Crustal structure between the Knipovich Ridge and the Van Mijenfjorden (Svalbard)

The Alfred Wegener Institute of Polar and Marine Research, the University of Bergenand the Hokkaido University acquired new seismic refraction data along a transect fromthe Knipovich Ridge to the inner Van Mijenfjorden in southern Svalbard. A close spacing ofon- and offshore receivers and a dense marine shot pattern provide the data for a high resolutionp-wave velocity model for geological interpretation. Additional new seismic reflection data(University of Bergen) yield structural information for a more reliable analysis.Crustal thickness along the Van Mijenfjorden is 33 to 34 km. Seismic velocities of 5.0 km/sare observed within the upper crustal section of the Tertiary Central Spitsbergen Basin.A Paleozoic sedimentary basin with a depth of 8 to 10 km is associated with the Nordfjorden Block.The seismic velocities are up to 6.0 km/s. Paleozoic sedimentary rocks are expected furtherto the west of the Hornsund Lineament since seismic velocities reveal a similar range here.West of the Bellsund the continental crust thins gradually over a 90 km wide rifted zone.The velocity structure within this section is very complex and comprises zones of decreasedvelocities below the West Spitsbergen Fold Belt (down to 20 km depth) and slightly elevatedvelocities (7.2 km/s) at the crust-mantle transition. The first structure is interpreted as intensivelyfractured rocks linked to post-Late Paleocene transpressive orogenic activity and subsequentlyaffected by transtension during break-up from Greenland. The faster deep-crustal velocities aresupposed to express magmatic intrusions of an unidentified origin. Melts could either be channelled by theSpitsbergen Shear Zone from more distant sources, or originate in magmatic interaction between the northern Knipovich Ridgeand the neighbouring young rifted crust.Oceanic crust each side of the Knipovich Ridge is thin (~3.5 km) and is characterised by theabsence of oceanic layer 3 (3.5/4.1 to 4.7 km/s). The oceanic section exhibits zones of verythin crust (~1 km) that are interpreted as fracture zones. Beneath these we observed decreasedmantle velocities (~7.3 km/s) indicating probable serpentinization of peridotites along thesefracture zones. Thickness variations further provide information about the segmentationand magma supply along the northern Knipovich Ridge.