REINTERPRETATION OF THE SEATTLE UPLIFT, WASHINGTON, AS A PASSIVE ROOF DUPLEX

Seismic reflection, tomography, aeromagnetic, geologic mapping, trench-log, seismicity, and borehole data provide evidence that the Seattle uplift is a passive-roof duplex. A roof duplex, also known as a triangle wedge, is defined as wedge bounded at top and bottom by thrust faults with opposite senses of vergence; in a passive-roof duplex the upper thrust moves only when the lower thrust slips. The Seattle fault is a south-dipping reverse fault that we propose is the leading edge of a 40-km-wide fold-and-thrust belt. The recently discovered, north-dipping reverse Tacoma fault is interpreted as a back thrust along the trailing edge of the belt, thus making the belt doubly vergent. Divergent, e.g. north- and south-dipping, seismic reflections from Tertiary sedimentary strata in both the Seattle and Tacoma basins, define triangle wedges that uplift and tilt sedimentary rocks in the hanging walls. Master floor thrusts defining the base of the wedges in the Seattle and Tacoma fault zones are interpreted as flattening updip into bedding plane thrusts and are thus blind structures. Along the eastern end of the Seattle uplift, between the Seattle and Tacoma faults, the seismic data image several shallow, short-wavelength folds that are more compatible with shallow thrusting than with a deeper reverse fault. From reflector truncations, at least 10 north-dipping thrust faults are inferred in the cores of these shallow folds. These faults, often interpreted as bedding plane thrusts, appear to root into shallow roof thrusts overlying the triangle wedges. Only a few of these shallow thrusts appear to have been activated during recent slip events on the underlying master floor thrusts of the Seattle fault, and all are currently aseismic, but they nevertheless pose a seismic hazard due to their ability to deform the surface over a broad area. These thrusts actively fold and uplift the seafloor at Alki, Restoration, Three Tree, and Robinson Points. These folds are evident in aeromagnetic data, where short-wavelength aeromagnetic anomalies in Puget Sound are compatible with the shallow folding of Miocene conglomerate rich in Crescent Formation clasts. The doubly vergent thrust fault system beneath the Seattle uplift provides an explanation for the widespread vertical offset of coastal marsh deposits at about 1100 years ago in central and southern Puget Sound.