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A Fluvial Record of Long-term Steady-state Uplift and Erosion Across the Cascadia Forearc High, Western Washington State

^* Department of Earth and Environmental Sciences, Lehigh University, 31 Williams, Bethlehem, Pennsylvania 18015
^** Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, Connecticut 06520-8109

Six late Quaternary river terraces, preserved along the Clearwater River in northwestern Washington State, provide a 140 ka record of long-term incision and uplift across the western side of the Cascadia forearc high. 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 is measured by the height of a strath relative to the adjacent modern river channel. The straths along the Clearwater show an upstream increase in bedrock incision, ranging from 0 at the coast to a maximum of 110 m in the headwaters. The incision at any point along the profile increases systematically with strath age. The calculated incision rates range from <0.1 m/ky at the coast, to 0.9 m/ky in the central massif of the Olympic Mountains. These rates are in close agreement with published long-term erosion rates estimated from fission-track cooling ages. The coincidence between bedrock incision rates and erosion rates suggests that over the long term (> 10 ky) the Clearwater River valley has maintained a steady-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 increase in the rate of rock uplift from the coast toward the central part of the range. These results are consistent with other evidence indicating a long-term steady-state balance between the accretionary influx and the erosional outflux for this part of the Cascadia subduction wedge since 14 Ma.

These results help show how terrace deposits form in tectonically active landscapes. 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 increased sediment supply due to local deglaciation and because of eustatic highstands that forced aggradation in the lower reach of the drainage and across the continental shelf as well. The fluvial system shows strong forcing by the glacial climate cycle. Even so, the river appears to have returned to the same valley profile during each cycle of strath cutting. Thus, bedrock incision is clearly unsteady at time scales shorter than the glacial climate cycle (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 analysis indicates that the accretionary flux into the wedge occurs mainly by frontal accretion and not by underplating. If accretion occurred entirely at the front of the wedge, the present west coast should be moving to the northeast at 3 m/ky, relative to a fixed Puget Sound. This prediction is in good agreement with offset of a 122 ka sea cliff preserved at the southwest side of the Clearwater valley profile. In this case, the long-term margin-perpendicular shortening would account for 20 to 35 percent of the geodetically-measured northeast-southwest shortening across the Olympic Mountains.

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