EOCENE BASIN RECORDS OF VOLCANISM, TOPOGRAPHY, AND TECTONICS IN THE NORTH AMERICAN CORDILLERAN HINTERLAND, SOUTHERN BRITISH COLUMBIA

Eocene volcanic and sedimentary rocks in southern British Columbia preserve a complex record of extensional tectonics associated with the evolution of the northwestern Cordilleran hinterland. Although the bulk rock geochemistry and paleobiota of these rocks are well-characterized, the lack of stratigraphic correlations and precise dating limit our understanding of the relationships between volcanism, extension, and sedimentation. To reconstruct the basin history, this study combines stable isotope and elemental geochemistry, U/Pb geochronology, and stratigraphy of Eocene strata and interbedded ignimbrites.

New U/Pb zircon ages indicate that basin sedimentation began prior to ~53 Ma, when an ignimbrite-generating magmatic pulse initiated in the south. This magmatic pulse propagated northward through 51 Ma. A regional unconformity followed the end of the ignimbrite pulse at ~47 Ma. δD_glass values from hydrated volcanic glass from these ignimbrites are proxies for Eocene waters, which correlate with depositional environment. Precipitation-hydrated units have δD_glass values of -186.5‰ ± 2.5‰ and -197.3‰ ± 2.0‰. Modeled Eocene lapse rates, calibrated to Eocene paleobotanical data and modern Coast Range surface waters, indicate that a high standing (2.8-3.0 ± 0.3 km) hinterland existed during the regional magmatic pulse, sedimentation, and extension. These results suggest that previous stable isotope paleoaltimetry studies overestimated elevations by ~1-2 km due to underestimation of regional isotopic lapse rates. Enriched δD_glass values from glasses deposited in lacustrine systems likely reflect enhanced evaporation during the Early Eocene Climatic Optimum (EECO). New U/Pb ages show deposition of the McAbee and Falkland beds occurred during the EECO and provide a terrestrial climate and fossil record during this period of high pCO₂ (600 to >1,300 ppm) and high temperatures at mid-latitudes and high elevations. These data further support the hypothesis that Eocene magmatism and the onset of core complex extension were driven by the propagation of a slab window across the region, and provide new temporal and topographic constraints on the associated basin formation.