Joint 69th Annual Southeastern / 55th Annual Northeastern Section Meeting - 2020

Paper No. 3-6
Presentation Time: 9:55 AM

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


WEBB, Laura E., Geology, University of Vermont, 180 Colchester Ave., Burlington, VT 05405, KARABINOS, Paul, Dept. Geosciences, Williams College, Williamstown, MA 01267 and KLEPEIS, Keith A., Department of Geology, University of Vermont, Trinity Campus, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405

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.