Studies of cleavage in the metasedimentary rocks of the northwest Adirondack Mountains, New York

Abstract

The pre‐Cambrian gneisses, schists, and marbles of the Grenville series, in the northwest Adirondack Mountains, have been complexly sheared and in part retaliated during metamorphism. The principal directions of shear that may be inferred from inspection of foliations and associated lineations are (1) northeast‐southwest (couple near horizontal) and (2) southeast‐northwest (vertical, couple). These directions are roughly subparallel respectively to the directions of gross lithologic trend and average dips.The lithologic trend and average dips are patterns imparted by the metasedimentary formations and rudely accordant orthogneisses. Some large‐scale shear at appreciable angles to the lithologic patterns appears in features of thick, mobile units such as marble, and in relatively brittle gneissic units which have ruptured and floated apart, as well as where large, discordant intrusives, or basement nodes seem to have obstructed or complicated the evolving, conventional pattern.If, instead of the gross process or patterns of shear, the details of the shear patterns are studied, the rock motions as reflected by foliations and lineations are more complex and commonly along surfaces at variance with individual beds. Thus, secondary foliations (cleavages) evolve only in part along or subparallel to bedding, and in considerable part across and at the expense of beds. Examples are discussed in which either key layers or folds appear in the rocks and seem to be directly associated with the shearing movements. These examples are used on the assumption that the form of the folds or the key layer contributes valuable data on the stage or degree of shear involved.The many other exposures of subparallel rock surfaces, in which no fold or key layer is apparent, are believed to have a common origin with, and fit within the limits of, types described. All prominent foliations of metamorphic origin (cleavages) are inferred to have evolved during successive, closely related stages of what was clearly a triaxial‐type deformation.