Growth of bending-moment faults due to progressive folding: Insights from sandbox models and paleoseismological implications

Bending moment normal faults (BMF) form during outer-arc extension of a folding layer. These faults, observed in contractional and extensional tectonic environments, can represent the only tectonic features potentially available for paleoseismological investigations (i.e., trenching) in order to date the movement of deeper structures. Yet, the reliability of BMF as a paleoseismological feature is debated in terms of its potential to continuously record coseismic deformation and their relationship with progressive flexure. We made use of analogue modeling in order to explore a simple model of progressive orthogonal flexure and the development of BMF. We collected data on mode of BMF inception and growth by analyzing time-series of cross section deformation, measuring dip-parallel BMF displacement profiles, and constructing a three-dimensional model of fault architecture by slicing of the experiment. Our results point to a model of BMF development typical of mechanically constrained faults, where limits on self-similar growth of faults is imposed once a maturity stage of displacement accumulation is attained. The BMF can accumulate displacement until the structure reaches a maturity stage when further slip is inhibited. This could result in an incomplete paleoseismological record of further fold movements. However, this fault stage can be recognized by the D-shaped displacement profile, typical of mature BMF. A simple two-dimensional constant-area balancing model is proposed, allowing us to derive incremental changes in flexuring, given the area of the collapsed sector with BMF and the thickness of the faulted layer. These results support the use of BMF as a paleoseismological proxy for deeper structures.