Late Quaternary tectonic deformation in the Cape Arago‐Bandon Region of coastal Oregon as deduced from wave‐cut platforms

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Abstract
The Cape Arago region of south central Oregon sits on the upper plate of the Cascadia subduction zone about 80 km east of the base of the continental slope. The style of late Pleistocene deformation along the Cascadia forearc near Cape Arago is well expressed by the altitudinal variation of a set of five uplifted wave‐cut platforms. These platforms record open folding, with a half wavelength of about 6–7 km, as well as late Quaternary offset on flexural‐slip reverse faults that parallel bedding in the underlying bedrock. The folds have produced both landward and seaward tilts to the uplifted wave‐cut platforms. Because the folds cut obliquely across the coastline, the magnitude of coastal uplift is variable. In the case of the lowest, 80 ka wave‐cut platform, this variable uplift has resulted in coastal deformation ranging from subsidence to a maximum uplift rate of 0.8 m/kyr. Quantitative analysis of the mechanism of flexural slip folding for the South Slough syncline near Cape Arago indicates that the late Quaternary strain rate has decreased in the last 200 kyr. Evidence of past great subduction‐style earthquakes, such as regional uplift and regional landward tilting of wave‐cut platforms, or regional submergence of coastlands, is lacking in the Cape Arago region. Instead, the deformational style is controlled by folding. Though localized folding is dominant, the occurrence of great subduction‐style earthquakes is not precluded because localized folding could occur concurrently with regional coseismic deformation.