|Institution:||Western Washington University|
|Keywords:||Fluvial geomorphology – Washington (State) – Olympic Mountains; Fluvial geomorphology – Washington (State) – Olympic Mountains – Mathematical models; Faults (Geology) – Washington (State) – Olympic Mountains; Subduction zones – Washington (State) – Olympic Mountains; Rock deformation – Washington (State) – Olympic Mountains; Geology; Olympic Mountains (Wash.); Academic theses|
|Full text PDF:||http://cedar.wwu.edu/wwuet/523|
Understanding topographic development in subduction zone forearcs requires comparison of deformation at short and long-term time intervals. We focus here on geomorphic records of uplift and incision in the Cascadia forearc of Washington State for comparison with short-term deformation driven by subduction zone coupling. We use surficial geologic mapping, optically stimulated luminescence dating, and surveyed terrace strath elevations to document fluvial incision and fault slip rates in the Wynoochee River valley in the southern Olympic Mountains. Results from 14 optically stimulated luminescence samples yield fluvial terrace age groupings of ~7-12 ka, ~14-18 ka, ~30-45 ka, and ~50-60 ka, which likely correspond to climate-modulated fluctuations in sediment supply within this repeatedly glaciated catchment. Calculated fluvial incision rates from surveyed terrace straths range between ~0.4 and 3.1 mm/yr, though incision in the upper reaches of the Wynoochee River is likely influenced by repeated valley aggradation, re-excavation, and isostatic rebound from glaciation, thereby hampering straightforward tectonic interpretation of calculation incision rates. Differential incision in the lower reaches of the river, however, is more straightforward to interpret and displays broad warping of the terrace tread and strath incision, potentially preserving a component of interseismic uplift. Terrace surfaces are vertically displaced by the active Canyon River fault, across which we estimate reverse dip-slip rates of ~0.1-0.3 mm/yr. We compare fault slip and incision results with a boundary element model constrained by GPS geodesy, estimating slip on the Canyon River fault and regional deformation in response to interseismic stress from the Cascadia subduction zone. The modeled reverse slip rates range between ~0.1-0.5 mm/yr, with lower rates of ~0.1-0.15 mm/yr near the Wynoochee River. Additionally, predicted uplift along the valley shows similar broad warping to the incision record in the lower reaches. The general agreement between observed long-term geologic record and modeled short-term deformation suggests that a portion of Cascadia interseismic strain may be permanent, manifested as fluvial incision and slip along upper-plate faults. Consistency between these signatures over time may indicate relative stability in the spatial pattern of subduction zone coupling over ~104 yr intervals. Advisors/Committee Members: Amos, Colin, Loveless, Jack, Clark, Douglas.