Crustal structure of western Washington State.
The primary results of the analysis of seismic, gravity, magnetic and geology data are as follows. The Puget basin is comprised of two subbasins with a maximum depth of 10 km. Seismic data clearly indicate an upper crustal high-velocity layer approximately 4 km thick at a depth of approximately 5 km. This interpreted ophiolitic body correlates with outcrops of mafic/ultramafic rocks along the western margin of the northern Cascades. Attenuation in the seismic data indicate discontinuous layers of volcanic sedimentary rock beneath the southern Cascades.
The middle crust consists of a mid-crustal reflector at approximately 20 km depth with laterally changing velocities from 6.4 km/s to 6.6 km/s north to south respectively. High middle crustal velocities beneath the southern Cascades is evidence of mafic-intrusive rock. Lower crustal velocities range from 7.1 km/s to 7.4 km/s. The Moho discontinuity includes a dome-like structure with up to 10 km of relief. The crust thins to approximately 36 km beneath the Puget basin and reaches thicknesses up to 50 km, in the southern Cascades volcanic arc.
Deep mantle reflectors indicate a low velocity layer interpreted as the top of the Juan de Fuca plate. The model shows that the Juan de Fuca plate is comprised of an oceanic crustal layer, 5-6 km thick with a velocity of 6.87 km/s, and a mantle layer beneath it. The subducting plate structure is similar to Moho structure; the top of the plate is 60 km deep beneath the Puget basin and 75 km deep at both the north and south ends of the model.
Crustal thickness interpreted from this and companion studies in the vicinity of western Washington state shows a general eastward thickening with a distinct zone of flattening in the west Puget Sound area, a relatively thin (36 km) crust beneath the Puget basin and a consistently thick crust along the Cascade Range volcanic arc.
The correlation between Moho and subducting plate geometry may imply that the Juan de Fuca and North America plates are coupled in some way. Coupling at the interface provides a mechanism for transmitting stress from the Juan de Fuca plate into the overriding North American plate and may explain the diffuse seismicity within the crust of western Washington state. (Abstract shortened by UMI.)
Gridley, James Michael, "Crustal structure of western Washington State." (1993). ETD Collection for University of Texas, El Paso. AAI9332750.