STRATIGRAPHY AND DUCTILE STRUCTURE OF THE PRESIDENTIAL RANGE, NEW-HAMPSHIRE - TECTONIC IMPLICATIONS FOR THE ACADIAN OROGENY

The Presidential Range of New Hampshire with its unique high relief co ntains exceptionally well exposed rocks of the Acadian orogenic hinter land, These rocks are within the Central Maine terrane, a belt of comp lexly metamorphosed and deformed Silurian and Devonian metasedimentary rocks, The Presidential Range lies between regions to the south domin ated by refolded nappes and synkinematic high-grade regional metamorph ism and regions to the northeast dominated by overlapping multiple def ormation and contact metamorphism produced by late- and post-Acadian p lutons, To better understand these complex tectonic variations and, in particular, to clearly and accurately determine the effects of the De vonian Acadian orogeny in a well exposed region, the bedrock geology i n 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 young est, are the Silurian (?) Rangeley, Perry Mountain, Smalls Falls, and Madrid Formations, and the Devonian (?) Littleton Formation, The Littl eton Formation has been subdivided into fifteen different members and three submembers based on variations in bedding style of the schists a nd quartzites and other lithologic peculiarities, The Madrid, Smalls F alls, and Ferry Mountains Formations are extremely thin, consistent wi th their deposition near the Silurian tectonic hinge, The Rangeley For mation is interpreted as an olistostromal melange and has been subdivi ded into three different members and six submembers on the basis of li thologic variations in the gneiss and subordinate units. The President ial Range preserves three phases of ductile folding (F1, F2, and F3) a nd 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 wes t-vergent (?) Greenough Spring thrust fault truncated Fl and F2 struct ures and juxtaposed nonmigmatized and migmatized rocks, F3 east-vergen t folding dominates the alpine zone and represents the final tightenin g of Acadian structures, which occurred after early granitic plutonism and prior to the emplacement of postkinematic granitic sheets. The st ructural and sedimentological evidence supports the existence of a sub duction system that was active beginning in the early Silurian on the east flank of the Bronson Hill Anticlinorium with a west-dipping subdu cting slab geometry. This system persisted throughout the Acadian.