SEISMIC-REFLECTION IMAGES BENEATH PUGET-SOUND, WESTERN WASHINGTON-STATE - THE PUGET LOWLAND THRUST SHEET HYPOTHESIS

Seismic reflection data show that the densely populated Puget Lowland of western Washington State is underlain by subhorizontal Paleogene an d Neogene sedimentary rocks deformed by west and northwest trending fa ults and folds. From south to north beneath the Lowland, features seen on the seismic data include: the horizontally-stratified, 3.5 lan thi ck Tacoma sedimentary basin, the Seattle uplift with south dipping (si milar to 20 degrees) strata on its south flank and steeply (50 degrees to 90 degrees) north dipping strata and the west-trending Seattle fau lt on its north flank; the 7.5 km thick, northward-thinning Seattle se dimentary basin; the antiformal Kingston arch; and the northwest trend ing, transpressional Southern Whidbey Island fault zone (SWIF). Interp leting the uplifts as fault-bend and fault-propagation folds leads to the hypothesis that the Puget Lowland lies on a north directed thrust sheet. The base of the thrust sheet may lie at 14 to 20 km depth withi n or at the base of a thick block of basaltic Crescent Formation; its edges may be right-lateral strike-slip faults along the base of the Ca scade Range on the east and the Olympic Mountains on the west. Our mod el suggests that the Seattle fault has a long-term slip rate of about 0.25 mm/year and is large enough to generate a M7.6 to 7.7 earthquake.