The viscosity of a partially molten layer in a paleo-orogenic plateau

Orogenic plateaus (e.g., Tibet, Altiplano) are characterized by broad, flat-top topography at high elevation and significantly increased crustal thickness. Partial melt is thought to weaken the middle crust of orogenic plateaus, and thus reduce the viscosity of the crust; however, the amount of partial melt and the magnitude of associated weakening remain unconstrained. The New England Appalachians represent an exposed mid- to lower crustal section of a paleo-orogenic plateau, similar to modern-day Tibet. In this study, we utilize the relationship between the spacing of deformation bands and the compaction length to constrain mid-crustal shear viscosity in a late Devonian migmatite. We find that the viscosity of the middle orogenic crust in the paleo-orogenic plateau of the New England Appalachians is 10^17–18 Pa∙s at ∼3–9% melt. This finding is consistent with geophysical models of orogenic channel flow and provides field-based evidence for a significant rheologic transition at low melt-fraction. Our results suggest that the key elements for the formation of a weak, mid-crustal layer in orogenic plateaus are an influx of water and temperatures near the hydrous granite solidus.