PALEOGEOGRAPHY OF THE LATE JURASSIC KLAMATH ARC FROM DETRITAL ZIRCON HAFNIUM ANALYSIS OF THE GALICE FORMATION, KLAMATH MOUNTAINS, OR AND CA

The Upper Jurassic Galice Formation, a metasedimentary unit in the western Klamath Mountains, formed within an intra-arc basin prior to and during deformation of the Nevadan orogeny. Recent detrital zircon U-Pb age analyses demonstrate a significant North American continental provenance component, consistent with tectonic models placing the Western Klamath terrane on the continental margin in Late Jurassic time. Isotopic analysis of Mesozoic detrital zircon from 11 samples collected from the Galice Formation reveal wide-ranging epsilon Hf values for Jurassic and Triassic grains, indicating a continental component in arc-related magmatism. Hf data from Jurassic plutons within the Klamath Mountains match some of the Galice detrital zircon, but cannot account for the most non-radiogenic Jurassic detrital grains. In fact, the in situ Cordilleran arc record does not provide a clear match for the wide-ranging isotopic signature of Triassic and Early Jurassic grains within the Galice Formation. In contrast, Jurassic detrital grains from the Great Valley Group and from the Cordilleran foreland basin do contain similar age and Hf populations of detrital zircon, suggesting shared sediment sources within the Cordilleran arc that are currently eroded, covered, and/or not yet sampled. Inferred Galice provenance within the Klamath Mountains and more distal sources suggest that the Galice basin received silicilastic turbidites that were fed by rivers draining the Klamath terranes and more distal rocks of the Cordilleran arc just prior to basin collapse. Thus, the Galice Formation contains a record of the fundamental shift in Cordilleran arc tectonics from a Jurassic transtensional system with a low-lying arc traversed by westward-flowing river systems, to a contractional system characterized by thrusting of the Western Klamath terrane underneath the Rattlesnake Creek terrane.