CRUSTAL STRUCTURE OF THE LOFOTEN CONTINENTAL-MARGIN, OFF NORTHERN NORWAY, FROM OCEAN-BOTTOM SEISMOGRAPHIC STUDIES

In 1988, an extensive geophysical experiment was performed on the Lofo ten passive continental margin, which was formed by the continental br eak-up between Greenland and northern Norway at 58 Ma. The geological units of the outer Lofoten margin are characterized by seaward-dipping reflectors (SDRs) and landward flood basalt, which extends up to 100 km landwards of SDRs. In this study, we obtain the P-wave velocity str ucture beneath the Lofoten Basin, the SDRs, and the landward flood bas alt by use of ocean-bottom seismograph refraction profiling, and we al so discuss the formation of the northern Norwegian passive continental margin. In the Lofoten Basin the crust is of oceanic type, consisting of sedimentary layers, oceanic layer 2 (4.9-5.5 km s(-1)), layer 3A ( 6.3-6.8 km s(-1)) and layer 3B (7.0-7.1 km s(-1)). Beneath the SDRs th e crustal layers are identical to those of the Lofoten Basin, but the thickness of the lower crust, which represents the same velocity as la yer 3B, increases to 5km towards the continent side, and a high-veloci ty lower crustal layer (7.3 km s(-1)) is formed at the base of the cru st. The ocean-continent transition zone is situated between the landwa rd side of the SDRs and the northward continuation of the Veiring Plat eau Escarpment. In this region the velocity of the lower crust gradual ly decreases and approaches the lower crustal velocity beneath the Lof oten Islands (6.8 km s(-1)). The model also indicates that the high-ve locity layer disappears in this region; Comparing our model with the c rustal structure on the Voring margin, it is clearthat the lower crust al body (greater than or equal to 7 km s(-1)) thickens southwards alon g the northern Norwegian continental margin. Recent results from petro logical and geophysical studies of the generation of the oceanic crust have shown that increasing the temperature of the upwelling asthenosp heric material increases the thickness of the oceanic crust. We interp ret this as that the oceanic crust in the southern area in the Voring- Lofoten margin was generated by hotter material than that of the north ern area.