Evidence of crustal flexure induced by fluvial incisions

Abstract

Fluvial erosion of the Earth's crust creates valleys ranging from hundreds of meters to a few kilometers wide. The resulting unloading is supported by the flexural rigidity of the continental lithosphere. As a result, the bending of the lithosphere around individual river valleys is usually imperceptible in the landscape, specially in tectonically quiescent environments. However, in the Recôncavo-Tucano-Jatobá (RTJ) basins, an elongated Cretaceous rift located in northeastern Brazil, there is evidence of upward bending of sedimentary strata at the edges of the river valleys, where parts of the syn- and post-rift Late Cretaceous sedimentary layers were eroded. To assess whether the flexural response of the lithosphere due to fluvial erosion is able to explain the observed bending along the RTJ basins, we conducted a series of numerical experiments to evaluate the amplitude and wavelength of the lithospheric flexure achieved in the thermomechanical scenarios for the lithosphere, while imposing the observed amount of denudation derived from field data. We conclude that the wavelength and amplitude of the flexural behavior of the lithosphere obtained in the numerical scenarios are consistent with the observed bending only when the upper crust is partially decoupled from the lithospheric mantle, a configuration observed when the lower crust has a low effective viscosity. We propose that a combination of thermal and lithological factors contributed to induce the lithospheric decoupling, including heating of the lower crust due to magmatic underplating and the blanket effect created by the thick sedimentary layer preserved along the rift. Additionally, we propose that the flexural response of the lithosphere to individual river incisions is only noticeable in the continental lithosphere when the flexural rigidity of the lithosphere is low, with an effective elastic thickness (Te) ≲5 km.