Mathilde Cannat

Daniel Sauter

Luc L Lavier

Manon Bickert

Ekeabino Momoh

Sylvie Leroy

Ekéabino Momoh

On spreading modes and magma supply at slow and ultraslow mid-ocean ridges

Keywords: slow and ultraslow mid-ocean ridges, divergent plate boundaries, detachment faults, melt supply, axial lithosphere, tectonic and magmatic seafloor spreading processes

published or not. The documents may come

INTRODUCTION

While all mid-ocean ridges are active volcanic chains, slow (<4 cm/yr) and ultraslow (<2 cm/yr) spreading ridges are also rift zones, where normal faults accommodate part of the plate divergence, interacting with magmatism to shape the newly formed oceanic lithosphere. These ridges are segmented, and significant along-axis variations in melt supply accompany this segmentation, with segment ends receiving less magma than the segments centers [START_REF] Lin | Evidence from gravity data for focused magmatic accretion along the Mid-Atlantic Ridge[END_REF]. At segment ends, and in whole segments of melt-poor slow and ultraslow ridge regions, large offset normal faults, also called detachment faults, accommodate a large part of the plate divergence [START_REF] Cann | Corrugated slip surfaces formed at ridge-transform intersections on the Mid-Atlantic Ridge[END_REF][START_REF] Cannat | Modes of seafloor generation at a melt-poor ultraslow-spreading ridge[END_REF][START_REF] Escartín | Central role of detachment faults in accretion of slow-spreading oceanic lithosphere[END_REF][START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF]. These detachments bring mantle-derived rocks up through the axial lithosphere, and the resulting seafloor therefore has an ultramafic, non-magmatic component [START_REF] Cannat | Emplacement of mantle rocks in the seafloor at mid-ocean ridges[END_REF]. By contrast, the centers of most slow and ultraslow spreading ridge segments undergo plate divergence through a combination of magma injection and several moderate offset normal faults distributed in the axial domain, and the seafloor exposes volcanic rocks [START_REF] Smith | Constructing the upper crust of the Mid-Atlantic Ridge: A reinterpretation based on the Puna Ridge, Kilauea Volcano[END_REF]. In the detachment-dominated mode of spreading, specific hydrothermal reactions, associated with the alteration of mantle peridotites, are favored, and feed original microbial communities [START_REF] Früh-Green | Serpentinization of oceanic peridotites: Implications for geochemical cycles and biological activity[END_REF]. Modes of spreading at slow and ultraslow spreading ridges therefore have a large impact not only on the composition of the oceanic lithosphere, but also on chemical exchanges between the solid Earth and the Ocean, and on the diversity of seafloor biology.

Many questions remain about oceanic accretion in the detachment-dominated mode. What controls the geometry and lifetime of detachments, and how do they root into the mantle? How do hydrothermal and magmatic processes operate in this mode and what are their time and space relations to detachment faulting? What is the architecture of the lithosphere that is accreted in the hanging wall plate? Although it is often assumed to be better constrained, there is also a lot we do not yet know about the volcanic spreading mode at slow and ultraslow ridges. While crustal architecture in this mode is commonly proposed to be similar to the layered magmatic crust formed at fast ridges, there is as yet limited direct evidence in support of this hypothesis, and at least one indication that it might not be the case: while volcanic slow spreading ridge regions have a substantial magma supply, they also have a much thicker (up to 7 km-thick at the Mid-Atlantic Ridge; [START_REF] Wolfe | Microearthquake characteristics and crustal velocity structure at 29 o N on the Mid-Atlantic Ridge: The architecture of a slow spreading segment[END_REF] seismogenic lithosphere, compared with the shallow depth to the axial melt lens of faster ridges [START_REF] Detrick | Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise[END_REF]. Finally, while magma supply is identified as a main control on which of the two spreading modes is activated at slow and ultraslow spreading ridges, and although this control has been tested in numerical models [START_REF] Buck | Modes of faulting at mid-ocean ridges[END_REF][START_REF] Olive | The structure of oceanic core complexes controlled by the depth distribution of magmaemplacement[END_REF][START_REF] Püthe | Dependence of mid-ocean ridge morphology on spreading rate in numerical 3-D models[END_REF][START_REF] Tian | Effects of axially variable diking rates on faulting at slow spreading mid-ocean ridges[END_REF][START_REF] Tucholke | Role of melt supply in oceanic detachment faulting and formation of megamullions[END_REF], there is no clear understanding as yet on how it operates in situ: by what melt supply-related mechanism(s) does a slow spreading ridge switch from the volcanic to the detachment-dominated mode of spreading?

We address several of these questions based on geological, geophysical and petrological data acquired over the course of several cruises in the eastern region of the Southwest Indian Ridge (SWIR; spreading rate of 14 mm/yr; [START_REF] Cannat | Modes of seafloor generation at a melt-poor ultraslow-spreading ridge[END_REF], east of the Melville Fracture Zone. This 660 km-long ridge region is a low magma supply end-member of the global mid-ocean ridge system [START_REF] Cannat | Spreading rate, spreading obliquity, and melt supply at the ultraslow spreading Southwest Indian Ridge[END_REF]. It is characterized by a pronounced focusing of the available melt to axial volcanos, separated by corridors of nearly-amagmatic detachment-dominated spreading where the seafloor exposes almost only ultramafic rocks [START_REF] Cannat | Modes of seafloor generation at a melt-poor ultraslow-spreading ridge[END_REF][START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF]. Volcanic areas show numerous and mostly spreading-perpendicular scarps and hummocky volcanic ridges, and correspond to more negative gravity anomalies, consistent with seismic crustal thicknesses up to 8 km [START_REF] Minshull | Crustal structure of the Southwest Indian Ridge at 66°E: Seismic constraints[END_REF]. Nearly amagmatic spreading corridors have, by contrast, a smooth topography, more positive gravity anomalies [START_REF] Cannat | Modes of seafloor generation at a melt-poor ultraslow-spreading ridge[END_REF], and a 2 to 5 km-thick seismic crustal layer that is interpreted as made primarily of serpentinized and fractured mantle-derived rocks [START_REF] Momoh | Quasi-3-D Seismic Reflection Imaging and Wide-Angle Velocity Structure of Nearly Amagmatic Oceanic Lithosphere at the Ultraslow-Spreading Southwest Indian Ridge[END_REF]. These rocks are proposed to have been exhumed by successive axial detachment faults, each cutting into the footwall of the previous fault, with an opposite polarity [START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF]. The eastern SWIR therefore offers opportunities to study the links between magma supply and seafloor spreading modes starting from a quasi melt-free regime, and transitioning to volcanic spreading as one nears the axial volcanos.

The formation of many distal continental divergent margins also involves tectonic extension with only incipient magmatism (eg [START_REF] Peron-Pinvidic | Structural comparison of archetypal Atlantic rifted margins: A review of observations and concepts[END_REF]. Specifically, seismic reflectors imaged in the Ocean Continent Transition (OCT) of the Iberian and Australian margins may also have formed by detachment faults that alternated polarity [START_REF] Gillard | Tectonomagmatic evolution of the final stages of rifting along the deep conjugate Australian-Antarctic magmapoor rifted margins: Constraints from seismic observations[END_REF][START_REF] Reston | Successive detachment faults and mantle unroofing at magmapoor rifted margins[END_REF].

Divergence rates and melt supply during the OCT phase, and during the following onset of indisputable seafloor spreading at these margins, are not strongly constrained. Our results and hypothesis on the nearly amagmatic seafloor spreading mode, and on its transition to volcanic seafloor spreading at an ultraslow ridge, may nonetheless inform studies of detachment-dominated divergence and mantle exhumation at the OCT phase, and of the transition from OCT to true seafloor spreading.

