<figure><img src="image_64.png" /><figcaption>Fig. 10: Successive parallel seismic reflection profiles from the NW (top) to the SE (bottom) showing the Mayotte Ridge mainly formed by the May2 volcanic series. The top seismic profile across the recent Fani Maoré volcano (indicated by a red star) is an MCS line. The following four are SISRAP profiles. Same caption as Fig. 8. See location in inset and in Fig. 1.</figcaption></figure>
facies and the step at the edges of the Mayotte Ridge, formed by the May2 facies, argue both for a stronger volcanic activity to the East of Mayotte relative to the southern part of Mayotte. The general WNW-ESE trend of this ridge, similar to the one of the recent EMVC, suggests a regional tectonic control of the emplacement of the volcanic activity (Feuillet et al., 2021). The Base May3 and Base May4 reflectors, 0.12 s TWT and 0.08 s TWT on average beneath TM, respectively (see α in Fig. 8-11A), are close in age, 9 and 8 Ma, respectively (see Supplementary Table 1). The single May3 reflector corresponds to a brief volcanic event affecting mostly the east of Mayotte Island, while the thickness of the May4 series suggests a longer-lasting volcanic phase.

## 5.2. Zélée & Geyser banks

South of the Zélée & Geyzer banks, we identified two volcanic series above the May1 reflectors, that we tied from the area south of Mayotte (Fig. 12). The base of the earlier series, Base Zélée1, corresponds to an almost continuous well-marked reflector, lying horizontally at 5.45 s TWT in its distal part. This reflector can be observed as far as 60 km away from the Zélée bank (see β in Fig. 12). The Top Zélée1 reflector is more discontinuous and undulating but can be confidently tracked due to the onlaps of the sedimentary reflectors and the associated typical chaotic facies, terminating at 40 km away from the Zélée bank (see α in Fig. 12). The maximum thickness of the Zélée1 chaotic facies is 1.2 km (0.6 s TWT with 4 km/s). In contrast, the Top Zélée2 is a strong reflector while the Base Zélée2 is discontinuous. Zélée2 thickness and its lateral extent are much smaller than for Zélée1 (~250 m and 10-12 km respectively, see α in Fig 12).

We estimated the Base of Zélée1, 0.26 s TWT on average beneath the TO horizon (see α in Fig. 12), at 32 Ma, in agreement with Leroux et al., (2020) who dated the main volcanic phase during Oligocene times. We note that the distal part of the Base Zélée2 reflector is at about the same depth, 5.2 s TWT, as the Base May2 reflector at 5.3 s TWT (Fig. 10 & 12) indicating that they are quasi-contemporary with an estimated age of 16 Ma and 21 Ma, respectively.

## 5.3. The Jumelles seamounts

As for the May4 series, the structure of the Jumelles seamounts is difficult to discern at depth as it is obscured by recent submarine volcanoes (Thinon et al., 2022). The
<figure><img src="image_65.png" /></figure>

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

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

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

Fig. 11: SISRAP seismic profiles with their interpretation to the north of Mayotte (A), south of Mohéli (B), north of the Jumelles seamount (C), and south of Grande Comore. See location in inset and in Fig. 1.
<figure><img src="image_69.png" /><figcaption>Fig. 12: NS trending SISRAP seismic profile south of the Zélée & Geyser banks (A) and its interpretation (B). See location in inset and in Fig. 1.</figcaption></figure>

volcanic Jum series of the Jumelles seamounts is recent with only a small sedimentary cover (Fig. 11C). It quickly thins and becomes concordant with the sedimentary units at ~15-20 km from the seamounts (see α on Fig. 11C). There, the Base Jum reflector is shallow at 4.75 s TWT depth, 0.04 s TWT above the TM horizon indicating an age of 4 Ma.

**5.4. Offshore Anjouan Island**

The seafloor and the volcanic series northeast of Anjouan Island are modified by numerous sills, volcanoes, and the associated seal bypass systems of the Mwezi province (Thinon et al., 2022). The Base Anj reflector is highly discontinuous, while the Top Anj reflector is more continuous and reflective. The Anj series reaches a maximum thickness of 800 m (0.4 s TWT with a 4 km/s) and thins northward, as far as 70 km from the Anjouan Island, where the Anj series becomes concordant with the sedimentary units (see α on Fig. 13). There, the Base Anj reflector is observed at 4.68 s TWT on average, just above the TM horizon, which gives an age of 4 Ma. This age is the same as the estimated age for the base of the volcanic series of the Jumelles seamounts.
<figure><img src="image_70.png" /></figure>

Fig. 13: (A) SW-NE trending SISRAP composite seismic profile north of Anjouan with its interpretation (B). The blue corresponds to the volcanic series between the Top Anj and Base Anj reflector. See location in inset and in Fig. 1.
## 5.5. Offshore Grande Comore Island

The Grande Comore (GrdCo) volcanic series is identified to the West and South of the island where there is little recent volcanism (Thinon et al., 2022). The SISRAP profile presented in Fig. 7D shows an area close to Grande Comore Island, where the top of the GrdCo volcanic series reaches the seafloor. The reflector at the base of the chaotic facies corresponding to the GrdCo series is well marked and becomes horizontal and concordant with the sedimentary units at ~60 km from the island (see α on Fig. 11D). It is identified at 4.98 s TWT depth, giving an age of 2±1.5 Ma.

## 5.6. Offshore Mohéli Island

We identified the Moh volcanic series south of Moheli Island. Although there is little recent volcanism observed at the seafloor in this area (Thinon et al., 2022), we note that there are many sills and seal bypass systems south of Moheli Island resulting in a scattered seismic image at depth. However, the Top Moh is easily identified as a continuous and strong reflector, while the Base Moh reflector is more discontinuous, especially near the island (Fig. 11B). The chaotic facies between the Top and Base Moh is up to 400m thick (0.2 s TWT with 4 km/s) and thins southward as far as 80 km away from the island, where Top Moh and Base Moh are almost joining, becoming horizontal and concordant with the sedimentary units (see α on Fig. 5). There, the Base Moh reflector is located at 0.11 s TWT on average below the TM horizon indicating a 9 Ma age. This is in agreement with the well-developed insular shelf around Moheli showing that it is significantly older than Anjouan and Grande Comore (Tzevahirtzian et al., 2020).

# 6. Discussion

## 6.1. Timing of the volcanism in the Comoros archipelago

We interpret the base of the deepest volcanic series, observed as far as ~125 km away from the islands, as marking the onset of the main volcanic phase, leading to their formation. We do not preclude earlier activity producing lava flows right beneath the islands that we could not map because of the blinding effect of the islands on our seismic reflection data. However, if such, we argue that this precursory activity must have been restricted locally as we did not find typical volcanic facies deeper than the identified oldest volcanic series.
<figure><img src="image_71.png" /><figcaption>Fig. 14: Map of the lateral extent of each volcanic series around the islands of the Comoros archipelago. Dots indicate the locations, along the seismic reflection profiles, where the reflectors of each volcanic series lose their reflectivity and become as reflective as the seismic reflectors of the sedimentary cover above and beneath (β). This loss of reflectivity is interpreted as a change in the sedimentary facies from layers of volcanic material to pelagic sediments.</figcaption></figure>

The regular deepening of the top of the volcanic facies and its progressive thinning away from the Grande Comore, Anjouan, and Mohéli islands do not indicate strong variations in the supply of magmatic material for these youngest volcanic edifices. By contrast, the oldest Zélée & Geyser banks and Mayotte Island (32 and 27 Ma), show distinct volcanic phases. The occurrence of sediments between two volcanic series and steps of the volcanic edifices at the edge of some the volcanic facies argues for changes in the supply of magmatic material for these two edifices. These latter also show different lateral extent for each volcanic facies (Fig. 14). The May1 series extends the furthest from the island (~125 km from Mayotte) and the Zélée1 series also extend far away from the Zélée & Geyser banks (~90 km) (Fig. 14). The following series May2 and Zélée2 (21 and 16 Ma), are present only locally: May2 is restricted to the east of Mayotte Island and Zélée2 to the west of the Zélée bank where it is almost
joining the May2 reflectors (Fig. 8-9). The base of the May3 and May4 series are very close in age (8 and 9 Ma) and could thus correspond to the onset of the same volcanic event. The larger thickness of the May4 chaotic facies relative to the thin May3 reflector would then indicate that this event was more intense to the north of Mayotte Island. At that time (8-9 Ma) volcanism became widespread, all along the archipelago from the Mohéli and Grande Comore to the west to the Jumelles seamounts to the East.

<figure><img src="image_72.png" /><figcaption>Fig. 15: Timing of the onset of volcanism for each island of the Comoros archipelago and comparison with the temporal distribution of volcanism in Madagascar and in the Rungwe volcanic province in Tanzania (using the age compilation of Michon et al., 2022).</figcaption></figure>

We thus propose two main periods in the evolution of the Comoros archipelago (Fig. 15). The first period corresponds to localized volcanic activity in Oligocene times: at Zélée & Geyser banks (Zélée1), and at Mayotte edifice (May1). From the average sedimentation rate, we calculate a time interval of 2 Ma between Top Zélée and Base May1 indicating a short period of relative magmatic quiescence before the outpouring of magmatic flows from Mayotte Island. Volcanism then began between the two existing volcanic edifices in Early Miocene times (May2 and Zélée2). About 10 Ma
later, in Late Miocene times, a second period started with distributed volcanism to the
North (May4), East (May3), and West of Mayotte Island (Moh), then at Anjouan Island
(Anj) and Jumelles seamounts (Jum) and finally starting also further to the Northeast
at Grande Comore (GrdCo) (Fig. 15). There seems thus to be a westward propagation
of the onset of volcanism along the Comoros archipelago.

Volcanism is still ongoing on Grande Comore Island (at the Karthala volcano) (Thivet et al., 2022), and Mayotte Island. There, radiometric ages determined from onshore lavas prove activity for the last 6 Ma (Berthod et al., 2021; Michon, 2016) and Anjouan Island for the last 1 Ma (Quidelleur et al., 2022). Volcanism between 3.8 Ma and 8 ka has also been reported on Mohéli Island (Rusquet et al., in rev.). Recent volcanism has also been observed in the Mwezi province to the northeast of Anjouan Island and in the N'Droundé province to the northeast of Grande Comore Island (Thinon et al., 2022). Therefore, while the onset of volcanism propagated westward since 32 Ma, volcanism is still active (at Karthala and Fani Maoré volcanoes), or at least took place in recent times (Holocene), all along the Comoros archipelago.

**6.2. Regional chronology**

The timing of the volcanism along the Comoros archipelago shows some
similarities in timing with the evolution of magmatism on Madagascar and along the
EARS.

The first occurrence of magmatism during Cenozoic times in Madagascar is
reported in the Upper Oligocene, at ~28 Ma (Bardintzeff et al., 2010). Michon et al.,
(2022) noted that Madagascar experienced discontinuous magmatic activity up to
15 Ma, followed by a period of quiescence over most of the Middle Miocene, and a
coeval resumption of activity around 12 Ma (Fig. 15). This onset of volcanism was also
coeval with the beginning of the doming of Madagascar which started during Early
Oligocene times and continued up to the late Miocene (Jiao et al., 2023). Recent
seismic tomographic studies have imaged low-velocity zones at 50–200 km depths
beneath the island, supporting the hypothesis that asthenospheric upwelling is the
source of the Late Cenozoic volcanism and uplift of Madagascar e.g. (Ebinger and
Sleep, 1998).

We note that while the initial and main phase of the volcanic edification of the
Glorieuses seamount occurred in late Turonian times, apparent uplift phases (related
to magmatic pulses) are recognized during Late Eocene, and Late Tortonian/Early
Messinian (~9.5 Ma-7 Ma) (Leroux et al., 2020). These two uplift phases are almost coeval with the two volcanic phases identified in the Comoros archipelago.

Although there are few radiometric ages for the volcanism in the southern Mozambique Channel, the main building phase of the volcanic edifices along the Quathlamba seismic axis is inferred to start in late Oligocene times, earlier than 20 Ma e.g.(Berthod et al., 2022). Late Cenozoic volcanism also occurred along the southern part of the Davie Ridge (Courgeon et al., 2018). This first phase in the southern Mozambique Channel was followed by an Upper Miocene-Pleistocene discontinuous activity (Berthod et al., 2022; O'Connor et al., 2019).

The Rungwe Province, in Tanzania, is the volcanic region within the western branch of the East African Rift that is closest to the Comoros archipelago. There, volcanism also started in the upper Oligocene (26 Ma) and then underwent a quiescence period during the Middle Miocene (between 17 and 9 Ma) followed by a resumption of activity in late Miocene times e.g. (Michon et al., 2022) (Fig. 15). Miocene to Holocene volcanism is also indicated in the Tanga Basin (offshore north-eastern Tanzania)(Mvile et al., 2023).

Interestingly, (Michon et al., 2022) noted that the different provinces of the EARS are all characterized by an Upper Oligocene-Lower Miocene discontinuous volcanism, followed by a significant hiatus during most of the Middle Miocene (12.5-15 Ma) and by a volcanic renewal during the Late Miocene.

We conclude that the volcanic evolution of the Comoros archipelago follows an evolution pattern in coincidence with Madagascar and with the EARS (both onshore and offshore). However, the reason for this remains to be explained.

