Bedrock fracture characterization of the New Hampshire State Route 111 bypass, Windham, New Hampshire

Abstract

Bedrock roadcuts developed with blasting along the New Hampshire State Route 111 bypass in Windham expose the metasedimentary Silurian Berwick Formation and intrusions of multiple phases of foliated to nonfoliated granite to granitic pegmatite of the Devonian New Hampshire Plutonic Suite. Fracture characterization at two roadway rock cuts (roadcuts) included measurement of fractures over a distance of approximately 225 and 85 meters. The Berwick Formation consists of medium-gray biotite-plagioclase-quartz granofels, biotite schist, and lesser calc-silicate rock. The Berwick Formation is locally sulfidic. Fresh, unweathered roadcuts are mostly gray but exhibit locally rusty weathering. The most conspicuous foliation in the region around the studied roadcuts is steeply northwest dipping to subvertical and northeast-southwest striking. Regionally, the strike of the foliation is consistently to the northeast-southwest, but the dip is locally variable to both the southeast and northwest. About 8 percent of the observed foliation surfaces exhibit limited fracture parting. The limited degree of parting agrees with observations for rocks within the garnet zone of metamorphism elsewhere in the Windham 7.5-minute quadrangle. The most prominent fracture trend is subvertical to steeply northeast-dipping and northwest-southeast striking (strike and dip of about 295°, 80°). The peak trend of steeply dipping fractures at the two exposures is 295°±12° and 289°±6°. Veins observed in the granite occur parallel to the peak fracture trend and consist primarily of quartz, tourmaline, and ankerite with minor amounts of sulfides (arsenopyrite, galena, and rare sphalerite), and trace amounts of apatite and rutile. The observed peak fracture trend at these roadcuts closely agrees with the most prominent fracture trend recognized within the Windham quadrangle. Gently south- to southeast-dipping and east- to northeast-striking fractures occur as joints and as parting fractures along a weak S 3 cleavage. Water-bearing fractures at one exposure occur along joints and gently dipping contacts between the Berwick Formation and the granite to granitic pegmatite of the New Hampshire Plutonic Suite. About 8 percent of the fractures are water-bearing and most water-bearing fractures are gently dipping to the southeast. Fracture data separated by rock type shows a similar distribution for steeply dipping northwest-striking trends, but with much fewer observed steeply north-dipping fractures in the granitic rocks. Both rock types show a cluster of gently south-dipping fractures. The granites show far fewer steeply dipping northeast-striking fractures, which reflects a greater degree of parting along the foliation in the metasedimentary rocks than in the granites. No foliation-parallel fractures were observed in the granites, but some contacts between granites and the Berwick Formation do exhibit parting. Fracture termination classification yields 3 percent abutting, 76 percent dead end, and 21 percent crossing (or throughgoing) fractures. Six brittle faults were observed, which strike northeast and most dip steeply to the northwest. Calculated paleostress tensors for the faults show an average stress field that is consistent with Late Triassic to Early Jurassic northwest-southeast extension associated with rifting of the New England crust during the initial opening of the Atlantic Basin. Fault data are consistent with brittle reactivation of the northeast striking and northwest dipping dominant foliation.