STRAIN PATTERNS IN THE TACONIC ALLOCHTHON OF THE NORTHERN APPALACHIANS AND THEIR BEARING ON DEFORMATION PARTITIONING IN LOW-GRADE METAMORPHIC ROCKS

The slate belt of the northern Taconic allochthon consists of a relatively homogeneous sequence of fine-grained siliciclastic rocks that were deformed in a retroarc setting during the last stage of the Early to Late Ordovician Taconic orogeny. We have mapped the strain over an along-strike length of about 60 km and have identified three structural domains. In the northern and southern domains, the map-view projection of the long axis of the strain ellipsoids is uniformly orthogonal to the structural grain. In contrast, in the central domain, the map-view projection of the long axis of the strain ellipsoids lies at a low to moderate angle to the structural grain. This geometry results from a change in both the orientation of the structural grain and the long axis of the strain ellipsoids. Within each structural domain, the strain magnitude is remarkably homogeneous. Complex three-dimensional deformation is commonly partitioned into regions of approximately strike-slip and dip-slip and/or simple-shear-dominated and pure-shear-dominated deformation. While the northern and southern domains of the Taconic slate belt are described by two-dimensional deformation that approximates simple shear, the obliquity between the structural grain and the strain in the central domain and the homogeneity of the strain imply three-dimensional deformation that has not been partitioned. The two-dimensional deformation in the northern and southern domains may therefore not be an expression of deformation partitioning over a region larger than the slate belt. This lack of deformation partitioning may be a consequence of the relatively homogeneous nature of the stratigraphy and the relatively low grade of the metamorphism. Regions like the Taconic slate belt that have escaped deformation partitioning may prove useful for estimating the orientation of the relative plate convergence vector for ancient orogens.