### 6.3. A nascent plate boundary since the Early Miocene?

While the earliest extension documented for the onshore EARS is evident in northern Kenya at ~30 Ma in the Oligocene (Morley et al., 1999), the major part of the offshore EARS developed mutually with the southward propagating onshore rift branches during the late Miocene and particularly during the Pliocene (Franke et al., 2015). The main rift phase initiated in Middle Miocene times and resulted in extension all over the EARS, from Afar up to the Mozambique channel (Michon et al., 2022). The resulting grabens then underwent a significant increase of subsidence around the Miocene - Pliocene transition (~5.7 Ma) (Michon et al., 2022). Volcanism and extension show coeval phases similar in magnitude and extent (Michon et al., 2022). Therefore,
(Michon et al., 2022) proposed that an Upper Eocene to Middle Miocene period of prevailing trap-scale rifting, with extension restricted to areas of continental flood basalts, is followed, since the Upper Miocene, by a period of plate-scale rifting that marks the onset of a widespread deformation along the EARS, including the offshore branches and Madagascar.

At first glance, the volcanic evolution of the Comoros archipelago would fit in such a scenario with the first localized volcanism at Zélée & Geyser and Mayotte, at Oligocene-Early Miocene times, followed by more widespread volcanism along the archipelago since the Late Miocene during the inferred period of plate-scale rifting. However, the general WNW-ESE trend of the Mayotte ridge, similar to the one of the recent EMVC, might indicate a regional tectonic control of the emplacement of the May2 volcanic series in Early Miocene times. In addition, we note that the volcanism propagated with a general westward trend, from Zélée & Geyser banks to the Grande Comore Island, suggesting an emergent link between the North Madagascar magmatic province and the Rungwe province in the EARS where volcanism began at late Oligocene times. Based on a high-resolution kinematic evolution model of the Southwest Indian Ridge, 52 Ma to present, DeMets et al., (2021) reconstructed the path of the Somalia Plate relative to Nubia and showed that divergence has occurred in Africa since 40 Ma, with little or no movement between 26 and 17-16 Ma, and a restart of the divergence after 17-16 Ma. These kinematics changes are coeval with the onset of the Arabia-Eurasia collision at the Eocene/Oligocene boundary (Song et al., 2023) and with the separation of Arabia from Africa with seafloor spreading initiating at 20-17 Ma along the Sheba Ridge (d'Acremont et al., 2006; Fournier et al., 2011; Gillard et al., 2021). We thus propose that the right-lateral southern boundary of the Somalia plate, previously suggested by Famin et al., (2020) to explain the present-day seismicity along the Comoros archipelago, may have been initiated to the east of the Comoros as early as at Early Miocene times. If so this would have been the result of a change of the Somalia plate motion. The volcanism then propagated westward in Late Miocene times.

## 7. Conclusions

Based on the interpretation of volcanic units in high-quality seismic reflection profiles, tied to a regional stratigraphy, we determine the evolution of the Comoros
Archipelago in the western Indian Ocean. The main results of our interpretation are as follows:

1. There are two main volcanic periods in the evolution of the Comoros archipelago: (1) a first period with localized volcanic activity at Zélée & Geyser banks and Mayotte edifice which began ~32 Ma, (2) a second period starting 8-9 Ma with distributed volcanism to the North, East, and West of Mayotte.

2. We thus confirm earlier investigations that the oldest volcanic edifices are the Zélée-Geyser seamounts and Mayotte Island, i.e. the eastern portions of the Comoros archipelago. If there was indeed an age progression, it went from east to west over time.

3. At about 8-9 Ma, volcanism became widespread, affecting the entire archipelago area from the northern part of Mayotte edifice to Mohéli Island in the west. This was followed by volcanism affecting Anjouan Island and the Jumelles seamounts at ~4 Ma and finally further to the northeast Grande Comore Island at ~1.5 Ma.

4. This timing of the Comoros volcanism shows striking similarities with the temporal evolution of magmatism at Madagascar and along the EARS, starting in the upper Oligocene, followed by a quiescence period during the Middle Miocene and by a resumption of activity in late Miocene times.

5. We suggest that along the Comoros archipelago, the developing right-lateral boundary to the south of the Somalian plate may have initiated as early as 21 Ma, at Early Miocene times.
Acknowledgments

