TRANSLATION AND DISPLACEMENT OF THE CHUGACH-PRINCE WILLIAM ACCRETIONARY COMPLEX, ALASKA

The Chugach-Prince William (CPW) terrane is dominated by Maastrichtian to Paleocene imbricated flysch. Grain ages of detrital zircons have remarkable continuity across the entire 2000 km belt and include abundant grains with ages close to the age of deposition, and thus the source undoubtedly included an active Late Cretaceous to Paleocene arc (~75-50 Ma). Basement units for the arc are Mesozoic (mainly Jurassic), with a minor fraction of Paleozoic (Devonian) and Precambrian rocks. Positive εHf isotopic values in Phanerozoic zircon are consistent with a relatively juvenile source, such as the Coast Plutonic Complex that intrudes Wrangellia and allied terranes.

Although uncommon, Precambrian detrital zircons define two cohorts. A western cohort with ages and isotopic signatures similar to northern Laurentian. These grains are dominated by populations between 1810-1870 Ma and 2520 to 2680 Ma and most grains have negative εHf(t) values. An eastern cohort has ages and an isotopic signature consistent with a southern Laurentian source – mainly Yavapai and Mojave and the Granite-Rhyolite province of the southwest US. These zircon are dominated by populations at ~1380 Ma, ~1485 Ma, and ~1722 Ma and εHf(t) values are mostly positive, ranging from +11.7 to –3.4.

We hypothesize that the majority of the flysch on the CPW terrane accumulated in an accretionary complex flanking what is now the Coast Plutonic Complex, but at an uncertain position along the Cordillera. However, zircon from the eastern cohort of the CPW appear to have been derived from rocks of the Sierran-S California-Peninsular Arc, which collapsed along a tectonic corridor at the Mojave breach. This source of SW Laurentian rocks provided sediment with a distinct Precambrian signature. Thus some of the CPW flysch was deposited in a trench adjacent to this point source and was subsequently displaced ~3200 km northward to Alaska, consistent with robust paleomagnetic data from several key locations in the CPW. The thermal history of the CPW terrane shows a distinctive high temperature resetting that can be attributed to intrusion of the near trench Sanak-Baranof plutons (63-48 Ma), but secondary cooling ages in the eastern part of the belt between about 28 and 35 Ma can be attributed to a late phase of strike slip faulting.