Mark T. Brandon, Arthur R. Calderwood; High-pressure metamorphism and uplift of the Olympic subduction complex. Geology ; 18 (12): 1252–1255. doi: https://doi.org/10.1130/0091-7613(1990)018<1252:HPMAUO>2.3.CO;2
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The discovery of the critical assemblage lawsonite + quartz + calcite indicates that a significant part of the Cenozoic Olympic subduction complex of northwestern Washington State formed by underplating at a depth of about 11 km. The deep structural level exposed in this area is attributed to the presence of a 10-km-high arch in the underlying Juan de Fuca plate. We postulate that this arch was formed when the southern Cordilleran coastline swung westward as a result of middle Miocene to recent extension in the Basin and Range province.