offshore granulites from the bay of biscay margins fission tracks constrain a proterozoic to tertiary thermal history over short distances partly as pebbles from albian conglomerates we chose the granulite samples for our fission track study anticipating that they have recorded most of the history of the continental crust in the area and because the origin and emplacement of these rocks is still not clear the results have important implications and show that fission track studies on offshore basement have great potential for furthering our under standing of continental margin pro cesses abstract zircon and apatite fission track ages were determined on granulites dredged along the bay of biscay margins a sample from ortegal spur iberia margin yielded 725 67 ma zircon a sample from le danois bank iberia margin yielded 284 58 ma zircon indicating post variscan cooling apatite from this sample gave 52 2 ma interpreted as final cooling after pyrenean thrust imbrication two other samples from le danois bank have early cretaceous apatite ages 138 7 and 120 8 ma interpreted to result from exhumation during rifting finally a granulite from goban spur armorican margin gave 212 10 ma apatite coinciding with a precursory rifting phase together with published radiometric results these data indicate a precambrian high grade terrane at the site of the current margins the distribution of the granulites on the seafloor reflects tectonic and erosional processes related to a mesozoic rifting and b early tertiary incipient subduction of the bay of biscay beneath iberia

cooling and uplift patterns in the lepontine alps south central switzerland and an age of vertical movement on the insubric fault line 96 new fission track ft apatite and zircon k ar and rb sr biotite and muscovite ages are presented for 19 samples mainly acid gneisses from a 40 km traverse through the lepontine alps in the maggia valley south central switzerland plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample the entire profile shows a fairly uniform late neogene recent mean uplift rate of 0 5 mm a confirmed by a gradient of ft apatite age with elevation cooling from higher temperatures occurred earlier in the south where uplift rates of 2 2 mm a in the steep belt root zone indicate 9 km early miocene uplift of the northern pennine block this uplift started before 23 ma and is interpreted as resulting from a major phase of backthrusting along the insubric line and as dating the formation of the mylonite belt estimated cooling rates constrain the timing of lepontine mid tertiary metamorphism 3 schematic models are proposed which also consider published rb sr k ar mica and hornblende and u pb monazite ages slow cooling differential initial heating and subsequent cooling of different parts of the central alps and post 38 ma cooling with syntectonic metamorphism at 27 ma are postulated as alternative interpretations of isotopic data and geologic evidence from extrapolation between k ar and rb sr mica ages and apatite ft ages 240 50 c is proposed as the closure temperature for the retention of fission tracks in zircon

u pb geochronology and hf nd isotope compositions of the oldest neoproterozoic crust within the cadomian orogen new evidence for a unique juvenile terrane new u pb dates combined with nd and hf isotopic data from rocks within the port morvan area of the baie de st brieuc region of brittany identify a unique portion of the neoproterozoic cadomia terrane two gneisses near port morvan yielded u pb dates of 754 6 0 8 ma and 746 0 0 9 ma ages that are more than 130 myr older than the oldest units formed during the main phase of early cadomian magmatism two trondhjemite boulders from the monogenetic facies of the cesson conglomerate yielded identical ages of 665 2 0 5 ma and 665 5 0 7 ma and a cobble from the polygenetic facies yields a 207pb 206pb date of 637 2 ma individual detrital zircons from a sandstone associated with the cesson conglomerates yield concordant u pb dates ranging from 650 3 ma to 624 1 0 6 ma initial nd values for the rocks in this region range from 5 0 to 6 6 indicative of a substantial input from depleted mantle initial hf values determined on zircons from these neoproterozoic rocks including the detrital zircons range from 6 7 to 14 5 consistent with the nd isotopic results maximum initial hf values for two 2 ga icartian gneisses considered basement to cadomia average 8 4 and 8 7 in contrast to the results of the port morvan rocks 616 608 ma syn tectonic intrusions from normandy and the british channel islands all have negative initial nd values 10 4 to 8 3 consistent with significant contamination by ancient crust such as the 2 ga gneisses the oldest arc related magmas should have interacted most extensively with cadomian basement buffering younger mantle derived magmas that were generated in subsequent magmatic episodes the rocks within the port morvan region are thus inconsistent as examples of the earliest cadomian intrusions as they show no evidence of interaction with 2 ga basement instead the older ages and mantle like isotopic composition of these rocks suggest they are part of an independent terrane that formed prior to and independently from the cadomian arc possible terrane scale structural boundaries have recently been identified including the newly recognized port morvan thrust fault and the nw dipping main cadomian thrust

cooling history of granulite samples from the ocean continent transition of the galicia margin implications for rifting abstract zircon and apatite fission track ages were obtained on twogranulite samples that were recovered from the sea floor in theocean continent transition area of the galicia margin northatlantic using the french submersible nautile zircon agesindicate that the rocks cooled through about 2508cincarboniferous to early permian time 307 42 ma and287 35 ma hence the granulites do not represent the preriftlower crust but were in an upper crustal position long beforeriftingstarted apatitesyieldedearlycretaceousages 126 6 7myr and 129 13 4 myr indicating cooling through 90 308ccoeval with the main rifting phase that preceded continentalbreakup we assume that the granulite samples originate from atectonic breccia cropping out near one of the sample locations this breccia formed along a synrift detachment accommodatingcontinentalbreakupandfinalexhumationofthegaliciamargin speridotite ridge terra nova 10 96 100 1998 ahedbhedcheddhedref markerfig markertable markerref end ref start correspondence tel fax 41 61 2673610 3613 e mail fuegenschuh ubaclu unibas ch

