DETAILED GEOLOGIC MAPPING - CENTRAL TO SOUTHEASTERN ADIRONDACKS, A SYNTHESIS

A swath of terrane measuring about 60 km by 27 km has been mapped at a scale of 1:24,000 and greater. Most of the Bad Luck Mtn, all of the Dutton Mtn, Minerva, Schroon Lake, Pharaoh Mtn, Chestertown (after Geraghty), Brant lake, Silver Bay, and portions of the Gore Mtn and North Creek 7.5’ quadrangles are included. When the geologic maps of these quadrangles are concatenated, several observations emerge that are not obvious otherwise.

1. The metasedimentary units are more sillimanite-rich in the east, more amphibolitic in the west, and thicker in the east. A marble unit is always found near the “base” of the metasedimentary section.

2. Massive, tabular bodies of pink, biotite-hornblende-garnet granitic gneiss separate zones of metasedimentary rocks that are often said to be infolds or keels. At the mapping scale, bilateral symmetry of infolds is hard to demonstrate. If F2 and later folding is removed, all of these bodies are nearly horizontal. Lateral extrusion of plastic material from a thickened deep crust late in an orogenic cycle may explain the granitic bodies (Molnar and Tapponier, 1975). Whether these bodies should be called the cores of recumbent antiforms remains to be discussed. It is suggested that the granitic sheets constitute a part of a tectono-stratigraphy that were emplaced along highly-rheid marble layers.

3. Most faults strike NNE, but in the east many strike NE in an area characterized by much meta-igneous rock. The Lake George AMCG pluton shows magmatic differentiation. Over 100 meters of underlying metasedimentary rocks contain ultramylonites and evidence of ductile thrusting. Rocks at the N edge of the pluton are flattened. Similar features are found at the southern border of the Minerva AMCG pluton. It is hypothesized that these bodies intruded along pre-existing ductile thrust zones in the deep crust (cf. Liang et al., 2015) since only the lower meter of so of each body shows evidence of simple shear. Flattening of country rock at the leading edge of these plutons suggests a zone of pure shear as well. The term “glossolith” (tongue rock) is proposed for such intrusives.