EVIDENCE FOR SALINIC AND ACADIAN REACTIVATION OF TACONIC THRUSTS ALONG THE WESTERN GREEN MOUNTAIN FRONT

Taconic thrust faults in Vermont have long been suspected to record multiple slip events, however, data revealing the timing of these postulated reactivations are largely lacking. We present the initial findings of a study seeking to constrain the timing of such events through integrated ⁴⁰Ar/³⁹Ar geochronology and microstructural analysis. We sampled two localities from the Rattlesnake Fault, the western frontal thrust of the Green Mountain massif near its southern end in Pownal, Vermont, where the fault places 1) Cambrian Cheshire Quartzite and 2) Neoproterozoic–Cambrian Dalton Fm. arenite on graphitic phyllite interpreted as either the Ordovician Walloomsac or Cambrian Nassau Fm. Upper plate rocks preserve S₂ mylonitic foliations in which asymmetric lozenges, mantled feldspar porphyroclasts and quartz oblique grain-shape fabrics indicate top-to-the-W shearing. Mica domains locally preserve evidence for an older S₁ foliation. Well-preserved D₂ quartz and feldspar microstructures suggest rapid cooling following temperatures of deformation in the range of 500–400°C. Evidence for overprinting by lower-T deformation is variable and includes subtle S₃ crenulation cleavage development, as well as Fe-rich fractures parallel to and cross-cutting S₂. Phyllite sampled in the lower plate directly below the Dalton Fm. has C’-S shear bands (S₂; top-to-the-W). The S₁ foliation is preserved locally as crenulations within S₂ microlithons and Fe-rich fractures are also observed. These observations integrated with the results of ⁴⁰Ar/³⁹Ar step-heating experiments on white mica and K-feldspar are consistent with a polyphase deformation history. Plateau and weighted mean ages of white mica from the Cheshire Quartzite and phyllite range from c. 419­–409 Ma and are interpreted to reflect the timing of D₂ top-to-the-W thrusting. These samples preserve age gradients that suggest Latest Devonian–Early Carboniferous partial resetting. Similar age gradients (c. 421–362 Ma) have been obtained from mylonitic Cheshire Quartzite in the hanging wall of the Hinesburg Thrust. In comparison, white mica and K-feldspar from the Dalton Fm. immediately above the thrust contact are completely reset with plateau-like segments at c. 356 and 350 Ma, respectively, and age gradients that suggest additional resetting at c. 250 Ma.