DETRITAL ZIRCON U-PB PROVENANCE OF EOCENE SEDIMENTS IN SOUTHERN BRITISH COLUMBIA AND NORTHERN WASHINGTON: RECORD OF A COLLAPSING OROGEN

Orogenic collapse of the southern Canadian Cordillera of British Columbia (BC) and northernmost Washington occurred during the Eocene and was accompanied by the formation of multiple sedimentary basins in the hinterland of the orogen. Eocene sedimentary strata consist of fluvial and lacustrine sediments, including clast- and matrix-supported cobble and pebble conglomerates, sandstones, and mudstones. Here, we present over 2,000 new detrital zircon uranium-lead (U-Pb) ages from this region that can be used to reconstruct sediment provenance and constrain maximum depositional ages. All 20+ samples contain large populations of detrital zircons with ca. 50 Ma ages (Early Eocene), with the exception of samples from Princeton and Merritt, BC, which are dominated by zircons with ages of ca. 130-170 Ma. Samples from Kelowna, BC, adjacent to the Shuswap Metamorphic Core Complex, contain ca. 50 Ma grains as well as a small population of ca. 140-170 Ma grains. Strata from the westernmost Fig Lake Graben, contain populations of ca. 50 Ma detrital zircons as well as grains with U-Pb ages between 130-300 Ma. The ca. 50 Ma detrital zircons are interpreted as having been derived from the Challis-Kamloops volcanic episode. Populations of older detrital zircons are interpreted as having been derived from sediment sources exposed in local, fault-bounded ranges, including Jurassic-Cretaceous plutons. Maximum depositional ages for Eocene sedimentary strata are typically between 52-48 Ma, suggesting relatively synchronous deposition across the area. An exception to this is Eocene sedimentary strata from Merritt, BC, which have maximum depositional ages of ca. 44 Ma. Differences in detrital zircon U-Pb ages between areas suggest the southern Canadian Cordillera was occupied by a series of discrete basins, separated by paleotopographic highs, during the Eocene. Changes in detrital zircon age populations within continuous sedimentary successions, like those in strata near Kelowna, BC, record progressive unroofing of nearby sediment sources.