugate volcanic rifted margins seafloor spreading and microcontinent insights from new high resolution aeromagnetic surveys in the norway basin we have acquired and processed new aeromagnetic data that cover the entire oceanic norway basin located between the more volcanic rifted margin and the jan mayen microcontinent jmmc the new compilation allows us to revisit the structure of the conjugate volcanic rifted margins and the spreading evolution of the norway basin from the early eocene breakup time to the late oligocene when the aegir ridge became extinct the volcanic margins in a strict sense that formed before the opening of the norway basin have been disconnected with the previous jurassic mid cretaceous episode of crustal thinning we also show evidence of relationships between the margin architecture the breakup magmatism distribution along the continent oceanic transition and the subsequent oceanic segmentation the norway basin shows a complex system of asymmetric oceanic segments locally affected by episodic ridge jumps the new aeromagnetic compilation also confirms that a fan shaped spreading evolution of the norway basin was clearly active before the cessation of seafloor spreading and extinction of the aegir ridge an important mid eocene kinematic event at around magnetic chron c21r can be recognized in the norway basin this event coincides with the onset of diking and increasing rifting activity and possible oceanic accretion between the proto jmmc and the east greenland margin it led to a second phase of breakup and microcontinent formation in the norwegian greenland sea 26 myrs later in the oligocene

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

pacific decadal variability paced by rossby basin modes abstracta systematic study is presented of decadal climate variability in the north pacific in particular the hypothesis is addressed that oceanic rossby basin modes are responsible for enhanced energy at decadal and bidecadal time scales to this end a series of statistical analyses are performed on a 500 yr control integration of the community climate system model version 3 ccsm3 in particular a principal oscillation pattern pop analysis is performed to identify modal behavior in the subsurface pressure field it is found that the dominant energy of sea surface temperature sst variability at 25 yr the model equivalent of the pacific decadal oscillation cannot be explained by the resonant excitation of an oceanic basin mode however significant energy in the subsurface pressure field at time scales of 17 and 10 yr appears to be related to internal ocean oscillations however these oscillations lack the characteristics of the classical basin modes and must either be deformed beyond recogni

mesozoic detachment faulting between greenland and norway inferences from jan mayen fracture zone system and associated alkalic volcanic rocks the spatial relation between the jan mayen fracture zone system and two occurrences of early tertiary mantle derived volcanic rock complexes indicates that an older continental fault system may have had hidden extensions in the deep lithosphere this is taken as indirect evidence that the inferred fracture zone precursor may have acted as a transfer fault system linking rifts formed by opposing simple shear detachment faulting through the lithosphere we infer that this contributed to continental rifting between greenland and norway in mesozoic to paleocene time before sea floor spreading began in the norwegian greenland sea region

late cretaceous cenozoic deformation of northeast asia abstract the plate tectonic paradigm implies rigid plates and narrow plate boundaries in contrast diffuse plate boundaries are common both in the oceans and continents r g gordon annu rev earth planet sci 26 1998 615 642 and their history is difficult to constrain especially in remote tectonically complex areas such as northeast asia m e chapman s c solomon j geophys res 81 1976 921 930 here we show how extensive north atlantic marine magnetic r macnab et al eos 76 1995 449 458 and gravity data d t sandwell w h f smith j geophys res 102 1997 10039 10054 can be used to unravel with tight confidence limits successive periods of deformation over 80 million years along the diffuse continental eurasian north american plate boundary a period of compression in the late cretaceous 14 mm yr in the laptev sea to 20 mm yr in kamchatka led to thrusting in the verkhoyansk mountains and was followed by extension from 68 to 40 ma when 400 km of extension was accommodated by the formation of a series of grabens including the moma rift system since 40 ma time varying compression and transpression along the moma rift system created strike slip faults thrusts and folds at rates up to 6 3 mm yr in the laptev sea region 600 km of extension from latest late cretaceous to present created the laptev sea and lena rift systems the deformation predicted by our model fits most geological features formed in the laptev sea and central northeast asia during late cretaceous cenozoic times the most recent deformation late miocene pliocene is not very well constrained since our model lacks data younger than 11 ma the deformation that occurred in kamchatka reflects a complex tectonic setting and our model s predictions are only tentative