new evidence for the origin of the porcupine median volcanic ridge early cretaceous volcanism in the porcupine basin atlantic margin of ireland 1 two dimensional pre stack depth migrated seismic reflection data gravity and velocity models are used to assess the nature and origin of a prominent buried ridge the porcupine median volcanic ridge pmvr within the porcupine basin offshore ireland the debate on the origin of the pmvr during the past 30 years has followed the evolution of the concept of continental margin genesis in this paper the origin of the ridge is evaluated on the basis of the internal geometry and velocity structure revealed by the seismic data implication of the presence of these type of ridges in hyper extensional rifted margins is discussed and compared with other margins the analysis indicates that the ridge is an extrusive volcanic ridge probably tholeiitic in composition constructed by stacked hyaloclastite deltas and topped by carbonate platforms the results invalidate previously proposed models involving highly rotated fault blocks and the serpentinite mud volcanism the extension magnitude analysis suggests a highly stretched setting where limited mantle serpentinization may have occurred but the architecture and velocity of the pmvr demonstrates that it is made of lower velocity materials than serpentinite during the opening of the north atlantic the pmvr represents the northern time equivalent magmatic event expressed along the newfoundland iberia galicia recorded by the j anomaly that originate from cretaceous volcanic deposits

magmatism and rift margin evolution evidence from northwest australia deep crustal seismic observations from northwest australia demonstrate that the cuvier margin is a volcanic passive margin that formed as greater india rifted away from australia in the early cretaceous formation of the cuvier basin and initial sea floor spreading resulted in the emplacement of exceptionally thick oceanic crust while contemporaneous spreading off the adjacent exmouth plateau formed normal thickness oceanic crust crust of normal thickness was generated in the cuvier basin within 3 4 m y after spreading began there is no evidence for hot spot activity with the appropriate timing or in the appropriate spatial pattern to account for the difference in volcanism along the margin we believe that rapid rifting of the cuvier margin created strong lateral temperature gradients in the subjacent upper mantle at the time of breakup this induced small scale convection in the upper mantle that increased the rate of upwelling and thus the volume of melt generated by decompression partial melting in turn this resulted in a larger volume of magma at the initiation of sea floor spreading and thicker oceanic crust was initially emplaced rifting on the exmouth plateau happened over a much longer period of time and no such convective partial melting occurred the volume of melt associated with rifting was therefore entirely the result of passive upwelling and a normal thickness of oceanic crust was emplaced immediately adjacent to the exmouth plateau

the evolution of amphibolites from site 1067 odp leg 173 iberia abyssal plain jurassic rifting to the pyrenean compression

the development of volcanic sequences at rifted margins new insights from the structure and morphology of the voring escarpment mid norwegian margin on the voring margin offshore mid norway paleogene continental breakup was characterized by the extrusion of large volumes of flood basalts erupted in different depositional environments the transition from subaerial to submarine emplacement environment is marked by the formation of the voring escarpment which records the early encroachment of flood basalt into the basin and the buildup of a lava delta system the increased availability of new and reprocessed high quality seismic data allows a more detailed characterization of the along strike and across strike continuity and variability of the different volcanic seismic facies units detailed seismic interpretation shows that the 350 km long ne sw trending voring escarpment is a prominent feature along the voring margin with a height ranging between 200 and 1600 m structurally the voring escarpment is segmented along strike into five segments e1 e5 with different controlling factors leading to variation in accommodation space relative sea level change and magma supply are the major controlling factors for segments e2 and e4 which are characterized by a well developed lava delta system and significant escarpment height tectonic movements along the jan mayen fracture zone resulted in second order segmentation of the e1 segment into pseudoescarpments with a very thin lava delta system and limited escarpment height segments e3 and e5 situated along the flanks of cretaceous paleocene highs are controlled by the structural highs which were possibly reactivated during breakup time our mapping results provide crucial information about the paleogeography and yield important information regarding the paleo water depth and depocenter locations prior to and during the breakup of the voring margin

extension of the exmouth plateau offshore northwestern australia deep seismic reflection refraction evidence for simple and pure shear mechanisms the response to extensional stresses that deformed and finally ruptured the northwest australian passive margin was achieved by an interplay of simple and pure shear mechanisms the results of a two ship seismic reflection and refraction experiment show that the outer exmouth plateau is dominated by large rotated blocks bounded by deeply penetrating normal faults some of which are heavily intruded magmatic underplating attended this extension but is absent in the central region where a set of prominent subhorizontal mid crustal arrivals are interpreted to be detachment surfaces extensive exploration seismic coverage drilling and sampling suggest that deformation on the outer plateau postdates that in the central plateau we propose that initial deformation of the region was dominated by a simple shear detachment system along low angle fault planes this passed laterally and temporally into a region of pure shear deformation characterized by high angle normal faults and magmatism and this became the locus of final rupture

late jurassic early cretaceous evolution of the eastern indian ocean adjacent to northwest australia over 9700 km of new aeromagnetic data were acquired off the northwest coast of australia and combined with existing magnetic data to map magnetic isochrons in the eastern indian ocean the isochrons were used to constrain a tectonic model of the evolution of the seafloor in the argo cuvier and gascoyne abyssal plains a complete set of anomalies from m26 through m16 was found in the argo abyssal plain trending generally n70 e spreading commenced in the center of the basin at or prior to m26 and propagated outward until at least m24 time anomalies m10 mo recording the separation of australia and india were found in the cuvier and gascoyne abyssal plains with a trend of about n30 e a significant crustal age discontinuity occurs in the vicinity of the joey rise where the two lineation sets converge because there appears to be no overlap of isochron ages in the two groups it is not necessary to postulate that a triple junction existed off northwest australia as has been previously suggested at m4 m5 time a 10 clockwise change in spreading direction occurred on the cuvier gascoyne spreading system this event triggered ridge jumps that transferred two pieces of the indian plate to the australian plate overlapping spreading on the forming and dying ridges curved fracture zones and lineations as well as fanned lineation trends suggest that the ridge jumps occurred by ridge propagation and that the transferred lithospheric blocks behaved as microplates for a brief interval of approximately 1 2 m y

