Constraining the relative positions of island arc terranes in the Canadian Cordillera using conodont oxygen isotope thermometry

The Canadian Cordillera is a composite of crustal terranes with various stratigraphic and tectonic histories. The relationships between these terranes, and their relative positions throughout the late Paleozoic and Mesozoic, remain contentious. In British Columbia and southern Yukon, the oceanic Cache Creek terrane is structurally juxtaposed between the Stikine and Quesnel island arc terranes; the exotic Wrangell island arc terrane lies further to the west. Multivariate statistical analyses of Late Triassic (late Carnian–early Norian) conodont faunas from the Stikine and Quesnel terranes and the North American margin have previously suggested that the terranes were likely situated to the south of their present location relative to the North American margin prior to accretion in the latest Triassic to Mid-Jurassic. An independent test of this hypothesis can be provided by oxygen isotope ( δ 18 O) analysis of conodont phosphate. Changes in δ 18 O seawater were likely minor in open, well-ventilated oceans, therefore analysis of δ 18 O preserved in Late Triassic (early Norian) conodonts of the Quesnel, Stikine and Wrangell terranes, as well as the northern part of the North American margin, may allow the relative seawater temperatures at the time of deposition of each of these tectonic entities to be determined for this time interval. Previous studies of the δ 18 O of conodonts have suggested that the seawater temperature of the Wrangell terrane was warmer than that of the North American margin during the early Norian. The present study expands on these preliminary results by extending the geographical range of the dataset. In addition to confirming higher seawater temperatures for the Wrangell terrane, the δ 18 O ratios of early Norian elements of the conodont genus Ancyrogondolella from both the Stikine and Quesnel terranes are notably lower than those of the same genus from the North American margin. If the salinity was consistent across the tectonic entities, then this may imply that the seawater temperatures of both the Stikine and Quesnel terranes were also warmer than that of the North American margin in the early Norian and suggests that these terranes, similar to the Wrangell terrane, were either located to the south of their present location in equatorial latitudes or oceanic currents were bringing warm water north at this time. A southern location for the terranes would be consistent with the results of the previous multivariate statistical analyses.