# Temporal and spatial variation of seafloor spreading at ultraslow spreading ridges: Contribution of marine magnetics

Fei Zhou, Jérôme Dyment

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Fei Zhou, Jérôme Dyment. Temporal and spatial variation of seafloor spreading at ultraslow spreading ridges: Contribution of marine magnetics. Earth and Planetary Science Letters, 2023, 602, 10.1016/j.epsl.2022.117957. insu-03947010

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1 Temporal and spatial variation of seafloor spreading at ultraslow spreading ridges: contribution of marine magnetics

3 Fei Zhou and Jérôme Dyment

4 Université Paris Cité, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France

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6 Abstract

7 The heterogenous magma supply at ultraslow spreading ridges creates diverse seafloor morphologies and lithospheric structures, which in turn generate a large variability in marine magnetic anomalies. The variability brings difficulties to interpret the evolution of oceanic lithosphere. On the other hand, different magnetic signatures of different seafloors provide an opportunity to identify the modes of seafloor spreading on the ultraslow spreading ridges. Here, we modeled several across-axis magnetic profiles selected from the Gakkel Ridge, Southwest Indian Ridge and Mid-Cayman Spreading Center to explore the lithospheric structure and seafloor spreading processes. Considering conjugate flanks, we observed three modes of seafloor spreading, Magmatic vs Magmatic, Magmatic vs Tectonic, and Tectonic vs Tectonic, on the three ultraslow spreading ridges. These three spreading modes reflect a strong, intermediate, and starved magma supply, respectively. Furthermore, four alternances of the different spreading modes were identified including the Magmatic vs Magmatic to Magmatic vs Tectonic, Magmatic vs Tectonic to Tectonic vs Magmatic, Tectonic vs Tectonic to Magmatic vs Magmatic, and Mixed. These alternances of spreading modes in across-axis direction suggest oscillations of the magma supply at different levels. Variation in the modes of seafloor spreading on four nearby profiles over the Mid-Cayman Spreading Center reveal the evolution of magma supply along the axis of this short ridge.
24 **Key words**

25 Ultraslow spreading ridges, Modes of seafloor spreading, Magma supply, Marine magnetics

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# 1 INTRODUCTION

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Strong variation of the magma supply at slow and ultraslow spreading centers results in a variety of crustal and lithospheric structures, some of them juxtaposing denudated mantle rocks and volcanic ridges whereas others cut-across previous detachment faults of opposed dip in a magma-starved spreading environment (Cannat et al., 2006). Studies on the slow-spreading Mid-Atlantic Ridge (MAR) found that detachment faults are widespread and account for >50% of the total lithospheric accretion (Smith et al., 2006; Escartín et al., 2008). Compared with slow-spreading centers, ultraslow spreading centers display a more heterogeneous along-axis magma supply, resulting in various contrasted spreading modes. The structural analysis at the easternmost SWIR identified four modes of seafloor accommodated by volcanic and detachment-fault seafloor, volcanic vs volcanic, volcanic vs corrugated, "smooth" (i.e., showing wide mantle exposures) vs corrugated, and volcanic vs smooth. (Cannat et al., 2006). A few near-axis detachment faults or Oceanic Core Complexes (OCC) were reported on the SWIR (e.g., Cannat et al., 2009; Dick et al., 2000; Tao et al., 2020; Wu et al., 2021; Zhou et al., 2022) and the Mid-Cayman Spreading Center (MCSC) (e.g., Connelly et al., 2012). In the across-axis direction, an alternance of detachment faulting and magmatic spreading was found at 64°E on the SWIR (Searle and Bralee, 2007), a "flip-flop" detachment faulting mode was suggested on the easternmost SWIR (Sauter et al., 2013), and a succession of oceanic core complexes was deciphered on the MCSC (Hayman et al., 2011; Grevemeyer et al., 2018). These studies indicate that lithospheric accretion shows different modes within a few ultraslow spreading segments. However, whether these modes are common on the other ultraslow spreading ridges and how do they evolve along- and across-axis still need to be clarified. We herein try to better characterize, through a modeling approach, the structure and mode of spreading at several representative
across-axis magnetic profiles on the three ultraslow spreading centers, i.e., Gakkel Ridge (GR), SWIR, and MCSC.

