Detrital Zircons Reveal Tectonic Translation of the Yakutat Terrane, Alaska

The Upper Cretaceous to Paleocene (~62 to 75 Ma) Yakutat Group consists of two distinct rock units: a flysch unit and a mélange unit. The location of deposition for these strata is unknown, but was probably far to the south of their present position. The origin of detrital zircons may be the key to understanding the age of these units, where they originated, and how they correlate to rocks to the south along the continental margin. Two samples were collected from remote and difficult to access areas in Glacier Bay National Park, and these samples can be compared to samples from Harlequin Lake, Russell Fiord, and Yakutat Bay to the north. For each sample, we determined the ages of the detrital zircons. A sample of Yakutat Group flysch (YGf) from the Grand Plateau Glacier is from sedimentary rock adjacent to the Grand Plateau pluton. It has a maximum depositional age (MDA) of ~66 Ma. The zircon age distribution suggests that most grains were formed in the mid-Cretaceous, and some were formed in the Jurassic. An unusual aspect of this sample is that about 23% of the zircon grains are Precambrian, and have two distinct populations with ages of ~1397 Ma and ~1702 Ma. A sample of sandstone from the Yakutat Group mélange (YGm) from Lituya Bay, was collected from an assemblage of dark lithic sandstones interbedded with basalt, and dark-gray bedded chert.  This sample has an MDA of ~108 Ma, and its grain-age distribution is dominated by Jurassic dates. Both samples can be correlated to similar dated units in the Yakutat Bay area. The YGf sample is correlative to the primary zircon facies common to rocks in both the Yakutat Group flysch and mélange, which we refer to as the Russell zircon facies. The YGm sample is more complicated, but it appears to belong to the Shelter Cove zircon facies. The Yakutat terrane has slid along the margin of the Cordillera, and candidate correlative rocks are to the south. Similar facies with identical grain-age distributions occur in the Western Mélange Belt (WMB) in the North Cascades foothills in WA. We evaluate the correlation and connection between the Yakutat and the WMB and how these units may have once been contiguous and separated by strike slip faulting at about 50 Ma. Since that time, the Yakutat block has slid north to Alaska, a remarkable journey of approximately 1,500 kilometers.