THE SWIFT SECTOR OF THE OWL

The Olympic-Wallowa Linement (OWL) extends northwesterly from ID to northwestern WA. The sector of OWL from Cle Elum in central WA to the Wallowa Mountains of OR (CLEW) is in the mid-Miocene Columbia River Basalt Group (CRBG). CLEW has fairly good topographic and structural defintion. OWL passes northwesterly out of CRBG at Cle Elum and traverses older rocks. From Cle Elum to the Puget Lowland OWL loses topographic and structural clarity. In this sector OWL parallels what is here recognized as the Seattle-Wenatchee-Kittitas fold and thrust belt (SWIFT).

SWIFT is largely underlain by middle to late Eocene unconformity-bounded formations. Regional stratigraphy and structure are obscured by a welter of local lithostratigraphic names, Pleistocene deposits, forests, and Puget Sound. 1:24,000 mapping 25 km west of Wenatchee outlines a major anticline with a southwesterly dipping reverse fault on its steeper northeasatern limb; antiformally folded Swauk and Teanaway Formations are faulted over the younger Chumstick (Roslyn-equivalent) Formation. This Blushastin anticline was formerly mapped as the southeastern segment of the Leavenworth normal fault zone. Analogous anticlines with southwesterly dipping faults are Eagle Creek (-10 km northwest of Wenatchee), Ainsley Canyon (from Cle Elum to Easton in Kittitas County), and Newport Hills with associated Seattle fault (in the Puget Lowland). Asymmetric folds do occur in the Oligocene volcaniclastic rocks of the Cascade Range (between the Ainsley Canyon and Newport Hills anticlines).

SWIFT is ~ 65 km wide between the crests of the Eagle Creek and Ainsley Canyon anticlines. Its exposed length is ~ 120 km. The Kittitas Valley syncline (a.k.a. Roslyn basin) is between the Blushastin and Ainsley Canyon anticlines. The Seattle basin is northeast of the Newport Hills anticline. The Eagle Creek, Blushastin, and Ainsley Canyon anticlines have cores of pre-Tertiary metamorphic rocks. Blushastin and Ainsley Canyon pass down-plunge into less steeply folded CRBG, which implies two major periods of folding. The pre-Oligocene Straight Creek fault displaces the western end of the Ainsley Canyon anticline and, hence, SWIFT, ~ 55 km.

Post-CRBG deformation in SWIFT is geologic evidence for the tectonic model of Wells et al. (1998) that OR is deforming the underbelly of WA. Neotectonic studies in northwestern WA should be quite SWIFT.