THE TAHUYA LINEAMENT: SOUTHWESTERN EXTENSION OF THE SEATTLE FAULT?

We present a tectonic model that suggests the Seattle fault zone is kinematically linked to the Tacoma and Saddle Mountain faults west of the Seattle uplift. The active east-striking Seattle and Tacoma faults separate the Seattle uplift from the Seattle and Tacoma basins to the north and south, respectively. These two faults lie beneath the most densely populated regions of Washington and likely accommodate much of the 4-6 mm/y of north-south horizontal compression currently underway in the Puget Lowland. The Seattle and Tacoma basins, clearly expressed in seismic tomography and gravity anomalies, extend eastward from Hood Canal across the Puget Sound. A smaller depression, the Dewatto basin, is located at the western terminus of the Tacoma fault. Immediately west of Hood Canal, the active northeast-striking Saddle Mountain fault may connect to the Seattle fault system. We propose that the Dewatto and Tacoma basins are kinematically linked, and that the west-directed Tacoma fault at its westerly terminus intersects a deeper north-striking fault along the western margin of the Seattle uplift, here termed the Tahuya fault. Geodetic measurements show the Olympic block, west of Hood Canal, is moving north-northeastward at 4-6 mm/y. We suggest that this migration of the Olympic block has caused the Tacoma basin to diverge around the southwest perimeter of the Seattle uplift, thereby forming the Dewatto basin. This tectonic process necessitates dip-slip and strike-slip components of motion along both the Tacoma and Tahuya faults. By palinspastically restoring regional magnetic anomalies, we identify 8.5 km of left-lateral offset on the Tacoma fault beneath the Tacoma Basin. High-resolution seismic-reflection data show evidence for late Quaternary deformation along the Tahuya fault and evidence that the Tahuya fault swings northeastward around Green and Gold Mountains at the westernmost part of the Seattle uplift and possibly connects with the Seattle fault. A cross-sectional model based on a ground-magnetic transect and constrained by seismic-reflection data is consistent with 3-4 km of vertical offset on the Tahuya fault. The kinematic link between the Seattle and Saddle Mountain faults suggests that northward motion of the Tacoma basin may be accommodated by both the Saddle Mountain and Tahuya faults.