Origin of Mid-Lithosphere Discontinuities in the South-Eastern US

Abstract:

A globally ubiquitous discontinuity has been observed within the continental lithosphere using multiple seismic techniques. Explanatory models include partial-melt or chemical stratification, anisotropy, and elastically-accommodated grain-boundary sliding; each with different implications for the evolution of the continental lithosphere. Interpreting the seismic results alongside other geophysical constraints e.g., conductivity and mineral physics, is valuable for testing for successful models. This is possible in the Southeastern US, where passive-source seismic and magnetotelluric experiments have been conducted. We compare our seismic results with magnetotelluric observations on the conductivity structure of the lithosphere in this region. We provide overlapping constraints on fine-scale lithospheric layering using a high-resolution Ps receiver function study. We develop an approach using a dual-bootstrap-beamform stacking that allows hypothesis testing and improved detection of weak conversions from the mid-lithosphere. The technique can also distinguish lithospheric conversions from reverberations in the overlying crust. We report a gradational and mostly-flat discontinuity at a depth of ~95 km. Taken together, these complementary geophysical constraints support a dominant role of sub-solidus grain-boundary sliding as a preferred mechanism for lithospheric layering in the south-eastern US.