Episodic Silurian-Devonian Orogenesis Due to Accretion of 3 Gondwanan Terranes

The Northern Appalachians evolved as an accretionary orogen prior to the Carboniferous-Permian assembly of Pangea. The Salinic (435-423 Ma), Acadian (421-400 Ma) and Neoacadian orogenies (395-350 Ma) are due to sequential accretion of Ganderia, Avalonia and Meguma, each having a different Paleozoic geological evolution, which led to a progressive eastwards (present coordinates) migration of the onset of collision-related deformation, metamorphism and magmatism. “Cratonization” of the accreted terranes also appears to become younger towards the east, consistent with episodic terrane accretion. Nevertheless, some terranes may have interacted with each other prior to their accretion to Laurentia.

Voluminous, syn-collisional felsic granitoid-dominated pulses associated with the Salinic and Acadian and perhaps also the Neoacadian are explained as products of slab-breakoff.

The Salinic orogeny was due to Late Ordovician-Early Silurian (450-423 Ma) closure of the Tetagouche-Exploits backarc basin, which separated the active leading edge of Ganderia (Popelogan-Victoria arc) from its trailing passive edge, the Gander margin. Salinic closure was initiated following accretion of the active leading edge of Ganderia to Laurentia and stepping back of the west-directed subduction zone. The Salinic orogeny was immediately followed by Late Silurian-Early Devonian accretion of Avalonia (421-400 Ma) and Middle Devonian-Early Carboniferous accretion of Meguma (395-350 Ma), which led to the Acadian and Neoacadian orogenies, respectively.

Each accretion took place after stepping-back of the west-dipping subduction zone behind an earlier accreted crustal ribbon. The Acadian orogeny was characterised by a flat slab setting after the onset of collision, which coincided with rapid southerly paleolatitudinal motion of Laurentia. Acadian orogenesis preferentially started in the hot and hence, weak backarc region. Subsequently it was characterised by a time-transgressive, hinterland migrating fold and thrust belt antithetic to the west-dipping A-subduction zone. The Acadian deformation front appears to have been closely tracked in space by migration of the Acadian magmatic front.