Lithospheric Thermo-compositional Structure as a Record of Tectonic History: A Comparison Between the Canadian Shield and the South American Platform.

Abstract:

The thermal and compositional structure of lithospheric keels underlying cratons, which are stable continental cores formed during the Precambrian, is still an enigma. Mapping lithospheric temperatures and compositional heterogeneities is essential to better understand geodynamic processes that control craton formation and evolution. Here we investigate the parts of the Canadian Shield in North America comprising the Superior Craton and surrounding Proterozoic belts, and the South American Platform, comprising the Central Brazilian and Atlantic shields. We model surface-wave dispersion curves from previous studies, which were regionalised based on cluster analysis and crustal structure. Next, we perform a grid search for sub-crustal thermal and compositional structures that are consistent with the average dispersion curve for each group. We apply independent constraints on crustal structure and use thermodynamic methods to map thermo-compositional structures into seismic velocity. Our results require vertical and lateral variations in compositional and thermal structures, which appear to reflect different stages of formation and modification of the lithosphere below the regions. We can distinguish compositional structures under different regions, including Archean cores, Archean/Paleoproterozoic collision belts, mid-late Proterozoic collision belts, and zones affected by rifting.