CONTINUED BEDROCK GEOLOGIC MAPPING IN THE PRESIDENTIAL RANGE, N.H.: A PROGRESS REPORT FOR EDMAP 2001

Bedrock geologic mapping done in north flank of the Presidential Range of New Hampshire during the 2001 field season was supported by the U.S. Geological Survey EDMAP program. Metasedimentary units mapped include a variety of members of the Littleton Formation, migmatized Littleton Formation, migmatized and rare unmigmatized Rangeley Formation. Notably absent were occurrences of the late Silurian Madrid and Smalls Falls Formations. Ordovician Ammonoosuc volcanics and Oliverian quartz monzonite, both of the Bronson Hill Anticlinorium were also mapped. The Ammonoosuc volcanics were subdivided into seven different members ranging from metamorphosed mafic lapilli tuff to a rusty weathering gneiss. Cross cutting all these units were sills of pegmatite, aplite, and medium- to fine-grained two mica and biotite granites and several basalt dikes.

The Littleton Formation had few exposures of original bedding, S0, with topping directions preserved; most of it was migmatized. Rangeley Formation migmatite was distinguished from the Littleton by the presence of calc-silicate lenses. The Littleton and Rangeley migmatites were often in contact suggesting the presence of an erosional unconformity, a pre-metamorphic slump fault, or syn-tectonic thrust. The P-T regime of these migmatites is currently being studied. D1 folds in the Littleton Formation were rare. Evidence of isoclinal F1 folding was common as S1 foliation was normally found parallel to S0. D4 folding was common in certain unmigmatized sections of the Littleton Formation.

The structure of Ammonoosuc volcanics is dominated by pervasive schistosity and a biotite or amphibole lineation. At least two, 1 to 2 m wide, mylonite zones were mapped within amphibolite members of the volcanics. The kinematics of these are currently being evaluating. An augen gneiss was mapped along the contact between the Ammonoosucs and the Rangeley-Littleton composite. This unit is likely a recrystallized mylonite and a significant regional structure. Previous studies show this boundary as the Mahoosuc Fault, interpreted to be either a low angle thrust fault or a normal gravity slide. Given the ductile nature of the augen gneiss and the strong mineral lineation, a syn-kinematic fault origin is favored.