Unlike the MCSC, the two other ultraslow spreading centers are located in areas with extreme weather conditions making geological and geophysical surveys difficult. Their lithospheric structure and spreading evolution deserve further studies, especially in off-axis areas. Fortunately, magnetic data are easy to acquire and record information on the structure and spreading history, making it an interesting tool to address the above-mentioned problems. Zhou and Dyment (2022) proposed a modeling method to interpret the variability of magnetic anomalies observed on the ultraslow spreading SWIR. Here we extend their work at a global scale by using sea-surface and aeromagnetic data on all acknowledged ultraslow spreading ridges (Dick et al., 2003; Snow and Edmonds, 2007; Hayman et al., 2011) and explore their structure and spreading modes, which in turn constrain the evolution of magma supply both in time and along axis.

## 2 GEOLOGICAL SETTING AND DATA SELECTION

The GR extends over 1800 km from the Lena Trough to the west to the continental margin of the Laptev Sea to the east (e.g., Michael et al., 2003). The slowest spreading ridge does not show any significant transform fault and provides ideal conditions for the studies of mantle heterogeneity (Liu et al., 2008), lithosphere structure (Jokat et al., 2003), and hydrothermalism (Edmonds et al., 2003). According to the amount of volcanism, the GR is divided into western volcanic zone (WVZ) (7° W–3° E), intermediate sparsely volcanic zone (SVZ) (3–29° E), and eastern volcanic zone (EVZ) (29–85° E) (Michael et al., 2003). The full spreading rate decreases from 12.7 km/Myr in the WVZ to 11.0 km/Myr in the EVZ, although the poor magnetic anomalies in the SVZ makes the spreading rate ambiguous there (Jokat et al.,
2003; Michael et al., 2003; O'Connor et al., 2021). We selected four representative profiles along the GR: two in the WVZ, one in the SVZ, and one in the EVZ, respectively (Fig S1). These aeromagnetic data were collected 300 m above the sea-surface with 250 m sampling distance and 9-17 km line spacing by the Project Magnet aircraft in 1974-1975 (Vogt et al., 1979). In general, the two profiles in the WVZ show better shaped magnetic anomalies than the one in the EVZ, the one in the SVZ being the most difficult to decipher.

The MCSC is located in the center of the Cayman Trough, a complex tectonic structure that separates the North American and Caribbean plates. This short spreading center strikes ~110 km from the northern Oriente Transform Fault to the southern Swan Islands Transform Fault (Harding et al., 2017). The axial valley depth reaches 5000-6000 m (ten Brink et al., 2002) indicating that the lithosphere formed from a cold mantle and low extent of melting (Klein and Langmuir, 1987). Magnetic anomaly shows that this ridge has spread for at least 45 Myr with an average full spreading rate of 15-17 km/Myr (Hayman et al., 2011; Grevemeyer et al., 2018). A large OCC, Mount Dent, rising ~3 km above the surrounding seafloor, was found in the central part of the western flank. This OCC hosts an ultramafic-hosted hydrothermal vent, the Von Damm Vent Field (VDVF), showing moderate temperature (~200° C) and serpentinization degree (Connelly et al., 2012; Harding et al., 2017; Szitkar and Murton, 2018). Sea-surface magnetic data across the Mid-Cayman Spreading Center were collected by Cruise Wilkes 933008 of the US Navy (Rosencrantz et al., 1988; Leroy et al., 2000) in 1973. We selected four profiles with a line spacing of ~10 km and interpolated the original data to a constant 600 m sampling interval (Fig S2).

The ultraslow spreading SWIR separates the African and Antarctic plates extending on 7700 km from the Bouvet Triple Junction (BTJ) to the west to the Rodrigues Triple Junction to
the east. Two major fracture zones (FZ), Andrew Bain FZ and Gallieni FZ, delimit the ridge into three main sections, with different spreading characters, melt supply, and mantle heterogeneity (Sauter and Cannat, 2010). The along-axis water depth, Mantle Bouguer Anomaly (MBA), and Na<sub>8,0</sub> content together show a stronger melt supply in the intermediate section between Andrew Bain and Gallieni FZ by comparison with the two other sections East of Gallieni FZ and West of Andrew Bain FZ (Meyzen et al., 2003; Sauter and Cannat, 2010). We selected magnetic anomaly profiles in all these three sections (Fig S3).

