Thermochronology and landscape evolution of the Serra do Caparaó region, an onshore segment of the Brazilian South Atlantic continental margin

Abstract

The origin and evolution of elevated passive continental margins have intrigued geoscientists since their prominent topography is more reasonably explained in the context of an active rift than a mature `passive’ setting. A classic example is the Brazilian continental margin which evolved from a rift in Cretaceous, ca. 130 Ma ago, yet presents elevations that reach more than 2000 m above sea level. The Brazilian South Atlantic marginal upwarp extends for more than 3000 km and shows substantial morphological variation. Nevertheless, geoscientists have been mainly focused on studying the escarpment-form segments (e.g., the Serra do Mar in southeast Brazil) and frequently assuming the Brazilian elevated continental margin as a well-defined and continuous marginal upwarp instead of a diversified and more complex landscape. Attempting to understand the landscape variation along the Brazilian South Atlantic passive margin, this work uses the low-temperature thermochronology to explore how time-space denudation patterns differ along the southeast Brazilian coast with a particular focus on the Serra do Caparaó segment; a relatively less-studied area found adjacent to the Serra do Mar and where the typical escarpment form is absent. Overall, the thermochronology analysis shows an asymmetric denudation profile in which AFT ages tend to increase with elevation and toward the continental interior. The continental drainage divide delimits a dissected coastal region (i.e., where AFT ages are generally younger or similar to the rifting event) from a less denudated continental interior area (i.e., where AFT ages are older than continental breakup). This asymmetric denudation profile is associated with the generalized differential exhumation of the coastal region due to the continued erosion and unloading isostatic rebound triggered by the continental breakup processes, which are expected in generic passive continental margins evolution models. However, the Brazilian continental margin shows AFT ages substantially younger than rifting event, suggesting that the margin has experienced increased post-rift exhumation. Thermal models frequently show a pulse of cooling occurring long after rifting event, indicating that an additional transient factor than continued rifting-related denudation may have influenced the exhumation of the margin. The post-rift exhumation seemed to have been controlled by the reactivation of lithospheric inherited structures that triggered the relative uplift/subsidence between basement blocks resulting in contrasting denudation patterns along the coastal region, including the onshore segment related to the Espirito Santo basin (i.e., the Serra do Capraó region). In this situation, the Serra do Capraó is currently a high-elevation coastal interfluve segment that includes relicts of ancient long-lived syn-rift coastal catchment areas (i.e., syn-fit less-denudated coastal interfluve region) that has been changed and reestablished during the post-rift reactivation. Finally, this study highlights that the thermochronology pattern corroborates the morphological differences along the margin, supporting that the Brazilian continental margin is not a single and continuous tectonic setting. Furthermore, besides the variety of proposed mechanisms operating over the post-rifting evolution of the margin, tectonic inheritance (i.e., structural framework, lithology, the effective flexural strength of the lithosphere) has strongly impacted the denudation pattern contributing to the geomorphic diversification along the Brazilian South Atlantic coast.