Interpretation of seismic-reflection profiles across the Gulf of Maine and Georges Bank reveals that the region underwent major crustal rifting during the Triassic Period. Three rift systems are delineated: the Fundy fault system extending southwest from the Bay of Fundy to the central Gulf of Maine, the Wilkinson basin system striking north-south in the western Gulf, and the Georges basin/Georges Bank system which parallels the Fundy fault system and underlies the southern Gulf and outer continental shelf. A detailed comparison between these three systems and the Triassic Newark Group onshore suggests similar tectonic histories. A stress-strain analysis of the Triassic fault pattern in the Gulf of Maine shows that the overall stress field associated with continental sepa ation in this area can be defined by a left-lateral shear couple. An attempt then is made to relate the observed structures and related stress conditions to a sequence of six tectonic episodes beginning with initial graben formation in Late Triassic and ending with regional collapse and the onset of geosynclinal deposition in the Middle Jurassic to Early Cretaceous.

In addition to Triassic rifting, the interpretation of the seismic-reflection profiles coupled with refraction data and the analysis of bedrock samples collected during the study by the submersible Alvin provides new insight into the distribution of coastal-plain sediments and the nature of Carboniferous-Permian tectonic activity. The restricted distribution of intermediate compressional wave velocities (4.6 to 5.1 km/sec) and that of late Paleozoic K-Ar radiometric ages to the northwestern Gulf of Maine, suggest the presence of a linear Carboniferous-Permian basin extending 250 km northeast from Boston into the gulf. This basin is thought to be part of a large right-lateral shear zone developed during the final phase of continental collision, when the proto-Atlantic Ocean was closed nd the continental landmasses slid past one another, creating a structural weakness that may have controlled subsequent continental separation in the Triassic.