Crustal Flow and Formation in Iceland from Radial Anisotropy

The formation of crust at mid-ocean spreading ridges is one of the fundamental processes of

plate tectonics. Iceland, which straddles the mid-Atlantic ridge and is uplifted by a convective

plume in the underlying mantle, has an active mature spreading ridge system exposed above

sea level. It therefore represents a unique opportunity to study a spreading ridge using landbased

methods. Furthermore, the crust in Iceland has an abnormal structure with a varying

thickness (up to 40 km in central Iceland) and a noticeable mid-crustal discontinuity at ~20km

depth. While this anomalous structure is usually attributed to the ridge-plume interaction, its

formation process and varying spatial distribution are highly debated. In this work, we explore

the velocity structure and radial anisotropy of the crust and upper mantle in Iceland using phase

velocity dispersion curves measurements from ambient noise for both Love and Rayleigh

waves. We find that while both Love and Rayleigh phase velocities are lower along the rift

zones, the SH and SV structures differ significantly with depth. In the uppermost crust we find

mostly negative radial anisotropy (SV > SH), which we attribute to the existence of vertically

oriented cracks and dykes which are formed due to the extensional regime of plate spreading.

In the lower crust however, we mostly observe positive radial anisotropy (SV < SH).

Furthermore, the radial anisotropy in the lower crust displays a weak azimuthal dependence

with lower velocities parallel to the plate spreading direction. These observations suggest that

the lower crust is built by layers of sills containing anisotropic minerals such as olivine and

that it flows horizontally with a component parallel to the spreading direction.