Subduction Research Articles

Low-δ18Ο in mississippian los leones granite (46°S). Tectonic implications for the understanding of late paleozoic evolution in patagonia

Los Leones Granite represents a calc-alkaline tonalite-granodiorite tabular stock of ∼3 km2 intruded within Paleozoic metasedimentary rocks from the Eastern Andean Metamorphic Complex. Two newly obtained zircon SHRIMP U-Pb ages reveal a Middle Mississippian (340-335 Ma) crystallization age, contrasting with previously accepted K-Ar Pennsylvanian age. Isotopic analysis indicate derivation from a depleted source with minimal influence of continental crust contamination (eHf(i) ∼14), which experienced hydrothermal alteration at high temperatures (δ18O ∼4.13). New data coupled with contemporaneous subduction-related rocks in Western Patagonia and Antarctica suggest the existence a broader Lower Carboniferous subduction zone extending at least from Northern Patagonia, to Western Antarctica. On the other hand, isotopic signatures depict a tectono-magmatic framework with a thermal configuration of the margin that was modified by the presence of a local slab tear or the progressive horizontalization of the isotherms as result of the ∼20 myr. of magmatic quiescence in the area.

Stratigraphic Determination Based on Shear Wave Velocity for Earthquake Disaster Mitigation in Lebong District

Bengkulu Province, including North Bengkulu and Lebong, is located in the subduction zone of the Indo-Australian and Eurasian plates, which makes the region prone to earthquakes. This research focuses on earthquake disaster mitigation in the Lemeu area, Uram Jaya District, and Lebong Regency. It identifies areas prone to earthquake shaking based on Vs stratigraphy, Vs30 distribution, and soil site class. The Vs value was measured directly using the Multichannel Analysis of Surface Wave (MASW) method for three passes. Each track has four stations, so there are 12 data collection stations. The station point is the center point of a series of 24 geophones. Each geophone has a recording time of 512 ms and a sampling rate time of 125 ms to obtain 4096 data for each geophone. The measurement data was processed using WINMASW 5.0 professional software to produce a 1D profile. From the stratigraphic Vs, Vs30 is calculated and used to determine the site class, and then a distribution map is made. The results show that this area consists of site classes D, C and B. Medium soil with a value range of 175 ≤ Vs30 ≤ 350, hard soil with a value range of 350 ≤ Vs30 ≤ 750 and soft rock with a value range of 750 ≤ Vs30 ≤ 1500. Of the 12 measurement points, 2 locations have relatively shallow Vs30 values, namely at points 4 and 11. This location has Vs30 values ranging from 221 - 258 m/s, so around this point, there is likely a high potential for building damage in the event of an earthquake.

Forearc crustal faults as tsunami sources in the upper plate of the Lesser Antilles subduction zone: the case study of the Morne Piton fault system

Abstract. In this study, alternatively to the megathrust, we identify upper-plate normal faults orthogonal to the trench as a possible tsunami source along the Lesser Antilles subduction zone. The Morne Piton fault system is such a trench-perpendicular upper crustal fault at the latitude of Guadeloupe. By means of seismic reflection, high-resolution bathymetry, remotely operated vehicle (ROV) imaging and dating, we reassess the slip rate of the Morne Piton fault since 7 Ma, i.e., its inception, and quantify an average rate of 0.25 mm yr−1 since ca. 1.2 Ma. This result divides by two previous estimations, increases the earthquake time recurrence and lowers the associated hazard. The ROV dive revealed a metric scarp with striae at the toe of the Morne Piton fault system, suggesting a recent fault rupture. We estimate a fault rupture area of ∼ 450–675 km2 and then a magnitude range for a maximum seismic event around Mw 6.5 ± 0.5, making this fault potentially tsunamigenic as the nearby Les Saintes fault responsible for a tsunami following the 2004 Mw 6.3 earthquake. Consequently, we simulate a multi-segment tsunami model representative of a worst-case scenario if all the identified Morne Piton fault segments ruptured together. Our model provides clues for the potential impact of local tsunamis on the surrounding coastal area as well as for local bathymetric controls on tsunami propagation. We illustrate that (i) shallow-water plateaus act as secondary sources and are responsible for a wrapping of the tsunami waves around the island of Marie-Galante; (ii) canyons indenting the shallow-water plateau slope break focus and enhance the wave height in front of the most touristic and populated town of the island; and (iii) the resonance phenomenon is observed within the Les Saintes archipelago, showing that the waves' frequency content is able to perturb the sea level for many hours after the seismic rupture.