This paper is a contribution of the COYOTES and SISMAORE teams
(http://www.geocean.net/coyotes/doku.php?id start). The processing and the detailed
analysis of these geophysical and geological data are carried out mainly in the
framework of the ANR COYOTES (ANR-19-CE31-0018, https://anr.fr/Projet-ANR-19-
CE31-0018) project funded by the French ANR (Agence Nationale de Recherche) and
the BRGM. The SISMAORE cruise was mainly funded by the Flotte Océanographique
Française (FOF) and by the BRGM. We thank CGG for allowing us to use the CGG
Geovation software in CNRS labs. We thank Captains P. Moimeaux and G. Ferrand,
the crews, and the technicians from the R/V Pourquoi pas? (FOF by
IFREMER/GENAVIR). Thanks to the BRGM regional department of Mayotte. Thanks
to REVOSIMA and DIRMOM for their assistance with the cruise during the COVID
sanitary crisis. Masquelet's Ph.D. is funded by Sorbonne Université, via the ANR
COYOTES. We thank TotalEnergies and TGS for permission to use some of their
seismic reflection profiles.
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<table><caption>Supplementary Material Table 1: Estimation of the ages of the distinct volcanic series</caption><thead><tr><th colspan="13">MAYOTTE 1</th></tr><tr><th>ID of the seismic profile</th><th>Location of the measurement point in the seismic profile (N° CDP)</th><th>TWT (s) of the Top Oligocene</th><th>TWT (s) of volcanic base of May1</th><th>Difference (s TWT) between the volcanic base of May1 and the Top Oligocene</th><th>ID of the seismic profile used to determine the interval velocities</th><th>Interval velocity used for the conversion time to depth (m/s twt)</th><th>Thickness (m) between the volcanic base of May1 and the Top Oligocene</th><th>Sedimentation rate (m/Ma) used to calculate ages</th><th>Difference (Ma) between the volcanic base of May1 and the Top Oligocene</th><th>Age of the Top Oligocene (horizon reference) (Ma)</th><th>Age of the base of the volcanic series of May1 (Ma)</th><th>Average age of the onset of the volcanism of May1 (Ma)</th></tr></thead><tbody><tr><td>MAOR21D01</td><td>1000</td><td>5.285</td><td>5.447</td><td>0.162 MAOR21D01</td><td></td><td>2000</td><td>162</td><td>30</td><td></td><td>5.40</td><td>23.03</td><td>28.43</td><td rowspan="27">27</td></tr><tr><td>MAOR21R116</td><td>3100</td><td>5.369</td><td>5.484</td><td>0.115 MAOR21D01</td><td></td><td>2000</td><td>115</td><td>30</td><td></td><td>3.83</td><td>23.03</td><td>28.66</td></tr><tr><td>MAOR21R117</td><td>5000</td><td>5.38</td><td>5.481</td><td>0.101 MAOR21D01</td><td></td><td>2000</td><td>101</td><td>30</td><td></td><td>3.37</td><td>23.03</td><td>26.40</td></tr><tr><td>MAOR21R003</td><td>7200</td><td>5.214</td><td>5.334</td><td>0.12 MAOR21D01</td><td></td><td>2000</td><td>120</td><td>30</td><td></td><td>4.00</td><td>23.03</td><td>27.03</td></tr><tr><td>MAOR21R004</td><td>1000</td><td>5.246</td><td>5.386</td><td>0.14 MAOR21D01</td><td></td><td>2000</td><td>140</td><td>30</td><td></td><td>4.67</td><td>23.03</td><td>27.70</td></tr><tr><td>MAOR21R114</td><td>1000</td><td>5.589</td><td>5.753</td><td>0.164 MAOR21D01</td><td></td><td>2000</td><td>164</td><td>30</td><td></td><td>5.47</td><td>23.03</td><td>28.50</td></tr><tr><td>MAOR21R110</td><td>0</td><td>5.592</td><td>5.679</td><td>0.147 MAOR21D01</td><td></td><td>2000</td><td>147</td><td>30</td><td></td><td>4.90</td><td>23.03</td><td>27.93</td></tr><tr><td>MAOR21R111</td><td>0</td><td>5.529</td><td>5.68</td><td>0.151 MAOR21D01</td><td></td><td>2000</td><td>151</td><td>30</td><td></td><td>5.03</td><td>23.03</td><td>28.06</td></tr><tr><td>MAOR21R002</td><td>16500</td><td>5.643</td><td>5.763</td><td>0.12 MAOR21D01</td><td></td><td>2000</td><td>120</td><td>30</td><td></td><td>4.00</td><td>23.03</td><td>27.03</td></tr><tr><td>MAOR21D08</td><td>23000</td><td>5.14</td><td>5.286</td><td>0.146 MAOR21D08</td><td></td><td>2000</td><td>146</td><td>30</td><td></td><td>4.87</td><td>23.03</td><td>27.90</td></tr><tr><td>MAOR21R009</td><td>16000</td><td>5.463</td><td>5.631</td><td>0.158 MAOR21D08</td><td></td><td>2000</td><td>168</td><td>30</td><td></td><td>5.60</td><td>23.03</td><td>28.63</td></tr><tr><td>MAOR21R009</td><td>10000</td><td>5.535</td><td>5.727</td><td>0.192 MAOR21D08</td><td></td><td>2000</td><td>192</td><td>30</td><td></td><td>6.40</td><td>23.03</td><td>29.43</td></tr><tr><td>MAOR21D09</td><td>1000</td><td>5.121</td><td>5.242</td><td>0.121 MAOR21D09</td><td></td><td>2000</td><td>121</td><td>30</td><td></td><td>4.03</td><td>23.03</td><td>27.06</td></tr><tr><td>MAOR21R012</td><td>11000</td><td>5.212</td><td>5.357</td><td>0.126 MAOR21D09</td><td></td><td>2000</td><td>126</td><td>30</td><td></td><td>4.20</td><td>23.03</td><td>27.23</td></tr><tr><td>MAOR21R013</td><td>12000</td><td>5.245</td><td>5.379</td><td>0.145 MAOR21D09</td><td></td><td>2000</td><td>145</td><td>30</td><td></td><td>4.83</td><td>23.03</td><td>27.86</td></tr><tr><td>MAOR21R015</td><td>1000</td><td>5.274</td><td>5.414</td><td>0.134 MAOR21D08</td><td></td><td>2000</td><td>134</td><td>30</td><td></td><td>4.47</td><td>23.03</td><td>27.50</td></tr><tr><td>MAOR21R010</td><td>1000</td><td>5.317</td><td>5.45</td><td>0.133 MAOR21D08</td><td></td><td>2000</td><td>140</td><td>30</td><td></td><td>4.67</td><td>23.03</td><td>27.70</td></tr><tr><td>MAOR21R0115</td><td>100</td><td>5.388</td><td>5.496</td><td>0.108 MAOR21D08</td><td></td><td>2000</td><td>133</td><td>30</td><td></td><td>4.43</td><td>23.03</td><td>27.46</td></tr><tr><td>MAOR21R113</td><td>4400</td><td>5.431</td><td>5.568</td><td>0.137 MAOR21D08</td><td></td><td>2000</td><td>108</td><td>30</td><td></td><td>3.50</td><td>23.03</td><td>26.63</td></tr><tr><td>MAOR21R038</td><td>1500</td><td>5.355</td><td>5.429</td><td>0.074 MAOR21D08</td><td></td><td>2000</td><td>74</td><td>30</td><td></td><td>4.57</td><td>23.03</td><td>27.60</td></tr><tr><td>MAOR21R008</td><td>sud7000</td><td>4.945</td><td>5.061</td><td>0.116 MAOR21D01</td><td></td><td>1900</td><td>110.2</td><td>30</td><td></td><td>3.67</td><td>5.333</td><td>9.01</td></tr><tr><td>MAOR21R013</td><td>nord1000</td><td>4.973</td><td>5.105</td><td>0.132 MAOR21D01</td><td></td><td>1900</td><td>125.4</td><td>30</td><td></td><td>4.18</td><td>5.333</td><td>9.51</td></tr><tr><td>MAOR21R010</td><td>6500</td><td>4.935</td><td>5.076</td><td>0.141 MAOR21D01</td><td></td><td>1900</td><td>133.95</td><td>30</td><td></td><td>4.47</td><td>5.333</td><td>9.80</td></tr><tr><td>MAOR21R117</td><td>4700</td><td>4.978</td><td>5.084</td><td>0.106 MAOR21D01</td><td></td><td>1900</td><td>100.7</td><td>30</td><td></td><td>3.36</td><td>5.333</td><td>8.69</td></tr><tr><td>MAOR21R111</td><td>4000</td><td>4.916</td><td>5.075</td><td>0.159 MAOR21D01</td><td></td><td>1900</td><td>151.05</td><td>30</td><td></td><td>5.03</td><td>5.333</td><td>10.37</td></tr><tr><td>MAOR21R111</td><td>2200</td><td>4.936</td><td>5.043</td><td>0.107 MAOR21D01</td><td></td><td>1900</td><td>101.65</td><td>30</td><td></td><td>3.39</td><td>5.333</td><td>8.72</td></tr><tr><td>MAOR21R004</td><td>4500</td><td>4.957</td><td>5.066</td><td>0.109 MAOR21D01</td><td></td><td>1900</td><td>103.55</td><td>30</td><td></td><td>3.45</td><td>5.333</td><td>8.78</td></tr></tbody></table>
<table><thead><tr><th colspan="14">ZEELE1</th></tr><tr><th>ID of the seismic profile</th><th>Location of the measurement point in the seismic profile (N° CDP)</th><th>TWT (s) of the Top Oligocene</th><th>TWT (s) of Zelee1</th><th>Difference (s TWT) between Zelee1 and the Top Oligocene</th><th>ID of the seismic profile used to determine the interval velocities</th><th>Interval velocity used for the conversion time to depth (m/s twt)</th><th>Thickness (m) between the volcanic base of Zelee1 and the Top Oligocene</th><th>Sedimentation rate (m/Ma) used to calculate ages</th><th>Difference (in age) between the volcanic reflector of Zelee1 and the Top Oligocene</th><th>Age of the Top Oligocene (horizon reference) (Ma)</th><th>Age of the volcanic base of Zelee1 (Ma)</th><th>Average age of the onset of the volcanism of Zelee1 (Ma)</th></tr></thead><tbody><tr><td>MAOR21R012</td><td>4000</td><td>5.282</td><td>5.547</td><td>0.265</td><td>MAOR21D01</td><td>2000</td><td>265</td><td>30</td><td>8.83</td><td>23.03</td><td>31.86</td><td rowspan="4">32</td></tr><tr><td>MAOR21R013</td><td>10000</td><td>5.29</td><td>5.544</td><td>0.254</td><td>MAOR21D01</td><td>2000</td><td>254</td><td>30</td><td>8.47</td><td>23.03</td><td>31.50</td></tr><tr><td>MAOR21R010</td><td>12500</td><td>5.244</td><td>5.459</td><td>0.215</td><td>MAOR21D01</td><td>2000</td><td>215</td><td>30</td><td>7.17</td><td>23.03</td><td>30.20</td></tr><tr><td>MAOR21R010</td><td>8000</td><td>5.321</td><td>5.634</td><td>0.313</td><td>MAOR21D01</td><td>2000</td><td>313</td><td>30</td><td>10.43</td><td>23.03</td><td>33.46</td></tr><tr><td>MAOR21R010</td><td>8500</td><td>5.402</td><td>5.638</td><td>0.236</td><td>MAOR21D01</td><td>2000</td><td>236</td><td>30</td><td>7.87</td><td>23.03</td><td>30.90</td></tr></tbody></table><table><thead><tr><th colspan="14">ZEELE2</th></tr><tr><th>ID of the seismic profile</th><th>Location of the measurement point in the seismic profile (N° CDP)</th><th>TWT (s) of the Top Oligocene</th><th>TWT (s) of Zelee2</th><th>Difference (s TWT) between Zelee2 and the Top Oligocene</th><th>ID of the seismic profile used to determine the interval velocities</th><th>Interval velocity used for the conversion time to depth (m/s twt)</th><th>Thickness (m) between the volcanic base of Zelee2 and the Top Oligocene</th><th>Sedimentation rate (m/Ma) used to calculate ages</th><th>Difference (in age) between the volcanic reflector of Zelee2 and the Top Oligocene</th><th>Age of the Top Oligocene (horizon reference) (Ma)</th><th>Age of the volcanic base of Zelee2</th><th>Average age of the onset of the volcanism of Zelee2 (Ma)</th></tr></thead><tbody><tr><td>MAOR21R010</td><td>10400</td><td>5.275</td><td>5.193</td><td>0.082</td><td>MAOR21D01</td><td>2000</td><td>82</td><td>30</td><td>2.73</td><td>23.03</td><td>20.30</td><td rowspan="2">16</td></tr><tr><td>MAOR21R010</td><td>10400</td><td>4.981</td><td>5.193</td><td>0.212</td><td>MAOR21D01</td><td>2000</td><td>212</td><td>30</td><td>7.07</td><td>5.333</td><td>12.40</td></tr></tbody></table><table><thead><tr><th colspan="14">JUMELLES</th></tr><tr><th>ID of the seismic profile</th><th>Location of the measurement point in the seismic profile (N° CDP)</th><th>TWT (s) of the Top Miocene</th><th>TWT (s) of Jum</th><th>Difference (s TWT) between Jum and the Top Miocene</th><th>ID of the seismic profile used to determine the interval velocities</th><th>Interval velocity used for the conversion time to depth (m/s twt)</th><th>Thickness (m) between the volcanic base of Jum and the Top Miocene</th><th>Sedimentation rate (m/Ma) used to calculate ages</th><th>Difference (in age) between the volcanic reflector of Jum and the Top Miocene</th><th>Age of the Top Miocene (horizon reference) (Ma)</th><th>Age of the volcanic base of Jum</th><th>Average age of the onset of the volcanism of Jum (Ma)</th></tr></thead><tbody><tr><td>MAOR21R014</td><td>2000</td><td>4.844</td><td>4.799</td><td>0.045</td><td>MAOR21D02</td><td>1800</td><td>40.5</td><td>30</td><td>1.35</td><td>5.333</td><td>3.98</td><td rowspan="3">4</td></tr><tr><td>MAOR21R015</td><td>500</td><td>4.816</td><td>4.771</td><td>0.045</td><td>MAOR21D02</td><td>1800</td><td>40.5</td><td>30</td><td>1.35</td><td>5.333</td><td>3.98</td></tr><tr><td>MAOR21R020</td><td>4000</td><td>4.808</td><td>4.756</td><td>0.052</td><td>MAOR21D02</td><td>1800</td><td>46.8</td><td>30</td><td>1.56</td><td>5.333</td><td>3.77</td></tr></tbody></table><table><tbody><tr><td>MAOR21R125</td><td>1000</td><td>4.81</td><td>4.773</td><td>0.037</td><td>MAOR21D02</td><td>1800</td><td>33.3</td><td>30</td><td>1.11</td><td>5.333</td><td>4.22</td><td></td></tr></tbody></table><table><thead><tr><th colspan="14">MAY4</th></tr><tr><th>ID of the seismic profile</th><th>Location of the measurement point in the seismic profile (N° CDP)</th><th>TWT (s) of the Top Miocene</th><th>TWT (s) of May4</th><th>Difference (s TWT) between May4 and the Top Miocene</th><th>ID of the seismic profile used to determine the interval velocities</th><th>Interval velocity used for the conversion time to depth (m/s twt)</th><th>Thickness (m) between the volcanic base of May4 and the Top Miocene</th><th>Sedimentation rate (m/Ma) used to calculate ages</th><th>Difference (in age) between the volcanic reflector of May4 and the Top Miocene</th><th>Age of the Top Miocene (horizon reference) (Ma)</th><th>Age of the volcanic base of May4</th><th>Average age of the onset of the volcanism of May4 (Ma)</th></tr></thead><tbody><tr><td>MAOR21D02</td><td>500</td><td>4.701</td><td>4.796</td><td>0.095</td><td>MAOR21D02</td><td>2000</td><td>95</td><td>30</td><td>3.17</td><td>5.333</td><td>8.50</td><td rowspan="4">8</td></tr><tr><td>MAOR21R045</td><td>5000</td><td>4.771</td><td>4.848</td><td>0.077</td><td>MAOR21D02</td><td>2000</td><td>77</td><td>30</td><td>2.57</td><td>5.333</td><td>7.90</td></tr><tr><td>MAOR21R125</td><td>500</td><td>4.761</td><td>4.837</td><td>0.076</td><td>MAOR21D02</td><td>2000</td><td>76</td><td>30</td><td>2.53</td><td>5.333</td><td>7.87</td></tr><tr><td>MAOR21R043</td><td>4000</td><td>4.765</td><td>4.831</td><td>0.066</td><td>MAOR21D02</td><td>2000</td><td>66</td><td>30</td><td>2.20</td><td>5.333</td><td>7.53</td></tr></tbody></table><table><thead><tr><th colspan="14">ANJOUAN</th></tr><tr><th>ID of the seismic profile</th><th>Location of the measurement point in the seismic profile (N° CDP)</th><th>TWT (s) of the Top Miocene</th><th>TWT (s) of Anj</th><th>Difference (s TWT) between the volcanic base of Anjouan and the Top Miocene</th><th>ID of the seismic profile used to determine the interval velocities</th><th>Interval velocity used for the conversion time to depth (m/s twt)</th><th>Thickness (m) between the volcanic base of Anjouan and the Top Miocene</th><th>Sedimentation rate (m/Ma) used to calculate ages</th><th>Difference (in age) between the volcanic reflector of Anj and the Top Miocene</th><th>Age of the Top Miocene (horizon reference) (Ma)</th><th>Age of the volcanic base of Anj</th><th>Average age of the onset of the volcanism of Anj (Ma)</th></tr></thead><tbody><tr><td>MAOR21R051</td><td>4800</td><td>4.734</td><td>4.675</td><td>0.059</td><td>MAOR21D02</td><td>1800</td><td>53.1</td><td>30</td><td>1.77</td><td>5.333</td><td>3.56</td><td rowspan="4">4</td></tr><tr><td>MAOR21R049</td><td>200</td><td>4.717</td><td>4.681</td><td>0.036</td><td>MAOR21D02</td><td>1800</td><td>32.4</td><td>30</td><td>1.08</td><td>5.333</td><td>4.25</td></tr><tr><td>MAOR21R052</td><td>3000</td><td>4.721</td><td>4.697</td><td>0.024</td><td>MAOR21D02</td><td>1800</td><td>21.6</td><td>30</td><td>0.72</td><td>5.333</td><td>4.61</td></tr><tr><td>MAOR21R055</td><td>500</td><td>4.72</td><td>4.671</td><td>0.049</td><td>MAOR21D02</td><td>1800</td><td>44.1</td><td>30</td><td>1.47</td><td>5.333</td><td>3.86</td></tr></tbody></table><table><thead><tr><th colspan="14">MOHELI</th></tr><tr><th>ID of the seismic profile</th><th>Location of the measurement point in the seismic profile (N° CDP)</th><th>TWT (s) of the Top Miocene</th><th>TWT (s) of volcanic base of Moheli</th><th>Difference (s TWT) between the volcanic base of Moheli and the Top Miocene</th><th>ID of the seismic profile used to determine the interval velocities</th><th>Interval velocity used for the conversion time to depth (m/s twt)</th><th>Thickness (m) between the volcanic base of Moheli and the Top Miocene</th><th>Sedimentation rate (m/Ma) used to calculate ages</th><th>Difference (in age) between the volcanic reflector of Moheli and the Top Miocene</th><th>Age of the Top Miocene (horizon reference) (Ma)</th><th>Age of the base of the volcanic series of Moheli</th><th>Average age of the onset of Moheli (Ma)</th></tr></thead><tbody><tr><td>MAOR21R090</td><td>500</td><td>4.955</td><td>5.056</td><td>0.101</td><td>MAOR21D08</td><td>1900</td><td>95.95</td><td>30</td><td>3.20</td><td>5.333</td><td>8.53</td><td rowspan="6">9</td></tr><tr><td>MAOR21R089</td><td>1700</td><td>5.031</td><td>5.151</td><td>0.12</td><td>MAOR21D08</td><td>1900</td><td>114</td><td>30</td><td>3.80</td><td>5.333</td><td>9.13</td></tr><tr><td>MAOR21D08 (1)</td><td>3100</td><td>4.921</td><td>5.056</td><td>0.135</td><td>MAOR21D08</td><td>1900</td><td>128.25</td><td>30</td><td>4.27</td><td>5.333</td><td>9.61</td></tr><tr><td>MAOR21D08 (2)</td><td>5200</td><td>4.994</td><td>5.115</td><td>0.121</td><td>MAOR21D08</td><td>1900</td><td>114.95</td><td>30</td><td>3.83</td><td>5.333</td><td>9.16</td></tr><tr><td>MAOR21D08 (3)</td><td>7600</td><td>5.053</td><td>5.153</td><td>0.1</td><td>MAOR21D08</td><td>1900</td><td>95</td><td>30</td><td>3.17</td><td>5.333</td><td>8.50</td></tr><tr><td>MAOR21R088</td><td>4800</td><td>5.041</td><td>5.151</td><td>0.11</td><td>MAOR21D08</td><td>1900</td><td>104.5</td><td>30</td><td>3.48</td><td>5.333</td><td>8.82</td></tr></tbody></table><table><thead><tr><th colspan="14">GRANDE COMORE</th></tr><tr><th>ID of the seismic profile</th><th>Location of the measurement point in the seismic profile (N° CDP)</th><th>TWT (s) of the base of Moheli</th><th>TWT (s) of volcanic base of Grande Comore</th><th>Difference (s TWT) between the volcanic base of Grande Comore and the base of Moheli</th><th>ID of the seismic profile used to determine the interval velocities</th><th>Interval velocity used for the conversion time to depth (m/s twt)</th><th>Thickness (m) between the volcanic base of Grande Comore and the base of Moheli</th><th>Sedimentation rate (m/Ma) used to calculate ages</th><th>Difference (in age) between the volcanic reflector of Moheli and the Top Miocene</th><th>Age of the base of Moheli (horizon reference) (Ma)</th><th>Age of the base of the volcanic series of Grande Comore</th><th>Average age of the onset of Grande Comore (Ma)</th></tr></thead><tbody><tr><td>MAOR21R061</td><td>2000</td><td>4.981</td><td>4.74</td><td>0.241</td><td>MAOR21D08</td><td>1900</td><td>228.95</td><td>30</td><td>7.63</td><td>9</td><td>1.37</td><td>1.5</td></tr></tbody></table>
CHAPITRE V. Héritage structural et domaine
crustal du bassin des Comores
**Introduction**

Contexte de l'article (soumis) pour le journal Tectonophysics sur la nature, la structure de la croûte et de l'héritage structural du bassin des Comores, intitulé :

Intra-oceanic emplacement of the Comoros Archipelago through inherited fracture zones

L'acquisition de la sismique réflexion profonde (sismique réflexion 960 traces, campagne SISMAORE) et de déploiement d'OBS, nous permet d'imager pour la première fois le domaine crustal à l'Est de Mayotte. Couplé à un autre jeu de données de sismique réflexion obtenu grâce à la participation de Total Énergies et de l'entreprise d'acquisition de données géophysique TGS, nous avons caractérisé le domaine crustal au Sud et au Nord de l'archipel des Comores, dans le but de connaitre la nature de la croûte, sa structure, et de comprendre la part de l'héritage structural sur la mise en place des structures volcaniques de l'archipel des Comores.
Intra-oceanic emplacement of the Comoros Archipelago through
inherited fracture zones

