Oceanic crust covers ~60% of the Earth’s surface and is continuously generating along the Mid-Oceanic Ridges (MORs). The MORs are partitioned into ridge segments of tens to hundreds kilometre long by oceanic transform faults (TFs), which are generally regarded as conservative plate boundaries with no lithosphere being neither created nor destroyed. Oceanic TFs comprise almost one-fifth of the total length of active global plate boundaries, releasing sixteen times more seismic energy than MORs and influencing the crustal accretion along spreading ridges. Despite this, the properties and nature of the deep lithosphere at oceanic TFs remain enigmatic.
The crustal accretion along MORs is known to be spreading-rate dependent. Along fast-spreading ridges, two-dimensional (2-D) sheet-like mantle upwelling creates relatively uniform crust. In contrast, the crust formed along slow-spreading ridges shows large along-axis thickness variations with thicker crust at centres of ridge segments, which is hypothesised to be due a three-dimensional (3-D) plume-like mantle upwelling with melt focusing to segment centres. However, this hypothesis has not been examined on the slow-spreading ridges in the equatorial Atlantic Ocean.
In this thesis, I perform 2-D ray-based travel time tomography of a ~855 km-long active-source seismic refraction profile to constrain the P- and S-wave velocity structures (Vp and Vs) of the lithosphere in the equatorial Atlantic Ocean. The tomographic velocity models enable me to investigate two important science questions: the properties of crustal accretion along slow-spreading ridges and the nature of the lithospheric mantle at oceanic TFs.
The Vp and Vs models across five ridge segments are obtained from tomography for crust with age varying from 8 to 70 Ma. Two categories of Vp structures are observed in the tomographic results. The dominant category exhibits a two-layered Vp structure with a ~1.9-2.3 km-thick upper crust of high Vp gradients (0.66-0.80 s-1) overlying a ~3.1-3.5 km-thick lower crust with significantly reduced Vp gradients (~0.13-0.17 s-1). These crust is characterized by Vp/Vs ratios <1.9, suggesting a mafic composition. These segments are interpreted as magmatically accreted. A small portion of the crustal segment shows Vp/Vs ratios >1.9 and a rapid increase in crustal Vp to ?7.7 km/s within ~2.2 km depth below basement, which is interpreted as a tectonically controlled crust with serpentinised peridotite in the upper crust. The crust throughout the five ridge segments has relatively uniform thickness of ~5.4-5.6 km with standard deviation of ~0.1-0.4 km, which is interpreted as due to a 2-D sheet-like mantle upwelling, contradicting previous hypothesis of 3-D plume-like mantle upwelling at slow-spreading ridges. This results also indicate that the lateral extend of magmatic accretion could be used to define the sheet-like mantle upwelling regions beneath the global ridge system. (Please refer to our preprint for more details: https://doi.org/10.21203/rs.3.rs-1366304/v1)
The tomographic Vp model also reveals the presence of a low-velocity anomaly extending down to ~60 km below sea level at the Romanche transform zone. Combined with the petrological constraints available from oceanic TFs and the modelled thermal structure for the Romanche TF, I interpret the low-velocity anomaly in the mantle at the Romanche transform zone as being caused by hydration and/or deformation of the upper mantle, where hydration leads to mantle alterations and hydrous mantle melting. The mantle serpentinisation occurs down to 16 km depth, followed by a hydrated and shear mylonite zone down to 32 km depth. A low-temperature water induced-melting zone is interpreted to reside below 32 km depth, elevating the lithosphere-asthenosphere boundary and hence thinning the lithosphere significantly at the transform zone. A thinned lithosphere will hence have a major impact on the dynamics of the ridge-transform system, and will influence the evolution of fracture zones and oceanic lithosphere. (Please refer to our accepted manuscript for more details: https://dl.ipgp.fr/sxmmv)