TRANSPORT STRATIGRAPHY OF THE YAKUTAT TERRANE, SOUTHERN ALASKA BASED ON DETRITAL ZIRCON FISSION-TRACK AND U/PB ANALYSIS

The Yakutat terrane was transported northwestward along the Transition and Queen-Charlotte Fairweather strike-slip fault system. Opinions vary concerning the total transport distance, but it may have originated as far south as Oregon and northwestern Washington. Two end member hypotheses, with no intermediate option, exist for the transport history of the Yakutat terrane through time. One involves the transport of the Yakutat terrane from a northerly position, and in this case the stratigraphy records a local in situ source. The second involves the transport of the terrane from the south, and the stratigraphy has a provenance that reflects considerable transport along the western edge of the Cordillera. To better understand provenance of the cover strata, the cooling ages (FT) and crystallization ages (U/Pb) of zircons was evaluated. Fission-track dating was first and then U/Pb analysis was conducted on the same grain. Paleocene to Eocene strata records a long-lived, non-volcanic source terrain that crystallized from ~50-220 Ma and cooled from ~48-110 Ma. The overall pattern of grain ages favors the northern transport option, based on the 1) uniform provenance of the Kulthieth and Poul Creek Formations; 2) the over all grain-age distribution which has common elements throughout; and 3) the distinct lack of volcanic zircons. The Southern transport path can be ruled out mainly due to the lack of volcanic grains that would be expected in the stratigraphy during continuous transport of the terrane along route. Miocene cooling of zircon in the Kulthieth and the Poul Creek Formations likely records erosion of plutons in the northern Coast Plutonic Complex and the Kuiu-Etoilin belt. Late Miocene deposition of the Yakataga Formation records a provenance signal of crystallization from ~50-53 Ma and cooled from ~17-20 Ma. The Yakataga Fm, clearly has a similar provenance to the Poul Ck and Kulthieth, but it also has a significant and distinctive grain age population that is much younger (20 Ma), and this deposition is likely associated with the Chugach accretionary complex and superimposed Sanak-Baranof Plutonic Belt (~50-58 Ma).