Complicated lithospheric structures beneath the contiguous US revealed by teleseismic S reflections

Abstract:

Lithospheric discontinuities, including the lithosphere-asthenosphere boundary (LAB) and the enigmatic mid-lithospheric discontinuity (MLD), hold important clues about the structure and evolution of tectonic plates. However, understanding lithospheric discontinuities is hindered by limitations in current seismic methods for imaging them. P- and S-receiver-function techniques (PRF and SRF), two traditional techniques to image Earth's deep discontinuities, have major shortcomings in imaging lithosphere discontinuities. PRFs suffer interference from reverberations generated at intra-crustal interfaces, whereas SRFs generally have depth resolution too low for imaging detailed structures within the lithosphere. Here we propose a new method using reflections generated by teleseismic S waves (hereafter teleseismic S reflections) to image lithospheric discontinuities, which is generally free of interference from multiple phases and has better depth resolution than SRFs. We apply this method to data collected by the Transportable Array and other regional seismic networks and obtain an unprecedentedly high-resolution image of the lithosphere beneath the contiguous US. We observe an interface with negative impedance contrast at 80--90 km across most of the Phanerozoic western and eastern US, which significantly shallows in regions with ongoing extension, e.g., the eastern and western boundaries of the Basin and Range Province, as well as fossil rifts, e.g. the Reelfoot Rift. We interpret this interface as the lithosphere-asthenosphere boundary below the western and eastern US. We observe complicated layering in the lithosphere of the cratonic central US, which might represent fossil sutures formed during the assembling of the North America cratons. Our results show general agreement with results from PRFs, which implies the possibility of constraining the properties of lithospheric discontinuities with joint analysis of teleseismic S reflections and PRFs.