Evolution of Tectonically Accreted Topmost Oceanic Lithospheric Mantle Based on Seismic Tomography Constraints at the Ultraslow‐Spreading Southwest Indian Ridge

Abstract

Oceanic lithosphere formed at the global mid‐ocean ridge system makes up >60% of Earth's solid surface. Its uppermost ∼6 km are accreted either magmatically, where mafic melt is available, or tectonically, where this melt is absent. The magmatically accreted lithosphere is known to evolve with age, as demonstrated by its velocity increase. However, whether and how the tectonically accreted lithosphere evolves remains poorly constrained, despite representing about 1/6 of the global seafloor accretion. Here, we provide the first constraints on the evolution of the tectonically accreted topmost lithosphere, composed of exhumed ultramafic rocks, at the ultraslow‐spreading Southwest Indian Ridge. The increase in seismic velocities from 0 to 6 Ma is ∼62% greater than in the topmost 6 km of magmatically accreted lithosphere, with most of this increase occurring at greater depths (1.5–4.5 km). Progressive serpentinization with aging of the topmost ∼2 km reduces velocities, partially counteracting the increase caused by crack closure and pore infilling. Thus, the overall alteration of the tectonically accreted lithosphere is likely greater than indicated by the velocity increase alone, implying that it evolves more rapidly than its magmatic counterpart. Our water content estimates further suggest that the upper 6 km of tectonically accreted topmost lithosphere can store up to three times more water than equivalent magmatically accreted lithosphere. This enhanced water storage may promote increased arc magmatism and intraslab seismicity at future subduction zones, when the Arctic, Atlantic and Indian Ocean basins begin to close, and tectonically formed lithosphere is recycled back into the Earth.