This study has three major goals: (1) Test for a dehydrated rheological litho- sphere beneath Iceland; (2) define the nature of Icelandic hotspot-ridge interaction; and (3) reconcile the ambiguities of previous seismic studies in the area. It has been proposed that the extraction of water from the mantle at the onset of partial melting increases the viscosity of the residue so much that it can resist convection. While such a dehydrated rheological lithosphere (DRL) can impact a wide range of processes in mantle convection, the existence of a DRL has yet to be tested against ob- servations. The Iceland hotspot is an outstanding site to perform this test because here the thickness of the DRL is likely to be most distinct from that of the (thin) thermal lithosphere, there is a source of convective mantle flow, and the area has an extensive, high-quality seismic data set. The source of convective flow is imaged seismically beneath Iceland as a low-velocity body, which is widely be- lieved to be a hot, plume-like upwelling in the upper mantle. The plume hypothesis predicts this up- welling to feed rapid mantle flow laterally along the Mid-Atlantic Ridge (MAR) but current seismic evidence for such a phenomenon is ambiguous. Arguments for a DRL come from geodynamic stud- ies of mantle plume-ridge interaction that require a DRL to hinder the rate of mantle upwell- ing/melting so as to successfully predict the thickness of Iceland?s crust. Seismic evidence, however, is contradictory. In support of a DRL, recent tomography studies reveal a layer of low velocities that extends south of Iceland with a width (>600 km) and thickness (>150 km) that are consistent with predictions of models with a DRL and distinguishably larger than those without one. The competing evidence comes from shear-wave splitting (SWS) and surface-wave measurements of seismic anisot- ropy that were interpreted to reflect crystallographic fabric due to shallow mantle flow, well within the hypothesized, non-convecting DRL. Anisotropy from SWS was also interpreted to be caused by large-scale mantle flow without the need of a mantle plume in the first place. The above goals motivate a new generation of seismic inversion scheme that directly tests the physical processes by integrating geodynamic models of mantle convection and seismic structure. Models of hotspot-ridge interaction will simulate a complete range of dynamic behaviors: from vig- orous (plume) to weak (non-plume) upwellings; and from cases with shallow lateral flow of low- viscosity mantle (no DRL) to deep lateral flow predicted for more viscous mantle (with a DRL). Pre- dicted patterns of crystallographic fabric, temperatures, and retained melt will then be used to com- pute 3D variations in elasticity tensors, from which we will generate synthetic seismograms. Misfits between the synthetic and real data will be used to identify the most and least probable geodynamics- based solutions. The ability to reject or confirm the mantle upwelling and along-axis flow that are predicted by plume theory will have broad importance to understanding hotspots. A positive test for a DRL will show that dehydration can dominate mantle rheology over a wide range of conditions, in- cluding the vigorous magmatism at Iceland. A negative result will lead to questions of the general importance of a DRL to mantle convection and could require a dramatic re-thinking of how melt is generated and transported along the Mid-Atlantic Ridge. Broader Impacts: This project has strong outreach potential because the topic of the origin of hotspots has far reaching importance to understanding absolute plate motions, mantle convection, and surface volcanism. The study will also advance a new generation of geodynamic-based seismic solution methods. The numerical codes developed will be made available to the community, such as through collaboration with the Computational Infrastructure for Geodynamics (CIG). Benefits to the broader community include the training of a graduate student and a post-doctoral scholar in interdis- ciplinary geophysics research, and supporting ongoing outreach activities of the PIs. Both of our uni- versities are located in ethnically diverse communities and are important contributors to the social, cultural, scientific, and technical resources of their respective regions. The proposed research will enrich our teaching and research programs, and provide advanced technological training and mentor- ing of science students with diverse cultural backgrounds.
