Surface wave tomography of the upper mantle beneath the Reykjanes Ridge with implications for ridge–hot spot interaction

Abstract

[1] We modeled fundamental mode Love and Rayleigh waves to study the seismic properties of the upper mantle beneath the Reykjanes Ridge. These waves were generated by regional earthquakes occurring in the North Atlantic to the south of Iceland and were recorded by stations located on Iceland. Over 12,000 measurements of the phase, group arrival time, and amplitude of narrow-pass-filtered waveforms (over the period range of 9.5-100 s) were used to solve for mantle shear wave velocity structure and anisotropy. In a vertical plane oriented normal to the ridge axis, the velocity structure contains a broad and deep low-velocity zone in the upper mantle beneath the ridge. A joint analysis of the seismic structure with gravity data reveals that the low velocities are consistent with elevated temperatures ($75°). Our study shows that plume material spreads broadly outward beneath the Reykjanes Ridge from Iceland and is not confined to a narrow lithospheric channel. At distances >200 km from the ridge, shear wave anisotropy indicates a predominant horizontal alignment of the fast axes of anisotropic crystals (mainly above 50 km depth), which can be interpreted as past, horizontal, ridge-perpendicular flow. Within ±200 km of the ridge the anisotropy indicates a general vertical alignment of the fast axes or an alignment such that the fast axes point along the ridge. The transition to this type of anisotropy coincides with the appearance of increased hot spot-ridge interaction $20 Myr ago, indicating that plume flow has largely disrupted mantle flow beneath the ridge since that time.

Investigating seismic anisotropy beneath the Reykjanes Ridge using models of mantle flow, crystallographic evolution, and surface wave propagation

“…1 To mimic the experiment of Delorey et al [2007], we considered the phase velocities of surface waves propagating in a single direction, Defined by Ito et al [1996]. b Radius is distance from the center where excess temperature decays to 1/e of the maximum.…”

“…[11] Similar to Delorey et al [2007], we jointly inverted the Love and Rayleigh phase velocities for Vp, Vsv, and radial anisotropy, (Vsh À Vsv)/ [(Vsh þ Vsv)/2], using a least squares iterative method (see supporting information Appendix A2). The variance, correlation length, and depth of grid points were chosen to produce solutions matching the characteristics of the input model.…”

“…[4] To explain the off-axis regions of negative radial anisotropy, at least three hypotheses involving mantle plume influence and LPO have been proposed: (1) high mantle buoyancy beneath the ridge, due to melt retention, produces convection cells with a strong upwelling limb beneath the ridge axis and two sinking limbs on the sides of the ridge, where the olivine a axes are aligned vertically [Blackman et al, 1996;Gaherty, 2001]; (2) strong Delorey et al [2007]. Red tick marks indicate the orientation of Rayleigh waves azimuthal anisotropy at 75 km depth as determined by Pilidou et al [2005].…”

“…The last hypothesis arises by the fact that the surface waves used in the seismic studies traveled roughly parallel to the Reykjanes Ridge, providing little azimuthal coverage [Gaherty, 2001;Delorey et al, 2007], therefore prohibiting an independent detection of azimuthal anisotropy. This hypothesis is supported by a local study that shows azimuthal anisotropy with the fast orientation of Vsv being roughly parallel to the RR in the shallow upper mantle beneath Iceland [Li and Detrick, 2003] and a larger scale study of the whole North Atlantic showing a similar sense of anisotropy over much of the length of the RR [Pilidou et al, 2005] (Figure 1a).…”

Regional distribution of volcanism within the North Atlantic Igneous Province

“…A strong influence to the south along the Reykjanes Ridge is well documented whereas the influence to north is significantly reduced (e.g. Vogt 1971;Ito 2001;Delorey et al 2007). This is typically attributed to a deflection of the Iceland mantle plume to the south, causing stronger plume-ridge interaction.…”