The bedrock of the Windham quadrangle in southeastern New Hampshire consists of deformed early Palaeozoic crystalline metamorphic and intrusive igneous rocks intruded by Mesozoic igneous dikes. Generally, less common northeast striking, steeply dipping fractures developed sub-parallel to the pre-existing tectonic foliation in the Palaeozoic rocks. Mesozoic lamprophyre and diabase dikes intruded along the northeast trending fractures, utilizing the pre-existing anisotropy in the crystalline rocks. Northwest striking, steeply dipping systematic joints and joint sets are the most prominent fractures in the area and, at least in part, post-date the Mesozoic dikes. Sub-horizontal sheeting joints occur in all rock types. Locally, the coincidence of the sub-horizontal fractures with a sub-horizontal Paleozoic cleavage suggests that some of the sheeting fractures utilized the pre-existing ductile anisotropy during unloading. Generally, the metasedimentary rocks show a less complex pattern of fracturing than the intrusive rocks suggesting that rock type is a controlling factor. Metasedimentary rocks in the biotite zone and well-foliated igneous rocks show a greater tendency to fracture along pre-existing bedding and foliation surfaces than metasedimentary rocks in the garnet zone and poorly foliated igneous rocks. A comparison of mapped fracture data and station fracture data indicates that either mapped data or station data can be used to identify regional fracture trends. Local fracture trends can not be identified by limited measurements at a few fracture stations, however, because they do not address spatial variability. Some fracture trends may be scale-dependant because they may be either unique to a local area or present only at regional scales.
Additional Publication Details
Contrasting methods of fracture trend characterization in crystalline metamorphic and igneous rocks of the Windham quadrangle, New Hampshire