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

Lydie-Sarah Gailler, Anna Martí, Jean-François Lénat

► To cite this version:

Lydie-Sarah Gailler, Anna Martí, Jean-François Lénat. Complex structure of Piton de la Fournaise and its underlying lithosphere revealed by magnetotelluric 3D inversion. Journal of Volcanology and Geothermal Research, 2018, 356, pp.200-210. 10.1016/j.jvolgeores.2018.03.006. insu-03708972

HAL Id: insu-03708972

https://insu.hal.science/insu-03708972v1

Submitted on 17 Jun 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Complex structure of Piton de la Fournaise and its underlying
lithosphere revealed by MT 3D inversion.

Lydie-Sarah Gailler¹, Anna Marti², Jean-François Lénat¹

¹ Université Clermont Auvergne, Laboratoire Magmas et Volcans - CNRS, UMR 6524, IRD, R 163, 6

Avenue Blaise Pascal TSA 60026 - CS 60026, 63178 AUBIERE Cedex, FRANCE

² Departament de Geodinàmica i Geofísica, Universitat de Barcelona, Spain

Corresponding author: lydiegailler@hotmail.fr

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.

32

33 **Keywords:** Piton de la Fournaise; magnetotelluric data; 3D inversion; hydrothermal system;
34 oceanic lithosphere; plumbing system.

35

# 1. Introduction

The architecture of volcanic systems is important to know because: (1) it provides information
on the construction and evolution of the volcanic structures; (2) it has an influence on the
magma paths (the conduits) and (3) it may generate its own volcano-tectonic activity through,
for example, gravitational instabilities. The need of high resolution imagery of the interior of
the volcanic systems is therefore obvious and is the main motivation for the application
geophysical prospecting methods in volcanology. Several geophysical techniques can be used
to image the structures and rocks physical properties at depth. The most common are
seismics, gravity and electrical resistivity measurements. The magnetotelluric (MT) method
has been used since the early 1980s in volcanic areas for both geothermal exploration and
volcanological studies. It is particularly well suited for volcanic studies (Piña et al. 2014 and
references therein) since the electrical conductivity is influenced by both temperature and
compositional variations. But it has also the capability to probe deep in the crust and the
mantle (Jones, 1999)
Electrical resistivity of crustal rocks generally varies over several orders of magnitude (10⁻⁴ to 100 kohm·m), depending on a wide range of petrological and physical parameters, such as bound versus free water, composition of the fluids, temperature, and the content of clay or metallic minerals. Resistivity is usually dominated by the ionic conductivity of the fluids present in the interconnected pores. In general, a basaltic shield consists predominantly of a pile of highly permeable lava flows. If not altered or water saturated, such formations generally have resistivity as high as 10–100 kohm-m. A large resistivity variation usually exists at the transition between the vadose zone, where rocks contain only some moisture, and the zone beneath the water table, where the rocks are fully water saturated. On oceanic islands, the basal water table consists of a lens of fresh water (Ghyben-Herzberg lens; e.g., see Vacher, 1988) floating over the seawater-saturated zone. The top of the freshwater lens generally takes place at an elevation of a few meters to a few tens of meters above the sea level. However, this model, initially derived for the Hawaiian Islands, may not apply when the hydraulic conductivity decreases significantly with depth. In this case, the water table forms a high elevation dome within the edifice. The latter model is preferred in the case of Piton de la Fournaise by Join et al. (2005). The resistivity of a basaltic pile saturated with fresh water is generally a few hundred ohm·m, but drops to a few tens of ohm·m or less when saturated with sea water. In addition to water saturation, the nature of the formation also plays a role in the resistivity distribution. In intrusion zones (central zone, rift zones), swarms of subvertical dikes can create impermeable barriers that impound bodies of water at high elevations (Stearns, 1942; Takasaki, 1981; Jackson and Kauahikaua, 1987). Alteration, particularly hydrothermal alteration, tends to lower the rock resistivity through the formation of highly conductive, hydrated minerals such as clay minerals and zeolites. Similarly, pyroclastic and breccia deposits generally have lower resistivity values than lava flows, because they typically contain hydrated minerals. Magma has a resistivity of only a few
ohm·m or less (Rai and Manghnani, 1977; Haak, 1982). The resistivity of the upper oceanic lithosphere underlying a volcanic edifice may be different and more complex than regular oceanic lithosphere. Constable and Cox (1996) indicate that the resistivity of an undisturbed upper oceanic lithosphere is more than 100 ohm·m. Increasing compaction with depth causes a decrease in the effective porosity and hence increases the resistivity. However, it may vary significantly depending on its temperature, and hence often its age: a young, hotter, lithosphere will be less resistive than its older counterpart (Nobes et al., 1992; Unsworth, 1994; MacGregor et al., 1998). The presence of conductive layers (e.g. fluid and/or mineral-filled fractures, magma) will also contribute to lowering the rock resistivity. In the case of hot spot volcanism, magmatic underplating is a common feature, and this has been suggested by Charvis et al. (1999) and Gallart et al. (1999) for La Réunion. Therefore the presence of magma beneath the crust can be investigated with the MT data. At greater depths, as the temperature increases, the mantle becomes more conductive. Laboratory experiments on basaltic and mantle rocks show the very low electrical conductivity of melts. For example, at the scale of the Hawaiian hot spot swell, 2D electrical models have shown a resistive lithosphere underlain by a conductive lower mantle, and a narrow conductive plume connecting the surface of the island to the lower mantle (Constable and Heinson, 2004).

In this work, we focus on La Réunion volcanic edifice, which has been extensively studied since decades, with a special interest given to Piton de la Fournaise volcano (see Lénat et al. (2012a and reference therein)). However, the deep architecture of Piton de la Fournaise and its underlying lithosphere remain poorly known. Previous geoelectrical studies carried out by Lénat et al. (2000) focused on the first kilometer or so of the active part of the volcano. The aim of the current study is to take advantage of a new large set of MT soundings acquired during a geothermal reconnaissance of Piton de la Fournaise in order to image the resistivity structure down to several to tens of kilometers. The good spatial coverage of the sites and the
lack of a preferred strike direction obtained from the dimensionality analysis allowed us to invert the data and obtain a 3D geoelectrical model of the volcanic edifice and its surrounding which is discussed in terms of large scale structures.

## 2. Geological and geophysical structure of Piton de la Fournaise and its basement

Piton de la Fournaise is located on the southeast of La Réunion Island (Fig. 1a), and is one of the world's most active basaltic shield volcanoes. Its evolution can be reconstructed by means of its deep valleys. The evolution can be divided into two main periods: the Ancient Shield (≥ 0.15 Ma) and the Recent Shield (< 0.15 Ma). At the base of the deep valleys (Rivière des Remparts, Rivière de l'Est and Rivière Langevin) (Fig. 1a), a differentiated unit called Pintades Unit (Bachlery, 1981; Bachlery and Mairine, 1990; Merle et al., 2010) is evidenced. It is dated from at least 530 kyr to 400/450 kyr, and is inferred by Smetiana (2011) to belong to a volcano which predates Piton de la Fournaise called Les Alizés volcano (Rançon et al., 1989; Malengreau et al., 1999; Lénat et al., 2001). The Pintades Unit is observed up to 1000/1200 m in elevation in the Rivière des Remparts, and therefore Piton de La Fournaise, rising at 2632 m, must be a relatively thin edifice (Merle et al., 2010).

Since about 290 kyr, the geological history of Piton de la Fournaise is summarized as a sequence of volcano-tectonic events with alternating episodes of volcanic destruction and reconstruction (Merle et al., 2010). Two calderas have been identified during the Ancient Shield period: the Rivière des Remparts caldera at about 250 kyr and the Morne Langevin caldera at about 150 kyr. During the Ancient Shield period, the volcanic center is inferred to have been located in what is presently the Plaine des Sables (Fig. 1). Bachlery and Mairine (1990) propose that the eastward displacement of the volcano to its present location (Fig. 1b) could have taken place at about 150 kyr, concurrently with the formation of the Morne
Langevin caldera, although it could be as young as the formation of the Plaine des Sables caldera, which formed at about 65 and/or 40 kyr (Gillot and Nativel, 1989; Bachèlery, P. and Mairine, 1990; Staudacher and Allègre, 1993; Merle et al., 2010). Letourneur et al. (2008) suggest the possibility of an intermediate location before the present one. The formation of the most recent, 8 km-wide caldera, called the Enclos Fouqué caldera (or, simply, Enclos), has been dated at about 4.5 kyr (Bachèlery, 1981; Staudacher and Allègre, 1993; Mohamed-Abchir, 1996), but the recent reappraisal of the existing radiocarbon data and new ages suggest two younger collapse episodes at around 3700, 3100 and 2300 kyr (Morandi et al., 2015). The present eruptive center is located on the Central Cone which has grown inside the Enclos caldera, which is breached to the east (the Grandes Pentes and Grand Brûlé areas). The relationships between the Enclos Fouqué, the Grandes Pentes and the Grand Brûlé are still debated (Bachèlery, 1981; Merle and Lénat, 2003; Michon and Saint-Ange, 2008).

