DEEP SEISMIC TRANSECTS ACROSS THE SHEARED WESTERN BARENTS SEA-SVALBARD CONTINENTAL-MARGIN

We have used recently acquired deep seismic reflection and refraction profiles together with a large amount of conventional MCS data to study the 1000 km long sheared western Barents Sea-Svalbard margin with the objective of determining its crustal structure and tectonic history. The margin developed from a megashear zone which linked the Norwegian-Greenland Sea and the Eurasia Basin during the Eocene opening. The margin is composed of two large shear segments and a central rifted margin segment associated with volcanism. The continent-ocean boundary is confined within a narrow zone (10-20 km) at the sheared margin segments but is more obscure and partly masked by volcanics at the rifted margin. The crustal structure and tectonic history differ significantly between the three margin segments: In the north, across the Hornsund Fault Zone the crustal thickness changes abruptly from more than 30 km thick continental crust on the Svalbard Platform to 6-8 km thick oceanic crust in the Greenland Sea. This margin segment experienced oblique continent-continent and partly continent-ocean shear with both transtensional and transpressional components during Eocene time while post-Eocene extension caused crustal stretching west of Svalbard. The volcanic province at the central rifted margin segment probably comprises an outer part of thick oceanic crust and an inner part of thinned continental crust covered by a mixture of volcanics and sediments. This formed at a releasing bend in the margin in response to Eocene-Oligocene extension and two phases of volcanism. In the south, the crustal thinning occurs over a wider area landward of the Senja Fracture Zone. Here, the margin bounds a basinal province in which as much as 18-20 km of sediments cover a highly attenuated crystalline crust. The Senja Fracture Zone evolved due to Eocene transtensional continent-continent shear followed by continent-ocean shear and has been passive since earliest Oligocene time. We conclude that the first-order crustal structure along the margin and its tectonic development is mainly the result of three controlling parameters: (1) the pre-breakup structure, (2) the geometry of the plate boundary at opening and (3) the direction of relative plate motion. The interplay between the direction of plate motion and the geometry of the plate boundary gave rise to striking differences in the structural development of the different margin segments.