Lithospheric structure beneath the High Lava Plains, Oregon, imaged by scattered teleseismic waves

Abstract

We compute high‐resolution seismic images from scattered wavefield to detect discontinuities beneath the High Lava Plains (HLP), using data recorded at a dense broadband array. Our images of the HLP and surrounding regions reveal (1) a prominent Moho discontinuity with varying depth, with thinnest crust of 35 km beneath the volcanic track, and thickened crust of ∼45 km beneath the Owyhee Plateau (OP); (2) distinct intracrustal velocity reversals beneath regions of pre‐2.0 Ma volcanism and within the OP; and (3) intermittent negative velocity discontinuities in the uppermost mantle beneath regions of Holocene volcanism and volcanic centers near Steens Mountain and Newberry volcano. These features exhibit remarkable similarity with those seen in the surface wave tomography and Ps receiver functions. We fail to find evidence for a ubiquitous regional lithosphere‐asthenosphere boundary (LAB). In concert with petrological constraints on the equilibration depths of primitive basaltic melts, our results suggest that the present‐day HLP mantle lithosphere is thin or absent, perhaps a consequence of episodes of extensive mantle inflow, lithospheric extension, and possibly melting induced by rapid slab rollback and trench retreat. It remains possible, however, that strong E‐W seismic anisotropy reported across this region may reduce the effective S‐wave velocity contrast to render the LAB less detectable. In contrast, the Owyhee Plateau exhibits a clear LAB, consistent with it being a block of older preexisting lithosphere. Our images demonstrate the complexity of mantle dynamics in the Cascadian back‐arc and the close casual link between subduction‐related processes and the origin of HLP volcanism.