Evolution of the Oceanic Lithosphere in the Equatorial Atlantic From Rayleigh Wave Tomography, Evidence for Small‐Scale Convection From the PI‐LAB Experiment

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2020-09, Vol.21 (9), p.n/a
• Main Authors: Harmon, Nicholas, Rychert, Catherine A., Kendall, J. Michael, Agius, Matthew, Bogiatzis, Petros, Tharimena, Saikiran
• Format: Article
• Language: English
• Subjects: Anomalies; Asthenosphere; Atlantic Ocean; Convection; Downwelling; Evolution; Fracture zones; Imaging techniques; Lava; Lithosphere; Ocean bottom seismometers; Ocean floor; oceanic lithosphere; Oceans; Permeability; PI‐LAB; Plate tectonics; Rayleigh waves; Regions; Ridges; Seismic velocities; Seismometers; shear velocity; Shear wave velocities; surface wave tomography; Tectonics; Tomography; Velocity; Volcanism; Wave velocity
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2020GC009174

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  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. Plain Language Summary Tectonic plates are created at mid‐ocean ridges. As the plates age and move over the weaker asthenosphere below, they thicken and subside. Yet there are few high‐resolution images of this process. We deployed 39 ocean bottom seismometers in the equatorial Mid‐Atlantic to image the seismic velocity of the tectonic plate and the asthenosphere below. We observe a tectonic plate that generally thickens with age, but the thickening is not simple and monotonic everywhere, and there may be evidence of drips coming off the base of the plate. We also see evidence for small pockets of melt and rising mantle far away from the ridge axis. Taken together this suggests that flow in the mantle is more complicated than previously thought. Small‐scale convection may occur beneath a range of seafloor ages, which may also affe