PRELIMINARY FINITE STRAIN AND FABRIC ANALYSIS OF CONGLOMERATE FROM THE SHAWANGUNK FORMATION; NORTHERN APPALACHIAN FOLD-THRUST BELT, HUDSON VALLEY, NEW YORK
To determine feasibility of a regional study to resolve low magnitude strains from primary fabrics in the Shawangunk Fm, three mutually perpendicular 50 x 75mm thin sections were prepared from seven oriented samples. The new computer program ELLIPSEFIT was used to digitize a series of polygons from grain outlines in photomicrographs of the thin sections. ELLIPSEFIT then calculated moment-equivalent ellipses from these polygons. The benefits of the ellipse-moment polygon fitting technique include allowing the use of irregular grains and significantly reducing operator bias. A hyperbolic vector mean calculated from the ellipses with a bootstrap error estimate was then used to un-strain the data, which were contoured on polar hyperboloidal projections to examine for fabric modes. 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. Although involved in fold-thrust deformation, preliminary results indicate the Shawangunk Fm accumulated only very low strains and that the long axes of strain ellipsoids do not consistently parallel the regional structural grain defined by the trends of fold axes and cleavage.