The locations of the magma reservoirs and transfer structures have been recently reviewed by Michon et al. (2016). Whereas most of the recent eruptive activity took place within the Enclos, either on the Central Cone or along the NE and SE rift zones, the presence of eruptive fissures and pyroclastic cones in other areas of the edifice indicates that magmas have reached the surface via different paths. In addition to the NE and SW rift zones that radiate from the Central Area of Piton de la Fournaise and extend beyond the Enclos (Fig 1a), a broad, diffuse N120 rift zone occurs between Piton des Neiges and Piton de la Fournaise volcanoes. It is marked by numerous (~160) cinder cones and lava flows. The age of this rift zone is not well defined, but eruptions began at least 29 ky ago (McDougall, 1971) and the last eruption is dated at 0.140 kyr (Morandi et al., 2015). Another concentration of cones is observed at the base of the subaerial south flank of Piton de la Fournaise. Michon et al. (2016) name this area the "South Volcanic Zone". Michon et al. (2016) propose to interpret the deep seismicity observed at La Réunion in connection with the distribution of the volcanic zones at the
148 surface. They use a compilation of the volcano-tectonic (VT) seismicity from 1993 to 2013.

149 Below a depth of 10-11 km (base of the crust, according to (Gallart et al., 1999)), most of the
150 VT events are located beneath the northwestern part of Piton de la Fournaise and beneath the
151 NW flank of Piton de la Fournaise and the Plaine des Cafres areas. They are generally deeper
152 (up to about 30 km) to the NW and become progressively shallower (up to 10-11 km) near the
153 presently active area of Piton de la Fournaise. No seismicity is observed from the base of the
154 crust to a region above 8 km in depth. Michon et al. (2016) suggest that this aseismic zone
155 could correspond to a zone of magma storage (underplating). They also propose that the deep
156 northwestern seismicity is organized along a N30-40 axis at depths greater than 20 km and
157 then rotates and concentrates beneath the N120 NW rift zone. Michon et al. (2016) link the
158 deep seismicity with magma transfer, and suggest a vertical ascent of magma between 30 and
159 20 km in depth. Earthquakes are observed from about 8 km in depth to the surface before and
during volcanic crises with an overall lateral migration of the seismic swarms along 10-11
160 km, from the Plain des Sables toward the presently active area of Piton de la Fournaise. In
161 addition, a high resolution survey of soil CO2 emissions carried out on the western flank of
162 Piton de la Fournaise (Boudoire et al., 2017) has evidenced that narrow zones of high fluxes
163 are aligned along a well-defined N135° axis coincident with seismic swarms detected during
164 the reawakening of the volcano (2014–2015). The authors have called this alignment the
165 Songit Lineament (Fig. 1a) which is consistent with the orientation of paleo-spreading axes.
166 They also suggest that this latter could result from the inheritance of the oceanic lithosphere.
167

168 Receiver function techniques have also provided information in depth (Fontaine et al., 2015).
169 A joint inversion of receiver function and surface wave dispersion suggests that thin
170 magmatic underplating may be present below La Réunion. Lateral variations of its thickness
171 suggest that this latter could result from an important interaction between the plume and
172

lithosphere. In addition, a low velocity layer interpreted as a zone of partial melt beneath the active volcano is evidenced at a depth of about 33 km.

173

174

At Piton de la Fournaise, recent geophysical explorations using electromagnetic surveys have provided new hydrogeological information (Join et al., 2005). Their hydrogeological model suggests the presence of a continuous aquifer which domes at 1800 m asl below the central active part. It deepens to between 1600 to 1400 m asl beneath the Plaine des Sables and is even deeper beneath the Fond de la Rivière de l'Est depression. A recent synthesis of all available data led Lénat et al. (2012 and reference therein) to propose a geological model of the infrastructure of Piton de la Fournaise and its basement (Fig. 1b). We summarize here the main points of this interpretative geological section, starting from the surface: (1) shallow accumulation of dense lava flows in paleo depressions, (2) presence of pervasive hydrothermal alteration, (3) the presumed interface with Les Alizés volcano, (4) presence of hypo-volcanic complexes (gabbros, cumulates) beneath the Plaine des Sables area, (5) possible ancient and recent magma reservoir at the interface between La Réunion and the oceanic crust, (6) no significant flexure of the lithosphere, and (7) possible magma underplating at the base of the crust.

175

176

177

178

179

180

181

182

183

184

185

186

187

188

The dense Ancient Shield hypovolcanic complex (ASC) beneath the Plaines des Sables-Enclos area is a major feature inside the edifice that creates a positive gravity anomaly. The location of the complex has been deduced from 2D ¾ and 3D gravity modelling (Gailler et al., 2009). Gabbro and cumulate xenoliths found in lava flows and tephra above this complex (Bachèlery, 1981) probably originated from it. The base of the volcanic edifice on the oceanic crust is inferred to be around -4 km in elevation, which is the level of the oceanic floor around La Réunion. This is a reasonable assumption given that seismic studies (De Voogd et al., 1999; Gallart et al., 1999) suggest that there is no lithospheric flexure beneath La Réunion.

189

190

191

192

193

194

195
196

Lastly, we note that the estimated Curie depth, based on analysis of the magnetic anomalies
(Gailler et al., 2016), shows a bulge in the Curie surface below the island, with its top shifted
offshore to the NE of Piton de la Fournaise. Nevertheless, several features as well as the deep
structure, beyond the volcanic edifice sensu stricto remains poorly imaged.

197

198

199

200

## 3. Resistivity data

201

202

203

In order to investigate the deep structure of the edifice, we have compiled all available
electrical data, especially magnetotelluric (MT) soundings which provide the greatest depth of
investigation in the western part of Piton de la Fournaise area.

204

### 3.1. Previous studies and new data

205

206

207

208

209

210

211

212

213

214

215

216

217

218

219

Electric and electromagnetic surveys have been carried out on La Réunion Island for
hydrogeologic studies (Courteaud, 1996; Schnegg, 1997), structural studies of Piton de la
Fournaise (Schnegg, 1997; Lénat et al., 2000) and geothermal prospecting (Benderitter and
Gérard, 1984; Benderitter, 1990; Smith, 1996). A compilation of these datasets has been
already provided by Gailler and Lénat (2012) to study the island at a larger scale. In this
study, we have mostly used new data acquired during a geothermal reconnaissance project
between 2001 and 2004. The main datasets (Fig. A1 in Supporting Information A) come from
a magnetotelluric (MT) survey carried out by Phoenix Geophysics in 2002 (34 collocated MT
soundings, AMT for audiomagnetotellurics and TDEM for Time Domain ElectroMagnetic
soundings) and an MT survey by PB Power (53 collocated MT and TDEM soundings). The
data were collected in the 10⁻³ to 10⁴ Hz frequency range. We have also used 24 AMT
soundings (in the range 1 to 10⁴ Hz) acquired by our group (here designated as UBP) in 2001
(7 collocated with shallow Direct Current (DC) older electrical soundings), 4 MT soundings
(in the range 10⁻³ to 10³ Hz) acquired in 2001 by University Pierre et Marie Curie (here
designated as Paris) and 3 old MT soundings acquired by the Bureau de la Recherche
220

Géologique et Minière (BRGM). The data of the latter (in the range 10⁻² to 10² Hz) were digitized from paper copies.

221

222

## 3.2. Data processing and analysis

223

The MT and AMT data were processed using WinGlink® software package from Geosystem.

224

When multiple sets of data were available for the same location (i.e. HF and LF for AMT and MT data), the data were combined into a single sounding. A first quality inspection based on the error bars and the consistency of the resistivity curves made us reduce the dataset to 108 sites located in figure 2. It can be observed that the density of soundings is much higher in the Plaine des Sables, since this area was the target of the geothermal reconnaissance survey.

225

226

227

228

229

However, parts of the Fond de la Rivière de l'Est, Plaine des Remparts and even the NW flank of Piton de la Fournaise Central Cone and Enclos are well enough covered by the data to allow us to expand our study to these areas.

