Despite mid-ocean ridges (MORs) generating ~60% of the Earth’s crust, their inaccessibility dictates that they remain one of the least understood tectonic features on Earth. Ultra-slow spreading ridges, an end-member of the plate spreading process, comprise 36% (Dick et al., 2003) of the MOR system and are especially poorly understood because of their remote location (in the Arctic and Indian Oceans) and relative lack of exploration. The purpose of this study is to understand the nature of faulting and associated plate spreading at ultra-slow spreading mid-ocean ridges, using new bathymetric data and dredge samples collected in 2019 and 2020 from the Southwest Indian Ridge (see below).
The Southwest Indian Ridge (SWIR):
- Has one of the slowest spreading rates in the world (~14 mm/yr; Cannat et al., 2008),
- Spreads primarily by normal faulting instead of magmatic accretion (Dick et al., 2003),
- Has a zone of seismicity as deep as 17 km below the seafloor in its slowest-spreading segments (see ~13.5°E; Schlindwein and Schmid, 2016; Grevemeyer et al., 2019),
- And is thought to have a very thick lithosphere and deep brittle-ductile transition.
TN365 Cruise to the SWIR ~45°E in Spring 2019 recovered many samples of peridotite mylonites (see left) along the ridge axis. The ductile deformation of these rocks must have occurred below the brittle-ductile transition, implying that they were then transported ~17 km or more to the surface. Long-offset normal faulting is the dominant mode of spreading along the ~45°E segment (Dick et al., 2019). However, no faults along this segment are observed to have offsets close to the ~17 km required by Grevemeyer and others’ (2016) seismic model, raising the question: How are peridotite mylonites unroofed from >17 km depth at ultra-slow spreading ridges?