INVESTIGATING WORK BALANCE MODELS FOR FAULT FORMATION USING ANALOG MODELING

A fundamental problem in our understanding of the evolution of fold-and-thrust belts lies in explaining why fault systems composed of multiple faults (e.g., fault duplexes or imbricate fans) form in some locations when a single fault could accommodate the same amount of slip. To explain the presence of duplexes and explore the conditions under which they form, several authors have turned to work balance models, where the total work of each type of fault system is calculated in order to determine the conditions under which each fault system is mechanically advantageous (Chapple, 1978; Davis et al., 1983; Stockmal, 1983; Decelles and Mitra, 1995). For example, Mitra and Boyer (1986) developed a numerical work balance model that balances the work of different components of the mountain belt to accomplish some finite strain:

W_total = W_{fault prop} + W_{int. deformation} + W_{friction +} W_gravity

This work balance model suggests multiple faults form due to strain hardening increasing the work of friction, as forming multiple faults requires more work (W_{fault prop.}) than forming a single fault. However, diffusional processes and fluids lead to strain softening, both of which are found within many thrust faults.

Recent research on modified work models indicate a different driver of fault generation—the gravitational work of moving a tapered wedge over a frontal ramp, where the work of gravity increases with each increment of slip. Because the orogenic wedge is thicker towards the hinterland, each increment of slip forward along the fault requires progressively more gravitational lifting.

Here, we present data from analogue sandbox modeling that illustrates the role of gravity on the deformation of a critically tapered wedge and the formation of faults. These experiments show that when a critically tapered wedge is thrust over a frontal ramp, the work of gravitational lifting is lower when multiple faults are allowed to form and higher when only a single fault forms. These results also indicate a connection between the shape of an orogenic wedge and the spacing between faults within that wedge, as a higher critical taper increases the favorability of forming new faults instead of slipping on existing faults.