PLATE DEFORMATION AND SUPERCONTINENT CYCLES

The Wilson cycle of ocean basin closure and opening, leading to supercontinent assembly and breakup, implies that continental margins are repeatedly weakened by thermal rejuvenation and fault reactivation during subduction, orogeny and rifting, leaving continental cores un-deformed. Such partitioning of deformation has led to the hypothesis of tectonic inheritance resulting from the combined effects of rheological heterogeneity and mechanical anisotropy. However, these parameters are poorly constrained on a global scale. We present global maps of T_e over continents from two approaches: Direct modelling of strength from thermo-rheological profiles and the estimation of T_e from the inversion of gravity and topography data. The two models show in general a good agreement, having equal means (at the 95% significance level) in about half of the continental areas. For the regions with the most robust determinations the relationship between the two models is close to linear. We find that the distribution of T_e is weakly bimodal with peaks at 30 and 150 km, corresponding to the distribution of Phanerozoic belts (T_e<40 km) and Proterozoic and Archean cratons (T_e>100 km). Variations of T_e with thermo-tectonic age and seismic velocity anomalies are consistent with T_e being controlled by differences in plate structure, consistent with regional findings. We show that directions of minimum rigidity consistently align with directions of large T_e gradients, perpendicular to major tectonic boundaries and sutures, indicating that T_e anisotropy is controlled by pre-existing structure. These results provide the first global observational constraints supporting the thesis that rheological heterogeneity and mechanical anisotropy play a dominant role in the deformation and evolution of continents by concentrating strain at pre-existing zones of weakness, thus stabilizing continental interiors during repeated supercontinent cycles.