Profiles 9-13 across the SWIR are located West of the Andrew Bain FZ, in the segment between 20-25° E, and were collected by Cruise Agulhas 44 of the University of Witwatersrand, South Africa, in 1986. Unlike most of the SWIR segments, this segment is long and orthogonal to the spreading direction (Dick et al., 2003). The mean axial valley depth is 4000 m (Sauter and Cannat, 2010), and the area displays a positive MBA (Grindlay et al., 1998). The selected profiles were interpolated at a constant 1 km sampling interval.

In the intermediate section, Profiles 14-15, from Zhou and Dyment (2022), were collected between Indomed and Gallieni fracture zones where the highest melt supply is observed along the SWIR, possibly in relation to the Crozet or Marion hot spots (Sauter et al., 2009; Zhou and Dick, 2013).

East of Gallieni fracture zone, the melt supply is the lowest and decreases eastward to the RTJ. We adopted Profiles 16-17 from Zhou and Dyment (2022).

## 3 MODELING METHOD

We assume the source of the magnetic anomalies in areas covered by magmatic crust to be made of three layers, from top to bottom a maximum 500 m-thick extrusive basalt layer, a
maximum 1500 m-thick gabbro layer, and a minimum 1000 m-thick peridotite layer that complement the two other layers to reach a 3 km total thickness (Fig 1A). To simulate the progressive diminution of magmatism toward the transition with the "tectonic crust", which exposes altered mantle material with possible gabbro intrusions on detachment surfaces, we apply a cosine tapper function to the maximum thickness Tm of the basalt and gabbro layers. The thickness T of these layers at a given distance x from the ridge axis within a magmatic crust area located between distances a and b from the ridge axis becomes

$$T = T_m \times \cos\left(\left|\frac{x-\frac{a+b}{2}}{\frac{a-b+w}{2}}\right| \times \frac{\pi}{2}\right),$$

where w is a weight modulating the width of the effective transition zone. We adopted a value of w = 0.05 km to insure a smooth transition within a relatively narrow zone, although sharper or smoother transitions would not significantly affect the calculated magnetic anomaly with a water depth of ~3000 m.

On seafloor formed by detachment fault spreading, the basalt and gabbro layers disappear, and the magnetic crust is only made of a 3000 m-thick peridotite layer with occasional 1000 m-thick intrusive gabbro bodies. All outcropping layers are topped by the bathymetry. Basalt and gabbro bear a remanent magnetization oriented along the local IGRF (International Geomagnetic Reference Field; Thébault et al., 2015). Their intensity of magnetization is kept proportional, with 15 A/m for basalt and 2 A/m for the underlying gabbro, and adjusted to fit the amplitude of near-axis magnetic anomalies on individual profiles, as profiles located at different latitudes require different intensity of magnetization to fit the magnetic anomalies. It should be noted that there is a tradeoff between the magnetization intensity and the thickness of each layer due to the non-uniqueness of potential field solution. These parameters therefore represent an
equivalent representation to facilitate the exploration of lateral variation of the lithosphere structure. The off-axis decrease of basalt layer magnetization caused by the alteration of titanomagnetite to titanomaghemite is taken into account by a decrease to half of the axial value. The transition between the near-axis and off-axis areas, determined by adjustment of the modelled and observed magnetic anomalies on each profile, lies 30-50 km away from the ridge axis. The magnetization of intrusive gabbro bodies is assumed to be 1-2 A/m stronger than that of gabbro formed during the magmatic spreading (Dick et al., 2000; Johnson and Pariso, 1993), because the former is more fractionated than the later. The peridotite layer bears an induced magnetization of 0.5 A/m that marginally contribute to the magnetic anomaly. The geomagnetic polarity time scale of Cande and Kent (1995) is used throughout this paper. The spreading rate of each spreading ridge is used to ascribe magnetic polarity of the basalt and gabbro along the modelled profiles.

