| Description |
The paleoseismology record of southeastern Alaska documents several megathrust earthquakes during the late Holocene, including the Mw 9.2 earthquake of 1964, when the Katalla River Valley was uplifted as much as 2 m (Plafker, 1965; Richards, 2000). The Katalla River Valley is exceptional because uplifted terraces and beach berms suggest net tectonic uplift relative to sea level during the late Holocene, when the greater coastal region experiences net subsidence. This study addresses the question: is the valley truly undergoing net tectonic uplift, and if so, what is the cause? Research methods included geomorphic analysis using LIDAR and other digital elevation data, sampling of Quaternary deposits by coring and hand trenching, radiocarbon dating of deposits, construction of paleogeography, and analysis of structural geology. There are four major results of this study. 1) Katalla behaves similarly to southeastern coastal Alaska during megathrust cycles: coseismic uplift is followed by interseismic subsidence. 2) In the last 7000 years, Katalla experienced net uplift with respect to sea level, and radiocarbon dates and stratigraphic profiles indicate tectonic uplift. 3) An uplift event documented in the stratigraphy of Katalla dated at 500 BP does not correlate with any known uplift events in southeastern Alaska. 4) The Ragged Mountain fault, which has been interpreted as both a thrust and normal fault, underwent extension in at least the last 10,000 years. There are three possible hypotheses for uplift in Katalla, each of which partially explains long-term extension along the Ragged Mountain fault: 1) uplift driven by buried imbricate thrusting, 2) uplift driven by slow aseismic anticlinal folding that is accommodating shortening, and 3) uplift driven by exhumation as the upper plate of the Ragged Mountain fault moves west. |