RESULTS

Along-axis extension, and estimated lifetime of successive axial detachments in a nearly amagmatic eastern SWIR spreading corridor

We focus on the nearly amagmatic corridor between the 64°E and 65.6°E axial volcanos (Figure 1). The smooth seafloor there forms 500 to 1000 m-high broad ridges, parallel to the ridge axis, which is nearly orthogonal to the north-south trending direction of plate divergence to the west of 65°E, and up to 25° oblique to the east (Figure 1). Following [START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF], we interpret these broad ridges (numbered from 1 to 8 going back in time; Figure 1) as formed by successive axial detachments between about 11 myrs-ago (#8) and the present (#1). The along flowline profile in Figure 2 summarizes this tectonic interpretation. Axial detachments #6 (which we propose was initiated about 5.6 myrs ago) to #1 (presently active) each cut into the footwall of the previous detachment, with an opposite polarity [START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF]. Proposed breakaways B7 and B8 are both located in the northern, African plate (Figures 1 and2) and the corresponding exhumed fault surfaces locally bear spreading-parallel corrugations [START_REF] Cannat | Oceanic corrugated surfaces and the strength of the axial lithosphere at slow spreading ridges[END_REF]. These two south-facing detachments face volcanic conjugate seafloor in the Australian plate and therefore formed under more magmatically active axial conditions than detachments #6 to #1 [START_REF] Cannat | Oceanic corrugated surfaces and the strength of the axial lithosphere at slow spreading ridges[END_REF].

We pick the top of the inward-facing (toward the axis) slope of each ridge as the best approximation (see Appendix Figure A1) for the location of the breakaway of the corresponding detachment fault, and the base of the outward-facing slope of the corresponding broad ridge as the best approximation for the location of the emergence of each fault at the seafloor. The along-axis extension of the eight identified detachments, based on the length of their breakaway (Figure 1 and Table 1), ranges between 25 km (B7) and 95 km (B4). While these breakaways are mostly straight, the inferred emergence traces show convex-outward undulations, with a typical wavelength of 15-20 km (Figure 1). These likely reflect along-axis changes in the rheology of the detachment footwall, impacting the fault's emergence angle. Most detachment-controlled ridges extend up to, and in the case of B5 (to the west) and B8 (to the west and east), into, adjacent volcanic areas (Figure 1). Breakaway ridges B4, and to a lesser extend B2, in the Antarctic plate, are long and follow the overall trend of the ridge axis, curving from E-W in the west, to ENE-WSW in the east (Figure 1). Breakaway ridges B5 and B7, in the African plate, are shorter and relayed to the east by broad ridges that trend NE-SW, oblique to the N-S spreading direction but parallel to the local trend of the ridge axis. A shorter ENE-WSW ridge, also interpreted as a detachment fault block, is found in an intermediate position to the east of B3 and B5 (Figure 1). These local complexities are interpreted as due to faulting modes specific to oblique nearly amagmatic spreading [START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF] and are beyond the scope of this paper. We thus focus the following analysis and discussions on the orthogonal-spreading part of the smooth seafloor corridor, to the west of 65°E (Figure 1).

The horizontal offset estimated for past detachments is the along-flowline distance between breakaway and emergence (reconstructed at the time of initiation of the next detachment). For detachment #1, which is still active and at an early stage, the proposed estimate takes mass-wasting into account and is explained in Appendix Figure A1. Estimated horizontal offsets for detachments along the 64.6°E flowline (Figure 2) range between 4 km (active detachment #1; Table 1), and ~21 km (detachments #2 and #8). At a spreading rate of 14 mm/yr [START_REF] Patriat | Reconstruction of the Central Indian Ocean[END_REF], these estimated horizontal offsets correspond to faulting durations (Table 1) between 300 kyrs (detachment #1, still active) and 1.5 myrs (detachment #2) for nearly amagmatic detachments 1 to 6, and of about 2.8 myrs, for corrugated detachments 7 and 8. For nearly amagmatic, flipping polarity detachments # 1 to 6, the horizontal distance between the emergence of a detachment and the location of the next breakaway is 2.8 to 5.6 km (Table 1), indicating that new detachments typically cut into footwall rocks that had been exhumed 200 to 400 kyrs previously.

Footwall to hanging wall: two stages in the generation of melt-starved oceanic lithosphere

A consequence of having successive detachments of opposite polarity that cut into their predecessor's footwall is that the inward-facing slopes of smooth seafloor breakaway ridges are made of material that was initially accreted and transported off-axis as part of the footwall of a detachment fault (Stage 1), then became part of the hanging wall of the next detachment fault (Stage 2). The oceanic lithosphere forming these inward-facing slopes is therefore predicted to have been generated in two stages and over the lifetime of two successive detachment faults.

Stage 2 involves the residence of previously accreted ultramafic seafloor, in the axial valley region as part of the hanging wall of the next axial detachment (Figure 3). Geological observations made in the present-day axial valley near 64.6°E indicate that Stage 2 results in the formation of small offset normal faults and of isolated volcanic patches and ridges, that are distributed in a > 10 km-wide axial valley (Sauter et al., 2013 and Figure 4). Seismic refraction and reflection results obtained in the same area [START_REF] Momoh | Quasi-3-D Seismic Reflection Imaging and Wide-Angle Velocity Structure of Nearly Amagmatic Oceanic Lithosphere at the Ultraslow-Spreading Southwest Indian Ridge[END_REF] show lower seismic velocities and several sub-horizontal and north-dipping reflectors in the basement of this near-axis domain, consistent with distributed faulting, hydrothermal alteration and magmatic intrusions in the hanging wall of active detachment #1.

The outward-facing slopes of smooth seafloor breakaway ridges record only Stage 1: they form in the footwall of a detachment, and are then captured, and moved off-axis into the footwall of the next (antithetic) detachment. Seafloor ages in the outward-facing slopes of detachment ridges therefore probably increase toward the axis (Figure 3). By contrast, most of the seafloor forming the inwardfacing slopes of these ridges has probably been modified by Stage 2 axial valley tectonics, hydrothermal alteration and sparse volcanism, over the lifetime of the next, antithetic detachment.

Seafloor spreading in the near absence of melt is therefore predicted to result in complex seafloor age patterns at scales of a few tens of kilometers or less (Figure 3). At larger scales, seafloor ages do nonetheless increase off-axis: pickings for the two best expressed magnetic anomalies in the area (3A and 5; Figure 1) are accordingly consistent with the ages predicted in Table 1 for breakaways B6 and B8.

This two-stage evolution has two predictable consequences, in addition to the generation of a complex lithosphere geology. One is the potential for thermal rejuvenation of the deep lithosphere during Stage 2, as older portions of the footwall of the first detachment find themselves on-axis again in the hanging wall of the next detachment. The other concerns the potential for nearly amagmatic oceanic lithosphere to bear magnetic anomalies. Ultramafic rocks that may have acquired a remanent magnetization upon serpentinization (eg [START_REF] Oufi | Magnetic properties of variably serpentinized abyssal peridotites[END_REF] in the footwall of the first detachment, would then be subjected to Stage 2 diking and volcanic eruptions in the hanging wall of the next detachment. For example, this could lead to the overprinting of a serpentinization-related magnetic signature recorded up to 2.9 myrs ago at depth in the footwall of detachment #3, by the magnetic signature recorded in volcanic rocks erupted up to 300 kyrs-ago in the hanging wall of detachment #2 (Figure 3). This, in addition to the overall low magnetization of serpentinized peridotites (refs), helps explain why the magnetic anomaly record is poor and ambiguous over the ultramafic seafloor of the study area [START_REF] Bronner | Magnetic signature of large exhumed mantle domains of the Southwest Indian Ridge – Results from a deep-tow geophysical survey over 0 to 11 Ma old seafloor[END_REF].

