ACCRETION AND TRANSLATION OF THE CHUGACH, PRINCE WILLIAM, AND YAKUTAT TERRANES IN ALASKA

The Chugach, Prince William, and Yakutat terranes are accreted units in southern Alaska that were defined nearly four decades ago. We propose a departure from the traditional view of the relationships between these three terranes based on our extensive U/Pb dating of detrital zircon, which facilitates a better understanding their translation history. As originally envisioned, the Chugach includes an older J-K mélange (McHugh Complex and correlatives, Chugach A) that sits structurally above the significantly younger turbidites of the Valdez Group and correlatives (Chugach B). However, we see no clear connection between the two units and treat them as distinct terranes. The structurally adjacent and slightly younger turbidites of the Prince William terrane (mainly Orca Group) is compositionally similar and shows clear continuity to the Chugach turbidites, but includes ophiolitic sequences (e.g. Resurrection and Knight Island). Together, we refer to these younger rocks (Chugach B+Orca) as the CPW turbidites, a single terrane. The Yakutat terrane has a complicated history and its relationship to the Chugach turbidites is critical in reconstructions. The Yakutat terrane is a composite terrane made of the Upper Cretaceous to Paleocene Yakutat Group (Yakutat A) and Paleocene-Eocene basalt (Yakutat B), and a sedimentary overlap (Kultieth, Poul Creek, and allied units). We exclude the “schist of Nunatak fiord” from Yakutat A because of their young maximum depositional age (~53 Ma) and show that these rocks are likely correlative to the CPW turbidites displaced from the schist of Baranof Island ~450 km to the south. The Yakutat Group has elements similar to Chugach mélange and turbidites, but we suggest they are different, in part because we show that the Yakutat mélange is significantly younger than the Chugach mélange and coeval with the Yakutat turbidites. Sandstones of the Yakutat Group (Yakutat A) have an identical provenance to the upper Nanaimo Group and the clastic facies in the Western Mélange Belt (WMB) in the Pacific Northwest. The provenance of all of these units is similar to the schists of southern California (Pelona, and allied units), and thus zircon grain-age data may indicate that the Yakutat Group has a translation history with ties first to southern California and then to the Pacific Northwest, and finally to Alaska.