TECTONIC HISTORY OF THE BRONSON HILL ANTICLINORIUM IN THE CONNECTICUT RIVER VALLEY, WESTERN NEW HAMPSHIRE AND EASTERN VERMONT

New geologic mapping supported by the National Park Service shows a complex four stage model of Paleozoic to Cenozoic tectonics. In the Bronson Hill anticlinorium (BHA), there are at least three structural levels, including the Monroe, Skitchewaug Mountain (SM), and Fall Mountain - Brennan Hill (FMBH) thrust sheets. The basal Monroe thrust sheet carries a deformed BHA stratigraphic sequence of the Ammonoosuc Volcanics (AV), Partridge Formation, Clough Quartzite, and Fitch and Littleton formations onto rocks of the Connecticut Valley – Gaspé trough during an early Acadian D1 nappe-stage event prior to peak metamorphism. Upper and lower plate truncations, mylonite, and local mélange characterize the basal Monroe fault. D2 deformation deformed the Monroe thrust, folded earlier isograds, initiated the major domes, and created the Meriden antiform. Lower greenschist facies D2 to D3 faults truncated peak-metamorphic assemblages, isograds, and D1 folds and faults. The SM level is a thrust sheet or fold nappe that encompasses the “big staurolite nappe” (BSN), an idea supported by the nearly identical P-T paths of the two nappes reported by Spear and others (2002, 2008). 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 hornblende cooling ages generally decrease to the east, 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 of 330-245 Ma also decrease eastward, but show local variations associated with newly mapped faults. Brittle faults, apatite fission track data, and WM ages define a protracted exhumation history from the Cretaceous through the Paleocene.