Imaging Subduction Beneath Mount St. Helens: Implications for Slab Dehydration and Magma Transport

Abstract

Mount St. Helens (MSH) is anomalously 35–50 km trenchward of the main Cascade arc. To elucidate the source of this anomalous forearc volcanism, the teleseismic‐scattered wavefield is used to image beneath MSH with a dense broadband seismic array. Two‐dimensional migration shows the subducting Juan de Fuca crust to at least 80‐km depth, with its surface only 68 ± 2 km deep beneath MSH. Migration and three‐dimensional stacking reveal a clear upper‐plate Moho east of MSH that disappears west of it. This disappearance is a result of both hydration of the mantle wedge and a westward change in overlying crust. Migration images also show that the subducting plate continues without break along strike. Combined with low temperatures inferred for the mantle wedge, this geometry greatly limits possible source regions for mantle melts that contribute to MSH magmas and requires lateral migration over large distances.

Shear Velocity Structure From Ambient Noise and Teleseismic Surface Wave Tomography in the Cascades Around Mount St. Helens

“…(k) Gradient model at 60‐km depth. In (h)–(k), purple contour shows the subducting oceanic crust (OC) at this depth, from Mann et al (). Yellow triangles show major volcanoes.…”

“…(c) E‐W Vs transect through the gradient model passing through MSH. Black line shows oceanic Moho from Mann et al (); plate interface is 6 km shallower. (d) Same for the Moho model.…”

“…The Moho discontinuity disappears within a couple kilometers west of the MSH edifice in Pn amplitudes (Brocher et al, ), PmP reflections (Hansen et al, ), and receiver functions (Mann et al, ). Likewise, the surface‐wave tomography presented here shows steep velocity gradients across the Moho to the east and very weak velocity gradients in the west, although the amplitude of the Moho step is sensitive to the initial model (Figures and S9).…”

“…Vertical slice follows Figure c, and top surface shows Vs at 2.5‐km depth. Blue/red shading overlay on cross‐section shows migrated receiver function image of Mann et al (). Colors on vertical slice are shaded combination of the surface‐wave color scale (SW) and that for receiver functions (RF).…”

Local Source Vp and Vs Tomography in the Mount St. Helens Region With the iMUSH Broadband Array

“…The downdip width of volcanism in this region is anomalously wide compared to the entire Cascades arc (Figure S1; Hildreth, ). Previous explanations for this behavior have included a slab tear or hole (Darold & Humphreys, ; Michaelson & Weaver, ), though analysis of receiver functions shows no evidence for a tear near MSH (Mann et al, ), or lateral migration of melt somewhere between the subducting slab and the upper crust (Hansen et al, ; Kiser et al, ). With regard to volcanism at MSH and Indian Heaven, upper crustal control has also been posited (Bedrosian et al, ).…”