Charles Masquelet*1, Louise Watremez2, Sylvie Leroy1, Daniel Sauter3,
Matthias Delescluse4, Nicolas Chamot-Rooke4, Jean Claude Ringenbach5, Isabelle
Thinon6 and Anne Lemoine6 and Dieter Franke7

1 Sorbonne Université, CNRS, Institut des Sciences de la Terre de Paris (ISTeP),
Paris, France

2 Université de Lille, CNRS, Université Littoral Côte d'Opale, IRD, UMR 8187 –
LOG – Laboratoire d'Océanologie et de Géosciences, Lille, France

3 Institut Terre et Environnement de Strasbourg (ITES), CNRS Université de
Strasbourg, Strasbourg, France

4 Laboratoire de Géologie de l'École Normale Supérieure (ENS), CNRS, PSL
Université, Paris, France

5 TotalEnergies SE, CSTJF, Pau, France

6 Bureau de Recherches Géologiques et Minières (BRGM), Orléans, France

7 Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Hannover,
Deutschland

Sorbonne Université, CNRS, Institut des Sciences de la Terre de Paris (ISTeP), 4
place Jussieu, Paris, France

<footnote>*Corresponding author: charles.masquelet@sorbonne-universite.fr</footnote>
**Abstract**

We shed light on the nature and structure of the crust surrounding the Comoros Archipelago, western Indian Ocean, offering insights into the region's geological history and volcanic island formation. Our comprehensive study encompasses the acquisition of new, deeply penetrating seismic data from the Sismaore cruise (refraction and reflection seismic), and the subsequent analysis of the characteristics and structure of the crust surrounding the Comoros Archipelago. Both reflection seismic imaging and the velocity structure using ocean bottom seismometers indicate that the Comoros Basin's crust exhibits oceanic characteristics, thus resolving previous controversies about its nature. The thickness of the oceanic crust ranges from 5.8-6.6 km to the north of the archipelago to 6-7.2 km within the Comoros Basin. The estimated roughness of the top basement in the Comoros Basin ranges from 110 to 200 m values typical of intermediate spreading ridges, such as the extinct spreading centre in the West Somali Basin.

The unloading depth of the basement of the Comoros Basin closely matches the expected water-loaded subsidence for an oceanic lithosphere of the Cretaceous or Jurassic age. In contrast, the West Somali Basin to the north of the Comoros Archipelago displays shallower basement depths, potentially linked to recent volcanic activity along the archipelago.

We propose that the pre-existing oceanic fracture zones in the West Somali Basin underwent reactivation, first during the Turonian period and later during the Late Eocene. These reactivated fracture zones are proposed to have acted as mechanically weak regions, influencing the emplacement of the volcanic islands of the Comoros Archipelago. Furthermore, the alignment of the changes of direction of these reactivated fracture zones may create weakened areas that likely played a crucial role in facilitating the emplacement of the Comoros Archipelago.
# 1. Introduction

The origin of the Comoros Archipelago (Fig. 1), in the northern Mozambique Channel, has been the subject of lively debate for some fifty years. It has long been interpreted as due to the impingement of a mantle plume under a moving oceanic lithosphere (Emerick and Duncan, 1982; Hajash and Armstrong, 1972). While hotspot volcanism may be supported by the enriched mantle signature of the lavas on Grande Comore Island (Class et al., 2009), the general E-W trend of the Comoros Archipelago does not match the northward absolute motion of the Somalia plate (Morgan and Morgan, 2007).

In May 2018, a major phase of seismo-volcanic activity gave birth to the large – 820 m high and 5 km in diameter - submarine Fani Maoré volcano, sitting at approximately 3500 m water depth, east of Mayotte Island (Feuillet et al., 2021; Lemoine et al., 2020; Masquelet et al., 2022; Thinon et al., 2022). The present-day active volcanism in both, Grande Comore Island and to the east of Mayotte Island together with widespread Holocene volcanic activity on Anjouan, Mohéli, and Mayotte Islands (Quidelleur et al., 2022; Rusquet et al., 2023) does not support a simple model of punctual and vertically rising mantle plume material piercing a moving lithosphere.

The recent seismo-volcanic crisis east of Mayotte Island together with previous sparse focal mechanisms along the Comoros Archipelago suggest mainly strike-slip faulting with a NE-SW normal component (Feuillet et al., 2021; Lemoine et al., 2020). This seismicity has been interpreted as resulting from a nascent plate boundary between the Somalia and Lwandle plates connecting the East African Rift System (EARS) to the Tertiary-to-Quaternary volcanism and rifting of Madagascar (e.g. (Famin et al., 2020; Feuillet et al., 2021; Thinon et al., 2022). However, there is no consensus over the offshore extent of the EARS. Deformation, identified onshore and on the seafloor, is thought to be either focused within a 150-200 km-wide right-lateral shear zone along the Comoros Archipelago (Famin et al., 2020; Feuillet et al., 2021; Thinon et al., 2022) or distributed within a ~600-km-wide zone of deformation positioned between the Rovuma microplate and the Somalian plate that extends across large swaths of Madagascar and the Comoros islands (Stamps et al., 2021).

Although crucial to understanding the origin of the archipelago, the nature of the crust beneath the Comoros volcanic islands and in the Comoros Basin, between the archipelago and Madagascar, is poorly known.
<figure><img src="image_73.png" /></figure>

**Figure 1:** Map of the study area offshore the Comoros archipelago (A). The orange lines indicate the 48-channel seismic reflection profiles and the green lines show the 960-channel seismic reflection profiles collected during the SISMAORE cruise (Thinon et al. 2020). Black lines indicate the TGS lines available for the study. The green triangles indicate the location of the OBSs (ocean bottom seismometers) of the SISMAORE cruise. N'Doundé and Mwezi volcanic provinces are shown in yellow. The blue lines correspond to the fracture zones from Davis et al. (2016). The green and orange dashed lines correspond to the COB (Continent-Ocean Boundary) of Roche and Ringenbach (2022) and Sinha et al. (2019), respectively. The black dashed line corresponds to the Davie Ridge. Thick red lines indicate the part of the profiles shown in the following figures of this paper. The background image is the satellite-derived free air gravity anomaly grid of Sandwell et al. (2014); (B) Location of the study area in the western Indian Ocean. The dashed green lines indicate the inferred plate boundaries after Michon et al. (2022); (C) Location of the OBSs of the SISMAORE cruise (green triangles) and the sonobuoys SB08 of Lort et al. (1979) and V449 of Coffin et al. (1986) (blue points). The numbering of the seismic reflection profiles (M01, M08 etc...) is also indicated. The background bathymetric grid is a compilation of several cruises after Masquelet et al. (submitted).
Fundamentally differing interpretations have been issued. Oceanic crust, continental crust and a wide continent-ocean transition were proposed in the Comoros Basin e.g. (Coffin et al., 1986; Lort et al., 1979; Nougier et al., 1986). In this paper, we first briefly review these interpretations, then present our newly collected seismic data set, show our results about the oceanic nature and structure of the crust around the Comoros Archipelago and finally discuss the implications of our findings for the emplacement of the magma forming the Comoros volcanic islands. We show that the location of the islands is controlled by the pre-existing segmentation of the oceanic crust of the West Somali basin.

## 2. Geological Background

North of the Comoros Archipelago, magnetic anomalies related to the spreading ridge of the West Somali Basin argue for a typical oceanic lithosphere e.g. (Cochran, 1988; Davis et al., 2016; Sauter et al., 2016; Ségoufin and Patriat, 1980). There, magnetic anomalies, from M24 (~152 Ma, according to the time scale of Ogg, (2020)) to M0 (~121 Ma) indicate slow to intermediate spreading rates (~30 to ~60 km/Ma full rates) up to Aptian times when spreading ceased in the West Somali Basin (Davis et al., 2016). The nature of the crust is less clear to the south of the archipelago. Rabinowitz et al. (1983) interpreted magnetic anomalies up to the M25 magnetic anomaly (~153 Ma) while Davis et al. (2016) and Cochran (1988) could not correlate any significant magnetic anomalies on a few magnetic profiles. From an early sonobuoy experiment using only two instruments, Lort et al. (1979) speculated that the Comoros Archipelago marks the boundary between oceanic crust to the north and continental crust to the south. Later, with over a hundred sonobuoy records Coffin et al. (1986) showed that both the West Somali and Comoros Basins are oceanic in character. However, at the same time, the observation of sandstone xenoliths found in lavas on the Comoros Islands (Flower and Strong, 1969) and the presence of a km-scale quartzite massif on the Anjouan Island have been used to suggest that the archipelago may be built on stretched continental crust (Montaggioni and Nougier, 1981). It was further proposed that magmatism along the Comoros Archipelago is controlled by deep lithospheric faults, linked to an inferred wide continent-ocean transition (Nougier et al., 1986; Tzevahirtzian et al., 2021). Based on a single receiver function analysis on Mayotte Island, Dofal et al. (2021) proposed a continental nature
of the crust beneath Mayotte. However, from a combined analysis of receiver functions and the joint inversion of receiver function and Rayleigh wave dispersion data, Dofal et al. (2023) could not discriminate the oceanic or continental nature of the crust beneath Mayotte Island. Recently, Roche and Ringenbach (2022) used seismic reflection profiles to show that oceanic crust is present at 13.5°S, to the southwest of Moheli Island. Wide-angle seismic data also showed the presence of oceanic crust on the eastern flank of the Davie Ridge, at 14.5°S (Vormann et al., 2020; Vormann and Jokat, 2021).

The Comoros Basin remains very little explored between the archipelago and the Madagascar margin, which may explain the debates regarding the origin of the Comoros Archipelago. To fill this gap, a regional-scale acquisition of geophysical and geological data was conducted off the Comoros Archipelago during the SISMAORE oceanographic cruise aboard “R/V Pourquoi pas?” in 2021 (Fig. 1), (Thinon et al., 2022, 2020). We present the collected seismic data set in the next section.

### 3. Data acquisition and processing

During the SISMAORE cruise 1480 km of deep multichannel seismic reflection data (960 channels) and 6800 km of lower penetration seismic reflection profiles (48 channels) were acquired. In addition, three OBS (Ocean-Bottom Seismometers) recorded the seismic shots along the multichannel line M01 high-resolution profile, located East of Mayotte (Fig. 1C). Published seismic reflection profiles within the Comoros Basin complete this new data set (Fig. 1). We also incorporated the seismic reflection profiles of the R/V Sonne SO-231 cruise (Franke et al., 2015; Klimke et al., 2016) and of the R/V L'Atalante PTOLEMEE cruise (Jorry, 2014; Leroux et al., 2020). We also used seismic multichannel lines from ION Geophysical™ partly published by Leroux et al. (2020) Sinha et al. (2019), and Roche and Ringenbach (2022) on the eastern flank of the Davie Ridge and in the Comoros Basin.

#### 3.1. Long-streamer 960-channel seismic reflection profiles

We collected a set of 12 multichannel seismic (MCS) profiles (6000 m long streamer with 960 channels spaced every 6.25 m). The seismic source comprised 16 airguns triggered every 40 s with a total volume of ~82 L (4990 cu. in.). The record length was 20 s with a sampling rate of 2 ms. The distance between each common
depth point (CDP) is 3.25 m. Super gather CDPs were built by merging four CDPs in
a bin of 12.5 m during processing, allowing for a fold of 60. Processing involved a
typical workflow, utilizing Geovation® software (developed by CGG) in the pre-stack
domain (including trace editing, amplitude correction, normal move-out correction, FK-
filtering, multiple attenuation, and predictive deconvolution). Masquelet et al. (2022)
and Masquelet et al. (submitted) detailed the seismic processing of this data set.

**3.2. Short-streamer 48-channel seismic reflection profiles**

A series of 110 48-channel seismic reflection profiles was collected using a 600 m-
long streamer (channels spaced every 6.25 m). The source comprised two airguns
with a total volume of 4.91 L (300 cu. In) shooting every 50 m (record length of 9 s and
inter CDP is 3.125 m). Geovation® software was used for standard processing
including 12.5 m binning (6-fold by stacking 2 adjoining traces), NMO correction, stack
and a Kirchhoff migration at 1500 m/s.

**3.3. OBS data set**

A set of three OBSs recorded the shots during the acquisition of the 960 channel
seismic reflection profiles (Fig. 1C). These three instruments were deployed along the
~290 km long M01 line, located East of Mayotte with a SSW-NNE orientation and
crossing the new Fani Maoré volcano (Masquelet et al., 2022). The OBSs also
recorded shots along crossing lines, allowing for the estimation of seismic velocities
along different directions.

