NEW INSIGHTS ON ACADIAN DEFORMATION AND REACTIVATION IN NORTHERN VERMONT FROM INTEGRATED STRUCTURAL AND GEOCHRONOLOGICAL STUDIES

The Appalachian mountains of New England record at least four orogenic events (Taconian, Salinian, Acadian, and Alleghenian), the effects of which can be difficult to distinguish in areas where they are superimposed. This study focuses on the Late Silurian-Devonian Acadian orogeny, which in Vermont is largely associated with metamorphism, folding, and syntectonic magmatism in the Connecticut Valley Trough (CVT). There is general agreement that the Acadian orogenic front migrated from SE to NW, overriding older (Ordovician) Taconic orogenic structures of the Laurentian margin west of the CVT. In these western regions, previous workers have proposed that Taconic thrust faults record multiple slip events, including events that reflect Acadian deformation. However, data revealing the timing of these postulated reactivations are largely lacking. This study presents integrated ⁴⁰Ar/³⁹Ar geochronologic, field and microstructural data that provide new insights into the distribution and timing of Acadian deformation in northern Vermont. Results from the Arrowhead and Hinesburg thrust fault zones, including a newly reported pseudotachylyte locality, support Late Devonian reactivation of Taconic (Ordovician) thrust faults and demonstrate that deformation in the western-most limit of the Acadian orogenic front locally included seismogenic faulting on pre-existing low-angle thrust faults. Taconic structures (S_1-2/F_1-2), including the Hinesburg thrust itself, are folded by N­–S-trending F₃ and E–W-trending F₄ folds that create a dome and basin fold interference pattern. In the hanging wall near the southern termination of the Hinesburg thrust, microstructures in the Pinnacle Formation record F₃ crenulations associated with second generation of biotite growth. ⁴⁰Ar/³⁹Ar data from biotite in these samples show a strong Acadian signal, including a plateau age of 386.6 ± 2.3 Ma (1σ). All apparent age spectra obtained in this study reveal a variety of age gradients that reflect superposed deformations. While localized recrystallization or Ar loss due to deformation may pose complications for modeling thermal histories, complex apparent-age spectra such as those obtained in this study have the potential to elucidate the timing of both ductile and brittle deformation events in polyphase tectonic histories.