Crustal structure of northwestern Svalbard and the adjacent Yermak Plateau: evidence for Oligocene detachment tectonics and non-volcanic breakup

North of Forlandsundet Graben the depth of the Moho varies between 23 and 28 km, and remains at this depth to the northern edge of the profile at 81°N. The crustal lithology off western Svalbard can be related to the basement province west of the Raudfjorden Fault Zone. Off the northern shoreline of Svalbard the structure of the Tertiary Dansk?ya Basin is mapped. Below this, a Late Silurian/Early Devonian basin, with seismic velocities between 5.1 and 5.8 km s-1 and a thickness of up to 8 km is present. A Palaeozoic sequence of up to 6 km thickness is expected below the Tertiary cover north of the Dansk?ya Basin. An earlier suggestion, that Oligocene rift processes affected the southern Yermak Plateau, is confirmed. A detachment structure is situated below the Palaeozoic Basin below Dansk?ya Basin, which is probably a consequence of simple shear tectonics. The middle crust exhibits low seismic velocities above the detachment fault. The lowermost crust beneath is slightly contaminated by mantle-derived melts, which is deduced by the slightly elevated velocities of the lowermost crust. These melts can be attributed to decompressive melting caused by modest uplift of the Moho during stretching. The velocity-depth model provides no evidence for large magmatic activity, which implies a non-volcanic rifted margin history. This leads to the assumption that the proposed Yermak Hotspot during the breakup of Svalbard from northern Greenland did not exist.