After clock-drift correction, the instruments were relocated using picks of the direct
wave from the shots within 10 km of their deployment location, the bathymetry grid and
averaged seismic velocities in the water column (1.505 km/s). The OBS records,
organized as receiver gathers, allowed for the picking of arrival times of several seismic
phases in the different geological layers down to the uppermost mantle (Appendices A
and B). We used a forward modelling program, Rayinvr (Zelt and Smith, 1992)
following a top-down layer stripping strategy to create velocity models reproducing the
picked data within their uncertainty range (Appendix C).
<figure><img src="image_74.png" /><figcaption>**Figure 2:** Results of the OBS data analysis. A: Velocity model along the M01 profile. B: Seismic reflection data overlain on the time-converted velocity model. C: Vertical velocity profiles beneath each OBS for each crossing shot line.</figcaption></figure>

We modelled 2 or 3 lines for each instrument. Here, we present the velocity model along M01, which crosses all 3 instruments, although each instrument can be considered as an independent ~1D model since no seismic ray crosses the entire model (Fig. 2A). This issue implies that the obtained velocities and thicknesses maybe
not so well constrained, but this is counterbalanced by the use of strong constraints
from the coincident high-resolution seismic reflection data. The initial velocity model
was built using the picks on the main geological interfaces, where available, and a first
estimation of (1) interval velocities in the sediments along these seismic reflection
profiles and (2) apparent velocities of corresponding refracted arrivals on the OBS
records used to convert these interfaces from two-way travel-time to depth. Rayinvr
modelling allows us to compute synthetic arrival times from ray-tracing through the
model, which we updated to reduce the misfit between computed and picked arrivals
(Appendix C). The final model is converted to two-way travel time to allow for a
comparison with coincident seismic reflection data (Fig. 2B). The vertical velocity
profiles (Fig. 2C) provide an assessment of the robustness of the velocity models. It is
noteworthy that a vintage sonobuoy profile (Lort et al., 1979) located only 7 km away
from the OBS3 shows comparable velocities in the sediment layers but was limited
down to the top of the crust (Fig. 2C).

**4. Method**

**4.1. Sedimentary sequence**

We use the seismic stratigraphy of Masquelet et al. (submitted), which is based on
the relatively well-known stratigraphy of the northern Mozambique Channel and the
West Somali Basin (Coffin and Rabinowitz, 1992; Franke et al., 2015; Mahanjane,
2014; Mougenot et al., 1986; Roche and Ringenbach, 2022). Using IODP Site 1476
and DSDP Site 242 (Hall et al., 2017; Simpson et al., 1974), located on the Davie
Ridge at the northern entrance of the Mozambique Channel and seismic reflection
profiles that intersect these sites, Masquelet et al. (submitted) identified the top
Miocene horizon and the top Oligocene horizon (TM and TO in Fig. 3) that were
extended towards the Comoros Archipelago, 500 km away. They also identified the
Cretaceous-Paleogene unconformity (K/Pg, Fig. 3), which is a distinctive event in the
seismic profiles across the West Somali Basin and offshore Majunga Basin (Franke et
al., 2015; Leroux et al., 2020; Mahanjane, 2014). They also interpreted the package of
high amplitude and low-frequency discontinuous reflectors located below the K/Pg
reflector, as the Turonian volcanism previously observed in the Diego and Majunga
basins offshore Madagascar (Coffin and Rabinowitz, 1992; Leroux et al., 2020).
<figure><img src="image_75.png" /></figure>

**Figure 3:** Seismic stratigraphy of the Comoros Basin from Masquelet et al. (submitted). The seismic character of interpreted key horizons is illustrated by a 960-channel seismic reflection profile; The Top Miocene (TM; 5.3 Ma), Top Oligocene (TO; 23.5 Ma), Cretaceous-Paleogene unconformity (K/Pg; 66 Ma), Turonian volcanism (TV; 89-93 Ma), and the top of the crust were identified in the Comoros basin and compared to the seismic stratigraphy of Franke et al. (2015) and Mougenot et al. (1986): BC/A0: Cretaceous/Tertiary boundary; TO/A1: Top Oligocene; LM/A3: Late Miocene; A4: Pliocene.

## 4.2. Internal seismic structure of the crust

Internal reflectivity of the crust imaged by seismic reflection shows a typical oceanic crust layering as defined during the 1972 Penrose Conference (Fig. 4) (Bécel et al., 2015; Mutter and Carton, 2013; Sauter et al., 2021). It includes, from top to bottom, the following successive seismic facies: (1) a reflective top of basement with some short reflections in the shallowest part of the crust, (2) a transparent unit in the upper crust, (3) a more reflective lower crust with dipping structures and (4) a set of horizontal reflectors typically interpreted as the oceanic Moho although various geological structures at the base of the crust may generate high-amplitude reflections (e.g. Collins et al., (1986)).
<figure><img src="image_76.png" /><figcaption>**Figure 4:** M09 960-channel seismic reflection profile east of Mayotte (A), and interpreted profile (see location in Fig. 1). The black and violet lines correspond to the top of the crust and the Moho, respectively. TO: Top Oligocene. K/Pg: boundary Cretaceous/Paleogene, TV: Turonian volcanism. (B) Close-up view of part of the M09 profile location shown in (A). (C) and (D) show other examples of typical oceanic crust (see location of the profiles in Fig. 1).</figcaption></figure>

We identified the sea-bottom multiple of the profile during the processing and took care of attenuating it using a demultiplex routine that filtered out low-velocity reflectors (i.e. velocities that are typical for shallow sediment layers rather than velocities expected near the Moho discontinuity). In addition, the velocity model requires two velocity gradients: a moderate velocity gradient below the basement and a low-velocity
gradient above the Moho. We have therefore ensured that all the reflectors at the base of the crust do not come from a multiple of the seafloor. This interpretation is confirmed by the OBS analysis, where geological layers were imaged down to the Moho or upper-most mantle (Fig. 2).

# 5. Results

## 5.1. Nature of the crust in the Comoros Basin

Both seismic reflection and refraction (OBS) data show evidence of the presence of oceanic crust South and SE of Mayotte (Figs. 4 and 5). Seismic reflection data show oceanic Moho reflections that are roughly parallel to the top of the crust (at tens of kilometres scale, Fig. 4). The oceanic Moho is generally marked by a couple of strong and continuous reflectors (Fig. 4A&B) at about 9.5 s TWTT. However, an up to 0.25 s TWTT thick package of laterally discontinuous reflectors may also be observed locally (Fig. 4C). It can also be weakly marked or absent, especially toward the volcanic islands or submarine volcanic provinces of the Comoros Archipelago postdating the emplacement of the oceanic crust.

The results obtained from the OBS data corroborate the interpretation of the oceanic nature of the crust. Indeed, the velocity models obtained for the three OBSs show a clear 2-layer basement, with velocities that are typical for the oceanic crust (Fig. 5). The upper-most basement layer shows velocities ranging from 5.25 to 6.70 km/s and strong vertical velocity gradients (0.3-0.55 s⁻¹), which correspond to seismic velocities in basalts, i.e., the oceanic Layer 2. The lower crustal layer shows higher velocities (6.75-7.2 km/s) with lower velocity gradient (~0.1 s⁻¹), typical for the gabbroic oceanic Layer 3 (Fig. 5). More than 700 wide-angle reflected arrivals were picked at the interface between Oceanic Layer 2 and Layer 3, and almost 2,300 at the Moho, thanks to the fact that incidence angle is much wider than on seismic reflection data (near-critical reflections, see modelling statistics for phases 6.2 and 7.2, respectively, in Appendix C).
<figure><img src="image_77.png" /></figure>

**Figure 5:** Vertical velocity profiles in the basement and uppermost mantle for OBS3 (A), OBS2 (B) and OBS3 (C), compared with data from the literature for oceanic crust, extended continental crust, sub-continental exhumed mantle and oceanic exhumed mantle.

The nature of the crust was debated around the Comoros Archipelago, including the location of OBS3. Fig. 5A shows a comparison of the crustal velocities beneath OBS3 with values of the literature. Lort et al. (1979), speculated that the crust could be continental based on velocities down to the top basement, while our study provides information down to the upper-most mantle. It is now clear that we observe a typical oceanic layering (Fig. 5A; White et al. (1992)). Indeed, the continental-type velocities for the extended crust are much lower than in the crust under OBS3 (Fig. 5A; Christensen and Mooney (1995)). It has also been discussed whether the area between the Madagascar margin and the ocean might be of transitional nature (Nougier et al., 1986). A comparison with continental (Dean et al., 2000), and oceanic exhumed mantle zones (Van Avendonk et al., 2006)(Fig. 5A) shows that the lower crustal velocities under OBS3 are too slow to be consistent with a partially serpentinized mantle.

## 5.2. Thickness of the crust
We picked the top of the basement and the base of the crust to obtain the crustal thickness at places where the oceanic Moho is well marked, by a set of highly reflective reflectors (Fig. 4). We then converted this TWTT thickness to kilometres using a 6.56 km/s mean velocity, which corresponds to the average seismic velocities that we found for crust from our OBS analysis.

Appendix D gives examples of crustal thickness measurements to the north (Table D1) and south (Table D2) of the Comoros Archipelago. Crustal thicknesses range between 1.8 and 2.07 s TWTT (5.9-6.8 km) to the north of Grande Comore. The crust is somewhat thicker in the Comoros Basin ranging between 1.95 and 2.25 s TWTT (6.4-7.4 km) (see for example Fig. 3). This agrees with our results of the OBS analysis in the Comoros Basin showing a ~6.5-7 km thick crust below the OBS3 and OBS2.

However, the crust is much thinner (~5 km) below OBS1 at the north end of the profile. Velocity modelling shows that the thickness of oceanic Layer 2 is homogeneously ~2-2.5 km, while the thickness of Layer 3 varies from ~2.5 km below OBS1, to the North, to 4-5 km below OBS2 and OBS3, toward the South (Fig. 5).

## 5.3 Top of the oceanic crust

### 5.3.1 Small-scale oceanic crust roughness

The variation of oceanic basement roughness at mid-oceanic ridges is primarily related to the spreading rate that mainly controls the thermal state of the lithosphere (Goff et al., 1997; Sauter et al., 2018b). Estimating the roughness of the oceanic crust (i.e. the root mean square deviation of residual basement relief) may thus provide insight into the spreading processes within the West Somali and Comoros Basins. We selected three seismic reflection profiles in line with the inferred spreading direction to estimate the roughness of the oceanic crust. After removing the NS regional deepening of the top basement, we obtained roughness values ranging from 110 to 200 m for the abyssal hill bathymetry along the three profiles (Fig. 6). Such roughness values are typical of slow to intermediate spreading ridges (Goff et al., 1997; Sauter et al., 2018b) (Fig. 6).
<figure><img src="image_78.png" /></figure>

**Figure 6:** Roughness estimation for a NS profile in the Comoros Basin (A; see location in Fig. 1) and plot of the roughness versus spreading rate (B). Depth values were obtained by converting TWTT using the velocity profile of OBS3. The standard deviation of the detrended profile in A gives a 120 m roughness value. The roughness values for the artifices are from Weigelt and Jokat (2001) and Ehlers and Jokat (2009). Those for the southwest Indian ultraslow-spreading ridge are from Sloan et al. (2012). Those for the slow-spreading Mid-Atlantic Ridge are from Goff (1991); Goff et al. (1997); Minshull (1999); Neumann and Forsyth (1995). The roughness value for the slow-spreading SWIR (before 24 Ma) is from Sauter et al. (2011). The roughness values for the intermediate-spreading southeast Indian ridge and fast-spreading East Pacific Rise are from Goff et al. (1997). The dashed grey line is the power-law curve fit of Malinverno (1991). The horizontal grey box shows the range of roughness values determined in our study area.
### 5.3.2 Regional depth variation of the top oceanic crust

Mapping the depth of the top basement along each seismic reflection profile (Fig 7), we found that the top basement is much deeper in the Comoros Basin (7 km mean depth) than in the West Somali Basin to the north of the Archipelago (6 km mean depth). Using the sediment thickness derived from the seismic profiles and a tentative range for their mean density (2.0 to 2.2), the unloaded depth would be 5.4 km for the Comoros Basin and 5.0 km for the region north of the Archipelago (Fig. 8).

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

**Figure 7:** Depth of the top of the crust (in second two-way travel time) obtained along the seismic reflection profiles. The blue lines correspond to the fracture zones from Davis et al. (2016). The background image is the satellite-derived free-air gravity anomaly grid of Sandwell et al. (2014).

Despite the large uncertainties inherent to such calculations, the unloaded depth in the Comoros Basin is not far from the asymptotic water-loaded subsidence inferred by Stein and Stein in their GDH1 plate cooling model (5560 m; Stein and Stein (1992)) (Fig. 8). Subsidence in the Comoros Basin is thus compatible with a Cretaceous or Jurassic age for the oceanic lithosphere. The West Somali Basin, north of the Archipelago, appears to be several hundred meters shallower (Fig. 8). Seismic profiles image a rather sharp step in the topography of the top basement between 12.2°S and 12.5°S, along an EW trending line through the Comoros Archipelago joining Mohéli
Island and Zélée Geyser banks (Figs 7 and 8). This shallower basement extends over a broad region, suggesting abnormal heating at mantle depth, possibly related to the recent volcanism discovered in the Mwezi and N'Droundé provinces to the northeast of Anjouan and Grande Comore islands and all along the Comoros Archipelago (Rusquet et al., 2023; Thinon et al., 2022).

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

**Figure 8:** Depth of the top basement obtained along a NS seismic reflection located at 44°E longitude and passing between Anjouan and Mohéli islands (green dots) (Figs 1 and 7 for location). The black dots show the depths obtained using the GDH1 model of Stein and Stein (1992). Orange dots show the depth of the top basement relieved of the weight of the sediments. Ages are calculated using the distance from the extinct axis and an intermediate 25 km/Ma half spreading rate.

### 5.4 Compressional structures

Two phases of compressional deformation to the North of the Comoros Archipelago are observed: the first is of Turonian in age and the second is Late Eocene.

