High-velocity breakup-related sills in the Voring Basin, off Norway

Multichannel seismic reflection profiles in the Hel Graben, Voring Basin, r eveal a sill complex at approximately 5 km depth. It is associated with exc eptionally high, 7.4 km s(-1), seismic wide-angle velocities. The existence of observable wide-angle arrivals shows that the sills act as efficient wa veguides. Seismic reflection data and amplitude modeling constrain the thic kness of individual sills to approximately 100 m. Sonic logs from sills of similar thickness on the nearby Utgard High show an average velocity of 7.0 km s(-1) Such high velocities require an olivine-gabbroic sill composition and emplacement under conditions which allowed growth of relatively large crystal sizes. A possible reason for such an emplacement environment is the Hel Graben's role as an intrusion center during breakup volcanism. This wo uld provide the necessary duration of the magmatic activity as well as loca lly increased melt volumes and cooling times. Sill complexes of this kind d ecrease the accuracy of determined velocity fields and crustal geometries b elow the top of the sill complex, affecting depth conversion and gravity mo deling. Furthermore, the results question the concept of lower crustal. bod ies as large-scale, homogeneous accumulations of mafic melt.