THE CONTINENT OCEAN TRANSITION OF THE LOFOTEN VOLCANIC MARGIN, N NORWAY/

A detailed model for the continent/ocean transition of the Lofoten pas sive volcanic margin, N. Norway, has been obtained by use of seismic w ide-angle data from Ocean Bottom Seismographs (OBSs), combined with re sults from other geophysical data. The OBS experiment across the trans ition zone involved dip- and strike-profiles, as well as a profile cro ssing the zone obliquely. The modelling reveals that this oblique prof ile is a key profile to the understanding of the transition zone, sinc e detailed crustal models are difficult to obtain by use of the dip-pr ofiles due to the presence of strong lateral variations in structures and seismic velocities. The Early Cenozoic extension episode on the ou ter Lofoten Margin ended with extensive submarine volcanism accompanie d by intrusions into the stretched continental crust and emplacements of magmatic bodies into the lower crust. The high magmatic activity ca n be explained by increased asthenosphere temperatures caused by the p roximity to the Iceland plume. From the seaward-dipping-reflectors (SD Rs) to the Voring Escarpment (VE) a landward decrease in the upper cru stal P-wave velocity from about 6.7 to 6.0 km/s is observed from the O BS data. This lateral velocity variation is interpreted to be caused b y a gradual landward decrease in the amount of high-velocity ''oceanic '' intrusions into the stretched continental crust. The same landward decrease in velocity is also observed in the lower crust; from about 7 .3 to 6.8 km/s. The modelling suggests that the high-velocity lower cr ustal body terminates landwards close to the VE. This lower crustal bo dy is interpreted as magmatic underplating of the crust associated wit h the Iceland plume. The Moho (upper mantle) velocity is inferred to b e 8.2 km/s from the shelf to the SDRs, and 7.9 km/s seward of the SDRs . This lateral velocity variation suggests that the upper mantle is of continental origin landward of the SDRs, and of oceanic origin from t he SDRs and westwards. This observation indicates that the lower crust al magmatic material is intruded laterally into the ductile lower crus t from the oceanic side. It is concluded that the transition from stre tched continental crust with non-observable amounts of magmatic intrus ions, to oceanic crust takes place over an about 50 km wide zone from the VE to the SDRs on the Lofoten Margin. Copyright (C) 1996 Elsevier Science Ltd