USING EARTHSCOPE TO UNDERSTAND CORDILLERAN OROGENESIS

The EarthScope initiative can address unresolved questions in Cordilleran orogenesis, a fundamental style of orogeny in which subduction, collision, and growth and stabilization of continents occur through cycles of accretion/contraction and collapse/extension. These cycles generate long, wide, and high mountain belts that influence crustal dynamics, surface processes, atmospheric circulation, and therefore global climate and geochemical cycles. In the northwest US, field and analytical studies of the partially eroded Cordilleran orogen have shown that successive accretion episodes culminated in formation of an Altiplano-type continental plateau that collapsed rapidly in the Paleocene-Eocene by upper crustal extension coupled with lower crustal flow. One hypothesis for the evolution of the orogen is that the deep crust was exhumed along the western (North Cascades) and eastern (Omineca) edges of the plateau to form metamorphic core complexes cored by migmatite domes. The intervening Intermontane belt may have subsided as the deep crust flowed toward the plateau margins. High-resolution seismology can help test this and other hypotheses by imaging the velocity structure of the lithosphere to identify major discontinuities (e.g. crust/lithosphere-scale faults). Seismic anisotropy measurements of the subcordilleran mantle can test whether the mantle thinned along with the crust or whether a substantial part of the lithospheric mantle was replaced by asthenosphere. Seismic reflectivity and anisotropy will test whether the flow of partially molten crust, as predicted by Ellipsis numerical models, drained material from under the Intermontane belt towards the eastern and western margins of the orogen. An EarthScope image of the Cordilleran lithosphere, integrated with numerical modeling and field-based structural and petrologic studies of exhumed orogenic crust, will provide new data for understanding first-order processes of Cordilleran tectonics.