In the Appalachians of mainland Canada and New England, Silurian/Early Devonian rocks are preserved in the Connecticut Valley–Gaspé and Merrimack troughs, and rest unconformably or in fault contact with older rocks belonging to Laurentia and to Gander/Avalon, respectively. The Silurian/Early Devonian rocks consist of marine clastic deposits with subordinate carbonates, lava flows and terrestrial deposits. The origin of these sedimentary basins is still poorly understood. Metamorphic ages and structures in the Laurentian margin, major unconformities and syn-sedimentary normal faulting in both troughs argue for significant crustal extension during deposition. The Connecticut Valley–Gaspé and Merrimack troughs are separated by inliers of pre-Ordovician to Ordovician rocks which we interpret as Silurian basement highs that would have been buried in Devonian times to form a composite sedimentary basin. Volcanic rocks are widely distributed in time and space in both basins, and are mostly subalkaline within-plate tholeiites, which is consistent with a tectonic setting involving crustal extension rather than with subduction. Granitic plutons are abundant in the Merrimack trough and attest to high temperatures at mid-crustal levels. It is suggested that crustal extension was responsible for the formation of both basins and that heating of the lower crust in the Merrimack trough during the Silurian was the result of synorogenic collapse likely triggered by delamination at the Laurentia–Medial New England boundary. Delamination of the subducted slab and the upwelling of the asthenosphere would have caused (1) isostatic uplift and formation of basement highs, (2) magmatism in the lower crust and regional-scale contact metamorphism in the upper crust, and (3) collapse of metamorphic terranes and the formation of subsiding sedimentary basins.

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