Water distributes pervasively across the Earth's crusts. They often come from rain fall near the surface, or carried by plate subduction into deeper levels in the form of mineral dehydration (Ni et al., 2017). Numerous studies have demonstrated that water has a pronounced weakening effect on the crustal strength. Water can either increase pore pressure, lower the effective normal stress and the frictional sliding resistance, or promote the creep behavior through processes like dissolution precipitation creep (DPC) (Alaoui, PhD thesis, 2024). However, the extent to which water weakens the strength of crustal fault rocks remains uncertain.

To address this question, we conduct shear tests on (70 vol.%) quartz + (30 vol.%) phlogopite mixture in a solid medium tri-axial deformation apparatus (modified Griggs rig) following Alaoui, (2024). Deformation conditions are 800°C, strain rate ~1×10-5 s-1, confining pressure 10 kbar and at variable water contents. Water content is controlled across four conditions: (1) super-dried, (2) oven-dried, (3) moisture-exposed, and (4) water added. To achieve these conditions, the simulated fault gouge is pretreated as follows: (1) super-dried samples are slightly compressed overnight at 200°C in an unsealed assembly, (2) oven-dried samples are stored in a ~110°C oven, (3) moisture-exposed samples are kept at room temperature and ambient humidity (30-40%), and (4) water-added samples have 0.1 wt.% water introduced to the moisture-exposed samples before testing. At the end of tests, we will compare the differential stress obtained at large strain and micro(nano)structures. Depending on the results, additional tests maybe performed using the mica-rich fault gouges to investigate the role of mica in strain accommodation and water redistribution at grain boundaries.