Along-axis transition from detachment-dominated to volcanic spreading

Figure 5 shows the transition, in the near-axis region, between nearly amagmatic detachmentdominated spreading (along-flowline profile 1), and more magmatic spreading near the 64°E axial volcano (profiles 3 and 4). Profile 1 has an asymmetrical topography, and the seafloor there exposes almost only serpentinized peridotite [START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF]. Faults in the axial valley floor have vertical offsets <200m; volcanism is sparse, forming isolated ridges and patches, and both rock sampling [START_REF] Paquet | Effect of melt/mantle interactions on MORB chemistry at the easternmost Southwest Indian Ridge (61°-67[END_REF][START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF] and seismic imaging [START_REF] Momoh | Quasi-3-D Seismic Reflection Imaging and Wide-Angle Velocity Structure of Nearly Amagmatic Oceanic Lithosphere at the Ultraslow-Spreading Southwest Indian Ridge[END_REF] indicate that magmatic rocks form a volumetrically minor part of the basement. Nearly all the plate divergence is therefore currently accommodated by axial detachment #1. By contrast, profiles 3 and 4, less than 30 km to the west, are located fully within volcanic seafloor, with several volcanic ridges, and fault scarps that have a maximum vertical throw of 1000 m (Figure 5c). The gravity anomalies are in the negative range (Figure 5b), consistent with a thicker crust [START_REF] Cannat | Modes of seafloor generation at a melt-poor ultraslow-spreading ridge[END_REF], and we see no evidence in these profiles that detachment #1 is continuing at depths beneath rafted blocks of volcanic seafloor [START_REF] Reston | Flipping detachments: The kinematics of ultraslow spreading ridges[END_REF][START_REF] Reston | The 3-D geometry of detachment faulting at mid-ocean ridges[END_REF]. The axial topography is symmetrical, and seafloor ages from magnetic anomalies [START_REF] Cannat | Modes of seafloor generation at a melt-poor ultraslow-spreading ridge[END_REF] increase monotonically with distance to the axis (Figure 5c). These characteristics are consistent with in situ eruption of the volcanics and are typical of seafloor spreading at moderate melt supply, with plate divergence being partitioned into magma emplacement and fault displacement distributed in the axial domain (eg [START_REF] Buck | Modes of faulting at mid-ocean ridges[END_REF].

Profile 2 is in an intermediate position and cuts the western end of detachment #1 (Figure 5a). The seafloor along this profile shows volcanic morphologies, with spreading perpendicular ridges that are clearly in-situ volcanic constructions (Figure 4 and [START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF], and fault scarps ≤ 200 m-high (Figure 5b); yet the gravity anomalies are in the positive range (Figure 5b). These characteristics are consistent with seafloor spreading at relatively low melt supply, with recent plate divergence being accommodated for a part by detachment #1, and for another part by magma emplacement and smaller offset normal faults distributed in the axial valley. The overall topography of the axial region along profile 2 is distinctly asymmetrical across-axis and similar to that of profile 1. This asymmetric topography is probably mostly due to the broad plate bending effect of the active detachment in the nearly amagmatic eastern domain of profile 1. The axial domain between profiles 2 and 1 is characterized by several volcanic ridges (Figure 4), that formed by local eruptions, on older volcanic seafloor in the west, and directly over ultramafic seafloor to the east [START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF].

Our near-axis observations therefore point to a gradual transition, over an along-axis distance of less than 30 km, from the detachment-dominated, nearly amagmatic mode of spreading, to a meltemplacement dominated, symmetrical, mode of spreading. Such a gradual transition indicates that the amount and the distribution of tectonic displacement (one axial detachment, and/or several distributed smaller offset normal faults) at slow spreading ridges are tuned with the melt supply at scales <10 km, so as to complement the melt emplacement contribution to the divergence of the two plates (Figure 6).

DISCUSSION

Full lithosphere sketches of an ultraslow and magma-poor ridge

Figure 6 illustrates the concept of a gradual transition from the magmatically robust volcanic spreading mode, to nearly amagmatic, detachment-dominated spreading, with tectonic displacement on faults being tuned over the width of the axial domain, to complement melt emplacement so that the two plates diverge at the spreading rate that is imposed by far-field plate tectonics forces. This is similar to the interpretation proposed by [START_REF] Macleod | Life cycle of oceanic core complexes[END_REF] for the along-axis transition from detachmentdominated spreading to more volcanic ridge domains at 13°N on the Mid-Atlantic Ridge (MAR); and an alternative to the rafting detachment model developed by [START_REF] Reston | The 3-D geometry of detachment faulting at mid-ocean ridges[END_REF] and by [START_REF] Reston | Flipping detachments: The kinematics of ultraslow spreading ridges[END_REF], in which axial detachments are proposed to continue at depth beneath rafted blocks of volcanic seafloor at slow and ultraslow ridges. We discard this alternative interpretation because of clear geological evidence that volcanic rocks in the study area were erupted in situ (Figure 4 Figure 6 shows the upper 10 kilometers of the axial domain. Microearthquake depths reported by [START_REF] Schlindwein | Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere[END_REF], indicate that the seismogenic lithosphere thickens over an along-axis distance of less than 50 km, from less than 10 km-thick beneath the 65.6°E axial volcano (Figure 1), to about 25 km-thick at the eastern edge of the 64.6°E nearly amagmatic corridor. Assuming that a similar thickening occurs with distance from the 64°E volcano (Figure 1), we infer that the volcanic seafloor at the longitude of profiles 3 and 4, and the nearly amagmatic seafloor at the longitude of profile 1 probably have seismogenic lithosphere thicknesses of about 10-15 km, and 20-25 km, respectively.

Transitioning from the two spreading modes therefore also involves thinning the axial seismogenic lithosphere by at least 10 km, over an along-axis distance of ~30 km. In the following paragraphs, we discuss full-lithosphere conceptual sketches based on the present-day geology of the eastern SWIR axis, for the nearly amagmatic and for the volcanic spreading modes.

Nearly amagmatic, detachment-dominated spreading mode. Detachment #1 (Figure 1) presently accommodates nearly all the plate divergence. It cuts through the thick axial brittle lithosphere and roots into ductile shear zones that are documented in dredged ultramafic samples (Bickert et al., in prep.) at the base of the plate (Figure 7a). This detachment is at an early stage (~300 kyrs of activity for a typical detachment lifetime of 1 to 1.5 myrs; Table 1) and has so far caused an estimated maximum footwall flexure of 15° (Appendix Figure A1). This flexure could at least partially be accommodated by reactivation of former detachment #2, accounting for the change of slope observed across the proposed emergence of detachment #2 (E2; Figures 1 and7a). Sub-horizontal and dipping seismic reflectors documented in the axial valley basement are interpreted as minor conjugate faults and magmatic intrusions in the hanging wall of detachment #1, some of which fed the sparse basalt patches observed at the seafloor [START_REF] Momoh | Quasi-3-D Seismic Reflection Imaging and Wide-Angle Velocity Structure of Nearly Amagmatic Oceanic Lithosphere at the Ultraslow-Spreading Southwest Indian Ridge[END_REF]. These basalts are compositionally distinct from those erupted in the nearby volcanic seafloor domains and, based on their composition, it is unlikely that they erupted from dikes propagating along-axis from these volcanic domains [START_REF] Paquet | Effect of melt/mantle interactions on MORB chemistry at the easternmost Southwest Indian Ridge (61°-67[END_REF].

Seafloor imagery (Sauter et al., 2013 and Figure 4) and submersible observations (Cannat et al., AGU abstract, 2017) indicate that most basalt patches are spatially associated with faults that guide melts in the upper levels of the lithosphere (Figure 7a). Melt infiltrations, and dikes intruded at greater depths and presumably for the most part into the footwall plate (Figure 7a) are also documented in peridotites samples from the smooth seafloor domains [START_REF] Paquet | Effect of melt/mantle interactions on MORB chemistry at the easternmost Southwest Indian Ridge (61°-67[END_REF].

Faults and the associated damage also probably channel hydrothermal fluids, so that serpentinization would occur for a good part in and next to the main axial detachment [START_REF] Andreani | Dynamic control on serpentine crystallization in veins: Constraints on hydration processes in oceanic peridotites[END_REF]Rouméjon and Cannat, 2014). Sample studies indicate that serpentinization at slow ridges is a multiphase reaction, occurring for the most part in the most kinetically favorable temperature range of 200-350°C, with an initial stage of formation of the serpentine mesh texture at low fluid-rock ratios, followed by several stages of veining and recrystallization of the initial serpentine at higher fluid-rock ratio [START_REF] Andreani | Dynamic control on serpentine crystallization in veins: Constraints on hydration processes in oceanic peridotites[END_REF][START_REF] Früh-Green | Serpentinization of oceanic peridotites: Implications for geochemical cycles and biological activity[END_REF]Rouméjon et al., 2014). Based on hypocenter depth distribution in melt-poor domains next to the 65.6°E axial volcano, [START_REF] Schlindwein | Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere[END_REF] proposed that serpentinization there initiates at depths of ~15 km below seafloor (Figure 7a).

