Gravity models across the northern flank of the Anglo-Brabant Massif in the southern North Sea, constrained by interpretations of deep seismic reflection profiles, suggest a structure in which the lower crust increases in thickness from south to north across a step in the Moho. To the south of this step a prominent reflector defines the top of a high density body in the middle crust, which also has a strong magnetic signature. By analogy with similar structures elsewhere, the modelled structures are interpreted as having formed prior to and during the collision of Armorica and Baltica following southerly subduction of the Tornquist Ocean beneath the former in Ordovician and Silurian times. It is hypothesised that the Anglo-Brabant Massif may have a compound origin in this collisional event and a period of northward-directed subduction in the Carboniferous.