Fracture development and diagenesis of Torridon Group Applecross Formation, near An Teallach, NW Scotland: millennia of brittle deformation resilience?

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 north–south. Set B has trimodal orientation defining three linked subsets that formed concurrently. Set L strike ranges from NE–SW to ENE–WSW, 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 2.3–17.0 GPa, most values <6.9 GPa) and moderate to high subcritical crack index (45–78), 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.

Appraisal of fracture sampling methods and a new workflow to characterise heterogeneous fracture networks at outcrop

“…So, to properly represent relative abundance between the fracture sets, multiple scanlines at different orientations should be used. Many studies have adopted this method (McQuillan, 1973;Hanks et al, 1997;Wennberg et al, 2007;Ortega et al, 2010;Hooker et al, 2011;Barbier et al, 2012;Ellis et al, 2012;Iñigo et al, 2012) whereby a scanline is set up perpendicular to the strike of each fracture set on the outcrop. For each scanline, fracture attributes are measured for that fracture set only.…”

“…To reduce orientation bias for linear scanlines many authors (McQuillan, 1973;Hanks et al, 1997;Wennberg et al, 2007;Ortega et al, 2010;Hooker et al, 2011;Barbier et al, 2012;Ellis et al, 2012) use a technique where a separate scanline is set up perpendicular to each fracture set present. We tested this method against our proposed workflow (Fig.…”

Influence of structural position on fracture networks in the Torridon Group, Achnashellach fold and thrust belt, NW Scotland

“…Fractures lack any systematic termination relationships between different fracture sets, meaning it is very difficult to determine relative ages. The Torridon Group foreland contains multiple fracture sets that appear not to be related to Moine Thrust Belt structures but instead formed either before or after folding and thrusting (Ellis et al, 2012). Measured intensities of these fractures are very low (<10 m/m 2 ).…”

“…Fracture aperture is also though to correlate to fracture lengths; longer fractures tend to have wider apertures. This relationship is shown by Vermilye and Scholz (1995) who use many field locations across North America, and Ellis et al (2012) who use data from the Torridon Group of NW Scotland.…”

Quaternary evolution of glaciated gneiss terrains: pre-glacial weathering vs. glacial erosion

“…The Lewisian gneiss cooled below c. 400 ° at c. 1500 Ma (e.g. Cliff et al, 1998) and has been accumulating brittle fractures ever since (Beacom et al, 2001;Pless, 2011 (Trewin, 2002;Woodcock and Strachan, 2002;Ellis et al, 2012). The vast majority of fractures thus pre-date glaciation, but repeated glaciation and deglaciation may have further opened some fractures.…”