Temperatures in the 200-350° C range could, however, also be found at shallower, near seafloor depths in the upflow zone of high temperature, black smoker-type, hydrothermal circulation cells. Such high temperature hydrothermal upflows have not been discovered so far in the area, and given that magmatic rocks are scarce, they should not represent the dominant hydrothermal regime. Yet black smoker-type circulations may develop transiently to cool isolated melt intrusions and trigger episodes of rapid serpentinization in the surrounding ultramafic rocks, both in the footwall and in the hanging wall plates (Figure 7a). Slower, lower temperature serpentinization, as yet not documented by sampling, may also occur in the tectonically damaged footwall and hangingwall domains.

At the current early stage of activity of detachment #1, the axial lithosphere, which we infer formed in the footwall of detachment #2 (Figure 3), is 0.6 to 1.5 myrs-old and could thus be thicker than if controlled solely by the active balance between present-day on-axis heat supply and heat loss. Heat in this spreading mode would be supplied primarily by conduction, by tectonic advection of hot mantle, and by serpentinization reactions. Although advected and latent heat from the sparse melt intrusions could support active black smokers, this efficient mode of hydrothermal cooling would be highly episodic and most hydrothermal circulations would occur at lower fluid-rock ratio in tectonically damaged domains. The resulting axial temperature gradient should therefore be nearly conductive (Figure 7c). Over time and as detachment #1 accommodates more displacement, greater footwall flexure will occur, lowering the fault emergence angle and causing enhanced internal deformation and serpentinization. Also, because of the near-zero melt input, the detachment system should migrate in the direction of the hanging wall and away from the axial region of higher heat input and thinner lithosphere, ultimately favoring the initiation of a new detachment [START_REF] Buck | Modes of faulting at mid-ocean ridges[END_REF].

More magmatically robust volcanic spreading mode. The gravity-derived crustal layer on-axis at the longitude of profiles 3 and 4 is 6 to 7 km-thick [START_REF] Cannat | Modes of seafloor generation at a melt-poor ultraslow-spreading ridge[END_REF]. Plate divergence there thus concerns an axial lithosphere that is thinner than in the nearly amagmatic setting sketched in Figure 7a, but still thicker than the crust. Hence, even if magmatic dikes and gabbro bodies do accommodate most plate divergence in the upper lithosphere, plate divergence acting on the deeper axial lithosphere still requires tectonic uplift of mantle material from the asthenosphere, and through-going melts may form mechanically weak intrusions in this axial lithospheric mantle [START_REF] Cannat | Emplacement of mantle rocks in the seafloor at mid-ocean ridges[END_REF]. Melts intruded higher up, into the brittle lithosphere, will also constitute weak zones while they cool, crystallize, and release the heat necessary to fuel vigorous black smoker-type hydrothermal circulation (eg [START_REF] Lowell | Characteristics of magma-driven hydrothermal systems at oceanic spreading centers[END_REF]. The axial lithosphere in this volcanic setting (Figure 7b) should therefore not just be thinner than in the nearly amagmatic detachment-dominated setting. It should also have a different type of geotherm (Figure 7c), with an upper brittle part that is more efficiently cooled by hydrothermal convection cells, and a lower part that is kept weak and thin by frequent melt injections. Temperatures in the upper domain are expected to be variable in space and time (Figure 7c), depending on the activation and geometry of black smoker circulations.

Whether magmatic intrusions that form in the lithospheric mantle are subsequently uplifted into the crust, or remain at deep levels of the new plate as it is transported off-axis, should depend on the distribution and offset of axial faults. In Figure 7b, we propose that several of the symmetricallydistributed axial normal faults observed in profiles 3 and 4 (Figure 5) root into the melt-intruded axial lithospheric mantle, and may thus lift some of its components into the crust. The axial domain of melt emplacement may thus be thinned by faults to produce the off-axis crust. Therefore, although the melt supply is probably such that the crustal layer off-axis is primarily magmatic, we predict that some of these magmatic rocks have initially been intruded in the lithospheric mantle, then tectonically uplifted into the crust by successive moderate offset normal faults, together with small volumes of their host mantle rocks. Exhumed mantle-derived rocks have actually been dredged in the volcanic seafloor domain a few km to the north west of profile 4 [START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF]. This calls for a discussion of the applicability of the popular M ratio concept to this, and to other thick lithosphere axial settings. The M ratio as defined by [START_REF] Buck | Modes of faulting at mid-ocean ridges[END_REF] describes the fraction of the total extension of the brittle lithosphere accommodated magmatically; (1-M) is accommodated by faults. If melt emplacement accommodates plate divergence over the full thickness of the brittle plate, as modelled for slow-spreading ridges by [START_REF] Buck | Modes of faulting at mid-ocean ridges[END_REF] and [START_REF] Tucholke | Role of melt supply in oceanic detachment faulting and formation of megamullions[END_REF], M is a direct function of the melt flux (m) per increment of plate separation (m can be expressed as the cumulated thickness of melt provided per increment of spreading). On the other hand, the M ratio does not directly reflect m if melts are either also emplaced below the plate on-axis (the common case at fast and many slow ridges), or emplaced only over a portion of a thick axial brittle plate (probably the common case at ultraslow mid-oceanic ridges) . Instead, M then also depends on the thickness of the brittle lithosphere on-axis (HB), and on the thickness of the domain of axial melt emplacement (HM). [START_REF] Olive | The structure of oceanic core complexes controlled by the depth distribution of magmaemplacement[END_REF] addressed this for the HM > HB case by defining a second ratio, MD for the contribution of melt emplaced in the ductile domain. In the HM < HB case, M can be expressed as m/HB. In the volcanic seafloor configuration of Figure 7b, this formula yields a M value of ~0.6, for HB = 12 km and assuming that the off-axis crustal thickness (~7 km) is a good approximation for m (ie that this offaxis crust is mostly free of mantle-derived peridotites, and that there is very little gabbro trapped in the off-axis mantle lithosphere).

Transition between the detachment-dominated and volcanic modes of spreading. Profile 2 (Figure 5) corresponds to an along-axis transition between the detachment-dominated (to the east) and volcanic (to the west) modes of spreading. Seafloor imagery and sampling [START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF] suggest that plate divergence at this longitude occurs by a combination of displacement along the westward continuation of detachment #1, along smaller offset normal faults in the axial valley, and by in-situ magmatic injections forming prominent spreading-perpendicular volcanic ridges (Figure 4). As a result, the geophysically-defined crustal layer should be more geologically composite than in the nearly amagmatic smooth seafloor configuration (Figure 7a), with proportions of serpentinized ultramafic rocks and magmatic intrusions that would depend both of recent magmatic input, and of whether the axial configuration in the recent past was more magmatic (in which case the crust offset by detachment #1 would be mostly magmatic), less magmatic (in which case this crust would be mostly ultramafic) or similarly magmatic.

In this intermediate spreading configuration, more melts would transit through and react with the lower lithospheric axial mantle than in the nearly amagmatic configuration. These deep melts would slowly release their intrinsic and latent heat into the surrounding mantle and would therefore keep the lower axial lithosphere thinner and hotter than in Figure 7a. A further increase in the melt supply would eventually allow for axial faults (and for the hydrothermal systems that develop in permeable brittle rocks) to reach down to very near these melt-rich and hot regions, leading to vigorous hydrothermal circulation. This configuration would ultimately resemble the more magmatically robust volcanic mode of Figure 7b. This leads us to conclude that the effect of melts emplaced in thick lithosphere, ultraslow spreading ridge settings goes beyond the mere mechanical accommodation of plate divergence. Melts thin and weaken the lithosphere (thereby modifying the thickness and strength of the rigid domain in which plate separation needs to be accommodated). Melts emplaced in and near the brittle lithosphere also modify the axial thermal regime by fueling vigorous, if transient, hydrothermal circulation, so that the upper and lower lithosphere are maintained at temperatures respectively cooler and hotter than the conductive geotherm. We propose that these combined effects (rheological and thermal) are key to understand how the two highly contrasted ridge settings proposed in Figure 7 transition along-axis over distances <30 km.