segmentation in gravity and magnetic anomalies along the u s east coast passive margin implications for incipient structure of the oceanic lithosphere segmentation is a characteristic feature of seafloor spreading along the global mid ocean ridge system while segmentation of active spreading centers has been the focus of much recent research the process by which a rifted continental margin develops into a segmented mid ocean ridge is still poorly understood in this study we investigate the segmentation character of the u s east coast margin through a modeling study of the margin basement structure magnetics and gravity anomalies the east coast margin is of particular interest because it is one of several rifted continental margins that display thick sequences of high seismic velocity igneous crust presumably formed during rifting the east coast magnetic anomaly ecma a distinct total field magnetic high running offshore along the margin is commonly located seaward of the thickest sections of the high velocity crust and displays segmentation on length scales 100 120 km similar to the segmentation observed at the mid atlantic ridge mar isostatic gravity anomalies were calculated by removing from free air gravity the predicted effects of seafloor sediments and a crust mantle model assuming local isostatic compensation the resultant residuals show a corridor of high anomaly running along the margin situated close to the maximum thickness of the high seismic velocity crust as determined from the two available seismic refraction lines reduction to the pole r t p of the total field magnetic anomaly shows that after the removal of skewness from the ecma the location of the isostatic gravity high is closely correlated to the ecma the isostatic gravity high is also segmented but in two distinct wave bands 100 150 km and 300 500 km the short wavelength 100 150 km segmentation in the r t p magnetic and isostatic gravity anomalies is similar in wavelength to segmentation in magnetization and mantle bouguer anomaly observed along the present day mar the 300 500 km segmentation in the along margin isostatic gravity anomaly is similar in length scale to both intermediate wavelength tectonic segmentation observed in the south atlantic and variations in lithospheric strength observed along the african margin furthermore two of the intermediate wavelength 300 500 km isostatic gravity lows correspond to the early traces of the kane and atlantis fracture zones suggesting that these two fracture zones may define boundaries of a single tectonic corridor in the north atlantic we hypothesize that the direct cause of the intermediate wavelength segmentation may be along margin variations in both the amount of underplated igneous crust and the strength of the lithosphere although the relative importance of these two effects remains unresolved our results imply that segmentation is an important feature of margin development and that segmentation at mature oceanic spreading centers may be directly linked to segmentation during continental rifting

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

north atlantic ocean deep water processes and depositional environments a study of the cenozoic norway basin abstract despite the enormous areas deep water basins occupy in modern oceans our knowledge about them remains poor at depths of greater than 2000 m the cenozoic norway basin in the northernmost part of the atlantic ocean is one such basin interpretation of 2d multichannel seismic data suggests a three stage evolution for the norway basin 1 eocene pliocene this time period is characterised by deposition of ooze rich sediments in a widening and deepening basin 2 early middle pleistocene a significant shift in sedimentary processes and depositional environments took place in the early pleistocene mass failures initiated on the norwegian continental slope and three early and middle pleistocene slide debrites with maximum thicknesses of 600 m and sediment volumes of up to 25 10 3 km 3 were deposited with ages estimated at c 2 7 1 7 ma 1 7 1 1 ma and 0 5 ma these slide deposits are among the largest identified worldwide and among the oldest mapped along the entire ne atlantic continental margin 3 late pleistocene present since c 0 5 ma the norway basin has been effected by glacigenic debris flows the storegga slide and hemipelagic glacimarine sedimentation these sedimentary processes were active during a time of repeated shelf edge ice advances along the ne atlantic continental margin this study shows that deep water basins represent dynamic depositional environments reflecting regional tectonic and climatic changes trough time

