Plate motion drivers: Geodynamical framework and statistical appraisal for the case of the Neogene Nazca–South America convergence

Recent high-resolution reconstructions of plate motions reveal a complex history of alternating slowdowns and speedups, often over short timescales (<5 Myr). These rapid changes offer an opportunity to reassess the geodynamic processes driving tectonic plates, which we explore using an analytical inverse framework. This approach, however, inevitably yields non-unique solutions when inferring the forces behind a motion change. We partly address this issue by focusing on forces capable of varying at rates consistent with rapid kinematic shifts, though the specific driver behind any change may remain ambiguous. We adopt a two-step methodology, using torque changes as intermediaries linking force variations to reconstructed absolute plate-motion changes. First, we employ an established method that combines rheological constraints with torque-balance principles to estimate the torque variation required for a given kinematic change. Second, we estimate torque-change vectors arising from a broad range of geodynamic scenarios - acting at plate boundaries (e.g., slab pull, interplate friction) and at the base of plates (e.g., asthenospheric flow). We then apply directional statistics to quantify the similarity between the motion-based torque-change distribution and each simulated vector. This comparison allows us to identify the location and direction of the force-change vectors most likely to produce the motion change of study. We apply this method to the Neogene Nazca-South America convergence. Our kinematic analysis reveals rapid slowdowns in the absolute motion of both plates and a pronounced Nazca speedup at similar to 10-12 Myr. Our geodynamic analysis indicates that the force variations driving the slowdowns are likely concentrated along the central segments of the shared convergent boundary. This result aligns with established hypotheses linking reduced convergence to Central Andes orogeny, thereby supporting our approach. Key advantages of this novel method include fast computation, explicit treatment of kinematic uncertainties, and broad applicability across tectonic settings.