Seismic Discontinuity Research Articles

Seismic crustal structure beneath Jeju Volcanic Island, South Korea from teleseismicP-receiver functions

SUMMARYJeju Island is an intraplate volcanic island with enigmatic origins, located on the continental shelf south of the Korean Peninsula. A dense temporary seismic array, operated on Jeju Island from 2013 to 2015, revealed several important constraints on the magma plumbing system of Jeju Island. In this study, we determined the deep crustal seismic structure beneath Jeju Island from the teleseismic P-to-S converted phases (receiver functions) recorded from 20 temporary and three permanent stations. We removed the contribution of near-surface reverberations in the resulting receiver functions by applying a resonance removal filter. We estimated crustal P-to-S velocity ratio (VP/VS) and discontinuity depth to provide teleseismic constraints on the composition and structure. We observed two major seismic discontinuities, which are the upper boundaries of a mid-to-lower crustal low-velocity zone (LVZ) and the Moho transition zone. The depth to the upper boundary of the LVZ is deep in the west and southeast (24–30 km) and shallow in the northeast (8–11 km). The LVZ can be interpreted as an extensively distributed residual magma plumbing system, with magma batches stalled at various levels and at various degrees of crystallization, consistent with the chemical diversity of Jeju magmas. The seismic characteristics of the Moho transition zone vary greatly among regions. The top interface of the Moho transition zone is at a wide range of depth (26–40 km), and is shallow at 26–29 km depths beneath central Jeju, suggesting a complex Moho topography. The presence of mafic cumulates and partially molten mushes may contribute to the observed shallow seismic discontinuity at a depth of 26–29 km. The lack of obvious crustal thickening below the shield volcano, Mt Halla, may be associated with mantle upwelling or presence of mafic underplating and cumulates below Jeju. Spatial variations of crustal VP/VS represent highly heterogeneous crustal composition, resulting from magma differentiation during the evolution of the island.

Metasomatized eclogite xenoliths from the central Kaapvaal craton as probes of a seismic mid-lithospheric discontinuity

The central region of the Kaapvaal craton is relatively understudied in terms of its lithospheric mantle architecture, but is commonly believed to be significantly impacted by post-Archean magmatism such as the ca. 2056 Ma Bushveld large igneous event. We investigate a collection of 17 eclogite xenoliths from the Cretaceous Palmietfontein kimberlite at the Western Limb of the Bushveld Complex for their mineralogical compositions (major and trace elements, plus Fe3+ contents), as well as stable oxygen and radiogenic Pb isotopic compositions to gain further insights into the nature and evolution of the central Kaapvaal cratonic mantle lithosphere. New U/Pb age determinations on mantle-derived zircon yield a magma emplacement age of ca. 75 Ma for the Palmietfontein Group-1 kimberlite, which means that the entrained eclogite xenoliths may record a protracted metasomatic history from the Proterozoic through to most of the Phanerozoic eon.Garnet δ18O values of up to 6.9‰ and positive Eu anomalies for the bulk rocks suggest seawater-altered oceanic crustal protoliths for the Palmietfontein eclogite xenolith suite, which is typical for the eclogitic components of the Kaapvaal root and other cratonic mantle sections worldwide. However, several features of the Palmietfontein eclogites are commonly not observed in other mantle-derived eclogite xenolith suites. Firstly, the samples studied yield relatively low equilibration pressures and temperatures between 2.7 and 4.5 GPa and 740–1064 °C, indicating a relatively shallow residence between 90 and 150 km depths. Secondly, euhedral coarse amphibole is present in several eclogite nodules where it is in equilibrium with ‘touching’ garnet, supporting eclogite residence within the amphibole stability field at uppermost lithospheric mantle conditions. Thirdly, primary omphacitic clinopyroxene is often overgrown by diopside, and is significantly enriched in incompatible trace elements. The clinopyroxene is also characterized by elevated 206Pb/204Pb of 17.28–19.20 and 207Pb/204 Pb of 15.51–16.27, and these Pb isotopic compositions overlap with those of Mesozoic Group-2 kimberlites from the Kaapvaal craton.Our results show that eclogites reside at ~85 km depth beneath the central Kaapvaal craton as part of a layer that corresponds to an approximately 50 km thick seismically-detected mid-lithospheric discontinuity. Mid-lithospheric discontinuities have been interpreted as metasomatic fronts formed by focussed crystallization of hydrous mineral phases from enriched volatile-bearing melts, and as such the strongly overprinted amphibole-bearing eclogite xenoliths from Palmietfontein may represent a physical expression of such seismically anomalous metasomatic layer at mid-lithospheric depth. Our Pb isotope data suggest that the focussed metasomatism can be attributed to volatile-rich melts reminiscent of potassic Group-2 kimberlites, which have been invoked in MARID-style metasomatic overprinting of the lower lithospheric mantle beneath the western Kaapvaal craton. However, the relatively low fO2 recorded by the Palmietfontein eclogites (minimum FMQ-4.5) suggests that the metasomatism at mid-lithospheric depth was less protracted compared to the more intensive and oxidizing metasomatism typically observed near the base of cratonic mantle roots. While it is possible that Proterozoic magmatic events were responsible for the focussed mid-lithospheric metasomatism of the Kaapvaal mantle, on the basis of the Pb isotope constraints the Palmietfontein eclogites were most likely overprinted during ca. 120 Ma Group-2 kimberlite magmatism.

