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

Hardman, K.; Holdsworth, R.E.; Scott, L.; Dempsey, E.; McCaffrey, K.J.W.

Abstract

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 (c. 1.55 Ga; ‘Assyntian’) strike-slip faulting. Deformation is characterized by sinistral reactivation of pre-existing NW–SE-trending features including intrusive contacts of (c. 2.4 Ga) mafic dykes and Paleoproterozoic ductile shear zone fabrics (c. 1.75 Ga). Reactivation occurred at palaeodepths of 10–15 km, where frictional–viscous deformation synchronous with co-seismic frictional melting led to cycles of millimetre- to decimetre-scale cavity dilation and collapse. Although individual melt-generating slip surfaces may have become rapidly welded, faulting was able to repeatedly localize 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, localized 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.

Citation

Hardman, K., Holdsworth, R., Scott, L., Dempsey, E., & McCaffrey, K. (2023). Mid-crustal reactivation processes linked to frictional melting and deep void development during seismogenic slip: examples from the Lewisian Complex, NW Scotland. Journal of the Geological Society, 180(2), https://doi.org/10.1144/jgs2022-037

• Journal Article Type: Article
• Acceptance Date: Nov 19, 2022
• Online Publication Date: Feb 17, 2023
• Publication Date: 2023
• Deposit Date: Jul 17, 2023
• Publicly Available Date: Jul 17, 2023
• Journal: Journal of the Geological Society
• Print ISSN: 0016-7649
• Electronic ISSN: 2041-479X
• Publisher: The Geological Society
• Peer Reviewed: Peer Reviewed
• Volume: 180
• Issue: 2
• DOI: https://doi.org/10.1144/jgs2022-037