Cartografia dos enxames de diques máficos da Província Borborema com base em dados aerogeofísicos e mapas auto-organizáveis

Abstract

Mapping widespread dyke swarms shed light on the tectonic processes that culminate in continental fragmentation, especially in the early phases of the crustal extension and magma emplacement. Airborne magnetic data are an effective geophysical tool to clarify the extent of dyke swarms in continental-scale areas due to the expressive magnetic contrast between magmatic bodies and host rocks. However, many geological features display similar magnetic patterns, making the qualitative interpretation of magnetic anomalies quite subjective and ambiguous. To improve and optimize the predictive mapping of dyke swarms, this research presents a multivariate analysis of airborne geophysical surveys, applying a Self-Organizing Maps (SOM) approach using two magnetic and three gamma-spectrometric variables. The SOM method was applied to investigate a set of mafic dyke swarms that intruded in the Neoproterozoic Borborema Province (BP), Parnaíba Basin (PB) and the São Francisco Craton (SFC) in NE Brazil. These dykes are part of a large magmatic event associated with the supercontinent Pangea breakup, which formed the Equatorial Atlantic Ocean in the Early Cretaceous, denominated EQUAMP. First, the SOM hyperparameters were defined by running the algorithm in a representative area in the central part of the BP. This training area worked as a SOM template, through which all data from the study area were processed. The SOM analysis identified seven different populations, according to responses found in the five geophysical input variables. Two of these populations were associated with the mafic dykes, reducing the subjectivity of the magnetic anomaly interpretation. These populations represent high SOM quantization error, which means that these groups are the most anomalous values, evidenced in airborne magnetic data. These results was checked during fieldwork, revealing that dyke swarms occur more widely than was previously known, throughout the BP, intruding the SFC, and showing some occurrences intro in the Paleozoic sedimentary infill of the eastern border of the PB. We also apply anomaly enhancement techniques to magnetic data to obtain the spatial distribution and depth estimate of the causative sources. A structural analysis was carried out, integrating magnetic patterns, field data and a compilation of previous geological maps to describe the detailed distribution of the dyke swarms in BP. Our analyses demonstrated that the dyke swarms extend over 6.8 x106 km2. The 1388 mapped mafic dykes are grouped in three distinct swarms: 1135 – Rio Ceará-Mirim (RCM); 168 – Canindé (CD); and 86 – Riacho do Cordeiro – RC. The dyke swarms show three preferential trends to E-W, NW-SE and NE-SW. The swarms intrude both the Precambrian basement units and the Cretaceous sedimentary basins. The geometry and structural analysis of the dykes allowed interpreting the paleostress field active at the intrusion time. To the RCM, the paleostress trajectories represent a N-S extension, rotating to NW-SE in the western portion of the swarm. While for CD, the paleostress orientation shows a NW-SE extension rotating to NE-SW. As for the RC, the trajectories were similar to those traced for the RCM, showing a predominant NW-SE extent.