Constraints on the Depth, Thickness, and Strength of the G Discontinuity in the Central Pacific From S Receiver Functions

AbstractThe relative motion of the lithosphere with respect to the asthenosphere implies the existence of a boundary zone that accommodates shear between the rigid plates and flowing mantle. This shear zone is typically referred to as the lithosphere‐asthenosphere boundary (LAB). The width of this zone and the mechanisms accommodating shear across it have important implications for coupling between mantle convection and surface plate motion. Seismic observations have provided evidence for several physical mechanisms that might help enable relative plate motion, but how these mechanisms each contribute to the overall accommodation of shear remains unclear. Here we present receiver function constraints on the discontinuity structure of the oceanic upper mantle at the NoMelt site in the central Pacific, where local constraints on shear velocity, anisotropy, conductivity, and attenuation down to ∼300 km depth provide a comprehensive picture of upper mantle structure. We image a seismic discontinuity with a Vsv decrease of 4.5% or more over a 0–20 km thick gradient layer centered at a depth of ∼65 km. We associate this feature with the Gutenberg discontinuity (G), and interpret our observation of G as resulting from strain localization across a dehydration boundary based on the good agreement between the discontinuity depth and that of the dry solidus. Transitions in Vsv, azimuthal anisotropy, conductivity, and attenuation observed at roughly similar depths suggest that the G discontinuity represents a region of localized strain within a broader zone accommodating shear between the lithosphere and asthenosphere.

Multidisciplinary Modeling of Mantle Lithosphere Structure Within the Superior Craton, North America

AbstractNew 3D multi‐azimuthal receiver function analysis identified four regional seismic discontinuities dipping at 7–13° within the mantle of the Superior craton of North America; most are discordant to known major upper crustal structures. Widely observed crustal‐scale structures with near‐vertical axial planes striking east‐west indicate that the most recent and dominant phase of folding and horizontal shortening strain occurred during the Kenoran (D2) crustal deformation concurrent with Au‐mineralization and peak metamorphism at 2.72–2.66 Ga. Two mantle discontinuities strike 065° and 249°, dipping to the southeast and northwest, respectively. These strikes roughly parallel the northern margin of the Superior craton and some intra‐cratonic features such as the axis of the Quetico Basin. Two discontinuities strike 355° and 187°, dipping to the east and west, respectively, and parallel to the western margin of the craton. Our new observations reveal neither moderately dipping, east‐west striking discontinuities nor coherent eclogitic layers characteristic of modern plate tectonic subduction zones. Prominent east‐ and west‐dipping mantle structures relate best to a Paleoproterozoic (Trans‐Hudson) deformation, which is rarely observed in the crust. A new analysis of mantle xenoliths and xenocrysts indicates that carbonatitic metasomatism predominates above some discontinuities where strongly localized conductivity occurs whereas kimberlitic metasomatism predominates below the discontinuities in the broadly conductive mantle.

