Crustal Structure of the Northern and Southern Jan Mayen Ridge Segments, Norwegian Sea, Based on Ocean Bottom Seismometer Data.

The Jan Mayen Ridge (JMR) is a submarine ridge trending south from the volcanic Jan Mayen island in the Norwegian Sea, towards Iceland. In the north, it is a distinct, single ridge, but in the south it is divided into several smaller ridges. JMR is interpreted as a micro-continent, being part of Greenland during the volcanically active rifting off Norway in the latest Paleocene. In the late Oligocene, JMR was rifted off Greenland when seafloor spreading shifted from the now extinct Aegir Ridge, to the presently active Kolbeinsey Ridge. The southern termination of the micro-continent is uncertain, though it may extend into the Icelandic shelf. Earlier studies of the northern ridge found extrusive volcanism, and an asymmetrical crustal root, displaced to the east. Two OBS profiles were shot across the northern and southern part in year 2000. The northern ( ∼69° N) terminates in the Jan Mayen Fracture Zone, and the southern ( ∼66.5° N) crosses the Aegir Ridge. The vertical and horizontal components were modeled by ray-tracing into two-dimensional velocity transects. In the north, a maximum crustal thickness of 16 km was found in a narrow root below the eastern part of the ridge. The P-wave velocity at the bottom of the eastern part of the root (7-7.2 km/s) indicates igneous rocks, while the western part (6.8 km/s) is typical for continental rocks, with a 40 km wide transition zone between. The supposed extrusive basalts do not stand out in the data, but may have a low velocity contrast to underlying pre-breakup sedimentary strata. The top oceanic basement is very rough near the Jan Mayen Fracture Zone, with upper basement P-wave velocity of 3.5-4 km/s. A slight increase in the Vp/Vs ratios indicates an increase in fracturing of the deep crust here. Adjacent to the JMR, the top oceanic basement becomes very smooth, and the velocity increases to 5.5 km/s. Average oceanic crustal thickness is 5.3 km. For the southern profile, the average thickness is 5.2 km around the Aegir Ridge. West of that, the crustal thickness increases smoothly to 12 km over a ∼90 km distance, and the crust appears to be of mafic, igneous composition. This is similar to the thick oceanic crust created immediately after continental breakup on the complementary Moere Margin in Norway. West of that region, a crustal root with a relief of up to 3.5 km is associated with a mid-crustal region (5-12 km depth) having a slight drop in P-wave velocity (-0.4 km/s), and Vp/Vs ratios (-0.03), is interpreted as a continental fragment. Towards the Iceland shelf, the results indicate a predominantly mafic igneous crust, with a Moho depth oscillating around 16 km. Relief to the Moho surface and mid-crustal velocity layers indicate that smaller continental fragments may be embedded in the igneous crust also here.