Collision along an irregular margin: a regional plate tectonic interpretation of the Canadian Appalachians: Discussion

Stockmal et al. recently presented a simple, three-dimensional, plate tectonic model by which they attempted to explain the pre-Carboniferous evolution of the northern Appalachians. This model, which in part follows the basic principles of the two-stage collision model of Colman-Sadd (1982), is mainiy based on the results of a marine seismic profile across northem Newfoundland and on large-scale geometrical considerations. Such models are useful because they provide geologists with a framework that can be tested against field data. However, in this case the existing geological data base, in Our opinion, is incompatible with the model. We will discuss the inconsistencies and present an altemative model. The main thesis of the Stockmal et al. model is that the Taconian and Acadian orogenies are explained in terms of continuous southeastward subduction of Iapetus oceanic lithosphere. During the collision between the Taconic magmatic arc and the irregular margin of North America the Benioff zone "clogged," first in Newfoundland and then in mainland Canada. The continuing convergence between the Taconic arc and North America in Newfoundland was then achieved by delamination of lithospheric mantle while B subduction continued between the Taconic arc and North America in mainland Canada. Evidence in support of this model includes the 300-400 km right-hand offset of equivalent tectono-stratigraphic zones between Newfoundland and mainland Canada and a difference in style and vergence of the Acadian structures in Newfoundland and New Brunswick. The idea of a diachronous collision is not a new one, but the sense of diachronism remains unresolved (see e. g . , Dallmey er 1977; Clague et al. 1981). Lux (1986) considered the allochthonous ophiolites in the Humber Zone and argued in favour of a north to south diachronism. His conclusion is consistent with the fact that the oldest fossil-dated, autochthonous flysch is younger in Quebec than in Newfoundland. Since there is no overstepping autochthonous cover in Quebec, Stockmal et al. argue that the timing of the final emplacement of the allochthons and the closure of Iapetus is not well constrained there, and they explain the Acadian orogeny in mainiand Canada as