HR: 1340h
AN: T53C-1435    [Abstracts]
TI: A Microstructural Study of the Extension-fracture to Shear-fracture Transition in Carrara Marble
AU: * Rodriguez, E
EM: erodriguez@geo.tamu.edu
AF: Center for Tectoniophysics, Department Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115
AU: Chester, J S
EM: chesterj@geo.tamu.edu
AF: Center for Tectoniophysics, Department Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115
AU: Chester, F M
EM: chesterf@geo.tamu.edu
AF: Center for Tectoniophysics, Department Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115
AB: Extension-fractures and shear-fractures produced in rock deformation experiments display distinctly different surface morphologies which, likely, reflects differences in the stress states at failure and details of fracture growth. To better understand the development of fractures under different states of stress, samples of Carrara marble were extended in a triaxial apparatus. Conditions tested covered the continuous transition from pure extension fracture under tensile loading, through a regime of hybrid fracture produced under mixed tensile and compressive loading, to shear fracture under purely compressive loading conditions. Small-scale roughness (smaller than grain size), characterized using laser profilometry and spectral analysis, gradually decreases across the extension-fracture to shear-fracture transition in directions parallel and perpendicular to the shear direction. In contrast, the large-scale roughness (greater than grain size) measured perpendicular to shear is relatively constant across the transition, in spite of the fact that undulations and grooves that are oriented parallel to the direction of shear are present on the surfaces of fractures formed under compressive loading conditions. The large-scale roughness measured parallel to shear initially increases and then decreases across the transition, reflecting the presence of macroscopic steps on the surfaces of the hybrid fractures. Steps, composed of treads and risers, display a right-stepping, left-lateral-shear, en echelon geometry where risers constitute restraining bends. Deformation in the rock neighboring the macroscopic failure surface, as displayed in petrographic sections cut perpendicular to the surfaces, includes pinnate extension fractures that are continuous with the treads on the surfaces. That the pinnate fractures extend across the surface of macroscopic failure and align with treads on this surface implies that the pinnate fractures formed prior to growth of the surface of macroscopic failure. The structures produced in the experiments are consistent with a step-crack model for fracture formation, where precursory, en echelon, tensile microfractures form and then link to create a through-going macroscopic fracture surface. The systematic decrease in the length and spacing of the stepped, precursory tensile cracks with increasingly compressive loading conditions across the extension-to-shear-fracture-transition corresponds to the progressive change in 1) the fracture surface morphology and 2) the macroscopic orientation of the fracture surfaces relative to far-field principal stress directions. These data suggest that a macroscopic extension fracture results from relatively unrestricted in-plane growth of precursory cracks under more tensile loading conditions. Shear fractures form under more compressive stress states where the growth of precursory cracks is restricted, and the formation and linking of stepped-cracks is favored by mechanical interaction of precursory cracks.
DE: 5104 Fracture and flow
DE: 8010 Fractures and faults
DE: 8020 Mechanics, theory, and modeling
DE: 8030 Microstructures
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: Fall Meeting 2005