Integrated geophysical model approach for Nazca Plate subduction in Ecuador

Integrated geophysical model approach for Nazca Plate subduction in Ecuador

Internal deformation of the North Andean Sliver in Ecuador-southern Colombia observed by InSAR

Summary In the Northern Andes, partitioning of oblique subduction of the Nazca plate beneath the South American continent induces a northeastward motion of the North Andean Sliver. The strain resulting from this motion is absorbed by crustal faults, which have produced magnitude 7 + earthquakes historically in the Andean Cordillera of Ecuador and southern Colombia. In order to quantify the strain in that area, we derive a high-resolution surface velocity map using InSAR time-series processing. We analyzed 6 to 8 years of Sentinel-1 data and combined different satellite line-of-sight directions to produce a reliable velocity map in the East direction. We use interpolated GNSS data to express the velocity map with respect to Stable South America and remove the long-wavelength pattern due to the post-seismic deformation following the 2016 Mw 7.8 Pedernales earthquake. The InSAR velocity map finds high E-W shortening strain rates along N-S trending structures within the Western Cordillera and the Interandean valley, with little deformation taking place east of them. This result strengthens the previous proposition of a ∼350 km long Quito-Latacunga tectonic block, forming a restraining bend in the overall right-lateral strike-slip fault system accommodating the northeastward escape motion of the North Andean Sliver. However, the high spatial resolution provided by InSAR indicates that previously proposed boundaries for this block need to be revised. In particular, InSAR results highlight high strain rate (>300 nstrain/yr) along undescribed active structures, south and west of the proposed limits for the Quito-Latacunga block, respectively in Peltetec and Ibarra regions. Interestingly, the two areas with the largest strain rates spatially correlate with the proposed areas of large historical earthquakes. Modeling of the InSAR and GNSS velocities in these areas suggests shallow coupling and high slip rates on structures which, previously, were not identified as active. We also demonstrate a slow-down of the shallow aseismic slip on the Quito fault after the Pedernales earthquake, suggesting that stress changes following large megathrust events might trigger transient slip behaviors on crustal faults. The high-resolution strain map provided by this work provides a new basis for future tectonic models in the Ecuadorian and southern Colombian Andes, and will contribute to the seismic hazard assessment in this highly populated area of the Andes.

Metasedimentary “carbon filter” and its implication for subduction zone carbon recycling

The movement of carbon in subduction zones plays a crucial role in regulating the global carbon cycle, controlling Earth's climate, and maintaining its habitability. Recent work suggests that only a fraction of the carbon released from subducting slabs at sub-arc depths is ultimately released from volcanic arcs, necessitating the existence of hidden carbon reservoirs within the slab-to-arc pathways. However, the precise location of these reservoirs remains enigmatic. Slab fluid serves as the primary medium for carbon transport in subduction zones; thus, a comprehensive understanding of fluid-rock interaction during slab fluid migration is essential for reconciling the carbon flux imbalance between the slab and the arc. In this study, we explore rock carbonation along a fluid conduit in the Southwestern Tianshan HP metamorphic belt in northwest China. Field evidence and petrologic observation reveal significant carbonation of a siliciclastic metasediment at its contact with a high-pressure garnet-bearing calcite (formerly aragonite) vein. We find that rock carbonation (by progressive Fe-bearing magnesite, dolomite, then aragonite precipitation) occurred when slab-derived carbonic fluids migrated through the metasedimentary sequence at approximately 80 km depth. Furthermore, modeling demonstrates that the metasedimentary layer atop the slab has the capacity to sequester 20%–50% of the fluid carbon from the ascending slab devolatilization flux. We propose that the metasedimentary veneer at the plate interface functions as a “carbon filter”, hindering the transfer of carbon from the slab to the arc and helping to reconcile the carbon flux imbalance between the amount released by the slab and that emitted by the arc. This study also provides insights into decarbonation efficiency and mechanisms, carbon-transfer pathways, and temporal aspects of the subduction zone carbon cycle.

