WESTERN CANADIAN CORDILLERAN TERRANES: NEW INSIGHTS FROM MICROPALEONTOLOGY AND TECTONOSTRATIGRAPHIC SEQUENCES

One of the longstanding questions regarding the accreted terranes on the western margin of cratonic North America has been with respect to their position relative to the craton. Are they far-traveled and truly exotic or are they pericratonic and share a long history with North America? This shared history has typically been recognized using similarity in biotic provinces revealed by fossils like fusulinaceans and conodonts. Another tool may be to be concentrate on the similarity of the temporal distribution within the stratigraphic succession and particularly on the unconformities that respond in part to tectonic events. Is there a shared tectonostratigraphic history? Deep crustal seismic studies have recently revealed that the Cache Creek, Quesnell, and Yukon-Tanana terranes represent thin slices thrust across an extensive wedge of Proterozoic basement rocks that extends much further west compared to previous models. Of these three terranes only the latter is typically regarded as pericratonic and only the Cache Creek has an “exotic” biologic signature. Permian fusulinaceans like Neoschwagerina and species of Jinogondolella from the Cache Creek Terrane are comparable to South China and Japan. This terrane may have been far-traveled across a paleo-equatorial region, finally docking with North America in the Jurassic. However, the position of the westernmost accreted terrane, Wrangellia, may constrain the lateral extent of this pathway since Wrangellia has Permian conodonts that are endemic to the craton of northwest Pangea suggesting that during the Permian Wrangellia was close to and at a comparable paleolatitude with respect to North America as it is today. The temporal extent of the stratigraphic succession within these terranes also reveals an interesting pattern. Pennsylvanian strata are present in the Cache Creek and Slide Mountain terranes as well as cratonic North America in western Canada, but absent from the in-between Quesnell and Kootenay terranes. This alternation of subsidence and inversion is reminiscent of the pattern suggested by intra-plate stress models. If so, it may suggest that there is a shared tectonostratigraphic history for at least the Upper Paleozoic between these accreted terranes that today overlie North American Proterozoic basement and a presumably nearby cratonic margin.