Finite strain heterogeneity and volume loss in slates of the Taconic Allochthon, Vermont, U.S.A.

Slates of the Taconic Allochthon have ellipsoidal reduction spots which have been used to determine the finite state of strain at seven sites. The strain is highly heterogeneous at all scales, ranging from several centimeters to many kilometers. Variations in XY ratios is small but variations in YZ ratios is large and this pattern is seen at all scales. The heterogeneity can be used to determine a strain path, which is nearly horizontal on a Flinn diagram. We propose that uniaxial flattening associated with late stage cleavage development was accommodated by volume loss, and that the most highly strained sites have experienced an average 55% volume loss, at a minimum. Explaining our results without volume loss requires a subhorizontal extension and creates strain compatibility problems. Consideration of the strain history of these rocks suggests that folding-related strains have been modified by later cleavage-related events, the last of which was uniaxial flattening accommodated by volume loss. The finite strains are reflective almost exclusively of the latest, cleavage producing strains.