Evolution of Strain Partitioning during Creep of Carrara Marble

Thursday, 18 December 2014: 11:20 AM
Alejandra Quintanilla Terminel1, J Brian Evans1, David Mainprice2 and Ricardo Lebensohn3, (1)Massachusetts Institute of Technology, Cambridge, MA, United States, (2)University of Montpellier II, Montpellier Cedex 05, France, (3)Los Alamos National Laboratory, Los Alamos, NM, United States
We measured the local strain and strain heterogeneity produced during creep deformation of split cylinders of Carrara marble under conventional triaxial loading to inelastic strains of 11% at a strain rate of 3x10-5s-1, confining pressure of 300 MPa and 400<T< 700ºC. A second suite of samples were deformed to strains of 11%, 22% and 36% at 600ºC and the same rate and confining pressure. Microscale strain mapping at a scale of 10 micrometers (MSSM) show that the partitioning of strain amongst twinning, dislocation slip, and grain boundary sliding mechanisms change with T. Preliminary results using a VPFFT model that gives more importance to dislocation slip predict strain heterogeneities with larger wavelength. Interestingly, at all T, although sliding occurred on some boundaries, on average, strain in regions near boundaries was less than that in grain interiors suggesting the formation of a core-mantle structure observed in naturally deformed rocks. The production of local crystallographic texture also depends on T (and presumably, partitioning amongst the mechanisms). In all the experiments, the texture index (TI) of local areas decreased after deformation, but the path of the evolution differed with changing T. Reductions in TI were greatest for samples deformed at 400ºC, where twin activity was greatest, and at 700ºC, where boundary sliding was more prevalent, and less for intermediate T. The wavelength and amplitude of the heterogeneity in local strain decreased with increasing strain at 600ºC, suggesting that the strain (and perhaps, structures) were being homogenized. From this data and previous observations, we conclude that the evolution of deformation structures in marble takes place over a substantial interval in strain; that the duration of this interval probably depends on strain rate, temperature, and pressure; and that extrapolation of mechanical behavior from lab to natural conditions will need to account for changes in strain partitioning.