internal structure of the 85 e ridge bay of bengal evidence for multiphase volcanism abstract the 85 e ridge located in the bay of bengal of the northeastern indian ocean is an enigmatic geological feature as it possesses unusual geophysical signatures the ridge s internal structure and mode of eruptions are unknown due to lack of deep seismic reflection and borehole data control here we analyze 10 km of long streamer seismic reflection data to unravel the ridge s internal structure and thereby to enhance the understanding of how the ridge was originated and grew over a geologic time seismic facies analysis reveals the ridge structure consisting of volcanic vent and several stratigraphic units including packs of prograding clinoforms the clinoform sequences are interpreted as volcanic successions and led to the formation of lava delta fronts interpreted features of lava fed deltas and intervening erosional surfaces and mass flows along ridge flanks suggest that the 85 e ridge is a volcanic construct and was built by both subaqueous and multiphase sub marine volcanism during the late cretaceous approximately from 85 to 80 ma at later time from oligocene miocene 23 ma onwards the ridge was buried under the thick sediments of the bengal fan system

crustal structure of edoras bank continental margin and mantle thermal anomalies beneath the north atlantic we discuss the structure of the continent ocean transition of the edoras bank area of the ne atlantic margin and the influence of the iceland mantle plume during continental breakup edoras bank lies close to the present day limit of influence of the iceland plume sequences of seaward dipping reflectors imaged on multichannel seismic profiles are well developed over much of the margin they form aerially extensive dipping sheets with individual reflectors imaged on margin parallel profiles as laterally continuous and subhorizontal features we interpret them as basalt flows extruded from linear fissure vents close to sea level at the time of continental breakup a crustal velocity model for the margin was developed by travel time modeling of wide angle refractions and reflections recorded on digital ocean bottom hydrophones and the structure confirmed by gravity modeling oceanic crust formed immediately after breakup is 10 km thick and exhibits classic layer 2 and layer 3 seismic velocities the crustal thickness increases to the se reaching 17 km beneath the continental fragment of edoras bank the continent ocean transition is dominated in the lower crust by a lens up to 8 km thick of high velocity material 7 2 7 5 km s 1 which exhibits a very low velocity gradient it extends over a 60 km wide region beneath the seaward dipping reflectors the high velocities and densities of the lower crust are probably caused by igneous rock with a high mgo content that underplated the margin or heavily intruded pre existing crustal material during continental breakup both the addition of large quantities of melt to the margin and the subsidence curves inferred from postrift sediments sampled by deep sea drilling project dsdp boreholes on the margin indicate the presence of abnormally hot mantle beneath the area at the time of continental breakup comparison of the structure and subsidence history of the edoras bank margin with other areas of the nw european margin allows us to deduce the spatial extent and temporal variation of the thermal anomalies caused by the iceland mantle plume during and following continental breakup we find that at the time of breakup a thin layer of anomalously hot asthenospheric mantle 150 c hotter than normal lay beneath the 2000 km long rift extending from the lofoten margin off norway in the north to the edoras bank margin in the south over a period of 5 10 m y following the onset of rifting the mantle thermal anomaly decayed rapidly to only 50 c or so above normal as the thin layer of abnormally hot mantle decompressed beneath the rift and produced a large pulse of magmatism

development of the continent ocean transform boundary of the southern exmouth plateau a two stage model is proposed to explain the principal tectonic and magmatic features observed in multichannel seismic reflection and refraction data across the southern transform margin of the exmouth plateau northwestern australia 1 the rifting stage in which detachment surfaces developed under conditions of extension at a high angle to the future transform and were later sheared by right lateral strike slip faulting final transform rupture was attended by large fault block rotation and mafic intrusions in conditions of pure shear 2 in the drifting stage as the oceanic ridge abutted the continent the continental rim was underplated at this location resulting in a 10 km thick 7 3 km s 3 g cm3 layer resulting in a permanent isostatic uplift of the crust and tilting of synrift sedimentary deposits this wedge extends laterally forming a thickened oceanic layer 3 transient heating of the continental lithosphere induced thermal uplift and erosion of up to 3 5 km of sedimentary units over a 50 km distance from the continent ocean contact

seismic image of the cantabrian mountains in the western extension of the pyrenees from integrated escin reflection and refraction data abstract integrated analysis of normal incidence and large aperture seismic reflection data collected in 1992 and 1993 within the spanish escin and complementary projects provide a first complete n s crustal transect across the northern iberian peninsula and continental margin images of the crustal structure of the cantabrian mountains and their transition to the duero basin and to the cantabrian margin are obtained from a a 65 km long vertical reflection profile escin 2 on land b a 200 km long reversed refraction profile and c wide angle recordings of the marine escin 4 profile consistent results between reflectivity pattern and velocity depth distribution reveal important lateral variations in the deep structure the reflective crust imaged in the escin 2 profile changes its attitude from sub horizontal beneath the duero basin to north dipping beyond the mountain front basement thrusts are observed in the upper crust merging into a detachment at 6 s twt and may have triggered the alpine uplift of the range the moho is identified at the bottom of the reflective lower crust and deepens from 12 to 15 s at the northern end of the profile about 35 km inland modelling of the refraction data laterally extends the seismic image and provides evidence for variscan crustal features beneath the duero basin northwards the velocity in the lower crust decreases and the moho constrained by the wide angle data from profile escin 2 deepens to about 60 km ending abruptly at the shoreline the velocity depth model is constrained along the asturian platform up to the continental slope where the crust mantle boundary is located at 24 km depth this margin moho shows a progressive deepening southwards and extends to the coast where it is found at 30 km depth the present seismic data support an important alpine reworking and thickening of the crust under the cantabrian mountains the onshore offshore transition is marked by an imbrication of two crusts of very different thicknesses this signature offers a strong parallelism with the one previously observed further east across the pyrenees in the ecors seismic profile