Insights Into Deep Mantle Thermochemical Contributions to African Magmatism From Converted Seismic Phases

AbstractThe contribution of mantle upwellings of varying spatial extent to Cenozoic magmatism across Africa is debated because geochemical and seismological tools used to interrogate them are primarily sensitive to either composition or temperature. Thermochemical conditions control the depth at which mantle materials undergo phase changes, which cause seismic discontinuities. Mapping seismic discontinuities across the mantle transition zone (MTZ) and below provides insight into the variable thermochemical nature of upwellings. We present observations of seismic discontinuities beneath Africa obtained from a compilation of P‐to‐s receiver functions (RFs; using Pds, PPds, and PKPds phases), recorded at seismograph networks across Africa between 1990–2019. We exploit a recent high‐resolution African continental P‐wavespeed model to migrate our RFs to depth in a common conversion point stack. Cenozoic magmatism along the East African Rift is largely underlain by a thin MTZ implying a contribution to rift magmatism from sources at or below MTZ depths. The Ethiopian rift is underlain by a depressed d410 and uplifted d660 indicating a moderate positive thermal anomaly at MTZ depths (∼100–150 K). The southern East African Rift displays a greater d410 depression and a regional d660 depression, suggesting a stronger thermochemical anomaly at MTZ depths. Here, seismic conversions at ∼1,025 km depth are collocated with slow wavespeeds within the African Superplume, corroborating evidence for a compositional anomaly. We suggest that the contribution of a purely thermal plume directly below Ethiopia augments conditions for mantle melting and rifting. Distinct upwellings may also affect the MTZ below Cenozoic magmatism in Cameroon and Madagascar.

Seismic Structure, Gravity Anomalies and Flexure Along the Emperor Seamount Chain

AbstractThe Hawaiian‐Emperor seamount chain in the Pacific Ocean has provided fundamental insights into hotspot generated intraplate volcanism and the long‐term strength of oceanic lithosphere. However, only a few seismic experiments to determine crustal and upper mantle structure have been carried out on the Hawaiian Ridge, and no deep imaging has ever been carried out along the Emperor seamounts. Here, we present the results of an active source seismic experiment using 29 Ocean‐Bottom Seismometers (OBS) carried out along a strike profile of the seamounts in the region of Jimmu and Suiko guyots. Joint reflection and refraction tomographic inversion of the OBS data show the upper crust is highly heterogeneous withPwave velocities <4–5 km s−1, which are attributed to extrusive lavas and clastics. In contrast, the lower crust is remarkably homogeneous with velocities of 6.5–7.2 km s−1, which we attribute to oceanic crust and mafic intrusions. Moho is identified by a strongPmParrival at offsets of 20–80 km, yielding depths of 13–16 km. The underlying mantle is generally homogeneous with velocities in the range 7.9–8.0 km s−1. The crust and mantle velocity structure has been verified by gravity modeling. While top of oceanic crust prior to volcano loading is not recognized as a seismic or gravity discontinuity, flexural modeling reveals a ∼5.0–5.5 km thick preexisting oceanic crust that is overlain by a ∼8 km thick volcanic edifice. Unlike at the Hawaiian Ridge, we find no evidence of magmatic underplating.

Seismic Diffraction Imaging to Characterize Mass‐Transport Complexes: Examples From the Gulf of Cadiz, South West Iberian Margin

AbstractMass‐transport complexes (MTCs) are often characterized by small‐scale discontinuous internal structure, such as slide blocks, rough interfaces, faults, and truncated strata. Seismic images may not properly resolve such structure because seismic reflections are fundamentally limited in lateral resolution by the source bandwidth. The relatively weak seismic diffractions, instead, encode information on subwavelength‐scale structure, with superior illumination. In this paper, we compare diffraction imaging to conventional, full‐wavefield seismic imaging to characterize MTCs. We apply a seismic diffraction imaging workflow based on plane‐wave destruction filters to two 2D marine multichannel seismic profiles from the Gulf of Cadiz. We observe that MTCs generate a large amount of diffracted energy relative to the unfailed confining sediments. The diffraction images show that some of this energy is localized along existing discontinuities imaged by the full‐wavefield images. We demonstrate that, in combination with full‐wavefield images, diffraction images can be utilized to better discriminate the lateral extent of MTCs, particularly for thin bodies. We suggest that diffraction images may be a more physically correct alternative to commonly used seismic discontinuity attributes derived from full‐wavefield images. Finally, we outline an approach to utilize the out‐of‐plane diffractions generated by the 3D structure of MTCs, normally considered a nuisance in 2D seismic processing. We use a controlled synthetic test and a real‐data example to show that under certain conditions these out‐of‐plane diffractions might be used to constrain the minimum width of MTCs from single 2D seismic profiles.

A thin mantle transition zone beneath the equatorial Mid-Atlantic Ridge.

