Influence of normal stress and temperature on water content of (smectite) clays and possible implications for frictional strength of fault gouge
Jong, S.M. de
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The slip behavior of large shallow crustal faults depends mainly on the mechanical properties of fault gouge, which generally includes a number of clay phases. The frictional properties of these clay minerals indicate that they can be very weak if water is present, with friction coefficients in the order of 0.2 (e.g. for smectite, chrysotile and talc). Whereas a weakening effect is evident, going from dry to wet conditions, the mechanism by which this occurs is not. In particular, the role of smectite clay, which is able to swell due to adsorption of water in the interlayer region, in promoting fault weakness is not fully understood yet. To attack this problem, new laboratory data are reported on the in-situ temperature- (20-180ºC) and normal stress-induced (to 41 MPa) dehydration behaviour of two well-known montmorillonites (Ca-SAz-1 and Na-SWy-1). In this way the differential stress threshold for the onset of interlayer water expulsion is investigated. We also analyze the interaction of surface adsorbed water molecules with the mineral surface for the non-swelling phyllosilicates: muscovite, kaolinite and pyrophyllite. The results of the present study are twofold. First, it is found that for our specific conditions (room relative humidity and temperature), a differential stress of 41MPa does not dehydrate the interlayer region of either of the montmorillonites studied. This state of stress approaches some of the thermodynamic calculations made to predict the stress threshold for the onset of dehydration. Second, we show explicitly the existence of strongly adsorbed water molecules, attached to the mineral surface, of clay minerals that cannot swell. This observation is consistent with a crustal weakening model that describes ongoing fault weakness as the result of growth of an interconnected network of weak clay minerals, characterized by surface adsorbed water films where the shear is located. Even at higher pressure and temperature, where capillary condensed water is driven-off, clay minerals can continue to be weak as a result of these more firmly attached surface adsorbed water films. The particular weakening effect of montmorillonite appears to have a direct and indirect basis. The direct water-assisted weakening mechanism is analogous to that seen in all charged (hydrophilic) sheet structured minerals, as a result of shear facilitated along surface adsorbed water films. The indirect water-assisted weakening mechanism follows from the possible expulsion of interlayer water, which results in an increase of the pore fluid pressure. We emphasize that to acquire reliable friction data on smectites, it is essential to use water-saturated or thoroughly dried (T>140ºC) conditions.