TECTONIC PERSPECTIVES ON THE MORETOWN FORMATION, VERMONT AND MASSACHUSETTS

The Moretown Fm is dominantly composed of interlayered massive and schistose quartz-rich metasedimentary rocks with a “pinstriped” tectonic fabric, intervals of black sulfidic rocks, and multiple generations of metaigneous rocks (L. Cambrian-Silurian). The Moretown Fm extends from northern VT to south-central MA and comprises the middle unit of the Rowe-Hawley Belt (RHB)- a tectonic assemblage of Cambrian –Ordovician oceanic and suprasubduction zone rocks juxtaposed in a Taconian arc-trench setting. Correlatives of the Moretown Fm extend into northern Conn. as the Taine Mountain Fm., but do not extend into S. Quebec. In northern VT, the Moretown Fm undergoes a facies change and is truncated by the Devonian Coburn Hill Thrust whereas in MA it is cut out along the Devonian Prospect Hill Thrust. This formation was also deformed and metamorphosed during the Acadian Orogeny.

From south-central MA to central VT the Moretown Fm is bounded to the east by the mafic and felsic forearc and arc metaigneous rocks of the Hawley Fm (MA) and Barnard Gneiss/North River Igneous Suite (VT); these metaigneous rocks have yielded L. Cambrian-E. Ordovician U/Pb zircon ages. In NW MA and S. VT, the Moretown/ Hawley-Barnard contact is a Devonian thrust fault (S. Newfane/Zoar Thrust). In north-central VT, the Moretown Fm is either directly against the RMC (Richardson Memorial Contact) or separated from it by black phyllites of the Cram Hill Fm. The RMC is a discontinuous Devonian fault at this latitude.

In north-central Vermont, the Moretown Fm is composed of two lithotectonic belts, an eastern belt of “pinstriped” granofels intruded by metadiabasic dikes and sills with a suprasubduction zone signature and a western belt of interlayered black phyllites, quartzites, and green granofels, that has no igneous rocks. The earliest structures (Taconian) are preserved in the western belt whereas these structures have been strongly overprinted by Acadian deformation in the eastern belt.

Tectonic models for the Moretown Fm can be post and/or pre subduction initiation. Post subduction initiation models are based on modern analogues in Taiwan or Barbados. Since minimum ages on suprasubduction zone intrusions in or near the Moretown Fm are L. Cambrian, it is possible that, at least, parts of the Moretown Fm predate the ~505 Ma Iapetan subduction initiation.