Birth of a large volcanic edifice offshore Mayotte via lithosphere-scale dyke intrusion

TL;DR: In this paper, the authors present geophysical and marine data from the MAYOBS1 cruise, which reveal that by May 2019, this activity formed an 820m-tall, 5'km³ volcanic edifice on the seafloor.

Abstract: Volcanic eruptions shape Earth’s surface and provide a window into deep Earth processes. How the primary asthenospheric melts form, pond and ascend through the lithosphere is, however, still poorly understood. Since 10 May 2018, magmatic activity has occurred offshore eastern Mayotte (North Mozambique channel), associated with large surface displacements, very-low-frequency earthquakes and exceptionally deep earthquake swarms. Here we present geophysical and marine data from the MAYOBS1 cruise, which reveal that by May 2019, this activity formed an 820-m-tall, ~5 km³ volcanic edifice on the seafloor. This is the largest active submarine eruption ever documented. Seismic and deformation data indicate that deep (>55 km depth) magma reservoirs were rapidly drained through dykes that intruded the entire lithosphere and that pre-existing subvertical faults in the mantle were reactivated beneath an ancient caldera structure. We locate the new volcanic edifice at the tip of a 50-km-long ridge composed of many other recent edifices and lava flows. This volcanic ridge is an extensional feature inside a wide transtensional boundary that transfers strain between the East African and Madagascar rifts. We propose that the massive eruption originated from hot asthenosphere at the base of a thick, old, damaged lithosphere.

Summary

A revised version of this paper has been submitted on December 15 2020 and is under review.

• How the primary asthenospheric melts form, pond 29 and ascend through the lithosphere is, however, still poorly understood.
• The authors document 30 an on-going magmatic event offshore Mayotte Island (North Mozambique channel), 31 associated with large surface displacements, very low frequency earthquakes and 32 exceptionally deep (25-50 km) seismicity swarms.
• The ridge is composed of many other recent edifices 42 and lava flows and is an extensional feature that opens inside a wide transtensional 43 boundary to transfer the strain between the East-African and Madagascar rifts.
• 53 Prior to this event, no recent eruption or significant seismic activity was reported around 54 Mayotte.

The discovery of the new volcanic edifice 70

• The French national research program “SISMAYOTTE” was launched in February 2019 to 71 determine the origin of the seismicity and deformation, to search for any seafloor volcanic 72 activity and to understand the scale, chronology and implications of the crisis.
• 80 A systematic 12 kHz multibeam echosounder survey east of Mayotte revealed a 820 m tall 81 new volcanic edifice (NVE) 50 km east of Mayotte .
• The NVE was detected by 82 comparing their data to those acquired during a 2014 survey by the French Naval Hydrographic 83 and Oceanographic Service (SHOM)19 .
• The edifice sits on an area that, in the 84 2014 seafloor topography, was locally almost flat at around 3300 m below sea level (bsl).

The Mayotte volcanic ridge 86

• The NVE has grown on the lower insular slope of Mayotte, near the end of a WNW-ESE 87 trending volcanic ridge (Mayotte ridge) emplaced on the submarine flank of Mayotte .
• This seismicity could indicate activation of pre-existing 228 subvertical faults 51 above a deep (> 55 km) depleting reservoir (R1,4), as has been observed 229 during caldera collapse events 52,53,54 but these faults would be much deeper than at any caldera 230 structures documented elsewhere.
• (Conductivity; Temperature; 316 Depth) equipped with an altimeter, an Aanderaa oxygen optode and a Seapoint Turbidity 317 Meter was mounted on a carousel with 16 ®Niskin sampling bottles (8L) to measure and 318 sample throughout the water column.

FIGURE CAPTIONS

• The volcanic ridge offshore Mayotte. a), also known as Figure 1.
• 800 earthquakes between 25 February and May 6 2019 located using OBSs and land stations, also known as Pink dots.
• Bathymetry from MAYOBS1 30-m resolution DTM and previous bathymetry-topography compilation 16,94 a) bathymetry (b) Backscatter seafloor reflectivity (white is highest reflectivity) from MAYOBS1 cruise.
• Arrows with colors with names: GNSS velocity vectors (mm/yr) and station names.
• The redish ellipse: Mayotte ridge, dashed circular area: old caldera structure in the morphology b) Cross-section (projection along azimuth 115 degree).

