Post-rift reactivation of the onshore margin of southeast Brazil: Evidence from apatite (U-Th)/He and fission-track data
Résumé
On the continental margin of southeast Brazil there is good geological evidence for post-rift tectonic reactivation, in the form of onshore Tertiary basins up to 800 m deep and Cretaceous to Paleocene alkaline intrusive bodies at the surface. With the aim of constraining the Mesozoic and Cenozoic exhumation history of this region, we present new apatite (U-Th)/He ages on basement samples, which one of us previously used for apatite fission track (AFT) analysis. The new He ages range from 115.5±2.2 Ma to 47.0±6.4 Ma, whereas the AFT ages range from 330.3±22 Ma to 64.5±3 Ma. Inverse thermal history modelling of all the data provides more detail on the post-rift cooling history than do the AFT data alone. Furthermore, models incorporating radiation damage effects on He diffusion produce thermal histories that are more consistent with independent geological evidence than models without these effects. The results imply that only a few samples record an early cooling phase, around the time of opening of the South Atlantic Ocean. The new (U-Th)/He data also imply a Late Cretaceous phase of exhumation between 90 and 60 Ma. Subsequently, the region underwent slow cooling (i.e. erosion rate). Two or three samples seem to indicate a period of rapid cooling during the Neogene, although this is at the limit of resolution of the methods. The post-rift cooling phases imply exhumation from depths between 2 and 5 km. This activity we attribute to compression in the South American plate, between the Andean subduction zone and the mid-Atlantic ridge. In southeast Brazil, compression acted on a crust that was thermally or mechanically susceptible to reactivation. During the Late Cretaceous, deformation in the study area was of regional extent, whereas during the Tertiary, it concentrated along pre-existing shear zones of the Precambrian Brasiliano orogeny.