Evolution of the Crustal and Upper Mantle Seismic Structure From 0–27 Ma in the Equatorial Atlantic Ocean at 2° 43′S

Abstract

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).

Discovery of distinct lithosphere-asthenosphere boundary and the Gutenberg discontinuity in the Atlantic Ocean

“…To convert the observed mantle reflections in the seismic image from two-way time to depth, we used colocated wide-angle tomography results for the crust and upper mantle from 2 Ma to 27 Ma ( 50 ). To incorporate for thermal effects near the ridge axis, we considered two distinct age ranges from 2 Ma to 10 Ma and 10 Ma to 27 Ma of the tomography velocities.…”

“…Varying the minimum mantle velocities from 8 to 8.3 km/s ( 50 ) and the maximum mantle velocities from 8.4 to 8.6 km/s ( 51 , 52 ) gives an average mantle velocity in the range of 8.2 to 8.35 km/s. In this range of average mantle velocity, we find that the depth of the reflection varies by a maximum of ±1.2 km in the oldest part of the profile where the reflections are deepest.…”

Lower oceanic crust formed by in situ melt crystallization revealed by seismic layering

“…1 . The crustal thickness in the study area is 5.6 ± 0.2 km 14 . As expected, the tomographic velocity model contains only the large-scale velocity structures but clearly shows upper crust with high velocity gradient down to ~2.4 km below the basement underlain by a low velocity gradient lower crust 14 .…”

“…1 ), covering 7 to 12 Ma old seafloor on the African plate. The velocity model obtained using the travel-time tomography 14 (see Methods ) is shown in Extended Data Fig. 1 .…”

“…When the magma supply is low, the tectonic process dominates, and the exhumation of mantle rocks in the crust and on the seafloor can occur along detachment faults 4 . Although the upper crustal structure and accretion process have been well studied 3 , 8 – 13 , our knowledge about the lower crust remains poorly constrained 14 . The lower crust could represent two-thirds of the oceanic crust 3 , therefore it is important to understand how this part of the crust is formed and evolves.…”