Figures

Figure 5: Regional volcano-tectonic setting of the submarine eruption offshore Mayotte. a) Volcano-tectonic setting of the new volcanic edifice (NVE). Bathymetry compiled from MAYOBS1 cruise 18, PTOLEMEE Cruise 95, and the General Bathymetric Chart of the Oceans (https://www.gebco.net). Global topography from SRTM GL1

Full text

This%version%of%this%paper%is%non-peer-reviewed%and%under%review%in%Nature%
Geoscience%
!
This!paper!has!been!submitted!for!publication!to!Nature!Geoscience!on!June!8!2020!
A!revised!version!of!this!paper!has!been!submitted!on!December!15!2020!and!is!under!
review.!
!
Any!comment!can!be!sent!to!the!!corresponding!author:!feuillet@ipgp.fr!

1
Birth of a large volcanic edifice through lithosphere-scale dyking offshore 1
Mayotte (Indian Ocean) 2
3
N. Feuillet
1,*
, S.J. Jorry
2
, W. Crawford
1
, C. Deplus
1
, I. Thinon
3
, E. Jacques
1
, J.M. Saurel
1
, A. 4
Lemoine
3
, F. Paquet
3
, C. Satriano
1
, C. Aiken
2
, O. Foix
1
, P. Kowalski
1
, A. Laurent
1
, E. 5
Rinnert
2
, C. Cathalot
2
. J.P. Donval
2
, V. Guyader
2
, A. Gaillot
2
, C. Scalabrin
2
, M. Moreira
1
, A. 6
Peltier
1
, F. Beauducel
1,4
, R. Grandin
1
, V. Ballu
5
, R. Daniel
1
, P. Pelleau
2
, J. Gomez
1
, S. 7
Besançon
1
, L. Geli
2
, P. Bernard
1
, P. Bachelery
6
, Y. Fouquet
2
, D. Bertil
3
, A. Lemarchand
1
, J. 8
Van der Woerd
7
. 9
10
1- Université de Paris, Institut de physique du globe de Paris, CNRS, F-75005 Paris, 11
France 12
2- IFREMER, Unité Géosciences Marines, Technopole La Pointe du Diable, 29280 13
Plouzané, France 14
3- Bureau de Recherches Géologiques et Minières - BRGM, DGR/GBS, F-45060 15
Orléans, France 16
4- Université Grenoble Alpes, IRD, ISterre 17
5- LIttoral ENvironnement et Sociétés (LIENSs) UMR7266, Université de La Rochelle - 18
CNRS, 2 rue Olympe de Gouges, 17000 La Rochelle 19
6- Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, 20
F-63000 Clermont-Ferrand, France, 21
7- Institut de Physique du Globe de Strasbourg UMR7516 CNRS Université de 22
Strasbourg, 5 rue René Descartes 67000 Strasbourg, France 23
24
25
26
27
Volcanic eruptions are foundational events that shape the Earth’s surface and provide a 28
window into deep Earth processes. How the primary asthenospheric melts form, pond 29
and ascend through the lithosphere is, however, still poorly understood. We document 30
an on-going magmatic event offshore Mayotte Island (North Mozambique channel), 31
associated with large surface displacements, very low frequency earthquakes and 32
exceptionally deep (25-50 km) seismicity swarms. We present data from the May 2019 33
MAYOBS1 cruise, which reveal that this event gave birth to a 820m tall, ~ 5 km
3
deep-34
sea volcanic edifice. This is the largest active submarine eruption ever documented. The 35
data indicate that deep magma reservoirs were rapidly drained through dykes that 36
intruded the entire lithosphere and that pre-existing subvertical faults in the mantle 37
were reactivated beneath an ancient caldera structure. 38