Spreading rate, melt supply, and the spreading mode at slow and ultraslow ridges

Spreading rate has a large impact on the thermal regime of mid-ocean ridges (eg [START_REF] Chen | A Nonlinear Rheology Model for Mid-Ocean Ridge Axis Topography The relative size of the decoupling chamber to the size lated to form the theory for the oceanic of the plastic failure zone within the brittle plate determines the sphere and an iteration sch[END_REF]. With a spreading rate about half of that of the MAR, the SWIR is expected to have a thicker axial lithosphere in places where the melt budget per unit of plate separation is the same, and a fortiori also in places where this melt budget is less (the axial lithosphere should be even thicker due to a lesser input of magmatic heat). In thick axial lithosphere settings, lithosphere thickness affects the (M) ratio between the magmatic and tectonic contributions to plate separation and could therefore be expected to be one of the main factors controlling the spreading mode [START_REF] Buck | Modes of faulting at mid-ocean ridges[END_REF]. We now assess this effect through a comparison of well-studied axial configurations at the MAR and SWIR.

The nearly amagmatic flip-flop detachment (after the terminology of [START_REF] Reston | Successive detachment faults and mantle unroofing at magmapoor rifted margins[END_REF] mode documented at the ultraslow SWIR has not so far been identified at slow ridges, an absence that probably results from more extreme degrees of along-axis melt focusing at ultraslow ridges, another consequence of having a very thick axial lithosphere [START_REF] Cannat | Melt supply variations to a magma-poor ultra-slow spreading ridge (Southwest Indian Ridge 61° to 69°E)[END_REF][START_REF] Standish | MORB generation beneath the ultraslow spreading Southwest Indian Ridge (9-25°E): Major element chemistry and the importance of process versus source[END_REF]. The other two modes of slow spreading accretion (volcanic and corrugated detachment modes) are common to both slow and ultraslow settings and therefore offer points of comparison. Microseismicity results acquired at 13°N at the Mid-Atlantic Ridge, and in the 49°E region of the SWIR indicate seismogenic lithosphere thicknesses of up to 15 km and 20 km respectively [START_REF] Parnell-Turner | Oceanic detachment faults generate compression in extension[END_REF][START_REF] Yu | Lithospheric Structure and Tectonic Processes Constrained by Microearthquake Activity at the Central Ultraslow[END_REF]. Corrugated, dome-shaped, core-complexes that form in the Antarctic plate at 49°39'E on the SWIR [START_REF] Zhao | Three-dimensional seismic structure of the Dragon Flag oceanic core complex at the ultraslow spreading Southwest Indian Ridge (49°39ʹE)[END_REF], therefore result from detachments that cut through some 20 km of brittle lithosphere, compared to up to 15 km for the 13°N MAR detachments (Figure 8). In the two cases, domal exposed detachment surfaces face volcanic seafloor in the other plate [START_REF] Macleod | Life cycle of oceanic core complexes[END_REF][START_REF] Zhao | Three-dimensional seismic structure of the Dragon Flag oceanic core complex at the ultraslow spreading Southwest Indian Ridge (49°39ʹE)[END_REF], and seismic crustal thicknesses are similar (with a large 2-5 km range of variation in the two settings). There is therefore no evidence for a significant contrast between the two settings in terms of the volume of melt supplied per increment of plate separation. Along the same line, the volcanic axial configuration in profiles 3 and 4 of Figure 4 resembles that observed at MAR segment centers, with a symmetrical topography, distributed faults and a crustal thickness of 6-7 km, yet it has an axial seismogenic plate thickness that is probably greater (10 to 15 km; [START_REF] Schlindwein | Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere[END_REF] than documented for volcanic domains of the MAR (less than 8 km; [START_REF] Wolfe | Microearthquake characteristics and crustal velocity structure at 29 o N on the Mid-Atlantic Ridge: The architecture of a slow spreading segment[END_REF], and more similar to that of corrugated detachment MAR domains (Figure 8).

These comparisons therefore suggest that the activation of the volcanic, or of the corrugated mode of spreading at slow and ultraslow ridges is connected more to the volume of melt per increment of plate separation, than to the thickness of the axial lithosphere. This is not fully consistent with the hypothesis, proposed by [START_REF] Buck | Modes of faulting at mid-ocean ridges[END_REF], that spreading modes are determined by the relative contribution of magma and faults to plate divergence, expressed as the M ratio. M is less, for a given magmatic contribution m (the cumulated thickness of melt provided to the ridge per increment of spreading), if the axial brittle plate is thicker. The volcanic and corrugated modes therefore both appear to be activated at lower M at ultraslow than at slow ridges. The explanation we now explore is that the impact of the axial lithosphere on spreading modes at slow and ultraslow ridges lies not just in its actual thickness, but also in its strength, and that this strength is affected by melt supply.

Detachment-dominated modes, corrugated or not, develop for substantial lithosphere thicknesses (Figure 8). MAR corrugated detachments are steep at depth (ca 70°; de [START_REF] De Martin | Kinematics and geometry of active detachment faulting beneath the Trans-Atlantic geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge[END_REF][START_REF] Parnell-Turner | Oceanic detachment faults generate compression in extension[END_REF] but, when mature, emerge at very low angles (<15°; [START_REF] Smith | Widespread active detachment faulting and core complex formation near 13°N on the Mid-Atlantic Ridge[END_REF], indicating low effective footwall flexural rigidity. Smooth seafloor detachments in nearly amagmatic corridors of the SWIR experience substantial, but probably not as large, footwall flexure (Momoh et al., in prep.).

Given that the axial lithosphere is not thin in either case, such large flexures call for mechanisms that weaken the detachment footwall as faulting proceeds, forming weaker domains in an otherwise thick mechanical lithosphere [START_REF] Buck | flexural rotation of normal faults[END_REF][START_REF] Lavier | Self-consistent rolling-hinge model for the evolution of large-offset low-angle normal faults[END_REF]. Most slow spreading ridge numerical models to date have implemented this effect by making rock strength a function of strain [START_REF] Lavier | Self-consistent rolling-hinge model for the evolution of large-offset low-angle normal faults[END_REF]. Geological ground truth comes from studies of samples from the exposed footwall of axial detachments. These studies show that strain localization occurs primarily in assemblages of one or several weak hydrous minerals (serpentine, chlorite, amphibole and talc; [START_REF] Boschi | Occurrence and significance of serpentinite-hosted, talc-and amphibole-rich fault rocks in modern oceanic settings and ophiolite complexes: An overview[END_REF][START_REF] Escartín | Constraints on deformation conditions and the origin of oceanic detachments: The Mid-Atlantic Ridge core complex at 15°45ʹN[END_REF][START_REF] Picazo | Deformation associated with the denudation of mantle-derived rocks at the Mid-Atlantic Ridge 13°-15°N: The role of magmatic injections and hydrothermal alteration[END_REF][START_REF] Schroeder | Strain localization on an oceanic detachment fault system[END_REF]) that form in the hydrothermally altered brittle lithosphere, at greenschists facies temperatures (<500°C). Talc, specifically, is significantly weaker than serpentine, and is not formed in substantial amounts from peridotites without the input of additional silica, via metasomatic hydrothermal fluids that have altered magmatic rocks, or directly from magmatic veins intruded in the peridotites [START_REF] Boschi | Occurrence and significance of serpentinite-hosted, talc-and amphibole-rich fault rocks in modern oceanic settings and ophiolite complexes: An overview[END_REF][START_REF] Picazo | Deformation associated with the denudation of mantle-derived rocks at the Mid-Atlantic Ridge 13°-15°N: The role of magmatic injections and hydrothermal alteration[END_REF]. Talc is uncommon in samples from the nearly amagmatic smooth seafloor (Rouméjon et al., 2014). It is by contrast common in samples from corrugated seafloor. Large footwall flexure in the corrugated detachment mode is therefore likely facilitated by talc forming from peridotite gabbro mixtures, while nearly amagmatic detachments would operate with more rigid footwalls [START_REF] Cannat | Oceanic corrugated surfaces and the strength of the axial lithosphere at slow spreading ridges[END_REF]. Higher melt supply, leading to a transition from the corrugated detachment to the volcanic mode, would also likely strengthen the brittle lithosphere [START_REF] Cannat | Oceanic corrugated surfaces and the strength of the axial lithosphere at slow spreading ridges[END_REF] because: 1-the proportion of mantlederived peridotites would become too small to form pervasive serpentine and or talc-bearing zones of weakness; and 2-new magmatic intrusions, which initially form weak melt-mush and ductile gabbro, turn, as they cool, into pristine and strong brittle magmatic rocks (Figure 8). More abundant melt, by contrast, weakens the ductile axial lithosphere. The nearly amagmatic detachment mode is therefore expected to be distinct from both the corrugated detachment, and the volcanic mode, in that it operates with a significantly stronger lower axial lithosphere (Figure 8), which would oppose large flexure and may therefore prevent the development of detachments [START_REF] Lavier | Half graben versus large-offset low-angle normal fault: Importance of keeping cool during normal faulting[END_REF]. Field evidence that detachments exist in the SWIR nearly amagmatic spreading corridors indicates that additional strain weakening mechanisms are activated. Microstructures in ductile and semi-brittle shear zones in dredged ultramafic samples show that dynamic grain size reduction in the ductile mantle lithosphere is a plausible candidate (Bickert et al., submitted).

