Archean continental terranes commonly have deep keels, but the whole upper mantle of western North America shows very slow seismic velocities independent of the age of the overlying crustal sections: Are seismic velocities tracking primarily the temperature or composition of the continental lithosphere? If western NA is underlain by mantle of similar age to the overlying crust, where is its expression in the seismic tomography? Does this imply that the lithospheric base of continental NA is as shallow as 100-150km west of the Rockies? Caption: Outline of the surface exposure of two areas of Archean crust in North America relative to the S-wave tomography at 120 km depth in the underlying mantle. Tomography base map from S. van der Lee et al., 2002, Variations in lithospheric thickness from Missouri to Massachusetts, Earth Plant. Sci. Lett, submitted.

US is a Natural Geologic Laboratory Plate Boundary Processes ♦ Convergence (Cascadia – Aleutians) ♦ Transform faulting (San Andreas) ♦ Extension (Basin and Range) Diversity of tectonic elements Hotspot ♦ intraplate seismicity ♦ Precambrian basement ♦ batholiths ♦ core complexes ♦ overthrust belts ♦ arc volcanism ♦ sedimentary basins 3.5 billion year record of plate evolution ♦ accretion ♦ sedimentary basins ♦ rifting ♦ volcanic arcs ♦ transition zones

Transportable Array

Facility Construction Progress Seismic Stations Backbone 18 Transportable Array 107 Flexible Array GPS Stations Volcanic Cluster 18 Extension Cluster 25 Transform Cluster 95 Subduction Cluster 34 Backbone Strain meters Borehole strainmeters 7 Long-baseline strainmeters 1 Magnetotelluric Stations SAFOD Phase II Drilling Complete

G. Rogers & H. Dragert (2003) Episodic Tremor and Slip on the Cascadia Subduction Zone: The Chatter of Silent Slip. Science, v. 300, p. 1942-1943. Published online 8 May 2003.

Paulson