The origin of ribbon lineation within the southern Adirondacks, U.S.A.

Abstract

Multiply deformed metamorphic rocks of the southern Adirondacks exhibit a pronounced linear fabric consisting of elongate, flat ribbons oriented parallel to early fold axes and lying within foliation planes. The ribbons, which are often monomineralic, consist of quartz, feldspar and mafic minerals. As seen in transition from least to most deformed rocks, these ribbons appear to be the result of elongation of grains, or grain aggregates, in response to a regional rotational strain which also rotated early fold axes into parallelism with the lineation. A consistent sense of asymmetry of feldspar tails with respect to foliation suggests that simple shear was the dominant component of strain. The long dimension of these ribbons is believed to mark the maximum elongation direction ( X) of the finite strain ellipsoid.