Anisotropic Layering in the North American Upper Mantle Using a Combination of Seismological Approaches

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The formation and evolution of the oldest parts of continents is still not fully understood, and in particular why they have cold but light roots in excess of 200 km thickness that have survived destruction over billions of years. Recent seismological studies of their fine scale structure has revealed the presence of fine scale structure within these roots. There is an indication that the shallower half has a distinct structure from the deeper part which suggests that they may have formed successively, through somewhat different processes. Applying state-of-the-art tools of seismic imaging to a high quality dataset (from the USArray seismic network of Earthscope) that allows unprecedented illumination of the deep structure of the conterminous United States, down to and beyond the continental root, this project aims at detailed mapping of the major structural elements and their transitions across the north American continent. This information will serve as a starting point for other geophysicists and geochemists to further address the fundamental questions of continental formation and coupling of tectonic plates with deep mantle flow. The methodologies developed will also serve as basis for studies in other parts of the world. The proposed work will support the career development of a post-doc and a graduate student at Berkeley, and a senior female seismologist, whose influence as role model may be appreciated. Evidence for widespread layering in the cratonic upper mantle in north America comes primarily from two types of seismic studies: 1) anisotropic tomography, and 2) receiver functions (RF). Both indicate the presence of a mid-lithospheric boundary (MLD) in cratonic areas. However, the nature of this MLD is not known nor is it clear whether the two types of data see the same boundary. The goal of this proposal is to combine high resolution continental scale anisotropic waveform tomography in the craton and proterozoic provinces of north America (NA), with 1D forward modeling of local layered anisotropic structure using a novel approach based on fully non-linear trans-dimensional Bayesian inversion, in order to map major structural and clarify the nature of lithosphere and asthenosphere layering east of the Rocky Mountain Front. It will shed light on the nature of the MLD and of the lithosphere-asthenosphere boundary, which is consistently detected as an anisotropic boundary, but is not consistently detected by RFs under the craton. The models and methodologies produced in this study will be made available to other investigators through the Earthscope/IRIS websites. As part of the proposed work, we will develop general methodologies for probabilistic integration of different data types, at different scales (frequencies). We thus expect the theoretical framework and algorithms developed here in the framework of trans-dimensional inference can be extended to include other types of data and will be useful to other researchers in seismology and Earth sciences.