Three-dimensional velocity structure of Siletzia and other accreted terranes in the Cascadia forearc of Washington

Eocene mafic crust with high seismic velocities underlies much of the Orego n and Washington forearc and acts as a backstop for accretion of marine sed imentary rocks from the obliquely subducting Juan de Fuca slab. Are-paralle l migration of relatively strong blocks of this terrane, known as Siletzia, focuses upper crustal deformation along block boundaries, which are potent ial sources of earthquakes. In a three-dimensional velocity model of coasta l Washington, we have combined surface geology, well data, and travel times from earthquakes and controlled source seismic experiments to resolve the major boundaries of the Siletz terrane with the adjacent accreted sedimenta ry prism and volcanic are. In southern Washington and northern Oregon the S iletz terrane appears to be a thick block (similar to 20 km) that extends w est of the coastline and makes a high-angle contact with the offshore accre ted sedimentary prism. On its east flank the high-velocity Siletz terrane b oundary coincides with an en echelon zone of seismicity in the are. In nort hern Washington the western edge of Siletzia makes a lower-angled, fault-bo und contact with the accretionary prism. In addition, alternating, east-wes t trending uplifts and downwarps of the Siletz terrane centered on the anti formal Olympic Mountains may reflect focusing of north-south compression in the northern part of the Siletz terrane. This compressional strain may res ult from northward transport and clockwise rotation of the Siletz terrane i nto the relatively fixed Canadian Coast Mountains restraining bend along th e coast.