In the context of lithosphere mechanics, the strength limits are bounded by two end-members. In one end-member model, the crust is strong and fails at high differential stresses (hundreds of MPa) consistent with the classical Christmas tree envelope and static friction governed by Byerlee's law. In the other end-member model, the crust is weak and fails at low differential stresses (tens of MPa) consistent with stress magnitudes that may result from topographic loading and tectonic forces. How significant are these end-member scenarios and how do they affect our perception of lithosphere dynamics on time scales ranging from a single earthquake to long-term processes such as orogeny? Can the end-members be reconciled or are they mutually exclusive? Do they reflect differences between continental interiors and plate margins or tectonically inactive and active regions?
Geomechanics is focused on providing the most accurate estimate of the present-day stress state, or quantifying criticality in the context of subsurface use. More complex questions can be addressed with numerical models. But how can the uncertainties of model parameters (material properties and structures) and calibration data (stress magnitudes) be quantified?
To address these fundamental questions, we invite contributions from observational, experimental, theoretical, and numerical studies that improve our understanding of the crustal stress state or expand the methodological repertoire. Highly appreciated are presentations about new methods and on strategies to reduce the uncertainties.
Fault gouges of granitoid composition represent the principal non-cohesive tectonites within fault zones in the continental crust. The spatial distribution and strength of granitoid fault gouges is therefore crucial for understanding how weak the upper continental crust could be due to the formation of fault zones. Although several laboratory investigations reported the mechanical weakening of granitoid gouges in shear experiments, the deformation mechanism responsible for such behavior remains not well understood.
To address this issue, we conducted two series of shear experiments on granitoid gouges by using a ring shear apparatus. The starting gouge powders were derived from crushed granitoid mylonite with a median grain size of 45 μm. In a first set of experiments, gouges were sheared at a sliding velocity of 100 μm/s for a displacement of 15 mm. Temperatures explored ranged from 20°C to 650°C in order to determine the temperature dependence of gouge strength. The second set of experiments is identical to the first ones, except that the applied sliding velocity was set at 1 μm/s to study how fault slip rate influences the strength of gouges.
We observe that differences in gouges strengths as a function of sliding velocity and temperature: At a sliding velocity of 100 μm/s, the steady-state shear stress (τ) remains relatively constant at τ=76-82 MPa over the entire temperature range. Contrastingly, at a sliding velocity of 1 μm/s the steady-state shear stress remains temperature-insensitive with τ≈75 MPa up to tempertures of 450°C, but decreases then to τ≈50 MPa at 650°C (Fig.1 a). Furthermore, the amount of decrease of shear stress is strain dependent (Fig.1 b). At even slower sliding velocity of 0.1 µm/s, the shear stresses decrease further to τ≈38 MPa.
Microstructurally, all gouges deformed at T≦450°C show typical cataclastic features, where angular clasts with grain size of ~10 μm are surrounded by a fine-grained matrix. Intergranular fracture arrays in Riedel- and Y-shears are well developed over the entire cross section, indicating homogeneous bulk deformation. In contrast, gouges sheared at 650°C with τ≈50 MPa show strain localization in a principal slip zone. It is shear plane parallel with widths up to ~50 µm. Inside the principal slip zone, all grains are dramaticly reduced to nm-size and tightly packed. No intergranular fracture arrays are observed. Outside the principal slip zone, rounded grains with size of ~5 μm are loosely packed, with meniscus cement growing in between. The aforementioned strain localization is enhanced at temperature above 450°C and slip rate below 1μm/s, suggesting that viscous creep mechanisms (e.g. pressure solution) control the deformation process at slow sliding velocities, which is not the case in fast rate experiments. Our results show that the activation of viscous creep mechanisms leads to significant fault zone weakening, while contrasts in grain size keep deformation localized.
Figure 1 Shear stress plotted as a function of temperature. Shear stress data collected at (a) 15mm displacement in steady-state, and at (b) 5mm displacement.
How to cite: Zhan, W., Nevskaya, N., Niemeijer, A., Berger, A., Spiers, C., and Herwegh, M.: Weakening of granitoid gouge in hydrothermal ring shear experiments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6615, https://doi.org/10.5194/egusphere-egu23-6615, 2023.
