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 s urrounding area are considered to be a fragment of a continent which w as separated from Greenland just prior to magnetic anomaly 6. This stu dy 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 basa ltic layer within the sedimentary section and extreme thinning of the continental crust towards the Iceland Plateau. The OBS data indicate t hat 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 thick ness of the continental lower crust varies significantly from 12 km be neath the Jan Mayen Ridge to almost zero thickness beneath the northwe stern 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) immediate ly oceanwards of the ocean/continent transition zone. Beneath the Icel and Plateau, the oceanic crust is thicker (9 km) than the typical thic kness 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 th e present study, it is proposed that the western part of the Jan Mayen Ridge may be referred to as a non-volcanic continental margin, genera ted by a long duration of rifting. Even if the asthenospheric material upwelling along the margin were hotter than normal, only small amount s of magmatic intrusions and extrusions would have been generated beca use of significant conductive cooling under the long duration of rifti ng.