Evidence for a narrow column of upwelling mantle beneath the Salton Trough from 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 broadband seismometers deployed by the Salton Trough Seismic Imaging Project to investigate lithospheric and asthenospheric structures that form during extension. We construct tomographic models for variations in seismic attenuation to discriminate between crustal and mantle signals with a transdimensional Bayesian approach that accurately separates shallow and deep signals, particularly when imposing a discontinuity based on previous results. 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. We consider three physical processes that can explain the quality factors allowed by the uncertainty – a “pre-melting” enhancement of attenuation at the minimally attenuating edges of our uncertainties, an enhancement of elastically-accommodated grain-boundary sliding during dehydration, or the presence of melt in an elongated geometry. Only the last mechanism provides a unified interpretation with variations in SKS splitting or can explain the modal quality factor, but the first two mechanisms cannot be rejected. However, we state with confidence that 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. This style of mantle flow commonly occurs beneath slow to intermediate spreading-rate mid-ocean ridges, and so we conclude that the Salton Trough has essentially evolved into a typical mid-ocean ridge in the asthenosphere despite the presence of both continental crust and lithosphere at shallower depths.