Adirondack Highlands–Northwest Lowlands “boundary”: A multiply folded intrusive contact with fold-associated mylonitization

The “boundary” between the Adirondack Highlands and the Northwest Lowlands near Harrisville, New York, is a multiply folded intrusive contact between metamorphosed sedimentary rocks of the Lowlands and metamorphosed intrusive rocks of the Diana Complex in the Highlands. This is shown by detailed geologic mapping and analysis of stratigraphy and minor structural features. Regionally developed protomylonitic foliation in the Diana Complex is an axial-plane foliation formed during isoclinal folding of dry quartz syenite at granulite-facies metamorphic grade. Lack of major post-intrusive fault displacement along the Diana Complex–Northwest Lowlands “boundary” is demonstrated by correlation of xenoliths in the Diana Complex with immediately adjacent metamorphosed sedimentary rock units of the Northwest Lowlands and by continuity of major folds across the “boundary.” A stratigraphic succession consisting of seven rock units has been recognized in Northwest Lowlands rocks of the Harrisville area. Rocks in these units include, from oldest to youngest(?): (1) garnet-sillimanite-cordierite gneiss; (2) rusty pelitic gneiss with biotite gneiss, pyroxene amphibolite, and quartzite; (3) graphitic calcite marble; (4) siliceous marble; (5) rusty calc-silicate rocks; (6) diopside gneiss, and (7) calc-silicate granulite. Four phases of folding are evident in these stratigraphic units, but only the last three phases affect, intrusive quartz syenite gneisses of the Diana Complex. First-phase isoclinal folds in bedding have a pervasive axial-plane foliation which is the regional foliation. Associated regional lineations are parallel to first-phase fold axes. Foliation and lineation formed during the first phase of folding were deformed by second-phase isoclinal folds, which have a weak to strong axial-plane foliation or slip cleavage. Quartz syenite of the Diana Complex intruded folded metamorphosed sedimentary rocks early during second-phase folding and was itself isoclinally folded somewhat later during the second phase. Isoclinal folding of quartz syenite gneiss is associated with development of protomylonitic to mylonitic axial-plane foliation, which is most intense in the northwest border zone of the Diana Complex, due to lithic contrast with rocks northwest of the border. Third-phase folds are isoclinal to open and are locally characterized by axial-plane slip cleavage or fracture cleavage. The axial surfaces of folds associated with the first three phases of deformation have a general northeasterly trend and northwesterly dip, but their axes have varied plunges. Two-dimensional heart-and-anchor and hook patterns are a product of interference of second- and third-phase folds. Fourth-phase deformation resulted in northwest-trending open folds with locally developed axial-plane slip or fracture cleavage. Interference of third- and fourth-phase folds is manifest in dome and basin patterns that are elongate to the northeast and overturned to the southeast, similar to dome and basin patterns throughout the Northwest Lowlands. Late-stage ductile shearing produced very fine-grained, thin mylonite and ultramylonite in shear zones that transect the preexisting protomylonite foliation in quartz syenite gneiss. Later brittle deformation resulted in closely spaced fractures, quartz and epidote veins, and local minor faults. However, no evidence for major faulting has been observed along this part of the Adirondack Highlands–Northwest Lowlands “boundary.”