FORMATION MECHANISM OF THE PELHAM DOME, WESTERN BRONSON HILL ANTICLINORIUM, CENTRAL MASSACHUSETTS

REED, Robert M., Dept. of Geological Sciences, University of Texas, Austin, TX 78712,
and WILLIAMS, Michael L., Dept. of Geology and Geography, University of Massachusetts, Amherst, MA 01003

The Pelham gneiss dome is an elongate, oval-shaped, structural dome, which is cored by Proterozoic and Ordovician gneisses and overlain primarily by weaker schists, quartzites, and amphibolites of Ordovician to Devonian ages. Kinematic analysis of rocks in the core and cover has shown a relatively homogeneous, subhorizontal, top-to-the-south shear to be the dominant fabric-producing element in the dome area. Kinematic indicators include asymmetric winged-inclusions and strain shadows, S-C fabrics, asymmetric folds and boudins, extensional crenulation cleavages, and quarter structures. The flow direction is parallel to a well-developed roughly N-S stretching lineation that is a prominent structural element in both basement and cover rocks. Although older deformational fabrics are present, the current dome-defining foliation and the stretching lineation result from the shearing, thus the shearing must be explicitly incorporated into any proposed dome-formation model.

Previous interpretations have been that the dome shape is the result of solid-state density-driven buoyant rise of the basement gneisses. However, there is no microfabric evidence for vertical motion during dome formation, which brings into question a buoyancy-based formation method. Several other mechanisms have also been proposed for the formation of gneiss domes including fold interference, mega-boudinage, and large-scale sheath-folding. The fold interference mechanism fails to account for the consistent subhorizontal shearing. The sheath fold mechanism accounts for the shearing but the pattern of foliation and layering within the dome is inconsistent with such a model. The mega-boudinage model requires opposite senses of shear at the ends of the dome, unlike the consistent sense seen in the Pelham dome. We propose a new model to fit the structural observations, where the gneisses coring the Pelham dome are a low-strain pod formed by strain partitioning within a large-scale shear-zone. This model could be considered asymmetrical mega-boudinage. This hypothesis accounts for the domal shape of the shear foliation as well as the consistent-sense of the shearing and its presence in both core and cover rocks.

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