COMPARISON OF AMS AND STRAIN-ANALYSIS RESULTS: RESOLVING SHORTENING DIRECTIONS IN LOW-STRAIN ROCKS OF A FOLD-THRUST BELT

Analysis of grain-scale deformation in competent structural lithic units within the sub-metamorphic portions of fold-thrust belts presents a challenge, because these units tend to develop only slight penetrative strains. Our study of the Late Silurian Binnewater Sandstone (quartz wacke) in the Appalachian fold-thrust belt of the central Hudson Valley, New York State, illustrates this problem. Although the unit is folded and incorporated in thrust sheets, normalized Fry strain analysis indicates that it accumulated only very low strains (generally < 3% shortening; max. 8%). Significantly, our results also show that the long axes of Fry strain ellipses in the plane of bedding do not consistently parallel the regional structural grain defined by the trends of fold axes and cleavage in the fold-thrust belt. 3-D strain analysis using the Rf/phi technique yields comparable results to the Fry analysis. Apparent strain intensities quantified by our Fry or Rf/phi analyses likely reflect a composite of at least two unrelated petrofabrics—depositional/compactional and tectonic. Samples with very low apparent strain intensities have relatively random fabric orientations, suggesting that these results are strongly influenced by primary and diagenetic fabrics, viscosity contrasts between markers and matrix, particle interactions, and/or changes in marker volume. Fabrics that roughly parallel the regional structural trends may reflect an amplification of the composite petrofabric where depositional/compactional and tectonic fabrics have similar trends. Analysis of the anisotropy of magnetic susceptibility (AMS) in the same unit yields different results from Fry or Rf/phi methods. Specifically, AMS measurements statistically define magnetic fabric ellipsoids (principal axes are K1 / K2 / K3; foliation is the K1-K2 plane) oriented such that the long axis of the ellipsoid closely parallels the regional structural grain of the fold-thrust belt and is perpendicular to shortening direction, even at localities where long axes of Fry ellipses or Rf/phi ellipsoids do not. Our results indicate that AMS is a better tool for resolving tectonic-fabric orientations in the Binnewater Sandstone, because the micro-scale fabric of the matrix may be a better passive strain marker than the distribution of detrital grains.