STRUCTURAL SETTING OF THE ROCKY POND SLICE, AN ENIGMATIC TECTONIC BLOCK IN EASTERN MASSACHUSETTS

The Rocky Pond Slice (RPS) is a NE-trending, fault-bounded block within the New England Appalachians of eastern Massachusetts, located between the Merrimack belt to the NW and the Nashoba terrane to the SE. The Nashoba terrane is an upper amphibolite-facies Cambrian-Ordovician arc-backarc complex and the eastern Merrimack belt consists of greenschist-facies Silurian to Early Devonian metasedimentary rocks. We used detailed structural mapping to investigate the relationships between units of the RPS and adjacent terranes.

The RPS consists of three metasedimentary units: the Boylston Schist, the Sewall Hill Formation and the Vaughn Hill Formation. These units are intruded by the Rocky Pond Granite in the north. The Boylston Schist and Sewall Hill Formations in the west were metamorphosed under amphibolite-facies conditions and the Vaughn Hill Formation in the east under greenschist-facies conditions. A moderately NW-dipping foliation exists throughout the RPS and is consistent with the regional fabric in the Nashoba terrane and Merrimack belt. The amphibolite-facies rocks are folded by cm- to m-scale isoclinal folds, and later refolded by map-scale moderately NW-plunging folds. Isoclinal fold hinge lines plunge moderately NNE in the southern RPS and gradually change to shallowly to moderately NW-plunging in the northwestern RPS.

The greenschist-facies eastern part of the RPS is separated from the high-grade Nashoba terrane by the NW-dipping Ball Hill Fault, and the amphibolite-facies western part of the RPS is separated from the low-grade eastern Merrimack belt by the Clinton-Newbury Fault. Both faults, and localized shear zones within the RPS, have complex polyphase histories with early sinistral shear being overprinted by normal movement. Local dextral and reverse shear sense indicators are present only along the intrusive contacts of the Rocky Pond Granite, but do not represent large offsets.

Substantial normal movement along the RPS-bounding faults may have been responsible for the juxtaposition of greenschist-facies rocks to the NW and amphibolite-facies rocks to the SE along both margins. Perhaps the RPS is a part of the Nashoba terrane that was down-dropped by normal movement. Another possibility is that the RPS is a higher grade part of the Merrimack belt that was brought up during convergence.