A fluvial record of long-term steady-state uplift and erosion across the Cascadia forearc high, western Washington State

Fj. Pazzaglia et Mt. Brandon, A fluvial record of long-term steady-state uplift and erosion across the Cascadia forearc high, western Washington State, AM J SCI, 301(4-5), 2001, pp. 385-431
Citations number
Categorie Soggetti
Earth Sciences
Journal title
ISSN journal
0002-9599 → ACNP
Year of publication
385 - 431
SICI code
Six late Quaternary river terraces, preserved along the Clearwater River in northwestern Washington State, provide a similar to 140 ka record of long- term incision and uplift across the western side of the Cascadia forearc hi gh. Terrace ages are constrained by weathering rind and radiocarbon dating and by correlation to dated coastal glacio-fluvial deposits and the global eustatic curve. The terraces overlie flat bedrock surfaces, called straths, which represent uplifted segments of the river channel. Bedrock incision i s measured by the height of a strath relative to the adjacent modem river c hannel. The straths along the Clearwater show an upstream increase in bedro ck incision, ranging from similar to0 at the coast to a maximum of 110 nr i n the headwaters. The incision at any point along the profile increases sys tematically with strath age. The calculated incision rates range from <0.1 m/ky at the coast, to similar to0.9 m/ky in the central massif of the Olymp ic Mountains. These rates are in close agreement with published long-term e rosion rates estimated from fission-track cooling ages. The coincidence bet ween bedrock incision rates and erosion rates suggests that over the long t erm (> similar to 10 ky) the Clearwater River valley has maintained a stead y-state profile defined by a long-term balance in the rates of incision and rock uplift. Upstream divergence of terraces is best explained by an incre ase in the rate of rock uplift from the coast toward the central part of th e range. These results are consistent with other evidence indicating a long-term ste ady-state balance between the accretionary influx and the erosional outflux for this part of the Cascadia subduction wedge since similar to 14 Ma. The se results help show how terrace deposits form in tectonically active lands capes. The dominantly fluvial Clearwater drainage was forming straths while alpine glaciers were advancing in adjacent drainages. In turn, the straths were buried during the transition to interglacial times because of increas ed sediment supply due to local deglaciation and because of eustatic highst ands that forced aggradation in the lower reach of the drainage and across the continental shelf as well. The fluvial system shows strong forcing by t he glacial climate cycle. Even so, the river appears to have returned to th e same valley profile during each cycle of strath cutting. Thus, bedrock in cision is clearly unsteady at time scales shorter than the glacial climate cycle (similar to 100 Ky) but appears to be relatively steady when averaged over longer time scales. A simple kinematic model is used to examine how uplift of the Cl. Our analy sis indicates that the accretionary flux into the wedge occurs mainly by fr ontal accretion and not by underplating. If accretion occurred entirely at the front of the wedge, the present west coast should be moving to the nort heast at similar to3 m/ky, relative to a fixed Puget Sound. This prediction is in good agreement with offset of a similar to 122 ka sea cliff preserve d at the southwest side of the Clearwater valley profile. In this case, the long-term margin-perpendicular shortening would account for 20 to 35 perce nt of the geodetically-measured northeast-southwest shortening across the O lympic Mountains.