Seismic Structure of the Upper Crust From 0-75 Ma in the Equatorial Atlantic Ocean on the African Plate Using Ultralong Offset Seismic Data

P. Audhkhasi (1) , S. C. Singh (1)

1 IPGP - Institut de Physique du Globe de Paris

The uppermost oceanic crust composes of Layers 2A and 2B, and the boundary between them is debated to be a lava/dike transition or a hydrothermal alteration boundary within the lava unit. Here, we present the analyses of ultralong multichannel seismic data along a 1,500 km long profile covering 0-75 Ma of the oceanic lithosphere on the African plate in the equatorial Atlantic Ocean. We find that the Layer 2A is observed along the whole profile, with its P velocity increasing from 2.5 km/s near the ridge axis to >4 km/s at ∼4 Ma with slight variations thereafter. We also find that the sediment thickness increases rapidly from 0 m at the ridge axis to 170 m at 4 Ma, suggesting that there is a link between the high sedimentation rate and the increase in Layer 2A velocity. These observations indicate that crust younger than 4 Myr may be influenced by active hydrothermal circulation. The observed thickness of Layer 2A decreases from ∼850 m near the ridge axis to ∼600 m at 15 Ma with no significant changes beyond. We also find an increase in Layer 2B velocity from 5.1 km/s at 4 Myr to 5.5 km/s at 46 Myr, suggesting that passive hydrothermal circulation may extend deeper than Layer 2A/2B boundary. We propose Layer 2A/2B boundary to be a lava/dike transition at the ridge axis and a hydrothermal alteration boundary within the extrusive section away from the ridge axis.

Mots clés: Equatorial Atlantic ocean; 0-75 Ma; Sediment cover; Layer 2A; Layer 2B; Hydrothermal circulation

Domaines: Planète et Univers [physics]

P. Audhkhasi, S. C. Singh. Seismic Structure of the Upper Crust From 0-75 Ma in the Equatorial Atlantic Ocean on the African Plate Using Ultralong Offset Seismic Data. Geochemistry, Geophysics, Geosystems, 2019, 20, pp.6140-6162. ⟨10.1029/2019GC008577⟩. ⟨insu-03586593⟩