INTERPRETING TECTONIC FABRICS FROM AMS AND NORMALIZED FRY MEASUREMENTS IN LOW-STRAIN SANDSTONE OF THE APPALACHIAN FOLD-THRUST BELT

Two methods are commonly used for characterizing grain-scale deformation in sandstone: the normalized Fry method which involves the visual measurement of center-to-center spacing between framework grains, and the AMS method which involves instrumental measurement of the anisotropy of magnetic susceptibility. In highly strained sandstone, both the long axis of the Fry-determined strain ellipse and the projection of the long axis of the AMS ellipsoid in the plane of bedding align well with regional structural trends as defined by cleavage, folds, and faults. This relationship implies that the orientation of the strain axes developed in response to tectonic shortening. Does such a relationship also hold for low-strain sandstone? To address this question, we measured Fry strain and AMS in low-strain sandstone from two localities in the Appalachian foreland fold-thrust belt: the Rosendale region of southeastern New York and the Lackawanna region of northeastern Pennsylvania. We found that the long axes of the AMS ellipsoids cluster closely (±8-17°) to the regional structural trends whereas the Fry axes show greater dispersion (±15-35°). This observation indicates that AMS is a more sensitive indicator than Fry analysis of grain-scale fabrics in low-strain sandstone. The sensitivity of the AMS method is likely due to the following: (1) AMS reflects the preferred orientation of matrix grains and the matrix strain may be greater than that of the bulk rock; (2) At low strains, Fry analysis is inaccurate because of primary irregularities in grain shapes; and (3) At the thin-section scale, the distribution of framework grains can be greatly influenced by primary depositional fabrics and/or by local particle interactions.