Rise and fall of the Acadian altiplano: Evidence for a Paleozoic orogenic plateau in New England

Abstract

High elevation orogenic plateaus are formed by a complex interplay of deep and surficial processes and influence a variety of Earth systems. However, few exposures of plateau mid-crust are presently recognized, hindering understanding of the deeper processes. We present evidence for the existence of an orogenic plateau during and after the Devonian Acadian orogeny whose mid-crustal roots are exposed in the New England Appalachians. The four-dimensional crustal evolution of this paleo-plateau is constrained by the integration of new petrologic and geochronologic databases, petrochronology, and geophysical imaging. Doubly thickened crust, widespread amphibolite to granulite-facies metamorphic conditions, a paleo-isobaric surface, and protracted mid-crustal anatexis indicate the development of a high elevation, low relief plateau by 380 Ma. 40Ar/39Ar thermochronology shows a distinct thermochronologic signature with very slow cooling rates of 2–4°C/m.y. following peak metamorphic conditions. Thermochronologic data, trace element and Nd isotope geochemistry, and monazite petrochronology suggest a 50 m.y. lifespan of the plateau. Orogen parallel ductile flow and extrusion of gneiss domes resulted in plateau collapse, crustal thinning, and homogeneous exhumation at ca. 330–300 Ma. Thinning and exhumation of the plateau crust may have led to the sharp 12–15 km step in Moho depth in western New England, possibly by reactivating the suture between Laurentia and accreted Gondwanan-derived terranes. The formation of the Acadian altiplano may have influenced Li-pegmatite genesis, foreland basin evolution, and Paleozoic paleoclimate, while its recognition may provide a window into the deeper processes of orogenic plateaus including partial melting, plutonism, and collapse by ductile extension.