Total magnetic intensity and seismic refraction are used to define tectonic features on the inner shelf beneath Block Island and Rhode Island Sounds. Magnetic lineaments suggest a tectonic pattern of rhombo-hedral basement blocks controlled by northwest and north-northeast to northeast fault sets. Similar patterns predominate in the basement complexes of Rhode Island and Connecticut.
From the geophysical data, the Narragansett Basin is inferred to continue at least 16 to 22 km off the Rhode Island coast. Here grabens or half grabens, filled with Pennsylvanian(?) metasediments may lie on either side of a central basement horst. Another Carboniferous basin may exist farther seaward on the Long Island platform.
The Narragansett Basin probably evolved in early Carboniferous time as a pull-apart basin between en echelon zones of left-lateral northeast-trending faults. Later episodes affecting the basin include a Permian compressional event, the consequence of a regional change to dextral shear along the same fundamental faults; a second phase of east-west extension in Late Triassic–Jurassic time; and a post-Cretaceous event during which regional fracture systems were reactivated by vertical movements.