Timing and spatial distribution of deformation in the Newfoundland Appalachians: a "multi-stage collision" history

Abstract

The Newfoundland Appalachians have been interpreted as an area where Lower Paleozoic plate convergence culminated in collision between an Ordovician volcanic chain and the North American craton hi Middle Ordovician times. Closure of the intervening proto-Atlantic (Iapetus) ocean was considered incomplete. Subsequent deformation gave rise to regional folding and faulting.Recent studies in the Newfoundland Dunnage zone have revealed that the deformation history is far more complex than previously recognized. Large-scale thrusting, folding and faulting occurred in Silurian-Devonian times. Furthermore, it has been suggested that the Dunnage zone is an allochthonous terrane underlain by dominantly continental crust rather than representing remnants of a "rooted" ocean basin.In view of these results a revision of tectonic scenarios and zonal subdivision is warranted and a "multi-stage collision" history will be discussed, with emphasis on the spatial distribution and significance of Silurian-Devonian deformation in central Newfoundland.Subduction in Lower Paleozoic times gave rise to the formation of a volcanic terrane; concurrently, to the southeast a marginal sea was formed (Mariana-type subduction). In Middle Ordovician times the volcanic terrane collided with the North American craton ("first-stage collision") and back-arc spreading terminated. Continued crustal shortening resulted in the formation of a Silurian accretionary terrane (telescoped marginal sea), and its subsequent deformation ("second-stage collision"). Devonian (-Carboniferous?) strike-slip faulting represents the third stage in the collision history.The model is applicable to large tracts of the Caledonian-Appalachian chain. Its main characteristics are: 1. (a) the revised zonal subdivision of the area is based on characteristics of Silurian and older rocks, rather than Middle Ordovician and older rocks only;2. (b) the central part of the orogen represents a telescoped marginal sea that formed to the southeast of the Ordovician volcanic chain, rather than a remnant of the incompletely closed Iapetus ocean;3. (c) the earliest deformation is progressively younger toward the southeast;4. (d) the Appalachian collision history is a result of the activity of a single deformation regime over a long period of at least 75 Ma.