Natural seismogenic pumping processes in near surface fractured basement gneisses

The fluid transport and storage properties of fractured crystalline rocks are relevant to understanding water and hydrocarbon resources and potential containment of radioactive materials and carbon dioxide. Crystalline lithologies have virtually non-existent primary porosity therefore any porosity and permeability is secondary, originating from fractures and/or surface weathering. Here, we report upon a study on the kinematics, geometry and topology of naturally formed fractures in the Lewisian Gneiss Complex (LGC) of NW Scotland. The fracture systems show good evidence for near surface seismogenic faulting, including the local generation of thin psuedotachylytes, reactivation and pulsed fluid transport that was likely to be epsiodic in nature. The evidence points toward the presence of short term high flux and long term persistent fluid transport pathways.

We studied exposures within and adjacent to the Canisp shear zone exposed on the coast near Achmelvich Bay, NW Scotland. At Alltan Na Bradhan, regionally recognised Neoproterozoic Stoer Group-age fractures associated with rifting are particularly well developed as haematite-stained faults, with many containing iron-stained breccias, cataclasites and epidote-mineralised ultra-cataclasites. Clastic material is found locally within these fractures consisting of fine red mudstone and sandstone of Stoer Group age. The Stoer Group-aged faults partly reactivated pre-existing foliation-parallel faults that locally carry pseudotachylytes. These so-called Late Laxfordian structures were extensively developed in the nearby Canisp shear zone and are themselves a brittle reactivation of earlier formed ductile shear zone fabrics. The Stoer Group age faults are mainly N-S or NW-SE trending (3 main sets) and display oblique/down dip normal-sense offsets up to a metre. 1D fault intensities show power law scaling with typically 10 structures per metre for 1mm apertures. In 2D, structures are highly connected in both map and section views. Fault rocks are chaotic breccias with local sediment fills and vuggy quartz-calcite-epidote-prenhite mineralised voids.

We interpret the Stoer Group age faults to be near-surface rift-related structures that cross cut and partially reactivate pre-existing faults. Importantly, we suggest that some of the structures were periodically highly dilational. Seismogenic slip along reactivated foliation-parallel faults led to instantaneous fracture dilation in fracture sets cross-cutting the foliation at high angles, leading to fracture wall implosion forming chaotic breccias up to 1 m across. The dilation likely sucked in surface water and sedimentary material from the surface a few tens to hundered of metres above. Elastic rebound triggered partial collapse of breccia-, sediment- and fluid-filled voids. The collpsed breccia fills then acted as natural props holding open the fractures systems thereby facilitating longer term fluid flow in otherwise impermeable basement.