NEW DATA AND IDEAS ON THE PALEOZOIC-TRIASSIC EVOLUTION OF THE INSULAR SUPERTERRANE OF THE NORTH AMERICAN CORDILLERA

We confirm earlier claims that Alexander and Wrangellia were amalgamated together into an Insular superterrane by at least the Pennsylvanian. Syn-tectonic, terrane-stitching Pennsylvanian to Permian plutons intruded during a penetrative phase of deformation. This deformation, which is in part coeval with the Browns Fork orogeny of the Farewell terrane, follows the Early Paleozoic Wales and Silurian Klakas orogenies.We propose that the Klakas orogeny led to amalgamation of the Early Paleozoic arc subterranes of SE Alaska with the St. Elias subterrane along a cryptic suture, which is largely masked by syn-orogenic Silurian turbidites (arc-trench gap-foreland basin). The St. Elias subterrane is mainly exposed in the Yukon and northern BC and is characterised by a well-preserved, Late Cambrian-Upper Silurian carbonate platform, which appears to rest conformably on Early to Middle Cambrian rift-related siliciclastics and volcanics. The inferred, late Cambrian rift-drift transition is unknown in Laurentia and suggests that the St. Elias subterrane is a tectonic tracer of its parent continent (Baltica and/or Siberia?). Most of the Late Permian to Middle Triassic is missing in the Insular superterrane, which probably reflects deformation-induced uplift and accompanying erosion. In addition, the Paleozoic amalgamation of Alexander and Wrangellia indicates the existence of a tectonic linkage between the Upper Triassic Karmutsen/Nikolai flood basalts in Wrangellia and the approximately coeval volcanic rock sand in the mineralised Tats Group and correlatives in Alexander. Published geochemical data indicate that the volcanic rocks in the Alexander metallogenic belt range from bimodal in the south to mainly basalt in the north. Compositions are generally transitional between arc and non-arc settings. Combined the upper Triassic volcanics probably formed in a rift-setting with the arc-component in the volcanics diminishing to the north and over time. Based on the existing data, we propose that the Insular superterrane collided with a ridge during the Middle-Late Triassic in its northern half, which formed a slab-window above which the flood basalts and the Tats volcanics were erupted. Over time the rift locally may even have progressed into a relatively narrow, Gulf of California-type oceanic basin.