Database for the bedrock geologic map of the Bellows Falls 7.5- x 15-Minute Quadrangle, Windham and Windsor Counties, Vermont, and Sullivan and Cheshire Counties, New Hampshire

The bedrock geology of the Bellows Falls 7.5 x 15 minute quadrangle, Vermont and New Hampshire, consists of polydeformed Ordovician to Devonian metasedimentary, metavolcanic, and metaplutonic rocks of the Connecticut Valley trough, Bronson Hill anticlinorium (or Bronson Hill terrane), and the Central Maine terrane. Previous work in this area includes a 1:62,500-scale published map and text by Kruger (1946), state geologic maps of New Hampshire (Lyons and others, 1997) and Vermont (Ratcliffe and others, 2011), and various maps and reports presented largely as parts of field trip guidebooks (e.g., Thompson and Rosenfeld, 1979; Chamberlain and others, 1988; Spear, 1992; Thompson and others, 1993). Armstrong (1997) completed a provisional open-file map of the geology of the Vermont part of the Bellows Falls 7.5 x 15 minute quadrangle, which is incorporated and revised on this map based on additional field work. This study recognizes three major structural levels from west to east, lowest to highest: (1) autochthonous rocks of the Connecticut Valley trough (CVT); (2) allochthonous rocks of the New Hampshire sequence and Bronson Hill arc in the Monroe thrust sheet, including the Skitchewaug nappe; and (3) allochthonous rocks of the Fall Mountain thrust sheet or nappe. The CVT consists of metasedimentary and metavolcanic rocks of Devonian Gile Mountain and Waits River formations, which are located west of, and within splays of the Westminster West fault zone. The CVT rocks are largely greenschist facies with most rocks in the biotite to garnet zones. The Monroe thrust sheet carries transported New Hampshire sequence, and include rocks previously described as the Cornish and Skitchewaug nappes (Thompson and others, 1968), and now interpreted to be largely at the same structural level (Walsh and others, in press). These rocks reached greenschist to amphibolite facies, with the lower biotite grade rocks occurring to the west near the Connecticut River. The eastern side of the CVT and the western side of the Skitchewaug nappe are dissected and deformed by multiple strands of the sinistral Westminster-West fault (Armstrong, 1997; McWilliams and others, 2013), an Alleghanian structure. The largest strand is marked by a significant zone of phyllonites—derived from various adjacent rock types from within the CVT—and chlorite grade retrogression. In the fault zone west of the Connecticut River, higher grade metamorphic assemblages are retrograded, deformed, and truncated in this wide zone. The Skitchewaug nappe shows an internal west-to-east increase in metamorphic grade from garnet zone to staurolite zone to sillimanite + muscovite zone near the Alstead dome. The Skitchewaug nappe exposes Ordovician to Devonian rocks of the New Hampshire sequence: Ordovician Partridge Formation, Silurian Clough Quartzite, Silurian Fitch Formation, and Devonian Littleton Formation. The Alstead dome is cored by Ordovician Ammonoosuc Volcanics and intruded by Oliverian Plutonic Suite trondhjemitic to granitic gneisses. The Ammonoosuc Volcanics is comprised of various amphibolites and hornblende schists. A metatuff mapped at the top of the Ammonoosuc Volcanics along the western flank of the dome yielded a Sensitive High Resolution Ion Microprobe (SHRIMP) U-Pb zircon crystallization ages of 455 ± 11 (Merschat and others, 2015; Valley and others, 2015, in press). Gneisses of the Oliverian Plutonic Suite are separated into two bodies: a smaller body to the north (~1.5 km long) and larger body extending southward beyond the quadrangle border. SHRIMP U-Pb zircon crystallization ages from these bodies are 448 ± 7 Ma and 452 ± 6 Ma, respectively (Merschat and others, 2015; Valley and others, 2015, in press). Map-scale truncations, a metamorphic break (staurolite against biotite and garnet zones), and mylonitic fabrics indicate a fault along the west side of the Skitchewaug nappe, which is mapped as the Northey Hill thrust. The structurally highest nappe, Fall Mountain, is floored by the Brennan Hill thrust (BHT) and contains sillimanite zone and higher-grade Silurian Rangeley Formation intruded by the ~400 Ma Bethlehem Granodiorite (Merschat and others, 2015). The BHT truncates units of the Skitchewaug nappe and juxtaposes the Bethlehem Granodiorite and migmatitic, sillimanite + K-feldspar zone Rangeley Formation over staurolite zone rocks of the Skitchewaug nappe. Reduction in grain size and an increase in the amount of biotite and muscovite in the Bethlehem Gneiss occur near the BHT. Mineral lineations plunge southeast, and kinematic indicators and fold patterns support NW-directed transport. The Fall Mountain nappe may be a west-directed sheath fold, similar to the Skitchewaug nappe and other F1-nappe stage folds (Walsh and others, in press). 40Ar/39Ar muscovite and amphibole ages across the nappes suggest Devonian to Mississippian cooling of the Bronson Hill anticlinorium. Amphibole from the Skitchewaug nappe in a window through the Fall Mountain nappe yields the oldest age at ~380 Ma, while amphibole age spectra from the Alstead dome yield ages of ~330 Ma. Muscovite ages from the Fall Mountain nappe and the Littleton Formation in the Monroe nappe in Vermont 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 Skitchewaug nappe 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 muscovite ages are probably related to late Alleghanian sinistral tectonics. 40Ar/39Ar muscovite ages from the Westminster-West fault zone indicate it is a sinistral Alleghanian fault at ~300 Ma (McWilliams et al., 2013). Extensional Mesozoic faults cut all structural levels. Mesozoic faults have normal dip-slip and strike-slip kinematics. Apatite fission track (AFT) data indicate that the brittle Ammonoosuc fault was active prior to about 100 Ma and experienced little to no re-activation in the Cretaceous, but other regionally significant older ductile faults such as the Northey Hill experienced late Cretaceous (less then 80 Ma) re-activation (Roden-Tice and others, 2009). Additional AFT data suggest some Cretaceous activity on regional brittle faults like the Grantham fault may have extended into the Paleocene (Schnalzer and others, 2015). Extensive brittle faults and slickensided foliation surfaces in the vicinity of the Westminster-West fault zone, especially along Interstate 91, attest to Mesozoic re-activation of earlier structures.