A tale of three collisions: terrane accretions and cryptic ocean closures in the Nova Scotia segment of the Appalachian orogen

Closure of the Iapetus and Rheic oceans occurred diachronously along the Appalachian orogen, and documentation of sutures is complicated by post-collisional deformation and by irregularities in the original Laurentian margin along which accretion of terranes occurred. In Nova Scotia, at least four cryptic terrane boundaries involved ocean closures but do not show the typical geological assemblages associated with subduction-related accretion of terranes. Three collisions discussed here are (i) between two Ganderian terranes (Aspy and Bras d’Or), (ii) between the Ganderian Bras d’Or terrane and Avalonian Mira terrane, and (iii) between the Meguma terrane and Avalonia in northern mainland Nova Scotia. In Cape Breton Island arc magmatism spanned the Ediacaran to Cambrian (620 Ma – 530 Ma) in both Aspy and Bras d’Or terranes, but only the Aspy terrane records arc magmatism in the Ordovician to Silurian. The Eastern Highlands shear zone (EHSZ) juxtaposed the Ganderian Aspy and Bras d’Or terranes at ca. 420-390 Ma. Minor magmatism at ca. 402 Ma likely occurred in a syn-collisional pull-apart basin that formed in an overall transpressional environment. No evidence is preserved in Nova Scotia of a magmatic or metamorphic event associated with collision of the Ganderian Bras d’Or terrane with the Avalonian Mira terrane, and the suture is not exposed at the surface. Geophysical data and clasts in a conglomerate overlying the suture constrain the location and age of the boundary, but its nature is not well understood. However, in Newfoundland this collision is marked by extensive subduction-related Silurian to Devonian magmatism and metamorphism, suggesting that in the Nova Scotian segment the collision was mainly transpressional. The accretion of the Meguma terrane to the southern Avalonian margin in Nova Scotia is also a well- documented transpressional collision. No subduction-related magmatism has been associated with the collision, but it was coeval with voluminous S-type magmatism throughout the Meguma terrane.  The transpressional character of these three accretionary events in Nova Scotia, in contrast to the equivalent events elsewhere in the northern Appalachians, suggests that the Nova Scotian segments of each collision may have repeatedly developed as transform boundaries.