Abstract - The growth of faults arising from the extension of rock volumes (i.e. normal faults) in periodically layered sequences with varying strength contrast and at varying confining pressure is modeled using the Distinct Element Method. The modeling reproduces many of the geometries observed in natural faults, including: (i) changes in fault dip due to different modes of failure in the strong and weak layers, (ii) fault bifurcation (splaying), (iii) the flexure of strong layers and the rotation of associated blocks to form normal drag and (iv) the progressive linkage of fault segments. The model results also suggest that faults in sequences with high strength contrast at low confining pressure are highly segmented due to different types of failure (extension vs. shear failure) in the different layers. The degree of segmentation decreases as the strength contrast decreases and confining pressure increases. Faults at low confining pressure localize as extension (Mode I) fractures within the strong layers and are later linked via shallow dipping faults in the weak ones. This leads to initial staircase geometries that, with increasing displacement, cause space problems that are later resolved by splaying and segmentation. As confining pressure increases the modeled faults show a transition from extension to hybrid and to shear fracture and an associated decrease in fault refraction, with a consequent decrease in fault surface irregularities. Therefore the mode of fracture in the strong layers of a mechanical multilayer at a particular confining pressure, exerts an important control on the final fault geometry.
Abstract of talk given to:
Mechanics of Materials: Perspectives and Recent Advances, Joint meeting of Irish Mechanics Society, the Swedish National Committee for Mechanics, and the Irish Society for Scientific Engineering & Computation, May 2007.