- Webb Hall 1100
Dr. Barbara Romanowicz, of UC Berkeley, will give this week's Speakers Club talk entitled Mantle plumes rooted at the core-mantle boundary: evidence from seismic waveform tomography. The talk is at 2:00 PM in Webb Hall 1100.
Many questions remain on the detailed morphology of convection patterns in the mantle. In particular, the existence of deep mantle plumes has been the subject of debate ever since they were proposed to explain the presence of hotspot volcanoes. With the advent of nu- merical methods for accurate seismic wavefield computations, it is now possible to apply the tools of waveform tomography to better detect the presence, throughout the mantle, of slow velocity anoma- lies, previously “hidden” by wavefront healing effects not captured by approximate wave propagation methods. Using waveform tomogra- phy based on the spectral element method (SEM), we have recently constructed a second-generation global, radially anisotropic, shear velocity whole mantle model, which shows better focused, finer scale low velocity structures both in the upper and in the lower mantle than in any previous global tomographic models. In particular, the lower mantle structure is dominated by vertically elongated struc- tures that form discrete “columns” rooted at the base of the mantle, positioned in the vicinity of major hotspots lying over the large lower mantle low shear velocity provinces. The vertical conduits are quite straight from the base of the mantle to 1000 km depth, but wider (500-1000 km) than expected from the standard “plume” model. Their character changes above this depth, as they seem to become narrower and meander across the upper mantle, where they appear to interact with secondary scale convection set off by plate motions. I will also discuss the significance of the apparent rheological bound- ary around 1000 km depth.