A Microstructural Study across the Extension to Shear Fracture Transition in Carrara Marble

Erika Rodriguez
Texas A&M University, Geology and Geophysics Department
College Station, TX
erodriguez@geo.tamu.edu

Extension and shear fractures are a common form of failure in rock deformation experiments and they occur under tensile and compressional stress states, respectively.

These fractures have been hypothesized to be end members of a continuum, suggesting the occurrence of hybrid fractures at the transition between tensile and compressive stress states. Observations of natural fractures thought to be hybrids and experimental attempts to produce hybrids in the laboratory have been inconclusive. Recently, Ramsey and Chester produced a continuous transition from extension to shear fracture with triaxial extension experiments using Carrara Marble, demonstrating that hybrid fracture surfaces can be formed in the laboratory. However, current hypotheses for their origin have not been tested and the micromechanisms responsible for their formation have not been identified.

The Carrara Marble samples generated by Ramsey and Chester will be used in this study to determine the microscopic structure of each fracture surface. The objectives of this study are to characterize the deformation mechanisms that operate to produce extension, shear and hybrid fracture modes, identify how these deformation mechanisms contribute to producing hybrid fractures and compare these results to different models of fracture propagation.

Profilometry will be used to quantify the morphology of the fracture surfaces. Scanning Electron Microscopy will be used to characterize the fine scale structures of the fracture surfaces, measure crack spacing/length ratios and infer the propagation direction of the fractures to test fracture propagation models for hybrid fracture development. Optical microscopy will be used to characterize the deformation mechanisms across the transition.