magmatic breakup as an explanation for magnetic anomalies at magma poor rifted margins during continental breakup the onset of seafloor spreading is thought to be marked by the first occurrence of a magnetic anomaly analysis of seismic and magnetic data from the iberia newfoundland continental rift system suggests that the first magnetic anomaly observed here instead represents a magmatic event that pre dates seafloor spreading

structure of the more basin mid norway continental margin abstract in the more basin offshore mid norway the basis of the cretaceous sequence reaches depths of more than 5000 m the basin is limited on its eastern margin by a system of extensional faults which probably were initiated in the triassic and which were active periodically throughout the aptian the internal basin floor is divided into subbasins by structural highs which are rotated fault blocks presumably rooted in the basement the main structuring of the basin started in late triassic and was followed by accelerating subsidence in the early cretaceous subsidence was interrupted by a mild phase of inversion in the latest cretaceous and a more pronounced inversional phase in the eocene miocene the latter developed folds with wavelengths in the order of kilometers and local low angle reverse faults and the inversion of depocenters

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

styles of extension offshore mid norway and implications for mechanisms of crustal thinning at passive margins 1 eocene magmatic breakup along the mid norway rifted margin was preceded by extreme jurassic cretaceous crustal thinning in a magma poor environment along the se borders of the rift top basement detachment faults with heaves on the order of 15 40 km evolved in at least two stages to become the boundaries between moderately thinned 20 30 km thick crust and a 100 200 km wide highly extended area with crustal thicknesses generally between 2 and 12 km under the present day more and voring basins in the footwalls of the basin flank detachments lower and middle crust was exhumed in extensional domes that became incised by a younger set of normal faults under the most highly thinned areas a more distal set of deep seated basin floor detachments incised and extended remnant crust and probably the upper mantle leaving as little as 5 km of continental crust to be preserved under thick synrift and postrift deposits we suggest that basin flank detachments such as the ones described above hold the potential to reduce the crustal thickness down to the critical value required for embrittlement of continental crust thus they prepare the ground for incision by the basin floor detachments which may become responsible for exhumation of deep crust or continental mantle if extension is allowed to proceed

a gravity and magnetic anomaly study of the extinct aegir ridge norwegian sea in 1990 the u s naval research laboratory mapped the extinct aegir ridge in the norwegian sea with seamarc ii side scan sonar hydrosweep multibeam seismics gravity and magnetics onboard r v maurice ewing we analyzed the 92 center beam bathymetry gravity and magnetic profiles in this study to examine slow spreading ridge processes and cessation of spreading about 25 ma in bathymetric expression the rift valley deepens and narrows southward while the rift mountain summits become higher south of 67 n the western rift mountains and flanks stand higher than the eastern mountains and flanks free air anomalies along the ridge axis show undulating intermediate wavelength 100 km anomalies superimposed on a regional decrease of the rift valley negative from 0 mgal in the north to 55 mgal in the south after removal of sediment effects results from three dimensional gravity modeling imply a variation of crustal thickness a new rotation pole for the movement of the jan mayen microplate relative to the eurasia plate from 50 ma to 25 ma was found at 64 34 n 13 w based on magnetic lineations and inferred discontinuity traces a plot of angular separation between significant anomaly peaks versus absolute age suggests an opening rate of 2 0 m y 36 ma followed by gradual deceleration to 0 at 25 ma full spreading rate between chrons c13n and c21n ranged from 13 mm yr in the north to 8 mm yr in the south