230

231

232

### 3.2.1. Dimensionality analysis

233

We have carried out a dimensionality analysis using the WALDIM code from Martí et al. (2009), which is based on the WAL invariants (Weaver et al., 2000). We used a threshold value of 0.2 and assumed an error percentage of 5%. As shown in Figure 3, the distribution of dimensionality shows a predominant 1D behavior for short periods (< 1s) and a few 2D and mainly 3D as periods increases beyond 1s, which seem to be reasonable. The behavior becomes fully 3D for periods longer than 10 s (Fig. 3), suggesting that a 3D inversion of the data is needed for a consistent interpretation of the overall dataset.

234

235

236

237

238

239

240

In order to better investigate the quality of the data and to provide a first general view of the resistivity distribution in depth, we provide maps of invariant phases for periods ranging from 10⁻⁴ to 10³ (Fig. 4a) using WINGLINK software. In such analysis, the invariant phase is calculated as the arithmetic mean of the phases elements, provides an image of the variation

241

242

243
244

of resistivity with the period, and presents the strong advantage to be unaffected by static
shift. To complete the data analysis, Bostick resistivity inversions are also presented showing
the main distribution of resistivity at fixed elevations (Fig. 4b). An overall good coherency is
evidenced between both representations (Fig. 4a and b). For the shortest period (10⁻⁴s), the
shallowest layer (2000 m asl) shows a very low invariant phase as well as high resistivity
values for most of the study area. At an intermediate range of periods (between 0.001 and 1s),
the invariant phase then progressively increases, especially in the area of Enclos Fouqué,
Plaine des Sables and Fond de la Rivière de l'Est. This feature could be correlated with the
decrease in resistivity observed from 1000 m to – 2000 m asl. Beyond a period of 100s and a
depth of -4000 m, the invariant phase scheme and the resistivity distribution are more
complex, with an overall increase in resistivity with depth.

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

Therefore, this analysis enables to delineate several interesting features: i) a resistive shallow
layer above 1000 m asl, ii) an intermediate conductive layer between 1000 and – 2000 m in
average, and iii) higher resistivity more in depth with both lateral and vertical heterogeneities
at depths greater than -4000 m asl. The 3D model will help depict the details of this general
description.

260

261

3.2.2. Sea water and topography artifacts

262

263

264

265

266

267

268

In an oceanic environment such as La Réunion with strong reliefs, it should be stressed that
the data artifacts associated to the sea, topography and bathymetry can be important. This is
especially the case for coastal soundings where the influence of the sea could be significant
and affect the interpretation of AMT and MT data. Pinã-Varas et al. (2014) have identified
such effects on the MT data by constructing a 3D conceptual geoelectrical model of Tenerife
Island, using known geological and geophysical data. Details on this synthetic test are given
in their study, suggesting that the MT responses are strongly distorted by topography and sea
effects at frequencies lower than 0.1 Hz. These effects will not be an issue as far as the bathymetry, topography and sea water are included in the model. A 3D approach should be, here again, preferred to provide reliable modeling of our dataset. 2D inversions have been also performed using Winglink software to investigate several profiles of interest. However, the structures are not fully reliable given the strong assumption of strike direction and TE and TM mode.

## 4. 3D inversion

### 4.1. Initial model

We have computed a 3D electrical resistivity model of Piton de la Fournaise area using the ModEm code (Egbert and Kelbert, 2012; Kelbert et al., 2014). The starting 3D mesh model was built using WINGLINK described by Mackie and Madden (1993) subsequently developed and implemented by Geosystem. The initial model was discretized onto a 79 (NS) x 70 (EW) x 78 (vertical) grid, where topography and bathymetry were defined and fixed using a digital elevation model. The resistivity value of the seawater was set to 0.33 ohm·m, and a thin layer of conductive sediments underwater was added in the initial model fixed at 200 ohm.m. The mesh was more refined in the first km (from 2600 m asl to -5000 m bsl) with the aim of obtaining a clear image of the area of hydrothermal alteration and eventually the presence of a magmatic reservoir more in depth. The inversions were undertaken using both the off-diagonal (Zxy, Zyx) and diagonal when available (Zxx, Zyy) components of the impedance tensor for a maximum of 15 periods in the period range between 0,0001 s to 1000 s, depending on the stations. A 5 % error floor in the impedance components was imposed during the inversion process. A smoothing of 0.2 was applied in all directions and an initial lambda value of 10 was chosen. The starting RMS was 9.45, while the final RMS is 1.50 after 63 iterations. Fig. B1 in the supplementary material B shows the comparison between the raw
293
294

295
296

297

data and model responses as apparent resistivity and phases curves for each site used in
inversion. Figure 5 shows an example for one site showing that the fits of the diagonals are
not perfect but acceptable. Several tests have been also done inverting either the full tensor or
only the off-diagonals. The resulting models are very similar, which confirms the overall good
consistency of the final 3D model described hereafter.

298

## 4.2. The 3D final model

299
300

301
302

303
304
305

In order to better visualize the 3D model, we present here two multi-segments sections crossing the main areas of interest, i.e. Plaine des Sables, Enflos Fouqué, Fond de la Rivière de l'Est and NW flank of Piton de la Fournaise (Fig. 6a and b). The geological studies described above and in figure 1b constitute a good reference model to start interpreting our 3D resistivity model. To the first order, the general structure can be described as follows:

(1) In the shallow resistive layer, the interface between the highly and moderately resistive values more or less mimics the topography.

306
307
308

(2) A very conductive layer, not continuous in the model is more superficial and thinner below the Plaines des Sables, western of the dense hypovolcanic complex modeled from gravity data (Gailler, 2010), and below the Enclos.

309
310

(3) In the deeper part of the edifice, the hypovolcanic complex is not individualized by a resistivity contrast.

311

(4) The lithosphere is globally moderately resistive but not fully homogeneous.

312

### 4.2.1. The shallow resistive layer

313
314

315
316

In detail, the shallow resistive layer can be separated into a highly resistive layer (~1000 to 10000 ohm·m) with a thickness of 1 km – 1.5 km, overlying a thinner and less resistive one (~100 to 1000 ohm·m). The shallower resistive layer is easily identified to piles of unsaturated lava flows. The underlying layer has resistivity values compatible with that of water-saturated
317

lava flows, although it is observed at a relatively high elevation, slightly higher than that of the regional water-table calculated by Join et al. (2005).

318

319

### 4.2.2. The intermediate conductive layer

320

A very conductive layer (~ 4 ohm·m or less to about 30 ohm·m) is observed below, with an average thickness of about 1 km. In the model, it does not form a continuous layer but appears as elongated patches. It is not possible to assume whether this discontinuous appearance is real or an artefact of the model due to the distribution and quality of the data. This very conductive layer is present in two different zones. The first one is at high elevation below the Enclos (we have to keep in mind that the Central Cone is less resolved because it is not covered by MT soundings), the Plaine des Sables, the Plaine des Remparts and the Fond de la Rivière de l'Est. The second one is just above the inferred oceanic floor on the NW flank of Piton de la Fournaise.

321

322

323

324

325

326

327

328

329

This conductor has been found in all the previous resistivity surveys (e.g. Benderitter and Gérard, 1984; Courteaud et al., 1997; Schnegg, 1997; Lénat et al., 2000)), in the different areas of Piton de la Fournaise. Various interpretations have been proposed to explain the presence of this conductor. Benderitter and Gérard (1984) attribute the very low resistivity to hydrothermal alteration whereas Courteaud et al. (1997) explain it by the presence of clay-rich brecciated material resulting from volcano-tectonic processes (caldera and flank collapses). For Lénat et al. (2000), the conductor, rising to a depth of about 300 m beneath the summit, is caused by the central hydrothermal activity within the Enclos. They suggest that in the Plaine des Sables it could also be associated with the hydrothermal activity of Piton de la Fournaise's old active center until the volcano center shifted to its present location in the Enclos (Bachèlery and Mairine, 1990). In the other zones, the geological nature of the conductor has not been identified.

330

331

332

333

334

335

336

337

338

339

340
341

Since the conductor is well above the expected level of sea water, the presence of which could
explain low resistivity values, such low resistivity values suggest the presence of hydrated
minerals, which could be found in landslide breccias, in hydrothermally altered zones, or in
thick pyroclastic layers. Therefore, the nature of the conductor may be different in each zone.

342

343

344

345

Below the NW flank of Piton de la Fournaise, the conductor above the preexisting sea floor coincides with the N120 rift zone (Michon et al., 2016; Chevallier and Bachèlery, 1981; Villeneuve and Bachèlery, 2006) characterized by a high density of large cinder cones. These are fed by primitive magmas through deeply rooted intrusions (Michon et al., 2016). The hydrothermal alteration caused by the swarm of the dikes in the rift zone is a possible explanation of this resistivity anomaly.

