A tale of two kinds of normal fault: the importance of strain weakening in fault development

We search for a description of fault formation consistent with the main features of two very different types of extensional faults: (1) large-offset, low-angle normal faults; (2) small-offset, high-angle normal faults. We use an advanced numerical model to predict how the pattern of faulting varies as a function of the imposed magnitude and rate of weakening of an extending Mohr-Coulomb layer. We assume that fault weakeing is due to reduction of cohesion with fault offset. Faults initiate and slip at high dip angles. When the fault offset is large (i.e. comparable with layer thickness) then the inactive footwall fault surface can be rotated to a flat orientation. We find two requirements for development of a large-offset fault. The magnitude of cohesion loss must be greater than c. 20% of the initial total extensional yield strength. Also, the rate of weakening with fault offset has to be moderate: the fault offset to lose cohesion has to be less than c. 2 km and more than c. 100 m, with the lower bound being harder to define. Using the same cohesion and rate of offset weakening, extension of a thick layer can lead to development of multiple, small-offset, high-angle faults rather than a single ‘low-angle’ fault. For cohesion reduction of 20 MPa a brittle lithosphere thicker than 20 km leads to multiple faults. Finally, we show that inclusion of thermal advection weakening can shift the transition to thinner layers for the same magnitude and rate of cohesion weakening