NEOTECTONIC VS. GLACIAL GEOMORPHOLOGY, SUSITNA RIVER LOWLAND AND MARGINS, ALASKA

The Susitna River lowland (SRL) is bounded to the N and W by the arcuate Alaska Range, to the E by the Talkeetna Mountains, and to the S by Cook Inlet. The SRL and its margins lie at the W boundary of the counterclockwise-rotating southern Alaska tectonic block, which is delimited on the N by the dextral Denali fault and on the S by convergence between the Pacific plate and Yakutat microplate with stable North America (Fletcher 2000). The northwestern SRL overlies a proposed transition from shallow to steep subduction (Ferris 2003) that may control fault and fold growth.

Neotectonic features in the SRL and its margins are obscured by glacial scouring and deposition. A regional ice sheet withdrew in late Wisconsinan (Kaufman and Manley, 2004), and Holocene valley glaciers have pushed into and retreated from the margins of the SRL multiple times (Calkin, 1988), both masking and providing datable markers for neotectonic faulting and folding.

Despite challenges caused by glaciation, systematic analyses of rivers and hills in the SRL show persistent faulting and fold growth in an arcuate band along the western and northern margins. Methods include: 1) Neural net classification of LandSat and Aster images to create maps of rivers and river gravels for analyzing changes in the planform geometry of river braiding, width, and sinuosity to identify probable regions of tectonic uplift; 2) multi-step image processing to map drumlin and moraine orientations and to separate glacial and tectonic features; 3) digital profiling using DEMs and DRGs and field profiling using a Trimble laser and real time GPS to measure fault offset, profile uplifted terraces, and quantify changes in slope and relief along laterally propagating folds and changes in river gradient across regions of proposed uplift.

Current results suggest: 1) the Pass Creek fault is an active normal fault with a probable component of left lateral slip; 2) the Castle Mountain fault is a right lateral fault; 3) Beluga Mountain and Kesugi Ridge form laterally propagating folds; 4) changes in longitudinal and cross-sectional geometry of several rivers imply active uplift along the N and W SRL. Correlation of the described geomorphology with limited gravity and seismic data and numerical models suggest a link between SRL neotectonics and the subduction angle of the N. American/Yakutat plates.