CRUSTAL STRUCTURE ALONG THE WEST FLANK OF THE CASCADES, WESTERN WASHINGTON

Knowledge of the crustal structure of the Washington Cascades and adja cent Puget Lowland is important to both earthquake hazards studies and geologic studies of the evolution of this tectonically active region. We present a model for crustal velocity structure derived from analys is of seismic refraction/wide-angle reflection data collected in 1991 in western Washington. The 280-km-long north-south transect skirts the west flank of the Cascades as it crosses three tectonic provinces inc luding the Northwest Cascades Thrust System (NWCS), the Puget Lowland, and the volcanic are of the southern Cascades. Within the NWCS, upper crustal velocities range from 4.2 to 5.7 km s(-1) and are consistent with the presence of a diverse suite of Mesozoic and Paleozoic metased iments and metavolcanics. In the upper 2-3 km of the Puget Lowland vel ocities drop to 1.7-3.5 km s(-1) and reflect the occurrence of Oligoce ne to recent sediments within the basin. In the southern Washington Ca scades, upper crustal velocities range from 4.0 to 5.5 km s(-1) and ar e consistent with a large volume of Tertiary sediments and volcanics. A sharp change in velocity gradient at 5-10 km marks the division betw een the upper and middle crust. From approximately 10 to 35 km depth t he velocity field is characterized by a velocity increase from similar to 6.0 to 7.2 km s(-1). These high velocities do not support the pres ence of marine sedimentary rocks at depths of 10-20 km beneath the Cas cades as previously proposed on the basis of magnetotelluric data. Cru stal thickness ranges from 42 to 47 km along the profile. The lowermos t crust consists of a 2 to 8-km-thick transitional layer with velociti es of 7.3-7.4 km s(-1). The upper mantle velocity appears to be an unu sually low 7.6-7.8 km s(-1). When compared to velocity models from oth er regions, this model most closely resembles those found in active co ntinental arcs. Distinct seismicity patterns can be associated with in dividual tectonic provinces along the seismic transect. In the NWCS an d Puget Lowland, most of the seismicity occurs below the base of the u pper crust as defined by a seismic boundary at 5-10 km depth and conti nues to 20-30 km depth. The region of transition between the NWCS and the Puget Lowland appears as a gap in seismicity with notably less sei smic activity north of the boundary between the two. Earthquakes withi n the Cascades are generally shallower (0-20 km) and are dominated by events associated with the Rainier Seismic Zone.