RISE AND FALL OF THE ACADIAN ALTIPLANO

The New England Appalachians record a full Wilson cycle and have been used as a type section and model of plate tectonics for decades. Multiple orogenic phases are recorded in the form of accreted terranes, polydeformed and metamorphosed rocks, and a thick wedge of sediment shed westward from the orogen. Surprising results from MacDonald et al. (2014) show that the Laurentia-Gondwana suture lies >50 km farther west than previously hypothesized. This suture may be related to a steep, 12-15 km step in Moho depth in western New England imaged by Li et al. (2018). To understand the tectonic significance of the Moho step and its relation to the Laurentia-Gondwana suture, we have developed a GIS database, integrating geologic, thermobarometric, geochronologic, and geochemical datasets with geophysical data. Barometric measurements show a broad zone of 0.6 GPa Acadian pressures in central New England east of the Moho step, corresponding to the enigmatic Central Massachusetts Metamorphic High of Robinson et al. (1986). These consistent pressures suggest a region of homogenous uplift. Ar/Ar ages record Taconic cooling west of the Moho step and Acadian cooling to the east. The 40-50 Myr gap between hornblende and muscovite ages shows that post-Acadian cooling rates were slow (3-4°C/Myr) also indicating that exhumation rates were slow. Uplift at ca. 330-310 Ma is signaled by high-Y monazite rims recording the breakdown of garnet. Barometric, chronologic, and petrologic data are consistent with the signature of an exhumed orogenic plateau (Rivers, 2012) as is the 50 Myr (380-330 Ma) lifespan of the Acadian altiplano. Collapse of the plateau was orogen-parallel and recorded by strong N-S lineations in the Pelham dome and central Massachusetts as documented by Massey et al. (2017). Our data suggest that plateau collapse causing crustal thinning of central New England established the Moho step. Low temperature thermochrometers show no gradient across the Moho step, indicating that the structure is Paleozoic. The Acadian altiplano may be important to the genesis of ca. 330 Ma critical mineral deposits emplaced along the Moho step and it may be a significant sediment source for black shales in the Catskill foreland basin. Ongoing mapping and geochronology aim to place constraints on crustal structures related to plateau rise and collapse.