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We present results of integrated 40Ar/39Ar geochronology and microstructural analyses of samples from Taconic thrust faults of the northern New England Appalachians that provide evidence for reactivation during the Acadian, Neo-Acadian, and Alleghenian orogenies. 40Ar/39Ar ages c. 420 Ma from western frontal thrusts of the Green Mountains and Berkshire Massif have been interpreted previously to reflect partial resetting of Taconic ages during Acadian metamorphism. In Massachusetts and southern Vermont, these W-directed thrusts transport Grenville basement and its cover sequences over Cambrian-to-Ordovician phyllites and graphitic schists. Our recent investigations of these faults, however, yield a suite of c. 420 Ma 40Ar/39Ar ages obtained from syn-tectonic mica in mylonites and footwall schist/phyllite that are interpreted, rather, to reflect a pulse of W-directed thrusting. This interpretation that these ages record the timing of deformation is based, in part, on the preservation of quartz and feldspar dislocation creep microstructures (i.e., lack of evidence for static recrystallization), as well as the regional distribution of these data relative to Acadian metamorphic isograds. These results align with recent findings for the timing of formation of the Green Mountain Anticlinorium in northern Vermont, as well as detrital zircon data that require isolation of the Catskill Basin from the Connecticut Valley-Gaspe Basin (CVGB) at the onset of deposition around that time. Mylonites and samples from the adjacent footwall schists and phyllites also locally record evidence for minor to wholesale resetting c. 355 Ma associated with a younger phase of ductile deformation. Further evidence for partial resetting of 40Ar/39Ar ages c. 250 is associated with hematite-rich seams parallel to the mylonitic foliation and cross-cutting fractures. We explore how these age populations relate to those obtained from, for example, the CVGB and Chester and Athens Domes, and their implications for correlating surface geology with results from seismic imaging of the lithospheric and mantle structure in the northern New England Appalachians.
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This content will become publicly available on December 9, 2025
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 over mid-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 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 deformational events associated with the late Salinic and/or Acadian orogenies. Preliminary studies from fieldwork and microstructural analysis of slabbed samples from transects across the Pine Hill thrust, where the lower Cambrian Dalton Formation is mapped as thrust over the Upper Ordovician Ira Formation, reveal at least four generations of foliation. The oldest tectonic foliation, S1, is parallel to primary compositional layering (S0) and is associated with isoclinal F1 folds. Moving from the Dalton Formation in the hanging wall towards the fault zone, S1 becomes progressively transposed into S2, marked by metamorphic compositional layering. Closer to the fault, S2 is crenulated, and S3 emerges as the dominant foliation, becoming the only foliation exhibited by the phyllonites in the fault zone. Finally, the youngest foliation, S4, is a localized crenulation cleavage developed in more pelitic material. These preliminary results suggest a complex deformation history, possibly involving multiple phases of post-Taconic motion on the fault during subsequent orogeneses. Further microstructural analysis and 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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- Award ID(s):
- 2147463
- PAR ID:
- 10596291
- Publisher / Repository:
- AGU Fall Meeting Abstracts
- Date Published:
- Journal Name:
- Transactions American Geophysical Union
- ISSN:
- 2379-6723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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