DETRITAL ZIRCON CONSTRAINTS ON NORTH AMERICAN TECTONISM AND SEDIMENT DISPERSAL DURING PALEOZOIC TIME (Invited Presentation)

Detrital zircons provide a powerful tool for reconstructing the Paleozoic tectonic evolution of circum-Laurentian orogens, as well as the dispersal of sediment from these orogens onto and in some cases across the cratonal interior. Timing of magmatism is recorded by U-Pb ages, whereas variations in epsilon Hf provide a record of crustal thickening (evolved values or “pull-downs”) versus crustal thinning (juvenile values or “pull-ups”). Hf isotope pull-downs recorded in detrital zircons from Paleozoic strata of northern Cordilleran terranes record crustal thickening during Early Ordovician, Late Silurian, Late Devonian/Early Mississippian, and Late Permian time. These pull-downs coincide with the M’Clintock (Ordovician), Caledonian/Klakas (Silurian), Ellesmerian/Antler (Late Devonian-Early Mississippian), and Klondike/Browns Fork/Sonoma (Late Permian) orogenies along the Arctic and Cordilleran margins. Intervening pull-ups coincide in some cases with recognized episodes of crustal thinning (e.g., opening of back-arc basins). Zircons derived from the Caledonides show a different history, with a pull-down during Early Silurian time that coincides with the collision of Laurentia and Baltica. Appalachian-derived detrital zircons pull down during mid-Ordovician time as result of Taconic deformation, but show little variation during Acadian and Alleghanian tectonism. The Ouachita-Marathon orogen is dominated by Neoproterozoic (Pan-African) orogenesis and followed by little evidence of crustal thickening/thinning during Paleozoic magmatism. The U-Pb/Hf characteristics of these source regions are sufficiently distinctive that they can be used to track the regional (and in some cases transcontinental) dispersal of sediment during Paleozoic time. Significant volumes of sediment were shed from the Caledonides during Ordovician time, from the Ellesmerian orogen during Late Devonian time, from the Appalachians during Carboniferous-Early Permian time, and from the Ouachita-Marathon orogen during Pennsylvanian-Permian time. The combination of U-Pb ages and Hf isotope variations in detrital zircons appears to provide a robust record of tectonic processes operating during both continental collision and accretion of arc terranes, as well as during intervening phases of crustal extension.