The location and degree of material transfer between the upper and lower mantle are key to the Earth's thermal and chemical evolution. Sinking slabs and rising plumes are generally accepted as locations of transfer1,2, whereas mid-ocean ridges are not typically assumed to have a role3. However, tight constraints from in situ measurements at ridges have proved to be challenging. Here we use receiver functions that reveal the conversion of primary to secondary seismic waves to image the discontinuities that bound the mantle transition zone, using ocean bottom seismic data from the equatorial Mid-Atlantic Ridge. Our images show that the seismic discontinuity at depths ofabout 660kilometres is broadly uplifted by 10±4 kilometres over a swath about 600kilometres wide and that the 410-kilometre discontinuity is depressed by 5±4kilometres. This thinning of the mantle transition zone is coincident with slow shear-wave velocities in the mantle, from global seismic tomography4-7. In addition, seismic velocities in the mantle transition zone beneath the Mid-Atlantic Ridge are on average slower than those beneath older Atlantic Ocean seafloor. The observations imply material transfer from the lower to the upper mantle-either continuous or punctuated-that is linked to the Mid-Atlantic Ridge. Given the length and longevity of the mid-ocean ridge system, this implies that whole-mantle convection may be more prevalent than previously thought, with ridge upwellings having a role in counterbalancing slab downwellings.

3-D and 5-D reconstruction ofPreceiver functions via multichannel singular spectrum analysis

SUMMARYThe receiver function (RF) method is fundamental in assessing mantle seismic discontinuity depths and reflectivities. Most of the current approaches rely on phase equalization, though in many applications, high levels of incoherent noise, incomplete and irregular sampling customarily interfere with the analysis of weak secondary phases. In recent years, advancements in the field of multidimensional seismic data processing have triggered a shift in interest towards its application to RFs, specifically to single station gathers that depend on a single spatial dimension. Our work generalizes the application of singular spectrum analysis to RFs that rely on two and four spatial dimensions recorded by dense seismic arrays. We adopt a multidimensional signal processing approach known as multichannel singular spectrum analysis. We develop a strategy to assemble and enhance 3-D and 5-D seismic volumes via matrix rank reduction and a reinsertion algorithm to simultaneously suppress random noise, retrieve absent observations and boost identifiability of secondary conversions. We provide informative synthetic examples to gain insight into the effectiveness and limitations of our approach. In the real data example, we improve weak conversions from the mantle transition zone (MTZ) recorded by the USArray in the Yellowstone area. The reconstruction algorithm accurately recovers the timing and polarity of conversions associated with the 410-, 520- and 660-km seismic discontinuities. Our investigation shows that the simultaneous processing of several spatial variables expedites signal restoration, particularly in directions where large recording gaps exist due to a lack of earthquakes, which aids the mapping and identification of the MTZ interfaces. This study presents a theoretical/practical framework for the reconstruction of multidimensional RF data, and its full potential can be exploited with dense acquisition available from the emerging seismic nodal arrays to improve passive seismic imaging.

Distinct slab interfaces imaged within the mantle transition zone

Oceanic lithosphere descends into Earth’s mantle at subduction zones and drives material exchange between Earth’s surface and its deep interior. The subduction process creates chemical and thermal heterogeneities in the mantle, with the strongest gradients located at the interfaces between subducted slabs and the surrounding mantle. Seismic imaging of slab interfaces is key to understanding slab compositional layering, deep-water cycling and melting, yet the existence of slab interfaces below 200 km remains unconfirmed. Here, we observe two sharp and slightly dipping seismic discontinuities within the mantle transition zone beneath the western Pacific subduction zone that coincide spatially with the upper and lower bounds of the high-velocity slab. Based on a multi-frequency receiver function waveform modelling, we found the upper discontinuity to be consistent with the Mohorovicic discontinuity of the subducted oceanic lithosphere in the mantle transition zone. The lower discontinuity could be caused by partial melting of sub-slab asthenosphere under hydrous conditions in the seaward portion of the slab. Our observations show distinct slab–mantle boundaries at depths between 410 and 660 km, deeper than previously observed, suggesting a compositionally layered slab and high water contents beneath the slab. Two seismic discontinuities in the mantle transition zone beneath the western Pacific represent subducted slab interfaces that could be the slab Moho and partially molten sub-slab asthenosphere, according to an analysis of seismic data.

