Seismic wavespeed images across the Iapetus and Tornquist suture zones

Closures of the Iapetus Ocean and the Tornquist Sea lead to the collision of the paleocontinents of Laurentia, Baltica and Eastern Avalonia during the Caledonian orogeny. It has been speculated that relicts of these two closures may be preserved within the crust or upper mantle. Over the past decades, numerous wide-angle seismic profiles were gathered in northwestern Europe to search for related subsurface features. Although active source studies revealed detailed crustal structures across the Iapetus and Tornquist suture zones, there are relatively few clear three-dimensional upper mantle images beneath this region. We use a new European crust and upper mantle model, EU30, determined based on continental scale, nonlinear adjoint tomography, to explore upper mantle structures across these two suture zones. Model EU30 reveals two fast anomalies within the upper mantle: one dips in a northwesterly direction down to approximately 400 km beneath the North Sea, and the other dips in a southwesterly direction down to nearly 250 km across the Tornquist Suture Zone. In addition, we observe a "gap" between the lithospheres of Laurentia and Eastern Avalonia across the Iapetus Suture Zone beneath the central British Isles. These seismic images suggest that heterogeneity related to the closures of the Iapetus Ocean and the Tornquist Sea have been preserved within the upper mantle over hundreds of millions of years. Citation: Zhu, H., E. Bozdag, D. Peter, and J. Tromp (2012), Seismic wavespeed images across the Iapetus and Tornquist suture zones, Geophys. Res. Lett., 39, L18304.