ESTIMATION OF VISCOSITY RATIOS OF CLASTS WITHIN THE DEFORMED POLYMICTIC SILURIAN UTSLETTEFJELL FORMATION, STORD, NORWEGIAN CALEDONIDES

One goal of structural geology is to determine crustal flow laws. Experimental studies are commonly used, however they are constrained by laboratory conditions such as strain rates. Field data may allow the extraction of additional information. Deformed conglomerates have been used to estimate viscosity ratios of clasts compared to the whole rock. Conglomerates of the Silurian Utslettefjell Fm, Stord, contain 8 lithologies within a feldspathic greywacke matrix. This area within the Norwegian Caledonides has experienced post-orogenic extension. Techniques include two ellipsoid calculation (Shan and Robin), and 3 bulk strain estimation methods: lithology-based, area-weighted, and normalized Fry. Oriented photographs were taken of 11 outcrop faces. Photographs were rectified to minimize lens distortion, clasts were traced using Inkscape, and strain analyses done using EllipseFit. A moment equivalent ellipse was generated for each shape and a mean ellipse determined from the eigenvectors of the mean shape matrix. The mean ellipse for each clast type in each section was used to calculate best-fit ellipsoids. Error analysis was performed using a Monte Carlo method with section data. Selection of 7 faces at higher angles to one another was required to avoid degenerate cases. Resulting ellipsoids have orientations close to one another. All are prolate, except for gabbro, with a moderately NE plunging principal strain axis and varying strain magnitude. The Robin method results in a slightly less prolate, slightly lower strain magnitude shape. Three methods were used to estimate bulk strain. The first used strain experienced by sandstone clasts due to their abundance and similarity in composition to the matrix. The second weighted the 2D shape matrix for each clast type based on area fraction and summed the weighted matrices giving a minimum estimate of bulk strain. Due to high residuals, the normalized Fry analysis was not deemed valid. Viscosity ratios for each clast type were calculated based on a logarithmic relationship between the axial ratios of the clast strain ellipsoid, and bulk strain, with a viscosity hierarchy of: epidosite > quartzite > jasper > granitoid > rhyolite > gabbro > sandstone > greenstone. Maximum elongation is consistent with lineations interpreted as due to orogenic shortening, rather than extension, however kinematic data is required to confirm this.