EVIDENCE FOR LIMITED VOLUME CHANGE DURING SLATY CLEAVAGE DEVELOPMENT IN THE TACONIC ACCRETIONARY WEDGE, NORTHEASTERN USA

Calculations of volume change in shear zones provide one means of understanding fluid flow in the crust, large-magnitude volume loss suggesting large fluid fluxes and constant-volume deformation suggesting closed-system behavior and diffusive mass transport. Although simple in theory, geometric techniques for the calculation of volume change are hampered by the scarcity of appropriate markers and the difficulty of separating the tectonic component from the total volume-change estimate. We have overcome these problems in the Taconic slate belt where strata contain both deformed graptolites and strain fringes around pyrite framboids and samples can be obtained from different structural positions around isoclinal folds.

On the basis of field relations and detailed microstructural observations, we have inferred the following sequence of events: (1) deposition of the graptolites; (2) dewatering of the strata as a result of (a) burial compaction and (b) tectonic consolidation; (3) flexural folding; and (4) slaty cleavage development during which time the graptolites underwent pyritization. The strain fringes formed during (4) and indicate top-to-the-west-northwest non-coaxial flow. The graptolites, which lie in the bedding plane, underwent changes in length during (2b) and (4).

Previously, we have shown that the amount of prelithification tectonic volume change can be estimated from samples where bedding and cleavage are parallel by removing the strain recorded by the strain fringes from that recorded by the graptolite thecal spacing. Here, we present new ideas about how the data sets can be combined to estimate the amount of volume change undergone by the strata during slaty cleavage development. The strain-fringe data are used to predict the length change undergone by graptolites in samples where bedding lies at an angle to cleavage. The predictions are compared to length changes determined by restoring the displaced pyrite blocks composing the graptolites and from graptolite thecal-spacing measurements. The results are inconsistent with large-magnitude volume loss and do not support large fluid fluxes through the Taconic accretionary wedge during slaty cleavage development.