The influence of ductile structure and rheological heterogeneity on brittle structures as exhibited by Avalonian granites in southeastern Massachusetts

The orientation and geographic distribution of joints, veins, and brittle faults show a conspicuous correlation with the heterogeneous distribution of foliation and lineation intensity in Neo-Proterozoic granites and their deformed counterparts in southeastern Massachusetts. Field mapping and stereonet analysis of brittle and ductile structural data collected during 1:24,000-scale geologic mapping of the Milford quadrangle yielded the following general observations, which suggest that the ductile deformational history of this region appears to have strongly influenced the later development of brittle structures in the same rocks:

• Regionally pervasive subvertical northeast-, north-south-, and east-west-trending fracture sets are dominant in otherwise undeformed portions of the granites.
• The above regional joint sets rotate into parallelism with lithologic and structural heterogeneities such as diabase dikes, shear zones, or xenoliths. Such features commonly have local joint sets parallel to them.
• Where the granites display a strongly developed fabric, the dominant joint sets are parallel and orthogonal to foliation and perpendicular to a pervasive northeast plunging lineation.
• Nearly all fractures observed that are not parallel to foliation, or part of the regional joint sets mentioned above, are either parallel or perpendicular to the regional lineation.

The behavior of these fractures in relation to ductile structure have implications for rheological constraints on tectonic models of the post-Alleghenian core of the New England Appalachians, hydrogeologic models of regional fractured bedrock aquifers, and the interpretation of topographic lineaments.