Relative timing of calcite twinning strain and fold-thrust belt development; Hudson Valley fold-thrust belt, New York, U.S.A.

Coarse-grained limestone samples were collected across the Hudson Valley Fold-Thrust Belt with the aim of determining the temporal and spatial relationship between calcite twinning strain and fold-thrust belt development. The majority of the samples have well defined oblate strain ellipsoids with the maximum shortening axes (e3) perpendicular to bedding strike, and e3 magnitudes that range from ∼ 1.5% to 6.5%. Whereas twinning magnitude does not vary systematically through the fold-thrust belt, twinning fabrics fall into two populations based on the presence of outcrop-scale deformational features. Calcite twinning strain preserved in samples from limestone layers absent of any small-scale penetrative deformation, such as cleavage and/or microfaults, are defined as component A strains. Component A strains generally have maximum shortening axes (e3) inclined consistently less than, but in the same E-W direction as, bedding dip. The second population of calcite twinning strain, population B, occurs in limestones pervasively cut by cleavage and/or microfaults. Component B strains has maximum shortening axes (e3) that are horizontal irrespective of bedding dip. Strain population A initiated as a pre-folding, layer-parallel shortening strain that was subsequently modified to its present orientation by active grain-scale rotation during flexural folding. Strain population B reflects post-folding superimposed homogeneous strain, and is restricted to limestone samples with relatively high clay content.