Subduction Underplating and Uplift in a Young Accretionary Complex

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The research involves a field-based study of the processes of subduction underplating, ductile thickening and uplift as exposed within the Olympic Core rocks, a Cenozoic accretionary complex in the Olympic Mountains of NW Washington. Geological and geophysical evidence indicates that the eastern Core rocks were accreted beneath the accretionary wedge at a distance of at least 50 km from the trench. Therefore, this area provides a unique opportunity to directly study deformational processes within the more landward and deeply seated part of a sediment-rich accretionary complex. The study will combine techniques of structural geology, metamorphic petrology and fission- track dating to determine (1) the process of underplating, whether by imbricate faulting or by wholesale flowage; (2) the amount and nature of post-accretion ductile strain using finite-strain measurements and deformational textures; (3) the maximum depth of structural burial as indicated by peak metamorphic grade; and (4) the time and rate of uplift using fission-track dating. Potential contributions include: (1) a better understanding of how accretionary wedges grow and how high- pressure metamorphic rocks are returned to the surface; (2) direct information about flow mechanisms within the wedge, which is critical to geomechanical modelling of wedges; and (3) new tools and concepts for mapping and interpretation of older subduction complexes.