RAIDERS OF THE LOST ARC: DETRITAL ZIRCON U-PB-HF PROVENANCE OF THE UMPQUA AND TYEE GROUPS AND IMPLICATIONS FOR LATE CRETACEOUS TO EOCENE PALEOGEOGRAPHY OF CASCADIA

Abundant 100–45 Ma detrital zircon (DZ) and mica in the middle Eocene Tyee Fm of SW Oregon have been used to suggest a distal, cratonic North America source in rocks of the Idaho Batholith (IB) and Challis volcanic field. However, north- to northwest-directed paleocurrents (modern coordinates) and sandstone petrography support a more proximal source in the Klamath Mountains (KM) province, despite a lack of preserved 100–45 Ma rocks in the KM. Here we present new DZ U-Pb and Lu-Hf isotopic data from the Tyee Fm and underlying Umpqua Grp to test competing provenance models and help to constrain Late Cretaceous to Eocene tectonic and paleogeographic reconstructions for Cascadia.

Statistically identical maximum depositional ages of ~46–45 Ma from the base and top of the 600–1,600 m-thick Tyee Fm imply rapid deposition (ca. 500 m/m.y.) and >150 km of delta progradation in <1–2 Myr, on par with the fastest documented rates for shelf-margin clinoform progradation (≤100 km/Myr). Preliminary ⁴⁰Ar/³⁹Ar dates from detrital mica in the Umpqua Group – which was unequivocally derived from the KM – range from 90–50 Ma, suggesting that similar mica dates in the Tyee Fm may have also been sourced locally rather than from the IB. DZ mixture modelling based on a regional compilation of >8,300 DZ dates supports that the Tyee Fm could have been derived entirely from the northern Sierra Nevada Arc and recycled Late Cretaceous strata in the KM (e.g., upper Hornbrook Fm, Dothan Fm). Most notably, a majority (62%) of the 90–50 Ma zircon in the Umpqua Grp and Tyee Fm have intermediate to primitive εHf values between -10 and +15, significantly higher than the more evolved εHf values of -30 to -10 in the IB that represent cratonic crust. These data appear to preclude an IB source for much of the Tyee Fm zircon, and require an alternative explanation.

These new insights suggest that most of the Late Cretaceous to early Eocene zircon in SW Oregon were derived from unknown “Lost Arc” sources constructed on primitive crust associated with accreted terranes in southern Oregon, NW Nevada, and/or NE California. We hypothesize that the Tyee Fm was sourced from a now dismembered Late Cretaceous-Paleocene forearc basin situated east of the KM, the remnants of which may include the Hornbrook-Ochoco basin and equivalents of the Western Mélange Belt in the North Cascades.