The Rocky Brook-Millstream Fault forms the break between the Tobique and Chaleur

subzones of the Chaleur Bay Synclinorium, and father southeast, constitutes the tectonic

contact between the Chaleur Bay Synclinorium and the Aroostook-Percé Anticlinorium (Fig).

Silurian stratigraphy of the Chaleur Group shows marked contrasts on opposite sides of the

fault (Wilson et al., 2005). However, the sequence of Simpsons Field–LaPlante–Free Grant

strata observed south of the fault is also observed north of the fault farther east, allowing the

post-Silurian dextral displacement to be estimated at 30 km (Dimitrov et al., 2004). The bulk

of this offset occurred prior to 380 Ma because the maximum dextral offset of the Nicholas

Dénys Granodiorite (381 ± 4 Ma) near the Ann’s Creek property is only one kilometre

(Walker et al., 1991). Stratigraphic evidence implies that the Rocky Brook-Millstream Fault

was active during (Ludlovian) deposition of the Simpsons Field Formation, and controlled

the deposition of the Chaleurs Group in the southeast (Walker et al., 1993; Dimitrov et al.,

2003).