Seismological evidence of a dehydration reaction in the subducting oceanic crust beneath western Shikoku in southwest Japan

2020 ◽  
Vol 224 (1) ◽  
pp. 151-168
Author(s):  
Katsuhiko Shiomi ◽  
Tetsuya Takeda ◽  
Tomotake Ueno
Keyword(s):  
Active Region ◽  
Oceanic Crust ◽  
Mantle Wedge ◽  
Shear Direction ◽  
Dehydration Process ◽  
Slab Surface ◽  
Dehydration Reaction ◽  
Southwest Japan ◽  
Waveform Data ◽  
Harmonic Decomposition

SUMMARY The Philippine Sea plate (PHS) is subducting beneath southwest Japan from the Nankai Trough. In this region, deep low-frequency earthquakes/tremors (LFEs) are active and their epicentres are distributed along the downdip of the source region of M8-class earthquakes that have occurred every 100–150 yr. The LFE activity may potentially be strongly related to the occurrence of great earthquakes between tectonic plates. To investigate the structural features around the LFEs, we applied teleseismic receiver function (RF) analysis to the seismograms observed at permanent and temporal seismograph stations in western Shikoku and we detected seismological evidence of a slab dehydration reaction linked to LFE activity. Based on the first-order harmonic decomposition of RFs, we first estimated the average plunge azimuth of the PHS beneath western Shikoku. Considering the backazimuth dependence of converted phase amplitude, we constructed the cross-section of the radial component RFs, excluding the incoming waveform data from the updip directions of the dipping slab. In this profile, the parallel negative and positive P-to-S converted phase alignment within a distance of 10 km were imaged. These phase alignments corresponded to the top of the slab and the slab Moho discontinuity, respectively. At the northern side of the profile, the landward (continental) Moho was also detected. In the region where LFEs were actively distributed, both the landward Moho and slab surface were unclear. The second-order harmonic decomposition of RFs for several kilometres above the slab Moho indicated that the anisotropic symmetry axes suddenly changed at the southern limit of the LFE active region; the fast axes in the region were normal to the trench though axes in the southern area were parallel. According to the thermal and pressure condition, a phase transition with a dehydration reaction can occur in the oceanic crust near the southern edge of the LFE active region. Once the dehydration process advances, released water causes the serpentinization of the mantle material near the slab surface and the corresponding seismic velocity decreases. The impedance contrast decreases at the boundary between the lower crust and the mantle wedge, as well as that between the mantle wedge and oceanic crust; therefore, the amplitudes of the converted phases at these boundaries also become small. Considering that serpentinite ordinarily has strong anisotropy with a seismic fast axis direct to the shear direction, all features observed are evidence of the dehydration process in the flat slab.

scholarly journals Seismic scatterers in the lower mantle near subduction zones

2019 ◽  
Vol 219 (Supplement_1) ◽  
pp. S2-S20 ◽  
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.

Development of Semi-Real-Time Tsunami Monitoring and Calculation System on Ocean-Bottom Stations off the Kii Peninsula, Southwest Japan

2016 ◽  
Vol 50 (3) ◽  
pp. 76-86
Author(s):  
Takeshi Nakamura ◽  
Toshitaka Baba
Keyword(s):  
Real Time ◽  
Pressure Gauge ◽  
Seismic Source ◽  
Ocean Bottom ◽  
Source Information ◽  
Southwest Japan ◽  
Tsunami Propagation ◽  
Waveform Data ◽  
Kii Peninsula ◽  
Calculation Results

