RECOGNIZING CRYPTIC OROGENIC SUTURES IN COLLISIONAL OROGENS: THE EXAMPLE OF CAPE BRETON ISLAND, NOVA SCOTIA, CANADA

The presence in Cape Breton Island of at least four fault-bounded terranes with different pre-Carboniferous stratigraphic, magmatic, metamorphic, structural, geophysical, isotopic, and tectonic characteristics has been recognized since the 1980s. These differences occur over a present-day cross-sectional distance of only 140 km, the narrowest part of the northern Appalachian orogen to record such diversity from Grenville province to Avalonia and possibly Meguma. The inferred sutures are cryptic, marked by Carboniferous faults, and lack any direct evidence for the former presence of intervening ocean crust. Mostly they are obscured by younger sedimentary cover. Proving that these young faults mark the sites of former subduction zones has been difficult, and hence alternative interpretations persist that they represent different parts of some smaller number of former continental fragments, or perhaps only a cross-section of a single continental fragment (Avalonia). An additional problem has been the documented occurrence of orogen-parallel translation of terranes relative to one another, so that now-adjacent terranes may have been separated by hundreds of kilometres at the time when they were forming. To try to resolve these issues, the approach has been to quantify all of the geological characteristics of the pre-Carboniferous rocks, in particular to identify the presence of continental margin sedimentary rocks of various ages, as well as subduction-related igneous rocks within each terrane, requiring the former presence of a plate convergence. The problem with this approach has been confirming that rocks of similar or overlapping ages formed in unrelated subduction zones, especially as isotopic characteristics are also ambiguous or overlapping. Comparisons with terranes in Newfoundland and New Brunswick where evidence of intervening oceanic rocks or subduction-related metamorphism is better preserved has provided perhaps the most convincing evidence. New U-Pb (zircon) ages have helped by leading to the recognition of Cambrian-Ordovician arc and possible back-arc units, strengthening the similarity of central Cape Breton Island to Ganderian central Newfoundland, and better constraining the ages of Neoproterozoic through Devonian subduction events.