Magnetic fabric, anisotropy of magnetic susceptibility (AMS), and inferred flow directions of dikes from Fogo Volcano, Cape Verde.

Fogo Island is an active volcano with at least 28 eruptions since 1460, the latest of which in 2014-2015. It is formed by a major conical and asymmetrical Quaternary strato-volcano with a summit depression (Chã das Caldeiras), within which a 1 km-tall cone rises to 2829 m, the maximum elevation of the island. The 8 km-wide Chã das Caldeiras is truncated by a large eastward flank collapse. Morphologically this depression is formed by two intersecting calderas, a northern and a southern one, separated by a spur (Monte Amarelo). The depression is surrounded on the north, west and south by an almost vertical wall (Bordeira) that reaches a maximum height of 1000 m. More than 500 dikes and sills, with widths ranging between 0.5m to 8m are exposed crosscutting the Bordeira wall, most of them broadly radial.
We studied 34 dikes along the base of Bordeira. The mean orientations of the dikes show two convergence areas. One, poorly constrained, located in the northern caldera, east of the Monte Amarelo spur, and another, well defined, broadly in the centre of southern caldera. These two convergence points are inferred as the origin of most of the radial dikes and suggest two distinct magmatic centres, which collapsed to form the calderas.
Magnetic susceptibility (MS) of the 34 studied dikes is lower than 140×10-3 SI units, without any noticeable systematic differentiation between the MS of the dikes of the different sectors.
Most of the dikes/margins (79%) show “Normal Magnetic Fabric”, defining an interpretable imbrication angle between the magnetic foliation plane (MFP) and dike margin. This behaviour shows that magnetic fabric (MF) orientation at the time of intrusion and late stages of cooling, was controlled by the dynamics of the magma flow. Just a few dikes mostly located in the intersection of the calderas display Intermediate or Inverse MF.
Thermo-magnetic k(T) analysis shows titanomagnetite as main magnetic carrier with compositions varying between Ti-rich to Ti-poor. Some samples show multiple magnetic phases identified by different Curie temperatures. Hysteresis measurements show that most of the samples fall in the PSD region with some samples showing a SD characteristic. This is consistent with the Intermediate and/or Inverse MF, observed in some margins, suggesting coexisting mixtures of inverse (SD grains) and normal (PSD and MD grains), magnetic fabrics.
Considering only the normal magnetic fabric margins (with usually triaxial or oblate AMS ellipsoid) and using the model of the imbrication of the MFP we obtain in 39 margins (57%), a reliable result that enables to infer a direction and sense of the magmatic flow. Results indicate an outward and mostly horizontal to near-horizontal flow, suggesting shallow magmatic sources. A few dikes show margins with asymmetric (“scissored”) magnetic fabrics which may indicate strike-slip shearing along the dike in the last phases of magma cooling.
This research has been funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – (https://doi.org/10.54499/UIDB/50019/2020), UIDP/50019/2020 (https://doi.org/10.54499/UIDP/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020). This work is a contribution to project GEMMA (Ref. PTDC/CTA-GEO/2083/2021)