Evolution of the oceanic lithosphere in the equatorial Atlantic From Rayleigh wave tomography, evidence for small‐scale convection From the PI‐LAB experiment

Harmon, Nicholas, Rychert, Catherine, Kendall, Mike, Agius, Matthew, Bogiatzis, Petros and Tharimena, Saikiran (2020) Evolution of the oceanic lithosphere in the equatorial Atlantic From Rayleigh wave tomography, evidence for small‐scale convection From the PI‐LAB experiment. Geochemistry, Geophysics, Geosystems, 21 (9), [e2020GC009174]. (doi:10.1029/2020GC009174).

Abstract

The oceanic lithosphere is a primary component of the plate tectonic system, yet its evolution and its asthenospheric interaction have rarely been quantified by in situ imaging at slow spreading systems. We use Rayleigh wave tomography from noise and teleseismic surface waves to image the shear wave velocity structure of the oceanic lithosphere‐asthenosphere system from 0 to 80 My at the equatorial Mid‐Atlantic Ridge using data from the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment. We observe fast lithosphere (VSV > 4.4 km/s) that thickens from 20–30 km near the ridge axis to ~70 km at seafloor >60 My. We observe several punctuated slow velocity anomalies (VSV < 4.1 km/s) in the asthenosphere between 50 and 150 km depth, not necessarily focused beneath the ridge axis. Some of the slow velocity regions are located within 100 km of the ridge axis, but other slow velocity regions are observed at distances > 400 km from the ridge. We observe a high velocity lithospheric downwelling drip beneath 30 My seafloor that extends to 80–130 km depth. The asthenospheric slow velocities likely require partial melt. Although melt is present off axis, the lack of off‐axis volcanism suggests the lithosphere acts as a permeability boundary for deeper melts. The punctuated and off‐axis character of the asthenospheric anomalies and lithospheric drip suggests small‐scale convection is active at a range of seafloor ages. Small‐scale convection and/or more complex mantle flow may be aided by the presence of large offset fracture zones and/or the presence of melt and its associated low‐viscosities and enhanced buoyancies.

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Contributors

Author: Nicholas Harmon

Author: Petros Bogiatzis

Author: Saikiran Tharimena

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