FRACTURE ARRAYS IN A SUBTHRUST SETTING: CAMBRIAN ERIBOLL GROUP SANDSTONES, NW SCOTLAND

Cambrian Eriboll Group Sandstones beneath the Moine Thrust Zone (MTZ), NW Scotland, contain arrays of opening-mode fractures that range in size from microfractures having lengths of microns to macrofractures having trace lengths of more than 100 m. Fractures can be divided into at least three regionally extensive sets based on crosscutting relations. From oldest to youngest these sets strike N to NNE, WNW, and WNW to NE. Microfractures and associated macroscopic opening-mode fractures (joints or veins) are sealed or locally lined with authigenic quartz that crosscutting relations and crack-seal texture suggests is in part contemporaneous with pore-filling quartz cement in the rock mass. Cumulative apertures along a line of observation record strains of as much as 4.9 percent for the oldest, N to NNE-striking fracture sets. Both small and large fractures are inclined normal to ESE-dipping beds. These fracture orientations with respect to tilted beds and local overprint of opening-mode fracture sets by faults associated with the MTZ, shows that N to NNE-striking fractures likely formed prior to tilting of the Cambro-Ordovician, possibly before emplacement of the MTZ. Based on increased abundance near the fault zone, some WNW-striking fractures may be associated with WNW emplacement of the MTZ. Some WNW- to ENE-striking fractures, which are youngest based on crosscutting relations, are locally bridged by quartz containing crack-seal texture but otherwise retain porosity in fractures having apertures >0.1 mm. Residual porosity in fractures implies that after they formed fractures cooled to less than a quartz accumulation threshold of about 80ºC. Preliminary diagenetic analysis of these patterns suggests that they formed in the Mesozoic or later and some of these open fractures are localized near probable Mesozoic or Tertiary WNW- to NE-striking normal faults.