volcanic margin formation and mesozoic rift propagators in the cuvier abyssal plain off western australia the western margin of australia is characterized by synrift and postrift magmatism which is not well understood a joint interpretation of magnetic anomaly satellite gravity anomaly and seismic data from the cuvier abyssal plain and margin shows that the breakup between india and australia started circa 136 ma m14 and was followed by two rift propagation events which transferred portions of the indian plate to the australian plate post breakup magmatism continued with the emplacement of the wallaby and zenith plateaus 17 18 km thick at their centers along a transform margin two narrow magmatic edifices adjacent to the wallaby plateau sonne and sonja ridges represent an extinct ridge and a pseudofault respectively they formed by excess volcanism probably by lateral migration of buoyant melt along upside down crustal drainage channels from the melt source underneath the wallaby plateau in a mantle plume scenario a small plume 400 km diameter located underneath the rift could have locally uplifted the bernier platform and exmouth sub basin in the early cretaceous and left a track consistent with the azimuth of the wallaby and zenith plateaus in this case ridge plume interaction would have caused two consecutive ridge propagation events towards the plume while the ridge moved away from the hotspot the abrupt end of the hotspot track west of the zenith plateau would be a consequence of the accelerating south eastward motion of the spreading ridge relative to the mantle after 120 ma leaving the mantle plume underneath the indian plate an alternative nonmantle plume scenario is based on the observation that between breakup and chron m0 120 ma the ocean crust in the southern cuvier abyssal plain was formed while the spreading ridge abutted indian continental crust small scale convection may have been initiated during rifting in the early cretaceous and maintained until the wallaby zenith ridge transform intersection passed by the eastern edge of indian continental crust at chron m0

archean protoliths within early proterozoic granulitic crust of the west european hercynian belt possible relics of the west african craton granulites from the bay of biscay area have been studied by using u pb and sm nd methods results on four granulites suggest two major events of crustal growth and high grade metamorphism the first is archean 2 7 ga and the second is early proterozoic 1 9 ga in the hercynian belt the oldest basement remnants known so far are about 2 0 ga and are located in the northern armorican massif the 2 7 ga orthogneiss protolith age from the bay of biscay is the first direct evidence of archean crust involved in the hercynian belt major events at 2 7 1 8 and 0 6 ga in the hercynian belt suggest that european precambrian microplate s were part of the west african craton where the same sequence of events is observed

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

seismic volcanostratigraphy of the gascoyne margin western australia abstract integrated interpretation of seismic gravity and magnetic data has revealed the presence of a large breakup related volcanic complex on the rifted gascoyne margin western australia using the concept of seismic volcanostratigraphy we have identified three main volcanic seismic facies units related to the extrusive volcanism 1 landward flows 2 seaward dipping reflections sdr and 3 volcanic protrusions additionally we have identified four seismic facies units related to the tectonomagmatic evolution of the margin 4 domes 5 moho 6 top underplating and 7 sill intrusions the galah rise comprises the majority of the volcanic complex and is dominated by 100 200 km long ne striking volcanic ridges surrounded by multiple sets of deep marine emplaced sdrs an estimated excess crustal volume of the 50 000 km 2 large galah rise is 350 000 km 3 the volcanic complex is located at the outer corner of a shear rift margin setting the magmatism is sparse on the shear margin massive near the outer corner and decreases ne wards along the rifted margin segment and away from the fracture zone the gascoyne margin is comparable to the volcanic voring margin ne atlantic in terms of dimensions structural setting and distribution of the volcanic seismic facies units however the along margin variations in magmatism are more prominent on the gascoyne margin

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

transform continental margins part 2 a worldwide review we present a global inventory of transform continental margins based on both a worldwide comparison of continent ocean boundary identifications with oceanic fracture zones traces and a compilation of published regional studies this inventory increases the number of identified transform margins from 29 to 78 these margins represent 16 of continental margins in cumulative length and 31 of non convergent margins we include morphological data published upper crustal sections continent to ocean transition locations and published moho shape data in the new database this review confirms that continent to ocean transitions are sharper at transform margins than at divergent margins it also emphasizes the structural diversity of transform margins associated with one third of transform margins we define marginal plateaus as a new type of relief that corresponds to a flat but deep surface inside the continental slope and that may be inherited from crustal thinning prior to transform faulting transform margin initiation appears to be favoured along propagating oceans and within cold and thick lithospheres

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

post rift flood basalt like volcanism on the newfoundland basin nonvolcanic margin the u event mapped with spectral decomposition abstract diabase sills emplaced at less than 5 km depth presently 7 km are imaged throughout the newfoundland basin as an extraordinarily high amplitude package of seismic reflections historically referred to as the u event the sills are extensive covering roughly 20 000 km2 and represent a post rift voluminous magmatic episode along an otherwise nonvolcanic rifted margin spectral decomposition is used to estimate a total magmatic volume at roughly 1000 2000 km3 on an individual basis the major sills are comparable to flows in flood basalt volcanic environments a proposed mechanism for generating these large volumes of magma which are also very limited in time to perhaps only two major intrusive episodes 2 sills is the approach of the canary and madeira hot spots suppressed volcanism while the hot spots traveled under full thickness continental lithosphere of newfoundland resulted in a local accumulation of magma which was suddenly released when the hot spots approached thinned lithosphere at the eastern edge of the grand banks the emplacement of the sills anticipated the arrival of the hot spots geographically

