REEVALUATION OF THE CHUGACH AND PRINCE WILLIAM TERRANES IN ALASKA

The Chugach and Prince William terranes are thought to be a composite terrane (CPW) that was an accretionary complex to Wrangellia (s. lato). We question this assessment, which is entrenched in the literature. Original terrane designations were based on general stratigraphic attributes and structural position along the southern Alaskan margin. Because almost all of the units lacked age control, the original stratigraphic correlation was simple, but blunt. The Chugach terrane contains several distinct tectonostratigraphic units and these can be divided into: 1) an older suite of units (mid K and older) that include mélange, volcaniclastic sediments, and local blueschist; and 2) younger units (Upper Cretaceous to Paleocene) that are mainly turbidites (Shumagin, Kodiak, Valdez, Sitka). These younger units share a similar provenance with turbidites of the Prince William terrane (Ghost Rocks, Orca, upper Sitka) that are mainly Paleocene-Eocene. In the last decade, we have dated 33,593 detrital zircons from 151 sites along nearly 2000 km of the southern Alaskan margin. Our extensive dating along the southern Alaskan margin allows for a new view that provides critical age control and aids correlation. Four observations concerning correlation of units fall out of the zircon data: 1) the Orca and the Valdez are similar, and are not distinctive enough in the field to be mappable units - they belong to one terrane; 2) rocks of the Orca Group include mafic volcanics and “ophiolites” (i.e. Resurrection, Knight, Glacier Is) erupted into the basin; 3) elements of the Chugach Metamorphic Complex, the Schist of Nunatak Fiord, and the Baranof schist contain young rocks (~50-60 Ma or “Orca”-like) and maximum depositional ages are commonly only slightly older (<5 Ma) than metamorphism; 4) the Yakutat Group is distinct and different from all CPW rocks with its own tectonic trajectory. We suggest that the original definition of the Chugach terrane and the ongoing lumping of Chugach and Prince William terranes (CPW terrane) has hindered tectonic reconstructions. The older Chugach rocks have an alliance with Wrangellia, and the younger Chugach and Prince William turbidites do not. Thus we are now focused on faults that separate the older units that are allied with Wrangellia and the younger more outboard units that we view as a suspect terrane.