CONCLUSIONS

We propose the following conclusions:

1-new oceanic lithosphere in the nearly amagmatic detachment-dominated mode is accreted over the lifetime of two successive detachment faults (ie over 1.5 to 3 myrs) and is for the most part made of material that experienced a two-stages tectonic, hydrothermal, and magmatic evolution: first in the footwall of one detachment fault, then in the hanging wall of the next detachment fault.

2-individual active detachments extend over the whole along-axis width of nearly amagmatic seafloor spreading corridors, up to 95 km in the case studied here. They accommodate up to 20 km of plate divergence, and probably cut through a very thick axial lithosphere (≥25 km). Breakaways are straight, cutting into ultramafic seafloor that was exhumed up to 400 kyrs previously in the former detachment fault's footwall. The emergence of mature detachment faults shows 15-20 km-long undulations, which we propose are due to along-axis changes in the rheology of the detachment footwall, impacting the fault's emergence angle.

3-the along-axis transition from nearly amagmatic, detachment-dominated spreading, to more magmatic spreading next to axial volcanic centers, occurs gradually over distances of < 30 km through a fine tuning of tectonic displacement on faults to the local rates of melt emplacement, so as to accommodate the plate divergence. We propose that this gradual transition involves two main processes that are both related to the increase of melt supply: 1-more melt reduces the need for displacement on faults as a mean to accommodate plate divergence; 2-more melt heats and weakens the deep axial lithosphere, thus reducing the thickness of the rigid domain over which divergence needs to be accommodated. 4-A comparison with the slow spreading MAR further suggests that the activation of the volcanic or detachment-dominated modes of spreading at slow and ultraslow spreading ridges is primarily controlled by the melt budget (the volume of melt supplied per unit of plate separation), over a range of axial lithosphere thickness (the thickness of the plate over which divergence must be accommodated, decreasing with spreading rate at a given melt budget). Spreading modes are therefore not determined solely by the relative contribution of magma and faults to plate divergence (the M ratio of [START_REF] Buck | Modes of faulting at mid-ocean ridges[END_REF], since this relative contribution also depends on axial lithosphere thickness . We propose that the rheology of the axial lithosphere is a key additional parameter, that also depends on the melt supply. Melt and melt products modify the strength of the brittle lithosphere, resulting in contrasted potential for strain localization and footwall flexure, the strongest configuration being that of the most magmatically active (volcanic) mode, and the weakest and most favorable to long-lasting detachments and large flexure, being that of the moderately magmatic (corrugated) detachment mode. Melt heat can also sustain vigourous, black smoker type hydrothermal circulations that impact the thermal state of the lithosphere and may favor enhanced hydrothermal alteration.

Finally, the presence of melt weakens the ductile mantle at the base of the brittle plate, so that the nearly amagmatic detachment mode probably operates on axial domains that are characterized by a strong ductile lithospheric mantle that is lacking in the more magmatically active corrugated detachment and volcanic modes of spreading.

In order to map past detachment faults in the nearly amagmatic spreading corridor of Figure 1, and to estimate their age and lifetime (Table 1), we picked the top of the inward-facing (toward the axis) slope of each detachment-controlled ridge as the best approximation for the breakaway, and the base of their outward-facing slope as the best approximation for the emergence. This approach yields uncertainties, primarily due to the effect of landslides, as illustrated in Figure A1 for presently active detachment #1. For young detachments, the (eroded) breakaways (B in Figure A1) may be a few hundred meters closer to the ridge axis than the top of the inward-facing slope of the detachment ridge, but this discrepancy should diminish and eventually disappear as flexure proceeds so that the outward facing slope of the detachment ridge steepens and also gets subjected to landslides (Figure A1f). Similarly, the emergence of an active detachment may be several hundred meters closer to the breakaway than the base of the detachment-controlled axial relief, which may correspond instead to the front of landslide deposits (Figure A1d). For past detachments, however, this domain has been captured into the footwall of a more recent detachment, probably causing flexure-related deformation and further obscuring the relation between the base of the slope and the past location of the emergence. We estimate that picking the emergence of past detachments at the base of the outwardfacing slopes is the most straightforward approach, although it may introduce errors of up to 2 km / 142 kyrs in the estimated displacement/duration of individual detachments. This is a large error on individual detachments. However, because seafloor spreading in the study area has almost fully been accommodated by detachment faulting from the initiation of detachment #6 (6.1 myrs ago; Table 1), to the present, individual errors made with our method on each detachment must be compensated on the others, within 2 km. Panels A1c to A1a are back in time, and panels A1e to A1f into the near future, assuming a progressive footwall flexure, resulting in a decrease of the faults emergence angle from an initial 50° dip (although not shown, we assume that fault dips also increase with depth); and a 20° slope stability angle for serpentinites [START_REF] Cannat | High-resolution bathymetry reveals contrasting landslide activity shaping the walls of the Mid-Atlantic Ridge axial valley[END_REF], which is maintained at each step by landslides so that the eroded cross sectionnal area equals that of landslide deposits. B is the location of the actual breakaway, and B' is the location of the top of the ridge-ward slope at each step. The distance between B and B' results from mass-wasting and is expected to decrease with increasing offset and flexure along the detachment. E is the emergence of the fault with respect to the hanging wall plate, and E' is the emergence from the landslide package. Based on this sketch, our best estimate for the horizontal offset/duration of detachment #1 are about 4 km/300 kyrs (Table 1). detachments in the 64.6°E nearly amagmatic corridor; the volcanic scarps and ridges in the more magmatically active volcanic domains to the east and west; the contours of these volcanic domains, and of several corrugated detachment surfaces and pickings for magnetic anomalies 0, 3A and 5 [START_REF] Cannat | Modes of seafloor generation at a melt-poor ultraslow-spreading ridge[END_REF]; the recovered lithologies in 16 dredges [START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF]; and the location of the cross-section shown in Figure 2. Detachment faults breakaways (B) and emergences are numbered from 1 (presently active) to 8 (initiated at the time of magnetic anomaly 5; ~11 Myrs ago). and axial magnetic anomaly pickings as in Figure 1) and on a residual mantle Bouguer anomaly map [START_REF] Cannat | Modes of seafloor generation at a melt-poor ultraslow-spreading ridge[END_REF]. Panel c: seafloor is colored as in Figure 2 for ultramafic seafloor, and in brown for volcanic seafloor; magnetic anomalies 0 (open star), A1 and A2 (closed stars, grey when picking is ambiguous) as in [START_REF] Cannat | Modes of seafloor generation at a melt-poor ultraslow-spreading ridge[END_REF]. Figure 6. 3D perspective sketch of the axial region between across-axis profiles 1 and 3 (in blue; location and tectonic interpretation of these profiles in Figure 5). The concept illustrated here is that tectono-magmatic processes that accommodate plate divergence are tuned so that along-axis variations of melt supply are compensated by greater tectonic displacement on faults. In the nearly amagmatic configuration of profile 1, most plate divergence occurs at a single detachment fault, while in the more magmatically active configuration of profile 3, only 30 km away along-axis, the same total amount of divergence occurs through distributed faulting and magmatic injections. This sketch only goes down ~10 km into basement and therefore does not fully reach to the base of the lithosphere, particularly in the nearly amagmatic configuration, where it probably is >25km-deep [START_REF] Schlindwein | Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere[END_REF]. Full plate 2D sketches for the configurations of profiles 1 and 3 are shown in Figure 7. 5). The base of the brittle lithosphere in a and b is drawn from constraints on the depth of seismicity [START_REF] Schlindwein | Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere[END_REF]. The crust (grey) is a low density, low seismic velocity layer. It is sketched on the basis of gravity and seismic velocity constraints [START_REF] Cannat | Modes of seafloor generation at a melt-poor ultraslow-spreading ridge[END_REF][START_REF] Momoh | Quasi-3-D Seismic Reflection Imaging and Wide-Angle Velocity Structure of Nearly Amagmatic Oceanic Lithosphere at the Ultraslow-Spreading Southwest Indian Ridge[END_REF], and formed by the combination of magmatic flows, intrusions, fault damage, and serpentinization. For clarity, the axial sketches only show the most recent magmatic flows, intrusions, and infiltrations, and the most recently serpentinized domains. See text for explanations. [START_REF] Parnell-Turner | Oceanic detachment faults generate compression in extension[END_REF][START_REF] Schlindwein | Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere[END_REF][START_REF] Wolfe | Microearthquake characteristics and crustal velocity structure at 29 o N on the Mid-Atlantic Ridge: The architecture of a slow spreading segment[END_REF][START_REF] Yu | Lithospheric Structure and Tectonic Processes Constrained by Microearthquake Activity at the Central Ultraslow[END_REF].

