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

Abstract

The Alfred Wegener Institute of Polar and Marine Research, the University of Bergen and Hokkaido University acquired new seismic refraction data along a transect from the Knipovich Ridge to the inner Van Mijenfjorden in southern Svalbard. A close spacing of on- and offshore receivers and a dense marine shot pattern provide the data for a high resolution p-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 s−1 are 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−1. Paleozoic sedimentary rocks are expected further to 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 decreased velocities below the West Spitsbergen Fold Belt (down to 20 km depth) and slightly elevated velocities (7.2 km s−1) at the crust-mantle transition. The first structure is interpreted as intensively fractured rocks linked to post-Late Paleocene transpressive orogenic activity and subsequently affected by transtension during break-up from Greenland. The faster deep-crustal velocities are supposed to express magmatic intrusions of an unidentified origin. Melts could either be channelled by the Spitsbergen Shear Zone from more distant sources, or originate in the magmatic interaction between the northern Knipovich Ridge and the neighbouring young rifted crust. Oceanic crust each side of the Knipovich Ridge is thin (∼3.5 km) and is characterised by the absence of oceanic layer 3 (3.5/4.1 to 4.7 km s−1). The oceanic section exhibits zones of very thin crust (∼1 km) that are interpreted as fracture zones. Beneath these we observed decreased mantle velocities (∼7.3 km s−1) indicating probable serpentinization of peridotites along these fracture zones. Thickness variations further provide information about the segmentation and magma supply along the northern Knipovich Ridge.