346

347

348

349

350

351

### 4.2.3. The base of the edifice and the transition with the crust

352

353

As indicated above, the base of the volcanic edifice on the oceanic crust is inferred to lie at around -4 km in elevation and the lithosphere does not show any significant flexure. A -4 km resistivity transition is clear in two areas only. The first one is beneath La Plaine des Sables-Fond de la Rivière de l'Est where the oceanic crust is more resistive (> 150 ohm.m) than the base of the volcanic construction (< 100 ohm.m). The second one is at the NW, where, as indicated above, a very conductive layer is present at the base of the volcanic construction. A third transition is suggested beneath the Rivière des Remparts, but in this area the model is poorly constrained by MT soundings.

354

355

356

357

358

359

360

The hypovolcanic dense complex of the Ancient Shield (ASC) of Piton de la Fournaise revealed by gravity in the interior of the edifice (Fig. 6 a and b) would be expected to show a higher resistivity that its surrounding, as it is the case for another intrusive complex beneath Piton des Neiges volcano (Gailler and Lénat, 2012). Here, the dense complex does not show a resistivity contrast with the rest of the lower part of the edifice. This lack of correlation between the resistivity distribution and the gravity model may be explained by two types of

361

362

363

364

365
366

reasons. The geometry and density distribution of gravity model may be biased by the fact
that constant density is assumed for both the intrusive complex and its surrounding. This
367
368 could be a too large simplification of the reality and the geometry of the dense body could be
369 significantly different if large density variations exist within the dense body and within its
370 surrounding. A second type of explanation would be to consider that parts of the dense body
371 are fractured and/or hydrothermally altered, and therefore less resistive. As a whole, the deep
372 part of the edifice seems rather homogeneous in resistivity and no structural features, such as
373 the transition with Les Alizés, is observed.

367

368

369

370

371

372

373

374

### 4.2.4. The oceanic lithosphere

375

The upper oceanic lithosphere beneath La Réunion is globally resistive with values up to
376 several thousand ohm·m (Fig. 6a and b). The boundary between the crust and the upper
377 mantle, estimated to be around –12 km (Gallart et al., 1999), is not marked in the model. At
378 this range of depth, resistivity contrasts are suggested only below Plaine des Sables and
379 Enclos area on the Bostick analysis (Fig. 4b). Beneath the NW flank, a pronounced
380 conductive anomaly of the crust extends at large depth. It starts at the NW of the Plaines des
381 Sables-Fond de la Rivière de l'Est area and deepens down to about 8 km at the border of the
382 model. It seems to more or less follow the N120 rift zone comprising a very conductive (< 5
383 ohm.m) zone in the upper 2 km that is distinct from a similar feature observed above in the
384 volcanic construction.

376

377

378

379

380

381

382

383

384

385

High resistivity values (>1000 ohm.m) below the W flank and the Rivière des Remparts from
386 about -8 to -16 km in depth, are not reliable, because this area is only covered by a few
387 number of soundings and lies at the edge of the model. The highly conductive area at a depth
388 of -16 km covering a large area below the Plaine des Sables-Fond de la Rivière de l'Est and
389 even part of the Enclos and the NW flank of Piton de la Fournaise on the Bostick analysis

386

387

388

389
390

(Fig. 4b) is also not reliable, because our study area is relatively narrow and may be not
adequate to resolve the resistivity distribution in this range of depths (Bhattacharya and
Shalivahan, 2002).

391

392

393

**5. Discussion and conclusion**

394

The interior of Piton de la Fournaise has been investigated by numerous geophysical and
geological studies. However, the information for all the methods become less precise with
depth and therefore the deep part of the volcanic construction and its interaction with the
lithosphere are poorly known. Our MT study offers the advantages of being able to probe both
the volcanic construction and the upper lithosphere and to use a different physical parameter
than those of the previous seismic, gravity and magnetic studies. An interpretative scheme is
presented in figure 6c, based on our 3D resistivity model and results from other studies.

395

396

397

398

399

400

401

The upper part of the edifice is very typical of basaltic shield volcanoes predominantly built
by highly permeable lava flows. Accordingly, the first layer of water unsaturated lava flows is
highly resistive. The fact that it evolves at depth to a more conductive layer (water saturated
lava flows) supports the hydrogeological model of Join et al. (2005) that postulates the
presence of a continuous high water table at Piton de la Fournaise. This contrasts with the
classical Hawaiian hydrogeological model where high elevation groundwater bodies are
disconnected and impounded by dike swarms or impervious layers.

402

403

404

405

406

407

408

At depth, within the edifice, the presence of more or less continuous highly conductive (< 5
ohm.m) patches is a special feature. Such low resistivity values imply the presence of highly
conductive fluids and/or minerals. High conductivity fluids should be hot and/or mineralized
ones. High conductivity minerals in this context are hydrated minerals commonly resulting
from hydrothermal alteration. The high conductivity layer is described in greater detail by
Lénat et al. (2000) beneath the Central Cone where it can be unambiguously linked to the

409

410

411

412

413
414

presently active hydrothermal system of the volcano. Beneath the Plaine des Sables-Fond de
la Rivière de l'Est, an hydrothermal origin of the conductor can only be presumed. Gravity
studies have shown that this area is underlain by a dense structure interpreted as a
hypovolcanic intrusive system (Gailler, 2010), source of heat and fluids for hydrothermal
activity. This complex would have formed during the Ancient Shield stage and, therefore, the
conductor beneath the Plaine des Sables would have developed during this period. The third
notable patch of high conductor in the study area is the one observed in the lower part of the
volcanic construction below the NW flank of Piton de la Fournaise (Fig. 6b). The fact that this
conductor is not a widespread but a localized structure suggests that it is not due to an
invasion of seawater but more likely to volcanic activity. Its location coincides with that of a
part of the N120 rift zone with numerous cinder cones. In this framework, it can be tentatively
explained by hydrothermalized and/or hot volume of rocks developing around a dense swarm
of cooling intrusions at the base of the edifice and, possibly in the upper part of the crust
where a similar conductor is observed. This system may be still active because a deep seismic
activity (between about 10 and 20 km) and CO₂ emanations are observed in this area (Liuzzo
et al., 2015; Michon et al.; 2016; Boudoire et al., 2017).

415

416

417

418

419

420

421

422

423

424

425

426

427

428

429

430

The dense hypovolcanic intrusive system found beneath the Plaine des Sable and the western part of the Enclos by Gailler (2010) is not characterized by a resistivity contrast with the rest of the edifice. Only overlying highly conductive layer may attest of its presence. Similarly, the interface between Les Alizés and Piton de la Fournaise volcanoes inferred by Lénat et al. (2012b) does not correspond to any obvious resistivity contrast in our model.

431

432

433

434

435

The upper lithosphere is markedly more resistive than the bulk of the volcanic construction (except for the upper part of this latter). However, one main resistivity anomaly is present in the crust, located beneath the NW flank. It spatially coincides, as indicated above, with the N120 rift zone assumed to be linked with present day seismic and degassing activity. It is

436

437

438
439

therefore appealing to suggest that the lowering of the resistivity of the crust in this area may
be due to the magmatic activity (magma intrusion, hydrothermal activity, presence of hot fluids
440
...). The NW flank therefore appears as a main area of interest in the study of Piton de la
441
Fournaise activity and its evolution as well. Also note that, at the base of the crust, the
442
underplating is not imaged in our 3D resistivity model. For further investigations in depth, the
443
study should be extended to the whole island to study the structures beneath Piton des Neiges
444
and the whole of the Piton de la Fournaise area.
445

440

441

442

443

444

445

446

Acknowledgments

447

We are very grateful to the 'Conseil Régional de la Réunion' for its financial support for the
acquisition of gravity and electromagnetic data in the framework of a geothermal exploration
project and for its encouragement to use the data for academic research purposes. All the
Geothermal Project participants are also thanked for their various contributions (E. Delcher,
B. Robineau, P. Andrieu, F. Ranvier, T. Souriot, X. Pierre, B. Lapierre, D. Runavot, L.
Fornot, O. Odon). The dataset will be available from the authors of this manuscript. We thank
the staffs of the Observatoire Volcanologique du Piton de la Fournaise (Aline Peltier, Valérie
Ferrazini, Nicolas Villeneuve), of Université de la Réunion (Laurent Michon, Anthony
Finizola), and of University of Barcelona (Pilar Queralt, Juanjo Ledo and Alejandro
Marcuello) for their contribution and discussions about this study. We also thank Naser
Meqbel for its help around 3D Grid software. Last we acknowledge Michael Walter, Volker
Rath and an anonymous reviewer for very insightful comments and suggestions.
448
449
450
451
452
453
454
455
456
457
458

448

449

450

451

452

453

454

455

456

457

458
460 **Figure captions**

461 **Figure 1: a)** 3D view of Piton de la Fournaise showing the main structural features and the
462 location of the main places discussed in the text. The study area is framed in red; **b)**
463 Interpretive W–E geological section of Piton de la Fournaise by Lénat et al. (2012a).
464 Coordinates in kilometers (WGS84, UTM40S).

