Continental delamination and the Colorado Plateau

TLDR

Continental lithosphere is in unstable equilibrium because its denser mantle layer sits atop a less dense asthenosphere, so that a conduit formed by cracking, slumping, or plume erosion can allow the dense lithospheric boundary layer to peel away, producing uplift, increased heat flow, reduced seismic velocities, and emplacement of volcanic and intrusive bodies. A one‑dimensional thermal model of new boundary‑layer formation predicts a half‑life of roughly 3 × 10⁷ years for the associated thermal anomaly and uplift. Geologic and geophysical data from the Colorado Plateau match this delamination hypothesis, indicating uplift by a delamination event about 30 million years ago and possibly a second event around 5 million years ago.

Abstract

Continental lithosphere is in unstable mechanical equilibrium because its mantle layer is denser than the asthenosphere. If any process such as cracking, slumping, or plume erosion initially provided an elongated conduit connecting the underlying asthenosphere with the base of the continental crust, the dense lithospheric boundary layer could peel away from the crust and sink. An analytic model for sinking velocities at the critical initial time shows that instability occurs if the effective viscosities of the lower continental crust and the rising asthenosphere are no more than 10 19 P. Analogies to subduction suggest that the mature instability would grow laterally at plate tectonic velocities; however, it would be almost aseismic. Loss of the cold mantle boundary layer would cause uplift, increased heat flow, reduced seismic velocities, and perhaps emplacement of basalt flows, mantle diatremes, and granodiorite sills. A one‐dimensional thermal model of the formation of a new boundary layer predicts a half life of about 3×10 7 years for this thermal anomaly and uplift. As an example, the geologic and geophysical data from the Colorado Plateau are shown to be consistent with the hypothesis that it was uplifted by a delamination event 30 m.y. ago and perhaps a second event about 5 m.y. ago.