u pb zircon and monazite geochronology of post collisional hercynian granitoids from the central iberian zone northern portugal abstract in the central iberian zone ciz of the iberian massif large volumes of granitoids were emplaced during the post collisional stage of the hercynian orogeny syn to post d3 the last ductile deformation phase twelve granitic units and a quartz monzodiorite were selected for a u pb zircon and monazite geochronological study they represent successive stages of the d3 event the ucanha vilar lamego sameiro and refoios do lima plutons are coeval 313 2 ma 319 4 ma 316 2 ma and 314 2 ma respectively and belong to the earliest stage later on the braga massif was emplaced its different units yielding the same age 309 3 ma for the braga granite 309 1 ma for the gonca granite and 311 5 ma for a related quartz monzodiorite the braga massif is subcontemporaneous with the agrela and celeiros plutons 307 3 5 ma and 306 2 ma respectively in agreement with field data the briteiros granite is younger 300 1 ma followed by the emplacement of the peneda geres massif geres paufito illa and carris granites the geres granite emplaced at 296 2 ma seems to represent a first magmatic pulse immediately followed by the intrusion of the paufito granite at 290 2 5 ma for the carris granite a minimum emplacement age of 280 5 ma was obtained based on these results the following chronology is proposed 1 syn d3 biotite granitoids 313 319 ma 2 late d3 biotite dominant granitoids 306 311 ma 3 late to post d3 granitoids ca 300 ma 4 post d3 granitoids 290 296 ma these chronological data indicate that successive granitic intrusions were emplaced in the ciz during a short time span of about 30 ma that corresponds to the latest stages of the hercynian orogeny a rapid and drastic change occurred at about 300 ma between a compressive ductile tectonic regime d3 ca 300 320 ma associated to calc alkaline monzonitic and aluminopotassic plutonism and a fragile phase of deformation d4 which controlled the emplacement of the subalkaline ferro potassic plutonism at 290 296 ma

geological setting and age of the flemish cap granodiorite east of the grand banks of newfoundland flemish cap a large isolated submarine knoll 600 km east of newfoundland consists of a central core of hadrynian rocks and an onlapping sequence of undisturbed to disturbed mesozoic cenozoic sediments the central core of the cap sampled with an electric rock core drill comprises pink fine to medium grained granodiorite dacite and volcanic siltstone the granodiorite samples collected in six out of eight cores are remarkably similar lithologically despite a separation of up to 65 km and probably represent a single pluton the aphanitic dacite and laminated cherty volcanic siltstone have been metamorphosed to subgreenschist prehnite facies but show no evidence of contact metamorphism u pb analyses of coarse and fine zircon fractions from a granodiorite core yielded upper intercepts of 751 and 833 ma respectively although not precise these ages probably represent the age of intrusion sometime in the 750 830 ma range and are older than those reported for similar hadrynian granitic rocks of e

magnetotelluric evidence for layered mafic intrusions beneath the voring and exmouth rifted margins abstract marine magnetotelluric mt surveys at two volcanic passive margins have revealed an enigmatic layer of extremely high conductivity 0 1 m at 10 km depth at the voring plateau off the northwest shelf of norway 2d inversion of data from nine sites along a 54 km line resolves a layer with a conductance of 10 4 s at the exmouth plateau off the northwest shelf of australia 2d inversion of 122 sites in 17 lines finds a similar layer at similar depth but an order of magnitude higher conductance at both plateaus the depth of the high conductivity layer coincides roughly with what seismic studies have identified as an assemblage of sills we propose that the extremely high conductance is due to well connected conductive cumulates e g magnetite precipitated in layered mafic intrusions in contrast to sill emplacement the nature of layered intrusion formation requires connection to a magma source over time such a connection would not be likely during rifting when the rift provides a preferential pathway for pressure release this implies emplacement prior to or during a pause in the early stage of continental breakup

rift shear architecture and tectonic development of the ghana margin deduced from multichannel seismic reflection and potential field data abstract the ghana margin displays one of the best known transform margins studies of the margin have provided the framework for a number of conceptual models aimed at understanding transform margin development worldwide however the deep structure of the margin is poorly known as knowledge is based only on wide angle refraction measurements obtained from two separate localities on the margin consequently complexities in the rift shear margin architecture have been overlooked by current interpretations of margin development based on combined analysis of a detailed grid of 2710 km multichannel mcs lines and potential field data we provide new insights into the structural architecture and tectonic development of the ghana margin in particular we outline the deep structure of the entire margin using a series of 2d gravity modelled transects constrained by mcs and published wide angle data our study reveals more complex rift shear margin architecture than previously envisaged we demonstrate that the main transform boundary representing the continental extension of the romanche fracture zone is actually composed of two distinct margin segments i e the ene wsw trending sheared margin segment of the cote d ivoire ghana ridge and the ne sw trending rift influenced sheared margin segment of the ghana platform these segments evolved under varying stress regimes and during different time intervals west of the transform margin divergent rifting during the early cretaceous initiated the development of the deep ivorian basin essentially as a single major pull apart structure however east of the shear zone oblique rifting resulted in the development of the eastern ghana slope basin as a composite of at least two coalescing pull apart basins displaced along strike slip faults our structural interpretation of the transform boundary geometry shows that the ridge and platform margin segments were each subjected to separate thermal influences from two different migrating spreading centres tectonic uplift of the ridge began through transpression during mid albian time following a change in relative direction of plate motion from ne sw to ene wsw however the ridge uplift was amplified by thermal heating from a previously undocumented spreading centre whose progressive westward migration along the ridge followed closely after the albian transpressional phase the structural architecture of the ghana margin resulted from a combination of factors notably pre existing basement structure plate boundary geometry the relative direction of plate motion and thermal heating

