THE LITHOSPHERE AND ASTHENOSPHERE UNDER THE GREAT PLAINS

The lithosphere of the North American Great Plains formed by Paleoproterozoic suturing of several Archean cratonic cores followed by accretion of multiple Proterozoic island arcs. To investigate the crust and upper mantle of the Great Plains and central U.S. we have developed a 3D shear velocity model from inversion of Rayleigh wave phase velocities measured from more than 200 teleseismic earthquakes recorded by USArray. Using a larger number of earthquakes, we have also made Sp and Ps receiver function (RF) common conversion point image volumes for this region. Crustal thicknesses were determined from H-k analysis of Ps RF receiver gathers, and manual picking of the image volumes.

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.