A NEW SYNTHESIS: CENTRAL TO SOUTHEASTERN ADIRONDACK MOUNTAINS, NEW YORK

A compilation of twenty-one 7.5-minute quadrangles at a scale of 1:62,500 shows several different geologic regimes. A shelf sequence of isoclinally folded metasedimentary strata interlayered with acidic metavolcanic strata trends E-W from the central Adirondacks to the vicinity of Pharaoh Mountain. The shelf sequence is bounded on the north by the main anorthosite massif and its offshoot intrusives and on the south by the Thirteenth Lake anorthosite dome. East of the Thirteenth Lake dome the shelf sequence contains a great deal more carbonate rock. The carbonate-rich zone has been intruded by steeply-dipping MCG bodies with associated lenses of anorthosite, leucogabbro, and ultramafic rocks. It extends along the southern border of the map area from the dome to the Schroon River fault.

The eastern part of the map area consists of a series of imbricate tectonic slices on ductile thrust zones and is called the SE allochthon. The SE allochthon is complex and is characterized by 10-15 times the volume of metagabbro intrusives relative to the shelf sequence, suggesting original proximity to an extensional axis. It also contains two (or more) major sills of the MCG series. An arcuate zone of biotite-quartz-plagioclase +/- potassium feldspar +/- sillimanite paragneisses along its western margin may represent a metamorphosed flysch deposit. Several exposures of granitic gneiss within the allochthon bear field evidence of almost complete anatexis of paragneisses.

The simplest history of the region involves early isoclinal and recumbent folding (F1) of the shelf sequence, followed by intrusion of the AMCG association, followed by open folding (F2) in the northern part of the map area becoming progressively tighter and overturned near the SE allochthon. An episode of ductile thrusting is roughly coeval with F2 folding. A final open folding (F3) with a N-S axial trend created an egg-carton pattern by interference with F2 folds.

The “Ticonderoga charnockite dome” appears to be an intrusive sheet that is domed by interference between F2 and F3 folds. The Springhill Pond and Thunderbolt Mountain metasedimentary/metavolcanic rocks mapped by Walton in the 1950’s appear to be another allochthon in the NE part of the map.