Complex structure of Piton de la Fournaise and its underlying lithosphere revealed by magnetotelluric 3D inversion

Abstract

La Réunion is a large volcanic construction resting on Paleocene oceanic crust. Through the 3D inversion of a large set of magnetotelluric (MT) soundings, our results reveal the general resistivity structure of the western part of Piton de la Fournaise volcano down to its base and the first ten kilometers or so of the underlying lithosphere. The upper resistive layer is associated to a superimposition of unsaturated and probably water-saturated lava flows with an averaged thickness of 1.5 km overlying more or less continuous highly conductive patches which imply the presence of highly conductive fluids and/or minerals. In the summit area and Enclos, this conductor is unambiguously attributed to the presently active hydrothermal system. In the Plaine des Sables, it is tentatively associated with the hydrothermal alteration beneath the ancient volcanic center. A third widespread conductive patch is observed below the NW flank of Piton de la Fournaise resulting more likely from volcanic activity since it coincides with part of the N120 rift zone. In the area beneath the Plaine des Sables where a dense intrusive complex has been inferred from gravity models, the resistivity is not significantly higher. The upper lithosphere is markedly more resistive than the bulk of the volcanic construction (except for the upper part of this latter) forming a virtual horizon around-4 km in depth. However, one main conductive anomaly is present in the crust beneath the NW flank along the N120 rift zone assumed to be linked with present day seismic and degassing activity. The NW flank therefore appears as a main area of interest in the study of Piton de la Fournaise activity and its evolution as well. We therefore demonstrate the capability of the method to image major shallow structures such as the hydrothermally altered zones, and deeper ones such as possibly the heterogeneity of the crust.

Imagery of internal structure and destabilization features of active volcano by 3D high resolution airborne electromagnetism

“…Our AEM global resistivity model is in good agreement with the geo-electrical models deduced from previous geophysical studies 17,20 , validating the global trend of our more detailed imagery. At the near-surface, the resistive layer - several hundred meters thick - corresponds to recent lava flows.…”

“…Among others, ground resistivity imagery techniques, such as 2D or 3D electrical resistivity tomography (ERT) 10,14,15 , magnetotellurics (MT) and transient electromagnetic soundings (TDEM) 16,17 , are renowned for being well-suited and reliable in such settings. While ERT oftenly provides high resolution imagery of the first hundred meters 18,19 , MT and TDEM provide a smoother image of the bulk resistivity up to few kilometres depth 20 . All these methods have been improved during the last decades to image resistivity variations in 3D 19,20 .…”

“…While ERT oftenly provides high resolution imagery of the first hundred meters 18,19 , MT and TDEM provide a smoother image of the bulk resistivity up to few kilometres depth 20 . All these methods have been improved during the last decades to image resistivity variations in 3D 19,20 . However, ground acquisitions in steep and harsh environments, as it is the case on the Piton de la Fournaise volcano (Fig.…”

Hydrothermal and Magmatic System of a Volcanic Island Inferred From Magnetotellurics, Seismicity, Self‐potential, and Thermal Image: An Example of Miyakejima (Japan)

“…Electrical resistivity values range from 2.5 to 2,200 Ω•m, and thus agree with the previous 1-D resistivity model obtained at Miyakejima by Zlotnicki et al (2003) from audio-magnetotellurics soundings. This resistivity range is also consistent with several volcanic islands (e.g., Gailler et al, 2018;Piña-Varas et al, 2018;Pryet et al, 2012;Revil et al, 2011) and continental arc volcanoes (e.g., Aizawa et al, 2005;Heise et al, 2016).…”

“…In Miyakejima, a sharp resistivity contrast exists between the superficial high-resistive materials ('u' unit) and the underlying conductive ones ('c' unit). Such vertical variation is often observed on active volcanoes that include significant topography and meteoric recharge (e.g., Aizawa et al, 2005;Gailler et al, 2018;Piña-Varas et al, 2014;Revil et al, 2011;Rosas-Carbajal et al, 2016;Triahadini et al, 2019;Usui et al, 2016). This change in electrical resistivity is generally interpreted as a shift from unsaturated to water-saturated regime, delimiting the water table boundary of the volcano (e.g., Aizawa et al, 2009;Hurwitz et al, 2003).…”