465 **Figure 2:** Location of the resistivity soundings used in this study. The area of interest is
466 framed in red. Coordinates in kilometers (WGS84, UTM40S). More details are given in
467 Supporting Information (Fig. S1).

468 **Figure 3:** Map of dimensionality for six different period bands, using WALDIM code (Martí
469 et al., 2009).

470 **Figure 4: a)** Map of invariant phases for periods ranging from 10⁻⁴ to 10³, using WINGLING
471 software. **b)** Bostick resistivity inversions are also presented showing the main distribution of
472 resistivity at fixed elevation, using WINGLING software.

473 **Figure 5:** Comparison of the apparent resistivity and phases between the model responses and
474 the raw data for the G10 sounding a) off-diagonal components; b) all components.

475 **Figure 6:** a) 3D model represented as a multi-segments section crossing the main area of
476 interest, i.e. Plaine des Sables, Enflos Fouqué, Fond de la Rivière de l'Est and NW flank of
477 Piton de la Fournaise. b) Interpretative scheme of the 3D model. ASC for Ancient Shield
478 Complex.
480 References

481 Bachèlery, P. (1981), Le Piton de la Fournaise (Ile de la Réunion): étude volcanologique,
482 structurale et pétrologique, 255 pp., Univ. Blaise Pascal, Clermont-Ferrand.

483 Bachèlery, P., and P. Mairine (1990), Evolution volcano-structurale du Piton de la Fournaise
depuis 0.53 Ma, in Le volcanisme de la Réunion, Monographie, edited by J.-F. Lénat, pp.
484
485 213-242, Centre de Recherches Volcanologiques, Clermont-Ferrand.

486 Bachèlery, P. and P. Mairine (1990), Evolution volcano-structurale du Piton de la Fournaise
depuis 0.53 Ma, in Le volcanisme de la Réunion, Monographie, edited by J.-F. Lénat, pp.
487
488 213-242, Centre de Recherches Volcanologiques, Clermont-Ferrand.

489 Battaglia, J., V. Ferrazzini, T. Staudacher, K. Aki, and J. L. Cheminée (2005), Pre-eruptive
490 migration of earthquakes at the Piton de la Fournaise volcano (Réunion island),
491 Geophys. J. Int., 161, 549-558.

492 Benderitter, Y. (1990), Études géoélectrique et électromagnétiques à la Réunion, in Le
493 volcanisme de la Réunion, monographie, edited by J. F. Lénat, pp. 29-42, Centre de
494 Recherches Volcanologiques, Clermont-Ferrand.

495 Benderitter, Y., and A. Gérard (1984), Geothermal study of Réunion Island:
496 audiomagnetotelluric survey, J. Volcanol. Geotherm. Res., 20, 311-332.

497 Bhattacharya B.B, Shalivahan (2002), The electric moho underneath Eastern Indian Craton,
498 Geophysica Research Letters, Vol. 29, NO. 10, 1376, 10.1029/2001GL014062.

499 Boudoire G., Liuzzo M., Di Muro A, Ferrazzini V., Michon L., et al. Investigating the
500 deepest part of a volcano plumbing system: Evidence for an active magma path below the
501 western flank of Piton de la Fournaise (La Réunion Island). Journal of Volcanology and Geothermal Research, Elsevier, 2017, 341, pp.193-207.<10.1016/j.jvolgeores.2017.05.026>.

504 Charvis, P., A. Laesanpura, J. Gallart, A. Hirn, J. C. Lepine, B. de Voogd, T. A. Minshull, Y. Hello, and B. Pontoise (1999), Spatial distribution of hotspot material added to the lithosphere under La Réunion, from wide-angle seismic data, J. Geophys. Res., B104,(2), 2875-2893.

508 Chevallier, L., and P. Bachlery (1981), Evolution structurale du volcan actif du Piton de la Fournaise. Ile de la Réunion, Océan Indien, Bull. Volc., 44 (4), 723-741.

510 Constable, S., and G. Heinson (2004), Hawaiian hot-spot swell structure from seafloor MT sounding, Tectonophysics, 389, 111-124.

512 Constable, S. C., and C. S. Cox (1996), Marine controlled source electromagnetic sounding: 2. The PEGASUS experiment, J. Geophys. Res., 101, 5519-5530.

514 Courteaud, M. (1996), Etude des structures géologiques et hydrogéologiques du Massif de la Fournaise par la méthode audiomagnétotellurique, 212 pp., Thèse d'Université, Univ. de la Réunion.

517 Courteaud, M., P. Bachlery, B. Robineau, and J. L. Join (1997), Les brèches de Saint-Gilles. Note technique géologique, Laboratoire des Sciences de la Terre, Univ. de La Réunion.

519 Egbert, G. D. and A. Kelbert, (2012), Computational recipes for electromagnetic inverse problems, Geophysical Journal International, 189, 251-267.
521

Kelbert, A., N.M. Meqbel, G. D. Egbert & K. Tandon, 2014. ModEM: A modular system for inversion of electromagnetic geophysical data, *Comp. Geosci.*, 66, 40-53, ISSN 0098-3004, http://dx.doi.org/10.1016/j.cageo.2014.01.010.

522

523

524

Fontaine, F.R., G. Barruol, H. Tkalčić, I. Wölbern, G. Rümpker, T. Bodin, M. Haugmard (2015), Crustal and uppermost mantle structure variation beneath La Réunion hotspot track, *Geophysical Journal International*, 203, Issue 1, 1, 107–126.

525

526

527

Gailler, L., J.F. Lénat, M. Lambert, G. Levieux, N. Villeneuve, and J. L. Froger (2009), Gravity structure of Piton de la Fournaise volcano and inferred mass transfer during the 2007 crisis, *J. Volcanol. Geotherm. Res.*, 184, 31–48, doi:doi:10.1016/j.jvolgeores.2009.01.024.

529

530

531

Gailler, L., J.F. Lénat, and R.J. Blakely (2016), Depth to Curie temperature or magnetic sources bottom in the volcanic zone of La Réunion hot spot, *Earth Planet. Sci. Lett.*.

532

533

Gailler, L.S. (2010), Structure interne d'un système océanique volcanique, La Réunion (Océan Indien) – Approches géophysiques, 471 pp., Université Blaise Pascal-Laboratoire Magmas et Volcans, Clermont-Ferrand.

534

535

536

Gailler, L.S., and J.F. Lénat (2012), Internal architecture of La Réunion (Indian Ocean) inferred from geophysical data, *J. Volcanol. Geotherm. Res.*, 221-222, 83–98.

537

538

Gallart, J., L. Driad, P. Charvis, M. Sapin, A. Hirn, J. Diaz, B. de Voogd, and M. Sachpazi (1999), Perturbation to the lithosphere along the hotspot track of La Réunion from an offshore-onshore seismic transect, *J. Geophys. Res.*, B, Solid Earth Planets, 104(2), 2895–2908.

539

540

541
542

Gillot, P. Y., and P. Nativel (1989), Eruptive history of the Piton de la Fournaise volcano, Réunion Island, Indian Ocean, J. Volcanol. Geotherm. Res., 36, 53–65.

543

544

Haak, V. (1982), Electrical Conductivity of Minerals and Rocks at Temperatures and Pressures, in Zahlenwerte und Funktionen aus Naturwissenschaften und Technik /Landolt-Brrnstein, vol. V/1b, edited by G. Angenheister, pp. 291–307, Springer, Berlin.

545

546

547

Jackson, D.B., and J. Kauahikaua (1987), The high-level water table beneath Kilauea Volcano, Hawaii. How Volcanoes Work, Hilo, Hawaii, January 19–25, 120.

548

549

Join, J.L., J. L.Folio, and B. Robineau (2005), Aquifers and groundwater within active shield volcanoes. Evolution of conceptual models in the Piton de la Fournaise volcano, J. Volcanol. Geotherm. Res., 147, 187–201.

550

551

552

Jones, A.G. (1999), Imaging the continental upper mantle using electromagnetic methods. Lithos 48, 57–80.

553

554

Lénat, J.F., P. Bachèlery, and O. Merle (2012a), Anatomy of Piton de la Fournaise volcano (La Réunion, Indian Ocean), Bull. Volcanol, 74:1945–1961. doi:10.1007/s00445-012-0640-y.

