Investigating Post-Taconic Deformation In The Pine Hill Thrust, Southern Vermont

The Pine Hill thrust, a western frontal thrust of the Green Mountain massif in southern Vermont, is characterized by reverse faults that place Precambrian basement rocks and the Cambrian Dalton Formation over Upper Ordovician rocks. Based on cross-cutting relationships, it has been considered a late-stage Taconic thrust. However, recent investigations in the western front of the Sutton Mountains, Green Mountain massif, and Berkshire massif of southern Quebec, Vermont, and Massachusetts, respectively, suggest fault displacement at ca. 420 Ma and younger. Therefore, motion on these faults may instead be associated with the late Salinic or early Acadian orogeny. This study investigates the hypothesis that the Pine Hill thrust records post-Taconic deformational events via the integration of detailed field structural mapping with thin section microstructural analysis of oriented samples to analyze the structural evolution of the rocks. LiDAR imagery was used to refine the detailed location of the thrust in the study area. The results allow targeting of meaningful 40Ar/39Ar geochronology samples for constraining the timing of deformation along the Pine Hill thrust. The transects across the Pine Hill thrust, where the lower Cambrian Dalton Formation is mapped as thrust over the Upper Ordovician Ira Formation, preserve at least three generations of foliation. In both the hanging wall (Cambrian Dalton Fm) and the footwall (Upper Ordovician Ira Fm), S2 is the dominant foliation and S1 is locally preserved in S2 microlithons. S3, the youngest foliation, is a crenulation cleavage most intensely developed in samples along the fault zone. These data suggest multiple phases of motion along the Pine Hill thrust. Based on relative age relationships, all three foliations (in the footwall) must be younger than the depositional age of the Ira Formation (< ca. 455 Ma). Quartz microstructures across the fault system are dominated by subgrain rotation recrystallization with local grain boundary migration and bulging recrystallization with no evidence of static recrystallization or recovery phases in microstructures, confirming that the rock records ductile deformation at greenschist-facies conditions. Although 40Ar/39Ar geochronology results are pending, the selected white mica samples tied to each foliation generation will enable direct linkage of fabric development to absolute ages once analyses are complete. These results show that the Pine Hill thrust is, rather, a steep, east-dipping reverse fault, that emplaced the Grenville basement and Cambrian Dalton Formation over the Upper Ordovician Ira Formation. The surface trace of the thrust coincides with locations at which the S3 foliation is most strongly developed. Further geochronology of these deformation fabrics will help establish the timing of deformation and its tectonic significance, helping to correlate surface geology with results from New England Seismic Transect (NEST) imaging of crustal and mantle lithospheric structure in the northern New England Appalachians.

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