THRUSTING IN THE BERKSHIRE MASSIF, WESTERN NEW ENGLAND APPALACHIANS

The Berkshire Massif in western Massachusetts and Connecticut is composed of Middle Proterozoic rocks that were thrust over Cambrian to Ordovician continental shelf deposits. It is widely accepted that westward-directed thrusting occurred during the Ordovician Taconic orogeny, yet the kinematics and age of deformation have not been well constrained. Ratcliffe and Harwood (1975) described zones of cataclasis and recrystallization in “blastomylonitic seams” along the western edge of the massif, but sense of shear indicators and microstructures have not been described in these rocks since their work. Furthermore, it may be possible to date fabric development and deformation with monazite.

We collected samples of deformed basement and cover rocks near thrust faults in the southwestern part of the massif. Rocks within a few meters of faults are mylonites with a strong, recrystallized, planar foliation. Strain decreases dramatically within tens of meters of faults, based on the appearance of deformation fabrics. Basement rocks approximately 20 m away from faults generally preserve their Proterozoic gneissic fabric, which is crosscut by distinct mica-rich shear bands with recrystallized feldspar and quartz. Biotite seams, quartz ribbons, and regions of fine-grained recrystallized feldspar define a sub-planar fabric that wraps around feldspar porphyroclasts. Porphyroclasts have short tails and theta- or sigma-type shapes, suggesting that recrystallization was very rapid compared to the strain rate. Asymmetric porphyroclasts, S-C fabrics, and extensional shear bands locally show east over west diplacement. Elongate quartz ribbons are composed of equant subgrains that are commonly recovered and strain-free, whereas the interiors of most feldspar porphyroclasts preserve evidence of deformation by grain-size reduction. Such grains are highly strained and show undulose extinction, suggesting that deformation was controlled by dislocation creep at temperatures above 500 degrees C. No evidence for cataclasis or other brittle mechanisms is preserved in these rocks.

It may be possible to identify monazite grains that grew during the development of thrust-related mylonitic fabrics and thereby date the age of thrusting and constrain the timing of the Taconic orogeny in this part of the northern Appalachians.