The magnetic crust, topped by the seafloor topography, is split into uniform cubic units, each with its own magnetization. We then calculate the magnetic effect of every unit to every observation point by using the expression of Bhattacharyya (1964). The first step is to assign the distribution of magmatic and tectonic seafloor according to the shape of the observed magnetic anomalies: 1) magmatic crust corresponds to stronger, well-shaped magnetic anomalies; 2) purely tectonic crust corresponds to low amplitude, almost flat anomalies; and 3) tectonic crust with intrusive gabbro bodies records weaker, poorly-shaped magnetic anomalies (Fig 1B). Then the magnetic polarity intervals are further slightly adjusted to improve the fit of the computed anomaly with the observed one. At the same time, the half-spreading rate is calculated for each period. While adjusting we try to keep a consistent full spreading rate, which means that a lower spreading rate on one flank should correspond to a coeval higher rate on the conjugate flank. We
conduct the iterations until the synthetic magnetic anomalies fit well the observed ones. The
resulting full spreading rates for the recent period fit well the full spreading rates given by the
MORVEL model of present-day plate tectonics (DeMets et al., 2010) on the GR and SWIR, but
the fit is less satisfying on the MCSC possibly reflecting the poor navigation of older surveys.

In order to illustrate and validate the proposed method, we compare the results from our
magnetic modeling with those from other geophysical studies in the same area. The MCSC is
selected as the best example due to many geophysical investigations in this area. One of our
magnetic profile cuts across the Mount Dent OCC which has been investigated by previous
studies (e.g., Hayman et al., 2011; Grevemeyer et al., 2018). Bathymetric and seismic data show
three separate OCCs near the Mount Dent OCC (Fig 1C). However, the lower resolution of the
magnetic modelling results in including the three OCCs in a unique episode of tectonic
spreading. A similar effect is also observed in the Longqi OCC on the SWIR (Fig S3G), where
the two nearby OCCs separated by a short volcanic spreading recognized on bathymetric and
seismic studies (e.g., Tao et al., 2020) are modeled in a unique tectonic period with intermediate
gabbro intrusions. Therefore, our method is able to distinguish the different modes of seafloor
spreading at a lower resolution than multibeam bathymetric surveys and seismic experiments.
Magnetic data being far more abundant than multibeam bathymetric and seismic data, the
method proves useful in providing a first-order estimate of the spreading modes over wide areas
(Zhou and Dyment, 2022).

## 4 RESULTS: MODES OF SEAFLOOR SPREADING AND THEIR ALTERNANCES

The modeling results of the 17 magnetic profiles offer the opportunity to compare the
types of seafloor - either magmatic or tectonic - on conjugate flanks at a given time (Fig S1-S3).
They therefore allow to determine the modes of seafloor spreading at that time, in a way similar
to what Cannat et al. (2006) did from bathymetric data. However, the large-scale shipboard and airborne magnetic surveys do not allow to distinguish between corrugated and smooth seafloor. Therefore in this study, we recognize both of them as a tectonic seafloor and accordingly get three modes of seafloor spreading, Magmatic vs Magmatic (MM), Magmatic vs Tectonic (MT), and Tectonic vs Tectonic (TT). For example, the Magmatic vs Tectonic mode means that one flank shows magmatic seafloor and the conjugate one tectonic seafloor. Furthermore, the spreading mode is not steady in the across-axis direction but varies in time. We categorize the evolution of spreading mode in four different alternances observed on the investigated profiles. They are Magmatic vs Magmatic to Magmatic vs Tectonic (MM-MT), Magmatic vs Tectonic to Tectonic vs Magmatic (MT-TM), Tectonic vs Tectonic to Magmatic vs Magmatic (TT-MM), and Mixed. The latter includes evolutions mixing the different modes of spreading without any dominant pattern and represent 4 of the 17 investigated profiles. They are not discussed further here.

