Abstract The Tornquist and Iapetus Suture Zones result from amalgamation of three plates (Laurentia, Baltica and Eastern Avalonia) during the Early Paleozoic Caledonian orogeny (∼440 Ma). We present a comparison of the velocity structure of the contrasting Proterozoic and Paleozoic lithosphere across the margins of Eastern Avalonia based on two deep seismic experiments, MONA LISA and VARNET in the SE North Sea and SW Ireland, respectively. Both velocity models show three different crustal types: (a) a high-velocity, three-layered shield type Proterozoic crust (in Baltica and Laurentia) to the north; (b) a transitional crust in the central part across the suture zones; and (c) Eastern Avalonian crust to the south. However, the sub-Moho velocities are ∼7.8 km/s under the ∼34-km-thick Baltica crust and ∼8.2 km/s under the 26-km-thick Eastern Avalonian crust on the MONA LISA-1 profile, in contrast to ∼7.8 km/s under the ∼31-km-thick Eastern Avalonian crust and ∼8.1 km/s under the ∼33 km thick Laurentian crust on the VARNET profile. These differences in the sub-Moho velocity structure are interpreted to be related to a change in subduction polarity between the Tornquist Sea and the Iapetus Ocean or in the direction of shearing in the mantle during collision tectonics.