Iron Formations as the Source of the West African Magnetic Crustal Anomaly

Nicolas Launay,Pierre Rochette,Yoann Quesnel,François Demory

doi:10.3389/feart.2018.00032

Abstract

The geological sources of major magnetic field anomalies are still poorly constrained, in terms of nature, geometry, and vertical position. A common feature of several anomalies is their spatial correlation with cratonic shields and, for the largest anomalies, with Banded Iron Formations (BIF). This study first unveils the magnetic properties of some BIF samples from Mauritania, where the main part of the West African magnetic anomaly is observed. It shows how strong the magnetic susceptibility and natural remanent magnetization for such rocks are. High Koenigsberger ratios imply that the remanent magnetization should be taken into account to explain the anomaly. A numerical modeling of the crust beneath this anomaly is performed using these constraints and both gravity and magnetic field data. A forward approach is used, investigating the depth, thickness and magnetization intensity of all possible crustal lithologies. Our results show that BIF slices can be the only magnetized crustal sources needed to explain the anomaly, and that they could be buried several kilometers deep. The results of this study provide a new perspective to address the investigation of magnetic field anomaly sources in other cratonic regions with BIF outcrops.

Highlights

The largest anomalies of the Earth’s magnetic field are often observed over old transition zones at the edges of cratonic regions
This anomaly reaches about 37 nT at satellite altitude. Another good example corresponds to the Bangui magnetic anomaly (Regan and Marsh, 1982; Ouabego et al, 2013) located between the North African and Congo cratons in the Panafrican Belt
An usual characteristic of these areas is the presence of Banded Iron Formations (BIF) (e.g., Alexandrov, 1973; Ravat et al, 1993; Schmidt et al, 2007)

Summary

INTRODUCTION

The largest anomalies of the Earth’s magnetic field are often observed over old transition zones at the edges of cratonic regions. Note that hysteresis was obtained on subsamples of the plug used for susceptibility, NRM and ARM measurements, leading to possible incoherent results for the same sample ID due to the heterogeneity of the rock at FIGURE 8 | Two possible simulation results for a model of the crustal structure along profile C beneath the WAMA, with induced magnetization only (a: M = 0 A/m) and adding remanence (b: M = 156 A/m). Due to the high susceptibility (>1 SI), a shape anisotropy effect will arise at the scale of the whole body imbedded as a slice in weakly magnetic crust, leading to a further deviation of magnetization within body elongation (see discussion on serpentinite magnetization in Rochette, 1994) This may mean that considering an induced magnetization in the ambient field direction is not straightforward for such rocks. Our results show a slightly better fit (RMS inferior by 1.5 nT) with only BIFs bodies, adjusted to compensate for the absence of deep source (present in Figure 9), meaning that a deep magnetized crust is not really needed in this case

DISCUSSION

Findings

CONCLUSIONS