Semi-brittle deformation of Solnhofen limestone: Initial porosity effects on strength

Semi-brittle deformation, characterized by the concurrent operation of cataclasis and crystal plasticity, plays a key role in constraining the strength of the middle crust. While the effects of temperature, pressure, fluid-abundance/pressure, and material properties (e.g., grain size) have been relatively well studied, the role of initial porosity in semi-brittle deformation remains poorly understood. Here, we performed a series of triaxial compression experiments on dry samples of Solnhofen limestone, which has an initial porosity of ~5.6% and an isotropic texture. Experiments were conducted at a range of confining pressures (P_c=30-300 MPa), temperatures (T=25 to 600 °C) and a constant strain rate of 1×10^-5 s^-1. Under these conditions, Solnhofen limestone mainly deforms in the semi-brittle regime associated with strain hardening, and brittle fracturing only occurs at low pressures (P_c≤50 MPa) and T <200 °C.

The strength, expressed as differential stress at a given strain, of Solnhofen limestone varies with imposed conditions. At 5% strain, the strength decreases with increasing temperature at all investigated pressures. In contrast, the pressure dependence of strength is temperature sensitive. At T =400 °C, the strength decreases significantly with increasing pressure from 30 to 300 MPa, in contrast to the positive pressure dependence observed for low porosity (~0.5%) Carrara Marble in the similar semi-brittle regime. This pressure sensitivity becomes less pronounced at 600 °C. Changes in porosity, determined from the pre- and post- deformation measurements, show that dilation and compaction are closely related to deformation mode. The extent in sample compaction correlates with the deformation ductility and becomes more marked with increasing temperature and pressure.

We speculate that the observed negative pressure dependence of strength during semi-brittle deformation arises from the presence of initial porosity and can be explained by the increasingly dominant role of plastic pore collapse. This hypothesis is supported by an additional experiment conducted at 400 °C, in which samples pre-compacted at 300 MPa for 3 h and subsequently deformed at 30 MPa exhibited higher strength than samples deformed directly at 30 MPa without a pre-compaction stage. Ongoing microstructural investigations will provide a basis for developing a microphysical model to better interpret deformation processes in rocks with intermediate porosity in the semi-brittle deformation regime.