Seismic tomography reveals contrasting styles of subduction-channel and mantle-wedge exhumation controlled by upper plate divergent motion

In fossil subduction zones associated with massive exhumation of (ultra)high-pressure ((U)HP) rocks such as the Western Alps, the geometry and behavior of subduction-channel and mantle-wedge rocks during exhumation are still poorly constrained by independent geophysical observations. Here we use a new local earthquake tomography model of the entire fossil subduction zone of the Western Alps based on data collected during the CIFALPS and CIFALPS2 passive seismic experiments, and the first receiver-function profile across the Ligurian Alps, to investigate the styles of subduction-channel and mantle-wedge exhumation as a function of increasing upper-plate divergence motion. In the northern Western Alps (low divergence), a thickened subduction channel can be detected, but no exhumed mantle wedge is found beneath the Gran Paradiso (U)HP dome. In the southern Western Alps (intermediate divergence), an exhumed mantle wedge is detected beneath the Dora-Maira (U)HP dome above a serpentinized subduction channel. In the Ligurian Alps (high divergence), an exhumed mantle wedge and a former subduction channel are detected at much shallower levels beneath the Voltri-Valosio (U)HP dome, and above a shallow-dipping lower-plate Moho. In this latter case, the lower boundary of the exhumed subduction channel is the most evident seismic-velocity interface, which may be easily misinterpreted as a true Moho. Similar Moho-like interfaces are found beneath the exhumed (U)HP domes of eastern Papua New Guinea and the Dabie Shan, which suggests that the results of the CIFALPS experiments may be used as a reference case for the interpretation of other (U)HP terranes worldwide.

Stratigraphic overview of the Neogene Transylvanian Basin, Romania

Although the Transylvanian Basin is often referred as part of the Miocene Pannonian Basin System, we point out important tectonic and sedimentary differences reflected in the stratigraphy. The Neogene of the Transylvanian Basin can be split into two tectonic megasequences: Lower Miocene and Middle to Upper Miocene. The Early Miocene onset of the back arc extension in the Pannonian Basin was not observed in the Transylvanian Basin, where shallow to deep marine sediments were deposited in a flexural basin. Diverse types of facies developed in the continental (e.g., Sânmihai) and marine (e.g., Coruş, Chechiş, Hida) Lower Miocene formations. Although these are poorly constrained to the standard biostratigraphy, recent 87 Sr- 86 Sr data support their limited correlation with the Burdigalian. The Middle Miocene extension in the Transylvanian Basin is very weak compared to that the Pannonian Basin. As the high-resolution Middle Miocene regional biostratigraphy of the Badenian and Sarmatian was calibrated by magnetostratigraphy and radiometric dating of the volcanic tuffs up to the Serravallian/Tortonian boundary, the influence of the regional tectonics on the basin's evolution and stratigraphy can be better understood. The regional tectonics produced a progressive basin isolation starting with the late part of the Middle Miocene. The uplift of the Carpathians isolated the basin at the beginning of the Pannonian and initiated the fan delta, lacustrine, and fluvial sedimentation. The basin was rapidly uplifted and eroded, while post-extensional subsidence was still ongoing in the Pannonian Basin. Endemic fossil assemblages allow only regional-scale correlations for this interval. The uppermost part of the stratigraphic record is represented by the volcano-sedimentary formations generated by the late evolution of the Eastern Carpathians. The sedimentation and subsidence pattern of the Transylvanian Basin reflect the effect of retreating subduction zone in the Eastern Carpathians.

Rheology modification in a subduction channel due to eclogite facies metasomatism (Rocky Beach Metamorphic Mélange, Port Macquarie, Australia)

The rheological properties of the interface between the down-going and overriding plates in subduction zones provides insight into how plate convergence is accommodated and the controls on seismic and aseismic slip. This interface is known as the subduction channel and exhumed examples provide the only direct information on deformation mechanisms and the impact of metamorphism on rheology. The Rocky Beach Metamorphic Mélange in eastern Australia is one such exhumed subduction channel, composed of eclogite, blueschist and greenschist facies blocks within a mélange matrix. Previous phase equilibria modelling indicates that high pressure blocks were subducted to ca. 100 km depth and then retrogressed during return flow and exhumation. We found that the rheology of blocks is modified by metasomatism, consistent with studies on other subduction channels. However, through comparison of blocks from different metamorphic grades we found that the effect of metasomatism on rheology varied depending on the pressure and temperature conditions of metasomatism. While unmetasomatised eclogites behaved as rigid objects in the mélange matrix, rocks with mineral assemblages that equilibrated during eclogite facies metasomatism accumulated significant strain, forming isoclinal folds and refolded folds. Deformation of these blocks began at eclogite facies and continued during return flow and retrogression to blueschist facies. At blueschist facies, metasomatised blocks developed mm-scale isoclinal folds with shearing parallel to fold limbs forming rootless isoclinal folds. At the transition between blueschist and greenschist facies, pressure solution became important, preferentially focusing along layers of lawsonite, dissolving it from the rock. At greenschist facies, dissolution-precipitation processes caused significant mass loss, producing mm-spacing between pressure solution seams and cuspate folds, analogous to dewatering structures in sediments. In the Rocky Beach Metamorphic Mélange eclogite facies metasomatism reduces the competence of rigid blocks, reducing overall subduction channel heterogeneity during return flow. We suggest that subduction channels that experience widespread eclogite facies metasomatism may be less likely to generate seismic slip during return flow, since the proportion of rigid blocks and block strength are both reduced.

