Plate boundaries are the locus of both focused heat flow and intense seismic activity. A critical problem in the dynamics of plate boundaries is to understand how deformation organizes internally to produce the macroscopic strength, structure and efficiency of heat and mass transport. To address these questions from an observational perspective, we need to extract as much information as possible from the seismic wave field about the viscosity structure. The core of this project is a methodology for calculating rheological properties across the broad spectrum of time scales relevant to geophysics, from seismic wave frequencies to strain rates associated with plate tectonics, referred to here as "Very Broadband Rheology". While developing the methodology, I am applying it to a range of (proto-) plate boundary settings in a comparative approach (e.g. East African Rift and the margins of the Colorado Plateau/Basin & Range provinces), in collaboration with seismologists, petrologists, geodesists and geodynamicists.
For a given set of thermodynamic variables (including state, compositional and structural variables), we calculate the elastic, anelastic and viscous properties. However, there is a great deal of incomplete understanding in all aspects of this problem, both in seismology (in the methodology of inversion of the seismic wave field for velocity and attenuation distribution in the Earth) and in mineral/rock physics (in the processes at the micro- and meso-scales that determine the mechanical properties across the spectrum of time scales). Constructing this methodology will illuminate what is not known and also what is important to know, and how sensitive our solutions and interpretations will be to each source of uncertainty. We can assess, for example, the relative severity of the uncertainties in the thermal structure compared to uncertainty in the effects of water and melt on viscosity, attenuation and elasticity. A flexible and open source methodology for this mapping will enable the community to assess the holes and limitations in our collective understanding. The educational components of this project include the development of a graduate-level course on the observational, theoretical and experimental background for this methodology.
