DETRITAL ZIRCON GEOCHRONOLOGY OF CORDILLERAN RETROARC FORELAND BASIN STRATA REVEALS CHANGES IN PROVENANCE, MAGMATISM, AND OROGENIC EXHUMATION ACROSS THE SEVIER TO LARAMIDE TRANSITION

Siliciclastic strata deposited in the retroarc region of the North American Cordillera contain detrital zircons with uranium-lead (U-Pb) age spectra that vary systematically throughout Jurassic to Eocene time. Changes in the relative abundance of specific age ranges during this period can be used to track orogenic exhumation and sediment dispersal pathways across the Sevier to Laramide transition. This study presents 8,717 U-Pb analyses from 95 detrital zircon samples of Jurassic-Eocene North American Cordillera foreland basin strata. 30 new samples were analyzed for this study. Each sample can be classified into one of six major provenance groups, whose age spectra suggest derivation from: 1) Mesozoic eolianites of the western United States, 2) Paleozoic passive margin strata of the western United States, 3) Paleozoic passive margin strata of western Canada, 4) the Mogollon Highlands, 5) the Cordilleran magmatic arc, or 6) Yavapai-Mazatzal Province crystalline basement rocks. Late Jurassic provenance is dominated by recycling of Mesozoic eolianites from sources in the Sevier thrust belt. Cretaceous-Eocene provenance is dominated by recycling of the passive margin, with increasing complexity upsection. The first appearance of Yavapai-Mazatzal dominated provenance indicates that basement-involved structural culminations were exposed by Campanian to Maastrichtian time. We interpret that retroarc region detrital zircons record a basin-wide unroofing sequence.

A composite age-probability plot of 1,539 young (<250 Ma) detrital zircons was used to construct a basin-averaged proxy for Cordilleran arc magmatic flux. This analysis reveals at least four age-abundance peaks (~160, 100, 75, and 50 Ma) that we interpret to represent periodic high-flux magmatism. Decreases in the recurrence interval between periods of high flux magmatism correlate with increases in convergence rate, suggesting that upper-plate and subduction processes were coupled.