DOCUMENTATION OF A MAJOR RECUMBENT ISOCLINE IN THE SARATOGA SPRINGS AREA: CONTINUATION OF A THRUST-FAULTED SEGMENT OF THE CANADA LAKE NAPPE, SOUTHERN ADIRONDACKS, NEW YORK

Mapping in the SE Adirondacks, NW of Saratoga Springs, reveals the presence of a S-dipping (~45°), ~ 1 km thick, quartzite-rich unit sandwiched between equally thick units of granodioritic-tonalitic orthogneiss. These E-W striking layers are over- and underlain, respectively, by garnetiferous, migmatitic metapelites that are ~1.5 km thick on either side of the enclosed sequences. The next units outward from the core consist of ~1 km thick, mylonitized, megacrystic granodioritic gneiss that is homogeneous except for the presence of very coarse, crosscutting pegmatites concentrated along its outer contacts, i.e., along the north contact on the structurally inverted northern limb and south contact of the southern limb. Outward from the pegmatite zones, there are garnetiferous migmatitic metapelites of undetermined thickness. The symmetry of the overall package of rocks documents that they define an overturned, E-W trending, isoclinal fold of major dimensions. The fold axis lies along strike of the large Canada Lake recumbent fold-nappe (F1) exposed ~50 km to the west. Moreover, the lithology and sequence of rocks in the present structure are almost identical to those near Canada Lake where the isocline is cored by the Irving Pond quartzite followed outward by charnockites, tonalites, metapelites, and megacrystic granitoids. Mapping was extended westward in order to match up contacts of relevant units. However, the resulting outcrop pattern precludes continuity and necessitates the presence of a low-angle, east-dipping fault about halfway between the two regions. Highly mylonitized quartzite, pegmatite, and garnetiferous leucogneiss mark the proposed trace of this wide (100s of meters) structure. Preliminary investigation suggests that the fault has a reverse throw and has repeated the Canada Lake nappe on the upper plate. We propose the name Fox Hill thrust due to good exposures at this locality in the Edinboro 7.5' quadrangle. The thrust has been folded by the regional, upright Gloversville syncline (F2). Widespread occurrence of muscovite in the hanging wall block suggests that it experienced late alteration by fluids, perhaps emanating from syntectonic pegmatites. It is probable that, as yet unrecognized, thrust faults underly much of the Adirondacks.