555

556

557

Lénat, J.F., D. Fitterman, and D.B. Jackson (2000), Geoelectrical structure of the central zone of Piton de la Fournaise volcano (Réunion), Bull. Volc., 62 (2), 75–89.

558

559

Lénat, J.F., B. Gibert-Malengreau, and A. Galdéano (2001), A new model for the evolution of the volcanic island of Réunion (Indian Ocean), J. Geophys. Res., B, 106,(5), 8645–8663.

560
561 Lénat, J.F., P. Bachèlery, and A. Peltier (2012b), The interplay between collapse structures, hydrothermal systems and magma intrusions: the case of the central area of Piton de la Fournaise volcano, Bull. Volcanol., 74, 407–421, doi:10.1007/s00445-011-0535-3.

564 Letourneur, L., A. Peltier, T. Staudacher, and A. Gudmundsson (2008), The effects of rock heterogeneities on dyke paths and asymmetric ground deformation: the example of Piton de la Fournaise volcano, J. Volcanol. Geotherm. Res., 173, 289–302.

567 Liuzzo, M., A. Di Muro, G. Giudice, L. Michon, V. Ferrazzini, S. Gurrieri (2005), New evidence of CO2 soil degassing anomalies on Piton de la Fournaise volcano and the link with volcano tectonic structures, Geochem. Geophys. Geosyst., 16, 10.1002/2015GC006032

571 MacGregor, L.M., S.C. Constable, and M. C.Sinha (1998), The RAMESSES experiment—III. Controlled-source electromagnetic sounding of the Reykjanes Ridge at 57°45′N, Geophys. J. Int., 135, 772–789.

574 Malengreau, B., J.F. Lénat, and J.L. Froger (1999), Structure of Reunion Island (Indian Ocean) inferred from the interpretation of gravity anomalies, J. Volcanol. Geotherm. Res., 88(3), 131–146.

577 Martí A, P. Queralt, J. Ledo (2009), WALDIM: a code for the dimensionality analysis of magnetotelluric data using the rotational invariants of the magnetotelluric tensor. Comput. Geosci. 35(12):2295–2303.

580 McDougall, I. (1971), The geochronology and evolution of the young volcanic island of Réunion (Indian Ocean), Geochem. Cosmochim. Acta., 35(3), 261–288.
582

Merle, O., and J.F. Lénat (2003), Hybrid collapse mechanism at Piton de la Fournaise volcano, Reunion Island, Indian Ocean, J. Geophys. Res., 108 (B3), 2166, doi:doi:10.1029/2002JB002014.

583

584

585

Merle, O., P. Mairine, L. Michon, P. Bachèlery, and M. Smietana (2010), Calderas, landslides and paleo-canyons on Piton de la Fournaise volcano (La Réunion Island, Indian Ocean), J. Volcanol. Geotherm. Res., in press.

586

587

588

Michon, L., and F. Saint-Ange (2008), Morphology of Piton de la Fournaise basaltic shield volcano (La Réunion Island): Characterization and implication in the volcano evolution, J. Volcanol. Geotherm. Res., 113(B03203), 1–19, doi:doi:10.1029/2005JB004118.

589

590

591

Michon, L., V. Ferrazzini, A. Di Muro (2016), Magma intrusions paths of Piton de la Fournaise. In: Bachèlery, P., Lénat, J.F., Di Muro, A., Michon, L. (Editor), Active Volcanoes of the Southwest Indian Ocean: Piton de la Fournaise and Karthala. Active Volcanoes of the World. Springer-Verlag Berlin and Heidelberg. pp. 91-106.

592

593

594

595

Mohamed-Abchir, A. (1996), Les Cendres de Bellecombe: un événement majeur dans le passé récent du Piton de la Fournaise, Ile de la Réunion, 248 pp., Université de Paris VII.

596

597

Morandi, A., A. Di Muro, C. Principe, L. Michon, G. Leroi, F. Norelli, and P. Bachèlery (2015), Pre-historic explosive activity at Piton de la Fournaise volcano, in Active Volcanoes of the Southwest Indian Ocean: Piton de la Fournaise and Karthala. Active Volcanoes of the World., edited by P. Bachèlery, J.F. Lénat, A. Di Muro, and L. Michon.

598

599

600

601

Nobes, D.C., L.K. Law, and R.N. Edwards (1992), Results of a sea-floor electromagnetic survey over a sedimented hydrothermal area on the Juan de Fuca Ridge, Geophys. J. Int., 110, 333–346.

602

603
604

Pinã-Varas P., L. Ledo, P. Queralt, A. Marcuello, F. Bellmunt, R. Hidalgo, M. Messeiller

605

(2014), 3-D Magnetotelluric Exploration of Tenerife Geothermal System (Canary

606

Islands, Spain), Surv. Geophys. 35:1045–1064, doi 10.1007/s10712-014-9280-4.

607

Rai, M.P., and M.H. Manghnani (1977), Electrical conductivity of basalts to 1550°C.

608

Chapman conference on partial melting in the earth's upper mantle proceedings. Oregon

609

Department of Geology and Mineralogy Bull. 96:219–232,

610

Rançon, J.P., P. Lerebour, and T. Augé (1989), The Grand Brûlé exploration drilling; new

611

data on the deep framework of the Piton de la Fournaise Volcano; Part 1,

612

Lithostratigraphic units and volcanostructural implications, J. Volcanol. Geotherm. Res.,

613

36(1-3), 113–127.

614

Schnegg, P.A. (1997), Electrical structure of Plaine des Sables caldera, Piton de la Fournaise

615

volcano (Réunion Island), Ann. Geofis., 40, 305–317.

616

Smith, D.K. (1996), Comparison of the shapes and sizes of seafloor volcanoes on Earth and

617

“pancake” domes on Venus, J. Volcanol. Geotherm. Res., 73(1-2), 47–64.

618

Staudacher, T., and C.J. Allègre (1993), Ages of the second caldera of Piton de la Fournaise

619

volcano (Réunion) determined by cosmic ray produced 3He and 21Ne, Earth Planet Sci

620

Lett, 119, 395–404.

621

Stearns, H.T. (1942), Hydrology of volcanic terranes, in Physics of the earth, vol IX.

622

Hydrology. Dover, New York, pp. 678–703.

623

Takasaki, K.J. (1981), Evaluation of major dike-impounded groundwater reservoirs, Island of

624

Oahu, US Geol. Surv. Open-File Rep. 81-1119146.
625

Unsworth, M.J. (1994), Exploration of a mid-ocean ridge with a frequency-domain EM system, Geophys. J. Int., 116, 447-467.

626

627

Vacher, H.L. (1988), Dupuit-Ghyben-Herzberg analysis of strip-island lenses, Bull. Geol. Soc. Am., 100, 580-591.

628

629

Villeneuve, N., and P. Bachèlery (2006), Revue de la typologie des éruptions au Piton de la Fournaise, *processus et risques volcaniques associés: Cybergeo: European Journal of Geography* (http://cybergeo.revues.org/2536),

630

631

632

De Voogd, B., S. Pou Palomé, A. Hirn, P. Charvis, J. Gallart, D. Rousset, J. Danobeitia, and H. Perroud (1999), Vertical movements and material transport during hotspot activity; seismic reflection profiling offshore La Réunion, J. Geophys. Res. B, Solid Earth Planets, 104,(2), 2855-2874.

633

634

635

636

Weaver, J.T., A.K. Agarwal, F.E.M. Lilley (2000), Characterization of the magnetotelluric tensor in terms of its invariants. Geophys. J. Int. 141(2):321-336. http://dx.doi.org/10.1046/j.1365-246x.2000.00089.x.

637

638
<figure><img src="image_2.png" /><figcaption>Figure 1</figcaption></figure>

<figure><img src="image_3.png" /></figure>
<figure><img src="image_4.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 3</figcaption></figure>
<caption>Figure 4</caption>

(a)

<figure><img src="image_6.png" /></figure>
(b)

<figure><img src="image_7.png" /></figure>
<caption>Figure 5</caption>

<figure><img src="image_8.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /></figure>
<figure><img src="image_15.png" /><figcaption>Figure 6</figcaption></figure>
(c)

<figure><img src="image_16.png" /></figure>
Supplementary Online material A

Details on MT-AMT data location

At the scale of the study area, the final dataset includes 118 MT and AMT soundings, and 17 electrical soundings from various organizations (Fig. A.1).

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>
<figure><img src="image_20.png" /></figure>

The soundings have a typical spacing of 700 m in the center of the survey area (2 by km²), increasing to 1000 m towards the periphery.

