GEOCHRONOLOGIC EVIDENCE FOR SALINIC THRUSTING AND ACADIAN REACTIVATION OF EXTERNAL BASEMENT MASSIFS IN WESTERN NEW ENGLAND AND OVERPRINTING OF THE ORDOVICIAN TACONIC THRUST BELT

We present new data from ⁴⁰Ar/³⁹Ar geochronology of samples from two exposures of the Rattlesnake Fault, the western frontal thrust of the Green Mountain massif near its southern end in Pownal, Vermont. At exposure 1, the Cambrian Cheshire Quartzite lies structurally above a graphitic phyllite interpreted as either the Ordovician Walloomsac or Cambrian Nassau Fm. At exposure 2, the Neoproterozoic to Cambrian Dalton Fm. is in the hanging wall above a similar phyllite. Samples of the Cheshire Quartzite in the fault zone record evidence of quartz deformation via subgrain rotation recrystallization and grain boundary migration. Small feldspars display core and mantle structures. Together these observations suggest temperatures of deformation at c. 500°C. White mica is present as fine grains along foliation planes and as small crenulated masses in thin seams. Step heating of white mica from the Cheshire Quartzite within the fault zone yielded a plateau age of 414.3 ± 3.6 Ma (2σ). Based on the lack of grain boundary area reduction and static recovery of quartz, we interpret the age to approximate the timing of deformation. Oblique grain-shape foliations defined by quartz in the dated sample suggest a top-to-the-W transport of the hanging wall. The Dalton Fm. records similar deformation mechanisms and kinematics as described above. A phyllite sample immediately below the Dalton Fm. at the fault contact displays C’-S fabric consistent with a top-to-the-W shear sense. The C’ shear bands cut an S₂ foliation in which microlithons locally preserve a crenulated S₁ foliation. White mica from this sample yielded a plateau age of 417.9 ± 3.8 Ma. An additional sample of the phyllite was collected from a lower structural level. Compared to the phyllite at the fault contact with the Dalton Fm., foliations at the lower level are generally more chaotic, quartz microlithons show greater grain size reduction, and the thin section is crosscut by microfaults. White mica from this sample yielded a weighted mean age of 398.7 ± 4.6 Ma. Although the footwall and hanging wall lithologies are similar to those at the Champlain Thrust, which is inferred to have formed during the Taconic orogeny, our new dates suggest that the Rattlesnake Fault is a late Silurian to Early Devonian Salinic thrust that may have been reactivated at c. 400 Ma during the Acadian orogeny.