HOW DOES NEW HAMPSHIRE ATTACH TO VERMONT? NEW INFORMATION ON A LONG-STANDING PROBLEM IN WESTERN NEW HAMPSHIRE

New 1:24,000-scale mapping in the Claremont, NH area reveals the complex nature of two particularly vexing problems. The Monroe Line (ML) and “Big Staurolite nappe” (BSN) are known regionally, but poorly understood geologic features in western New Hampshire. The ML is either an unconformity or an early Acadian thrust fault that separates rocks of the Vermont sequence (VTS) from those of the New Hampshire sequence (NHS). The ML in the Claremont area is a boundary between the Ordovician Ammonoosuc Volcanics and Partridge Formation (NHS) and the Devonian phyllitic rocks of the Gile Mountain (VTS) or Littleton Formations (NHS). Discontinuous “pods” or slices of the Silurian Clough Quartzite occur along the contact, which is parallel to the oldest recognized fabric (S1). Upper and lower plate truncations along the contact favor a D1 thrust fault model. Much of the early structural history (S1/D1) of the ML is obscured by pervasive ductile NE-SW fabric (D2). D1 and D2 structures in the eastern part of the area are deformed by D3 doming, and all rocks are deformed by late E-NE trending open folds. To complicate matters, Mesozoic normal faults locally truncate the ML.

Mapping shows that the limit of the “Big Staurolite” nappe is sub-parallel to parallel to the D2 fabric. Near the contact, the staurolite schist is mylonitic and random staurolite crystals (4 to 7 cm long) become aligned and plunge towards the SE before disappearing over a distance of about 10 m. Staurolite-bearing rocks decrease in abundance along the west side of Green Mountain and completely disappear to the south where Clough Quartzite is directly in contact with non-staurolite bearing Littleton Formation. Additionally, rocks on either side of the staurolite-limit are metapelite and quartzite with a shared early metamorphic history. Both sides of the contact experienced early garnet-grade metamorphism overprinted by retrograde greenschist facies assemblages. In the staurolite zone, staurolite overgrew garnet, it encapsulated inclusions that retain an early fabric and it was deformed by the NE striking D2 fabric. These data suggest that the contact may be regionally significant, and could perhaps correlate with the basal thrust of the Skitchewaug slice exposed to the southwest in VT, but its significance as a major tectonic boundary is speculative.