Tectonic geomorphology and the record of Quaternary plate boundary deformation in the Olympic Mountains

We use Quaternary stratigraphy to reconstruct landscape evolution and measure tectonic deformation of the Olympic Mountains section of the Pacifi c Northwest Coast Range. An important motivation for understanding orogenesis here, and throughout the Coast Range, is the concern about the relationship of active deformation to seismic hazards associated with the Cascadia subduction zone. There is also much interest in apportioning the nature of the deformation, whether cyclic or permanent, whether it involves mainly shortening parallel or normal to the margin, and how the deformation on the proversus retrowedge sides of the orogen compare. Pre-Holocene stratigraphy and structure provide the only records of suffi cient duration to separate long-term permanent deformation from earthquake-cycle elastic deformation. For this reason, active-tectonic studies have focused on deformation of Quaternary deposits and landforms, which are best preserved along the Pacifi c Coast and offshore on the continental shelf. At least four major glacial advances are recorded in the valley and coastal deposits along the western margin of the Olympic Peninsula. Both numeric and relative dating, including soils of these deposits, establish a stratigraphic anchor that is used to document the relationship between margin parallel and margin normal deformation in 38 F.J. Pazzaglia et al.