A FAR-TRAVELED MIGRATION HISTORY FOR THE YAKUTAT TERRANE, SOUTHERN ALASKA

The mobile Yakutat Terrane, a crustal block that migrated from the south through the Eocene until it collided with Alaska, is still uplifting the St Elias-Wrangell Mountains and the Alaska Range today. Two competing hypotheses exist regarding the origin of and distance traveled by the terrane as: 1) excised from the southeast Alaska/British Columbia coast when a transform fault boundary between the Pacific and North American plates jumped eastward sometime between ~50 and 30 Ma; and, 2) far-traveled, in some cases indicating as much as 4000 km of terrane translation with a possible origin near the California/Oregon border. Here we present two lines of evidence to describe the tectonic migration of the Yakutat Terrane since the early Eocene. First, we describe the presence of an intensely weathered isochronous paleosol in continental Early Eocene strata at many places in North America including at multiple locales in Alaska. The paleosol includes a siderite spherule-bearing horizon from which δ¹⁸O values were obtained to construct a paleolatitudinal gradient for Early Eocene North America. Comparison of the paleosol siderite spherule δ¹⁸O composition from the mobile Yakutat Terrane to the North American paleolatitudinal gradient indicates that during the Early Eocene the terrane existed at ~41º paleonorth. Second, we link the aforementioned evidence to our stratigraphic descriptions and chronostratigraphic constraints of the Paleogene Kootznahoo Formation, southeast Alaska archipelago. We use the results of detrital zircon assays by other researchers to aid interpretation of our stratigraphy - a shift from early to middle Eocene forearc basin sedimentation to Late Eocene/Oligocene strike-slip subsidence and sedimentation is linked to northward passage of the Yakutat Terrane past the stationary Kootznahoo basin. We favor a far-traveled migration pathway for the terrane.