The Presidential Range of New Hampshire with its unique high relief contains exceptionally well exposed rocks of the Acadian orogenic hinterland. These rocks are within the Central Maine terrane, a belt of complexly metamorphosed and deformed Silurian and Devonian metasedimentary rocks. The Presidential Range lies between regions to the south dominated by refolded nappes and synkinematic high-grade regional metamorphism and regions to the northeast dominated by overlapping multiple deformation and contact metamorphism produced by late- and post-Acadian plutons. To better understand these complex tectonic variations and, in particular, to clearly and accurately determine the effects of the Devonian Acadian orogeny in a well-exposed region, the bedrock geology in the alpine zone of the Presidential Range was mapped in great detail. The results of the stratigraphic and ductile structural analyses are presented here.
Five metasedimentary formations have been recognized and constitute a conformable stratigraphy, which, from oldest to youngest, are the Silurian (?) Rangeley, Perry Mountain, Smalls Falls, and Madrid Formations, and the Devonian (?) Littleton Formation. The Littleton Formation has been subdivided into fifteen different members and three submembers based on variations in bedding style of the schists and quartzites and other lithologic peculiarities. The Madrid, Smalls Falls, and Perry Mountains Formations are extremely thin, consistent with their deposition near the Silurian tectonic hinge. The Rangeley Formation is interpreted as an olistostromal melange and has been subdivided into three different members and six submembers on the basis of lithologic variations in the gneiss and subordinate units.
The Presidential Range preserves three phases of ductile folding (F1, F2, and F3) and a single phase of thrust faulting (T1). Well-constrained southeast-, east- or northeast-facing directions and consistent asymmetry of F1 nappe limbs suggest that nappe vergence was easterly. Severe refolding of some nappes resulted during F2 collapse of the nappe pile. The west-vergent (?) Greenough Spring thrust fault truncated F1 and F2 structures and juxtaposed nonmigmatized and migmatized rocks. F3 east-vergent folding dominates the alpine zone and represents the final tightening of Acadian structures, which occurred after early granitic plutonism and prior to the emplacement of postkinematic granitic sheets.
The structural and sedimentological evidence supports the existence of a subduction system that was active beginning in the early Silurian on the east flank of the Bronson Hill Anticlinorium with a west-dipping subducting slab geometry. This system persisted throughout the Acadian.