DETRITAL ZIRCON FACIES OF THE ALLOCTHONOUS WESTERN U.S. CORDILLERA

U-Pb detrital zircon data from rocks of the allochtonous western US Cordillera are divisible into four principal age spectra, here termed detrital zircon facies: characteristic age-probability distributions defined by the presence of grains within specific age ranges. A minimum of four distinct detrital zircon facies are represented. (1) Paleoproterozoic-Archean (PPA) facies is chiefly found in Paleozoic accretionary complexes and is defined by Paleozoic, early Proterozoic (ca. 2.0–1.6 Ga), and late Archean ages (ca. 2.7–2.3 Ga). PPA facies may represent translated NW Laurentian-affinity crust or fragments which are allochtonous to the western US. (2) Mixed Proterozoic (MP) facies is found in Early to Late Jurassic-age basins and is defined by grains spanning ~ 1.8 Ga–160 Ma from sources including the Yavapai-Mazatal provinces, intracontinental-anorogenic granites, the greater Grenville province, Appalachian Lower Paleozoic and Neoproterozoic rocks, and the East Mexico and early Mesozoic Cordilleran arcs. In “allochthonous” Cordilleran basins, MP facies represents long-distance transport and recycling of grains sourced from Pangean orogenic complexes, as well as SW Laurentian Precambrian basement and active marginal arcs. (3) Jurassic–Triassic (JTr) facies is found in Late Jurassic to Early Cretaceous basins of the western US Cordillera and is defined by Late Jurassic Cordilleran arc ages (peak ~ 150 Ma) with fewer Triassic ages (peak ~ 230 Ma) and a general lack of Precambrian ages. JTr facies represents initial isolation of marginal basins from the Laurentian craton and records early construction of the Andean-type margin. (4) Mixed Mesozoic (MM) facies is found in Late Cretaceous basins and is defined by Jurassic and Cretaceous Cordilleran arc ages (~185–140 and ~120–110 Ma) with no Precambrian ages. MM facies records the two main phases of the Mesozoic Cordilleran arc and continued isolation of marginal basins from the craton. In the western US Cordillera, detrital zircon facies vary over >100 Ma in a systematic and predictable manner based on depositional age and contintal-scale tectonic setting. This suggests that at an ~ 2nd-order stratigraphic scale, detrital zircon age distributions are controlled by the full integration of orogenic, erosion, sediment-transport, and depositional systems.