Thermo-compositional structure of the North and South American cratonic lithosphere

It remains unanswered how cratons - the ancient cores of the continents - formed, remained stable, and occasionally lose their deep roots. In this thesis, I analyse geophysical data from North and South America to constrain the thermo-compositional structure of their cratonic lithosphere. For northeastern North America, I model Rayleigh-wave phase velocities, and for eastern South America, I jointly invert Rayleigh-wave group velocities, topography, and geoid height. I use a grid search to find structures that match the data, searching a large set of plausible shield geotherms and a comprehensive set of compositional structures with the option of metasomatic minerals and eclogite as seismic slow and fast compositions. The data require larger variations in cratonic thermal and compositional structures than often considered; structures that can be correlated to tectonic evolution stages. Craton assembly appears to often involve subduction, where we find evidence of a diachronous secular evolution during the Proterozoic from (shallow) subduction where eclogitised crust was preserved in the lithosphere, to (steeper) subduction leaving only a volatile-altered shallow lithosphere. Thick roots (350-150 km) remain under much of the North American Craton and part of the South American Platform. These regions include preserved Archean/Paleoproterozoic cores and neighbouring regions with roots metasomatized throughout much of their depth by plume activity/rifting. Evidence of root loss/erosion is observed mainly under South America, which we attribute to major plume and subduction interaction during the Phanerozoic. Our results, confirmed by preliminary waveform analysis, indicate that metasomatism is a widespread feature of cratonic lithosphere, particularly at shallow depths. The main implications of this work are that the cratonic lithosphere preserves more signatures of its tectonic evolution than previously realised and likely holds larger quantities of volatiles than often assumed, at a level that affects its density, viscosity, and possibly solid-earth volatile cycling.Open Acces