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

Abstract

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 1,214 nT. 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.

Characterizing the Source of the Eastern Galicia Magnetic Anomaly (NW Spain): The Role of Extension in the Origin of Magnetization at the Central Iberian Arc

“…In this regard, Martínez Catalán et al. (2018) already suggested that the origin of the EGMA was related with the extensional tectonics characteristic of the late stages of Variscan evolution in the present N Iberian Massif. These authors used the high‐resolution map of the absolute magnetic anomaly of Figure 3 to carry out 2‐D models showing that the anomaly could be mimicked by means of magnetic blocks parallel to extensional detachments, being the one associated to the Viveiro Fault the most important in terms of induced and remanent magnetization.…”

“…Detailed surveying has included the acquisition of ground‐based magnetic data and collection of samples for characterization studies. The map resulting from land acquisition (Martínez Catalán et al., 2018) is shown in Figure 3, with the location and number of paleomagnetic sites in color coded labels according to their Königsberger ratios (Qn). Thorough tracking of the anomaly and later mapping showed that not only late Carboniferous migmatites and inhomogeneous granites where highly magnetic (Ayarza & Martínez Catalán, 2007), but Neoproterozoic and early Cambrian metasediments (quartzites, schists, and gneisses) were also found to be magnetic in some areas (Figures 4c and 4d).…”

“…Martínez Catalán et al. (2018) took into account preliminary results of the remanence studies presented here and included in the EGMA models a reverse polarity remanence of J = 0.267 A/m, D = 156°, and I = −28°. These data are an average of the measured values, although remanence is actually undersampled and very heterogeneous.…”

“…Nevertheless, the fact that the anomaly continues and performs high amplitudes (Figure 1b) indicates that detachments still exist and deepen to the S as does the dome. The radially averaged power spectrum calculated from the aeromagnetic signature of the whole EGMA finds magnetic sources as deep as 12 km (Martínez Catalán et al., 2018), indicating that buried contributions to the anomaly do exist.…”

“…In this paper, we present a detailed analysis of the magnetic properties of the rocks that, according to Ayarza and Martínez Catalán (2007) and Martínez Catalán et al. (2018) are the source of the EGMA. These rocks are characterized by, at least, two diachronous populations of magnetic minerals that exhibit high magnetic susceptibilities and intense magnetic remanence, a very high AMS factor, and high δ 18 O ratios.…”