transform tectonics and thermal rejuvenation on the cote d ivoire ghana margin west africa formation of a pronounced basement ridge along many transform continental margins has been attributed to a variety of processes during continental break up including transpressional crustal thickening and thermal rejuvenation and igneous underplating during passage of a spreading ridge odp drill holes on the cote d ivoire ghana margin now provide the first opportunity to quantify the vertical motions along this type of margin apatite fission track dating of detrital sands suggests that large amounts of erosion occurred on the flanks of an intra continental wrench zone that predated margin formation rapid cooling of 120 c at 120 115 ma corresponds to erosion of 3 5 5 km along the conjugate brazilian margin reflecting c 1 km of tectonically driven uplift subaerial erosion and isostatic uplift due to unloading following rift initiation at 120 ma aptian an oceanic spreading axis passed adjacent to this part of the margin at 90 ma cenomanian maximum uplift during the ridge transform intersection was 390 m considerably less than the 2000 m predicted by heat conduction models in local isostatic equilibrium the modern ridge is partially the product of thicker crust 22 km underlying the ridge than the adjacent deep ivorian basin 19 km and partially related to flexural unloading of the transform ridge between the end of intra continental wrenching and ridge transform intersection flexural coupling between the continental and oceanic plates since ridge transform intersection has caused a progressive depression of the offshore margin estimated at about 650 m in the study area

tertiary evolution of the eastern indonesia collision complex abstract eastern indonesia is the zone of interaction between three converging megaplates eurasia the pacific and indo australia the geological basis for interpretations of the tertiary tectonic evolution of eastern indonesia is reviewed and a series of plate tectonic reconstructions for this region at 5 million year intervals covering the last 35 million years is presented the oldest reconstruction predates the onset of regional collisional deformation at this time a simple plate configuration is interpreted consisting of the northward moving australian continent approaching an approximately e w oriented southward facing subduction zone extending from the southern margin of the eurasian continent eastwards into the pacific oceanic domain beginning at about 30 ma the australian continental margin commenced collision with the subduction zone along its entire palinspastically restored northern margin from sulawesi in the west to papua new guinea in the east from this time until ca 24 ma the australian continent indented the former arc trend with the northward convergence of australia absorbed at the palaeo northern boundary of the philippine sea plate the present day palau kyushu ridge at ca 24 ma the present day pattern of oblique convergence between the northern margin of australia and the philippine sea plate began to develop at about this time a large portion of the palaeogene colliding volcanic arc the future eastern philippines began to detach from the northern continental margin by left lateral strike slip from ca 18 ma oblique southward directed subduction commenced at the maramuni arc in northern new guinea at ca 12 ma the sorong fault zone strike slip system developed effectively separating the philippines from the indonesian tectonic domain the sorong fault zone became inactive at ca 6 ma since which time the tectonics of eastern indonesia has been dominated by the anticlockwise rotation of the bird s head structural block by some 30 40 contemporaneously with post 18 ma tectonism the banda arc subduction collision system developed off the northwestern margin of the australian continent convergence between indo australia and eurasia was accommodated initially by northward subduction of the indian ocean and subsequently since ca 8 ma by the development of a second phase of arc continent collision around the former passive continental margin of nw australia

seismic response and construction of seaward dipping wedges of flood basalts voring volcanic margin geological and geophysical data from the 900 m of volcanic basement drilled at ocean drilling program site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins the main part of the drilled section corresponding to a prominent seaward dipping reflector sequence consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments the basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity pore aspect ratio distribution and alteration stacks of laterally continuous basalt flows appear to have thin layer transverse isotropic properties for typical wavelengths in multichannel seismic data vertical seismic profiling and average sonic log velocities are similar 3 77 km s and 3 88 km s respectively while comparable refraction velocities are 10 20 higher synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces though the most continuous reflectors are related to thick flows seismic models based on the characteristic velocity and density basalt flow distributions and site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows the base of the dipping reflector sequence reflector k correlates with flows in the lower part of upper series basalts while the transition from basaltic to underlying dacitic andesitic lavas correlates with a locally defined reflector a model for the breakup related volcanism includes 1 prebreakup dacitic andesitic volcanism 2 early breakup basaltic volcanism infilling the prebreakup relief 3 main breakup stage with intense focused volcanism large subsidence and lava pounding and 4 late breakup volcanism during a period of decreased subsidence and local off axis activity

deformation and exhumation in timor distinct stages of a young orogeny abstract timor island in the outer banda arc bordering the timor sea preserves the orogenic product of an arc continent collision between the australian plate and the banda arc that commenced after 10 9 9 8 ma gts2004 but emerged above sea level only 3 1 ma ago the orogenic pile includes large tracts of material from the australian margin including the permian to middle jurassic gondwana megasequence and the late jurassic to early late miocene australian margin megasequence which occur in thrust slices in addition material from the banda arc side of the plate margin referred to as the banda terrane occurs throughout the island and includes both seafloor metamorphosed igneous material and cover sediments also in thrust sheets however the distribution of thrust slices is unclear in many areas perhaps because only the uppermost nappes of the thrust pile are currently emergent and also because the thrust piles have been disrupted by later high angle faulting evidence from east timor suggests that the major break between deformed pre collisional strata and the relatively undeformed overlying deposits was during the late miocene 9 8 5 5 ma we present evidence for the timing of three distinct phases of orogenic development as determined from east timor including initial collision and emplacement of the early nappes creating loading and diapirism within the 9 8 5 5 ma interval a tectonic quiet interval 5 5 ma 4 5 ma that extended for about a million years during the middle of the collision and may represent the time of locking of the subduction system and a post 4 5 ma phase of uplift unroofing and further diapirism in response to isostatic rebound our conclusions offer an alternative model for the evolution of this part of the banda arc

