TIMING OF DEPOSITION, DEFORMATION, METAMORPHISM, AND PLUTONISM IN THE CENTRAL MAINE AND MERRIMACK BELTS OF SOUTHERN MAINE AND NEW HAMPSHIRE

Stratified rocks of southwestern Maine and southeastern New Hampshire are separated into the Central Maine and Merrimack belts. Both have contrasting histories of deposition, metamorphism, deformation, and plutonism. They continue to raise questions about the nature of their depositional site(s) and boundaries (e.g., one or two basins separated by a positive tectonic element, as in the Portland-Bath area and farther northeast, and/or faults); the significance of somewhat different sediment sources (Central Maine predominantly from the west; Merrimack mostly from the east); and their response to diachronous tectonism.

In the Merrimack belt the Eliot and Kittery Formations form a conformable sequence. The Berwick Formation is enigmatic: Does it belong to one, the other, or to both belts? In the absence of paleontological data and minimum age estimates based on dated cross-cutting plutons, recent LA-ICP-MS and ID-TIMS data from detrital zircons suggest that deposition of all three units occurred in middle Silurian time (≥426 Ma; Sorota, 2012 and unpub.). When combined with the oldest intrusive rock, the Newburyport pluton (418Ma) that cuts the Eliot and Kittery, the minimum time interval between deposition, Salinic metamorphism and deformation and early Devonian magmatism is ~10Ma.

Conformable formations in the Central Maine belt are closely correlated with the Silurian to Early Devonian stratigraphic sequence in Rangeley, Maine, and their distal counterparts. In the area of this study the Rindgemere, Gully Oven, Towow, “Madrid-equivalent” and East Rochester Formations are intruded by numerous Paleozoic granite plutons with ages ranging from 384 to 287Ma (Middle and Late Devonian to Carboniferous). The oldest granite intruding this sequence brackets a longer ~15-20Ma interval between deposition and widespread metamorphism and deformation of the Acadian Orogeny. This episodic(?) east to west progressive event affected rocks of both the Central Maine belt and the Merrimack belt. It is perhaps best documented by a late, generally northeast-trending, upright fold system common to both belts that overprints one or more earlier fold sets of Acadian and/or Salinic age. Sediment age, structural style(s) and magmatic record help to distinguish the history of these two belts.