MASSACHUSETTS BAY LINEAMENT: A SUBMERGED, 100-KM-LONG, NORTHWEST-TRENDING BRITTLE FAULT ZONE EAST OF BOSTON, MA?

Enhancements of recently available digital sonar data from Massachusetts Bay have revealed a 50-km-long lineament, herein named the Massachusetts Bay lineament (MBL), which extends east-southeast from northern Boston to Stellwagen Bank. The MBL on the west side of the bay is characterized by a 10-km-wide depression along which there is a zone of multiple, WNW-trending, rectilinear, elevated block-like features that are 200-400 m wide, up to 12 m high, and up to 2 km long, some of which can be traced almost to the coast at Boston. Along the west side of this 10-km-wide depression and just east of Nahant Island is a zone of WNW-trending, 2- to 4-km-long, 4- to 7-m-high, northeast-side-up scarps which may be fault related. To the southeast, the MBL is characterized by a linear, 10-km-wide canyon that is tilted down to the northeast and is bisected by a series of three linear, left-stepping, submerged ridges. Only a few of the elevated blocks, like those to the northwest, were noted in this area because of thicker seafloor sediments. A local depression occurs along the most eastern left-step along the submerged ridges, an observation which is consistent with a pull-apart basin if this part of the MBL is a left-stepping fault zone that has undergone sinistral motion. Sinistral motion on this proposed fault zone would be favored by the MBL’s west-northwest orientation relative to the ENE-oriented axis of the regional maximum horizontal compressive stress field. Sinistral motion on the proposed fault zone is also favored by an apparent ~5-km-wide sinistral offset of a zone of NE-trending sonar lineaments across the MBL ~8 km east of Nahant Island. Further studies of the NE-trending zone of lineaments are needed, however, to confirm this hypothesis and to determine the timing of displacement along the MBL.

About 10 km north of Cape Cod, the MBL is collinear with a 50-km-long, NW-trending, linear positive gravity anomaly interpreted from the regional gravity image that was illuminated from the northeast, suggesting that the MBL may be at least 100 km long. Inland, a 10-km-long linear positive magnetic anomaly interpreted from the regional aeromagnetic image suggests that the MBL may extend through the northern Boston area. Seismicity near the MBL is sparse, however, suggesting that the proposed fault zone is either locked or is currently inactive.