Late Jurassic paleogeography of the US Cordillera from detrital zircon age and hafnium analysis of the Galice Formation, Klamath Mountains, Oregon and California, USA

The Upper Jurassic Galice Formation, a metasedimentary unit in the Western Klam-ath Mountains, formed within an intra-arc basin prior to and during the Nevadan orog-eny. New detrital zircon U-Pb age analyses (N = 11; n = 2792) yield maximum deposi-tional ages (MDA) ranging from ca. 160 Ma to 151 Ma, which span Oxfordian to Kim-meridgian time and overlap Nevadan con-tractional deformation that began by ca. 157 Ma. Zircon ages indicate a significant North American continental provenance component that is consistent with tectonic models placing the Western Klamath terrane on the continental margin in Late Jurassic time. I3f isotopic analysis of Mesozoic de-trital zircon (n = 603) from Galice samples reveals wide-ranging & epsilon;I3f values for Jurassic and Triassic grains, many of which cannot be explained by a proximal source in the Klamath Mountains, thus indicating a com-plex provenance. New U-Pb ages and I3f data from Jurassic plutons within the Klamath Mountains match some of the Galice Forma-tion detrital zircon, but these data cannot ac-count 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 Trias-sic and Jurassic grains. When compiled, Galice samples indicate sources in the Sierra Nevada pre-batholithic framework and ret-roarc region, older Klamath terranes, and possibly overlap strata from the Blue Moun-tains and the Insular superterrane. Detrital zircon age spectra from strata of the Upper Jurassic Great Valley Group and Mariposa Formation contain similar age modes, which suggests shared sediment sources. Inferred Galice provenance within the Klamath Mountains and more distal sources suggest that the Galice basin received siliciclastic turbidites fed by rivers that traversed the Klamath-Sierran arc from headwaters in the retroarc region. Thus, the Galice For-mation contains a record of active Jurassic magmatism in the continental arc, with sig-nificant detrital input from continental sedi-ment sources within and east of the active arc. These westward-flowing river systems remained active throughout the shift in Cor-dilleran arc tectonics from a transtensional system to the Nevadan contractional system, which is characterized by sediment sourced in uplifts within and east of the arc and the thrusting of older Galice sediments beneath older Klamath terranes to the east.

Automated summary

The article investigates the Upper Jurassic Galice Formation in the Western Klamath Mountains and finds that it was deposited between ca. 160 Ma to 151 Ma, overlapping with the Nevadan orogeny. The provenance of the sediment sources for the formation is diverse, including the Sierra Nevada pre-batholithic framework, older Klamath terranes, and possibly the Blue Mountains and Insular superterrane. These findings suggest that the Galice Formation records active Jurassic magmatism in the continental arc, with significant detrital input from continental sediment sources to the east.