TECTONIC INHERITANCE IN THE NORTH ATLANTIC: A CONSEQUENCE OF EPISODIC OROGENIC COLLAPSE?

The eastern margin of North America displays an obvious spatial association between the location of present continental margins and rocks affected by older orogens. The region comprises two complete Wilson cycles that terminated in collisional orogens and a modern continental margin. A prominent feature of the North Atlantic margin is a series of crustal-scale structural boundaries in the Grenville and Appalachian orogens and their seaward prolongations that coincide with fracture zones in the modern ocean crust. In contrast, some oceanic transform margins are older and do not correspond to pre-existing lineaments—whereas some pre-existing Paleozoic boundaries are not expressed in Mesozoic rift structures or modern fracture zones.

The spatial relationship between fracture zones of the North Atlantic and major structural boundaries in the Grenville and Appalachian orogens implies the dominant role of tectonic inheritance. Four observations, however, are inconsistent with mechanisms involving only ancient collisional events: 1) the structural fabric in the older orogens is parallel to the vector direction of transform motion and seafloor spreading; 2) the offshore trace of transforms is colinear with major crustal boundaries; 3) pre-existing faults are not reactivated inboard of the rifted continental margin; and 4) transform faults do not extend outboard from suture zones. Several lines of evidence suggest that rapid uplift and collapse are associated with terminal phases of the Grenville and Appalachian orogens. These observations indicate that crustal boundaries formed by collisional orogenesis were reactivated during orogenic collapse and are responsible for partitioning strain throughout the lithosphere and producing a pervasive fabric of aligned olivine crystals in the mantle. These heterogeneous zones of weakness in the mantle act as structural guides that remain tectonically active over long-time scales and indicate that tectonic processes in the continental crust can generate fabrics in the mantle lithosphere that are subsequently duplicated in the oceanic crust. The persistence of a zone of mantle anisotropy associated with orogenic collapse suggests that the structure of the modern North Atlantic Ocean was predetermined by tectonic processes in the Proterozoic.