Deformation and hydrothermal alteration of gabbroic rocks in the Vema oceanic transform fault

The oceanic lithosphere along oceanic transform faults (OTFs) forms at ridge–transform intersections (RTIs) through the interplay of magmatic, tectonic, and hydrothermal processes, and can subsequently evolve via deformation within the transform fault zone itself. We investigate these processes along the southern side of the Vema OTF, which segments the slow-spreading Mid Atlantic Ridge (MAR), thus focusing on a magmatically robust RTI that contrasts with most MAR transform faults.

Significant magmatic supply to the MAR segment south of Vema is indicated by a well-developed basaltic upper crustal section exposed in the transform wall and by abyssal ridge morphology of the adjacent seafloor. The south wall of the Vema transform, and to a lesser extent its valley, have been extensively sampled. Gabbros crop out primarily at the base of the wall. Submersible observations document a steep, transform-parallel fault contact between gabbros and foliated serpentinised peridotites further down the wall.

We studied gabbroic rocks from 25 dredges and 2 dives from the base of transform wall, with more deformed ones mainly collected from depths greater than 4000 m below sea level and towards the western part of the OTF. Twenty-one representative samples were selected for petrological, geochemical and thermobarometric analyses, allowing us to identify four successive deformation regimes.

(1) A high-temperature viscous regime characterized by mylonitic shear bands with brown amphibole (Amp), ilmenite-magnetite, plagioclase (Pl), clinopyroxene (Cpx), orthopyroxene (Opx) ± apatite. Amp-Pl thermobarometry indicates deformation at ~850-950℃. Mineral textures and Ti-rich amphibole suggest melt-assisted deformation.

(2) A high-temperature semi-brittle regime marked by shear zones, cataclastic zones and fractures containing green-Amp, secondary Pl, sphene-ilmenite and Cl-rich apatite. Amp-Pl thermobarometry leads to temperatures of ~650-750℃ and pressure of 1.5-3 kbar. Significant amount of chlorine (700-2400 ppm) together with low Ti (0.065- 0.23 a.p.f.u) in the green hornblende suggest a hydrothermal fluid origin.

(3) A medium temperature semi-brittle regime with formation of green-Amp, chlorite, and sphene within fractures Cl-rich green Amp (up to 6000 ppm) again involves hydrothermal fluids. Amp-Pl thermometry gives temperatures of around 500 ℃ consistent with greenschist facies assemblage.

 (4) A low-temperature brittle regime characterized by fracturing and brecciation, with syn/post deformational globular zeolite crystallization, reflecting interaction with seawater at ~200 ℃.

These gabbros formed at the magmatically robust east-RTI, although actual contacts have not been observed, they crop out adjacent to and structurally below a well-documented upper crustal sequence of basalt lava and dikes. This suggests crystallization, and subsequent deformation, at relatively shallow depths for these gabbros, consistent with the low pressure estimated from mineral thermobarometry. Our interpretation is that the succession of deformation regimes documents the deformation style and hydrothermal alteration at relatively shallow depths in the transform. And that the large range of temperature covered by the 4 deformation regimes corresponds mostly to progressive cooling and hydrothermal alteration of lower crustal rocks during aging and lateral transport away from the RTI, with limited contribution from tectonic exhumation within the transform fault.