PRELIMINARY CRUSTAL THICKNESS ESTIMATES FROM CHEMICAL MOHOMETRY ACROSS THE CHALLIS-KAMLOOPS MAGMATIC BELT IN SOUTHEASTERN BRITISH COLUMBIA AND NORTHEASTERN WASHINGTON STATE

Preliminary Moho depth estimates based upon whole-rock geochemical data using the chemical mohometry method of Luffi and Ducea (2022) were derived from a broad transect through southeastern British Columbia and northeastern Washington that includes the deeply exhumed Shuswap complex. Several geodynamic triggers for the post-orogenic extensional collapse of this region have been proposed, including changing Pacific basin – North America plate convergence, mid-crustal channel flow, lithospheric delamination, opening of a slab window or slab gap, slab rollback, and underplating by a mantle plume. Moho depth calculations using the chemical mohometry indicate crustal thicknesses of 42 - 59 km and paleoelevations of 2.4 - 4.9 km above sea level during the period of Eocene Challis-Kamloops group magmatism (Ypresian, ~53 - 47 Ma). These values are consistent with previous estimates for this region based on stable isotopes, structural restoration, metamorphic petrology and fossil leaf physiognomy. The preliminary results suggest that the crust was still >50 km thick until near the end of the magmatism, and began to thin in its waning stages, implying that significant crustal extension may have continued after the major period of ductile core complex footwall extension and exhumation from mid-crustal depths. The chemical mohometric depth estimates generally have errors of <7% (+/- ~4 km). When combined with high-precision U-Pb zircon CA-TIMS dating of the Challis-Kamloops group, these data provide the opportunity to constrain crustal thickness evolution at a temporal and spatial scale that allows quantitative evaluation of existing extensional models.