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.

Plain Language Summary

Oceanic crust emplaced along mid-ocean ridges shows significant variations in the crustal thickness and velocity structures that might be controlled by spreading rate, aging or changes in ridge crest segmentation. Here, we have analyzed seismic refraction data along a 600 km long profile covering 0–27 Ma to study the evolution of crust and upper mantle in the equatorial Atlantic ocean. The seismic velocities in the crust rapidly increase up to 6 Myr and remain fairly constant up to 27 Myr. Crustal thickness varies between 5.6 and 6 km due to the heterogeneous accretion of crust at slow-spreading ridges. At the ridge-axis, low seismic velocities in the mid-to-lower crust indicate higher temperatures greater than 1,200°C and the presence of partial melt. Our results suggest that most of the evolution in the crust and upper mantle occurs in the first 10 Myr and the structures at older ages beyond 10 Myr could shed light on the history of the past spreading processes and tectonics at mid-ocean ridges.