An experimental and field study of ductile deformation in clastic rocks

Naturally deformed clastic rocks such as breccias and conglomerates provide a useful way to study strain and, indh:ectly, the rheology of rocks. However plastic deformation processes may potentially compromise many of the commonly used methods of determining strain in such rocks at high temperatures, since many of these strain analysis techniques rely upon changes in object e.g. grain shape, orientation and distribution. The work reported in this. thesis explores this matter quantitatively by performing a number of deformation experiments on synthetic calcite 'conglomerates' at temperatures where the rate-dependence of grain size-sensitive deformation processes in the clast and matrix can beú varied. In order to control the microstructures of the experimental samples, specimens were fabricated by mixing granulated Solnhofen limestone with powders of Chelometric grade calcite. The mixtures were tumbled for 3 hours duration to ensure uniform intermixing of the powders, and were then hot-isostatically pressed at a temperature of 700úC under a confining pressure of 190 MPa for 72 hours. The resulting samples were fully dense and consisted of sub-angular Solnhofen limestone clasts 60-90J.Ull in diameter, in a foam-textured matrix of Chelometric grade calcite with a grain size of22.2J.Ull ñ7.5J.Ull. All experiments were conducted at temperatures ranging between 400ú -700úC, and at a constant confining pressure (158 MPa) and strain rate (3.0xlO-4 S-I). An initial set of deformation experiments were performed upon pure Solnhofen limestone and Chelometric grade calcite to establish the flow behaviour of the clasts and matrix, respectively. Subsequent experiments were performed upon samples in which the volume fraction of clasts and matrix varied in the range 9:91 to 37:63. In order to assess the accuracy of strain measurements made on deformed conglomerates and, in particular, the potential complicating influence of grain size-sensitive flow on those measurements, the Rf/J and Fry techniques have been used to determine the clast and matrix strains, respectively, in the experimental samples. Detailed microstructural analyses were performed using an automated Electron Backscatter Diffraction (EBSD) technique. The experimental results obtained emphasize the significant role that viscosity contrasts, volume fraction and microstructure have in controlling the bulk strengths of polyphase materials. The results also show that at 550úC there is no strain partitioning between the clast and matrix despite the large viscosity contrast between them whereas at 700úC there is a small but consistent strain partitioning. A rigorous interrogation of the results using the law of mixtures has shown that the component of strain which is accommodated by grain boundary sliding can be quantified in these experiments. Microstructural analysis has shown clearly a temperature dependent change in the development of fabric intensity with strain which may provide a new insight into inferring true bulk strains in rocks deformed at high temperatures.EThOS - Electronic Theses Online ServiceGBUnited Kingdo