BRITTLE STRUCTURES AND THE GRANTHAM FAULT WITHIN THE SOUTHERN HALF OF THE 7.5-MINUTE GRANTHAM QUADRANGLE, NEW HAMPSHIRE

The 7.5-minute Grantham quadrangle located in the southwestern New Hampshire contains rocks of the Bronson Hill Anticlinorium (BHA), the Croydon Dome, and the New Hampshire Plutonic Suite. These rocks were metamorphosed and deformed during multiple Paleozoic orogenies. The bedrock geology has been covered in areas of lower elevations by surficial deposits. The Grantham Fault, a Mesozoic normal fault separates the Bronson Hill Anticlinorium and the Croydon Dome from the Bethlehem Gneiss, part of the New Hampshire Plutonic Suite. The brittle geologic history of this area is recorded in parting fractures, fractures that are perpendicular to foliation, and symmetric fractures. The fracture analysis has provided evidence for the fracture system of the field area.

The Grantham Fault is characterized by kink folding and silicified zones of almost pure quartz that are chaotic in orientation. There are small areas of the silicified zones that are brecciated. The portion of the quadrangle east of the Grantham Fault is Devonian Bethlehem Gneiss, which has a generally subhorizontal foliation with 48% of these foliations displaying parting displacement. Parting fractures are observed at almost every outcrop, but there are also fractures that are perpendicular to foliation commonly influencing the topography. The outcrops closer to the fault are more variable in fracture orientation. Many of these fractures are perpendicular to foliation. Steeply dipping fractures that strike approximately 185 degrees are prevalent near the Grantham Fault. West of the Grantham Fault there is a diversity rock types, represented by the Ordovician Ammonoosuc Formation, Silurian Clough and Fitch Formations, Devonian Littleton Formation, and the Croydon Dome rocks. These formations have a greater number of fractures that are perpendicular to foliation, as well as symmetric fractures that have spacing of approximately 1 m. A much lower percentage, approximately 20 percent, of these foliations have parting fractures.