THE LITHOSPHERE AND ASTHENOSPHERE UNDER THE GREAT PLAINS
The Ps RFs give an average crustal thickness of ~45±10 km in the central portion of the study region. We observe 1) NE-SW trending zones of increased crustal thickness (~53 km) associated with the boundaries between accreted Proterozoic terrains, 2) regions of whole crustal thrust faulting in the TransHudson region, and 3) an intermittent Ps conversion from a high velocity lower crustal layer that underlies parts of the northern and eastern study area.
Striking features in the 3D Vs model are the high velocities (Vs > 4.7 km/s) at the very top of the mantle of the Superior Province, and low velocities (locally Vs < 4.4 km/s) beneath the southern Rocky Mountain front. In cross section the 3D velocity model shows a low velocity channel (Vs < 4.5 km/s) that thins and deepens eastward away from the Rockies. Beneath the Rockies, low velocities occupy almost the entire upper 200 km of the mantle. The channel axis deepens from 100-125 km beneath the Rockies to 150-175 km depth beneath the Gulf of Mexico coastal plain, and to 200 km beneath the TransHudson Province. The channel is weakest (4.5 < Vs <4.6 km/s), thinnest (~50 km) and deepest (> 200 km) beneath the southwestern corner of the Superior province. The Sp RF images reflect the basic channel shape, with zones of long (> 200 km) layered positive and negative events appearing in the lithospheric mantle above the channel. The shape of the lithosphere-asthenosphere boundary reflects the elevation gradient of the Great Plains, suggesting a causal relationship.