STRUCTURE AND TIMING OF NAPPE EVOLUTION IN THE BRONSON HILL ANTICLINORIUM, BELLOWS FALLS AND ALSTEAD 7.5–MINUTE QUADRANGLES, NH–VT

Geologic mapping, U-Pb geochronology and ⁴⁰Ar/³⁹Ar thermochronology are combined to examine the evolution of nappes exposed in the Connecticut Valley, Bronson Hill, and central Maine terranes in SW NH and adjacent VT, Bellows Falls and Alstead 7.5–minute quadrangles. Three polydeformed nappes containing Ordovician to Devonian rocks were recognized (from west to east, lowest to highest): Monroe (or Cornish), Skitchewaug (SK) and Fall Mountain (FM) nappes. The Monroe nappe contains Bt and Grt zone rocks of the Bronson Hill and Connecticut Valley terranes dissected and deformed by multiple strands of the sinistral Alleghanian Westminster–West fault. The overlying SK nappe of the Bronson Hill terrane shows an internal west-to-east increase in metamorphic grade from St zone to Sil + Ms zone near the Alstead dome, cored by 452 ± 6 Ma and 448 ± 7 Ma (SHRIMP U-PB ages) trondhjemitic gneisses. Map-scale truncations, a metamorphic break (St over Bt and Grt zones), and mylonitic fabric indicate a fault at the base of the SK nappe. FM, the structurally highest nappe, is floored by the Brennan Hill thrust (BHT) and contains Sil zone and higher grade Silurian Rangeley Formation intruded by the ~400 Ma Bethlehem Gneiss. The BHT truncates units of the SK nappe and juxtaposes the Bethlehem Gneiss and migmatitic, Sil + Kfs zone Rangeley Formation over lower grade rocks of the SK nappe. The presence of the BHT is further indicated by both a local reduction in grain size and an increase in the amount of Bt and Ms in the Bethlehem Gneiss. Mineral lineations plunge SE and fold patterns support NW-directed transport.

New ⁴⁰Ar/³⁹Ar muscovite and amphibole ages across the nappes suggest Devonian to Mississippian cooling of the Bronson Hill anticlinorium. Amphibole from the SK nappe in a window through the FM nappe yields the oldest age at ~380 Ma, while amphibole age spectra from the Alstead dome yield correlation ages of 328 ± 2, 333 ± 3, and 334 ± 2 Ma. Muscovite ages from the FM nappe and the Littleton Formation in the Monroe nappe in VT, yield ages of 316-335 Ma, while ages near the Alstead dome are younger, ~300 Ma. Collectively, the ⁴⁰Ar/³⁹Ar cooling ages suggest peak metamorphism in the FM and SK nappes occurred prior to ~380 Ma, but the main phase of nappe formation did not occur until the Mississippian at about 330 Ma with local differential cooling.