Magnetic Anomalies in Extensional Detachments: The Xistral Tectonic Window of the Lugo Dome (NW Spain)

The Eastern Galicia Magnetic Anomaly (EGMA) is a conspicuous feature of the aeromagnetic map of Iberia, which structurally overlaps the Lugo and Sanabria domes, two spatially linked late Variscan extensional structures that delineate a segment of the Ibero-Armorican Arc. In the northern part of the Lugo Dome, the Xistral Tectonic Window exposes a deep section of the continental crust, which includes several extensional detachments and associated shear zones. The aim is to check whether or not there is a correlation between the extensional process and the EGMA. A map of the magnetic anomaly of the Xistral Tectonic Window acquired on land improves the resolution of the aeromagnetic data and shows that the EGMA is actually composed by several shorter wavelength anomalies reaching amplitudes up to 1,214nT. Their location in relation to outcropping rocks and structures, together with 2-D modeling, shows a conspicuous link between the anomalies and the extensional detachments. On a larger scale, the EGMA forms the northern part of a largeanomaly occupying the core of the Central Iberian Arc. This anomaly can be related to extension caused by gravitational collapse of the thickened Variscan crust, implying that like the EGMA, local anomalies might reflect extensional structures, many of which may remain buried. Our study may shed light on the origin of magnetic anomalies in other gneiss domes as it shows that they can be related with the doming process instead of being inherited from older rocks presently outcropping at the core of the domes.