PALEOGENE RESTORATION OF THE YAKUTAT GROUP IN ALASKA TO THE NANAIMO GROUP AND WESTERN MÉLANGE BELT IN THE PACIFIC NORTHWEST

New U/Pb dates on detrital zircon from sandstones clarify the age and stratigraphic relationships between units that define the Yakutat terrane, the adjacent Chugach-Prince William terrane, and terranes to the south in the Pacific Northwest (WA and southern BC). The composite Yakutat terrane has two basement units: a Paleocene oceanic basaltic unit that was overthrust in the Paleogene by an Upper Cretaceous-Paleocene flysch and mélange of the Yakutat Group. The Yakutat Group includes relatively coherent flysch and a highly disrupted sedimentary mélange. The mélange is dominated by sandstones, most of which are arkosic with white mica, and they also contain slices and phacoids of marble/greenstone, metaigneous blocks, and chert. Detrital zircons from the Yakutat Group flysch (YGf) are identical to the primary zircon facies of the Yakutat Group mélange (YGm), which we refer to as the Russell zircon facies. Based on the youngest cohort of detrital zircon, the Russell zircon facies in the mélange and the flysch has a Maximum Depositional Age (MDA) range of 60.4 ± 1.8 to 72.0 ± 2.2 Ma, and hence they were deposited at the same time. The primary grain-age populations and overall pattern of the grain-age distributions overlap with primary populations at 73-74 Ma, 90-91 Ma, 1370-1380 Ma, and 1705-1710 Ma. Both units have Precambrian zircons with Cretaceous metamorphic rims that grew between 67 and 90 Ma and were derived from a source terrane exhumed rapidly at ~85-88 Ma. We conclude that the arkosic sandstones in the YGm have been tectonically kneaded into the mélange fabric, probably in the Paleocene. Hafnium isotopes in Cretaceous zircons from this source terrane are distinctive because they are relatively non-radiogenic (negative eHf), so the Cretaceous igneous rocks in the arc were partly built on Precambrian rocks. Our new zircon data indicate that the Yakutat Group is not correlative to rocks of the Chugach terrane, and this stratigraphic reassignment affects many published models that relay on continuity of Yakutat basement rocks and the CPW. The similarity of the Yakutat Group to the Western Mélange Belt and the upper Nanaimo Group in the Pacific Northwest and southern Vancouver Island suggests that the three units were contiguous and separated by about 1500-1700 km along a margin-parallel strike-slip fault since the Eocene.