Six late Quaternary river terraces, preserved along the Clearwater River in northwestern Washington State, provide a ϳ ϳ140 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 ϳ ϳ0 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 ϳ ϳ0.9 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 (>ϳ ϳ10 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 ϳ ϳ14 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 (ϳ ϳ100 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 ϳ ϳ3 m/ky, relative to a fixed Puget Sound. This prediction is in good agreement with offset of a ϳ ϳ122 ka sea cliff preserved at the southwest side of the Clearwater valley profile. In this case, the long-term margin-pe...

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[1] We integrate existing and new geologic data [REtreating TRench, Extension, and Accretion Tectonics (RETREAT project)], particularly on the origin, growth, and activity of the mountain front at Bologna, Italy, into a new model that explains Apennine orogenesis in the context of a slab rollback -upper plate retreat process. The Bologna mountain front is an actively growing structure driving rock uplift $1 mm/year, cored by a midcrustal flat-ramp structure that accommodates ongoing shortening driven by Adria subduction at a rate of $2.5 mm/year. The data we use are assembled from river terraces and associated Pleistocene growth strata, geodesy including releveling surveys, reinterpretation of published reflection lines, and a new high-resolution reflection line. These data constrain a simple trishear model that inverts for blind thrust ramp depth, dip, and slip. Apennine extension is recognized both in the foreland, as high-angle normal faults and modest stretching in the carapace of the growing mountain front, and in the hinterland, with larger normal faults that accomplish some crustal thinning as the upper plate retreats. This coevolution of extension and shortening shares some notable characteristics with other basement-involved collisional orogens including the early Tertiary Laramide orogeny in the American West and the Oligocene to Miocene evolution of the Alps. We propose a possible relationship between underplating and the development of the Po as a sag basin as a Quaternary phenomenon that may also apply to past periods of Apennine deformation (Tortonian). Continued shortening on the structure beneath the Bologna mountain front represents by far the most important and underappreciated seismogenic source in the front of the northern Apennines.

[1] The incision of rivers in bedrock is thought to be an important factor that influences the evolution of relief in tectonically active orogens. At present, there are at least six competing models for incision of bedrock rivers, but these models have received little quantitative testing. We statistically evaluate these models using observations from the Clearwater River in northwestern Washington State, which crosses the actively rising forearc high of the Cascadia margin. A previous study has used fluvial terraces along the Clearwater to estimate bedrock incision rates over the last $150 kyr. They show that incision rates have been steady over the long-term (>50 kyr), consistent with other evidence based on isotopic cooling ages, for steady long-term (>1 Myr) erosion rates. The steady state character of the river allows us to use the relatively simple time-invariant solutions for the various incision models and also to estimate long-term sediment discharge along the river, which is a critical variable for some incision models. An interesting feature of the Clearwater River is that it has a downstream decrease in the rate of incision, from $0.9 mm/yr in the headwater to <0.1 mm/yr at the coast. None of the incision models, including the shear stress model, successfully accounts for this relationship. This result may be due to the simple way in which these models are used, commonly without consideration for the distribution of discharge with time, and the variable capacity of the river channel to contain peak flows along its course. We suggest some general improvements for the incision models, and also guidelines for selecting those rivers that will allow good discrimination between competing models.

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