AbstractWe developed a semi-real-time calculation and data monitoring system that measures pressure perturbations at ocean-bottom pressure-gauge stations deployed off the Kii peninsula in southwest Japan in order to identify tsunami signals associated with earthquakes. The system automatically calculates geodetic deformations and tsunami propagation immediately after getting seismic source information on hypocenter, magnitude, and mechanism. The calculation results for transoceanic tsunamis can be available in approximately 20 s after getting source information to output waveform data by executing the optimized parallel calculation code on our computer server SGI UV2000 with a 32-core processor unit. The system also provides tide-removed and filtered waveform data at ocean-bottom stations, enabling the calculation results to be compared with actual tsunami arrivals. System operations began in July 2015 and have been applied to tsunamigenic earthquakes in the Pacific Ocean. The system is effective in identifying tsunami signals and automatically predicting tsunami propagation in offshore areas, which may be useful for further data analyses on tsunami propagation.

scholarly journals ASPECTOS ENERGÉTICOS E ELETRÔNICOS DA ZEÓLITA H-ZSM-5 NA AÇÃO CATALÍTICA DA REAÇÃO DE DESIDRATAÇÃO DE ÁLCOOIS

2020 ◽  
Author(s):  
Gustavo Gomes de Sousa ◽  
José Roberto dos Santos Politi
Keyword(s):  
Reaction Mechanism ◽  
Fossil Fuels ◽  
Acid Catalysis ◽  
High Efficiency ◽  
Dehydration Process ◽  
Dehydration Reaction ◽  
Alcohol Dehydration ◽  
Catalytic Behavior ◽  
Zeolite Cavity ◽  
Computational Methodology

Due to the growth of ecological concerns and the need to reduce dependence on fossil fuels, the dehydration of alcohols by acid catalysis has been used for the production of various hydrocarbons. Inside this theme, the H-ZSM-5 zeolite has been widely used as a catalyst for this reaction because its high efficiency. Thus, in order to understand the catalyzed reaction mechanism of the alcohol dehydration reaction, this work used the computational methodology ONIOM to study the catalytic behavior of the H-ZSM-5. It was modeled the dehydration reaction process for several alcohols (ethanol, propanol, isopropanol, butanol and 2-butanol) by modeling these alcohols within the zeolite cavity. The study was divided into 3 stages: the adsorption and protonation of alcohols by zeolite, the description of the hydroxyl outlet, and the formation of the double bond. The analysis of the results indicates that the first stage of the reaction occurs with the contact of alcohol with the zeolite cavity, where acid hydrogen promotes the protonation of alcohols, occurring differently for each alcohol. The dehydration process occurs, preferably, via E2 type elimination mechanisms. However, the profile of the energy curves indicates that for larger alcohols, the mechanism is intermediate between the elimination mechanisms E2 with some features of E1 (E2[E1]). Therefore, the zeolite converts alcohols to hydrocarbons in a specific way. Primary, lower-chain alcohols follow E2 mechanism, while secondary and longer-chain alcohols react by a slightly different mechanism, namely E2[E1].

scholarly journals Geochemical distinction between altered oceanic basalt- and seafloor sediment-derived fluids in the mantle source of mafic igneous rocks in southwestern Tianshan, western China

2021 ◽  
Author(s):  
Li-Tao Ma ◽  
Li-Qun Dai ◽  
Yong-Fei Zheng ◽  
Zi-Fu Zhao ◽  
Wei Fang ◽  
...  
Keyword(s):  
Trace Elements ◽  
Oceanic Crust ◽  
Mantle Source ◽  
Mantle Wedge ◽  
Igneous Rocks ◽  
Western China ◽  
Arc Magmatism ◽  
Oceanic Basalt ◽  
Southwestern Tianshan ◽  
Seafloor Sediment

