Two fundamental fault patterns accommodate extension in the brittle, upper crust: horst-and-graben geometry and book-shelf geometry. With a horst-and-graben geomety, neighboring normal faults are inclined in opposite directions; with a book-shelf geometry, neighboring faults are inclined in the same direction. Numerical models and scaled physical models with dry sand commonly produce a horst-and-graben geometry. The book-shelf geometry only develops under specific, asymmetric boundary conditions. Scaled physical models with wet clay, however, produce a book-shelf geometry with either symmetric or asymmetric boundary conditions. These observations raise the following questions. Is the book-shelf geometry in nature generally the result of fundamentally asymmetric stress patterns? What are the significant differences in the mechanical behavior of sand and clay? Can numerical models reproduce more clay-like behavior? Is the mechanical behavior of many rocks in nature more clay-like than sand-like?
Numerical, sand, and clay models, with a range of boundary conditions, are underway to address these questions. The analyses of these models has several objectives: (1) to define which numerical parameters best reproduce the fault patterns observed in sand and clay models; (2) to examine the transfer of deformation with depth in the sand and clay models; and (3) to numerically simulate low-viscosity behavior, anisotropic behavior, and pore-pressure changes near faults to better understand the results of the clay models. The ultimate goal of this collaborative numerical/experimental research is to quantify and understand the ranges of geologic conditions and material behaviors that can produce the geometries of faults observed in natural zones of continental extension. The project has economic impact because extensional fault systems are commonly associated with hydrocarbon-producing regions. Also, constraining fault and lithospheric properties may improve the understanding of earthquake dynamics.
