Exploring the Controls on Fault Generation and Spacing within Duplexes and Imbricate Fans Using Numerical and Analog Models.

David Brink-Roby, Marshall University
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Abstract

Duplexes and imbricate fans are fundamental structural elements of fold-and-thrust belts, influencing hydrocarbon trapping, groundwater flow, mineral resource formation, seismic hazards, and mountain belt evolution. They also provide an important mechanism for maintaining critical taper within Coulomb wedges. While critical taper theory predicts the overall geometry of orogenic wedges, it does not explain why new thrust faults propagate to form duplexes rather than continued slip along existing faults.

To investigate the controls on thrust fault propagation and spacing, we developed a numerical work model based on the framework of Mitra and Boyer (1986):

W_total = W_fault-propagation + W_internal-deformation + W_friction + W_gravity

Previous models generally attribute new fault propagation to increasing frictional resistance along existing faults, commonly associated with strain hardening caused by grain-size reduction. However, this mechanism requires frictional work to increase sufficiently to offset the additional work required for fault propagation and internal deformation associated with multiple thrust faults—a requirement that becomes increasingly difficult to satisfy in the presence of elevated pore-fluid pressures and ductile deformation.

We propose an alternative mechanism in which the geometry of the basal glide plane governs the formation and spacing of thrust faults by controlling the work performed against gravity. As the dip of the basal glide plane increases, the gravitational work required to transport rock along a single thrust increases more rapidly than that required by a duplex system. Above a critical basal slope, propagating a new thrust becomes mechanically favorable because the reduction in gravitational work exceeds the additional work required for fault propagation and internal deformation. Conversely, at lower basal slopes, continued slip along a single fault remains energetically preferred.

To evaluate this hypothesis, we conducted analogue experiments using a sand box model. Orogenic wedge models containing basal glide planes of varying dip were deformed under identical boundary conditions, and the resulting fault geometries were analyzed. The experiments demonstrate a systematic relationship between basal glide-plane dip and thrust-fault spacing, providing experimental support for a gravity-driven control on duplex formation and the spacing of imbricate thrust systems.

Session

Deformation in the upper crust