3D STRAIN FROM RUTILATED QUARTZ AND ITS RELATIONSHIP TO QUARTZ CRYSTALLOGRAPHIC PREFERRED ORIENTATIONS

Strain analysis provides a means to assess variations in deformation along the strike of a mountain belt. The extended or folded acicular inclusions of rutile that may occur in quartz can be used to quantify the three-dimensional strain within a rock. We have studied rutile inclusions within quartz from 10 samples from a 75 km strike-parallel transect from the Antietam Formation, an Early Cambrian orthoquartzite exposed in the Blue Ridge of central VA. The observed deformation is spatially heterogeneous, with along-strike variation in both the magnitude and geometry of deformation. Both prolate and oblate strain shapes are observed and the maximum stretching direction is variably aligned, from parallel to perpendicular to bedding. Maximum stretch values vary from 1.2 to 1.6. The 3D strain solutions are used to orient quartz crystallographic preferred orientation (CPO) data collected through electron backscatter diffraction. Usually, CPO results are interpreted with respect to the macroscopic foliation and lineation directions; however, these features are generally lacking in hand samples of the Antietam Formation. When aligned parallel to the principal directions determined from the strain analysis, the quartz CPO data show coaxial type 1 cross-girdles with small circles oriented around the maximum shortening direction. Opening angle thermometry suggests this strain occurred under relatively low temperatures (275-375°C).