DETRITAL ZIRCONS FROM THE NANAIMO BASIN, VANCOUVER ISLAND, BRITISH COLUMBIA: AN INDEPENDENT TEST OF LATE CRETACEOUS TO CENOZOIC NORTHWARD TRANSLATION

The Cretaceous development of the North American Cordillera is disputed. Paleomagnetic and paleobotanical observations require large-scale dextral displacements of terranes along the western margin of North America during the Cretaceous-Paleogene. A lack of corroborating geological evidence for large-scale (>1500 km) displacements has prevented the integration of paleomagnetic data into most contemporary tectonic models. Detrital zircons from the Cretaceous Nanaimo forearc basin, British Columbia, Canada, provide an independent test of its Late Cretaceous paleogeographic position.

Campanian and Maastrichtian rocks of the Nanaimo Basin are dominated by Mesozoic detrital zircon populations (modes at ~148 and ~88 Ma) indicating the majority of the sediment derives from the Coast Plutonic Complex (CPC) to the east. Maastrichtian rocks also contain Paleoproterozoic (~1700 Ma) and Mesoproterozoic (~1390 Ma) zircon populations that derive from source areas east of the CPC. LA-ICP-MS depth profiling reveals thin rims (n=313) on some Mesozoic grains and all >300 Ma grains from Maastrichtian samples. The majority of rims yield dates between 95 and 71 Ma (mode 87 Ma), and U/Th ratios of >10, suggesting the source area for these grains underwent metamorphism at that time.

The ages of Proterozoic detrital zircon populations in Maastrichtian rocks are consistent with derivation from one of two regions; the Mojave-Sonoran region of southwestern North America or the Lemhi sub-basin of Idaho. The timing of metamorphism in the source area recorded by rim growth on detrital grains apparently precludes derivation from the Lemhi sub-basin region, as metamorphism there had ended by 83 Ma. However, rim growth does overlap with metamorphism of the Pelona, Orocopia and Rand (POR) schists in the Mojave-Sonoran region (95 to 50 Ma). It seems plausible that detrital zircon populations from the Nanaimo Basin with Late Cretaceous rims derive from higher-grade equivalents of the POR schists, such as the schist of the Sierra de Salinas and Swakane Gneiss. We speculate that these schists were tectonically exhumed from the base of the lithosphere via extension associated with rifting between the Insular superterrane and the Mojave-Sonoran region at ~71 Ma.