TRACKING CRUSTAL THICKNESS IN THE NEW ENGLAND APPALACHIANS: IMPLICATIONS FOR AN ACADIAN ALTIPLANO AND THE REGIONAL METAMORPHIC FIELD GRADIENT

Determining when and how the crust is modified during orogenic events is critical for building four dimensional models of orogenesis and crustal evolution. The tectonic history of the New England Appalachians is dominated by a series of Paleozoic tectono-thermal events but many aspects remain poorly understood due to polyphase deformation, high-grade metamorphism, and along-strike variability. The composition of igneous rocks in convergent margins is sensitive to crustal thickness presenting a means to see through the complex overprint in order to constrain temporal and spatial thickness changes. Geochemical analyses from syn-collisional plutons were used to reconstruct paleo-crustal thickness using trace element (Sr/Y, La/Yb; Hu et al., 2017) and thermo-isotopic (Nd; DePaolo et al., 2019) proxies. Systematic spatial and temporal trends suggest two periods of crustal thickening in the Acadian orogeny, each with distinct characteristics. The first occurred between 425 and 400 Ma, resulted in thickening to ~40 km, and progressed from SE to NW, compatible with existing sedimentological and structural evidence. The second occurred between 400 and 380 Ma and with significant N-S variability. Minimal crustal thickening is indicated in northern New England while crustal thicknesses to the south nearly doubled. Crustal thicknesses of 55 to 70 km in central and southern New England were maintained for at least 30 m.y., strongly supportive of models for a long-lived orogenic plateau (the Acadian altiplano). Intriguingly, the variability in along strike paleo-crustal thicknesses shows a correlation with the presently exposed metamorphic gradient (e.g. Robinson et al., 1998), with higher metamorphic grade corresponding to greater paleo-crustal thicknesses. We suggest that isostatic and gravitational instability of the overthickened crust of central and southern New England underwent orogenic collapse, exhuming more deeply buried rocks, whereas process did not occur to the same degree in the thinner, presumably more stable, crust of northern New England. Thus, first order variations in crustal thickness are directly related to the present metamorphic field gradient.