HR: 0800h
AN: T41F-1299    [Abstracts]
TI: Relations Between Microstructure and Strength in Carrara Marble During Semi-brittle Flow
AU: * Evans, B
EM: brievans@mit.edu
AF: Mass. Inst. Tech., 54-718, Cambridge, MA 02139 United States
AU: Rybacki, E
EM: uddi@gfz-potsdam.de
AF: GeoforschungsZentrum, GeoForschungsZentrum Potsdam Projektbereich 3.2 Telegrafenberg D425, Potsdam, D-14473 Germany
AU: Dresen, G
EM: dre@gfz-potsdam.de
AF: GeoforschungsZentrum, GeoForschungsZentrum Potsdam Projektbereich 3.2 Telegrafenberg D425, Potsdam, D-14473 Germany
AB: Deformation in metasediments during orogenic events is often localized within limestone and marble formations. Such faults and shear zones in carbonate rocks are likely to be important in determining overall rock strength, in the partitioning of strain, and in setting the style of deformation. Fortunately, there is an extensive data base bearing on the mechanical properties of carbonate rocks in general, and of Carrara marble in particular. This body of work suggests that at laboratory strain rates, and temperatures from room temperature to 800K, deformation occurs by a mixture of mechanical processes including microfracturing, mechanical twinning, and dislocation creep. Unfortunately, many aspects of the rheology are still poorly understood, and a thorough correlation between the micromechanical processes and rock strength has not been established. We completed a suite of conventional triaxial mechanical tests over a broad range of conditions: 300 to 900K, confining pressures of 50 to 300 MPa, and strain rates of 10$^{-3}$ to 10$^{-6}$ s$^{-1}$. The tests extend and overlap conditions in previously published work and provide a matrix of samples that can be used to examine the relationship between microstructure and rheology. Because the material is continually hardening as it deforms, it seems clear that some microstructural variable(s) important in determining mechanical state are changing; candidates include crack density, twin density and spacing, grain size, or some aspects of dislocation structure. Both the overall strength and the rate of hardening of the rock increase with decreasing temperature and increasing strain rate. Such characteristics suggest that a rate-dependent process like dislocation creep is important. However, the overall strength and the hardening rate are also pressure dependent, at least within the temperature range from 300 to 700K. Thus, strain must also involve local dilatancy or microfracturing. Interactions among the three mechanisms occur and are probably mechanically important. In addition to the well-known correlation between peak strength and dislocation density, detailed transmission electron microscopy suggests that dislocation patterning occurs at all conditions and that interactions between mechanical twins and dislocations are an important part of the evolution of the microstructure. Qualitatively, micro fracture density decreases with increasing temperature, but the correlation between specific surface area and confining pressure or strain rate is not as strong.
DE: 8159 Rheology--crust and lithosphere
DE: 5104 Fracture and flow
DE: 3902 Creep and deformation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting