hide
Free keywords:
-
Abstract:
In the last 10 years, global seismic tomographic models have shown that some subducting slabs pond in the extended upper mantle transition zone (ETZ, between 660 and 1000 km depth), as do most “fat” plumes that originate at the core-mantle boundary. These plumes extend quasi-vertically across the lower mantle, but have more complicated pathways in the upper mantle, where they are more difficult to resolve, since these plumes become thinner, likely due to a lower ambient viscosity. Recent results from full waveform tomography in the Atlantic and Indian Oceans show a variety of behaviors in the ETZ, with plumes meandering towards hotspot volcanoes at the earth’s surface, and in the process, ponding at different depths in the upper mantle. Rarely does a plume extend vertically into the deep mantle directly beneath a hotspot. This is also the case beneath some well resolved continental plumes such as Yellowstone. I will review these results and their geodynamic significance. In particular, I will discuss evidence for secondary scale convection across the ETZ in ocean basins, with cells aligned with absolute plate motion and a periodicity of ~2000 km. Combining evidence from seismic tomography and the analysis of meso-scale geodetic signals in the Pacific Ocean suggests the presence of lenses of partial melt atop the 410 km discontinuity, in the upwelling limbs of the secondary scale convection, likely due to dehydration melting.
