Abstract
The Eocene volcanic and sedimentary rocks of southwestern Canada preserve a complex record of extensional tectonics associated with the evolution of the western Cordillera hinterland. Potential mechanisms driving magmatism, rapid extension, and surface subsidence and uplift in the area include the opening of a slab window, slab rollback, and mantle lithospheric delamination. To test these models, this study combines stable isotope and elemental geochemistry, U/Pb geochronology, and stratigraphy of ignimbrites and sediments. New U/Pb zircon ages indicate basin sedimentation began prior to the initiation of an ignimbrite flare-up event in the south at ~53 Ma and propagating northward through 51 Ma. A regional unconformity and terrestrial sedimentation followed the end ignimbrite flare-up event at ~47 Ma. δDglass values from ignimbrites represent Eocene meteoric water isotope compositions, which correlate with depositional environment. Precipitation-hydrated units have δDglass values of -186.5‰ and -197.3‰ ±2.5‰. In comparison to a previously reported near shore value δDprecipitation of -55 ± 2‰, these values indicate a high standing (2800-3000 m ± 300 m) hinterland existed synchronously with the regional ignimbrite flare-up, sedimentation, and core complex extension. Enriched δDglass values from glasses deposited in lacustrine systems reflect local evaporation, likely enhanced during the Early Eocene Climatic Optimum. The formation of a slab window is the most likely mechanism driving early to middle Eocene magmatism and basin accumulation within the southern Canadian Cordillera hinterland.