Western Boundary of the Seattle Uplift, Washington

Lidar topographic scarps and aeromagnetic anomalies define a northeast-striking, en echelon sequence of faults along the southeastern Olympic Peninsula of Washington, all active in Holocene time and possibly linked kinematically with the Seattle fault. The northeast-striking Saddle Mountain fault, the northernmost fault in the sequence, was first recognized in the early 1970s and is now well mapped in the Hoodsport area on the basis of lidar surveys, aerial photography, and trench excavations. Drowned trees and trench excavations demonstrate Holocene deformation on the Saddle Mountain fault approximately contemporaneous with the MW 7.5 Seattle fault earthquake 1100 years ago and with a wide variety of other fault and landslide activity observed over much of the eastern Olympic Peninsula and central and southern Puget Lowland. The northwest-striking Frigid Creek and Canyon River faults, lying 4 km and 27 km to the southwest of Saddle Mountain, respectively, also show evidence of late Holocene deformation. A detailed analysis of aeromagnetic data suggests that the Saddle Mountain fault extends at least 35 km, from 6 km southwest of Lake Cushman to the latitude of the Seattle fault as mapped east of Hood Canal. Regional aeromagnetic data also indicate that the Seattle fault may extend westward across Hood Canal and into the Olympic Mountains, where it terminates near the magnetically inferred northern end of the Saddle Mountain fault. The en echelon alignment of the Saddle Mountain, Frigid Creek, and Canyon River faults, all active in late Holocene time, reflects a >45-km-long, northeast-trending zone of deformation that may accommodate the northward shortening of Puget Lowland crust inboard of the Olympic massif. In this model, the Seattle fault and Saddle Mountain deformation zone form the northern and western boundaries of the northward advancing Seattle uplift.