2
39
40
The new volcanic edifice is located at the tip of a 50 km-long volcanic ridge on the 41
eastern insular slopes of Mayotte. The ridge is composed of many other recent edifices 42
and lava flows and is an extensional feature that opens inside a wide transtensional 43
boundary to transfer the strain between the East-African and Madagascar rifts. A hot 44
asthenosphere at the base of a thick damaged lithosphere could be at the origin of this 45
massive eruption. 46
47
Since May 10 2018, Mayotte Island (Comoros archipelago, north Mozambique Channel 48
between Africa and Madagascar, Figure 1a) has experienced a major magmatic event off its 49
eastern coast. This event generated more than 11000 detectable earthquakes (up to Mw 5.9), 50
surface deformation rates of up to 200 mm/year and unusual very low frequency (VLF) 51
earthquakes
1,2,3
. As of December 2020 (the time of writing), Mayotte is still deforming and 52
both VLF events and earthquakes with Mw up to 5 are still being recorded. 53
Prior to this event, no recent eruption or significant seismic activity was reported around 54
Mayotte. Only two earthquakes were detected within 100 km of the island by the global 55
network since 1972
4
and the most recent volcanic exposure is a 4-6 kyr-old pumice layer 56
sampled in the lagoon surrounding the island
5
. 57
Recent geodynamic reconstructions suggest that the archipelago was built on ~150 Ma old 58
oceanic lithosphere accreted to accommodate the opening of the Western Somali Basin
6
. This 59
Comorian volcanism may result from partial melting of the base of this old oceanic 60
lithosphere in interaction with plume material
7,8,9
possibly super plumes originating from 61
Africa
10,11,12
. This volcanism may have been controlled by reactivation of the fractures zones
13
62
or by diffuse zones of right-lateral shear deformation
14
. Subaerial volcanic activity on Mayotte 63

3
island began 11 My ago
13
. Well-preserved cones, tuff rings and maar craters in the 64
Northeastern part of the island (on Petite Terre and in and around Mamoudzou
15,7
and further 65
offshore
16
(Figure 1b) testify to relatively recent (probably Holocene
7
) subaerial explosive 66
volcanic activity. Gas emissions on Petite-Terre with a high percentage of carbon dioxide and 67
helium indicate magma degassing
17
. 68
69
The discovery of the new volcanic edifice 70
The French national research program “SISMAYOTTE” was launched in February 2019 to 71
determine the origin of the seismicity and deformation, to search for any seafloor volcanic 72
activity and to understand the scale, chronology and implications of the crisis. As part of this 73
program, we 1) set up seismic and Global Navigation Satellite System (GNSS) stations on 74
Mayotte and Grande Glorieuses Islands, 2) deployed Ocean Bottom Seismometers (OBS) 75
with attached Absolute Pressure Gauges (APG) around the seismic swarm area, and 3) 76
acquired high-resolution marine data (bathymetry, seafloor and water column backscatter, 77
sub-bottom, magnetic and gravity profiles), rock dredges and CTD (Conductivity-78
Temperature-Depth)- Rosette during the MAYOBS1 cruise aboard the R/V Marion 79
Dufresne
18
. 80
A systematic 12 kHz multibeam echosounder survey east of Mayotte revealed a 820 m tall 81
new volcanic edifice (NVE) 50 km east of Mayotte (Figure 1). The NVE was detected by 82
comparing our data to those acquired during a 2014 survey by the French Naval Hydrographic 83
and Oceanographic Service (SHOM)
19
(Figure 2a). The edifice sits on an area that, in the 84
2014 seafloor topography, was locally almost flat at around 3300 m below sea level (bsl). 85
The Mayotte volcanic ridge 86
The NVE has grown on the lower insular slope of Mayotte, near the end of a WNW-ESE 87
trending volcanic ridge (Mayotte ridge) emplaced on the submarine flank of Mayotte (Figure 88