Plate tectonics through Earth’s history: constraints from the thermal evolution of Earth’s upper mantle

ABSTRACT Temporal changes in Earth’s tectonic style play a crucial role in understanding the planet’s evolution. Modern-style tectonics is characterized by the formation of basaltic crust at divergent plate boundaries and its subsequent recycling at subduction zones, accompanied by wedge mantle formation and arc magmatism. It is commonly believed that the secular cooling of the mantle modified the tectonic style from stagnant lid or heat pipe on an early hotter Earth to horizontal tectonics during Meso-Neoarchean. However, various field, petrographic, and geochemical studies suggest that the onset of plate tectonics ranges from Hadean to Neoproterozoic. In this study, we re-evaluate the primary magma temperature (mantle potential temperature, Tp) of the upper ambient mantle, spanning from Eoarchean to Neoproterozoic. We used basalts from several Archean and Proterozoic greenstone belts worldwide, along with Proterozoic ophiolites, to (re)calculate the Tp using the FRACTIONATE-PT method. We observed a Tp range of 1444–1639°C during the Archean and 1414–1611°C during the Proterozoic. These findings indicate a strong correlation with previously estimated Tp values obtained from PRIMELT3 method. This further highlights strong internal consistency among different methods and supports models of a hot ambient mantle during the Archean and Proterozoic. We further reviewed numerical models regarding the effect of mantle temperature on the viability of early Earth plate tectonics. Such model results, composition of Archean continental crust, recent crustal growth models, and field evidence are consistent with the operation of plate tectonics on an early hotter Earth. The transition from a hotter mantle to a colder one from Eo-Neoarchean resulted in thinner oceanic lithosphere and a less depleted lithospheric mantle. Subduction of this thinner oceanic lithosphere led to modern-style tectonics. The intensity and style of plate tectonics on the early Earth differed from modern tectonics, which emerged during the Neoarchean.

Rate-and-state friction of epidote gouge under hydrothermal conditions and implications for the stability of subducting faults under greenschist metamorphic conditions

Epidote is a common hydrous mineral present in subduction zones subject to greenschist metamorphic conditions – and potentially an important control on the fault stability-instability transition observed under greenschist facies. We explore controls on this transition through shear experiments on simulated epidote gouge at temperatures of 100–500 °C, effective normal stresses of 100–300 MPa and pore fluid pressures of 30–75 MPa. We use rate-and-state friction to define these controls of temperature, effective stress and pore fluid pressure on gouge stability. Experimental results indicate that the epidote gouge is frictionally strong (μ ∼ 0.73) and the frictional strength is insensitive to variations in temperature or pressure. With increasing temperature, the epidote gouge exhibits a first transition from velocity-strengthening to velocity-weakening at sub-greenschist conditions (T < 100 °C) before transitioning to velocity-strengthening under greenschist metamorphic conditions (T > 300 °C). Elevating the pore fluid pressure or decreasing the effective stress promotes unstable sliding. The transition in gouge rheology at varied temperatures and pressures is explained by the competition between granular flow-induced gouge dilation and pressure solution-induced gouge compaction. Our results demonstrate that the rate-and-state frictional stability of epidote gouges support the potential for a fault stability-instability-stability transition for subduction under greenschist metamorphic conditions.

