From rheology to plate break-up: from self-weakening to deformation localization in the lithospheric mantle – RheoBreak

The RheoBreak project aims at understanding the dynamical process of plate break-up, i.e. the generation of a low-viscosity plate boundary from a previously rigid plate interior. Based on preliminary results, we hypothetize that olivine ductile deformation can alone cause break-up at mantle-scale, through feedbacks between deformation and dynamics leading to a localized self-weakening of the cold lithospheric mantle, associated to the dependency of mantle viscosity on strain rate and on temperature.

The RheoBreak project comprises 3 work packages :

- WP1 “Evaluating the potential of any mantle rheology to localize deformation”. Visualizing the bifurcation between self-weakening and self-hardening paths in neighbouring regions in a temperature-strain-rate-viscosity domain gives clues to quantify how much a rheology is prone to localization. We will investigate various rheological parameterizations compatible with crystal-scale flow laws, to reconcile these experimental data with large-scale dynamics of plate break-up as simulated in numerical models. Indeed, if the “yield stress” mantle rheology has successfully generated self-consistent plate tectonics atop mantle convection in previous studies, it is not compatible with experimental data of micro-scale deformation which predict dislocation creep, i.e. temperature and strain-rate dependent viscosity, at the depths expected at plate base (~30-100 km).

- WP2 “Triggering plate break-up through deformation localization in various geodynamical contexts”. We will design several set-ups of regional-scale models of subduction, rifting, or plume to force large intraplate stresses. We expect different scales, magnitudes and patterns of asthenosphere flows, plate deformation and heat transport, resulting in various efficiency of the localizing feedback. We will then run whole-mantle convection models in which several local dynamics are expected to coexist
and to interact, to estimate a frequency of plate break-up depending on the Rayleigh number.

- WP3 “Evaluate the use of space gravity data to evaluate lithosphere thermal structure, a proxy for intraplate deformation." Outstanding gravimetry and gradiometry data have recently been measured by the GOCE space mission. In addition to the high spatial resolution, the acquisition of gravity gradients provide a unique opportunity to study structures at intermediate scales (100-1000km). Preliminary results show indeed that plate thinning is measurable by gravity gradients, but not detectable from gravity vector. We will first analyze synthetic gravimetry and gradiometry signals computed from the thermo-mechanical models of WP2, before applying an inversion procedure to natural cases, i.e. from space gravity data to plate thermal and weakening structure.