We selected several representative profiles to better explain how the modes of seafloor spreading alternate with each other. Figure 2 focuses on the MM-MT alternance. Fig 2A shows an example from the SWIR at 22°E. In this and the following figures, the top panel shows across-axis lithospheric structure, the middle-top panel shows types of seafloor within each period with red for magmatic seafloor and blue for tectonic seafloor, the middle-bottom panel shows the magnetic polarity and therefore the age, and the bottom panel shows observed (in blue) and modelled (in red) magnetic anomalies. Before ~ 9.02 Ma, both flanks are accreted by magmatic spreading, hence a Magmatic vs Magmatic (MM) mode. From ~ 9.02 to ~ 2.58 Ma, the lithosphere shows magmatic spreading on one flank and tectonic spreading on the other flank, corresponding to a Magmatic vs Tectonic (MT) mode. After ~ 2.58 Ma, the MM mode
resumes until now. Therefore, the MM and MT modes of seafloor spreading alternate in time, forming a MM-MT alternance. Another example comes from a profile from MCSC at 18.3°N (Fig 2B). On this profile, both flanks show magmatic seafloor before ~9.7 Ma corresponding to a MM mode. Between ~9.7 and ~3.8 Ma, magmatic seafloor is observed on one flank and tectonic seafloor on the conjugate flank, resulting in a MT mode. After ~3.8 Ma, the MM mode resumes with a possible short MT.

A first example of the MT-TM alternance is from the SWIR at 49.65°E (Fig 3A). The profile extends to ~ 11 Ma in the northern flank and ~ 15 Ma in the southern flank, allowing us to identify the modes of seafloor spreading within 11 Ma. During this period, seafloor spreading is mostly accommodated by MT mode, with only three short MM intervals between 0 – ~1.1 Ma, ~ 4 – 4.8 Ma, and ~9.35 Ma – 11 Ma. The magmatic and tectonic flanks change with time, resulting in a MT-TM alternance in this area. In another example from the Gakkel Ridge at 3°W (Fig 3B), the modeling result shows a clear MT-TM alternance from ~ 16.7 Ma to 1.9 Ma.

Two profiles in Fig 4 illustrate the TT-MM alternance. Fig 4A is an example from the SWIR at 50.5°E. Before ~ 8.3 Ma, both flanks were mostly accommodated by tectonic seafloor, although the southern flank shows a short episode of magmatic spreading, resulting in a TT mode. The area experienced a short MT mode episode between ~8.3 and ~ 5.89 Ma, then both flanks showed magmatic seafloor and therefore a MM mode. The profile is showing a clear alternance of TT mode to MM mode, as also suggested by the bathymetry Sauter et al., 2009). Another example from the Gakkel Ridge at 42°E displays a similar evolution (Fig. 4B). Between ~13.1 and 8.7 Ma, spreading was magmatic with a MM mode, then between ~8.7 and 4.18 Ma, it turned to tectonic with a TT mode, before resuming to a MM mode after ~ 4.18 Ma.
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The three spreading modes and their three main alternances are consistently observed on the three ultraslow spreading ridges, the long GR and SWIR and the short MCSC.

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# 4 DISCUSSION

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## 4.1 Alternances of spreading modes reflect temporal variation of the magma supply

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Different modes of seafloor spreading indicate different levels of magma supply. When the magma supply is high, a magmatic crust is created on both flanks, resulting in the MM mode of spreading. When the magma supply is intermediate, a magmatic crust is created on one flank only and tectonic processes generate the conjugate flank (Buck et al., 2005), resulting in the MT mode of spreading. When the magma supply is low, tectonic processes generate the both flanks (for instance by the “flip-flop” detachment fault spreading observed on the easternmost SWIR; Sauter et al., 2013), resulting in the TT mode. Therefore, changes of seafloor spreading modes in the across-axis direction reflect the variation of magma supply in time.

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The MM-MT alternance suggests a magma supply varying between a high level, sufficient to generate a fully magmatic crust on two flanks, and an intermediate level, generating a magmatic crust on one flank only. For example, the MM-MT alternance observed on Fig 2A suggests that the magma supply on the SWIR at 22°E was high before ~ 9.02 Ma and decreased to an intermediate between ~ 9.02 Ma and 2.58 Ma, before regaining a high level afterward.

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The MT-TM alternance suggests a regular magma supply of intermediate level to steadily generate a magmatic crust on one flank and tectonic spreading on the conjugate flank. Fig 3B clearly shows that the Gakkel Ridge at 3°W consistently exhibited a magmatic flank and a tectonic flank between ~ 16.7 and 1.9 Ma, although the magmatic spreading switched from the southern to northern flank at ~ 9.75 Ma.

