Deep seismic reflection and magnetic data suggest that the Gulf of Maine is underlain by four crustal blocks of differing reflection and magnetic character. Two of these blocks, the Gulf of Maine fault zone and adjacent central plutonic zone, can be correlated with Avalonian rocks in southern New England and New Brunswick. The boundary between them, the Fundy fault, projects onshore near the Ponkapoag fault in southeastern Massachusetts in a region where no major crustal boundary has (yet) been identified. A third block, called the southern plutonic zone, is interpreted as Avalonian, although the reflection and magnetic data are ambiguous. The fourth block, along the rifted continental margin, is correlated with Meguma rocks of Nova Scotia. The late Paleozoic Variscan front crosses the Gulf of Maine within the Gulf of Maine fault zone and indicates significant compressional movement rather than strike-slip.
The Moho surface throughout the region is essentially flat and may have been produced by Mesozoic crustal extension. Associated modification of the lower crust was minimal in the northern Gulf and may have been moderate in the central and southern Gulf. The Franklin rift basin formed by reactivation of the inferred Avalon-Meguma boundary beneath Georges Bank as a low-angle detachment.