Application of a Premelting Model to the Lithosphere‐Asthenosphere Boundary

AbstractA seismic discontinuity known as the G‐discontinuity, which has been prevailingly detected in the oceanic mantle, is considered to provide evidence of a sharp lithosphere‐asthenosphere boundary (LAB). We investigate whether this sharp LAB is explained by grain boundary premelting, which causes a solid‐state weakening of rock from just below the solidus temperature. We use an experimentally derived mechanical model describing the effect of premelting and calculate vertical profiles of and for various sets of oceanic geotherm and solidus values. Based on the realistic volatile distribution and solidus profile obtained from a model of fractional decompression melting, we show that even when the oceanic upper mantle is melt free, geotherm sufficiently approaches the solidus at various plate ages, resulting in a steep reduction in due to the anelastic effect of premelting. A small amount of volatile adsorbed to grain boundary plays an important role in this result. The predicted seismic discontinuity sharpness (mostly 15 km) is consistent with seismological observations (≤15 km), predicted discontinuity depth (along 1300°C isotherm) is deeper than observations (along 1100°C isotherm), and predicted velocity reduction amplitude (1%–2%) is considerably smaller than observations (≥5%). Because the predicted velocity reduction amplitude is subject to uncertainty resulted from the limited experimental data, further study is needed for the mechanical effect of premelting. We further discuss a discrepancy of seismological results from receiver function and SS precursor studies and from surface wave studies, making it difficult to explain both data consistently.

Global variations of Earth's 520- and 560-km discontinuities

We investigate seismic discontinuities in the mantle transition zone (MTZ) by analyzing SS precursors recorded at global seismic stations. Our observations confirm the global existence of the 520-km discontinuity. Although substantial regional depth variations in the 520-km discontinuity are generally correlated with temperature in the mid-MTZ, they cannot be fully explained by the Clapeyron slope of the wadsleyite-ringwoodite phase transition, suggesting both thermal and compositional heterogeneities in the MTZ. A second discontinuity at ∼560-km depth, previously interpreted as splitting of the 520-km discontinuity, is most commonly detected in cold subduction zones and hot mantle regions. The depth separation between the 520- and 560-km discontinuities varies from ∼80 km in cold regions to ∼40 km in hot areas. The exsolution of calcium-perovskite (Ca-pv) from majorite garnet has been proposed to explain the velocity and density changes across the 560-km discontinuity. However, the gradual exsolution of perovskite and partitioning of Ca and Al between perovskite and garnet appear inconsistent with the relatively “sharp” discontinuity in seismic observations and thus need to be revisited in the future. Nevertheless, because the only known transition in major minerals at this depth in the MTZ is the formation of Ca-pv, the existence of the 560-km discontinuity may imply localized high calcium concentrations in the mid-MTZ possibly related to the recycling of oceanic crust.

Feedback of Slab Distortion on Volcanic Arc Evolution: Geochemical Perspective From Late Cenozoic Volcanism in SW Japan

AbstractSouthwest Japan is an island arc formed by subduction of the Philippine Sea (PHS) plate. The Quaternary magmatism in this region is characterized by eruptions of high‐Sr andesites and dacites, considered to have been derived by melting of the PHS plate. The loci of these volcanoes spatially coincide with seismic discontinuities of the subducted PHS plate. Thus, the magmatism is interpreted as the result of slab melting at the plate tears. However, the processes that promote slab tearing remain unclear. In this study, we applied geochronological and geochemical analyses to late Cenozoic volcanic rocks in southwest Japan as tracers of slab morphology. Two different magma types, ocean‐island basalt (OIB) and island‐arc basalt (IAB), have occurred over 12 million years (Myr). These two magmas are attributed to different integrations of melts extracted from an originally fertile mantle; the OIBs from high temperature melt (1,300–1,400°C) were extracted at a depth of 40–80 km, whereas the IABs were extracted from a shallower, lower temperature region (30–60 km, 1,200–1,350°C). Secular change in Sr enrichment of IAB likely arose due to a transition of slab‐derived fluids, incorporated into magmas as they formed, from water‐ to melt‐dominant one. Progressive shallowing of the subducted PHS plate is responsible for secular change in the properties of slab‐derived fluids as well as rollback of OIB volcanoes. Production of chemically variable magmas in the Chugoku district is the surface expression of distorting slab morphology by interaction between mantle and the subducting plate.

