Silurian-Devonian rocks in the northwestern mainland Appalachians form part of an overstep sequence deposited across the vestiges of Iapetus after the collision of Laurentia and Gondwana. They occur in three synclinoria (Gasp¿-Connecticut Valley, Merrimack-Aroostook, and Fredericton synclinoria), separated by two anticlinoria (Munsungun-Pennington-Boundary Mountain-Bronson Hill and Miramichi). Silurian rocks in Gasp¿-Connecticut Valley and Merrimack-Aroostook synclinoria consist of shelf and trough sediments derived mainly from Laurentia to the northwest, with some local contributions from an ephemeral landmass (Appalachia) and from the Miramichi Highlands. In contrast, Early Devonian rocks in all three synclinoria are mainly flysch derived from the southeast in Avalonia. Interbedded volcanic rocks are predominantly Late Silurian-Early Devonian in age. They are generally bimodal with the mafic rocks exhibiting a change from transitional alkalic-tholeiitic to tholeittic around the Siluro-Devonian boundary, broadly coincident with the Salinic disturbance in the Gasp¿ Peninsula. The geochemical characteristics of the basalts indicate that they were erupted in a continental intraplate environment with melting migrating upward through time across the garnet to spinel phase boundary as stretching increased.

Thermal uplift during rifting is inferred to have led to erosion associated with the Salinic disturbance. The start of the volcanism appears to coincide with a switch from sinistral to dextral transpression along the orogen that may be related to the change from clockwise to anticlockwise rotation of Laurentia relative to Gondwana. The effects of dextral transpression vary with the trend of the orogen; intense deformation and metamorphism occurred in the Central Mobile belt opposite the New York and St. Lawrence promontories, whereas rifting developed in the Quebec Reentrant, leading to thinning of the crust, upwelling of the asthenosphere, melting, and magmatism. ¿ American Geophysical Union 1994