ABSOLUTE FINITE STRAINS FROM RECONSTRUCTIONS OF FRACTURED PYRITIZED GRAPTOLITES AND VOLUME CHANGE IN THE TACONIC SLATE BELT, EASTERN NEW YORK

The interpretation of volume change values obtained using geometric techniques requires an understanding of deformation history. Three different geometric markers in black slate of the Mount Merino Formation in the Taconic slate belt can be analyzed to determine absolute finite strains for volume change analysis: (1) graptolite thecal spacing; (2) strain fringes around pyrite framboids; and (3) fractured pyritized graptolites. Previously, we have shown that the strain fringes record slaty cleavage formation, and that discrepancies between graptolite thecal spacing and strain fringe results can be explained by shortening of the graptolites during prelithification tectonic consolidation. Large volume losses calculated from graptolite thecal spacing, therefore, include fluid loss from porosity reduction. Here, we show how reconstructions of the fractured pyritized graptolites can be used to determine volume change undergone by the strata solely during slaty cleavage formation.

Graptolites were analyzed from three different structural positions around an isoclinal syncline. Where bedding and cleavage are nearly parallel, pyrite blocks forming the graptolites exhibit chocolate tablet boudinage. Where bedding and cleavage lie at very high angles, the pyrite blocks have rotated and imbricated to allow shortening of the graptolites. Restoration of the displaced pyrite blocks to their original positions yields values for extension along bedding, from which the three principal strains are calculated. Cleavage does not wrap around the graptolites, indicating that the graptolites record the same amount of strain as the matrix.

Preliminary analyses of the pyritized graptolites give absolute changes in length that are lower than those given by the strain fringes and volume change values that are inconsistent with large magnitude volume loss. The discrepancy in strain values implies that pyritization of the graptolites occurred after the onset of slaty cleavage formation. The volume change values, therefore, are consistent with (1) large magnitude volume loss if the volume loss occurred early in slaty cleavage formation or (2) limited to no volume change throughout slaty cleavage formation. We favor the latter because there is no apparent causal link between pyritization and a change in deformation process.