Influence of strain history on the mechanical and micro-fabric evolution of calcite rocks: insights from torsion experiments

Different types of deformation experiments were conducted on specimens of Carrara marble at a temperature of 1000 K, 300MPa confining pressure, and 3 × 10−4s−1 shear strain rate. Microstructural examinations were carried out using light and electron microscopy. Fabric analysis was performed through electron backscattered diffraction (EBSD) measurements. The stress-strain relationships from the experiments indicate that the mechanical response of Carrara marble depends on the deformation history. After a first ductile deformation event the marble becomes permanently softened and is able to undergo plastic strain at relatively lower stresses compared to previously undeformed marble. After shear strain reversal of γ=2 or less the original microstructure is restored; in highly re-strained samples (γ = ±3,4 and 5) dynamic recrystallization mechanisms create an evident foliation whose angle in respect to the shear plane defines a shear sense criterion which is in agreement with the reversed sense of shearing. After a minimum shear strain reversal of γ=3 the pre-existing foliation is totally overprinted. The fabric produced during strain reversal experiments shows features, which are comparable with those of single-stage experiments; but the strength of the crystallographic preferred orientation (CPO) is significantly lower, indicating complex microstructural interactions during plastic reactivation. Our tests indicate that the strain weakening in Carrara marble is predominantly caused by grain size refinement through dynamic recrystallization. The development of a crystallographic preferred orientation contributes towards only about one third of the total weakening observe

Frequency-dependent seismic attenuation in shales: experimental results and theoretical analysis

Samples of shales from the Ordovician Bongabinni and Goldwyer source rock formations were recovered from the Canning Basin (Western Australia). Attenuation was experimentally measured on preserved plugs from these formations in the frequency range between 10−2 and 102 Hz. Samples cored with different orientations with respect to the sedimentary bedding were prepared and tested in their native saturated state and after drying in the oven at 105 °C for 24 hr to assess the effect of fluids and of the sediment anisotropy on attenuation. To aid the interpretation of the experimental results, the clay-rich samples were characterized in terms of mineralogy, water content, porosity, permeability and microstructure. The two shales have significantly different quality factors; and this is seen to be dependent on both the saturation state of the samples and the propagation direction of the oscillatory signal. The attenuation coefficient for compression/extension parallel to bedding is less than that vertical to bedding in both the preserved and partially dehydrated situations. No frequency dependency is observed in the preserved samples within the range of frequencies explored in this study. On the other hand partially saturated samples show peaks in attenuation at around 40 Hz when the stress perturbation is transmitted normal to the macroscopic bedding. The interpretation of the attenuation measurements in terms of well-established theoretical models is discussed in view of the physical characteristics and microstructure of the tested rock

Influence of strain history on the mechanical and micro-fabric evolution of calcite rocks: insights from torsion experiments

ISSN:1661-8734ISSN:1661-872

Frequency-dependent seismic attenuation in shales: Experimental results and theoretical analysis

ISSN:0956-540XISSN:1365-246