Abstract The role of subducting oceanic crust-derived fluids in generating mafic arc magmatism has been widely documented. However, the subducting oceanic crust is generally composed of basaltic igneous crust and seafloor sediment, which may give rise to different compositions of liquid phases causing metasomatism of the mantle wedge. Because of the similarity in enrichment of fluid-mobile incompatible elements in the two sources of subduction zone fluids, it has been a challenge to distinguish between them when studying the products of mafic arc magmatism. This difficulty is overcome by a combined study of whole-rock Li isotopes and zircon O isotopes in addition to whole-rock major-trace elements and Sr-Nd-Hf isotopes in Late Paleozoic mafic igneous rocks from southwestern Tianshan in western China. Zircon U-Pb dating yields consistent ages of 313±3 Ma to 305±1 Ma for magma crystallization. The mafic igneous rocks exhibit arc-like trace element distribution patterns and depleted whole-rock Nd-Hf isotopes but slightly high (87Sr/86Sr)i ratios of 0.7039 to 0.7056. They also show positive zircon εHf(t) values and slightly higher zircon δ18O values of 5.2-7.6‰. There are covariations of whole-rock Sr isotopes with Th/La and Rb/Nb ratios, indicating a contribution from terrigenous sediment-derived fluids to their mantle source in addition to basaltic igneous crust-derived fluids. Based on the slightly higher zircon δ18O values but variably lower whole-rock δ7Li values of -0.8 to 3.5‰ for the target rocks than those of mantle respectively, both altered oceanic basalt- and terrigenous sediment-derived fluids are identified in the mantle source of these mafic igneous rocks. Model calculations for trace elements and Sr-Nd-Li isotopes further confirm that the geochemical compositions of these mafic igneous rocks can be explained by chemical reaction of depleted MORB mantle peridotite with the mixed fluids to generate ultramafic metasomatites at subarc depths. Therefore, chemical metasomatism of the mantle wedge is a key mechanism for the incorporation of crustal components into the source of arc-like mafic igneous rocks above oceanic subduction zones.

scholarly journals 3-D thermal regime and dehydration processes around the regions of slow earthquakes along the Ryukyu Trench

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Nobuaki Suenaga ◽  
Shoichi Yoshioka ◽  
Yingfeng Ji
Keyword(s):  
Phase Transformations ◽  
Oceanic Crust ◽  
Thermal Regime ◽  
Mantle Wedge ◽  
Plate Interface ◽  
Okinawa Island ◽  
Hydrous Minerals ◽  
Ryukyu Trench ◽  
Temperature Range ◽  
Slow Earthquakes

AbstractSeveral interplate seismic events, such as short-term slow slip events (S-SSEs) and low-frequency earthquakes (LFEs), have been identified in the Ryukyu Trench, southwestern Japan. As one of the specific characteristics of this seismicity, the depths at which S-SSEs occur at the plate interface beneath Okinawa Island are approximately 5–10 km shallower than those beneath the Yaeyama Islands. To elucidate the cause of this difference in depth, we constructed a three-dimensional, Cartesian thermomechanical subduction model and applied the subduction history of the Philippine Sea (PHS) plate in the model region. As a result, the interplate temperatures at which S-SSEs take place were estimated to range from 350 to 450 °C beneath Okinawa Island and from 500 to 600 °C beneath the Yaeyama Islands. The former temperature range is consistent with previous thermal modelling studies for the occurrence of slow earthquakes, but the latter temperature range is by approximately 150 °C higher than the former. Therefore, explaining how the depth difference in S-SSEs could be caused from the aspect of only the thermal regime is difficult. Using phase diagrams for hydrous minerals in the oceanic crust and mantle wedge, we also estimated the water content distribution on and above the plate interface of the PHS plate. Near the S-SSE fault planes, almost the same amount of dehydration associated with phase transformations of hydrous minerals from blueschist to amphibolite and from amphibolite to amphibole eclogite within the oceanic crust were inferred along Okinawa Island and the Yaeyama Islands, respectively. On the other hand, the phase transformations within the mantle wedge were inferred only beneath the Yaeyama Islands, whereas no specific phase transformation was inferred beneath Okinawa Island around the S-SSE occurrence region. Therefore, we conclude that dehydrated fluid derived from the oceanic crust at the plate interface would play a key role in the occurrence of S-SSEs.

