Magma-poor spreading at the Southwest Indian ridge: new insights from multichannel seismic reflection data and implications for magma-poor rifted margins

Observations at active magma-poor mid-oceanic ridges during ultraslow spreading (< 20 km/Myr full rate) are crucial for understanding the oceanization processes taking place during tectonic plate breakup. Particularly along magma-poor rifted margins, where subcontinental mantle is exhumed prior to the onset of oceanic spreading. It is hypothesized that this exhumation, occurring along detachment faults, is accompanied by a progressive increase in the magmatic budget, ultimately leading to the formation of a spreading ridge. These exhumation processes are believed to be similar to those observed in magma-poor areas along ultra-slow-spreading ridges, such as the easternmost part of the Southwest Indian Ridge (SWIR).

There, dredging revealed an oceanic basement composed of serpentinized exhumed mantle intruded by gabbros and locally overlain by variable amounts of basalts (Sauter et al., 2013). The morphology of the serpentinite ridges allowed to propose a "flip-flop" evolution of the detachment faults, characterized by alternating fault vergences. In this study, we analyse large-scale seismic reflection profiles of the Sismosmooth cruise (2014), over a series of peridotite ridges formed by flip-flop detachment processes. The absence of sedimentary cover allows for direct observation and ground-truthing of the nature of the exhumed basement at the seafloor (dredges, sub-marine images, bathymetry, TOBI side-scan sonar data). However, seismic reflection data are challenging to interpret due to the high impedance contrast between the water column and the basement, which limits wave penetration in the basement (Canales et al., 2004).

Our objective is to identify new criteria for identifying flip-flop detachment faults in contexts where the basement surface is covered by sediments, i.e. at continental margins. We also aim at identifying differences between flip-flop faulting at mid-ocean ridges and magma-poor rifted-margins. Detachment fault blocks in the easternmost SWIR form large amplitude, regularly spaced (11-18 km), mostly rounded and asymmetric ridges that expose serpentinized peridotites, locally with a thin basaltic cover. Seismic reflection data shows that the reflective top basement is locally affected by normal faults dipping mostly toward the ridge axis. Deep reflectors parallel to the top basement (~0.8 s TWT below top basement) occur locally, mostly beneath the inward-facing slopes of ridges, where the basement top is concave. We propose that they result from magma entrapment in the axial rift, when a new, antithetic, detachment fault cuts the previous one. Higher heat flow and hydrothermalism in the fault damage zone could prevent melt ascension to the seafloor.

We next look for these features (smooth reflective top basement ridges and reflectors ~0.8 s TWT below top basement) in seismic reflection profiles acquired across magma-poor rifted margins where flip-flop processes are suspected. We propose an interpretation of smooth basement ridges in the most distal magma-poor rifted margins as proto-oceanic or oceanic domains. We apply this approach to the Iberia and Antarctica fossil margins and show how this new criteria, allowing us to propose that flip-flop detachment processes took place during or directly after the final breakup of the lithospheric mantle, may help map and interpret key domains of the most distal part of magma-poor rifted margins.