A melt dynamic defined tectonic plate, evidence from the PI-LAB experiment at the Equatorial Mid Atlantic Ridge

In plate tectonic theory a weak asthenosphere facilitates the motions of the rigid plates. Partial melt could weaken the mantle, but the existence of persistent melt is controversial. Melt has been interpreted in a wide variety of characters and locations with no consensus. We use a combination of seismic S-to-P receiver functions and surface waves, with a range of resolutions and sensitivities, to image 0 – 80 Myr old lithosphere and the underlying asthenosphere near the equatorial Mid-Atlantic Ridge. We image a tectonic plate thickness that increases with age in one location but undulates in another location. We image melt in two ways: 1) ponded beneath the plate and 2) in punctuated regions of ascending partial melt several hundred kilometres off the ridge axis. This suggests melt persists over geologic timescales, although its character is dynamic, with implications for the lithosphere-asthenosphere boundary (LAB) and the driving forces of the plates. Ascending melt intermittently feeds melt at the base of the plate. Associated melt-enhanced buoyancy would enhance ridge-push in driving plate motions; whereas, ponded melt would reduce the resistance of the plates to motion. This suggests melt dynamics plays a larger role in controlling plate tectonics than previously thought.

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