The western extension of the Seattle fault: new insights from seismic reflection data

Seismic reflection profiling across the Seattle fault west of Puget Sound, western Washington State, reveals near-surface deformation as far west as Hood Canal. North of Green Mountain and in line with the trend of the Seattle fault, folding within late Pleistocene sediments and a deflected stream channel indicate active deformation. Here, we observe folding in late Pleistocene deposits overlying more steeply dipping Tertiary strata previously inferred to be part of the Seattle monocline. The greater than 70 km long dominantly east- west striking Seattle fault follows the contours of the northwest edge of Green Mountain and locally strikes south-southwest, or a ~25 degree rotation in local fault trend. Exposures of Eocene bedrock (Crescent Fm) south of the fault indicate deeper exposures than along the eastern portions of the Seattle fault. This deeper exposure, accompanied with greater degrees of deformation than observed to the east, suggest higher uplift rates above the Seattle fault west of Puget Sound. Seismic profiles west of Green Mountain, along strike with the Seattle fault west near the west end of the Seattle Basin, show a complex pattern of late Quaternary and older deformation. However, these profiles extend beyond the inferred limits of the Seattle Basin and do not show the characteristic monocline and deformation front that defines the Seattle fault to the east. A west-northwest striking fold within Late Quaternary sediments, steeply dipping Tertiary strata, and a southwest-striking magnetic lineament all suggest a complex and possibly distributed pattern of deformation along the west part of the fault. The Seattle fault continues west beyond its mapped extent north of Green Mountain; it may merge with structures that control the Dewatto Basin along the south edge of the Seattle uplift, including the Frigid Creek or Saddle Mountain faults west of Hood Canal.