Natural fault and fracture network characterization for the southern Ekofisk field: A case study integrating seismic attribute analysis with image log interpretation

Production from the Ekofisk Chalk Field in the North Sea is believed to be significantly influenced by the presence of a connected fault and fracture network. In the current study, we create a 3D seismic discontinuity cube which is representative of this network within the southern part of the Ekofisk Field. This is done using a multiscale workflow which integrates seismic fault and fracture detection with borehole image log interpretation from three horizontal well sections. The results show that faults and fractures are prevalent in the Ekofisk Formations. Within the study area, faults are mainly organised in three orientations: 1) WNW-ESE, 2) NNE-SSW and 3) NNW-SSE. Smaller E-W striking faults are also observed. The interpreted fractures show a similar pattern and are organized in four orientation groups: NW-SE, WNW-ESE, ENE-WSW and NE-SW. The analysis of seismic discontinuity data (i.e. faults and fractures detectable on seismic) indicates that most small-scale discontinuities occur in proximity to large faults, and that the Lower Ekofisk Formation is characterized by more widespread – and a higher intensity of small-scale seismic discontinuities. It is also demonstrated that along each studied well section, the extracted seismic discontinuities show a qualitative correlation with the image log interpretation. This correlation suggests that the 3D seismic discontinuity cube can serve as a proxy for the fault and fracture network in the southern part of the Ekofisk Chalk Field. Following from our key findings, we conclude that the presented workflow and results could provide a starting point for future studies assessing the impact of natural fractures in the Ekofisk – and other complex reservoirs.

Orphaning Regimes: The Missing Link Between Flattened and Penetrating Slab Morphologies

Slab orphaning is a newly discovered phenomenological behaviour, where the slab tip breaks off at 660 km depth and is abandoned by its parent slab. Upon orphaning, subduction continues uninterrupted through the lateral motion of the parent slab above 660 km depth. In this work, we present a regime diagram for the range of conditions under which slabs can orphan at mid-mantle depths. We observe that a viscosity jump at 1000 km, separate from the endothermic phase change responsible for the 660 km seismic discontinuity, is necessary for orphaning, as is slab and mantle weakening between 660 and 1000 km. We show that orphan slabs, similar to other deep slab morphologies, can be the end result for a wide range of physical parameters governing slab dynamics: Slab orphaning persists across wide variations in slab dip, slab strength, Clapeyron slope and overriding plate nature. The diversity in orphan slab sizes and orphaning length scales is tied to the orphaning regime space, which describes a hitherto unexplored region between deflected and penetrating deep-subduction modes that can be found in the transition zone between upper and lower mantle. Orphaning provides a simple dynamic link between the well known deflection and penetration, and provides one possible way for subducting slabs to switch from direct penetration to deflection, littering the mantle with abandoned fragments.

Seismic discontinuities in the lithospheric mantle at the Dead Sea Transform

SUMMARYThe Dead Sea Transform (DST) was formed in the mid-Cenozoic, about 18 Myr ago, as a result of the breakaway of the Arabian plate from the African plate. Higher resolution information about the sub-Moho structure is still sparse in this region. Here, we study seismic discontinuities in the mantle lithosphere in the region of the DST using a modified version of the P- and S-receiver function methods. We use open data from permanent and temporary seismic stations. The results are displayed in a number of depth profiles through the study area. The Moho is observed on both sides of the transform at nearly 40 km depth by S-to-p and in P-to-s converted signals. The lithosphere–asthenosphere boundary (LAB) on the eastern side of the DST is observed near 180–200 km depth, which is according to our knowledge the first LAB observation at that depth in this region. This observation could lead to the conclusion that the thickness of the Arabian lithosphere east of the DST is likely cratonic. In addition, we observe in the entire area a negative velocity gradient at 60–80 km depth, which was previously interpreted as LAB.

Autocorrelation Reflectivity of Mars

AbstractThe seismic structure of the Martian interior can shed light on the formation and dynamic evolution of the planet and our solar system. The deployment of the seismograph carried by the InSight mission provides a means to study Martian internal structure. We used ambient noise autocorrelation to analyze the available vertical component seismic data to recover the reflectivity beneath the Insight lander. We identify the noise that is approximately periodic with the Martian sol as daily lander operations and the diurnal variation in Martian weather and tides. To investigate the seismic discontinuities at different depths, the autocorrelograms are filtered and stacked into different frequency bands. We observe prominent reflection signals probably corresponding to the Martian Moho, the olivine‐wadsleyite transition in the mantle, and the core‐mantle boundary in the stacked autocorrelograms. We estimate the depths of these boundaries as ~35, 1,110–1,170, and 1,520–1,600 km, consistent with other estimates.