Granitic magmas: possible and impossible sources, water contents, and crystallization sequences

1976 ◽  
Vol 13 (8) ◽  
pp. 1007-1019 ◽  
Author(s):  
Peter J. Wyllie ◽  
Wuu-Liang Huang ◽  
Charles R. Stern ◽  
Sven Maaløe
Keyword(s):  
Continental Crust ◽  
Oceanic Crust ◽  
Mantle Wedge ◽  
Regional Metamorphism ◽  
Mantle Peridotite ◽  
Ocean Crust ◽  
Crustal Rocks ◽  
Subducted Oceanic Crust ◽  
Water Contents ◽  
Phase Relationships

The calc-alkalic rocks of batholiths or their precursors may be generated in deep continental crust, in subducted oceanic crust, in the mantle wedge above, or in processes involving material from all three sources. For the series gabbro–tonalite–granite, we have phase relationships with excess H2O to 35 kbar (3500 MPa), and the H2O-undersaturated liquidus surfaces mapped with contours for H2O contents and with fields for near-liquidus minerals. Isobaric diagrams with low H2O contents provide grids potentially useful in defining limits for the H2O content of magmas, based on the sequence of crystallization. Conclusions from the experimental framework include: (1) The H2O content of large granitic bodies is less than 1.5%. (2) Primary granite magmas can not be derived from the mantle or subducted ocean crust. (3) Primary granite magmas with low H2O content are generated in the crust, and erupted as rhyolites. (4) Primary tonalite and andesite are not generated from mantle peridotite; the H2O contents required are unrealistically high. (5) Primary tonalite and andesite are not generated in the crust unless temperatures are significantly higher than those of regional metamorphism. (6) Subducted ocean crust yields magmas with intermediate SiO2 content, but not primary tonalite and andesite. (7) Batholiths are produced from crustal rocks as a normal consequence of regional metamorphism, with the formation of H2O-undersaturated granite liquid and mobilized migmatites. Some batholiths receive in addition contributions of material and heat from mantle and subducted ocean crust.

Seismic Imaging, Arc Magamtism and Megathrust Earthquake under the Western Pacific Subduction Zone

2020 ◽  
Author(s):  
Zhi Wang ◽  
Jian Wang
Keyword(s):  
Partial Melting ◽  
Subduction Zone ◽  
Oceanic Crust ◽  
Mantle Wedge ◽  
Western Pacific ◽  
Arc Magmatism ◽  
S Wave ◽  
Slab Melting ◽  
Megathrust Earthquake ◽  
The Western Pacific

<p>Arc magmatism and megathrust earthquake occurrence in a subduction zone are deemed to attribute to many factors, including structural heterogeneities, fluid contents, temperature, depth of subducting oceanic crust, and etc. However, how these factors affect them is unclear. The extensive arc magmatism observed on the island arcs is considered to be an indicator on chemical exchange between the wedge mantle and the surface in a subduction zone. Megathrust earthquake frequently occurrence is also considered to be attributed to the slab melting and associated interplate coupling of the subducting plate. The Western Pacific subduction zone is regarded as one of the best Laboratory for seismologists to examine these processes due to the densest seismic networks recording numerous earthquakes. Some of the previous studies suggest that the island-arc magmatism is mainly contributed to the melting of peridotite in the mantle wedge due to fluids intrusion from the dehydration process associated with the subducting oceanic crust. They further argued that the oceanic plate could only provide water to the overlying mantle wedge for arc magmatism, but not melt itself due to that it is too cold to melt at a depth between 100 and 200km. However, some of other studies revealed that the hydrated basalt derived from the mid-ocean ridge will be melted with high T and water saturated on the upper interface of the sbuducting plate in the mantle wedge. We consider that the three-dimensional (3-D) P- and S- wave velocity (Vp, Vs) and Poisson’s ratio (σ) structures of the subduction zone, synthesized from a large-quantity of high-quality direct P-, and S-wave arrival times of source-recive pairs from the well located suboceanic events with sP depth phase data could provide a compelling evidence for structural heterogeneity, highly hydrated and serpentinized forearc mantle and dehydrated fluids in the subduction zones. In this study, we combined seismic velocities and Poisson’s ratio images under the entire-arc region of the Western Pacific subduction zone to reveal their effects on megathrust earthquake generation and arc magmatism. We find that a ~10 km-thick low-velocity layer with high-V and high-Poisson’s ratio anomalies is clearly imaged along the upper interface of the subducting Pacific slab. This distinct layer implies partial melting of the oceanic crust due to the deep-seated metamorophic reactions depending on the source of fluids and temperature regime. Such a process could refertilize the overlying mantle wedge and enrich the peridotite sources of basalts under the island arc. Significant low-V and high-Poisson’s ratio anomalies were observed in the mantle wedge along the volcanic front, indicating melting or partial melting of peridotite-rich mantle and then yield tholeiitic magma there. The present study demonstrates that the combined factors of fluid content, mineral composition and thermal regime play a crucial role in both slab melting and arc-magmatism under the Western Pacific subduction zone.</p>

