Frictional strength of ground dolerite gouge at a wide range of slip rates

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We conducted a series of rotary-shear friction experiments on ground dolerite gouges, in which the amount of adsorbed moisture increases with grinding time (t[gr]), at room temperature and humidity, a normal stress of 2 MPa, and constant equivalent slip rates (V[eq]s) ranging from 20 µm/s to 1.3 m/s. Their frictional strength changed with V[eq] and t[gr] in three different ways depending on V[eq] and the gouge temperature (T). At V[eq] ≤ 1.3 cm/s, T did not exceed 80°C, and the steady state friction coefficient (μ[ss]) ranged from 0.59 to 0.80. μ[ss] changes little with V[eq], while μ[ss] at a given V[eq] systematically increases with t[gr] probably due to moisture-adsorbed strengthening of gouges. At V[eq] = 4 cm/s, T exceeded 100°C, and dehydration of gouges resulted in roughly the same μ[ss] values (0.60–0.66) among gouges with different periods of t[gr]. At V[eq] ≥ 13 cm/s, T reached 160–500°C, and μ[ss] dramatically decreases with V[eq] to 0.08–0.26 at V[eq] = 1.3 m/s, while μ[ss] at a given V[eq] systematically decreases with t[gr]. At these fast V[eq]s, dehydration of gouges likely occurred too fast for water vapor to completely escape out from the gouge layer. Therefore, faster dehydration at faster V[eq] possibly resulted in a larger pore pressure increase and lower frictional strength. In addition, because gouges with longer periods of t[gr] contain larger amounts of adsorbed moisture, they became weaker due to larger increases in pore pressure and hence larger amounts of reduction in frictional strength.

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