|Fracture development and diagenesis of Torridon Group Applecross Formation, near An Teallach, NW Scotland: millennia of brittle deformation resilience?
Magdalena A. Ellis1,2, S. E. Laubach1, P. Eichhubl1, J. E. Olson3 and P. Hargrove1,4
1Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78713, USA
2ExxonMobil Exploration Company, Houston, TX 77098, USA
3Petroleum and Geosystems Engineering, University of Texas at Austin, Austin, TX 78713, USA
4BP America, Houston, TX 77079, USA
Abstract.The Late Proterozoic Torridon Group Applecross Formation in the foreland of the Moine Thrust Belt, NW Scotland, contains deformation bands, three fracture sets (from oldest to youngest A, B, and L) defined by orientation, crosscutting relations, and progressively less quartz cement in younger sets, and joints. Set A crosscuts deformation bands and strikes northsouth. Set B has trimodal orientation defining three linked subsets that formed concurrently. Set L strike ranges from NESW to ENEWSW, in parent crack wing crack arrays that formed progressively; these are more abundant near small-displacement, oblique-slip faults that offset the overlying Cambrian Eriboll Formation and the Moine Thrust Belt. Applecross sandstones have low fracture abundance, possibly a consequence of low elastic moduli (Young’s modulus 2.317.0 GPa, most values <6.9 GPa) and moderate to high subcritical crack index (4578), resulting from compacted soft lithic grains and clay-mineral cements. Low abundance contradicts models that postulate persistent incipient failure by subsurface fracture. The fracture sequence resembles that found in the overlying Cambrian Eriboll Formation quartzarenites, implying that no widespread late Proterozoic fracture sets exist in this part of the Applecross Formation, an uneventful record for a rock profoundly resistant to brittle deformation.