A window into magma-starved upper oceanic lithosphere: Structure and geomorphology of a young active mid-ocean ridge detachment fault footwall at the Southwest Indian ridge

Abstract:

Mantle-derived rocks commonly form the new seafloor at slow-spreading ridges. The area near 64°E at SWIR gives an opportunity to study the structure of the upper lithosphere beneath a N-dipping old detachment fault (DF), exhumed by an active S-dipping young (~300 kyrs; Cannat et al., 2019) DF. This active fault creates a 2700m high relief over about 7km, from the fault’s emergence near the bottom of the axial valley (~5000 mbsl) to the top of the footwall (~2200 mbsl).

HR bathymetry and ROV video reveal the upper slope of the footwall (2200-2700 mbsl) comprises several kilometer-sized benches of coherent blocks, probably resulting from a complex history of mass-wasting of the breakaway and N-slopping older seafloor. Some S-dipping scarps that limit the benches bear decameter scale triangular faces and conical debris deposits. These are interpreted as due to recent mass-wasting of older scarps. Scarps that limit other benches lack these features and are interpreted as younger. Broad slope parallel ridges (300m wide, 370m high) characterize the middle and lower slopes (2700-5000 mbsl). ROV videos reveal that these ridges show frequent outcrops with 2 dominant fabrics:

1. S-dipping, slope-parallel fault planes and cleavages that locally correspond with hectometer-sized expanses of corrugated seafloor, probably formed in the presently active DF.
2. NW, N, NE to E dipping cleavages correspond to the internal fabric of the footwall, possibly related to the previous N-dipping DF.

The entire footwall including most mass wasting-related structures is cut by arrays of nearly ridge-parallel (EW) N-facing scarps (up to 3km long, <30m high). Most of these scarps expose outcrops with NW, N, NE to E dipping cleavages and are interpreted as due to deformation of the footwall in response to tectonic bending, possibly guided by the preexisting NW, N, NE to E dipping fabrics. One of these N-dipping scarps hosts the Old city hydrothermal field (Cannat et al., Goldschmidt Conference 2019).