Figs 9 and 10 show an example of compressional deformation affecting the sedimentary cover North of Mayotte Island. Slight inverse faulting resulted in a small dome affecting mainly the early sedimentary layers while faults are hardly observed in the crust (Fig. 8). The dome is located above a basin within the crust and is sealed by post-Turonian layers. Other such basins are observed along profile M02 and correspond to elongated NNW-SSE trending gravity lows in the satellite-derived free-air gravity anomaly grid (Fig. 9). Although obscured by the thick sedimentary cover
<figure><img src="image_81.png" /><figcaption>**Figure 10:** Identification of reactivated fracture zones (Fz1, Fz2a, Fz2b) observed on three EW trending seismic reflection profiles (A) and location map with the fracture zones drawn in white dashed lines (B). The dipping reflectors (black lines) in the 48-channel seismic reflection profiles (MR05 and MR01) are interpreted as the top of the oceanic crust.</figcaption></figure>
these gravity lineaments mark the fracture zones of the extinct seafloor spreading
centre within the West Somali Basin (Davis et al., 2016). More severe compressional
features are found to the north of Grande Comore. Fig. 11 shows two profiles across
such a compressional structure. The main episode of deformation is clearly marked by
a dense network of inverse faults both in the sedimentary cover and in the crust.
Oligocene sedimentary layers seal these structures. The amplitude of the uplifted
areas reaches 0.3-0.4 s TWTT (300-400 m with a 2 km/s interval velocity). The EW
trending profile shows a set of strong reflectors occurring ~1 s below the top basement
that is located beneath the uplifted area and a former small basin within the crust.
These reflectors connect the oceanic Moho both to the East and West of the deformed
area. We thus interpret them as corresponding to the deformed oceanic Moho.

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

**Figure 11:** Sections of a EW trending seismic reflection profile (A) and a NS trending seismic reflection profile (B) north of the Grande Comore Island (see location in Fig. 1). TO: Top Oligocene, TV: Turonian Volcanism. The green horizon shows the last sedimentary layer affected by the compressional deformation. The thick black and the violet lines correspond to the top of the oceanic crust and the Moho, respectively. Thin black lines indicate faults.

The crust would then be thinner below the uplifted area than ~20 km away to the east
or west where the crustal thickness is normal (Fig. 11 A). Such thinner crust is typically
observed at transform faults at the end of oceanic ridge segments. We note that both
the top basement and Moho are shallower to the east of the deformed area than to the
west which could indicate an oceanic lithospheric compartment of younger age to the
east indicating that the profile is on either side of a fracture zone. Moreover, a NS
fracture zone has been mapped at the edge of the deformed area in the satellite-
derived free-air gravity anomaly grid (Davis et al., 2016)(Figs 1 and 10). Therefore, we
suggest that together, the former basin within a thinner crust separating a deeper
segment from a shallower one, indicates a fracture zone, inherited from the paleo-
spreading centre of the West Somali Basin, that has been reactivated. As some small
faults reach the seafloor, we further propose that such reactivation may have continued
until recent times. We note that the Turonian phase is also observed in the easternmost
part of this EW profile with a small doming of the early sediments (Fig.11A). Similar
uplifts and faulting geometry affecting the early sediments have been described to the
southwest of Mohéli Island (Klimke et al., 2016). In the NS trending profile, the
compressive structure shows (Fig. 11 B) a set of strong reflectors within the crust
above the oceanic Moho and also locally within the uplifted area. We interpret these
reflectors as indicating magma intrusions along faults typical of the lower oceanic crust
(Sauter et al., 2021) and possibly up to the seafloor. As magma is unlikely to be present
120 Ma after the cessation of the spreading, these intrusions may be rather related to
the magmatic province around the Comoros Archipelago which would then extend
200 km more to the north than previously mapped (Thinon et al., 2022; Tzevahirtzian
et al., 2021).

**6. Discussion**

**6.1. Nature of the Comoros Basin**
We found evidence for oceanic crust in the Comoros Basin, in agreement with early studies (Coffin et al., 1986), and have thus resolved past controversies about the nature of the crust around the Comoros Archipelago.

The Moho seismic reflection is weak in some parts of the study area. This is not surprizing as there are many places, away from the axis of mid-oceanic ridges, where the crust-mantle transition shows no distinguishable reflection (Mutter and Carton, 2013). Either the Moho interface is quite variable in structure, resulting in a gradual transition zone broad enough that it does not generate a vertical incidence response, and/or the scattering and absorbing properties in the crustal section lead to variable imaging conditions at the Moho level (Mutter and Carton, 2013). In our survey area, the volcanic overprinting enhances the variability of both the sedimentary cover and the crustal structure (Masquelet et al., 2022). Locally absent Moho reflections were also noted in the northern part of the West Somali Basin where the variable seismic signature of the base of the crust was attributed to faulting, deformation and/or intrusions (Sauter et al., 2018a; Sinha et al., 2019). Furthermore, no evidence for a clear oceanic Moho reflection has been reported by Klimke et al., (2016), east of the Davie Ridge where oceanic crust is inferred from wide-angle data (Vormann et al., 2020; Vormann and Jokat, 2021).

Because our data set does not include profiles right across the islands, we cannot rule out the possibility of some isolated continental blocks below the Comoros Islands as it was hypothesized beneath some volcanic islands surrounded by oceanic crust like Iceland or Mauritius (Foulger et al., 2020). Our data set further shows that the recent Fani Maoré volcano, which crowns the Mayotte volcanic edifice 50 km to the east of Mayotte Island, does not sit directly on the top of the basement but on a >2 km thick sedimentary unit above the oceanic crust (Masquelet et al., 2022). We conclude that, if present below the islands of the archipelago, such continental blocks would be of very small extent (<30-40 km) which seems unlikely when compared to typical microcontinents such as the undisputed examples of Jan Mayen and the Seychelles (Péron-Pinvidic and Manatschal, 2010).

The obtained 6-7 km crustal thicknesses in the Comoros Basin are in agreement with the 6.72 km average thickness for oceanic crust >125 Ma old (Christeson et al., 2019).
<figure><img src="image_83.png" /><figcaption>**Figure 12:** Schematic structural map of the West Somali and Comoros Basins. The black arrows show the paleo-spreading direction. The thin red line corresponds to the location of the extinct spreading ridge. The black dashed line corresponds to the location of Davie Ridge. The blue lines are the fracture zones identified by Davis et al., (2016). The dashed green lines indicate the extent of the volcanic zones around each island (Masquelet et al., submitted). The dashed red lines delimit the area where the fracture zones show a distinct change of direction. The stars indicate the location of the reactivated fractures zones shown in Figs 9-11. The background image is the satellite-derived free-air gravity anomaly grid of Sandwell et al. (2014).</figcaption></figure>

Similar crustal thicknesses were obtained both from seismic reflection profiles and wide-angle seismic data on the eastern flank of the Davie Ridge between 13.5 and
14.5°S (Roche and Ringenbach, 2022; Vormann et al., 2020; Vormann and Jokat, 2021). This range of thickness together with the estimated roughness of the top basement (110-200 m) is typical for spreading ridges with a relatively high magmatic budget and minor contribution of tectonism (Goff et al., 1997). This also agrees with the intermediate spreading rate indicated by the magnetic anomalies for the extinct spreading centre of the West Somali Basin (Davis et al., 2016).

Locally the crust may be much thinner (e.g. ~5 km below OBS1). The velocity models show that while oceanic layer 2 does not change along profile M01, Layer 3 is ~2 km thinner below OBS1. Because a thinner layer 3 is often observed at segment ends of slow to intermediate spreading ridges e.g. (Minshull et al., 2006), this thickness variation of layer 3 may indicate the presence of a fracture zone to the northern end of the M01 profile, beneath OBS1. We note that the top of the crust along the M01 profile is shallowing northward with two topographic steps between OBS3 and OBS2 and below OBS1 separating sections at more or less constant depth (Fig. 2). We interpret this stepwise depth variation as indicating successive sections of oceanic lithosphere of different age and thus the presence of fracture zones that offset segments of the extinct spreading ridge of the West Somali Basin. Moreover, a basin is well imaged beneath OBS1 on profile M02 that also corresponds to a gravity lineament revealing the paleo segmentation of the extinct spreading centre (Figs. 9 and 10). Some of these fracture zones were reactivated locally.

## 6.2. Pre-existing structures in the Comoros and West Somali Basins and emplacement of the Comoros Archipelago

Both reactivation of NS tectonic features and volcanic activity occurred along the Davie Ridge in the Late Cretaceous after the cessation of the southward drift of Madagascar (Bassias and Leclaire, 1990; Mascle et al., 1987). The reactivation of former extensional faults is also recorded for the on- and offshore Majunga Basin during the Late Cretaceous (Razafindrazaka et al., 1999). Transpressional events have also been evidenced during the end of the Cretaceous until the Paleogene along other NS trending major fracture zones close to the East African margin such as the Seagap (offshore central Tanzania) and Walu faults (i.e. the northern extension of the Davie Ridge (Roche and Ringenbach, 2022). Here we show that such a reactivated fracture zone is also observed, in Late Eocene times, to the north of the Comoros Archipelago, away from these main transform faults. Several islands of the
archipelago, such as Grande Comore, Anjouan and Mayotte, show an elongated shape, several rift zones, numerous dikes and alignments of offshore volcanoes with a main NS to NNW-SSE orientation, together with some minor orientations (Famin et al., 2020) (Fig. 12). In the same way, the recently discovered N'droundé Ridge province (see Fig. 1 for location) and Domoni ridge between Grande Comore and Mohélie islands show the same ~NS orientation (Thinon et al., 2022). (Famin et al., 2020) showed that the maximum horizontal stresses of the paleostress tensors obtained from fault analysis at Mayotte, Anjouan and Mohéli islands are consistently oriented NNW-SSE.

We also note that several post-seafloor-spreading volcanic edifices of Late Cretaceous age were found aligned with fracture zones of the extinct spreading centre both in the northern part of the West Somali Basin (Cruciani and Barchi, 2016) and on the eastern flank of the Davie Ridge (Klimke et al., 2016). It has also been suggested that deep lithospheric melt was channelized along shear zones up to shallow deformation corridors to feed volcanic edifices along pre-existing oceanic fracture zones of the West Somali Basin (Sauter et al., 2018a). We, therefore, propose that the emplacement of the volcanic edifices of the Comoros Archipelago is partly controlled by mechanically weak zones along pre-existing fracture zones of the oceanic crust.

Several analyses of the satellite-derived gravity data have resulted in maps of gravity lineaments related to fracture zones, which all show a change in spreading direction at about 12°S from NW-SE to the south, to NS in the West Somali Basin (Davis et al., 2016; Phethean et al., 2016; Reeves, 2018; Sauter et al., 2018a) (Fig. 12). We note that oceanic crust is thinner to the north of the archipelago while it is thicker to the south, and also that the depth of the oceanic crust is shallower in the north than in the south (Fig. 7). Different thicknesses and depths could suggest a change of magma budget related to the change of spreading direction. This change is also well defined by the striking bend in the Davie Ridge 12-14°S, which shows diffuse extension relative to the more focused extension further north and south, in the Kerimba and Lacerda Miocene to Pliocene narrow grabens, respectively (Franke et al., 2015) (Fig. 10). The islands of the Comoros Archipelago show a general EW alignment at the same latitude (12-13°S) with NS gravity lineaments to the north and NW-SE ones to the south (Fig. 12). We compare this geodynamic context with the so-called Buried Hills to the southeast of Sri Lanka. These magmatic structures form an elongated NE-
SW trending chain between the NS-oriented 85°E Ridge and the Afanasy Nikitin Seamounts (Altenbernd et al., 2020). They are located at a border between swaths of oceanic crust formed by seafloor spreading in two different directions. It was thus suggested that they may have been set up along a line of pre-existing structural weakness (Altenbernd et al., 2020). The reactivation of oceanic fracture zones has also been inferred to explain the formation of the Bathymetrists seamounts in the Central Atlantic (Van Der Zwan et al., 2023). There, a change in motion of the African plate 60-40 Ma ago, together with the reactivation of the fracture zones of the strongly sheared equatorial Atlantic, created pathways in the lithosphere and possibly enhanced magmatism (Van der Zwan et al., 2023). We thus suggest that the segmentation of the extinct spreading centre of the West Somali Basin strongly controls the emplacement of the Comoros Archipelago. This does not preclude other controlling mechanisms such as those suggesting a nascent plate boundary between the Lwandle and Somalia plates (Famin et al., 2020; Feuillet et al., 2021). The reactivation phase at Late Eocene time, coeval with the onset of the Arabia-Eurasia collision at the Eocene/Oligocene boundary (Song et al., 2023) may have triggered the weakening of the fracture zones in the Somali and Comoros Basins. This phase would then result in the emplacement of volcanic edifices along a weakened area where these fracture zones change direction favouring a nascent plate boundary.

7. Conclusions

1) The crust of the Comoros Basin is decidedly oceanic in character resolving controversies of the past.

2) The crust is thinner (5.76-6.62 km) to the north of the Archipelago and thicker (6-7.2 km) in the Comoros Basin and displays a 110-200 m top basement roughness. Such crustal thickness and top basement roughness are typical of crust formed at intermediate spreading ridges such as the extinct spreading centre of the West Somali Basin.

3) The Comoros Basin unloaded depth closely matches the asymptotic water-loaded subsidence predicted by the Stein and Stein (1992) plate cooling model, indicating potential Cretaceous or Jurassic age for the oceanic lithosphere, while the West Somali Basin north of the archipelago appears to have shallower basement, possibly linked to abnormal mantle heating related to recent
volcanism in the Mwezi and N'Drounde provinces northeast of Anjouan and
Grande Comore islands, respectively.

4) The pre-existing fracture zones of this extinct spreading centre of the West Somali Basin have been reactivated first at Turonian time and then at Late Eocene time. They may have served as mechanically weak zones controlling the emplacement of the volcanic islands of the Comoros Archipelago.

