Force changes in response to fault growth and out-of-sequence thrusting in brittle compressional analogue models

Much of our current understanding of fold-and-thrust belt (FTB) structure and evolution is based on the critical taper theory. Even though this concept successfully explains the first-order relationship between FTB dynamics and shape via certain physical parameters, it does not account for transient force changes associated with the FTB's internal dynamics, including lateral fault growth or out-of-sequence thrusting. We implement a novel measurement technique to analogue modelling that uses an array of five force sensors aligned at the backstop to characterize the evolution of force. Combined with optical data that monitors surface deformation, this approach provides a methodological framework for capturing second-order force variations associated with non-coaxial deformation within FTBs.

This novel approach enables the prediction of lateral thrust fault growth prior to surface emergence and of out-of-sequence reactivation of earlier-formed thrusts, thereby informing FTB evolution.  We examine these relationships by assessing force responses to thrust propagation across pre-existing basement steps, with the ramp angle as the main variable. Our results indicate that a 90° ramp angle generates a pronounced second-order force component, which correlates with enhanced lateral fault variability and associated out-of-sequence thrusting driven by back-thrust activation. A similar structural and force response is observed for a 25° ramp angle, whereas 60° ramp angles produce negligible force disturbances and almost no out-of-sequence thrusting. These results demonstrate the sensitivity of FTB dynamics to structural inheritance.