A NEW MODEL FOR THE DOMINANT CONTROL ON FAULT PROPAGATION IN DUPLEXES: INSIGHTS FROM NUMERICAL AND ANALOG APPROACHES

Duplexes found in fold and thrust belts are critical to numerous geologic problems and processes: e.g., hydrocarbon production, groundwater distribution, mineral resource formation, seismic hazards, and the evolution of mountain belts. However, existing models do not adequately explain why duplexes occur. Comparisons of work for the same amount of offset between a single-thrust structure and a duplex would seem to indicate that duplexes require increased work due to the propagation of new faults and further internal deformation. Our research shows that, under certain circumstances, duplexes can hold a significant mechanical advantage over deformation along a single fault. In showing this, our research proposes a new model for the dominant control on the propagation and spacing of new faults during the formation of duplexes and imbricate fans.

To explore the controls on fault generation and spacing of thrust faults within fold and thrust belt systems, we employed a numerical work model based on the work of Mitra and Boyer (1986):

W_total= W_{fault prop.}+W_{int. defm.}+W_friction+W_gravity

Previous studies based on this model have hypothesized that increased friction resulting from strain hardening, caused by grain size reduction within fault zones, drives new fault propagation. However, increasing frictional work from strain hardening along a single fault enough to overcome the increased work associated with fault propagation and internal deformation along multiple faults poses a numerical problem. The problem is intensified when considering the effects of fluids and ductile deformation mechanisms.

Our research further advances previous numerical work models by proposing a new dominant control on the propagation of new faults and the formation of duplexes. To test our hypothesis, we conducted analogue experiments using a box model developed by Grudovich et al. (2023). Using this analogue model, we varied key parameters and recorded the resulting fault systems from each experiment. The results clearly demonstrate the relationship between our hypothesized key parameters and fault spacing within orogenic wedges.