age of seychelles india break up many continental flood basalt provinces are spatially and temporally linked with continental break up establishing the relative timing of the two events is a key step in determining their causal relationship here we investigate the example of the deccan traps and the separation of india and the seychelles whilst there has been a growing consensus as to the age of the main phase of the deccan emplacement 65 5 1 ma chron 29r the age of the rifting has remained unclear we resolve this issue through detailed seafloor magnetic anomaly modeling supported by wide angle and reflection seismic results of the north seychelles and conjugate laxmi ridge gop rift margins and geochemistry and 40ar 39ar geochronology of rocks from the north seychelles margin we show that syn rift volcanics offshore the seychelles islands in the form of seaward dipping reflectors were most likely erupted during chron 28n and the first organized seafloor spreading at the carlsberg ridge also initiated during this chron at 63 4 ma the severing of the seychelles occurred by a south eastward ridge propagation that was completed by the start of chron 27n 62 ma a brief pre 28r phase of seafloor spreading occurred in the gop rift possibly as early as 31r 32n 71 ma initial extension at the margin therefore preceded or was contemporaneous with the deccan emplacement and separation of the seychelles was achieved less than 3 5 ma afterwards this is the shortest time interval between flood basalt emplacement and break up yet reported for any continental flood basalt rifted margin pair a contributing factor to the apparently short interval in the deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere however we conclude that external plate boundary forces rather than the impact of a mantle plume were largely responsible for the rifting of the seychelles from india

imaging the pulsing iceland mantle plume through the eocene the temperature of mantle plumes may vary on geologic time scales from a few million years to tens of millions of years in special circumstances such as near iceland in the north atlantic where the plume underlies an oceanic spreading center temporal variations in the oceanic crustal thickness provide a sensitive proxy for the mantle temperature if as is likely the crustal thickness is controlled primarily by passive decompression of mantle rising beneath the spreading center we show from both seismic reflection imaging and wide angle ocean bottom seismometer data from the norwegian sea that the temperature of the iceland mantle plume decreased by 50 c over the first 5 m y following continental breakup and then oscillated by 25 c over an 3 m y period similar temperature variations on a 3 6 m y time scale creating strong lineations in the gravity field are inferred from the regional north atlantic they occur both in the period immediately following breakup and at the present day reykjanes ridge south of iceland where they create v shaped ridges as the mantle thermal anomalies propagate away from the center of the plume beneath iceland we propose that mantle plume temperature variations of 25 c have occurred in the iceland plume with a similar amplitude and frequency since at least 49 ma and are likely to be a feature of all mantle plumes

interrelation between rifting faulting sedimentation and mantle serpentinization during continental margin formation including examples from the norwegian sea the conditions permitting mantle serpentinization during continental rifting are explored within 2 d thermotectonostratigraphic basin models which track the rheological evolution of the continental crust account for sediment blanketing effects and allow for kinetically controlled mantle serpentinization processes the basic idea is that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur the isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions a systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs increased sedimentation rates shift this critical stretching factor to higher values as sediment blanketing effects result in higher crustal temperatures sediment supply has therefore through the temperature dependence of the viscous flow laws strong control on crustal strength and mantle serpentinization reactions are only likely when sedimentation rates are low and stretching factors high in a case study for the norwegian margin we test whether the inner lower crustal bodies lcb imaged beneath the more and voring margin could be serpentinized mantle multiple 2 d transects have been reconstructed through the 3 d data set by scheck wenderoth and maystrenko 2011 we find that serpentinization reactions are possible and likely during the jurassic rift phase predicted thicknesses and locations of partially serpentinized mantle rocks fit to information on lcbs from seismic and gravity data we conclude that some of the inner lcbs beneath the norwegian margin may be partially serpentinized mantle

