February 6 @ 4:00 pm - 5:00 pm CST
EEPS Seminar: Nicholas Mancinelli, Department of Earth, Environmental and Planetary Sciences, Brown University
Unraveling Mantle Mixing And Lithosphere-Asthenosphere Coupling From Scattered Seismic Signals
The scales of mixing in Earth’s deep mantle and the coupling of the tectonic plates to the deeper convective system bear consequence on our understanding of Earth history, evolution, and bulk chemistry. Energy radiated from large earthquakes illuminates Earth’s deep interior: the elastic vibrations we record at the surface is a superposition of scattered and transmitted waves, resulting in rich, complex waveforms that we must untangle to elucidate constraints on whole Earth structure. I first discuss how the frequency dependence of scattered-wave amplitudes can be related to scales of geochemical heterogeneity in the mantle. I explore one model in particular, where the heterogeneity is dominated by the contrast between basalt/eclogite and harzburgite in a system of chaotic convection. I address outstanding challenges to this interpretation, including determining the depth extent and lateral variability of such heterogeneity. In the second part, I show how numerical models of shear-to-compressional wave conversions place constraints on a transition fundamental to the theory of plate tectonics—that is, from the rigid lithosphere (i.e. the tectonic plate) to the more pliable asthenosphere below. In particular, I focus on the lithosphere-asthenosphere transition beneath cratons where the thickness of the lithosphere has for decades remained the subject of great debate. The results suggest that in a vast majority of cratonic regions the lithosphere-asthenosphere transition is characterized by a thermal boundary layer, in contrast to oceanic and Phanerozoic regions where the transition could be controlled by the ponding of partial melt beneath the solidus. Finally, I conclude with a discussion of future research plans, highlighting recent development of scattered-wave sensitivity kernels for higher dimensional inversion of shear-to-compressional wave observations.