Crustal Structure of the Greenland-Iceland Ridge from Joint Refraction and Reflection Seismic Tomography

We report a reanalysis of active-source wide-angle seismic data collected along the Greenland-Iceland Ridge during the 1996 Seismic Investigation of the Greenland Margin (SIGMA) experiment. Interpreting the crustal structure of volcanic rifted margins has suffered from nonuniqueness because thick crust at the continent-ocean transition may not be totally of igneous origin. In this regard, the Greenland-Iceland Ridge presents a unique opportunity because, together with the Faroe-Iceland Ridge, it is generally considered to constitute the Iceland hotspot track, and as such, the bulk of its crust may be considered to be of igneous origin. From 15 ocean-bottom instruments deployed along a 290-km-long refraction transect, we collect 5,383 Pg and 1,118 PmP travel times, and we use joint refraction and reflection seismic tomography with adaptive importance sampling to invert them to construct a two-dimensional compressional wave speed (Vp) model across the Greenland-Iceland Ridge. Based on the covariation of crustal thickness and Vp, the western part of the transect may be almost entirely continental. Even the eastern part could include a significant fraction of preexisting continental crust. When considered together with the seismic structure of the Icelandic crust and its geochemical characteristics, our results suggest that the putative Iceland mantle plume could be considerably weaker than commonly assumed.