4
1). The NVE and many other volcanic features along the ridge are highly reflective in seafloor 89
imagery (Figure 1c and extended data Figures 1, 3) indicating recent volcanic activity all 90
along the ridge. The ridge is 50 km long, extending from the most recent subaerial cones and 91
maar craters on Grande-Terre and Petite-Terre islands (MPT Volcanic zone) to the NVE 92
(Figure 1b). It is segmented into two main parts: an upper slope volcanic zone (western 93
segment) and a mid- to lower-slope zone (eastern segment). The eastern segment trends 94
N130°E and is made of many constructional features similar to mafic submarine eruption 95
features observed elsewhere
20,21,22
: cones up to 2 km-wide and 500 m-high, probably 96
monogenetic; high backscatter zones with smooth bathymetry, which could correspond to 97
recent lava flows; elongated ridges with steep slopes and varying orientations, which could 98
result from dykes in more sedimented areas (Figure 1 and extended data Figure 2d,e). 99
The western segment is made of volcanic features having more complex morphologies and 100
emplaced along different directions (Figure 1b and extended data Figure 2b,c). The main 101
features are: i) Two N40°E and N120°E trending sets of cones and lava flows, with high 102
backscatter, northeast and southeast of Petite-Terre, respectively. These sets converge to 103
prolong the onshore maar craters of Petite-Terre and may have emplaced along pre-existing 104
fractures or faults; ii) a horse-shoe shaped edifice (the Horseshoe) with a 3.5 km wide cone, 105
steep slopes and a large collapse-induced scar. East of the Horseshoe, several smaller cones 106
and volcanic features are aligned E-W, suggesting eruptive fissures. Large lava flows 107
characterized by high backscatter and rough bathymetry likely originate from this fissure 108
system. iii) a 4 km-wide circular structure (the Crown), whose rim is crowned by seven 1 km-109
wide, 100-150m high volcanic cones. Their arrangement suggests typical post-caldera domes 110
23,24
. West of the Crown, submarine canyons and slope failure scars all terminate at a N-S 111
trending slope break that may be controlled by faulting. The Crown appears to be located in a 112

Frequently asked questions

Q1. How many VLF earthquakes were detected between February 25 and April 24, 2019?

326Eighty-four very low frequency (VLF) earthquakes were detected between February 25 and 327April 24, 2019, using an amplitude trigger on ocean bottom hydrophones recordings, filtered 328between 0.05 and 0.10 

Q2. What was the seismic network used during the 2019 314 cruise?

The seismic network used during 323the two month deployment included OBSs, onshore local and regional stations (up to 500km 324 distance). 

Q3. What are the events occurring in the first six weeks of the crisis?

Colored circles are events occuring in the first six weeks of the crisis, white circles are earthquakes in the intervening 8 months. 

Q4. What is the likely explanation for the seismicity detected under the Mayotte ridge?

Mantle seismicity has been detected beneath Kilauea, Loihi 41-199 43 and La Réunion 44 volcanoes, where it has been interpreted as failure of the brittle 200lithosphere induced by magma migration through long-lived tectonic structures or by the 201islands’ loading. 

Q5. What is the fit-model for the GNSS data?

Best fit-models with 1σ uncertainties of the GNSS data for one isotropic point source and a triple volumetric discontinuity pCDM source. 

Q6. What is the lava flow rate above the swarm1?

234The VLF events, located above swarm1, may be generated by the resonance of a fluid-filled 235(magma, gas or hydrothermal) shallower cavity or a fluid-filled crack, most probably at the 236base of the crust. 

Q7. What is the en-echelon arrangement of the Mayotte ridge?

The 255left-lateral en-echelon arrangement of these features resembles that of extensional tectonic 256structures in a context of oblique extension (i.e in segmented and diffuse strike-slip fault 257 systems 67 or highly-oblique rifting 68, 69,70 ,71. 

Q8. What is the cause of the 225swarm 1 earthquake?

This stationary seismicity could be caused by stress 224perturbation along pre-existing structures and/or fluid (gas, magma or water) motions. 

Q9. What is the way to measure the seismicity of the Mayotte ridge?

The earthquakes show strike-slip focal mechanisms compatible with a least 207compressive principal stress orthogonal to the eastern segment of the ridge (extended data 208Figure 8). 

Q10. Who was responsible for the installation of new GNSS stations in 347Mayotte?

ALM 346and JVW were responsible for the installation of new seismological and GNSS stations in 347Mayotte and of data acquisition onshore. 

Q11. how long did the lava flow rate between the start of the eruption and their survey last?

On the basis of this 216 assumption, the authors estimate a minimum mean lava flow rate of ~180m3s-1 between the start of 217the eruption on the seafloor and their survey (~ 11 months). 

Q12. What is the acoustic effect of the lava plumes?

The 241 acoustic plumes emanating from the overlying Horseshoe edifice may result from actively 242 degassing of this shallower reservoir. 

Q13. What is the effect of the earthquakes on the NVE?

The local stress probably 218decreased considerably once the magma path to the NVE was opened, as is observed during 219 many eruptions involving dyke propagation 50, which would explain why no earthquakes were 220detected beneath the NVE during the OBS deployment, which started in late February 2019.