The IMPULSE experiment: Oceanic crust formed beneath the Reykjanes Ridge at 60° N

There are relatively few measurements of oceanic crust formed at the Reykjanes Ridge south of Iceland. During the IMPULSE experiment of 2024, we acquired two wide-angle seismic profiles using dense arrays of ocean bottom seismometers (OBSs). One profile, presented here, deployed 89 OBSs along an approximately 400 km flow line centered on the ridge axis at 60°17’ N, extending to plate ages of over 18 million years on either side. The second profile consists of 51 OBSs deployed along a 550 km axial chron line. Travel times of crustal (Pg) and mantle (Pn) refractions, and the wide-angle reflections from Moho (PmP) were picked and inverted using the TOMO2D software package to map crustal and upper mantle structure along the flow line. The results reveal an igneous crustal thickness varying between 6 and 9 km at intervals of 25-50 km from the ridge axis. Seismic velocities near the base of the thickest crust reach ~7.5 km/s away from the ridge axis, but can be as low as ~7.1 km/s for the thinnest crust on the profile. Variations of both crustal thickness and seismic velocity with distance are similar on either side of the ridge axis, suggesting that they are controlled by axial processes. At the ridge axis, the crust is approximately 9 km thick. However, lower crustal velocities within 10 km of the ridge axis are ~0.5 km/s slower than those observed at locations with similar crustal thicknesses only 50 km away. This observation suggests that the thick axial crust is anomalously hot, consistent with the diminished earthquake seismicity observed along this segment of the Reykjanes Ridge. Our results support the hypothesis that a hot transient pulse of asthenosphere lies beneath the Reykjanes Ridge at 60° N.