Flattening fabrics in the northern Appalachian foreland fold-thrust belt? Unexpected results of finite strain and fabric analysis in the Shawangunk Formation; Hudson Valley, New York

Mapping in the Appalachian fold-thrust belt suggests that the Shawangunk Formation, a silica-cemented, quartz pebble conglomerate, strongly influenced the structural scale of regional folding and faulting. Exposures of the Shawangunk Fm near New Paltz and Rosendale, New York provide excellent opportunities to map macro-scale strain gradients in a wide range of geologic structures. However, to resolve the nature of grain-scale deformation, we conducted an analysis of penetrative, 3D finite strain fabrics within the unit. Interestingly, although the Shawangunk Fm is strongly deformed, our results suggest the unit accumulated only very low penetrative grain-scale strains. Additionally, the orientation of strain ellipsoids poorly correlates with the regional fold axes and cleavage trends, suggesting that strain is associated with deposition and compaction. Most importantly, our analysis documents an enigmatic flattening fabric that has not been previously reported in similar rocks within the Appalachian Mountains. Our presentation details a strain analysis of three mutually perpendicular 50 x 75 mm thin sections prepared from seven oriented samples. Outlines of grain boundaries were traced from photomicrographs of thin sections with the computer program ELLIPSEFIT. Moment equivalent ellipses were then calculated from these polygons. The benefits of the ellipse moment polygon fitting technique include allowing the use of irregular grains and significantly reducing operator bias. Strain ellipsoid axial ratios were calculated from the three section-ellipses for each sample and plotted on Flinn and Hossack-type graphs. Strain axial orientations were plotted on a spherical equal area projection using the program ORIENT to determine orientation of deformation