Dados da Submissão

Título

Relating differential crustal architecture to passive margin evolution: a case study from the Colatina Fracture Zone (SE Brazil) using apatite fission-track thermochronology

Texto do resumo

The Colatina Fracture Zone (CFZ) defines a distinct NNW-SSE-oriented linear zone of fractures and brittle faults that represents an inherited weak zone in the current crustal structure of the (Pre)Cambrian Araçuaí Orogen. In the Early Cretaceous, the CFZ was reactivated during rifting of West Gondwana and subsequent opening of the South Atlantic Ocean, as evidenced by the emplacement of dykes along its structural network and the development of major depocenters of the Campos Basin in the offshore segments of the CFZ. Previous thermochronological studies have demonstrated that the CFZ was also rejuvenated during the drift phase of the South Atlantic. However, a number of questions regarding differential surface uplift and basement exhumation between the CFZ and its surrounding areas, such as the Doce River Valley (DRV), are still unresolved. In this study, we aim to investigate the CFZ as a distinctive structure in the tectonic rejuvenation of the passive margin of southeast Brazil. Rock samples from the CFZ and DRV were collected in the states of Minas Gerais and Espírito Santo and underwent conventional fragmentation, magnetic and heavy liquid separation processes for apatite fission track analysis (AFT). The method consists of highlighting, measuring and counting spontaneous tracks (accumulated in the crystal structure of apatite due to the spontaneous fission of 238U), induced tracks (results of nuclear irradiation) and known patterns in apatite grains. Central ages were calculated using IsoplotR software and models with time-temperature (t-T) trajectories of Markov Chain Monte Carlo (MCMC) were generated using QTQt PC v 5.6.0. In the DRV, samples yield AFT central ages from 87 to 97 Ma with mean track lengths (MTL) from 12.6 to 13.3 µm. In contrast, in the CFZ, AFT central ages from 70 to 83 Ma with MTL values from 13.2 and 13.4 µm are obtained. The correlation between AFT age and elevation suggests that the tectonic development of these regions was markedly different and uncoupled. The thermal history models from the AFT data further constrain this differential evolution. On the one hand, thermal history modeling for the DRV indicates a slower and protracted cooling since the incipient Atlantic rifting in the Early Cretaceous. On the other hand, the models for CFZ reveal a rapid cooling phase between the Late Cretaceous to the Paleocene. In the DRV, the observed basement cooling was most probably triggered by erosion of the uplifted rift shoulder generated by Gondwana break-up. The more recent, Late Cretaceous – Paleocene rock cooling, localized in the CFZ, was synchronous with a major phase of the Andean orogeny. This suggests that reactivations and erosional exhumation of the CFZ basement could be a consequence of far-field propagation of intraplate compressional stress. The higher susceptibility of the CFZ to reactivating over its surroundings shows that structural inheritance is a key factor in the differential tectonic evolution of passive margins. Further research on the Late Cretaceous – Paleocene reactivation in the CFZ's offshore extension may be crucial for the exploitation of hydrocarbons in the Campos and Espírito Santos basins.

Palavras Chave

South Atlantic; reactivation; structural inheritance; Araçuaí orogen