THE RISE AND FALL OF EASTERN NORTH AMERICA’S OROGENIC PLATEAUX: FROM THE ADIRONDACKS TO THE ACADIAN ALTIPLANO

The formation and evolution of eastern North America has involved multiple cycles of mountain building, terrane accretion, and orogenic collapse from the Proterozoic to Paleozoic. Constraining the mechanism(s), rates, and timing of uplift and collapse of these orogens is critical data for understanding the crustal evolution of eastern North America and collisional tectonics in general. Here, we focus on two key orogenic events interpreted to have involved orogenic plateaux: the Mesoproterozoic Grenville orogeny and Paleozoic Acadian altiplano. We integrate igneous whole-rock geochemical crustal thickness proxies (chemical mohometry) with metamorphic P-T-t paths to quantitatively track spatial and temporal changes in paleo-crustal thickness. Data from the Grenville orogeny suggest crustal thickening to ~50 km by ~1180 Ma followed by rapid thinning to ~35 km by ~1150 Ma. Crustal thinning (~15 km) is greater than the degree of exhumation (~6 km), consistent with lower crustal thinning (lithospheric delamination?) associated with ~1150 Ma anorthsite-mangerite-charnockite-granite (AMCG)-suite magmatism. The crust re-thickened to ~55-60 km by 1080 Ma followed by gradual thinning to ~40 km during 1050-1020 Ma orogenic collapse. Geochemical data from the Acadian orogeny indicate crustal thickening to 60-70 km by 380 Ma. 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. Crustal thinning of the Acadian altiplano during orogenic collapse (340-310 Ma) likely involved ductile flow and, possibly, lithospheric delamination. Combining chemical mohometry with P-T-t histories provides insights on the delicate balance of variables (including rheology, temperature, shortening) beneath the current level of exposure of metamorphic terranes through the lifetime of plateaux. Quantifying the rates of lower crustal thickening and thinning provides insight into previously cryptic processes such as lithospheric foundering and lower crustal growth, ductile flow, and recycling.