Eclogitization of the Subducted Oceanic Crust and Its Implications for the Mechanism of Slow Earthquakes

Until recently, the Nisutlin allochthonous assemblage, a part of the Yukon–Tanana composite terrane interpreted as trench mélange from a late Paleozoic – Mesozoic arc system, was the only tectonic assemblage known to include subducted material in the northern Cordillera. The discovery of eclogitic rocks in two parts of a klippe of the Anvil allochthonous assemblage, which comprises mafic ophiolitic rocks, above the Cassiar terrane west of the Tintina fault confirms other evidence that subducted oceanic crust was also returned to the surface. The eclogitic rocks have been largely retrograded by postsubduction metamorphism. Their existence is interpreted as additional evidence of the link between nappes above the Cassiar terrane and their inferred root, the Teslin suture zone. The Nisutlin and Anvil allochthonous assemblages can now be interpreted, not simply as crustally metamorphosed assemblages with minor, structurally interleaved high-pressure components, but as deeply metamorphosed and intensely strained slices of continental and oceanic crust switched from subducting slab to overriding plate and returned to the surface during collision of the arc with the North American margin.

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Downdip velocity changes in subducted oceanic crust beneath Northern Japan—insights from guided waves

Geophysical Journal International ◽

10.1093/gji/ggu206 ◽

2014 ◽

Vol 198 (3) ◽

pp. 1342-1358 ◽

Cited By ~ 10

Author(s):

Tom Garth ◽

Andreas Rietbrock

Keyword(s):

Oceanic Crust ◽

Guided Waves ◽

Subducted Oceanic Crust ◽

Velocity Changes ◽

Northern Japan

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Seismic array detection of subducted oceanic crust in the lower mantle

Journal of Geophysical Research Atmospheres ◽

10.1029/2007jb005263 ◽

2008 ◽

Vol 113 (B6) ◽

Cited By ~ 39

Author(s):

Sebastian Rost ◽

Edward J. Garnero ◽

Quentin Williams

Keyword(s):

Oceanic Crust ◽

Lower Mantle ◽

Seismic Array ◽

Subducted Oceanic Crust ◽

Array Detection

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Seismic scatterers in the lower mantle near subduction zones

Geophysical Journal International ◽

10.1093/gji/ggz241 ◽

2019 ◽

Vol 219 (Supplement_1) ◽

pp. S2-S20 ◽

Cited By ~ 3

Author(s):

Satoshi Kaneshima

Keyword(s):

Oceanic Crust ◽

Lower Mantle ◽

Subduction Zones ◽

P Waves ◽

Waveform Data ◽

Basaltic Rocks ◽

Short Period ◽

Subducted Oceanic Crust ◽

Conversion Depth ◽

Seismic Networks

SUMMARY We investigate the global distribution of S-to-P scatterers in the shallow to mid-lower mantle beneath subduction zones, where deep seismicity extends down to the bottom of the upper mantle. By array processing broadband and short period waveform data obtained at seismic networks, we seek anomalous later phases in the P coda within about 15–150 s after direct P waves. The later phases usually arrive along off-great circle paths and significantly later than S-to-P conversion from the ‘660 km’ discontinuity, often show positive slowness anomalies relative to direct P, and do not show a conversion depth that is consistent among nearby events. They are thus adequately regarded as scattered waves, rather than conversion at a global horizontal discontinuity. The S-to-P scattered waves often show amplitudes comparable to ‘S660P’ waves, which indicates that a spatial change in elastic properties by several percent occurs at the scatterers as abruptly as the post-spinel transformation and should arise from compositional heterogeneity. We locate prominent S-to-P scatterers beneath Pacific subduction zones and beneath southern Spain. Nearly half of 137 S-to-P scatterers located in this study and previous studies by the authors are shallower than 1000 km, and the number of scatterers decreases with depth. Scatterers deeper than 1800 km are rare and mostly weak. We examine relations between the locations of the scatterers and recently subducted slabs inferred from seismic tomography. The scatterers of mid-mantle depths, deeper than about 1000 km, are located distant from tomographic slabs. On the other hand, the majority of shallower scatterers are located beneath the slabs rather than near their fastest portions, which would indicate that chemically heterogeneous materials are not extensively entrained within thickened and folded slabs when the slabs impinge on the lower mantle. We also find scatterers near the locations where basaltic rocks of recently subducted oceanic crust are expected to exist, which suggests that oceanic crust is not delaminating when slabs impinge on the lower mantle.

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Stability of the hydrous phases of Al-rich phase D and Al-rich phase H in deep subducted oceanic crust

American Mineralogist ◽

10.2138/am-2019-6559 ◽

2019 ◽

Vol 104 (1) ◽

pp. 64-72 ◽

Cited By ~ 10

Author(s):

Xingcheng Liu ◽

Kyoko N. Matsukage ◽

Yu Nishihara ◽

Toshihiro Suzuki ◽

Eiichi Takahashi

Keyword(s):

Oceanic Crust ◽

Rich Phase ◽

Subducted Oceanic Crust

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Velocity and density characteristics of subducted oceanic crust and the origin of lower-mantle heterogeneities

Nature Communications ◽

10.1038/s41467-019-13720-2 ◽

2020 ◽

Vol 11 (1) ◽

Cited By ~ 2

Author(s):

Wenzhong Wang ◽

Yinhan Xu ◽

Daoyuan Sun ◽

Sidao Ni ◽

Renata Wentzcovitch ◽

...

Keyword(s):

Phase Transition ◽

Oceanic Crust ◽

Seismic Tomography ◽

Lower Mantle ◽

Shear Velocity ◽

Calcium Ferrite ◽

Velocity Anomaly ◽

Mantle Heterogeneities ◽

Subducted Oceanic Crust ◽

Low Shear

AbstractSeismic heterogeneities detected in the lower mantle were proposed to be related to subducted oceanic crust. However, the velocity and density of subducted oceanic crust at lower-mantle conditions remain unknown. Here, we report ab initio results for the elastic properties of calcium ferrite‐type phases and determine the velocities and density of oceanic crust along different mantle geotherms. We find that the subducted oceanic crust shows a large negative shear velocity anomaly at the phase boundary between stishovite and CaCl2-type silica, which is highly consistent with the feature of mid-mantle scatterers. After this phase transition in silica, subducted oceanic crust will be visible as high-velocity heterogeneities as imaged by seismic tomography. This study suggests that the presence of subducted oceanic crust could provide good explanations for some lower-mantle seismic heterogeneities with different length scales except large low shear velocity provinces (LLSVPs).

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