Luiz Rogério Bastos Leal

Evolução crustal do setor ocidental do Bloco Arqueano Gavião, Cráton do São Francisco, com base em evidências U-Pb, Sm-Nd e Rb-Sr

The western part of the Gaviao Block (Sao Francisco Craton) is one of the oldest fragments of the South American Platform. It comprises the Riacho de Santana Gneissic Migmatitic complex, the Santa Isabel complex and the Riacho de Santana greenstone belt. T DM ages of country rocks of these units reveal that the main accretion periods were: 3.9 Ga, 3.2 - 3.0 Ga and 2.6 Ga. The Gneissic Migmatitic complex yields U-Pb age of 3648 ± 69 Ma, while the negative eNdt value is compatible with crustal reworking. The Rb-Sr isochron provided age of 3247 ± 120 Ma, tentatively interpreted as migmatization event. U-Pb data indicate that crystallization of a rock from the Santa Isabel Complex occurred at 2954 ± 100 Ma ago, with migmatization at 2748 ± 100 Ma ago (Rb-Sr isochron). This complex displays T DM ages of 3.3 - 3.1 Ga and variable eNdt parameters (-4.7 to +0.3), which suggest juvenile and reworking processes from Mesoarchean protholiths with short crustal residence time. Relative the metamafic (tholeiitic) rock from the Riacho de Santana greenstone belt, at least one of the samples indicates anomalous fractionation, yields a 2.6 Ga T DM age whereas the U-Pb crystallization age is 2218 ± 18 Ma. This suggests crustal contamination of the particular mantle source. Shortly after, the country rocks were intruded by the (U/Pb = 2054 ± 8 Ma) Guanambi batholith which was mainly derived from Archean protholiths (T DM ages of 2.8 - 2.7 Ga). Eventually, tectonic and thermal overprints took place in the study area during Meso- and Neoproterozoic periods, as indicated by K-Ar apparent ages and lower intercept age of a U-Pb discordia line from rocks of the Riacho de Santana Gneissic Migmatitic Complex. We believe that these younger episodes reflect the onset of the Northern Espinhaco intracratonic system and the Neoproterozoic dynamics, respectively, as similar as recorded over the Paramirim Corridor.

Evaluation of susceptibility for terrain collapse and subsidence in karst areas, municipality of Iraquara, Chapada Diamantina (BA), Brazil

The morphological evolution of the karstic systems is associated with a set of physical and chemical processes, triggered by the dissolution of the rocks, related to percolation of groundwater and surface water, which consequently open underground voids and carve out peculiar forms of relief. Due to environmental and geotechnical aspects, this system is naturally more fragile and vulnerable than other natural systems and, therefore, has increasingly received the attention of the scientific community over the past decades. The objective of the study was to delimit zones with varying degrees of susceptibility for collapses and subsidence of sinkholes in the municipality of Iraquara, Chapada Diamantina (BA), Brazil, and to understand their geological and morphological determinant factors. Geological data, karst phenomenon map, and visual analysis in the field were used to categorize zones with different types of susceptibilities to the nucleation of new sinkholes based on a Hazard Index. This index was defined from the sum of geological hazard factors, lineament density, and sinkhole density. The areas that presented the highest susceptibility for terrain collapse and subsidence corresponded to regions where carbonate rocks outcrop, with high density of photolineaments and 2.62 sinkholes/km2. Processes associated with terrain collapse and subsidence in karst areas consisted of a combination of various factors, hindering precise predictions. However, zones of different types of susceptibilities to terrain collapse and subsidence can be delimited when the relationships between these processes and their factors are understood. The Hazard Index proposed does not provide quantitative values for the probability of hazard susceptibility, but rather indicates areas that are more susceptible to terrain subsidence and collapse.