The Passamaquoddy Bay region of southwest New Brunswick and northeast Maine is known for its moderate seismicity, yet the structures responsible for these earthquakes are not well understood. To gain insight on the tectonic history of this region, marine magnetic surveys were conducted at speculated intersections of faults and the Early Jurassic Ministers Island dyke. Analysis of the vertical gradient of the magnetic field strongly suggests that the 189 ± 8 Ma Ministers Island dyke is dextrally offset 370 m by the Oak Bay Fault below the St. Croix River. This sense of motion points to a reversal of fault movement compared to that recorded in much older (Silurian) rocks nearby. The interpretation is supported by magnetic modelling, constrained by direct measurements of the magnetic susceptibility and natural remanent magnetization of the dyke. In Passamaquoddy Bay, however, farther to the east, there is no evidence that slight deviations in the trend of the dyke anomaly or its termination result from post-emplacement faulting. Furthermore, the data suggest that a sub-parallel mafic dyke on the northern shore of Passamaquoddy Bay is either a distinct dyke system or a left stepover emplacement of the Ministers Island dyke, as opposed to a faulted offset as previously speculated. This study illustrates how focused high-resolution magnetic surveys can strengthen interpretations of faulting in areas of geological ambiguity.

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