Frictional Properties of Simulated Chlorite Gouge at Hydrothermal Conditions: Implications for Subduction Megathrusts

Abstract

Chlorite is abundant at hypocentral depths in subduction zones and is likely to play a key role in controlling megathrust slow slip and catastrophic rupture. However, no data exist on the frictional properties of chlorite(-rich) fault rocks under the hydrothermal conditions relevant for the subduction seismogenic zone. We report results from experiments conducted under such conditions, using chlorite powders prepared from single crystal clinochlore (Mg-chlorite), as well as limited experiments using a stack of single crystal sheets. Shear experiments were carried out at effective normal stresses (σ n ) of 100 to 400 MPa, pore fluid pressures (P f ) of 50 to 220 MPa, and at temperatures (T) of 22 to 600°C, using stepped displacement rates (v) from 0.3 to 100 μm/s. The gouges are characterized by a coefficient of friction (μ) of 0.2-0.3 at T ≤ 400°C and 0.3-0.4 at 500-600°C, while (a-b) values showed positive values for nearly all conditions tested, except at 300°C. Microstructures of gouges sheared at T ≤ 300°C show evidence for widespread comminution, compared with a lower porosity at 600°C. Experiments using a stack of single crystal sheets showed μ ≤ 0.008 at low displacements (<3 mm) followed by hardening, while microstructures are suggestive of slip along (001), folding and tear of cleavage planes, and gouge production. Our results have important implications for the mechanisms controlling megathrust fault slip under greenschist facies conditions in a subduction zone and shed new light on the strain accommodation mechanisms within sheared gouges versus single crystals composed of phyllosilicates.

Fault Instability in a Geothermal Reservoir Facilitated by Low-grade Metamorphism

“…Conversely, chlorite gouge is frictionally weak (µ ~ 0.35) and promotes strong velocity strengthening. These frictional characteristics are generally consistent with previous results and highlight the role played by mineral structure in fault friction (Bos & Spiers, 2002;Okamoto et al, 2019;Tesei et al, 2012) ( Figure S12).…”

Low‐Temperature Frictional Characteristics of Chlorite‐Epidote‐Amphibole Assemblages: Implications for Strength and Seismic Style of Retrograde Fault Zones

“…The conclusion that chlorite is associated with low friction and velocity strengthening may also apply to deeper conditions. Pure chlorite is found to be weak and slide stably under almost all laboratory conditions up to 400 MPa effective stress and 600 °C (Okamoto et al, 2019). This suggests that the effect of chlorite should be to weaken and stabilize faults from greenschist conditions all the way to the surface.…”

Hydrous oceanic crust hosts megathrust creep at low shear stresses

“…In NMR amphibolite, slip along the phyllosilicate foliae is likely to be an important deformation mechanism, given the contrast between strong albite and the weak phyllosilicate minerals (9,13). The CPO of muscovite and chlorite (Fig.…”

“…Taking the frictional strengths of chlorite (13) and gabbro (9) as lower and upper bounds on the frictional strength of the oceanic crust, and assuming a thermal gradient of 20°C/km and a nominal pore fluid factor of  = 0.8, the frictional-viscous transition for hydrated oceanic crust is expected to occur in the range of 370° to 450°C at shear stresses between 60 MPa and 25 MPa (Fig. 6, B and C).…”

“…(B) The piezometer and geothermometer estimates define a strength-temperature curve for hydrous oceanic crust, which is far weaker than dry diabase (9). Coulomb strengths with effective friction  eff = 0.12 and  eff = 0.04 represent the cohesion-less strength of unaltered oceanic crust (9) and chlorite (13), respectively (assuming a pore fluid factor  = 0.8). The sketched strength curve suggests a frictional-viscous transition near ~400°C.…”