THE MISSING, NEAR-MISSING, AND CRYPTIC UPPER CRUST OF THE ACADIAN ALTIPLANO

Evidence for the ~380–330 Ma Acadian altiplano—a hypothesized ~50–70 km thick orogenic plateau stretching from Connecticut to Maine—includes a paleo-isobaric surface, widespread high-grade metamorphism, protracted regional anatexis, and slow cooling. Exposures at the surface today are primarily mid-crustal metamorphic and plutonic rocks; there are no known surviving supracrustal rocks that were once deposited atop the plateau.

Within the footprint of the altiplano, low-grade, upper-crustal metasedimentary rocks are preserved as roof pendants and xenoliths in at least 12 of New England’s Jurassic and Cretaceous igneous complexes. Detrital zircon data from the Intervale Clay Slate mega-xenoliths and Terrace Mountain roof pendants, both in NH, suggest correlation with Silurian-Devonian units 100+ km to the NE in Maine. Clasts of slate and phyllite in conglomerates of the Mesozoic Hartford Basin and other age-equivalent basins may provide the same kinds of insights as the xenoliths and roof pendants. Beyond the footprint, detritus from the altiplano’s supracrustal succession probably survives in one or more of the younger Paleozoic basins (e.g., Narragansett, Appalachian, and Maritimes) to the SE, SW, and NE. The regional paleocurrent pattern (n >30,000) in the Maritime Basin’s vast, ~6-km-thick Pennsylvanian–Permian fluvial succession points directly away from the altiplano.

Judging from modern analogs (Andes, Tibet, Iran) the altiplano was at >3 km elevation and was likely the site of an arid, endorheic basin system with a thick accumulation of clastics, volcanics and evaporites. Witzke’s (1990) Laurentia-scale synthesis of climatically sensitive sedimentary rocks suggests that the altiplano region drifted through the southern dry belt from ~380–330 Ma, then northward into the equatorial wet belt from ~330–250 Ma, where erosion rates would have been far greater. The putative evaporites are important because they are an essential factor in a new model for late Paleozoic lithium pegmatite genesis at ~10 km depth in the altiplano. But because of the solubility of evaporite minerals, evidence bearing on the former presence of evaporites in a long-vanished source area is likely to be elusive. Detrital clay minerals known to occur in continental evaporite sequences (e.g., Mg-smectites) may prove helpful.