PERMO-TRIASSIC OROGENY IN THE NORTHERN CORDILLERA: SONOMA NORTH?

The Late Permian to Early Triassic Sonoman Orogeny of the western US is characterized by the emplacement of a relatively intact oceanic slab (Golconda Allochthon) eastwards over the Antler overlap sequence. Models proposed for the Sonoman Orogeny include: 1) closure of a back-arc basin behind a W-facing arc; or 2) collision of an E-facing arc with the western edge of the continent. Subsequently, a W-facing Middle Triassic arc was built along the margin. Recent work in the Yukon demonstrates that the timing and style of latest Paleozoic deformation along the Ancestral North American margin is similar to that of the Sonoman Orogeny. In the northern Cordillera, mid-Paleozoic back-arc extension led to formation of the Slide Mountain Ocean (SMO) behind a W-facing arc built on a rifted ensialic fragment (Yukon-Tanana Terrane, YTT). The SMO started to close in late Paleozoic time by W-dipping subduction under YTT and the attached remnants of the SMO, generating the Late Permian Klondike arc on YTT. Metaluminous intrusions ranging in age from ~220-185 Ma subsequently intruded YTT. Emplacement of the YTT/SMO allochthon onto North America was previously thought to have occurred in Early Jurassic time. New mapping, structural studies, U-Pb zircon dating and detrital zircon studies, indicate:1) the Klondike arc rocks range in age from 264-255 Ma; 2) arc rocks were ductily deformed twice by 250 Ma; and 3) detritus from the YTT is first recognized on the western margin of North America by Early Triassic (Smithian) time, well before the previously proposed Early Jurassic time of allochthon emplacement. We argue that latest Permian ductile deformation preserved in the Klondike arc records a collision between the YTT/SMO allochthon and the thinned western edge of the North American plate. By Early Triassic time sediments were being shed from the uplifted and deformed hinterland eastwards across the suture into a peripheral foreland basin built on the remnant Cordilleran miogeocline. The subsequent W-facing mid-Late Triassic magmatic arc in YTT (Quesnellia) is therefore a continent margin arc developed on the new North American continental margin. Our data indicate late Paleozoic tectonism affected both the north and south portions of the Cordilleran margin and suggest Permo-Triassic orogeny was not an isolated event to the southwestern US.