EDMAP BEDROCK GEOLOGIC MAP OF THE SOUTHERN HALF OF THE 7.5-MINUTE GRANTHAM QUADRANGLE, SULLIVAN COUNTY, NH

The southern half of the 7.5-Minute Grantham quadrangle, located in southwestern New Hampshire, was mapped at 1:24,000-scale in ArcGIS. The Bronson Hill Anticlinorium and the Croydon Dome are exposed in the western portion of the Grantham quadrangle. The Bronson Hill Anticlinorium consists of metavolcanic and metasedimentary rocks of the Ordovician Ammonoosuc Formation (Oam), Silurian Clough (Scg) and Fitch (Sf) Formations, and the Devonian Littleton (Dl) Formation. The granodioritic Croydon Dome is part of the Oliverian suite. The western margin of the Croydon dome and Bronson Hill Anticlinorium preserves a high strain zone that is identified by augen gneiss within the Croydon dome and intensely foliated Bronson Hill rock units. Foliations within the high strain zone parallel the Croydon dome margin and Bronson Hill units are truncated, indicating tectonic emplacement. The eastern margin of the Croydon Dome is predominantly cut by the Mesozoic Grantham Fault. Where the eastern margin is preserved, the Croydon Dome rocks and the Bronson Hill rocks are foliated and the foliation is aligned with the margin. The Grantham Fault is a NE-SW-trending Mesozoic normal fault that separates the Bronson Hill Anticlinorium and the Croydon Dome from the Bethlehem Gneiss. Narrow elongate silicified zones and kink folds define the Grantham Fault. The Bethlehem Gneiss of the New Hampshire Plutonic Suite is a medium- to fine-grained granodioritic to granitic gneiss. Gneissic foliation is defined by biotite and numerous outcrops display crenulation. Open upright folds, with fold axes that trend NE-SW and plunge shallowly, warp the foliation. Outcrop-scale fold axis calculations show a NE-SW trend with a subhorizontal plunge. The large-scale folding within the Bethlehem Gneiss is controlling the topography. NE-SW trending syntectonic left-lateral shear bands and mafic enclaves are abundant throughout the Bethlehem Gneiss. There are also substantial pegmatites and quartz veins transecting the Bethlehem Gneiss.