Hydrothermal Alteration-Induced Weakening in Experimentally Deformed Fault Gouges

In the granitoid crust, phyllosilicate-rich fault gouges are prevalent in mature fault zones undergoing hydrothermal alteration and often exhibit lower frictional strength compared to framework minerals (e.g., qtz, fds) under deformation at room temperature. However, the mechanical behavior and deformation mechanisms of altered gouges under hydrothermal conditions are not fully understood so far.

To investigate these effects, we conducted a series of experiments on three types of fault “gouge” material using a ring shear deformation apparatus. We used gouge mixtures obtained from (i) crushed granitoid ultramylonite, (ii) biotite- and (iii) muscovite-bearing gouges to represent quartzofeldspartic materials with (i) no alteration, (ii) high-temperature and (iii) low-temperature alteration, respectively (see Table 1 for the mineralogy). Deformation temperatures (T) ranged from 20-650°C, with a sliding velocity kept at 1 μm/s, and an imposed effective normal stress and pore fluid pressure at 100 MPa. At large shear strain (g ≈ 22-25) and T = 20-450°C, granitoid gouges consistently showed higher shear stresses (t = 73-81 MPa) than muscovite- (t = 47-69 MPa) and biotite-bearing gouges (t = 44-56 MPa). Granitoid gouges showed a decrease in t at T ≥ 450°C, while mica-rich gouges showed an increase in t with T at all tested conditions. Microstructurally, all gouges experienced strain localization into relatively fine-grained and dense principal slip zones (PSZs) at elevated T. The presence of newly percipitated minerals (e.g. bt, qtz) suggested the operation of dissolution-precipitation creep (DPC). However, the PSZs of granitoid and mica-rich gouges exhibited distinctive geometric features in their microstructure at 650°C. Granitoid gouges showed PSZs with ultrafine-grained (≤ 1 μm) relicts of porphyroclasts sparsely distributed within a dense matrix. In contrast, the PSZs of mica-rich gouges showed the anastomosing P-foliation of aligned micas with intervening shear band cleavages. Within these localized domains, quartz in mica-rich gouges exhibited larger grain sizes (1-4 μm) compared to those in granitoid gouges.  

Our observations indicate that in all tested gouges, frictional deformation gives way to grain-size sensitive creep mechanism as T rises, leading to the formation of fine-grained PSZs. We suggest that the ultrafine grain sizes in granitoid gouges promote DPC-accommodated viscous granular flow more efficiently, leading to the low shear stresses. In contrast, the strengthening of altered gouges with T was attributed to two factors: a less efficient DPC-assisted deformation due to generally larger grain sizes, and a less efficient viscous granular flow due to the development of foliation and shear bands inclined to the shear direction. Therefore, the mechanical behaviour of granitoids along the retrograde hydration-path depends not only on the evolving mineralogy, but also on microstructures and grain sizes.

Table 1. List of Samples Used in This Study and Their Mineralogy According to Quantitative XRD.

Sample	Composition (wt%)	Altreation type
Granitoid ultramylonite	37% qtz, 49% fds, 8% bt, 6% ep	No alteration
Biotite-bearing natural fault gouge	35% qtz, 4% fds, 37% phl, 21% mus, 3% smc	High-temperature
Muscovite-bearing natural fault gouge	39% qtz, 5% fds, 38% mus, 11% ser, 6% chl, 1% cal	Low-temperature

Qtz:quartz, fds: feldspar, bt: biotite, ep: epidote, phl: phlogopite, mus: muscovite, ser: sericite, smc: smectite, chl: chlorite, cal: calcite