STRUCTURAL GEOLOGY AND TECTONIC GEOMORPHOLOGY OF THE KATALLA RIVER VALLEY, ALASKA: THE CASE FOR UNUSUAL STRUCTURE IN A MEGATHRUST SETTING

The Katalla River Valley (KRV), located in southeastern Alaska, lies on the boundary of the Yakutat Terrane and the North American Plate. The valley overlies the subduction megathrust and is bounded to the west by the tectonically active Ragged Mountain Fault (RMF), which is marked by ~30 km long normal fault scarps, but also juxtaposes older rocks of the plate margin over the Yakutat terrane. Former beach ridges, lagoons and terraces of both fluvial and marine origin provide a geomorphic record of tectonic uplift and subsidence associated with paleo-earthquakes, while geodetic data suggests that this area lies near the boundary of a small tectonic block located within the upper plate of the megathrust. The Yakutat Terrane is accreting to, and subducting beneath North America at about the same rate as the Pacific Plate (~55my^-1). Known ages of megathrust events along the subduction zone are 3800 BP, 3200 BP, 2600 BP 2100 BP, 1500 BP, 900 BP and 1964 AD, but the KRV also has a record of an out-of-sequence uplift event (~500 BP). Ragged Mountain and the KRV undergo net tectonic uplift, while adjacent coastlines undergo net subsidence (eg, the Alaganik Slough to the west and Puffy Slough to the east).

This unusual coastal geomorphology and uplift history suggests that the structural geology is more complicated than expected by slip along the megathrust alone. Structural explanations for uplift are development of blind imbricate faults, which may be arrayed in an antiformal stack, or duplexing structure in the crust above the megathrust. Growth of this antiformal fault stack may also be responsible for the anomalous normal fault scarp along the RMF.