TRACING SUTURES IN AN ACCRETIONARY OROGEN: DIACHRONOUS PALAEOZOIC ACCRETION OF PERI-GONDWANAN TERRANES IN THE NORTHERN APPALACHIANS AND CALEDONIDES

The term "suture" originated before plate-tectonics and originally referred to a line of ophiolitic fragments within an orogen, separating zones of contrasting affinities. Reconstructions of the Caledonides and northern Appalachians go back to the early years of plate tectonics, when sutures were identified as sites of subduction and ocean closure. Published reconstructions over the last 50 years differ in the positioning of Laurentian and Gondwanan components and in the correlation of major sutures between these components.

Most modern models for the evolution of the northern Appalachians and Caledonides envisage an Ordovician arc-continent collision producing the Taconian/Grampian orogeny. This was followed by subduction polarity reversal, and the accretion of terranes of peri-Gondwanan origin during the Salinian and Acadian orogenies. In such a scenario, sutures may be identified in multiple positions: along the original arc-continent collisional interface; along the line marking subduction polarity reversal; along the inboard edges of subduction complexes; or along lines where oceanic slabs have been emplaced over allochthonous terranes. Using published stratigraphic evidence and detrital zircon provenance data from units of known depositional age, the timing of arrival of allochthonous units at the Laurentian margin between the Early Ordovician and Early Devonian can be constrained. Several of the accreted terranes do not extend over the entire length of the orogen, with the result that the lines separating them change character along strike from terrane-bounding sutures to simple thrust faults in an accretionary complex. The domain Ganderia consists of at least four separate terranes that share a common origin on the continental margin of Gondwana, but were separated by back-arc oceanic crust as they crossed the Iapetus Ocean and collided diachronously with the Laurentian margin.

In identifying lines as sutures, it is important to take this along-strike variability into account. In the past, sutures have been correlated based on either their age, or on the nature (Laurentian vs. Gondwanan) of the terranes they separate. In a non-cylindrical orogen, these two correlation methods will produce different results, and must be clearly distinguished.