THERMOISOTOPIC AND PETROCHRONOLOGIC CONSTRAINTS ON CRUSTAL EVOLUTION OF THE ACADIAN OROGENY

Determining when and how the crust is modified during orogenic events is critical for building 4D models of crustal evolution, especially in multistage orogens such as the Appalachians. In order to define changes in crustal thickness and their implications for the tectonic history we integrate geochemical and isotopic data with independent constraints from petrochronology, geothermobarometry, and rock fabrics. Crustal thickness estimates derived from Nd isotopes in the thermo-isotopic framework of DePaolo et al. (2019) suggest relatively uniform crustal thickness (~30 km) across New England in the early Acadian, ca. 420-400 Ma. Significant crustal thickening (to ≥55 km) in central and western New England occurred by 380 Ma, an apparent crustal thickening rate of 1 km/m.y. In contrast, the crust of northern New England remained ca. 35 km thick. Development of thickened crust in central and western New England by 380 Ma is supported by independent thermobarometry and monazite petrochronology. The regional homogeneity of crustal thickness estimates, 0.6 GPa pressures, and ⁴⁰Ar/³⁹Ar cooling dates, support the development of a broad orogenic plateau by 380 Ma. Crustal thinning began ca. 330 Ma, as recorded by garnet breakdown and ⁴⁰Ar/³⁹Ar thermochronology. Late N-S lineations suggest that the collapse was orogen parallel. The orogenic plateau existed for at least 50 m.y. (380-330 Ma), a duration similar to that of other more recent plateaus. Furthermore, our results support the Paleozoic development of a 12-15 km offset in Moho depth in western New England. Ongoing monazite, xenotime, and zircon U-Th-Pb petrochronology will be used to further constrain the timing of metamorphic fabrics and reactions associated with crustal modifications.