origin and models of oceanic transform faults abstract mid ocean ridges sectioned by transform faults represent prominent surface expressions of plate tectonics a fundamental problem of plate tectonics is how this pattern has formed and why it is maintained gross scale geometry of mid ocean ridges is often inherited from respective rifted margins indeed transform faults seem to nucleate after the beginning of the oceanic spreading and can spontaneously form at a single straight ridge both analog and numerical models of transform faults were investigated since the 1970s two main groups of analog models were developed thermomechanical freezing wax models with accreting and cooling plates and mechanical models with non accreting lithosphere freezing wax models reproduced ridge ridge transform faults inactive fracture zones rotating microplates overlapping spreading centers and other features of oceanic ridges however these models often produced open spreading centers that are dissimilar to nature mechanical models on the other hand do not accrete the lithosphere and their results are thus only applicable for relatively small amount of spreading three main types of numerical models were investigated models of stress and displacement distribution around transforms models of their thermal structure and crustal growth and models of nucleation and evolution of ridge transform fault patterns it was shown that a limited number of spreading modes can form transform faults microplates overlapping spreading centers zigzag ridges and oblique connecting spreading centers however the controversy exists whether these patterns always result from pre existing ridge offsets or can also form spontaneously at a single straight ridge during millions of year of accretion therefore two types of transform fault interpretation exist plate fragmentation structures vs plate accretion structures models of transform faults are yet relatively scarce and partly controversial consequently a number of first order questions remain standing and significant cross disciplinary efforts are needed in the future by combining natural observations analog experiments and numerical modeling

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

redistribution of the lithosphere deformation by the emplacement of underplated mafic bodies implications for microcontinent formation abstract sedimentary basin migration and microcontinent formation could be the result of deformation redistribution in which the emplacement of an underplated mafic body upmb may play an important role in this study a 2d finite element model is used to examine the redistribution of the deformation that can be triggered by a upmb it is shown that three modes of deformation redistribution can exist 1 a shift completed mode where the deformation is completely redistributed into the newly weakened region 2 a transition mode where the deformation is redistributed but extension is accommodated by thinning in both regions 3 a shift failed mode where the deformation is not redistributed whether or not the deformation is redistributed depends on the configuration of the upmb and on the initial rheological heterogeneity in the initially deformed region however it becomes difficult for any upmb to initiate the redistribution of the deformation once the stretching factor in the first deformed region exceeds a critical value the microcontinent formations observed in the indian ocean in the norwegian greenland sea in the red sea afar region and in the western mediterranean are particularly discussed in relation to the transition mode providing some important geodynamic implications

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

fracture zones in the north atlantic from combined geosat and seasat data we present a new tectonic fabric map of the northern and central north atlantic ocean floor from 10 s to 66 n based on combined geosat and seasat altimetry data short wavelength vertical gravity along track was derived by hilbert transforming and high pass filtering geosat and seasat deflection of the vertical profiles the along track gravity data were used in conjunction with magnetic anomaly data to locate a large number of fracture zones as well as extinct spreading ridges and v shaped structures we traced many fracture zones more continuously and closer to adjacent continental margins than was previously possible we also mapped a number of unnamed fracture zones and fracture zone segments many of which were previously uncharted we have evaluated the agreement between the mapped fracture zones and synthetic flow lines as predicted from recent plate models for the opening of the north central north and south atlantic most fracture zones in this area correlate quite well with synthetic flow lines supporting the proposition that geosat seasat altimetry data serve as a useful tool to map fracture zones where insufficient bathymetric data are available many fracture zones including some small offset fracture zones in the central north atlantic have been stable and follow tectonic flow lines for tens of millions of years however many other fracture zones do not reflect a railroad track pattern of plate motions the geometry of many small and medium offset fracture zones indicates that the ridge axis discontinuities that produce them can migrate along the ridge causing off axis traces that deviate from tectonic flow lines velocity space diagrams constructed using a recent relative and absolute present day plate motion model indicate that some low angle v shaped troughs and ridges are likely formed by migration of the plate boundary relative to a mesospheric reference frame other low angle v shaped structures reflect slow migrations oscillations of small offset ridge axis discontinuities that are distinct from both tectonic flow lines and v shapes reflecting absolute plate motions high angle v shaped structures are the result of fast migrating ridge axis discontinuities propagating rifts