Hot springs reflect the flooding of slab-derived water as a trigger of earthquakes

In subduction zones, water expelled from the subducting slab is believed to be involved in seismic activity. However, little is known about its quantity and flow processes. Here, we show that the Arima hot springs in western Japan contain high concentrations of water derived from the subducting Philippine Sea slab. A long-term record spanning over half a century reveals that the fraction of slab-derived water exhibited a temporary surge in the year preceding and/or subsequent years of the 1995 Kobe (Hyogo-ken Nanbu) earthquake. In total, an estimated 2.6–4.2 × 105 cubic meters of slab-derived water was introduced in conjunction with the earthquake. We infer that the earthquake was triggered by a flood-like release of water from the slab or by the bursting of clogged flow paths to the hypocenter. Our findings highlight the importance of hydrologically slab-connected hot springs for understanding ultradeep water cycles and their causal relationships with seismic phenomena.

Optimal design and placement of a combined trench and submerged breakwater system on the coastal area of Aceh

Tsunami waves are currently one of the most devastating water hazards that threaten regions or countries near the subduction zones, including Indonesia. In this research, a novel coastal protection system combining a submerged breakwater and a trench is proposed to mitigate tsunami occurrences. The model is developed based on the Shallow Water Equations, which are then solved analytically using the Separation Variables Method, and numerically using the Staggered Finite Volume Method. The wave transmission coefficient, which indicates the wave reduction rate generated by the system, is expected to be the outcome of both approaches. The numerical scheme is validated against the analytical solutions to verify its accuracy in estimating wave reduction by the system. Further, an optimization technique is integrated into the model to obtain the optimal size and position of each structure (submerged breakwater and trench) on the real bathymetry of Aceh's coast, which has been impacted by a deadly tsunami occurrence before. The results of this study are expected to be useful in the future development of coastal protection systems against tsunami waves, especially in Indonesia.

The effect of antigorite dehydration on velocity structure and water migration in subduction zones

The water migration in subduction zones, primarily driven by the phase transition in hydrous minerals, can give rise to hydrated regions with reduced velocity. A fundamental element in comprehending and deciphering these low-velocity zones revolves around acquiring insights into the stability and elasticity of relevant hydrous minerals. As one of the main water carriers in shallow areas, antigorite can dehydrate to form talc, forsterite, and fluid (talc–bearing peridotites) in deep areas of subduction zones, and then the talc thus serves as one of the minerals that can bring water to the deep Earth. Here, the elasticity of talc up to 24 GPa and forsterite up to 12 GPa are calculated by using the first principles method. The result supposes that the talc structure transforming from talc I to talc II is at a pressure between 6 GPa and 8 GPa, impacting the trend of elastic wave velocity in response to pressure. Furthermore, the elastic wave velocity of forsterite can be significantly affected by iron concentration. Meanwhile, a variation velocity model with antigorite consumption and talc content is set up for talc-bearing serpentinized peridotite based on the elastic properties of talc and forsterite in this study, and antigorite in Wang et al. (2022). The results of our model demonstrate a decrease in the low-velocity anomaly in subduction zones, particularly in deep regions or areas with higher initial serpentinization degrees. The results also suggest that the mode of antigorite dehydration can diminish the estimation of water content transported to depths of subduction zones, such as the Mariana Trench and Northern Japan subduction zones. The mode of antigorite dehydration thus provides a useful tool for constraining the composition, seismic velocity structure, and water migration in subduction zones.

Tracing the Oxidizing State and Element‐Mobilizing Fluids in Continental Subduction Zones: Insights From the Granitic Melt‐Eclogite Interface

AbstractFluids in subduction zones significantly influence element mobility, isotope fractionation, and mass transfer. However, unraveling the source, composition, and redox state of fluids in continental subduction zones poses a significant challenge. This study focuses on a granitic melt‐eclogite contact interface, along with adjacent granite and eclogite from the Sulu ultrahigh‐pressure metamorphic belt in East China. The interface exhibits complex mineral assemblages, enriched rare earth elements (REEs), and high field strength elements (HFSEs). Zircon grains from the interface show an age of ∼217 ± 9 Ma, slightly later than peak metamorphism, along with the presence of coesite inclusions. These findings suggest that the interfacial fluid likely formed from the mixing of granitic anatectic melt and aqueous fluid from the eclogite during the initial exhumation of the Sulu terrane. The interaction resulted in the eclogite acquiring substantial REEs and HFSEs, suggesting the interfacial fluid's effective element‐transporting capability and potential supercritical fluid properties. Zircon Ce anomaly and Fe3+/Fe2+ oxybarometer data indicate a highly oxidizing interfacial fluid, analogous to arc magmas in oxygen fugacity. This led to the preferential loss of isotopically heavier Cr from the eclogite during fluid‐eclogite interaction, evidenced by heavier Cr isotopic compositions in the interface (δ53Cr = −0.04 to −0.05‰) compared to adjacent eclogite (δ53Cr as low as −0.11‰). In summary, our results highlight the presence of strong oxidizing and element‐mobilizing fluids in continental subduction zones, offering insights into supercritical fluid recognition and the genesis of oxidizing arc magmas in subduction zones.