Thermoelasticity and stability of natural epidote at high pressure and high temperature: Implications for water transport during cold slab subduction

Epidote is a typical hydrous mineral in subduction zones. Here, we report a synchrotron-based single-crystal X-ray diffraction (XRD) study of natural epidote [Ca1.97Al2.15Fe0.84(SiO4)(Si2O7)O(OH)] under simultaneously high pressure-temperature (high P-T) conditions to ~17.7 ​GPa and 700 ​K. No phase transition occurs over this P-T range. Using the third-order Birch-Murnaghan equation of state (EoS), we fitted the pressure-volume-temperature (P-V-T) data and obtained the zero-pressure bulk modulus K0 ​= ​138(2) GPa, its pressure derivative K0' ​= ​3.0(3), the temperature derivative of the bulk modulus ((∂K/∂T)P ​= ​−0.004(1) GPa/K), and the thermal expansion coefficient at 300 ​K (α0 ​= ​3.8(5) ​× ​10−5 ​K−1), as the zero-pressure unit-cell volume V0 was fixed at 465.2(2) Å3 (obtained by a single-crystal XRD experiment at ambient conditions). This study reveals that the bulk moduli of epidote show nonlinear compositional dependence. By discussing the stabilization of epidote and comparing its density with those of other hydrous minerals, we find that epidote, as a significant water transporter in subduction zones, may maintain a metastable state to ~14 ​GPa along the coldest subducting slab geotherm and promote slab subduction into the upper mantle while favoring slab stagnation above the 410 ​km discontinuity. Furthermore, the water released from epidote near 410 ​km may potentially affect the properties of the 410 ​km seismic discontinuity.

Decarbonation of Stagnant Slab in the Mantle Transition Zone

AbstractPhase relations of carbonated basalts have been investigated at 13–20 GPa and 1200–1600°C, to model the decarbonation process of stagnant slab in the mantle transition zone (MTZ). Two synthetic mixes with CO2 contents of 2.5 wt% (PC‐a) and 5.0 wt% (PC‐b) were used as the starting materials. The estimated solidus was ~1350°C at the top of the MTZ (~13–15 GPa), which declined to ~1250°C for pressures of above ~15–16 GPa for both mixes. The average slab geotherms are lower than the obtained solidus, creating a carbonate‐bearing stagnant slab, followed by decarbonation of stagnant slab with increased residence time. Dehydration of stagnant oceanic lithosphere could induce decarbonation of the upper oceanic crust for temperatures below the solidus of carbonated basalts. The resulting carbonate melt is highly reactive with the ambient mantle, producing a carbonated domain in the MTZ or at the top of the 410‐km seismic discontinuity. A portion of carbonate melt could ascend to shallow depth because of low density and low viscosity and bring oceanic crust signatures into the source of some volcanoes after complex interactions with the surrounding mantle. On the example of Eastern Asia, decarbonation of stagnant slab is inferred to be a possible prerequisite for the formation of the big mantle wedge and triggered the intraplate volcanoes of Eastern Asia.

Seismic geologic structure characterization using a high-order spectrum-coherence attribute

Characterization of seismic geologic structures, such as describing fluvial channels and geologic faults, is significant for seismic reservoir prediction. The coherence algorithm is one of the widely used techniques for describing discontinuous seismic geologic structures. However, precise coherence attributes between adjacent seismic traces are difficult to compute due to the nonstationary and non-Gaussian property of seismic data. To describe seismic geologic structures accurately, we define a high-order spectrum-coherence (HOSC) attribute. First, we have developed a time-frequency (TF) analysis method to compute a constant-frequency seismic volume with high TF resolution, i.e., the second-order synchrosqueezing wave packet transform. Then, we developed a coherence approach by combining the mutual information (MI) calculation and coherence algorithm based on the eigenvalue computation (C3). To improve computational efficiency, we adopt the information divergence instead of the eigenvalue calculation of the C3-based algorithms. By applying our coherence algorithm to constant-frequency seismic volumes, we obtain the HOSC attribute. To test the validity of the proposed workflow, we evaluate the HOSC attribute using synthetic data. After applying our workflow to 3D real seismic data located in eastern China, the HOSC attribute characterizes seismic geologic discontinuities and subtle features clearly and accurately, such as fluvial channels and subtle faults.