Crustal and upper mantle structure in the intracratonic Parana Basin, SE Brazil, from surface wave dispersion using genetic algorithms

摘要

The mechanisms for the formation of intracratonic basins remain a matter of dispute. More information about deep crustal structures, especially in the poorly known intracratonic Parana Basin, may contribute to a better understanding of their origin. A high-gravity anomaly along the basin axis has been suggested as due to a high-density lower crust caused by underplating. To test this hypothesis, S-wave velocity profiles in the Parana Basin were studied using interstation surface wave phase velocities. Ps—P time differences from receiver functions were used as constraints to select models with good fit. Genetic algorithms (GA) were used to find the best fitting model and the range of acceptable models. A trial-and-error jumping iteration was developed to improve the GA inversion efficiency. The average S-wave velocities of the lower crust are less than approximately 3.8 km/s, lower than the global average of 4.0 km/s for platform areas. Near the basin center, where flood basalt thickness is maximal, similar low S-wave velocities were found, suggesting that underplating was not significant or widespread in the Parana Basin and that a high density layer in the lower crust cannot be used to explain the high-gravity anomaly at the basin axis. No significant difference could be detected in the upper mantle between the axis and the border of the basin. The average upper mantle S-wave velocity is 4.65 km/s, a typical value for cold platform areas.