Linear Viscous Flow of Nanocrystalline Fault Rocks at 300 – 500˚C

TitleLinear Viscous Flow of Nanocrystalline Fault Rocks at 300 – 500˚C
Publication TypeConference Proceedings
Year of Conference2019
AuthorsSun, H, Peç, M
Conference NameAGU Fall Meeting
PublisherAmerican Geophysical Union
Conference LocationSan Francisco, CA

Fault slip under a broad range of conditions is accommodated in a thin volume of nano-crystalline to amorphous materials derived from the wall rock. These materials contribute to the strength and stability of a fault, however their rheological properties are only poorly constrained. To prepare nano-crystalline fault rocks, we ball milled Verzasca Gneiss for ~15 minutes. The initial grain size of all minerals was ~100nm after milling. This powder was sheared at temperature, T = 200, 300 and 500˚C, confining pressure, Pc = 500MPa, shear strain rate, γ ̇ ≈ 10-3s-1, to finite shear strains, γ = 0 − 5. Steady state flow is achieved at 300 and 500˚C, while the rocks strain harden at 200˚C. The nano-crystalline fault rocks are an order of magnitude weaker than their micro-crystalline counterparts and exhibit a linear stress – strain rate relationship with a stress exponent, n ≈ 1 at 300 and 500˚C. Strain localizes along R and R’ shear bands delimiting regions with uniform optical birefringence. Microprobe measurements reveal a homogeneous chemical composition comparable to rhyolite, appropriate for the mixing of all minerals present in the granitoid rock. Raman spectra of the fault rock material show broadening of main fused silica bands (1150cm-1 − 1650cm-1), diminishing of quartz peak at 467cm-1 and crystalline peaks of other minerals with increasing temperature. TEM observations document that the fault rock is formed of intimately intermixed ~40 nm equi-axed grains of silicates and elongated grains of micas with very low porosity at 500˚C. Our results document that nano-crystalline fault rocks formed by comminution can cause extreme strength loss at low temperature conditions if an kinematically viable failure plane can form. Diffusion creep can be an important mechanism accompanying cataclastic flow even at low temperatures provided the diffusion distances are short.