STRUCTURAL AND THERMOCHRONOLOGIC EVOLUTION OF THE FALL MOUNTAIN KLIPPE AND ALSTEAD DOME, BRONSON HILL ANTICLINORIUM, NH–VT

Ongoing mapping and ⁴⁰Ar/³⁹Ar thermochronology examine the evolution of thrust nappes of the Bronson Hill anticlinorium exposed in SW NH and adjacent VT, in the Bellows Falls and Alstead 7-5 minute quadrangles. This study recognizes three polydeformed thrust nappes or sheets containing rocks of the Ordovician Bronson Hill arc and their Silurian to Devonian cover. Thrust sheets are delineated on the basis of the truncation of units, the presence of mylonitic fabrics along the base of the thrust sheets, and inverted metamorphic sequences. The lowest Monroe (Cornish) thrust sheet contains Bt and Grt zone rocks. The overlying Skitchewaug thrust sheet shows an internal west-to-east metamorphic increase from the St zone to the Sil + Ms zone. The base of the Skitchewaug thrust sheet exhibits footwall and hanging-wall truncations, a metamorphic break (St over Bt and Grt zones), and protomylonitic fabric. The Brennan Hill thrust sheet, including the Fall Mountain klippe (FMk), is the structurally highest and contains Sil + Ms zone Silurian Rangeley Formation intruded by the ~407 Ma Bethlehem Gneiss. The FMk is juxtaposed against Skitchewaug thrust sheet and contains Bethlehem Gneiss and migmatitic, Sil + Kfs zone Rangeley Formation. The base of the FMk shows significant grain size reduction and an increase in the modal proportion of Bt and Ms in the Bethlehem Gneiss, and Sil + Kfs rocks over lower grade rocks. Mineral lineations plunge NW-SE and fold patterns support NW-directed thrust transport.

New ⁴⁰Ar/³⁹Ar muscovite and amphibole ages from across the upper thrust sheets suggest Devonian to Mississippian cooling of the Bronson Hill anticlinorium. Amphibole from a window through the FMk yield the oldest age at ~380 Ma, while amphibole age spectra from near the Alstead dome reproduce with correlation ages of 328 ± 2 and 334 ± 2 Ma. Muscovite ages from the FMk and the Littleton Formation in the Monroe thrust sheet in adjacent VT , yield ages of 316-335 Ma, and ages near the Alstead dome are younger at ~300 Ma. Collectively, the ⁴⁰Ar/³⁹Ar cooling ages suggest initial emplacement of the thrust sheets occurred by 380 Ma, but much of the area remained at depth until the Mississippian at about 330 Ma. This late stage exhumation may record final stages of dome formation during a protracted cooling event.