crustal structure and evolution of the outer more margin ne atlantic abstract five regional ocean bottom seismic profiles from the northwestern part of the more margin ne atlantic have been modeled with regard to crustal structure and the interpretations have been supported by the use of multi channel seismic and potential field data the chi squared values uncertainty for the studied profiles are generally in the range 1 3 and the uncertainty in the moho depth is estimated to 1 km the continent ocean transition is found to be relatively sharp 10 40 km wide expressing a change in physical properties corresponding to felsic continental crust along the landward part of the marginal high vp 6 2 6 5 km s and thick mafic oceanic crust further northwestwards the early eocene breakup was associated with excessive magmatism and the models document direct continuity between this magmatism and a lower crustal high velocity layer in the more basin vp 7 1 7 3 km s suggesting a genetic link the jan mayen lineament is observed across the entire study area as northeastward termination of the lower crustal body as well as structuring in the top of the crystalline basement and the moho we present a speculative model involving lower crustal bodies of eclogites vp 8 km s inferred in the easternmost part of the more basin linking the jan mayen lineament to the post caledonian nordfjord sogn detachment

lower crustal bodies in the more volcanic rifted margin geophysical determination and geological implications abstract understanding nature structure and age of lower crustal bodies lcbs and their relation to the crustal structure of the more margin mid norwegian margin and in a more general way of magma rich rifted margins is a key issue to decipher the tectono magmatic processes found in volcanic rifted margins in light of 2d potential field modelling combined with reflection and refraction seismic data we reinvestigated the crustal nature of the more margin and adjacent jan mayen corridor in the proximal domain our study shows that the lcbs most likely represent inherited crustal bodies and not necessarily rift related serpentinised mantle as previously proposed to fit all geophysical observations both lower and middle crustal layers need to be preserved over a large part of the more basin for the distal margin the interpretation of the lcbs is more difficult our preferred interpretation is that they are mainly made of boudins of hyper extended pre rift lower continental crustal rocks more or less intruded by early tertiary magmatic material our seismic magnetic and gravity data does not easily support large scale exhumation of serpentinised mantle in the inner and is unlikely in the outer parts of the more basin the deep structures of the mid norwegian magma rich rifted margin result from the poly phase stretching and thinning of complex inherited crustal structures locally intruded by early tertiary magmatic material

reevaluation of magnetic chrons in the north atlantic between 35 n and 47 n implications for the formation of the azores triple junction and associated plateau 1 in this paper we present a new magnetic compilation for an area of the north atlantic located between 35 n and 47 n and up to anomaly 33r we also present a strategy to pick magnetic isochrones and compute finite rotation poles this technique is based on a continuous reduction to the pole technique and some basic assumptions regarding the direction of the remanent magnetization vector a cost function that measures the misfit between interpreted and rotated isochrones and the systematic exploitation of the parameter space is used to compute the best set of finite eulerian rotations for the chrons 5 6 6c 11 12 13 18 20 21 22 23 24 25 26 28 29 30 32 33 and 33r this set of chrons and poles is used to discuss the evolution of the north atlantic close to iberia and in particular the onset and early development of the azores triple junction area we show that the relative motion between the eurasian and the african plates can be coherently described in terms of rigid plate kinematics respecting both the anomalies shapes and the precise location of the main structural elements of the area the pico fracture zone the east azores fracture zone and the gloria fault we distinguish from the magnetic point of view two different areas of the azores plateau the south azores domain where almost undisturbed nnw magnetic lineations can be found and the azores domain close to the topographic highs and with no systematic magnetic stripping with the exception of a few recent lineations probably matuyama and brunhes we present an approximate reconstruction of the plate configuration after chron 18 to conclude that the attachment of iberia to eurasia was younger than previously thought lower miocene triggering the formation of the azores domain in which stretching took place essentially in the last 20 ma at an average rate of 3 8 mm a and progressively attaching the south azores domain to the african plate by a northward progression of the triple junction