Complicated lithospheric structure beneath the contiguous US revealed by
teleseismic S reflections
Abstract
Lithospheric discontinuities, including the lithosphere-asthenosphere
boundary (LAB) and the enigmatic mid-lithospheric discontinuities
(MLDs), hold important clues about the structure and evolution of
tectonic plates. However, P- and S-receiver-function techniques (PRF and
SRF), two traditional techniques to image Earth’s deep discontinuities,
have some shortcomings in imaging lithosphere discontinuities. Here, we
propose a new method using reflections generated by teleseismic S waves
(hereafter S reflections) to image lithospheric discontinuities, which
is less affected by multiple phases than PRFs 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 new
high-resolution images of the lithosphere below the contiguous US.
Beneath the tectonically active Western US, we observe a negative
polarity reflector (NPR) in the depth range of 60–110 km, with greatly
varying amplitude and depth, which correlates with active tectonic
processes. We interpret this feature as the lithosphere-asthenosphere
boundary below the Western US. Beneath the tectonically stable Central
and Eastern US, we observe two NPRs in the depth ranges of 60–100 km
and 100–150 km, whose amplitude and depth also vary significantly, and
which appear to correlate with past tectonic processes. We interpret
these features as mid-lithospheric discontinuities below the Central and
Eastern US. Our results show reasonable agreement with results from
PRFs, which have similar depth resolution, suggesting the possibility of
joint inversion of S reflections and PRFs to constrain the properties of
lithospheric discontinuities.