Seismic evidence for a cold serpentinized mantle wedge beneath Mount St Helens

Hansen, S. M.; Schmandt, Brandon; Levander, A.; Kiser, E.; Vidale, John E.; Abers, G. A.; Creager, K. C.

doi:10.1038/ncomms13242

Cited by 54 publications

(52 citation statements)

References 42 publications

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“…Previous studies have inferred that the Moho reduction is entirely due to a slow, serpentinized forearc mantle wedge (Bostock et al, ; Brocher et al, ; Hansen et al, ). However, they only constrain velocity or impedance contrasts rather than absolute velocities, so they cannot distinguish low upper‐mantle velocities from high lower‐crustal velocities.…”

Section: Discussionmentioning

confidence: 98%

“…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).…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2019

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Mount St. Helens (MSH) lies in the forearc of the Cascades where conditions should be too cold for volcanism. To better understand thermal conditions and magma pathways beneath MSH, data from a dense broadband array are used to produce high‐resolution tomographic images of the crust and upper mantle. Rayleigh‐wave phase‐velocity maps and three‐dimensional images of shear velocity (Vs), generated from ambient noise and earthquake surface waves, show that west of MSH the middle‐lower crust is anomalously fast (3.95 ± 0.1 km/s), overlying an anomalously slow uppermost mantle (4.0–4.2 km/s). This combination renders the forearc Moho weak to invisible, with crustal velocity variations being a primary cause; fast crust is necessary to explain the absent Moho. Comparison with predicted rock velocities indicates that the fast crust likely consists of gabbros and basalts of the Siletzia terrane, an accreted oceanic plateau. East of MSH where magmatism is abundant, middle‐lower crust Vs is low (3.45–3.6 km/s), consistent with hot and potentially partly molten crust of more intermediate to felsic composition. This crust overlies mantle with more typical wave speeds, producing a strong Moho. The sharp boundary in crust and mantle Vs within a few kilometers of the MSH edifice correlates with a sharp boundary from low heat flow in the forearc to high arc heat flow and demonstrates that the crustal terrane boundary here couples with thermal structure to focus lateral melt transport from the lower crust westward to arc volcanoes.

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Section: Discussionmentioning

confidence: 98%

Section: Discussionmentioning

confidence: 99%

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

et al. 2019

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“…During the iMUSH project, a variety of multiscale seismic data were obtained including the following: an active source experiment with a total of 23 explosive sources and ~5000 geophone recording sites [ Kiser et al ., ], continuous recording by 70 broadband seismographs from 2014 to 2016 [ Ulberg , ; Crosbie , ], and a 2 week deployment of 904 continuously recording geophones within 15 km of MSH [ Hansen and Schmandt , ]. Existing products from the ongoing research project include 2‐D traveltime tomography of the crust [ Kiser et al ., ], reflection imaging of the Moho [ Hansen et al ., ], and detection and location of local microseismicity [ Hansen and Schmandt , ]. New structural imaging based on the iMUSH data has so far focused on depths greater than ~4 km.…”

Section: Introductionmentioning

confidence: 99%

Ambient noise tomography across Mount St. Helens using a dense seismic array

et al. 2017

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We investigated upper crustal structure with data from a dense seismic array deployed around Mount St. Helens for 2 weeks in the summer of 2014. Interstation cross correlations of ambient seismic noise data from the array were obtained, and clear fundamental mode Rayleigh waves were observed between 2.5 and 5 s periods. In addition, higher‐mode signals were observed around 2 s period. Frequency‐time analysis was applied to measure fundamental mode Rayleigh wave phase velocities, which were used to invert for 2‐D phase velocity maps. An azimuth‐dependent traveltime correction was implemented to mitigate potential biases introduced due to an inhomogeneous noise source distribution. Reliable phase velocity maps were only obtained between 3 and 4 s periods due to limitations imposed by the array aperture and higher‐mode contamination. The phase velocity tomography results, which are sensitive to structure shallower than 6 km depth, reveal an ~10–15% low‐velocity anomaly centered beneath the volcanic edifice and peripheral high‐velocity anomalies that likely correspond to cooled igneous intrusions. We suggest that the low‐velocity anomaly reflects the high‐porosity mixture of lava and ash deposits near the surface of the edifice, a highly fractured magmatic conduit and hydrothermal system beneath the volcano, and possibly a small contribution from silicate melt.

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“…By contrast, the forearc Moho is absent in much of the region, leading to interpretations of widespread serpentinization of the shallow forearc that require mantle temperatures too cold to allow melting (Bostock et al, 2002;Brocher et al, 2003), extending to the edifice at MSH. To explain this contradiction, Hansen et al (2016) proposed that melt migrates laterally from the backarc.…”

Section: Introductionmentioning

confidence: 99%

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

Mann

Abers

Crosbie

et al. 2019

Geophysical Research Letters

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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.

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Seismic evidence for a cold serpentinized mantle wedge beneath Mount St Helens

Cited by 54 publications

References 42 publications

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

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

Ambient noise tomography across Mount St. Helens using a dense seismic array

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

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