THE FALL MOUNTAIN NAPPE: A POSSIBLE ACADIAN D1 MAP-SCALE SHEATH FOLD IN THE BRONSON HILL ANTICLINORIUM, SOUTHWEST NEW HAMPSHIRE

The Fall Mountain nappe, located in the Bronson Hill anticlinorium in southwestern New Hampshire, juxtaposes the Bethlehem Granodiorite (BG) and migmatitic, sillimanite + K-feldspar-zone Rangeley Formation over staurolite and garnet zone rocks of the New Hampshire sequence. The inverted metamorphism and the contrasting stratigraphy support the interpretation of an allochthonous origin of the thrust nappe­­­­. Recent geologic mapping has identified a high-strain zone along the base of the BG and a regional NW–SE trending mineral lineation in the Acadian D₁ Fall Mountain nappe. F₁ fold axes are sub-parallel to the mineral lineation and support a sheath fold geometry for the D₁ nappe. The high-strain zone along the base of the Fall Mountain nappe is marked by a reduction in grain size and an increase in the amount of biotite and muscovite in the BG. The foliation is defined by biotite, muscovite, and ribbons of recrystallized quartz. Feldspars, which in BG typically are 0.5–1 cm across or larger, are reduced to 1–3 mm porphyroclasts with tails of biotite, muscovite and recrystallized quartz ribbons; recrystallized tails and lenses of plagioclase extend from the ends of plagioclase porphyroclasts. The mineral lineation is an aggregate lineation of quartz, biotite, muscovite and plagioclase, and plunges gently to the WNW and ESE. Kinematic indicators suggest top-to-the-WNW. In underlying nappes, the mineral lineation and elongate quartz pebbles also plunge gently to the NW or SE; kinematic indicators and axial surfaces of folds suggest NW-directed transport. ⁴⁰Ar/³⁹Ar data from amphibole from a window through the nappe yield ages of ~370 Ma and, combined with microstructures, suggest emplacement at lower amphibolite facies consistent with published monazite ages of 420–355 Ma from the Fall Mountain nappe. Similar structural patterns also occur in the underlying Skitchewaug nappe indicating that major D₁ sheath folds, as well as other non-coaxial folds, may be common during the Acadian throughout the Bronson Hill anticlinorium.