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 prefolding, layer-parallel shortening strain that was subsequently modified to its present orientation by active grainscale rotation during flexural folding. Strain population B reflects post-folding superimposed homogeneous strain, and is restricted to limestone samples with relatively high clay content.