Crider, J.G., Kattenhorn, S.A., and Pollard, D.D. (1998)
Fault-related deformation around surface-breaking normal faults: how folding and fissuring depend upon boundary conditions
Lithostatic compression and tectonic tension play competing roles in the process of normal faulting: one tending to hold the fault surfaces in contact and increase frictional resistance to slip; the other tending to pull the fault surfaces apart and reduce friction. In addition, the tectonic tension resolves into shear stress on the fault plane that drives slip. Both have a profound effect on the near-surface stress field and distribution of displacement discontinuity (opening and slip) across the fault, and consequently on fault-related deformation. We postulate an isotropic lithostatic stress that increases linearly from zero at the Earth's surface in proportion to the weight of the overlying rock. At the surface, this lithostatic stress would have no impact on faulting behavior. At 1 km depth, however, this stress would produce a confining pressure of about -25 MPa. If we superimpose on the lithostatic stress a uniaxial tectonic tension that is constant with depth, and on the order of 10 MPa, a 1 km-tall fault cutting the surface will experience tension in the upper 2/5 and compression in the lower 3/5. Given a simple friction law, where resistance to slip is proportional to normal stress across the fault, the friction on the fault would increase linearly with depth where the surfaces are in contact. However, if the fault surfaces are not in contact in the upper part of the fault due to fissuring, there will be no friction in this region.
We use a 3-D boundary element numerical code to compare fault-related deformation for surface-breaking normal faults with deformation associated with deeply-buried normal faults. Calculated maximum tensile stress around the faults predicts fault-related jointing at the upper tipline of a buried fault. However, for a surface-breaking fault, which has no rock above its upper tipline, joints develop at the lateral tiplines. As a consequence, predicted joint orientations differ between surface-breaking and buried faults. Slip distributions and displacement fields also differ, and thus does fault-related folding. For the study of near-surface, fault-related deformation (topographic deflection or fracturing), or where small changes in the stress perturbation are important, care should be taken to define appropriate boundary conditions in numerical models that capture the effects of proximity to a free surface.
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