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The TT-MM alternance suggests a starved magma supply to generate a fully tectonic spreading, occasionally interrupted by intermittent magma bursts to generate a fully magmatic spreading. The two magmatic episodes observed on the GR at 42°E (Fig. 4B) have a duration of 4.4 Myr and more than 4.18 Myr. Conversely, the magmatic episode observed on the SWIR at 50°.5°E (Fig. 4A) is almost 8 Myr long. While the influence of the Marion or Crozet hotspot has been invoked to explain the unique long magmatic episode at the SWIR (Sauter et al., 2009; Zhang et al., 2011), the multiple occurrences of magmatic bursts at the GR may be explained by the accumulation and sudden release of melt at the lithosphere asthenosphere boundary (LAB). In this model, the cold and thick lithosphere would hamper the melt upward migration and favor its accumulation near the LAB, in a stage when seafloor spreading is displaying the TT mode. The magma, possibly accumulated in large magma chambers like the one observed at the SWIR 50.5°E (Jian et al., 2017), may suddenly be released and start an episode of magmatic seafloor spreading, sustaining the MM mode for several million years. Anomalously deep (4000 m) - and therefore powerful - explosive volcanism has indeed been documented from craters and pyroclastic sediments on the eastern GR (Sohn et al., 2008).

## 4.2 Seafloor spreading modes reflect along-axis magma supply

The studied profiles on the GR and SWIR are too spread apart to allow significant profile-to-profile correlation. Conversely, the four profiles 5-8 cutting across the MCSC are close enough to recover the spreading history at the scale of the ridge segment. Fig 5 shows the variation of seafloor type on each flank and spreading modes on two flanks in time. On each panel, columns on the sides are the types of seafloors on western and eastern flanks, respectively, in which the red patterns represent magmatic seafloor, and the blue patterns represent tectonic seafloor. The middle panel shows the modes of seafloor spreading considering scenarios of two
291 flanks, in which the black patterns represent MM mode, the yellow patterns represent MT mode,
292 and the green patterns represent TT mode.

293 The four profiles of the MCSC show a consistent variation of the spreading modes and in
294 turn a consistent evolution of the magma supply. The alternance of spreading modes is
295 dominantly MM-MT, except for Profile 7 in the central part that displays a TT mode in place of
296 the MT mode observed on adjacent profiles. The MM-MT alternance suggests that magma
297 supply was originally high and decreased to an intermediate (or, on Profile 7, low) level. This
298 decrease happened at different ages on the four profiles with a north-south progression from ~ 11
299 Ma on the northern Profile 5, to ~ 9.7 Ma on Profile 6, to ~ 8.7 Ma on Profile 7, to ~7.25 Ma on
300 the southern Profile 8. This indicates that the magma supply first diminished in the north and its
301 restriction gradually propagated to the south to affect the full segment at ~ 7.25 Ma. During this
302 episode of lower magma supply, a MT mode was sustained at the ends of the segment, whereas a
303 TT mode existed at the center. This status was maintained for ~ 4 Myr until the magma supply
304 recovered, first at the segment center at ~ 4.18 Ma, then progressively toward the segment ends,
305 turning the whole segment to the MM mode in about 3 Myr. This example suggests that, if the
306 magnetic data are sufficiently dense, they can help understanding variations of spreading mode
307 and evolution of the magma supply both in time and along axis.

