TECTONIC EVOLUTION AND EXHUMATION OF THE LOWER STRUCTURAL LEVELS OF THE BRONSON HILL ANTICLINORIUM IN SOUTHWESTERN NEW HAMPSHIRE AND ADJACENT VERMONT

Mapping and geochronology from Lebanon to Alstead, NH support a tectonic model that involves Ordovician arc volcanism followed by four stages of Paleozoic to Cenozoic deformation. In the Bronson Hill anticlinorium (BHA), mapping confirms at least three structural levels, previously considered fold nappes, and now recognized as the Monroe, Skitchewaug Mountain (SM), and Fall Mountain - Brennan Hill (FMBH) thrust nappes, or thrust sheets. The lowermost thrust sheet is floored by the Monroe fault which carries a deformed stratigraphic sequence of Ammonoosuc Volcanics (AV), Partridge Formation, Clough Quartzite, and Fitch and Littleton formations. The Monroe thrust sheet placed the BHA rocks over rocks of the Connecticut Valley – Gaspé trough during an early Acadian D1 nappe-stage event prior to the peak of metamorphism. Upper and lower plate truncations, mylonite, and local mélange characterize the Monroe fault. D2 doming deformed the Monroe thrust, folded earlier isograds, initiated the major domes, and created the Meriden antiform. Lower greenschist facies D2 to D3 faults truncate peak-metamorphic assemblages, isograds, and D1 folds and faults. Mapping shows that the SM thrust sheet may encompass the “big staurolite nappe” (BSN), an idea supported by the nearly identical P-T paths reported by Spear and others (2002, 2008) for the SM thrust sheet and the BSN. The FMBH thrust sheet is the structurally highest and contains Sil-Ms zone Silurian Rangeley Formation intruded by ~407 Ma Bethlehem Gneiss.

Zircon U-Pb SHRIMP analyses from 9 igneous rocks in the BHA yield Ordovician ages for both the AV (~460 Ma) and the cross-cutting Oliverian Plutonic Suite (475-448 Ma). U-Pb SHRIMP and LA-ICP-MS analyses constrain an Early Devonian deposition age for the Littleton Formation between ~408 Ma (youngest zircon population) and ~395 Ma (age of cross-cutting granite). ⁴⁰Ar/³⁹Ar Hbl cooling ages generally decrease to the east, moving from ~380 Ma in VT to ~330 Ma near Alstead, NH, and imply that much of western NH remained at depth until the Mississippian. White mica (WM) ages range from 330-245 Ma and also decrease eastward, but show local variations associated with mapped structures. Apatite fission-track data support Cretaceous to Eocene (D4) brittle faulting and, along with the WM data, define a protracted exhumation history.