High b-values in the leaky segment of the Tjörnes Fracture Zone north of Iceland: are they evidence for shallow magmatic heat sources?

Abstract

Near the seamount Hóll in the Tjörnes Fracture Zone (TFZ) north of Iceland two anomalies of high seismic b-values have been found in a depth shallower than 10 km. According to the results of similar b-value anomalies beneath volcanic edifices such as Mt. St Helens a magma chamber was suspected in the region. On first view, the existence of a long-living magma body between the island of Kolbeinsey and Hóll seamount appears to be confirmed by reflection seismic surveys conducted in the area with shallow water depths of 400 m. They show a clear deep reflector. However, the reflector is best imaged in a migrated record at 1500 m/s, i.e. the speed of sound in water, and it can be modeled by single scattering from the bathymetry in the area, which leads to the conclusion that the apparent reflection images the seafloor rather than a magma chamber. The recent volcanism along Manareyjar Ridge with the last eruption in 1868/69 and the hydrothermalism east of Grimsey Island would be trivially explainable by a magma chamber in the subsurface. However, there is no evidence for a magma chamber in this more southern part of the Grimsey Lineament (GL). Even in a volcanic area such as the GL as the leaky part of the TFZ, neither the existence of high b-values nor an apparent deep reflector prove the existence of a magma chamber. Instead, the seismic imaging of magma chambers in areas of intermediate to slow spreading ridge areas appears to be questionable.