LAUSR

The oceanic lithosphere (crust and upper mantle) is formed along the mid-ocean ridges (MOR) in a 5–20 km wide zone above a mantle melting zone extending down to ∼60–100 km depth (Langmuir & Forsyth, 2007). The newly formed lithosphere subsides as it moves away from these axes and is cooled by conductive and convective processes as it interacts with ocean water (e.g., Hasterok, 2013; Lister, 1972; Stein & Stein, 1994). Although the composition and structure of crust and upper mantle is set at the MOR axis, the thermal and chemical properties likely evolve with age (Stein & Stein, 1994). Except in the regions of seamounts, which account for 5%–10% of the seafloor, the remaining 90% of ocean floor is void of intra-plate magmatism (McClain & Orcutt, 1989). This implies that hydrothermal processes are the predominant factor for crustal evolution and conductive cooling is the main factor for lithospheric mantle evolution. We can thus identify two major types of processes affecting the properties of lithosphere, the processes that contribute to its formation at the ridge axis (e.g., magmatic, tectonic, hydrothermal phenomena) and the processes that contribute to its evolution with age (e.g., conductive cooling and hydrothermal circulation). Abstract We present seismic tomographic results from a unique seismic refraction and wide-angle survey along a 600 km long flow-line corridor of oceanic lithosphere ranging in age from 0 to 27 Ma in the equatorial Atlantic Ocean at 2° 43′S. The velocities in the crust near the ridge axis rapidly increase in the first 6 Myr and then change gradually with age. The upper crust (Layer 2) thickness varies between 2 and 2.4 km with an average thickness of 2.2 km and the crustal thickness varies from 5.6 to 6 km along the profile with an average crustal thickness of 5.8 km. At some locations, we observe negative velocity anomalies (∼−0.3 km/s) in the lower crust which could be either due to chemical heterogeneity in gabbroic rocks and/or the effects of fault related deformation zones leading to an increase in porosities up to 1.6% depending on the pore/crack geometry. The existence of a low velocity anomaly beneath the ridge axis suggests the presence of partial melt (∼1.3%) in the lower crust. Upper mantle velocities also remain low (∼7.8 km/s) from ridge axis up to 5 Ma, indicating a high temperature regime associated with mantle melting zone underneath. These results suggest that the evolution of the crust and uppermost mantle at this location occur in the first 10 Ma of its formation and then remains unchanged. Most of the structures in the older crust and upper mantle are fossilized structures and could provide information about past processes at ocean spreading centers.