Synopsis

A field and petrographic study of Lewisian- and Torridonian-hosted clastic dykes near Gairloch, NW Scotland indicates that these generally trend NE and align in an up to 1 km wide NE trending fracture zone extending over a distance of approximately 10 km. Clastic dykes are not pure opening mode fractures, but possess a component of horizontal shear displacement (strike-slip) as well. Margins of dykes are sharp and unaltered and clasts of lithified, host sandstones are found within those dykes hosted by the Diabaig and Applecross Formations sandstones. This suggests that fracturing and clastic filling took place when a considerable amount of Torridonian sediment was already deposited and lithified and a source from the upper Torridon Group (Aultbea and/or Cailleach Head Formations) is inferred. We suggest that fracturing within the Lewisian basement and lithified lower Torridon Group (Diabaig and Applecross Formations) sediments took place under the influence of an extensional strike-slip regime causing forceful downward intrusion of water and sediment due to: (a) suction of water from the Torridonian sediments into the fractures created in the underlying, dry basement, and (b) loading of unlithified sediments on top of fractures in the lithified lower Torridon Group (Diabaig and Applecross Formations) strata.

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