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Master of Science (M.S.)
Year Degree Awarded
Month Degree Awarded
A microfabric study conducted in the northern part of the Pelham gneiss dome, central Massachusetts, has shown that the dome fabric is dominated by a relatively homogeneous, pervasive top-to-the-south shearing. Evidence consists of numerous kinematic indicators including: S-C fabrics, asymmetric winged porphyroclasts, asymmetric strain shadows, asymmetrically-boudinaged pegmatites, and asymmetric folds. Shear-related fabrics are present throughout the exposed section, from the lowest exposed gneissic rocks to the uppermost units in the cover section. Some textural evidence exists to suggest that this shearing diminishes slightly in intensity with structural depth. Reconnaissance indicates that shear fabrics are present throughout the Pelham dome and perhaps in adjacent areas of the Bronson Hill anticlinorium.
Porphyroblasts within pelitic units contain inclusion trails which preserve evidence for at least one, and probably two, previous foliations occurring at a high angle to the shear fabric. Both the shear fabric and the one certain previous fabric seem to have formed at similar amphibolite-grade metamorphic conditions. Kyanite, the peak metamorphic mineral present in these assemblages, seems to have formed both before and after the shearing, with the majority overgrowing the shear-related foliation.
Fabrics within the Pelham dome are inconsistent with the Pelham dome having formed through solid-state diapirism. The concentric, oval foliation and shallow N-S lineation are clearly associated with top-to-the-south subhorizontal shearing. Several alternative mechanisms for dome formation have been examined, and none has proven completely satisfactory. The most likely mechanism is that the Pelham dome is a lowstrain pod within a very large ductile shear regime.
Interpretations of the tectonics of the deformation seen here are complicated by uncertainty over the age of deformation. It is probable that some component of the deformation is Pennsylvanian in age (Gromet and Robinson, 1990), but other elements are probably Acadian. Serious questions about timing, style of deformation, and direction of shear undermine the direct correlation of shearing in the Pelham dome and shearing in the Willimantic dome suggested by Getty (1990).
Michael L. Williams
George E. McGill