A COMPARISON OF RF/PHI AND AMS FABRICS WITHIN STRATA INVOLVED IN THE VARISCAN FOLD-THRUST BELT; WESTERN IRISH NAMURIAN BASIN, COUNTY CLAIRE, IRELAND

Analysis and comparison penetrative R_f/Phi and anisotropy of magnetic susceptibility (AMS) fabrics in rocks deformed in fold-thrust belt deformation in western Ireland suggests that despite being involved in map-scale structures, R_f/Phi fabrics are generally consistent with deposition and compaction whereas AMS fabrics may provide a better proxy for tectonic shortening directions. Deformation in the Western Irish Namurian Basin (WINB) involves the thick sequences of siltstone and fine-grained sandstone of the Shannon and Central Clare Groups. Between the Loop Head Peninsula and the Cliffs of Moher, these strata are involved in an East-West trending array of nine first-order, kilomerter-scale thrust-related folds. R_f/Phi and AMS fabrics were measured in five oriented samples collected from the Ross, Tullig, and Kilkee Formations. The results of 3D R_f/Phi analysis conducted using the EllipseFit (Vollmer, 2017) computer program indicate that grain-based fabrics are very weak (R 1.019 - 1.14) and predominatly oblate (k 0.19 - 0.71). Only one sample yielded a prolate fabric (k 1.21). X axes orientations in most of the R_f/Phi fabrics trend nearly perpendicular to the ENE structural grain defined by the trends of thrust fault-related structures. Analysis of the mean coercivities and dispersions in these samples suggests the magnetic mineralogy includes magnetite, hematite, and goethite. Magnetite ranges from pseudo-single (Mr/Ms 0.1 - 0.5, Hr/Hc 2 - 4) to multi-domain (Mr/Ms values << 0.01, Hr/Hc > 10). AMS analysis measured strongly oblate in all samples except the prolate Rf/Phi sample. The corrected degree of anisotropy (P_j) is low and varies from 1-1.06. With bedding restored to horizontal, the long (K1) and intermediate (K2) axes of the AMS ellipsoids are subhorizontal and the short axes (K3) are subvertical. Interestingly, the K1 axes orientations measured at two of the sample locations do appear to loosely correlate with the ENE structural grain, which suggests that the AMS fabrics may reflect some component of layer-parallel shortening. In this case, AMS fabrics may be a more sensitive proxy for tectonic fabrics than those measured using traditional methods of grain-scale deformation.