A Thermo‐Compositional Model of the Cratonic Lithosphere of South America

N.‐P Finger,W D Mooney,M Tesauro,C Haeger,M K Kaban,M Thomas

doi:10.1029/2020gc009307

Abstract

AbstractNew seismic data on the crust and upper mantle of South America have recently become available and describe its deep structure with unprecedented resolution. Here, seismic data are combined with gravity data and mineral physics constraints to develop self‐consistent models of the temperature, composition, and density of the South American lithospheric and sub‐lithospheric upper mantle. A new model of the crustal structure was developed, including sediment and average crustal density and depth to the Moho. This crustal model was used to correct the gravity field and obtain the residual topography. Then, an initial temperature model, derived from seismic tomography and mineral physics, was improved through an iterative process based on the joint inversion of gravity and residual topography. The results reveal deep cratonic roots present under wide parts of the Amazon, São Francisco, and Paranapanema Cratons, but not under the Rio de la Plata and Parnaíba Cratons. These cratons probably never developed lithospheric roots or were rejuvenated during subsequent tectonic events. We hypothesize that the root of the northwestern Amazon craton was removed by upwelling of hot mantle material under the Guyana highlands. Temperature and density anomalies evident beneath the São Francisco Craton appear to be connected with those of the Paranapanema Craton, which could indicate a westward shift of its root. Depletion south of the Paranapanema Craton probably denotes remnants of a previously larger craton that was rejuvenated by the upwelling of hot mantle material leading to emplacement of the Paraná Flood Basalts.

Highlights

Tectonic processes, which shape the surface of the Earth and cause its seismic activity, are driven by strong heterogeneities of density and temperature in the mantle that are not visible at the surface
At 50 km depth, seismic velocities and resulting temperatures might be biased by incorrect Moho positioning in the tomography model or the deep crustal roots present in some parts of the area
The lithosphere is probably thin in these areas with temperatures >1300°C at 200 km, including the Andean Forelands and most areas along the Trans-Brasiliano Lineament (TBL), where thermal densities are negative at all depths as well

Summary

Introduction

Tectonic processes, which shape the surface of the Earth and cause its seismic activity, are driven by strong heterogeneities of density and temperature in the mantle that are not visible at the surface. Density increases with decreasing temperature, cratons are often neutrally or even positively buoyant. This is caused by iron depletion due to partial melting and melt fractionation over time (Jordan, 1978). Seismic velocities are less sensitive to these compositional density variations than the gravity field, which in turn does not allow robust estimations of temperature. Multiple methods hold the potential to construct a more comprehensive model of the lithosphere and upper mantle. We combine gravity and seismic data, constrained by mineral physics, to model the lithospheric and sub-lithospheric upper mantle of South America using an approach that has been applied to other continents (e.g., Kaban et al, 2014; Tesauro et al, 2014a)

Methods

Results

Discussion

Conclusion