Lessons Learned from Three-Dimensional Inversion of Long Period Magnetotelluric Arrays in Argentina

Abstract:

Common assumptions about the ability of a magnetotelluric (MT) array to resolve structure based on skin depth arguments and site coverage are not always true. This is because electromagnetic (EM) energy propagates horizontally as well as vertically. Thus energy entering through resistive windows can illuminate targets from the side and even from below. This also implies that structure far outside an array can sometimes be reliably imaged. These concepts will be illustrated using strategies developed to assess model feature validity in three-dimensional (3D) inversions of two large MT arrays in Argentina. We demonstrate the existence of electrically conductive plumes (possibly buoyant and conductive due to water content not temperature) that appear related to interaction of subducted slabs and the mantle transition zone (MTZ), to back-arc intraplate volcanism and to episodes of "flat" subduction in which the plate dip is substantially less than typical. Although these plumes are confirmed only in the area we have studied, we suspect that they are a worldwide phenomenon that could result in an entirely new paradigm for how the subduction process evolves after slabs leave the shallow Earth and in particular how they interact with the MTZ.