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

Multichannel seismic reflection profiles in the Hel Graben, Voring Basin, reveal a sill complex at approximately 5 km depth. It is associated with exceptionally high, 7.4 km s(-1), seismic wide-angle velocities. The existence of observable wide-angle arrivals shows that the sills act as efficient waveguides. Seismic reflection data and amplitude modeling constrain the thickness 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 would provide the necessary duration of the magmatic activity as well as locally increased melt volumes and cooling times. Sill complexes of this kind decrease the accuracy of determined velocity fields and crustal geometries below the top of the sill complex, affecting depth conversion and gravity modeling. Furthermore, the results question the concept of lower crustal. bodies as large-scale, homogeneous accumulations of mafic melt.