Recycled Oceanic Crust in the Source of the Intraplate Changbaishan‐Tianchi Volcano, China/North Korea

AbstractContinental intraplate volcano is an ideal probe to unravel the composition and structure of the deep Earth. The intraplate Changbaishan‐Tianchi volcano was one of the most hazardous eruptions on the Earth's planet. The long‐term activity of this volcano from the Pleistocene to 946 CE has erupted materials with a broad compositional range from basalt to rhyolite, which are expected to be associated with the continuous northeastward subduction of the Pacific plate, but the magma source remains controversial. In this paper, we present a comprehensive data set of in situ zircon Hf and O isotope data, combined with whole‐rock element and Sr‐Nd‐Pb isotope compositions, for selected eruptions of the Changbaishan‐Tianchi volcano, aiming to provide new insights into their magma source and the associated geodynamics. Radiogenic isotopic ratios and incompatible trace element compositions indicate that the erupted volcanic rocks at different stages, although with a varied differentiation degree, were derived from a common magma source characterized by a mixture of DM and EM1 end‐members. Zircon Hf and O isotopes are both relatively homogeneous for different lithologies and eruption stages, with the εHf(t) values varying between −5 and +5, and δ18O values between 3.58‰ and 5.97‰. Modeling of source mixing indicates that high‐temperature altered oceanic crust materials are an important component in the source of Changbaishan‐Tianchi volcano, likely derived from an ancient stagnant slab that has been reactivated by the subduction of the Pacific plate. This study demonstrates that the recycling of deeply subducted oceanic crust is potentially an important source and trigger for continental intraplate volcanism.

Elasticity of epidote at high pressure and its implications for the velocity anomaly in subduction zone

Hydrous minerals play a critical role in modifying the physical and chemical properties of the Earth’s interior. Among those, epidote is an important hydrous mineral in greenschist and blueschist phases of the metamorphosed subducting crust at shallow depth (30-60 km). Here, we measured the compressional (P) and shear (S) wave velocities of a polycrystalline epidote sample at pressures up to 7 GPa and room temperature by means of ultrasonic interferometry. The obtained sound velocities and elastic moduli of epidote increase monotonically with pressure. Finite strain analysis on those data set yielded the elastic moduli and their pressure derivatives of epidote at ambient condition as follows: KS0=115.2GPa, G0=66.7GPa, Ks′=4.6, G′=1.1. Using the elastic properties of epidote, we set up a model to better understand the velocity jumps in the subducted oceanic crusts concerning the blueschist-eclogite transition at 60-90 km depths. Our results indicate that the calculated P and S wave velocity jumps are in good agreement with those seismic observations in the typical subduction zones such as northeastern Japan and southwestern Japan. The eclogitization from epidote bearing blueschist may provide an explanation for the wave velocity anomalies occurred in those regions.

Disentangling the Roles of Subducted Volatile Contributions and Mantle Source Heterogeneity in the Production of Magmas Beneath the Washington Cascades

AbstractThe compositional diversity of primitive arc basalts has long inspired questions regarding the drivers of magmatism in subduction zones, including the roles of decompression melting, mantle heterogeneity, and the amount and composition of slab‐derived materials. This contribution presents the volatile (H2O, Cl, and S), major, and trace element compositions of melt inclusions from basaltic magmas erupted at three volcanic centers in the Washington Cascades: Mount St. Helens (two basaltic tephras, 2.0–1.7 ka), Indian Heaven Volcanic Field (two &lt;600 ka basaltic hyaloclastite tuffs), and Glacier Peak (late Pleistocene to Holocene basaltic tephra from Whitechuck and Indian Pass cones). Compositions corrected to be in equilibrium with mantle olivine display variability in Nb and trace element ratios indicative of mantle source variability that impressively spans nearly the entire range of arc magmas globally. All volcanic centers have magmas with H2O and Cl contributions from the downgoing plate that overlap with other Cascade Arc segments. Volatile abundances and trace element ratios support a model of melting of a highly variably mantle wedge driven by a subduction component of variably saline fluid and/or slab partial melt. Magmas from Glacier Peak in northern Washington have unusually high Th/Yb ratios that are similar to Lassen region basalts, indicating possible contributions of “subcreted” metasediments that geophysical data suggest are not present beneath central Oregon and southern Washington. This data set adds to the growing inventory of primitive magma volatile concentrations and provides insight into spatial distributions of mantle heterogeneity and the role of slab components in the petrogenesis of arc magmas.

