PALEOTOPOGRAPHY OF THE KLAMATH MOUNTAINS PROVINCE REVEALED BY U-PB DETRITAL ZIRCON AGES AND HF ISOTOPES FROM THE LATE CRETACEOUS TO EOCENE MONTGOMERY CREEK FORMATION

Early Eocene (ca. 54-49 Ma) docking of the oceanic Siletzia terrane resulted in syn- to post-collisional shortening, extension, magmatism, and rotation in the forearc. However, paucity of a Tertiary rock record in the Klamath Mountains Province (KMP) of California and Oregon hinders our understanding of how the KMP was affected by collision of Siletzia in the north. Most modern studies conclude that Eocene detrital zircons (DZ) in the CA and OR forearc were derived from sources in western Idaho that shed sediment through large west-flowing paleorivers, but this hypothesis has not been rigorously tested. The Montgomery Creek Formation (MCF) in northern California contains Late Cretaceous to Eocene fluvial conglomerate and sandstone that unconformably overlie Mesozoic rocks of the eastern KMP. Prior workers concluded that Ar-Ar ages of detrital micas from the MCF indicate a source in the Idaho Batholith (Renne et al., 1990), though clast lithologies and southerly paleocurrents support a KMP source (Aalto, 1988). To test these alternate hypotheses, we collected three sandstone samples from the basal to upper MCF for U-Pb DZ age dating (total n=873) and Hf isotopes (n=94). The basal MCF yields a DZ age peak of ~95 Ma with εHf values of -5 to +9, and a ~160 Ma peak with positive εHf values, most closely matching sources in the Sierra Nevada and KMP. A maximum depositional age (MDA) of 89.1 ± 0.7 Ma for the basal MCF is coeval with the Upper Hornbrook Formation (Surpless, 2015). The middle and upper MCF samples yielded: Eocene MDAs of 42.9 ± 0.3 and 37.3 ± 0.3 Ma, respectively; sharp age peaks of 43-49 Ma with εHf values from -20 to 0; secondary age peaks 85-100 Ma with negative εHf values; and broad age peaks from 100 to 200 Ma with εHf of +5 to +12. The 43-49 Ma zircon grains could have been derived from either Challis (Idaho) or Clarno (Oregon) volcanic eruptive centers. In the latter interpretation, the 85-100 Ma DZ grains with negative εHf values (representing only 6-10% of grains in the middle and upper MCF samples) could be explained by recycling of Hornbrook Formation sediments that were eroded during Early Tertiary mountain building. In either scenario, the data are consistent with an emerging new perspective that the northern KMP formed a long-lived topographic and orogenic highland produced by Eocene collision of Siletzia with North America.