BEDROCK GEOLOGIC MAP OF THE BELLOWS FALLS AND ALSTEAD 7.5–MINUTE QUADRANGLES, NH–VT: INSIGHT INTO THE TECTONICS OF THE BRONSON HILL ANTICLINORIUM

A new 1:24,000-scale, bedrock geologic map of the Bellows Falls and Alstead 7.5–minute quadrangles, NH–VT, recognizes three polydeformed nappes of the Bronson Hill anticlinorium containing Ordovician to Devonian rocks—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 Devonian Gile Mountain, Waits River and Littleton formations. The Monroe nappe is dissected and deformed by multiple strands of the sinistral Alleghanian Westminster-West fault. The overlying SK nappe shows an internal west-to-east increase in metamorphic grade from St zone to Sil + Ms zone near the Alstead dome, which is cored by Ammonoosuc Volcanics that are intruded by 452 ± 6 Ma and 448 ± 7 Ma (SHRIMP U-Pb ages) granitoid gneisses. Map-scale truncations, a metamorphic break (St against Bt and Grt zones), and mylonitic fabric indicate a fault along the west side of the SK nappe. The structurally highest nappe, FM, 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 St zone rocks of the SK nappe. Reduction in grain size and an increase in the amount of Bt and Ms in the Bethlehem Gneiss occur near the BHT. Mineral lineations plunge SE and fold patterns support NW-directed transport. Extensional Mesozoic faults cut all of the nappes.

⁴⁰Ar/³⁹Ar Ms 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 ages of ~330 Ma. Ms 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 data suggest peak metamorphism in the SK prior to ~380 Ma followed by emplacement of the FM between 335–380 Ma. The Alstead dome may have formed at ~330 Ma or later, and local late fabrics and younger Ms ages are probably related to late Alleghanian sinistral tectonics.