Brittle failure in a 3-D strain field: Gruinard Bay, Northwest Scotland

David Healy

Department of Earth Sciences, University of Liverpool, Liverpool L69 3GP, UK

dhealy@liverpool.ac.uk

Many previous studies of deformed conglomerates have focussed on plastic deformation of clasts and matrix. This poster describes fractured pebbles within Triassic conglomerates from the eastern margin of the Minch Basin at Gruinard Bay, Scotland. The clasts were derived locally and deposited in decimetre to metre thick sheets of conglomerate. The pebble lithologies comprise rare Lewisian gneiss and amphibolite, red and grey Torridonian sandstones, white quartzites from the Cambrian Eriboll Sandstone Formation and grey limestones from the Ordovician Durness Limestone Formation. Steeply dipping, closely spaced fractures are confined to the pebbles, and often filled with white carbonate cement. In most pebbles the strike distribution of the fractures is bimodal. The coarse sandy, carbonate cemented matrix is intact except for mm wide white carbonate veins and metre to decametre-scale normal faults. Fractures within pebbles very rarely trace out into the matrix. An initial interpretation was that the fractures were tensile (mode I) and probably hydrofractures in the presence of a carbonate saturated fluid. However, detailed analysis of many pebble fractures in the field and in thin section reveals shear offsets along most of the fractures, especially when measured in vertical sections. The curviplanar form of the fractures and lack of plumose markings on exposed surfaces are also inconsistent with a tensile hydrofracture origin. The steep dips of the fractures may suggest a hybrid tensile-shear fracture origin, but the exact status of this failure mode is questionable (Engelder, 1999). A simpler explanation, consistent with all of the field and thin section evidence, is that the pebble fractures are primary shear fractures (mode II) with a significant component of positive dilatation. Deformation bands measured in sandstone interbeds also exhibit positive dilatation, and show a quadrimodal pattern of orientations. Normal faults throughout this Triassic section show a similar quadrimodal pattern and the lack of fault rocks implies minimal cataclasis. The deformation recorded by the fractured pebbles can be characterised as (primary) shear fracture with positive dilatation in a three-dimensional strain field. The two-dimensional Andersonian model of faulting based on the Mohr-Coulomb failure criterion is insufficient to explain these findings.

Figure 1. Fractured and veined Torridonian sandstone cobble in Triassic conglomerate (cm scale). Laide jetty, Gruinard Bay.

Figure 2. Quadrimodal deformation bands in coarse Triassic sandstone. Lens cap for scale. Udrigle, Gruinard Bay.