Mid-crustal reactivation processes linked to frictional melting and deep void development during seismogenic slip: examples from the Lewisian Complex, NW Scotland

Exhumed examples of ancient fault voids formed during seismic slip at depths > 10 km are well preserved in the Assynt Terrane of the Lewisian Complex, NW Scotland. They are interpreted to have formed during regional Mesoproterozoic (ca 1.55 Ga; ‘Assyntian’) strike-slip faulting. Deformation is characterised by sinistral reactivation of pre-existing NW-SE trending features including intrusive contacts of (ca 2.4 Ga) mafic dykes and Paleoproterozoic ductile shear zone fabrics (ca 1.75 Ga). Reactivation occurred at palaeodepths of 10-15km, where frictional-viscous deformation synchronous with co-seismic frictional melting led to cycles of mm to dm-scale cavity dilation and collapse. Although individual melt-generating slip surfaces may have become rapidly welded, faulting was able to repeatedly localise along adjacent pre-existing planar anisotropies favourably oriented for slip leading to the creation of a mesh of foliation-parallel melt generation surfaces linked by foliation-perpendicular dilational voids. The latter features are filled by chaotic clast-supported wall rock collapse breccias, localised injected frictional melts and hydrothermal mineralization. The fills act as natural props, holding cavities open and preserving them as long-term, pipe-like fluid flow conduits. These exhumed features are likely to be typical of multi-rupture seismogenic fault systems formed by direct reactivation of pre-existing basement structures