TECTONOSTRATIGRAPHIC FRAMEWORK AND PALEOZOIC EVOLUTION OF PERICRATONIC TERRANES IN THE NORTHERN CORDILLERA

The pericratonic terranes of the northern Cordillera (including Yukon-Tanana [YTT] and Slide Mountain [SM] terranes) consist of complexly deformed and metamorphosed mid- to late Paleozoic continental margin, arc and marginal basin assemblages. They underlie large portions of the northern Cordillera, but yet they remain some of its most poorly understood elements. Recent regional mapping of YTT and related terranes in Yukon, northern B.C. and eastern Alaska provides the basis for a new tectonostratigraphic framework of the pericratonic realm.

YTT of Yukon and B.C. consists of four tectonic assemblages of regional extent. They comprise a basal siliciclastic assemblage of continental margin affinity (Snowcap assemblage) overlain by three unconformity-bounded, mid- to late Paleozoic volcano-sedimentary successions of continental arc and back-arc affinities (Finlayson, Klinkit and Klondike assemblages). Coeval oceanic rocks of the SM assemblage record contemporaneous development of a marginal ocean basin. Much of east-central Alaska that was formerly assigned to YTT is now re-interpreted as part of the parautochthonous continental margin.

The Paleozoic tectonic evolution of the northern pericratonic realm includes: 1) Middle to Late Devonian regional extension and rifting of part of the distal continental margin (Snowcap assemblage); 2) Late Devonian onset of arc and back-arc magmatism in both YTT and distal portions of the North American margin (NAM; eastern Alaska, Selwyn basin, Kootenay terrane of southern B.C.); 3) Early Mississippian opening of the SM marginal ocean between the YTT arc and NAM (including pericratonic rocks of eastern Alaska); 4) superimposed Carboniferous arc magmatism in YTT (Finlayson and Klinkit assemblages), punctuated by at least one episode of regional, intra-arc deformation; and 5) Middle to Late Permian arc magmatism and development of HP rocks along the eastern edge of YTT. The Middle Devonian to Early Permian evolution of YTT is interpreted to have occurred above an east-dipping subduction zone, with slab-rollback as driver of regional extension. The Middle Permian marked a change to westard subduction of SM lithosphere beneath YTT. Closure of the SM ocean and accretion of YTT to the NAM plate are indicated by occurrence of YTT detritus in Triassic clastic strata of the NAM.