NEW CONSTRAINTS FROM THE CHALLIS-KAMLOOPS GROUP ON CRUSTAL EVOLUTION DURING EARLY EOCENE EXTENSION IN SOUTHEASTERN BRITISH COLUMBIA AND NORTHEASTERN WASHINGTON

Southern British Columbia and northern Washington underwent profound post-orogenic extension, mid-crustal exhumation, basin formation and magmatism in the Early Eocene. This is recorded in the volcanic and sedimentary rocks of the Challis-Kamloops Group. High-grade metamorphic exhumed core complex infrastructure has been studied in great depth, but much less so the associated supradetachment basins. Chronologic ties between core complex footwall and hanging wall, and the regional chronostratigraphy of the supradetachment basins have not been established with high precision. Vigorous debate continues over geodynamic drivers of post-orogenic extensional collapse.

To better understand geodynamic drivers of early Eocene lithospheric collapse, we are taking a three-pronged approach to creating new regional constraints on crustal evolution, utilizing CA-ID-TIMS U-Pb zircon geochronology to refine regional chronostratigraphy of the supra-detachment basins; geochemical estimation of crustal thickness, and new whole-rock geochemical constraints on the evolution of magmatism. Twelve new high-precision ages refine the regional chronostratigraphy, previously based mainly upon lithostratigraphy and low-precision K-Ar geochronology. Combining these three methods at the regional scale will better elucidate the spatial and temporal distribution of extension, magmatism and sedimentation and the contribution of various geodynamic mechanisms during the extensional period.

Results to date indicate that crustal thickness reached a maximum of ~62 km and elevation ~4.9 km prior to onset of crustal collapse. Our new ages, when integrated with extant regional U-Pb geochronology, support extensional collapse in two (or more) distinct phases, consistent with data from the footwall domain. The first and larger was between ~52.1 and ~50 Ma with total crustal thinning of ~18-20 km, associated with ductile-brittle detachments. A second brittle low-angle normal fault phase with 7-10 km of thinning occurred between ~49 Ma and at least 46.2 Ma. The basal “O’Brien Creek” or “Kettle River” tuff has been found to comprise at least three eruptive events over 360 kyr. In the hyperextended region, volcanism in the Republic Graben appears to have preceded the westerly White Lake Basin or easterly Priest River Complex.