The 1995 Deep Probe seismic investigation of the western North American lithosphere is
unique among modern seismic refraction studies on this continent due to its scale and spatial
sampling (Figure 1). It provides seismological observations of the lithosphere between the fine
scales imaged by regional reflection/refraction crustal studies and the continental scale features
imaged by teleseismic earthquake studies. The Deep Probe corridor approximately follows the
110th meridian, spanning ~29 degrees from near the US-Mexican border to Great Slave Lake.
From north to south, the recording profile crosses the Archean Hearne and Wyoming provinces,
the Cheyenne belt (a Proterozoic age crustal suture; Karlstrom and Houston, 1984), and the
Proterozoic terranes of the southern Rocky Mountains and Colorado Plateau.

        The Hearne province of central-south Alberta is the westernmost extension of the Canadian
shield. Based on basement drillcore studies and gravity and aeromagnetic signatures, the Hearne
province comprises several distinct Archean domains (Ross et al., 1991). A prominent crustal
feature is the Vulcan structure, which separates two Archean domains and may be the boundary
between the Wyoming and Hearne provinces. The sedimentary section in southernmost Alberta
and northern Montana indicates that this region was largely a stable topographic high for the past
1.5 Ga. The Wyoming province appears to be composed of fragments of back-arc basins, island
arcs, and micro-continents, invaded by Late Archean granites but largely stable by 2.7-2.6 Ga
(Houston et al., 1993). Several Proterozoic island arc terranes accreted to the southern Wyoming
province building the interior of the southwestern U.S. from 2.0-1.3 Ga (Karlstrom and Bowring,
1988). The resulting continent, Laurentia, was rifted in Late Proterozic to early Cambrian time,
producing the modern North American craton (e.g. Hoffman, 1988).

        During the Phanerozoic the western margin of North America was uplifted and modified by a
succession of orogenic events (Burchfiel and Davis, 1975; Ye et al., 1996). Only the Ancestral
Rockies (Pennsylvanian) and Laramide (Late Cretaceous through mid-Eocene) orogenies greatly
deformed the lithosphere in our study area. The Ancestral Rockies event affected the Proterozoic
terranes south of the Cheyenne Belt, producing the Uncompahgre uplift. The Laramide produced
basement uplifts and deep sedimentary basins throughout the Proterozoic terranes and the
southern Wyoming province. Subsequently, the Colorado Plateau was uplifted, and the
southernmost part of our profile underwent Basin and Range extension (e.g. Schneider and
Keller, 1994).

        The goals of the seismic investigation were to contrast the lithospheric structure of the
relatively stable Hearne and northern Wyoming provinces with that of the southern Wyoming
province and the Proterozoic terranes affected by the Laramide, and particularly the structure on
either side of the Cheyenne belt. The seismic data provide a continental-scale P-wave velocity
model for the crust and upper mantle to depths of ~150 km. We identify two major lateral
changes in velocity structure in the crust and one in the mantle. All are associated with the
boundaries between the geological provinces, presumably indicating major differences in
lithospheric evolution.