GEOCHRONOLOGIC CONSTRAINTS ON THE TIMING OF DEFORMATION IN THE FOOTWALL OF THE PROSPECT ROCK FAULT IN NORTH-CENTRAL VERMONT

The Prospect Rock Fault (PRF) is key to our understanding of the regional tectonic evolution during the Taconic, Salinic, and Acadian orogenies, and may have played an important role in the exhumation of blueschist and eclogite-facies rocks in the Tillotson Peak Complex (TPC) during the Taconic Orogeny. The TPC is in the footwall of the PRF in the eastern limb of the Green Mountain Anticlinorium. In the TPC, the dominant foliation is S₂ and E-W trending F₂ folds parallel L₂ lineations, which run orthogonal to regional N-S trending folds associated with the Taconic Orogeny. The PRF itself is folded by F₂ folds. Presently, there is a lack of consensus about the role of the PRF in the exhumation of the TPC, and studies have not reconciled the formation of the E-W folds and lineations to a regional model.

Oriented samples and structural data were collected from the footwall of the PRF over several transects. Samples were processed into orthogonal thin sections for microstructural analyses and for ⁴⁰Ar/³⁹Ar step-heating of white mica. The dominant foliation in the PRF samples, S₂, is defined in thin section by mica and quartz microlithons, and oriented mica grains. S₁ is only locally preserved in some mica domains and albite/garnet inclusion trails. S₃ appears as crenulations of S₂, with no significant new mineral crystallization. In the field, L₂ lineations are defined by mineral and quartz rods, and L₃ lineations are defined as intersection lineations on S₂ surfaces.

⁴⁰Ar/³⁹Ar analyses yielded plateau ages ranging from 458.6 ± 2.0 Ma to 420.0 ± 2.7 Ma (1σ). The oldest plateau ages are just slightly younger, yet concordant, with published and new ⁴⁰Ar/³⁹Ar ages from the TPC and come from the structurally highest portions of the footwall in the northern part of the study area. Virtually all apparent age spectra show age gradients. The relationships between ages and microstructures are consistent with younger ages being associated with increased presence of S₃ foliations. Our results suggest the PRF played a role in exhumation of the TPC and ages obtained are closely aligned with deformation ages constrained from ⁴⁰Ar/³⁹Ar dating in southern Quebec for the Taconic D₂ and Salinian D₃ deformation. These dates may aid to further correlation of tectonostratigraphic models between southern Quebec and New England.