High-pressure fluids generated by metamorphic dehydration reactions can have profound effects on both rock strength and the stability of faults. A decrease in effective pressure so produced could induce earthquakes at intermediate depth in the subducting slab, either by embrittlement of an intact rock mass or by the reactivation of preexisting faults. The purpose of this study was to experimentally investigate both processes using gypsum as an analog material. Constant strain rate, relaxation, and friction tests were performed on both intact cylindrical samples and saw cut aggregates. Both room temperature and dehydration conditions were investigated for comparison. The maximum confining pressure and temperature were 50 MPa and 150 degrees C, respectively and the strain rate was varied between 1.5 x 10(-6) and 1.5 x 10(-5) s(-1). This investigation shows that dehydrating gypsum is an excellent analog material for studies on reaction-induced weakening. However, the lack of important brittle features like shear fractures and stress drops implies that intact gypsum aggregates do not show dehydration embrittlement. In contrast, this study demonstrates that depending on both the level of fault normal stress and the strain history, dehydration-induced unstable slip on preexisting faults is possible. Although some of the observations might be particular to gypsum, the results also indicate that strong mechanical locking of the fault may continue to promote ductile flow unless new, more favorably oriented faults are formed.
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