5) We suggest that the weakened area formed by the alignment of changes of direction of these fracture zones may have favoured the emplacement of the Comoros Archipelago.
Acknowledgements

This paper is a contribution of the COYOTES and SISMAORE teams
(http://www.geocean.net/coyotes/doku.php?id start). The processing and the detailed
analysis of these geophysical and geological data are carried out mainly in the
framework of the ANR COYOTES (ANR-19-CE31-0018, https://anr.fr/Projet-ANR-19-
CE31-0018) project funded by the French ANR (Agence Nationale de Recherche) and
the BRGM. The SISMAORE cruise was mainly funded by the Flotte Océanographique
Française (FOF) and by the BRGM. We thank CGG for allowing us to use the CGG
Geovation software in CNRS labs. We thank Captains P. Moimeaux and G. Ferrand,
the crews, and the technicians from the R/V Pourquoi Pas? (FOF by
IFREMER/GENAVIR). Thanks to the BRGM regional department of Mayotte. Thanks
to REVOSIMA and DIRMOM for their assistance with the cruise during the COVID
sanitary crisis. OBS facilities are from the IPGP instrumental network. Masquelet's
Ph.D. is funded by Sorbonne Université, via the ANR COYOTES. We thank
TotalEnergies and TGS for permission to use some of their seismic reflection profiles.
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# Supplementary Material

## Appendix A: Seismic refraction phase identification and description

<table><caption>Table A1: Description of the phase codes used in Appendices B and C and corresponding picking uncertainties</caption><thead><tr><td>Phase code</td><td>Description</td><td>Picking uncertainty (ms)</td></tr></thead><tbody><tr><td>1.0</td><td>Direct wave (water)</td><td>10</td></tr><tr><td>2.1</td><td>Refraction in the first sediment layer</td><td>40</td></tr><tr><td>2.2</td><td>Reflection at the top of basalts</td><td>40</td></tr><tr><td>3.1</td><td>Refraction in basaltic layer</td><td>40</td></tr><tr><td>3.2</td><td>Reflection at the base of basalts</td><td>60</td></tr><tr><td>4.1</td><td>Refraction in the second sediment layer</td><td>40</td></tr><tr><td>4.2</td><td>Reflection at base of the 2<sup>nd</sup> sediment layer</td><td>60</td></tr><tr><td>5.1</td><td>Refraction in the third sediment layer</td><td>40</td></tr><tr><td>5.2</td><td>Reflection at the top basement</td><td>60</td></tr><tr><td>6.1</td><td>Refraction in the upper crust</td><td>60</td></tr><tr><td>6.2</td><td>Mid-crust reflection</td><td>80</td></tr><tr><td>7.1</td><td>Refraction in the lower crust</td><td>80</td></tr><tr><td>7.2</td><td>Moho reflection</td><td>80</td></tr><tr><td>8.1</td><td>Refraction in uppermost mantle</td><td>100</td></tr></tbody></table>

### 1. Details on the seismic refraction data modelling

We perform forward velocity modelling of the refraction phases from three OBSs using Rayinvr (Zelt & Smith, 1992). This method is particularly adapted when a limited number of instruments are deployed and tomography cannot be used. The distance between instruments allows for 1D velocity models in the vicinity of the instruments, and the absence of crossing rays increases the non-uniqueness of the models. However, we use the multichannel seismic reflection images to constrain the geometry of the main - fortunately sub-horizontal - reflective interfaces down to the basement. In addition, refraction shots are available perpendicularly to line M01 crossing the Fani-Maore volcano (see Appendix C), again with constraints from coincident seismic
reflection images. The velocity model is built from top to bottom using 7 layers (excluding water) detailed in Table A1.

Four layers are needed to model the velocity gradients in the sediments. The second and fourth layers feature magmatic intrusions in the form of sills or flows in the vicinity of OBS 1 and 2. In the second layer of the model, the basaltic layer (4-4.5 km/s) discussed in Masquelet et al. (2022) on the basis of NMO velocity analysis is confirmed by a clear « moustache style » phase and a shadow zone on OBS 2 (Figures B3 and B4). This shadow zone indicates a much lower and more conventional sedimentary velocity below, in the third layer. The deepest « above basement » velocities are modelled in layer 4. For OBS 1 and 2, although the top of layer 4 is unambiguously mapped following a sediment facies in the reflection seismic profiles (Figures 4 and 8), it is not possible to model velocities lower than 4.5 km/s (max. 5.2 km/s), and this remains valid in both directions (Appendix C3 and C4). Low frequencies and sills are visible in the seismic images where these high velocities are modelled (Figures 2 and 8) while the basement reflection amplitude is weak. In contrast, the basement is obvious beneath OBS 3 (Figure 2) and the sedimentary seismic facies is more typical. Below basement, two layers (5 and 6) are necessary to model the crustal structure. OBS 1 and 2 are located close to the islands, in volcanic areas, whereas OBS 3 is located further south above a thick sedimentary basin. Despite these different environments, both crustal layers can be modelled with similar velocity structure for all three OBSs, including on lines perpendicular to the main Fani-Maore line (Appendix C). Layer 5 embeds a relatively high velocity gradient starting at 5.5 km/s while layer 6 features a low velocity gradient around 7 km/s. The base of the crust is constrained by wide angle PmP Moho reflections and Pn mantle refraction phases. These observations lead to a modelled crustal thickness decreasing toward the north from OBS 3 to 1.

Masquelet, C., Leroy, S., Delescluse, M., Chamot-Rooke, N., Thinon, I., Lemoine, A., Franke, D., Watremez, L., Werner, Ph., Paquet, F., Berthod, C., Cabiativa Pico, V. & Sauter, D. (2022). The east-Mayotte new volcano in the Comoros archipelago: structure and timing of magmatic phases inferred from seismic reflection data. *Comptes Rendus. Géoscience, 354*(S2), 65-79.

Zelt, C. A., & Smith, R. B. (1992). Seismic traveltime inversion for 2-D crustal velocity structure. *Geophysical journal international, 108*(1), 16-34.
Appendix B: OBS records, picked and computed arrival times

<figure><img src="image_84.png" /><figcaption>Figure B1: Uninterpreted seismic record for OBS01 along M01 profile (top), and interpreted section (bottom). The interpreted section shows picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases).</figcaption></figure>
<figure><img src="image_85.png" /><figcaption>Figure B2: Uninterpreted seismic record for OBS01 along M02 profile (top), and interpreted section (bottom). The interpreted section shows picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases).</figcaption></figure>
<figure><img src="image_86.png" /><figcaption>Figure B3: Uninterpreted seismic record for OBS02 along M01 profile (top), and interpreted section (bottom). The interpreted section shows picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases).</figcaption></figure>
<figure><img src="image_87.png" /><figcaption>Figure B4: Uninterpreted seismic record for OBS02 along M10 profile (top), and interpreted section (bottom). The interpreted section shows picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases).</figcaption></figure>
<figure><img src="image_88.png" /></figure>

Figure B5: Uninterpreted seismic record for OBS03 along M01 profile (top), and interpreted section (bottom). The interpreted section shows picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases).
<figure><img src="image_89.png" /><figcaption>Figure B5: Uninterpreted seismic record for OBS03 along M08 profile (top), and interpreted section (bottom). The interpreted section shows picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases).</figcaption></figure>
<figure><img src="image_90.png" /><figcaption>Figure B5: Uninterpreted seismic record for OBS03 along M12 profile (top), and interpreted section (bottom). The interpreted section shows picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases).</figcaption></figure>
Appendix C: Ray-tracing, fits and modelling statistics for each
OBS along each profile

<figure><img src="image_91.png" /><figcaption>Figure C1: Ray-tracing, fits and modelling statistics for the OBS1, along M01 profile. The top left panel shows the relief map with M01 profile highlighted in red and OBS1 shown as a yellow star. Bottom left panel shows the modelling statistics for each seismic phase in the model, with N, the number of picks used in the model, t<sub>RMS</sub>, the root-mean squared travel-time misfit between picked and computed travel-times, and χ<sup>2</sup>, the normalised misfit. The phase codes are presented in Appendix A. Top right panel presents the picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases). Middle right panel shows the ray tracing in the model for each phase. Dashed lines correspond to the model interfaces. Lower right panel shows the corresponding ray coverage in the model, with the colour-scale on the right.</figcaption></figure>
<figure><img src="image_92.png" /><figcaption>Figure C2: Ray-tracing, fits and modelling statistics for the OBS1, along M02 profile. The top left panel shows the relief map with M02 profile highlighted in red and OBS1 shown as a yellow star. Bottom left panel shows the modelling statistics for each seismic phase in the model, with N, the number of picks used in the model, t<sub>RMS</sub>, the root-mean squared travel-time misfit between picked and computed travel-times, and χ<sup>2</sup>, the normalised misfit. The phase codes are presented in Appendix A. Top right panel presents the picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases). Middle right panel shows the ray tracing in the model for each phase. Dashed lines correspond to the model interfaces. Lower right panel shows the corresponding ray coverage in the model, with the colour-scale on the right.</figcaption></figure>
<figure><img src="image_93.png" /><figcaption>Figure C3: Ray-tracing, fits and modelling statistics for the OBS2, along M01 profile. The top left panel shows the relief map with M01 profile highlighted in red and OBS2 shown as a yellow star. Bottom left panel shows the modelling statistics for each seismic phase in the model, with N, the number of picks used in the model, t<sub>RMS</sub>, the root-mean squared travel-time misfit between picked and computed travel-times, and χ<sup>2</sup>, the normalised misfit. The phase codes are presented in Appendix A. Top right panel presents the picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases). Middle right panel shows the ray tracing in the model for each phase. Dashed lines correspond to the model interfaces. Lower right panel shows the corresponding ray coverage in the model, with the colour-scale on the right.</figcaption></figure>
<figure><img src="image_94.png" /><figcaption>Figure C4: Ray-tracing, fits and modelling statistics for the OBS2, along M10 profile. The top left panel shows the relief map with M10 profile highlighted in red and OBS2 shown as a yellow star. Bottom left panel shows the modelling statistics for each seismic phase in the model, with N, the number of picks used in the model, t<sub>RMS</sub>, the root-mean squared travel-time misfit between picked and computed travel-times, and χ<sup>2</sup>, the normalised misfit. The phase codes are presented in Appendix A. Top right panel presents the picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases). Middle right panel shows the ray tracing in the model for each phase. Dashed lines correspond to the model interfaces. Lower right panel shows the corresponding ray coverage in the model, with the colour-scale on the right.</figcaption></figure>
<figure><img src="image_95.png" /><figcaption>Figure C5: Ray-tracing, fits and modelling statistics for the OBS3, along M01 profile. The top left panel shows the relief map with M01 profile highlighted in red and OBS3 shown as a yellow star. Bottom left panel shows the modelling statistics for each seismic phase in the model, with N, the number of picks used in the model, t<sub>RMS</sub>, the root-mean squared travel-time misfit between picked and computed travel-times, and χ<sup>2</sup>, the normalised misfit. The phase codes are presented in Appendix A. Top right panel presents the picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases). Middle right panel shows the ray tracing in the model for each phase. Dashed lines correspond to the model interfaces. Lower right panel shows the corresponding ray coverage in the model, with the colour-scale on the right.</figcaption></figure>
<figure><img src="image_96.png" /><figcaption>Figure C6: Ray-tracing, fits and modelling statistics for the OBS3, along M08 profile. The top left panel shows the relief map with M08 profile highlighted in red and OBS3 shown as a yellow star. Bottom left panel shows the modelling statistics for each seismic phase in the model, with N, the number of picks used in the model, t<sub>RMS</sub>, the root-mean squared travel-time misfit between picked and computed travel-times, and χ<sup>2</sup>, the normalised misfit. The phase codes are presented in Appendix A. Top right panel presents the picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases). Middle right panel shows the ray tracing in the model for each phase. Dashed lines correspond to the model interfaces. Lower right panel shows the corresponding ray coverage in the model, with the colour-scale on the right.</figcaption></figure>
<figure><img src="image_97.png" /><figcaption>Figure C7: Ray-tracing, fits and modelling statistics for the OBS3, along M12 profile. The top left panel shows the relief map with M12 profile highlighted in red and OBS3 shown as a yellow star. Bottom left panel shows the modelling statistics for each seismic phase in the model, with N, the number of picks used in the model, t<sub>RMS</sub>, the root-mean squared travel-time misfit between picked and computed travel-times, and χ<sup>2</sup>, the normalised misfit. The phase codes are presented in Appendix A. Top right panel presents the picked arrival times with their uncertainties (colour bars) and computed arrival times in the model (black dots). The colours correspond to the phase codes presented to the right (from shallow to deeper phases). Middle right panel shows the ray tracing in the model for each phase. Dashed lines correspond to the model interfaces. Lower right panel shows the corresponding ray coverage in the model, with the colour-scale on the right.</figcaption></figure>
## CHAPITRE VI. *Conclusion/ Perspectives*
# I. Synthèse des résultats / Conclusion

Cette étude de l'archipel et du bassin des Comores a constitué une avancée dans la compréhension du domaine marin de l'archipel des Comores, basée sur l'interprétation sismique, et sur la caractérisation des structures, que ce soit dans le domaine sédimentaire ou crustal.

Pour rappel, les résultats de cette thèse sont répartis en trois chapitres constituant trois publications scientifiques publiée, en révision ou en préparation.

## A. Le volcanisme des Comores

L'étude sur la première image passant à l'aplomb du nouveau volcan sous-marin nous aura permis de caractériser la structure interne du volcan Fani Maoré et la couverture sédimentaire en dessous.

