Fluid-assisted deformation processes at the roots of oceanic transform faults.

Oceanic Transform Faults (OTFs) are major plate boundaries that regularly offset the axis of Mid-Oceanic Ridges and are able to generate Mw 7 earthquakes. Yet, very little is known on the evolution of fault slip mechanics at depth, mainly due to rare exposures of deep sections at the seafloor. The Atobá Ridge is one of the rare structures where the roots of an active transform fault are exposed and accessible. This transpressive ridge is part of the northern transform fault of the St. Paul transform system in the Equatorial Atlantic (Maia et al., 2016). There, ultramafic mylonites are tectonically exhumed along the inner thrust faults of a positive flower structure.

Here we study the deformation mechanisms in ultramafic mylonites and ultramylonites sampled at the Atobá ridge. We show that all samples experienced ductile deformation at 750-900°C in the spinel stability field, resulting in a pervasive grain size reduction. We propose fluid-assisted dissolution-precipitation creep as the main deformation mechanism, leading to dissolution of orthopyroxene and formation of lens-shaped olivine and interstitial minor phases’ neoblasts (pyroxenes, spinel, and amphibole). Orthopyroxene neoblasts formed by this mechanism mimic the Crystallographic Preferred Orientation of the olivine neoblasts. Fluid-assisted dissolution-precipitation creep allows deformation of stiff minerals at significant lower stresses and temperatures than dislocation creep, possibly leading to an intense strain localization. This mechanism, previously reported in ophiolites and orogenic contexts (Hidas et al., 2016; Prigent et al., 2018), is described for the first time in the oceanic transform environment. Similar microstructures have been observed in mylonites from other OTFs, suggesting that this mechanism could be more widespread and could even represent one of the main deformation law in the lower oceanic lithosphere, with important implications on the mechanics and structures of (oceanic) transform faults and long-lived detachments.

This work is supported by PRIN2017KY5ZX8.

REFERENCES

Maia, M., Sichel, S., Briais, A., Brunelli, D., Ligi, M., Ferreira, N., Campos, T., Mougel, B., Brehme, I., Hémond, C. and Motoki, A., 2016. Extreme mantle uplift and exhumation along a transpressive transform fault. Nature Geoscience, 9(8), pp.619-623.

Hidas, K., Tommasi, A., Garrido, C. J., Padrón-Navarta, J. A., Mainprice, D., Vauchez, A., Barou, F., & Marchesi, C. (2016). Fluid-assisted strain localization in the shallow subcontinental lithospheric mantle. Lithos, 262(October), 636–650. https://doi.org/10.1016/j.lithos.2016.07.038

Prigent, C., Guillot, S., Agard, P., & Ildefonse, B. (2018). Fluid-assisted deformation and strain localization in the cooling mantle wedge of a young subduction zone (Semail ophiolite). Journal of Geophysical Research: Solid Earth, 123. https://doi.org/10.1029/ 2018JB015492