Modeling Subduction With Extremely Fast Trench Retreat

AbstractThe Tonga‐Kermadec subduction zone exhibits the fastest observed trench retreat and convergence near its northern end. However, a paradox exists: despite the rapid trench retreat, the Tonga slab maintains a relatively steep dip angle above 400 km depth. The slab turns flat around 400 km, then steepening again until encountering a stagnant segment near 670 km. Despite its significance for understanding slab dynamics, no existing numerical model has successfully demonstrated how such a distinct slab morphology can be generated under the fast convergence. Here we run subduction models that successfully reproduce the slab geometries while incorporating the observed subduction rate. We use a hybrid velocity boundary condition, imposing velocities on the arc and subducting plate while allowing the overriding plate to respond freely. This approach is crucial for achieving a good match between the modeled and observed Tonga slab. The results explain how the detailed slab structure is highly sensitive to physical parameters including the seafloor age and the mantle viscosity. Notably, a nonlinear rheology, where dislocation creep reduces upper mantle viscosity under strong mantle flow, is essential. The weakened upper mantle allows for a faster slab sinking rate, which explains the large dip angle. Our findings highlight the utilizing rheological parameters that lead to extreme viscosity variations within numerical models to achieve an accurate representation of complex subduction systems like the Tonga‐Kermadec zone. Our study opens new avenues for further study of ocean‐ocean subduction systems, advancing our understanding of their role in shaping regional and global tectonics.

Investigation of changes in the cumulative number of magnetic anomalies before and after earthquakes using satellite data

Iran is always at risk of destructive earthquakes due to its location on the Alpine-Himalayan seismic belt. Many significant earthquakes have occurred there so far, which have caused a lot of financial and human losses. The plateau consists of a composite system of collision-oblique transpressive fold-and-thrust mountain belts with active reverse and strike-slip faulting, range-and-basin terrains, active subduction zones, recent volcanic activity, variable crust thicknesses and rigidity, and relatively stable aseismic blocks of different dimensions with low.topographic relief and nearly flat areas., as a result, attention to earthquakes and research in its various fields is necessary and inevitable in Iran. This study investigated Iran’s earthquakes from 2014 to 2021 with a magnitude above 5.5. A total of 30 earthquakes were investigated, of which 21 cases were considered due to solar activities and the simultaneous occurrence of some earthquakes. For this purpose, Swarm Alpha and Swarm Charlie satellite data belonging to the European Space Agency have been used vector magnetic field of all the paths that passed near the place of the earthquake was used between two months before the occurrence and one month after it. First, by performing the first stage of processing for the paths that are close to the earthquake in terms of time, the vivid anomalies caused by the event are identified in the y component of the magnetic field. Then, using these anomalies, a threshold value for the standard deviation is defined for each event so that the anomalies with a higher standard deviation than this threshold value are examined and analyzed. Finally, the occurrence of the studied earthquakes was predicted by fitting the sigmoid function to the cumulative number of anomalies. Because the sigmoid function is not a dynamic function, at the same time, a cubic function has been used to estimate the critical time of the system so that when the sigmoid function cannot fit the data, the cubic function can do this. A first-order curve has also been used to show the proper fit of the data with the sigmoid and cubic functions. And also, C-factor has been used to show the strong compatibility of sigmoid and cubic functions with the cumulative number of anomalies. In the last stage, an analysis has been done under the title of “Confutation analysis” in such a way that the said algorithm is applied to the areas where the earthquake did not occur, and it is expected that the cumulative number of anomalies drawn, will be closer to the linear state than when the earthquake occurred. Using the existing database, 90% of the occurrence of earthquakes studied in this study, pre-indication abnormalities are observed a few hours to a month before the incidence of earthquakes.Also, a Confutation analysis was performed on three cases of earthquakes to show that the obtained results were not random and the change in the cumulative number of magnetic anomalies could be caused by the earthquake.