Thermal gradients in the lower lithosphere are inferred to be higher at higher melt supply (see Figure 7c and text). Lithosphere strength (not scaled) is a function both of lithosphere thickness, and of two types of weakening mechanisms: hydrothermal alteration yielding weaker minerals (simplified here as a choice between serpentine or mechanically weaker talc; see text); and melt injection, which initially weakens the plate as the melt cools, then could make it locally stronger (if cutting though weak, hydrothermally altered exhumed mantle; see text). These weakening mechanisms differ between the 3 spreading modes and are proposed to contribute to the transition from one mode to another at varying melt supply (see text).

  and recent submersible observations; Cannat et al., AGU abstract 2017).

Figure A1 .

 A1 Figure A1. Conceptual sketches illustrating the evolution of presently active detachment #1. This detachment is at an early stage. Its proposed present-day configuration is shown in A1d, with the measured axial bathymetric profile in blue (no vertical exaggeration). The dip (~35°) and location of the emergence are based on recent submersible observations (Cannat et al., AGU abstract 2017).

  along the profile of Figure2.

FIGURE CAPTIONS Figure 1 .

 CAPTIONS1 FIGURE CAPTIONS

Figure 2 .

 2 Figure 2. Tectonic interpretation of the along flowline cross-section located in Figure 1, modified after (Sauter et al., 2013), showing the proposed sequence of successive detachments leading to the continuous exposure of ultramafic rocks in the two diverging plates in smooth seafloor domain (detachments #6 to 1), or in the northern, African plate, facing volcanic seafloor in the Antarctic plate (corrugated-volcanic seafloor domains; detachments #8 and 7). Stars: pickings of anomaly A5 in nearest shipboard magnetic profile[START_REF] Cannat | Modes of seafloor generation at a melt-poor ultraslow-spreading ridge[END_REF].

Figure 3 .

 3 Figure 3. Tectonic interpretation of the along flowline cross-section located in Figure 1. Zoom to the near-ridge region, showing the ages estimated (text andTable 1) for the initial exposure of ultramafic rocks in the footwall (FW) of each detachment, and for subsequent tectonics, incipient volcanism, and hydrothermal alteration in the hanging wall (HW) of the next detachment.

Figure 4 .

 4 Figure 4. Perpective views of the near axis region in the 64.6°E nearly amagmatic corridor. abathymetry (depth scale as in Figure1) with proposed detachment fault breakaways B1 and B2, dredge tracks (in grey, recovered rocks are > 90% ultramafic;[START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF]), and outlines of the TOBI (Towed Ocean Bottom Instrument) survey[START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF]. b-TOBI reflectivity image drapped on bathymetry. Hummocky basaltic outcrops rest on less reflective ultramafic seafloor. They tend to form spreading-perpendicular ridges (a1-a2 and b1-b2), that align with small offset faults of the ultramafic basement (a2 to a4 and b2 to b3; these faults are more visible in the bathymetry of panel a). The location of the emergence of detachment #1 is based on recent submersible observations(Cannat et al., AGU abstract 2017).

Figure 5 .

 5 Figure 5. Tectonic interpretation of 4 along flowline cross-sections distributed over the transition from nearly amagmatic seafloor in the east (profile 1), to fully volcanic seafloor in the west (profiles 3 and 4). Panels a and b show the location of these profiles on bathymetry (depth scale, tectonic features

Figure 7 .

 7 Figure 7. No vertical exaggeration, full plate 2D conceptual sketches of the ridge axis (a and b) and of axial geotherms (c) for across-axis profiles 1 and 3 (location in Figure 5). The base of the brittle

Figure 8 .

 8 Figure 8. Conceptual sketches of axial geotherms (a) and lithosphere strength profiles (b) for the nearly amagmatic smooth seafloor detachment mode, the weakly magmatic corrugated detachment mode, and the magmatically more robust volcanic spreading mode, at slow and ultraslow ridges. Depth to the base of the brittle lithosphere is based on constraints on the depth of seismicity at the MAR and SWIR (Parnell[START_REF] Parnell-Turner | Oceanic detachment faults generate compression in extension[END_REF][START_REF] Schlindwein | Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere[END_REF][START_REF] Wolfe | Microearthquake characteristics and crustal velocity structure at 29 o N on the Mid-Atlantic Ridge: The architecture of a slow spreading segment[END_REF][START_REF] Yu | Lithospheric Structure and Tectonic Processes Constrained by Microearthquake Activity at the Central Ultraslow[END_REF].

TABLE 1 .

 1 Characteristics of successive axial detachments as measured and estimated (see Appendix)

Acknowledgements:

We thank our two reviewers for their work and several helpful comments. We acknowledge partial funding by CNRS-INSU post cruise research grants obtained in 2015 and 2017 and by ANR project "Ridge-Factory-Slow". This is IPGP paper #4025.

APPENDIX

Mapping of axial detachment faults and estimating their age and lifetime in melt-poor oceanic lithosphere


Marc Khalife
email: marc.khalife@aphp.fr

Pascal Khalife

Hugues Pascal-Moussellard

Dr Marc Khalifé

M.Sc Pascal Khalifé

Sacrum osteotomy to change pelvic parameters: Surgical technique and 3D modeling

Keywords: Sacrum osteotomy, pelvic parameters, pelvic incidence, sagittal alignment

come

INTRODUCTION

These past years, sagittal balance has been an increasing concern for spine surgeons, especially regarding deformity correction [START_REF] Roussouly | Classification of the normal variation in the sagittal alignment of the human lumbar spine and pelvis in the standing position[END_REF][START_REF] Laouissat | Classification of normal sagittal spine alignment: refounding the Roussouly classification[END_REF]. Relationship between pelvic parameters and morphology of the spine has been demonstrated and is now commonly admitted [START_REF] Legaye | Pelvic incidence: A fundamental pelvic parameter for three-dimensional regulation of spinal sagittal curves[END_REF]. Some clinical situations, such as great sagittal imbalance, high-grade isthmic spondylolisthesis or sacral malunion could require a sacral osteotomy in order to decrease pelvic parameters, horizontalize the sacrum or correct sacral malunion.

Here is described a novel technique to perform a two-stage sacral osteotomy to decrease pelvic parameters with a lumbo-pelvic construct, with first a sacral slope decrease, then a pelvic tilt decrease.

SURGICAL TECHNIQUE

Patient is in prone position, ideally on an orthopedic table. Posterior median approach extends from L3-L4 level to S2-S3.