The data, available as EDI files, were processed using WinGLink™ Version 2.6.02 provided by Geosystem society. In addition to treatment and modelling options, this software also provides features of a geo-referenced database for all geophysical measurements. When multiple sets of data were available for a sounding (i.e. HF and LF for AMT and MT data), the combination of the data was also performed with this software.
Supplementary Online material B

Raw data versus model responses

Fig. B.1: Comparison between the raw data and model responses as apparent resistivity and phases curves for each site used in inversion.
Site: 2

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

Total RMS = 20196.14

Zxy RMS = 20404.41

Zyx RMS = 19985.71
Site: 3

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

Total RMS = 20560.45

Zxy RMS = 20339.08

Zyx RMS = 20779.46
Site: 18

<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

Total RMS = 21298.52

Zxy RMS = 21243.25

Zyx RMS = 21353.66
Site: G02

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>

Total RMS = 16595.32

Zxy RMS = 16033.28

Zyx RMS = 17138.94
Site: G03

<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /></figure>

Total RMS = 16040.16

Zxy RMS = 16495.12

Zyx RMS = 15571.92
Site: G05

<figure><img src="image_36.png" /></figure>

<figure><img src="image_37.png" /></figure>

<figure><img src="image_38.png" /></figure>

Total RMS = 18732.56

Zxy RMS = 19105.57

Zyx RMS = 18351.98
Site: G06

<figure><img src="image_39.png" /></figure>

<figure><img src="image_40.png" /></figure>

<figure><img src="image_41.png" /></figure>

Total RMS = 15633.57

Zxy RMS = 15428.56

Zyx RMS = 15835.92
Site: G08

<figure><img src="image_42.png" /></figure>

<figure><img src="image_43.png" /></figure>

<figure><img src="image_44.png" /></figure>

Total RMS = 12092.52

Zxy RMS = 12525.79

Zyx RMS = 11643.14
Site: G09

<figure><img src="image_45.png" /></figure>

<figure><img src="image_46.png" /></figure>

<figure><img src="image_47.png" /></figure>

Total RMS = 15210.51

Zxy RMS = 14903.62

Zyx RMS = 15511.34
Site: G10

<figure><img src="image_48.png" /></figure>

<figure><img src="image_49.png" /></figure>

<figure><img src="image_50.png" /></figure>

Total RMS = 15949.68

Zxy RMS = 16038.32

Zyx RMS = 15860.54
Site: G25

<figure><img src="image_51.png" /></figure>

<figure><img src="image_52.png" /></figure>

<figure><img src="image_53.png" /></figure>

Total RMS = 16749.41

Zxy RMS = 18478.39

Zyx RMS = 14820.07
Site: G26

<figure><img src="image_54.png" /></figure>

<figure><img src="image_55.png" /></figure>

<figure><img src="image_56.png" /></figure>

Total RMS = 18982.16

Zxy RMS = 20157.87

Zyx RMS = 17728.65
Site: G27

<figure><img src="image_57.png" /></figure>

<figure><img src="image_58.png" /></figure>

<figure><img src="image_59.png" /></figure>

Total RMS = 16839.70

Zxy RMS = 17219.77

Zyx RMS = 16450.85
Site: G28

<figure><img src="image_60.png" /></figure>

<figure><img src="image_61.png" /></figure>

<figure><img src="image_62.png" /></figure>

Total RMS = 16537.34

Zxy RMS = 17289.55

Zyx RMS = 15749.23
Site: G29

<figure><img src="image_63.png" /></figure>

<figure><img src="image_64.png" /></figure>

<figure><img src="image_65.png" /></figure>

Total RMS = 14708.17

Zxy RMS = 13939.03

Zyx RMS = 15439.04
Site: G30

<figure><img src="image_66.png" /></figure>

<figure><img src="image_67.png" /></figure>

<figure><img src="image_68.png" /></figure>

Total RMS = 14179.85

Zxy RMS = 14620.19

Zyx RMS = 13692.35
Site: G31

<figure><img src="image_69.png" /></figure>

<figure><img src="image_70.png" /></figure>

<figure><img src="image_71.png" /></figure>

Total RMS = 17140.71

Zxy RMS = 16530.46

Zyx RMS = 17729.97
Site: G32

<figure><img src="image_72.png" /></figure>

<figure><img src="image_73.png" /></figure>

<figure><img src="image_74.png" /></figure>

Total RMS = 16158.40

Zxy RMS = 16013.28

Zyx RMS = 16302.23
Site: G33

<figure><img src="image_75.png" /></figure>

<figure><img src="image_76.png" /></figure>

<figure><img src="image_77.png" /></figure>

Total RMS = 19592.65

Zxy RMS = 20550.51

Zyx RMS = 18585.49
Site: G34

<figure><img src="image_78.png" /></figure>

<figure><img src="image_79.png" /></figure>

<figure><img src="image_80.png" /></figure>

Total RMS = 15834.54

Zxy RMS = 15926.74

Zyx RMS = 15741.81
Site: G35

<figure><img src="image_81.png" /></figure>

<figure><img src="image_82.png" /></figure>

<figure><img src="image_83.png" /></figure>

Total RMS = 15338.90

Zxy RMS = 15802.70

Zyx RMS = 14860.64
Site: G36

<figure><img src="image_84.png" /></figure>

<figure><img src="image_85.png" /></figure>

<figure><img src="image_86.png" /></figure>

Total RMS = 15956.09

Zxy RMS = 15017.94

Zyx RMS = 16842.07
Site: G37

<figure><img src="image_87.png" /></figure>

<figure><img src="image_88.png" /></figure>

<figure><img src="image_89.png" /></figure>

Total RMS = 15411.35

Zxy RMS = 15950.64

Zyx RMS = 14852.48
Site: G38

<figure><img src="image_90.png" /></figure>

<figure><img src="image_91.png" /></figure>

<figure><img src="image_92.png" /></figure>

Total RMS = 17188.67

Zxy RMS = 18076.96

Zyx RMS = 16251.89
Site: G39

<figure><img src="image_93.png" /></figure>

<figure><img src="image_94.png" /></figure>

<figure><img src="image_95.png" /></figure>

Total RMS = 14490.77

Zxy RMS = 14886.06

Zyx RMS = 14084.39
Site: G40

<figure><img src="image_96.png" /></figure>

<figure><img src="image_97.png" /></figure>

<figure><img src="image_98.png" /></figure>

Total RMS = 15332.18

Zxy RMS = 15116.79

Zyx RMS = 15544.59
Site: G41

<figure><img src="image_99.png" /></figure>

<figure><img src="image_100.png" /></figure>

<figure><img src="image_101.png" /></figure>

Total RMS = 16716.44

Zxy RMS = 17499.59

Zyx RMS = 15834.42
Site: G42

<figure><img src="image_102.png" /></figure>

<figure><img src="image_103.png" /></figure>

<figure><img src="image_104.png" /></figure>

Total RMS = 15693.14

Zxy RMS = 16494.77

Zyx RMS = 14848.29
Site: G43

<figure><img src="image_105.png" /></figure>

<figure><img src="image_106.png" /></figure>

<figure><img src="image_107.png" /></figure>

Total RMS = 15880.53

Zxy RMS = 16607.10

Zyx RMS = 15119.09
Site: G44

<figure><img src="image_108.png" /></figure>

<figure><img src="image_109.png" /></figure>

<figure><img src="image_110.png" /></figure>

Total RMS = 15637.37

Zxy RMS = 16340.34

Zyx RMS = 14901.27
Site: G45

<figure><img src="image_111.png" /></figure>

<figure><img src="image_112.png" /></figure>

<figure><img src="image_113.png" /></figure>

Total RMS = 15393.25

Zxy RMS = 15206.71

Zyx RMS = 15577.55
Site: G46

<figure><img src="image_114.png" /></figure>

<figure><img src="image_115.png" /></figure>

<figure><img src="image_116.png" /></figure>

Total RMS = 14380.60

Zxy RMS = 13629.48

Zyx RMS = 15094.40
Site: G47

<figure><img src="image_117.png" /></figure>

<figure><img src="image_118.png" /></figure>

<figure><img src="image_119.png" /></figure>

Total RMS = 16398.34

Zxy RMS = 16715.07

Zyx RMS = 16075.38
Site: G48

<figure><img src="image_120.png" /></figure>

<figure><img src="image_121.png" /></figure>

<figure><img src="image_122.png" /></figure>

