Fabrics within a 1-km-wide shear zone along the eastern margin of the Bronson Hill anticlinorium in central Massachusetts provide important information on the regional tectonic developement during the Devonian Acadian orogeny. Two distinct mineral lineations observed within the zone are each defined by alignment of coarse sillimanite needles and elongated quartz and feldspar grains and aggregates. Both lineations are associated with shearing and development of mylonitic fabrics. The character of the two lineations and related fabrics differs mainly in orientation and in the relationship of each lineation to the local orientation of quartz c axis fabrics. The interpreted earlier lineation has a steep west plunge and the later lineation has a shallow plunge and north-south to southwest trend, parallel to the regional trend of the orogen. Extensive dynamic recrystallization of feldspar, the possible activity of prism slip in quartz, and the mineral assemblages in apparent equilibrium during deformation suggest that both lineations probably formed under upper amphibolite facies conditions, just after the thermal peak of Acadian metamorphism. A progressive change between the two deformation fabrics is suggested by evidence of overlap between the two, by their striking similarity in style and metamorphic conditions of formation, and by the similarity in orientation of their bulk strain axes, with a change only in transport direction between them.

Kinematic indicators associated with each phase of lineation development are consisent throughout the shear zone. The earlier phase has a west-side-up sense of shear and the later phase indicates west-side-north movement. These phases are correlated with the regional fackfold and dome stages of Acadian deformation. The kinematics and relationships between them suggest a revised tectonic model for the later part of the Acadian orogeny and for development of orogen-paralell lineations in this region. In this model, uplift and thickening of basement gneisses in the Bronson Hill anticlinorium during the backfold stage was impeded by gravitational instability in a thermally weakened crust. This led to a change from subvertical to subhorizontal extension. East-west Acadian shortening was still in progress, constraining this extension to a north-south direction, parallel to the trend of the orogen. ¿ American Geophysical Union 1993