Nous avons donc identifié des réflecteurs internes de volcan. Nous avons comparé des profils bathymétriques avant et après la mise en place du volcan Fani Maoré conjoint au profil de sismique réflexion. En projetant ces profils de bathymétrie, convertie en temps grâce à une loi de vitesse, nous avons identifié sur le profil de sismique réflexion une paléo-bathymétrie datant d'avant l'édification de Fani Maoré. Une ride volcanique a également été identifiée depuis le volcan vers Mayotte (EMVC : East Mayotte Volcanic Chain ; Feuillet et al., 2021, Thinon et al., 2022). Il est possible que cette paléo-bathymétrie corresponde à la ride Est-Mayotte. Ceci nous a permis de distinguer le matériel magmatique apporté lors de l'édification du volcan Fani Maoré.

Dans la partie plus profonde du profil sismique, on a constaté que le volcan est posé sur une couche sédimentaire de 2 à 2,5 km d'épaisseur, témoignant d'une activité sédimentaire importante. Nous avons également identifié une couche de basalte de 800 m d'épaisseur sous le volcan sous-marin. Cette couche de basalte a été interprétée comme une phase volcanique majeure d'édification de l'île de Mayotte, étant donné la proximité avec l'île volcanique. Une stratigraphie sismique cohérente du bassin a été réalisée afin de dater les structures observées dans celui-ci, basée sur deux puits (IOPD1476A et le DSDP242) localisés proche de la ride de Davie (Figure 6 Chapitre IV). Cela nous a permis de dater l'initiation du volcanisme entre 26
et 28 Millions d'années, contraignant l'âge d'initiation de Mayotte, jusqu'à présent datée au maximum à 10,6 Ma et estimé par Michon (2016), à 20 Ma.

***Eléments de discussion :*** La stratigraphie sismique de notre étude possède trois approximations.

Nous avons toujours utilisé un taux de sédimentation constant lors de la conversion secondes temps double - profondeur en mètres.

(1) Notre taux de sédimentation moyen utilisé est de 30 m/Ma, ce qui est caractéristique du taux de sédimentation sur une plaine abyssale (Hall et al., 2017). Cependant, cela introduit une erreur dans la datation des structures du bassin basé sur la stratigraphie sismique. Le taux de sédimentation d'un bassin n'est jamais constant au cours de l'histoire géologique car le flux sédimentaire du bassin est dépendant en grande partie des apports, et donc des reliefs, de la tectonique à l'échelle locale et lithosphérique.

(2) L'utilisation de profils sismiques pour propager les marqueurs stratigraphiques dans le bassin est très utile et nécessaire mais peut introduire également un biais d'interprétation car nous avions plusieurs jeux de données de sismique réflexion de résolutions différentes.

(3) L'interprétation stratigraphique effectuée au Nord de l'archipel est plus difficile car la colonne sédimentaire change brutalement d'épaisseur. C'est la raison pour laquelle plusieurs marqueurs stratigraphiques n'ont pas été retrouvés au Nord. Et ceux existant possèdent une moins bonne confiance que ceux étendus dans le bassin des Comores.

Pour défendre notre interprétation, des arguments peuvent être avancés. En effet, plusieurs carottages ont été réalisés dans le cadre de la campagne SISMAORE et ont apportés des taux de sédimentation au nord et au sud de l'archipel entre 27 et 35 m/Ma pour les 2 premiers millions d'années ce qui est cohérent avec les taux de sédimentation obtenus dans les puits DSDP et IODP de la ride de Davie. Par ailleurs, ces taux de sédimentation sont très constants dans la période échantillonnée par la carotte.

Nous avons réalisé une corrélation stratigraphique dans le bassin de Morondava grâce à des profils de sismique réflexion de TGS et à partir de la stratigraphie établie par Delauney (2018), à partir de forages récemment datés par TotalEnergies. Nous avons corréélé cette stratigraphie avec la stratigraphie des puits DSDP242 et IODP 1476A et avons trouvé les mêmes résultats pour les marqueurs du toit du Miocène et celui de l'Oligocène.
## B. Le volcanisme régional du bassin des Comores

L'étude approfondie de la partie volcanique autour de Mayotte a permis d'identifier plusieurs phases volcaniques en lien avec l'édification de Mayotte, daté à 27, 21 et 9 Ma, grâce à une stratigraphie sismique cohérente dans le bassin des Comores. Au Nord-Est de Mayotte, nous avons également identifié et daté des phases d'édifications des bancs volcaniques de Geyser & Zélée à 32 et 16 Ma. Enfin, nous avons caractérisé les phases majeures d'édifications des îles de Mohéli, Anjouan, Grande Comores et des rides volcaniques des Jumelles, et nous avons datés le début de ces constructions avec la stratigraphie sismique à 9 Ma pour Mohéli, 4 Ma pour Anjouan et les Jumelles, et 1,5 Ma pour Grande Comore. Nous avons distingué deux périodes volcaniques majeures dans l'évolution de l'activité volcanique de l'archipel des Comores. Une première période comprenant l'édification de la phase principale de Mayotte et de Geyser et Zélée dans la période entre 32 et 16 Ma et une seconde période avec un début d'un phase volcanique fin Miocène/ Pliocène (9-2 Ma).

Ces âges de début de volcanisme coïncident avec les âges du volcanisme Néogène de Madagascar (Bardintzeff et al., 2010; Michon et al., 2022) et avec l'activité volcanique d'une branche Sud-Ouest du Rift Est Africain, le bassin de Rungwe (Figure 4 Chapitre III). Durant cette période, une activité volcanique a été également notée hors du bassin des Comores, au niveau de la ride de Davie et dans le bassin du Mozambique. Ces deux périodes coïncident avec les périodes de volcanisme et tectonique du Rift Est Africain et du nord de Madagascar. Cela nous permet de suggérer le développement de la limite de la plaque Lwandle à 21 Ma.

**Eléments de discussion** : Au Nord de l'île de Mayotte, l'interprétation est moins précise pour plusieurs raisons. (1) L'épaisseur de la colonne sédimentaire change brusquement (voir les éléments de discussion de la stratigraphie sismique). (2) La présence quasi permanente de phases volcaniques et magmatique (présence de sills) ne permet pas une interprétation très précise des marqueurs sédimentaires et volcaniques. Si le volcanisme de Mayotte est assez bien contraint, en revanche, dans l'archipel, les autres îles ne sont pas aussi bien caractérisées.

En effet, la densité des profils de sismique réflexion n'est pas la même autour de Mohéli, Anjouan et Grande Comore. Et, pour les îles de Grande Comore et Mohéli, seul le domaine au sud est caractérisé car il était difficile d'étendre les marqueurs stratigraphiques au Nord et à l'Est, à cause de la présence de la ride de Chisiwani (Figure 1 Chapitre IV).
Ainsi les données géophysiques permettent la caractérisation des structures majeures dans un domaine difficile d'accès et /ou bien trop grand pour l'échantillonnage, mais, l'imagerie dans le domaine volcanique reste limitée et d'autres méthodes géophysiques devraient être envisagées.

## C. Domaine crustal, héritage structural et lien avec le volcanisme des Comores

L'acquisition de profils de sismique réflexion 960 traces a permis de d'identifier la nature de la croûte dans le bassin des Comores et de caractériser les structures crustales existante le long de ces profils. La croûte est océanique entre Mayotte et Madagascar et dans le bassin des Comores. Cependant, la profondeur de son toit au Nord des Comores est anormalement faible par rapport à un modèle de subsidence thermique normale (Stein and Stein, 1992), impliquant soit un budget magmatique différent dans la dynamique d'océanisation du bassin ou bien un support thermique externe, comme un réchauffement anormal du manteau lié à un volcanisme récent. Une interprétation et cartographie des profils au nord de Mayotte a établi la présence de zones de fractures réactivées au Turonien puis à l'Éocène supérieur. De même, nous pouvons constater dans tout l'archipel des direction préférentielles de la morphologie des îles volcaniques et des édifices volcaniques sous-marins vers le NNW-SSE voire N-S, comme dans l'exemple de la nouvelle province volcanique de N'Droundé (Figure 12 Chapitre IV).

En outre, nous avons constaté la présence d'un zone fracture identifiée par Davis et al. (2016), sous le nouveau volcan Fani Maore, lui-même reposant sur une ride volcanique ancienne (Ride Est Mayotte). Enfin, nous notons que l'archipel des Comores s'est édifié le long du changement de direction des zones de fractures (Figure 12 Chapitre IV). De ces observations, nous suggérons que les zones de fractures présentes dans le domaine crustal, ont contrôlé la mise en place des îles de l'archipel des Comores et du volcanisme récent. La phase de réactivation à l'Éocène supérieur, coïncidant avec le début de la collision Arabie-Eurasie à la limite Éocène/Oligocène (Song et al., 2023) pourrait avoir déclenché l'affaiblissement des zones de fractures dans les bassins de Somalie et des Comores. Cette phase aurait alors entraîné la mise en place d'édifices volcaniques le long d'une zone affaiblie où ces zones de fracture changent de direction en faveur d'une frontière de plaque naissante.
## Eléments de discussion :

Nous avons montré la présence d'un toit de la croûte anormalement plus haut par rapport à un modèle standard de subsidence thermique d'une croûte océanique jurassique de l'Atlantique. Nous avons suggéré la présence soit d'une différence de taux de formation de la croûte océanique ou bien un support thermique en dessous de cette zone. Afin d'imager en profondeur cette zone il serait intéressant d'acquérir des profils de tomographie le long et perpendiculairement par rapport à cette zone de faiblesse, afin de vérifier cette hypothèse d'un support thermique sous l'archipel des Comores et de caractériser la dynamique mantellique.
## II. Perspectives

Suite à cette étude sur la caractérisation de structures volcaniques et sédimentaires dans le domaine marin, autour de l'archipel des Comores et Mayotte, dans le bassin de Somalie, nous suggérons plusieurs axes de recherches.

### A. Chronologie des structures du bassin des Comores

Afin d'obtenir le plus d'informations, et contraindre plus précisément la stratigraphie et donc l'histoire de l'archipel et du bassin des Comores, il serait pertinent de réaliser un forage au sud de l'archipel sur des profils de sismique réflexion déjà acquis. Un forage au Nord de l'archipel permettrait également de connaître l'histoire sédimentaire et par la même occasion volcanique et tectonique du nord de l'archipel, et permettrait d'expliquer la différence d'épaisseur de sédiments entre le Sud et le Nord de l'archipel.

### B. Structures sédimentaires et lien avec le volcanisme

Les structures volcaniques identifiées dans le bassin des Comores sont souvent liées à des figures sédimentaires caractéristiques qu'il serait intéressant de mieux appréhender. En effet, nous avons pu caractériser au moins deux transports gravitaires (Mass Transport Complex) à l'Est de Mayotte et au Sud de Mohéli (voir Chapitre IV, Figure 8, 9 et 10 du papier) grâce à des faciès caractéristiques de ces structures (MTD :Mass Transport Deposits).

Nous avons vu que ce MTC s'est étendu sur des dizaines de kilomètres, symétriquement et radialement autour de la Ride Est Mayotte. Cependant, nous ne nous sommes pas attardés sur la cause de formation de ce MTC. De plus, nous avons pu constater la présence d'autres MTCs de plus petite taille dans le bassin des Comores. La datation de ces structures permettrait de comprendre leur mise en place et leur relation avec l'activité volcanique des Comores, comme cela est montré dans la province volcanique des Canaries, autant dans le domaine marin (León et al., 2019) qu'à terre (Paris et al., 2017)

### C. Structures volcaniques et tectoniques
Jusqu'à présent, seul Rusquet et al 2023 ont obtenu un âge sur une des rides volcaniques de l'archipel (la ride de Chistwani).

Il serait intéressant de faire des dragages sur les autres rides afin de contraindre un âge lié à leurs mises en place. Il serait également intéressant d'utiliser de la sismique réfraction pour caractériser la colonne sédimentaire et éventuellement les structures crustales (anciennes zones de fractures ?) sous ces rides afin de confirmer le contrôle de l'héritage sur la mise en place de ces structures volcaniques. Nous n'avons pas observé de croûte continentale près des Comores, cependant, nous n'avons aucun profil passant sur la ride et ayant suffisamment de résolution pour voir en profondeur. La sismique réfraction pourrait répondre à cette question. Ainsi, une acquisition de plusieurs profils de sismique réflexion multitraces grandes profondeurs parallèles aux Nord de la province volcanique de Mwezi, sur une direction Est-Ouest et recoupant les profils de TGS, permettraient de mieux caractériser :

* La dynamique et l'histoire sédimentaire de la zone au nord des Comores

* Une cartographie plus poussée des zones de fractures et possiblement l'identifications d'autre phases de volcanisme.

* Une couverture bathymétrique plus au Nord permettrait aussi de connaitre l'extension du volcanisme et des structures tectoniques en lien avec les déformations régionales.

Une étude du passage entre l'archipel des Comores et la ride de Davie serait également pertinent d'un point de vue structural. Identifier les structures tectoniques présentes entre la ride de Davie et l'archipel des Comores permettrait d'affiner cette zone de déformation diffuse et cette limite de plaque immature entre la plaque Somalienne et la plaque Lwandle.

## D. Contraintes géodynamiques et cinématique

Comme j'ai pu le constater une grande inconnue de cette cinématique du bassin des Comores est l'âge de première croûte océanique.

Pour pallier ce manque, nous suggérons l'acquisition de profils de magnétisme au niveau de la marge malgache et plus loin dans le bassin, en espérant avoir un signal suffisamment important pour établir un modèle malgré la « Jurassique quiet zone ». La connaissance de la marge malgache pourrait apporter une contrainte temporelle et structurale sur la dynamique d'ouverture du bassin de Somalie.
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