When continents break apart, the rifting is sometimes accompanied by the production of large volumes of molten rock. The total melt volume, however, is uncertain, because only part of it has erupted at the surface. Furthermore, the cause of the magmatism is still disputed-specifically, whether or not it is due to increased mantle temperatures. We recorded deep-penetration normal-incidence and wide-angle seismic profiles across the Faroe and Hatton Bank volcanic margins in the northeast Atlantic. Here we show that near the Faroe Islands, for every 1 km along strike, 360-400 km(3) of basalt is extruded, while 540-600 km(3) is intruded into the continent-ocean transition. We find that lower-crustal intrusions are focused mainly into a narrow zone approximately 50 km wide on the transition, although extruded basalts flow more than 100 km from the rift. Seismic profiles show that the melt is intruded into the lower crust as sills, which cross-cut the continental fabric, rather than as an 'underplate' of 100 per cent melt, as has often been assumed. Evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures, with no requirement for either significant active small-scale mantle convection under the rift or the presence of fertile mantle at the time of continental break-up, as has previously been suggested for the North Atlantic Ocean.

[1] We show new crustal models of the Hatton continental margin in the NE Atlantic using wide-angle arrivals from 89 four-component ocean bottom seismometers deployed along a 450 km dip and a 100 km strike profile. We interpret prominent asymmetry between the Hatton and the conjugate Greenland margins as caused by asymmetry in the initial continental stretching and thinning, as ubiquitously observed on ''nonvolcanic'' margins elsewhere. This stretched continental terrain was intruded and flooded by voluminous igneous activity which accompanied continental breakup. The velocity structure of the Hatton flank of the rift has a narrow continent-ocean transition (COT) only $40 km wide, with high velocities (6.9 -7.3 km/s) in the lower crust intermediate between those of the continental Hatton Bank on one side and the oldest oceanic crust on the other. The high velocities are interpreted as due to intrusion of igneous sills which accompanied the extrusion of flood basalts at the time of continental breakup. The variation of thickness (h) and P wave velocities (v p ) of the igneous section of the COT and the adjacent oceanic crust are consistent with melt formation from a mantle plume with a temperature $120-130°C above normal at breakup, followed by a decrease of $70-80°C over the first 10 Ma of seafloor spreading. The h-v p systematics are consistent with the dominant control on melt production being elevated mantle temperatures, with no requirement for either significant active small-scale mantle convection under the rift or of the presence of significant volumes of volatiles or fertile mantle.

Int he summero f2002,newdatawerea cquired alongwide-anglea ndnormal-incidence profiles extendingf rom the Hatton Basin,a cross the adjacent Hatton Bankvolcanic continentalm arginandi nto the IcelandB asin. Eighty-ninefour-component ocean-bottom seismometers (OBS)w ered eployed alongatransect runningacross the continentalmarginandalongtwo strike lineslocated abovethe region ofthickest extrusiveand intrusivei gneous rock on Hatton Banka ndover44 Ma oceanic crust neart he endofthe maindipp rofile.The seismic profilingwaso ptimized for large offsetOBS arrivals withalow-frequencyairgun source centred at 10 Hz. Wide-angleseismic energyp enetrated the extruded basalt,the underlyingc rust andwell into the upper mantle.Initialmodellingresults from Hatton Basinshow a , 4kmthick sedimentary sequence andprovide anew sub-sediment crustalt hickness estimateof , 15km. Thiscrustalt hickness suggests astretchingf actor oftwo relativeto the continentalcrust underRockall Bank.Thesenewresults havebeenintegrated withanestablished modelfor the region to demonstratethe variations ofcrustalstructureacross the margin. Futureprocessingand modellingofthe extensived atasetcollected int he 2002 surveyw ill allow refinement ofthe crustals tructure modelacross the stretched continentalcrust beneathH atton Basinandthe adjacent continentalm argin. In particularitw ill place betterconstraintso nt he intrusivea nde xtrusivei gneous componentsp roduced during continentalbreak-up int he Palaeogene.

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