interpretation of a vertical seismic profile conducted in the columbia plateau basalts seismic reflection data are often of poor quality when recorded in areas where volcanic rocks are present at or near the surface in order to investigate this phenomenon a vertical seismic profiling vsp experiment was conducted in the columbia plateau basalts so that the behavior of seismic energy in subsurface volcanic rocks could be observed directly thus giving insight into data acquisition in volcanic terrains the lithologic section at the vsp site consists of low velocity 400 m s to 900 m s alluvium in the uppermost 50 m beneath which are layers of high velocity about 5800 m s high density basalts interbedded with clay layers with much lower velocities about 1700 m s and densities these large velocity and density contrasts dramatically influence wave generation and propagation in spite of the small source borehole offset 61 m large amplitude s waves are generated by the downgoing p waves when they reach a shallow 250 m clay basalt boundary these s waves in turn generate strong

crustal structure of the rifted volcanic margins and uplifted plateau of western yemen from receiver function analysis we analyse p wave receiver functions across the western gulf of aden and southern red sea continental margins in western yemen to constrain crustal thickness internal crustal structure and the bulk seismic velocity characteristics in order to address the role of magmatism faulting and mechanical crustal thinning during continental breakup we analyse teleseismic data from 21 stations forming the temporary young conjugate margins laboratory yocmal network together with gfz and yemeni permanent stations analysis of computed receiver functions shows that 1 the thickness of unextended crust on the yemen plateau is 35 km 2 this thins to 22 km in coastal areas and reaches less than 14 km on the red sea coast where presence of a high velocity lower crust is evident the average vp vs ratio for the western yemen plateau is 1 79 increasing to 1 92 near the red sea coast and decreasing to 1 68 for those stations located on or near the granitic rocks thinning of the crust and by inference extension occurs over a 130 km wide transition zone from the red sea and gulf of aden coasts to the edges of the yemen plateau thinning of continental crust is particularly localized in a 30 km wide zone near the coastline spatially co incident with addition of magmatic underplate to the lower crust above which on the surface we observe the presence of seaward dipping reflectors sdrs and thickened oligo miocene syn rift basaltic flows our results strongly suggest the presence of high velocity mafic intrusions in the lower crust which are likely either synrift magmatic intrusion into continental lower crust or alternatively depleted upper mantle underplated to the base of the crust during the eruption of the sdrs our results also point towards a regional breakup history in which the onset of rifting was synchronous along the western gulf of aden and southern red sea volcanic margins followed by a second phase of extension along the red sea margin

a review of cretaceous smooth slopes extensional basins along the iberia eurasia plate boundary how pre rift salt controls the modes of continental rifting and mantle exhumation this article points out for the first time a striking correlation between the paleogeography of upper triassic deposits and the mode of crustal stretching around and inside the northern iberia plate during the cretaceous transtensional event first we propose a review of the architecture of the basins which opened during the mid cretaceous times along the iberia eurasia plate boundary similarly to the emblematic parentis basin these basins exhibit a peculiar synclinal shaped profile and are devoid of prominent block faulting the top of the basement is characterized by gentle slopes which dip symmetrically toward the center of the basins as revealed by recent comparisons with geologically constrained rifting models established from the north pyrenean zone this architecture results from the thinning of a heterogeneous continental crust under greenschist facies conditions basement deformations are thus dominantly ductile and are characterized by large scale boudinage hyper thinning and subsequent lateral extraction bulk deformation of the upper and middle crust leads to the formation of anastomosed shear zones and the development of mylonitic fabric tectonic lenses consisting of crustal material remain welded on top of the exhuming mantle the common character shared by all the pre rift sequences of the studied basins is the presence of a thick low strength upper triassic evaporites and clays layer belonging to the keuper group i e pre rift salt and clay unit in the studied basins efficient decollement along the pre rift salt and clay unit triggers mechanical decoupling and gliding of the pre rift cover that remains in the center of the basin thus during the early rifting phase the basement undergoes thinning while the pre rift cover remains preserved in the basin center in response to hyper thinning and horizontal extraction of the continental crust hot mantle is exhumed beneath the pre and syn rift cover subsequent thermal exchange i promotes ductile deformation of the basement and ii induces the development of ht lp metamorphic conditions in the pre rift sediments and at the base of the syn rift flysch levels this thermal event is well recorded in the axial portion of the pyrenean realm future north pyrenean zone as well as in the pre rift sediments of the cameros basin northern spain the architecture of the smooth slopes type basins thus contrasts with the structure of iberia newfoundland atlantic margins which are characterized by i top basement detachment faults accommodating crustal extension through rotation and translation of undeformed basement blocks and ii the individualization of continental extensional allochthons tectonically emplaced over exhumed lower crust or mantle rocks finally using recent paleogeographic reconstructions we show that the distribution of the pre rift salt and clay unit remarkably matches the distribution of the pyrenean and peri pyrenean smooth slopes type basins this allows for the first time to propose a genetic link between the distribution of evaporite bearing pre rift sedimentary formations and the development of smooth slopes basins

modelling the δ18o value of cave drip water and speleothem calcite abstract stable isotope signals recorded in speleothems have provided important insights about past climate variability in recent years quantitative reconstruction of mean annual temperature and the amount of precipitation however remains difficult because the stable isotope signals are influenced by various processes here we present a drip water model which shows how these climate parameters affect the oxygen isotope signal of cave drip water in the model the dependence of the 18 o value of drip water on mean annual temperature is established by correlation to the amount of winter precipitation and winter temperature application of the model to two caves in western germany reveals a strong influence of winter rainfall on the oxygen isotope composition of cave drip water in this region assuming equilibrium isotope fractionation between drip water and calcite we provide a function relating the 18 o value of speleothem calcite to mean annual surface temperature this function shows a clear anticorrelation between temperature and the 18 o value of speleothem calcite which has been previously reported for several caves in central and northern europe by inverse application of this function we tentatively reconstruct average temperatures for the period between 6 and 1 5 ka from the 18 o signals of two stalagmites from atta and bunker cave western germany the resulting temperature curves are very sensitive to the value used for the correlation between the amount of winter precipitation and winter temperature since this correlation was probably not constant in the past the reconstructed temperature curves are associated with substantial uncertainty

