The denudation history of the onshore continental margin of SE Brazil inferred from apatite fission track data

Abstract

A suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast Brazil are presented. Some of the results reflect resetting related to the eruption of the Paraná continental flood basalt (∼130 Ma), and four others are directly related to Late Cretaceous magmatism (∼80 Ma). The majority of the results were obtained from Precambrian basement rocks and sediments from the Paraná Basin. The fission track ages broadly increase inland from 60–90 Ma on the coastal plain to >300 Ma in the continental hinterland. The track length distributions show a characteristic trend, being negatively skewed at the coast and showing increasing bimodality inland, except for the oldest samples which are more unimodal. These trends do not reflect rift‐related heating and resetting but are the result of protracted denudation since the opening of the South Atlantic. The results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland. The topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology. For the region further south, model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long‐wavelength topography, unless the flexural rigidity is more than 1025 N m. In this case the initial elevation of the rift margin would need to be >2.5 km in order to accommodate the amount of denudation inferred from the fission track data. However, it is possible that the effective flexural rigidity varies across the margin, becoming greater in the continental interior. Alternatively, the broad scale wavelength topography may be the result of magmatic underplating related to the Paraná continental flood basalt event. If so, estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5.