Nanomechanics of hydrous, seismogenic crustal faults : Microphysical changes and phase transitions

Abstract

Earthquakes can affect rocks over large scales, ranging from kilometres down to nanometres. Fault damage often localises within a narrow zone of a few millimetres, the fault core, and studies published during recent years suggest that the micro-, and nanoscale processes within these zones may even control and dictate macroscopic earthquake mechanics. Such zones of localised deformation can form ultra-polished fault surfaces, termed fault mirror surfaces, that were proposed to present nanogranular surface coatings resulting from seismic events. To address the hypothesis of nanomaterials controlling fault behaviour throughout the seismic cycle, the present research aims to provide a comprehensive investigation into processes related to deformation on the micro-, and nanoscale inside the fault core directly on, or below the principal slip surfaces. To achieve this, we conducted state-of-the-art, multi-scale electron microscopy, mass and vibrational spectroscopy, and stable-isotope geochemistry, to unravel micro-, and nanoscale deformation and transformation processes during coseismic fault gouge deformation. With the present work, we focused on three natural fault mirror surfaces and one carbonate fault-gouge deformation experiment. We also investigated the impact of mineral reactions on rock deformation under hydrous conditions. Decarbonation below the excepted decarbonation temperature is the critical factor to drive co-, and post-deformational fluid-rock interactions in hydrous carbonate fault zones. Coseismic, hydrous deformation of carbonates proceeds through a fast sequence of fluid-mediated exchange reactions which leaves us with the conclusion that hydrous (natural) carbonate deformation in fault cores under upper-crustal conditions cannot be described by only solid-state deformation mechanisms. In addition to the physico-chemical interactions of decarbonation products and fluids, dislocation creep appears to contribute to cataclastic processes in fault damage zones also at lower temperatures than previously estimated for the activity range of laboratory observed slip systems. Crystal-slip system analyses suggest that fluids may influence the slip systems activated during deformation and that temperature estimates for such slip systems cannot be extrapolated from low to high strain rates without restrictions. Cyclic repetition of deformation and annealing presents an alternative mechanism to fracturing of nanostructure formation for fault rocks which ultimately appears to strengthen the material close to the principal slip surface. Finally, we propose that the syn-, and post-deformational fluid-rock interactions have a strong influence on fault slip and that such interactions with fluids are the governing factors that will control fault rheology in the systems studied here. The collective evidence of this thesis suggests that the physico-chemical interactions of carbonate-deformation products with fluids have a stronger impact on fault-gouge deformation and fault rheology then previously considered.