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

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 sea floor 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.

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 affect the way the lithosphere cools and ages. Thus, our result has implications for how tectonic plates evolve and interact with the mantle below and how plate tectonics works to cool the earth since its formation billions of years ago.