scholarly journals Role of warm subduction in the seismological properties of the forearc mantle: An example from southwest Japan

2021 ◽  
Vol 7 (28) ◽  
pp. eabf8934
Author(s):  
Changyeol Lee ◽  
YoungHee Kim
Keyword(s):  
Subduction Zones ◽  
Mantle Wedge ◽  
Numerical Models ◽  
Thermal Structure ◽  
Free Fluid ◽  
Southwest Japan ◽  
Seismic Properties ◽  
Delay Times ◽  
Forearc Mantle ◽  
Nonvolcanic Tremors

A warm slab thermal structure plays an important role in controlling seismic properties of the slab and mantle wedge. Among warm subduction zones, most notably in southwest Japan, the spatial distribution of large S-wave delay times and deep nonvolcanic tremors in the forearc mantle indicate the presence of a serpentinite layer along the slab interface. However, the conditions under which such a layer is generated remains unclear. Using numerical models, we here show that a serpentinite layer begins to develop by the slab-derived fluids below the deeper end of the slab-mantle decoupling interface and grows toward the corner of the mantle wedge along the interface under warm subduction conditions only, explaining the large S-wave delay times in the forearc mantle. The serpentinite layer then allows continuous free-fluid flow toward the corner of the mantle wedge, presenting possible mechanisms for the deep nonvolcanic tremors in the forearc mantle.

scholarly journals Slab melting and slab melt metasomatism in the Northern Andean Volcanic Zone : adakites and high-Mg andesites from Pichincha volcano (Ecuador)

2002 ◽  
Vol 173 (3) ◽  
pp. 195-206 ◽  
Author(s):  
Erwan Bourdon ◽  
Jean-Philippe Eissen ◽  
Marc-André Gutscher ◽  
Michel Monzier ◽  
Pablo Samaniego ◽  
...  
Keyword(s):  
Partial Melting ◽  
Oceanic Crust ◽  
Mantle Wedge ◽  
Geochemical Characteristics ◽  
Geodynamic Setting ◽  
Volcanic Zone ◽  
Slab Melting ◽  
The Andes ◽  
Peridotitic Mantle ◽  
Slab Melt

Abstract Situated in the fore-arc of the Northern Volcanic Zone (NVZ) of the Andes in Ecuador, Pichincha volcano is an active edifice where have been erupted unusual magmas as adakites and high-Mg andesites. The particular geodynamic setting of the ecuadorian margin (i.e. the flat subduction of the Carnegie Ridge) suggests that thermo-barometric conditions for the partial melting of the oceanic crust are accomplished beneath this volcano. Pichincha adakites possess all the geochemical and isotopic characteristics of slab melts described in various other arc settings. High-Mg andesites with geochemical characteristics close to those of adakites present strong enrichments in MgO that suggest that, once they were produced by ca. 10 % partial melting of the downgoing subducted slab, some adakites en route to the surface strongly interacted with the peridotitic mantle wedge. Adakitic magmas could then represent, as in many other arcs where slab melting occurs, the principal metasomatic agent of the mantle in the NVZ in Ecuador.

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