the volcanic margins of the northern labrador sea insights to the rifting process 1 a new interpretation of seismic reflection data on the continental margin of northern labrador shows basement structures similar to those observed on other volcanic rifted margins seaward dipping reflections inner flows volcanic plateaus and lava deltas are observed on these data magnetic chron 27n 61 ma is coincident with the volcanic plateaus connecting these features to the paleocene volcanism farther north in davis strait therefore we are able to extend the region of volcanism about 500 km south along the margin similar structures are also observed on the conjugate west greenland margin below the volcanic plateaus gravity and wide angle seismic data show thick igneous crust which is fairly symmetrically distributed across the conjugate margins however the geometry of the thinned continental crust is not symmetrical a narrower zone of thinning is observed off northern labrador the thick igneous crust lies seaward of thinned continental crust which exhibits little or no underplating by the magmatic event this observation is compatible with recent models for the formation of nonvolcanic margins followed by a magmatic event late in the rift evolution in mesozoic time rifting of the cold thick cratonic lithosphere occurred between greenland and north america with minimal volcanic activity then in late cretaceous time the zone of rifting narrowed and localized seaward of the shelf creating a relatively narrow zone of lower lithospheric thinning and mantle upwelling into which the paleocene magmas were eventually channeled this scenario is supported by studies of mesozoic igneous rocks on land in coastal west greenland and labrador

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

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

seismic volcanostratigraphy of the norwegian margin constraints on tectonomagmatic break up processes voluminous volcanism characterized early tertiary continental break up on the mid norwegian continental margin the distribution of the associated extrusive rocks derived from seismic volcanostratigraphy and potential field data interpretation allows us to divide the more voring and lofoten vesteralen margins into five segments the central more margin and the northern voring margin show combinations of volcanic seismic facies units that are characteristic for typical rifted volcanic margins the lofoten vesteralen margin the southern voring margin and the area near the jan mayen fracture zone show volcanic seismic facies units that are related to small volume submarine volcanism the distribution of subaerial and submarine deposits indicates variations of subsidence along the margin vertical movements on the mid norwegian margin were primarily controlled by the amount of magmatic crustal thickening because both the amount of dynamic uplift by the icelandic mantle plume and the amount of subsidence due to crustal stretching were fairly constant along the margin thus subaerial deposits indicate a large amount of magmatic crustal thickening and an associated reduction in isostatic subsidence whereas submarine deposits indicate little magmatic thickening and earlier subsidence from the distribution of volcanic seismic facies units we infer two main reasons for the different amounts of crustal thickening 1 a general northward decrease of magmatism due to increasing distance from the hot spot and 2 subdued volcanism near the jan mayen fracture zone as a result of lateral lithospheric heat transport and cooling of the magmatic source region furthermore we interpret small lateral variations in the distribution of volcanic seismic facies units such as two sets of inner seaward dipping reflectors on the central voring margin as indications of crustal fragmentation

age and geochemistry of magmatism on the oceanic wallaby plateau and implications for the opening of the indian ocean the temporal relationship between tectonic and volcanic activity on passive continental margins immediately before and after the initiation of mid ocean ridge spreading is poorly understood because

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

seismic structure of the northern continental margin of spain from escin deep seismic profiles abstract by the end of the carboniferous the crust of the continental shelf in northwestern spain was made up of deeply rooted structures related to the variscan collision from permian to triassic times the tectonic setting had changed to mainly extensional and the northern iberian continental margin underwent rifting during late jurassic early cretaceous times along with sea floor spreading and the opening of the bay of biscay until the late cretaceous subsequently the northern iberian margin was active during the north south convergence of eurasia and iberia in the tertiary a multichannel seismic experiment consisting of two profiles one north south escin 4 crossing the platform margin offshore asturias and another escin 3 crossing the platform margin to the northwest of galicia was designed to study the structure of the northern iberian margin the escin 4 stacked section reveals inverted structures in the upper crust within the le danois basin north of the steep continental slope escin 4 shows a thick sedimentary package from 6 to 9 5 s two way travel time twt within this latter package a 40 km long north tapering wedge of inclined mainly south dipping reflections is thought to represent a buried alpine age accretionary prism in the north western part of the escin 3 escin 3 1 stacked section horizontal reflections from 6 5 to 8 5 s correspond to an undisturbed package of sediments lying above oceanic type basement in this part of the line a few kilometres long strong horizontal reflection at 11 2 s within the basement may represent an oceanic moho reflection also a band of reflections dips gently towards the southeast from the base of the gently dipping continental slope the part of escin 3 line that runs parallel to the nw galicia coast escin 3 2 is characterized by bright continuous lower crustal reflections from 8 to 10 s beneath the lower crustal reflectivity a band of strong reflections dips gently toward the southwest from 10 to 13 5 s the part of escin 3 that parallels the northern margin escin 3 3 shows good reflectivity in all levels upper crustal reflections image the sedimentary fill of probable mesozoic to recent basins mid crustal reflectivity is characterized by dipping reflections until 8 s that are probably related to compressional variscan features the lower crustal level shows layered reflections between 8 and 12 s dipping reflections are found below the continental moho