First stage: Decreasing Sacral Slope

The first stage of the surgical technique consists in implanting pedicle screws in L4, L5 and S1. A Smith-Petersen-type osteotomy and a discectomy are performed at the L5-S1 level in order to allow greater motion during later maneuvers. Two temporary rods are then put in place before achieving a distraction maneuver, which is locked by tightening the locking screws. This distraction creates kyphosis in the L5-S1 level thus decreasing sacral slope. It is mandatory to lock this distraction so the closing effect of the osteotomy to be performed is not transmitted to L5-S1 causing lordosis thus sacral slope increase. Two iliac screws are then implanted in both iliac wings.

Second stage: Decreasing Pelvic Tilt

Once the implants put in place, a complete laminectomy of S1 and S2 is performed, that allows locating and protecting neurological structures: dural sack, S1 and S2 nerve roots. This osteotomy procedure is risky and must be performed after thorough preparation because of the numerous noble structures that surround the sacrum. Indeed, there are the sacral nerve roots exiting the sacrum through the anterior foramens, L4 and L5 nerve roots located on the anterior aspect of the sacrum alas and sacro-iliac joints. Regarding blood vessels, there are the common iliac vessels on the superior part of the sacrum alas and, lower, the internal and external iliac vessels.

The osteotomy is then performed using a bone chisel, starting with the two transversal cuts. It is a PSO-type osteotomy (Pedicle Substraction Osteotomy), more common in lumbar spine surgery, with anterior apex and posterior basis. The angulation between the two cuts is determined according to the correction needed. The second part of the osteotomy consists in performing two vertical cuts. These cuts start at the basis of the S1 facets on the external aspect, and end at the distal transversal cut, on the internal aspect of the sacral foramens. S1 and S2 nerve roots must be individualized and dissected far enough anteriorly in the sacral foramen as they can be injured during this procedure.

The sacral body is then emptied between the two transversal cuts and the anterior cortical hinge is cautiously ruptured. There is now an experimental "U-shape" sacrum fracture and a lumbopelvic disjunction.

The osteotomy is then closed, using the orthopedic table by moving hips in extension, as well as using the ancillaries classically used in spine surgery to put contraction between pelvic construct (double iliac anchorage) and the spinal one (L4-S1 screws). Thanks to the instrumentation going up to L4, it is possible to close in compression using a domino connector. Thus, by contracting posteriorly on the osteotomy site, pelvic tilt decreases through a posterior shift of the femoral heads. After the osteotomy closure, the L5-S1 level can be contracted to increase lordosis.

3D MODELING

Osteotomy simulation

In order to confirm our hypothesis, we first performed tridimensional reconstructions of the lumbar spine and pelvis, made from CT-scan images of a healthy individual (SolidWorks, v. 2016). We were then able to simulate she sacral osteotomy that has been planned, step by step, as follows: a) Initial parameters This is the tridimensional model used, at t0 (Figure 1). Pelvic parameters values (pelvic incidence (PI), sacral slope (SS) and pelvic tilt (PT)) were as follows:

PI= 38° / SS= 31° / PT=7° b) After distraction in L5-S1
View of the pelvis after distraction at the L5-S1 level. The transversal cuts are also represented, randomly set at 25° (Figure 2).

PI= 38° / SS= 25° / PT=13° c) Posterior view of the osteotomy

This is a posterior view of the sacrum representing the vertical cuts of the osteotomy, starting lateral to S1 pedicles and ending medial to dorsal sacral foramens (Figure 3).

d) Pelvis after the osteotomy

This is the final image after sacral osteotomy contraction (Figure 4). The goal of decreasing pelvic incidence, and all pelvic parameters is achieved.

PI = 20° (-15°) / SS= 25° (-6°) / PT= -5° (-11°)

Mathematic formula

Same CT-scan images of a healthy low-PI individual were used to define a new twodimension pelvis. Initial pelvic parameters were increased manually to obtain a high-PI pelvis, closer to the indications of performing the osteotomy, and geometrical constraints were fixed in the baseline 2D model to avoid manual measuring for each sacral osteotomy angle (SOA) increment and associated bias.

Several osteotomies with various angles have been simulated using this twodimension pelvis defined by the following pelvic parameters: a pelvic incidence of 70°, a sacral slope of 52° and pelvic tilt of 18° (Figure 5). Theses osteotomies were located between S1 and S2, as described in the surgical technique. The series of osteotomies, ranging from 1 to 38 degrees, with an increment of 1°, have been simulated to assess the evolution of PI in relation to sacral osteotomy angle.

A linear relationship was observed between PI and SOA, according to the following equation:

With: PI0: the initial pelvic incidence PIF: the resulting pelvic incidence

There was a strong relationship between this linear model and values observed after simulation, with an R 2 of 0.9993. A calculation of the error was performed for each simulated osteotomy based on the calculation of PIF with the linear equation and the actual measured values. The mean error measured was 1.06% with a standard deviation of 0.44.

The relevant parameter for surgical planification being the SOA, the equation to use to plan sagittal balance improvement is the following:

With PI: the desired PI variation.

CADAVER STUDY

The osteotomy previously described has then been performed on a cadaver.

Implants have been set in place after exposure (Figures 6a and6b). Then, the Smith-Petersen osteotomy and discectomy at the L5-S1 level were performed (Figure 7). After isolation of the neural elements (Figure 8), the horizontal cuts were made (Figures 9a andb). The vertical cuts were then performed, by protecting L5, S1 and S2 nerve roots (Figure 10). The rods and connectors have then been set and contraction was performed thus achieving osteotomy closure (Figure 11).

Videos of contraction and distraction of the osteotomy site show interesting mobility resulting from the osteotomy.

DISCUSSION

  Several pelvic osteotomies have already been described. Certain techniques were initially designed in pediatric surgery in the treatment of hip dysplasia such as Chiari's or Salter's[START_REF] Patil | The Chiari medial displacement osteotomy[END_REF][START_REF] Schulze | Salter innominate osteotomy : Indications, surgical technique, results[END_REF]. More recently, several authors described first sacral vertebra osteotomy techniques, in a fashion similar to pedicle substraction osteotomies (PSO) performed in spine deformity corrections. Hsieh et al. published in 2007 a PSO performed at the S2 level with a L2-Iliac construct to correct a sacrum kyphotic malunion [6]. The osteotomy extended laterally from one sacro-iliac joint to the other. The patient observed two weeks bed rest after surgery. A 23° decrease of the PI has been noted. Ozturk et al. described, a decade later, an osteotomy of S1 with L5 lower endplate resection (the patient previously had L5-S1 ALIF cage) [7]. The technique then follows similar aspects with vertical cuts along dorsal foramens and L5-S3 laminectomy. They decreased PI by 15°. Vanaclocha et al. published a feasibility study of S1 pedicle substraction osteotomies on 12 cadavers [8]. The biomechanical study showed greater mechanical features of the construct if an ALIF was associated, two iliac screws were implanted on each side instead of one and if an iliac transverse connection device was added. PI was decreased by 16°. Bodin et al. published in 2014 a study comparing different pelvic osteotomies performed on patients for correction of spinal deformities: Bilateral Salter's, modified Salter's, Chiari's and S1 PSO [9]. Five patients were treated with S1 PSO, with no complications except transient L5 palsy in two patients. They concluded that the most efficient techniques were S1 PSO and then bilateral Salter's osteotomy. The osteotomy proposed here and performed on specimen results from a thorough reflection about pelvic parameters and tridimensional reconstruction with the definition of a mathematic formula describing the linear relationship between osteotomy angle and PI change. The results of both mathematical formula and specimen surgery exhibited significant PI decrease. These results show that this technique achieves its main goal, namely decreasing pelvic parameters. The next step is to perform this osteotomy on patients to analyze its morbidity. CONCLUSION This preliminary work shows that this technique is efficient to decrease pelvic parameters by decreasing first sacral slope and then pelvic tilt, resulting in a pelvic incidence decrease. A linear relationship has been exhibited between osteotomy angle and PI decrease, as per the following formula: osteotomy angle = PI change/0.84 These results have to be confirmed by performing this technique on live patients. DECLARATIONS Funding: None. Conflicts of interest/Competing interests: Pascal Khalifé is employed as a biomechanical engineer and owns shares of NovaSpine. Marc Khalifé owns shares of NovaSpine. NovaSpine does not present competing interests with the present study.

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