Abstract
The South American Transcontinental Drainage Divide (TDD) is roughly oriented NE-SW from its Atlantic termination in SE Brazil to its NW termination in the limits between the southwestern edges of the Amazon Craton and the Andean Foreland. Based on a weak spatial coincidence with 90 - 70 Ma igneous intrusions, the origin and stability of the TDD have been attributed to intraplate magmatic activity. Using geomorphic analysis of drainage divides, we infer an overall southward migration of the TDD, even in areas with constrained uplift associated with intrusions. Most of the TDD is also the main drainage divide of the Paraná River Basin (PRB). Our analysis of the PRB divides reveals wholesale basin shrinkage, consistent with the southward migration of the TDD. We infer that the basalts of the Serra Geral Group induce topographic inversion of the PRB. Additionally, the PRB's thicker continental crust contributes to its higher elevation. Non-uniform dynamic subsidence rates since 50 Ma likely lowered relief in the basin headwaters near the TDD and enhanced across-divide relief asymmetry. Together, these internal and external factors keep the PRB elevated, with low relief, and make it topographically vulnerable to long-term drainage area loss. These base-level controls explain the position and mobility of the TDD.