Total RMS = 18339.42

Zxy RMS = 17670.07

Zyx RMS = 18985.18
Site: G49

<figure><img src="image_123.png" /></figure>

<figure><img src="image_124.png" /></figure>

<figure><img src="image_125.png" /></figure>

Total RMS = 15208.89

Zxy RMS = 15280.30

Zyx RMS = 15137.14
Site: G50

<figure><img src="image_126.png" /></figure>

<figure><img src="image_127.png" /></figure>

<figure><img src="image_128.png" /></figure>

Total RMS = 14461.07

Zxy RMS = 14280.00

Zyx RMS = 14639.91
Site: G51

<figure><img src="image_129.png" /></figure>

<figure><img src="image_130.png" /></figure>

<figure><img src="image_131.png" /></figure>

Total RMS = 14848.28

Zxy RMS = 14359.02

Zyx RMS = 15321.93
Site: G52

<figure><img src="image_132.png" /></figure>

<figure><img src="image_133.png" /></figure>

<figure><img src="image_134.png" /></figure>

Total RMS = 16028.94

Zxy RMS = 16114.83

Zyx RMS = 15942.59
Site: MT16

<figure><img src="image_135.png" /></figure>

<figure><img src="image_136.png" /></figure>

<figure><img src="image_137.png" /></figure>

Total RMS = 16398.04

Zxy RMS = 17957.32

Zyx RMS = 14673.99
Site: MT19

<figure><img src="image_138.png" /></figure>

<figure><img src="image_139.png" /></figure>

<figure><img src="image_140.png" /></figure>

Total RMS = 15821.50

Zxy RMS = 15692.19

Zyx RMS = 15949.77
Site: MT20

<figure><img src="image_141.png" /></figure>

<figure><img src="image_142.png" /></figure>

<figure><img src="image_143.png" /></figure>

Total RMS = 20672.62

Zxy RMS = 17998.55

Zyx RMS = 23038.36
Site: MT21

<figure><img src="image_144.png" /></figure>

<figure><img src="image_145.png" /></figure>

<figure><img src="image_146.png" /></figure>

Total RMS = 17862.84

Zxy RMS = 18187.76

Zyx RMS = 17531.91
Site: X03

<figure><img src="image_147.png" /></figure>

<figure><img src="image_148.png" /></figure>

<figure><img src="image_149.png" /></figure>

Total RMS = 15019.14

Zxy RMS = 14354.29

Zyx RMS = 15655.79
Site: X05

<figure><img src="image_150.png" /></figure>

<figure><img src="image_151.png" /></figure>

<figure><img src="image_152.png" /></figure>

Total RMS = 17940.69

Zxy RMS = 16463.21

Zyx RMS = 19305.43
Site: X06

<figure><img src="image_153.png" /></figure>

<figure><img src="image_154.png" /></figure>

<figure><img src="image_155.png" /></figure>

Total RMS = 18029.86

Zxy RMS = 19210.54

Zyx RMS = 16766.24
Site: X07

<figure><img src="image_156.png" /></figure>

<figure><img src="image_157.png" /></figure>

<figure><img src="image_158.png" /></figure>

Total RMS = 13583.37

Zxy RMS = 12967.00

Zyx RMS = 14172.95
Site: X11

<figure><img src="image_159.png" /></figure>

<figure><img src="image_160.png" /></figure>

<figure><img src="image_161.png" /></figure>

Total RMS = 15324.60

Zxy RMS = 13576.03

Zyx RMS = 16893.14
Site: X12

<figure><img src="image_162.png" /></figure>

<figure><img src="image_163.png" /></figure>

<figure><img src="image_164.png" /></figure>

Total RMS = 19107.14

Zxy RMS = 19227.66

Zyx RMS = 18985.86
Site: X14

<figure><img src="image_165.png" /></figure>

<figure><img src="image_166.png" /></figure>

<figure><img src="image_167.png" /></figure>

Total RMS = 14438.90

Zxy RMS = 14829.41

Zyx RMS = 14037.53
Site: X15

<figure><img src="image_168.png" /></figure>

<figure><img src="image_169.png" /></figure>

<figure><img src="image_170.png" /></figure>

Total RMS = 15184.08

Zxy RMS = 15589.95

Zyx RMS = 14767.07
Site: X16

<figure><img src="image_171.png" /></figure>

<figure><img src="image_172.png" /></figure>

<figure><img src="image_173.png" /></figure>

Total RMS = 16329.50

Zxy RMS = 15238.98

Zyx RMS = 17351.61
Site: X19

<figure><img src="image_174.png" /></figure>

<figure><img src="image_175.png" /></figure>

<figure><img src="image_176.png" /></figure>

Total RMS = 18192.31

Zxy RMS = 17442.28

Zyx RMS = 18983.14
Site: X20

<figure><img src="image_177.png" /></figure>

<figure><img src="image_178.png" /></figure>

<figure><img src="image_179.png" /></figure>

Total RMS = 21161.00

Zxy RMS = 13001.47

Zyx RMS = 26954.36
Site: X21

<figure><img src="image_180.png" /></figure>

<figure><img src="image_181.png" /></figure>

<figure><img src="image_182.png" /></figure>

Total RMS = 18083.14

Zxy RMS = 17642.59

Zyx RMS = 18551.81
Site: X22

<figure><img src="image_183.png" /></figure>

<figure><img src="image_184.png" /></figure>

<figure><img src="image_185.png" /></figure>

Total RMS = 18245.60

Zxy RMS = 20432.66

Zyx RMS = 15757.86
Site: X23

<figure><img src="image_186.png" /></figure>

<figure><img src="image_187.png" /></figure>

<figure><img src="image_188.png" /></figure>

Total RMS = 18875.07

Zxy RMS = 17704.10

Zyx RMS = 19977.52
Site: X27

<figure><img src="image_189.png" /></figure>

<figure><img src="image_190.png" /></figure>

<figure><img src="image_191.png" /></figure>

Total RMS = 18886.54

Zxy RMS = 18043.71

Zyx RMS = 19693.33
Site: X28

<figure><img src="image_192.png" /></figure>

<figure><img src="image_193.png" /></figure>

<figure><img src="image_194.png" /></figure>

Total RMS = 19157.47

Zxy RMS = 18360.72

Zyx RMS = 19922.39
Site: X29

<figure><img src="image_195.png" /></figure>

<figure><img src="image_196.png" /></figure>

<figure><img src="image_197.png" /></figure>

Total RMS = 18356.35

Zxy RMS = 17857.94

Zyx RMS = 18841.59
Site: X30

<figure><img src="image_198.png" /></figure>

<figure><img src="image_199.png" /></figure>

<figure><img src="image_200.png" /></figure>

Total RMS = 15183.62

Zxy RMS = 15004.09

Zyx RMS = 15361.06
Site: X31

<figure><img src="image_201.png" /></figure>

<figure><img src="image_202.png" /></figure>

<figure><img src="image_203.png" /></figure>

Total RMS = 17432.71

Zxy RMS = 16696.87

Zyx RMS = 18085.40
Site: X32

<figure><img src="image_204.png" /></figure>

<figure><img src="image_205.png" /></figure>

<figure><img src="image_206.png" /></figure>

Total RMS = 17754.72

Zxy RMS = 18425.72

Zyx RMS = 17057.35
Site: X33

<figure><img src="image_207.png" /></figure>

<figure><img src="image_208.png" /></figure>

<figure><img src="image_209.png" /></figure>

Total RMS = 15148.42

Zxy RMS = 15818.73

Zyx RMS = 14387.06
Site: X34

<figure><img src="image_210.png" /></figure>

<figure><img src="image_211.png" /></figure>

<figure><img src="image_212.png" /></figure>

Total RMS = 17292.40

Zxy RMS = 16252.76

Zyx RMS = 18272.99
Site: X35

<figure><img src="image_213.png" /></figure>

<figure><img src="image_214.png" /></figure>

<figure><img src="image_215.png" /></figure>

Total RMS = 17449.80

Zxy RMS = 16734.63

Zyx RMS = 18075.41
Site: X40

<figure><img src="image_216.png" /></figure>

<figure><img src="image_217.png" /></figure>

<figure><img src="image_218.png" /></figure>

Total RMS = 14628.71

Zxy RMS = 14129.51

Zyx RMS = 15111.43
Site: X41

<figure><img src="image_219.png" /></figure>

<figure><img src="image_220.png" /></figure>

<figure><img src="image_221.png" /></figure>

Total RMS = 12094.18

Zxy RMS = 13192.71

Zyx RMS = 10885.36
Site: X87

<figure><img src="image_222.png" /></figure>

<figure><img src="image_223.png" /></figure>

<figure><img src="image_224.png" /></figure>

Total RMS = 16765.96

Zxy RMS = 15966.60

Zyx RMS = 17528.92