Formation of Nanocrystalline and Amorphous Materials Causes Parallel Brittle‐Viscous Flow of Crustal Rocks: Experiments on Quartz‐Feldspar Aggregates

Abstract

Relative motion of tectonic plates is accommodated along lithosphere-scale shear zones. The strength and stability of these shear zones control large-scale tectonics and the location of earthquakes (Bürgmann & Dresen, 2008;Molnar, 2020). Laboratory-derived strength profiles of the lithosphere postulate that the strength in the upper crust is controlled by frictional sliding along preexisting fractures, while the strength of the lower crust and upper mantle is controlled by viscous flow of rocks (Brace & Kohlstedt, 1980;Goetze & Evans, 1979;Kohlstedt et al., 1995). In this traditional view of the strength of the lithosphere, the transition from frictional sliding to viscous flow is abrupt and occurs at the intersection of Byerlee's rule with a dislocation creep flow law for a mineral of choice deforming at a constant strain rate. This sharp

Nanometric flow and earthquake instability

“…1a). Note that microcrystalline experiments develop patches of nanocrystalline material with increasing strain that interconnect at peak stress and allow for failure 24,30 . All experiments were conducted in a general shear geometry at temperatures, T = 200, 300 and 500 °C and confining pressures, P c = 500 MPa at a constant displacement rate of ~10 −3 mm s −1 , corresponding to a shear strain rate, _ γ; of ~10 −3 s −1 .…”

“…Fault weakening occurs due to the intrinsic low viscosity of the nanocrystalline fault rocks. Shearing of coarse-grained fault rocks locally leads to comminution and patchy production of nanocrystalline material 22,30 , the volume occupied by these nanocrystalline materials increases with increasing work and homologous temperature 24,45 , eventually reaching a percolation threshold. Although the extremely finegrained material is rate-strengthening, it has a much lower viscosity (Figs.…”

“…Localization in the high-strain domains is a prerequisite for numerous weakening mechanisms such as thermal pressurization 3,4 and shear heating 5,6 that may induce weakening and may lead to an earthquake instability 7 . Furthermore, the material where strain localizes becomes profoundly transformed; high local work input induced by localization promotes comminution 8 , metamorphic reactions [9][10][11] , microstructural transformations 12,13 , phase transitions 14,15 and melting [16][17][18] that can locally weaken the fault rocks over a broad range of pressure, temperature (P-T) and strain rate conditions and produce nanocrystalline to amorphous materials in nature [19][20][21] as well as in experiments 16,[22][23][24][25] .…”

“…The principal problem in determining the rheology of nanocrystalline fault rocks is in isolating their intrinsic properties from the mechanical signal which is dominated by coarser-grained material in low velocity, small-displacement experiments 22,24,26 . In high velocity, large-displacement experiments, temperature transients make it difficult to untangle the effect of heating from the effects of nanomaterial formation 23,[32][33][34][35] .…”

Locally Resolved Stress‐State in Samples During Experimental Deformation: Insights Into the Effect of Stress on Mineral Reactions

“…The modeled principal stresses, pressure, differential stress, and deformation rates also show heterogeneous patterns as observed in the experimentally produced phase and grain‐size distribution. In fact, in experimental studies, it is common to observe shear bands and zones of localized deformation not randomly distributed but rather following specific patterns (Holtzman et al., 2003; Marti et al., 2020; Pec & Al Nasser, 2021; Pec et al., 2016). In this work, we showed that the distribution of shear stress and strain is more pronounced along the short diagonal in the central part of the sample, that links the forcing‐block inner tips.…”