MYRIAM: Open‐Source Software to Estimate Torque Variations Associated With Plate‐Motion Temporal Changes

Tectonic plate motions are a prime constraint on lithosphere dynamics and on the torques acting upon plates. Researchers typically test hypotheses on the controlling torques via forward computer models, which allow accepting or rejecting hypotheses on the basis of the fit of model-output plate velocities to kinematic reconstructions. Such models typically require a significant amount of input information (e.g., tectonic boundaries, structure of the lithosphere, lateral variations of rheology, among others) to appropriately model the tectonic system, and obtain sufficiently-realistic realizations of plate motions. Alternatively, the inverse problem approach takes the difference between the plate torque-balance at two distinct moments in time. This results in an equation in which (a) torques that have not varied through time are canceled out and (b) torques that instead have varied through time are linked to the resulting plate-motion change through a term that accounts for the plate shape and the rheology of the underlying asthenosphere. This approach sacrifices the capability to describe the different individual forces/torques acting upon a plate. Instead, it focuses on determining torque temporal variations, which significantly reduces the amount of input knowledge required. In this technical note we introduce MYRIAM, an open-source software that implements such an inverse approach. MYRIAM takes plate-motion temporal changes, and outputs an estimate of the torque variation required upon a plate to generate the input kinematic change. MYRIAM is released as an open-source repository hosted at GitHub, complete with source code and executable files for Windows and Unix-based operating systems. Plain Language Summary The outermost layer of the Earth, known as the lithosphere, is fragmented into plates that are in motion relative to each other. Through geological history, the motion of tectonic plates did not remain steady, and in fact important changes are now known to have occurred relatively fast, within a few million years. By virtue of Newton's laws of motion, these changes are all necessarily caused by the variation in one or more forces acting upon the plates, and therefore disrupt their force balance. Changing the motion of a plate is not an easy feat-for plates it requires overcoming the frictional resistance exerted by the underlying asthenosphere, a low-viscosity layer beneath the plates. The resulting motion change will depend on (a) the mechanical properties of the asthenosphere and (b) the amount of plate surface interacting with said asthenosphere (i.e., the basal area of the plate). All these considerations are taken into account by MYRIAM, an open-source, computationally-inexpensive software that estimates the force/torque variations required to produce a given change of plate motion.