INTERACTION OF MAGMATISM AND DEFORMATION DURING POST-OROGENIC DELAMINATION IN THE NORTH AMERICAN CORDILLERA DURING THE EOCENE-OLIGOCENE

The Eocene-Oligocene extensional deformation and magmatism, occurring after ~125-55 Ma contraction, is commonly interpreted through the lens of subducting slab models. An alternative model is presented, in which the post-orogenic deformation/magmatism is a result of mantle delamination below a relatively narrow (~100 km wide) orogenic plateau that extended from central Idaho to northernmost Mexico during the Late Cretaceous-Paleocene. Another zone of major uplift occurred just south of the Lewis and Clark zone in Idaho and Montana, and extended ESE toward the Black Hills, SD. The former is a result of an endload on the western margin of North America; the latter is associated with the post-85 Ma clockwise rotation of the BMAL (Blue Mountains and adjacent Laurentia) block rotation in the Pacific Northwest, accommodated by sinistral movement on the Lewis and Clark zone.

The directional delamination model proposes the following. The thickened lithosphere in Idaho - an area of high elevation - would collapse first, facilitated by the reversal of shear sense (dextral) on the Lewis and Clark zone. This pattern explains the >1000 km long (WA to SD), ESE-oriented zone of magmatism in the Early Eocene (Challis magmatism in Idaho), that is parallel to the Lewis and Clark zone. Starting at ~50-45 Ma, magmatism and extensional deformation sweeps southward, following the region of thickened lithosphere. Although this zone is approximately 200-500 km wide now, it was significantly narrower prior to both Eocene-Oligocene and Miocene-present (Basin and Range) extension. Hence, it is extremely unlikely to be the margin of an orogenic-scale slab. Instead, it may be better to view the process as the “peeling off of the thickened lithospheric Band-Aid”. This is supported by the thickened (and dangling) lithosphere mantle that still exists in central Idaho. Here, slow seismic velocities start at the base of the crust and extend vertically downward to ~300 km depth as a curtain. As the lithosphere peeled off southward, there was upwelling of the asthenosphere leading to increased magmatism; the magmatism and local high elevation lead to extensional deformation.