308 5 CONCLUSIONS

309 Sea surface magnetic anomalies represent a tool to study the structure and evolution of
310 the lithosphere. In this study, we selected 17 profiles on three ultraslow spreading ridges, the
311 Southwest Indian Ridge, the Gakkel Ridge, and the Mid-Cayman Spreading Center, to model the
312 across-axis lithospheric structure and the modes of seafloor spreading. Three spreading modes,
313 Magmatic-Tectonic (MT), Magmatic-Magmatic (MM), and Tectonic-Tectonic (TT), are
identified from the analysis of magnetic profiles on conjugate ridge flanks, reflecting different levels of magmatism at the time of lithospheric accretion. Subsequently, four evolution patterns, MM-MT, MT-TM, TT-MM, and Mixed, are recognized along the three investigated ultraslow spreading ridges. While the MT-TM mode represents a steady evolution around an intermediate level of magmatism and the MM-MT mode an oscillation between high and intermediate levels of magmatism, the more contrasted TT-MM mode involves the occurrence of magmatic bursts in an otherwise magmatically starved environment. Such bursts may reflect the influence of a nearby hotspot or mark the accumulation and sudden release of melt at the Lithosphere Asthenosphere Boundary. Finally, dense magnetic profiles over the short Mid-Cayman Spreading Center reveals a consistent evolution of the spreading modes and of the magma supply, demonstrating that our approach allows depicting the variation of magmatism both in time and along axis.

## ACKNOWLEDGEMENTS

We thank the crews and science teams who acquired the aeromagnetic and sea-surface marine magnetic data used in this study. These data are available at NCEI (www.ngdc.noaa.gov/) databases, and the bathymetric data of Figs. 1-3 are available at MGDS (https://www.marine-geo.org/index.php). We thank Dr. Mathilde Cannat and Dr. Zhiteng Yu for useful discussions. The Editor, Dr. Florent Szitkar and an anonymous reviewer are gratefully acknowledged for useful comments that significantly improved the manuscript. Fei Zhou is funded by China Scholarship Council (201906170047). This is IPGP contribution xxxx.
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FIGURE CAPTIONS:

487 Fig. 1. Magnetic modeling: A. Configuration and B. Flow chart of the procedure (See text for details). C. Comparison of our modelling of a magnetic profile with results from bathymetric and seismic data on the MCSC. The solid black shapes show the location of three OCCs interpreted by Grevemeyer et al. (2018).

492 Fig. 2. Alternance of Magmatic vs Magmatic to Magmatic vs Tectonic (MM-MT). Two examples from: A. SWIR at 22°E, and B. MCSC at 18.3°N. Top panel, structure, Middle-top panel, mode of seafloor spreading, with MM, Magmatic vs Magmatic; MT, Magmatic vs tectonic; TM, Tectonic vs Magmatic, and TT, Tectonic vs Tectonic; Middle-bottom panel, magnetic polarities; Bottom panel, observed (blue) and modelled (red) magnetic anomalies.

498 Fig. 3. Alternance of Magmatic vs Tectonic to Tectonic vs Magmatic (MT-TM). Two examples from: A. SWIR at 49.65 °E, and B. Gakkel Ridge at 3°W. Panels as in Fig. 2.

501 Fig. 4. Alternance of Tectonic vs Tectonic to Magmatic vs Magmatic (TT-MM). Two examples from: A. SWIR at 50.5 °E, and B. Gakkel Ridge at 42 °E. Panels as in Fig. 2.

504 Fig. 5. Modes of seafloor spreading and their alternances for the four nearby profiles crossing the MCSC. The side columns display types of seafloors on each flank, the middle column the mode of spreading with time. The red dashed curves underline possible correlations between profiles suggesting consistent changes of magma supply within the segment.
A

Sea surface

Basalt

Gabbro

Peridotite

500 m

1500 m

1000 m

Magnetic polarity

+

-

+

B

Parameters

Assign the distribution of magmatic vs tectonic seafloor

Adjust intervals of magnetic polarities

Calculate spreading rate in each episode

Calculate magnetic anomaly and compare with observed anomaly

Lithospheric structure

C

<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>
<figure><img src="image_4.png" /></figure>

Southwest Indian Ridge (22°E)

<figure><img src="image_5.png" /></figure>

Mid-Cayman Spreading Center (18.3°N)
<figure><img src="image_6.png" /><figcaption>Souwest Indian Ridge (49.65°E)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Gakkel Ridge (3°W)</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Southwest Indian Ridge (50.5 °E)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Gakkel Ridge (42°E)</figcaption></figure>
<figure><img src="image_10.png" /></figure>

North

Mid-Cayman Spreading Center

South

Types of spreading on one flank

Magmatic spreading

Tectonic spreading

Spreading modes on two flanks

Tectonic-magmatic

Tectonic-tectonic

Magmatic-magmatic