A constitutive law for low-temperature creep of water-saturated sandstones

An accurate predictive model for the long-term strength of the continental lithosphere is important in a range of geophysical and geodynamic problems. While laboratory experiments are consistent with Mohr-Coulomb brittle faul ting in the cold, upper continental crust, there is increasing evidence tha t time-dependent processes may also be important in these rocks, even at lo w temperature. However, there is some ambiguity as to the exact form of the constitutive law for describing time-dependent behavior of upper crustal r ocks. Here we present results of room temperature creep experiments on a su ite of water-saturated sandstones spanning a range of petrophysical and the ological properties and underlying deformation mechanisms. On physical and microstructural grounds our analysis suggests that a modified power law cre ep, of the form (epsilon) over dot = A'(sigma (d) - sigma (f))(eta'), where sigma (d) is the differential stress and sigma (f) is the long-term failur e (fundamental) strength, provides a more complete description of the exper imental data. In particular, the parameters can be used to differentiate be tween sandstone types, with A', sigma (f), and eta' varying systematically with cementation state, rock rheology, and confining pressure. The fundamen tal strength (sigma (f)) for time-dependent deformation varies much more th an the other parameters of the distribution, making it a potentially sensit ive indicator of underlying creep mechanisms. Further tests would be needed to prove the constitutive law on a wider range of rock types and to prove that the three-parameter model is statistically better in the general case.