Late Quaternary deformation of marine terraces on the Cascadia Subduction Zone near Cape Blanco, Oregon

The Cape Blanco region of south coastal Oregon sits on the upper plate of the Cascadia subduction zone about 60–70 km east of the base of the continental slope. Though the region has no historic coseismic deformation, late Quaternary deposits and landforms show abundant evidence of uplift, folding, and faulting. A set of five late Quaternary marine terraces and one uplifted Holocene beach berm are preserved in the Cape Blanco region. Stratigraphic and altitudinal surveys of these physiographic features, combined with several numerical and correlation age determinations, permit a reconstruction of tectonic deformation near Cape Blanco in approximately the last 200,000 years. The most cogent aspects of the neotectonics in this part of the Cascadia subduction zone are that an east‐west trending anticline deforms the lower three terraces (80–125 ka) as well as accounting for uplift of the Holocene storm berm. Latest movement on the anticline was no more than 2,000 years ago, but the anticline also deforms sediments as old as Middle Miocene. The anticline is consistent with a north‐south principal contraction axis at Cape Blanco in about the last 100,000 years. An older terrace at Cape Blanco (approximately 200 ka) is offset by two reactivated faults whose movements prior to the Eocene served to suture several terranes to the North American continent. Tectonic tilting in the older terrace is not consistent with a north‐south principle axis of contraction, but the axis of tilt is parallel to north‐northwest striking structures developed in the Quaternary fold belt offshore on the continental shelf. The observed deformation indicates that regional stress in the vicinity of Cape Blanco in the late Quaternary has been accommodated by a variable pattern of strain.