Dynamic upwelling beneath the Salton Trough imaged with teleseismic
attenuation tomography
Abstract
The Salton Trough is one of the few regions on Earth where rifting is
sub-aerial instead of sub-marine. We use the relative attenuation of
teleseismic P phases recorded by the Salton Trough Seismic Imaging
Project to investigate lithospheric and asthenospheric structures that
form during extension. Map-view analysis reveals stronger attenuation
within the Salton Trough than in the adjacent provinces. We then
construct tomographic models for variations in seismic attenuation with
depth to discriminate between crustal and mantle signals with a damped
least-squares approach and a Bayesian approach. Synthetic tests show
that models from damped least-squares significantly under-estimate the
strength of attenuation and cannot separate crustal and mantle signals
even when the tomographic models are allowed to be discontinuous at the
lithosphere-asthenosphere boundary. We show that a Bayesian approach
overcomes these problems when inverting the same synthetic datasets, and
that shallow and deep signals are more clearly separated when imposing a
discontinuity. With greater than 95% confidence, the results reveal
first, that attenuation occurs primarily beneath the LAB; second, that
the width of the attenuative region is narrower than the rift at 120 km
depth; and third, that the strength of attenuation requires that the
attenuative feature represents a melting-column similar to those beneath
mid-ocean ridges. The narrow width of the melting-column below the
volatile-free solidus is inconsistent with models for passive upwelling,
where flow is driven only by rifting. Instead, we attribute the
generation of incipient oceanic crust to mantle upwelling focused by
buoyancy into a narrow diapir.