Crustal Layer Research Articles

Spectral determinants of biological soil crusts in the Gurbantungut Desert

ABSTRACT As thick crustal layers form on dryland surfaces, they affect the spectral information that is originally dominated by sand or rock. The spectral characteristics of organic matter replace the mineral elements as prominent features. In this case, the growth patterns and spectral characteristics of biological soil crusts (BSCs) can be observed. Satellite spectral data have been used for BSC spatial information extraction. However, the dynamic changes in BSCs can affect the spectra. Two aspects are involved here: moisture change and BSC growth. When these changes are superimposed with BSC succession, they lead to an increase in spectral complexity. This study explored three BSC types, including algal crust, lichen and moss, and discussed their spectra. By selecting BSC samples at different succession states and by combining coverage and simulated precipitation, the response of the spectra to BSC coverage and the spectral characteristics of BSCs under dry and wet conditions were measured and analysed. In addition, the spectral index variations caused by coverage and moisture of three types of BSCs were discussed, where the spectral indices include the normalized difference vegetation index (NDVI), brightness index (BI), crust index (CI), and biological soil crust index (BSCI). The results showed that the succession, moisture and growth of BSCs were the main factors affecting their spectra. BSC types can be distinguished in a particular climatic context to determine the degree of BSC succession. Precipitation in the monitoring areas needs to be considered to avoid the effects of dry and wet BSC variations on remote sensing monitoring. The coverage of different types of BSCs in mixed pixels can be determined by multiple indices. The results of this study will provide a basis for monitoring BSCs using satellite spectral information to guide regional ecological management.

Singularity of lithosphere mass density over the mid-ocean ridges and implication on floor depth and heat flow

The relation of heat flow and floor depth across the mid-ocean ridges versus lithosphere age can be described by linear functions of square root of age according to plate thermal conductive Half Space Models (HSM). However, one of the long-standing problems of these classical models is the discrepancies between predicted and observed heat flow and floor depth for very young and very old lithosphere. There have been several recent attempts to overcome this problem: one model incorporates temperature- and pressure-dependent parameters and the second model includes an additional low-conductivity crustal layer or magma rich mantle layer (MRM). Alternatively, in the current paper, the ordinary density of lithosphere in the plate conductive models is substituted with a reduction of lithosphere density towards axis that features the irregularity and nonlinearity of plates across the mid-ocean ridges. A new model is formulated incorporating the new form of density for predicting both peak heat flow and floor depth. Simple solutions of power-law forms derived from the model can significantly improve the predicting results of heat flow and floor depth over the mid-ocean ridges. Several datasets in the literature were reutilized for model validation and comparison. These datasets include both earlier datasets used for original model calibration and the more recently compiled high-quality datasets with both sedimentary and crustal loading corrections. The results indicate that both the heat flow and the slope (first order-derivative) of sea floor approach infinity (undifferentiability or singularities) around the mid-ocean ridges. These singularities are partially due to the boundary condition as it has been already known in the literature and partially to the reduction of density of lithosphere as discovered for the first time in the current research.

Differentiate responses of soil nutrient levels and enzymatic activities to freeze-thawing cycles in different layers of moss-dominated biocrusts in a temperate desert.

The biological soil crust, a widespread phenomenon in arid and semi-arid regions, influences many ecological functions, such as soil stability, surface hydrology, and biogeochemical cycling. Global climate change has significantly altered winter and spring freeze-thaw cycles (FTCs) in mid and high-latitude deserts. However, it is unclear how these changes will affect the biological soil crust and its influence on nutrient cycling and soil enzyme activity. We conducted this study in the Gurbantunggut Desert, a typical temperate desert, using the moss crust as an example of an evolved biological soil crust. Simulating the effects of different FTC frequencies (0, 5, and 15 times) on soil carbon, nitrogen, phosphorus-related nutrients, and extracellular enzyme activities allowed us to understand the relationship between soil environmental factors and nutrient multifunctionality during FTC changes. The results showed that recurrent FTCs significantly increased the accumulation of carbon and phosphorus nutrients in the soil and decreased the effectiveness of nitrogen nutrients. These changes gradually stabilized after 15 FTCs, with available nutrients showing greater sensitivity than the previous full nutrient level. FTCs inhibited carbon, nitrogen, and phosphorus cycle-related hydrolase activities and promoted carbon cycle-related oxidase activities in the crust layer. However, in the 0-3 cm layer, the carbon and phosphorus cycle-related hydrolase activities increased, while peroxidase and urease activities decreased. Overall, the nutrient contents and enzyme activities associated with the carbon, nitrogen, and phosphorus cycles were lower in the 0-3 cm layer than in the crust layer. In addition, the multifunctionality of nutrients in the soil decreased after 15 FTCs in the crust layer and increased after 5 FTCs in the 0-3 cm layer. Structural equation modeling showed that FTC, soil water content, pH, available nutrients, and extracellular enzyme activity had opposite effects on nutrient multifunctionality in different soil layers. The change in nutrient multifunctionality in the crust layer was primarily caused by changes in total nutrients, while soil water content played a greater role in the 0-3 cm layer. Regardless of the soil layer, the contribution of total nutrients was much higher than the contribution of available nutrients and extracellular enzyme activity. In conclusion, it is essential to consider different soil layers when studying the effects of global climate change on the nutrient cycling of the biological soil crust.

Next-generation seismic model of the Australian crust from synchronous and asynchronous ambient noise imaging

The proliferation of seismic networks in Australia has laid the groundwork for high-resolution probing of the continental crust. Here we develop an updated 3D shear-velocity model using a large dataset containing nearly 30 years of seismic recordings from over 1600 stations. A recently-developed ambient noise imaging workflow enables improved data analysis by integrating asynchronous arrays across the continent. This model reveals fine-scale crustal structures at a lateral resolution of approximately 1-degree in most parts of the continent, highlighted by 1) shallow low velocities (<3.2 km/s) well correlated with the locations of known sedimentary basins, 2) consistently faster velocities beneath discovered mineral deposits, suggesting a whole-crustal control on the mineral deposition process, and 3) distinctive crustal layering and improved characterization of depth and sharpness of the crust-mantle transition. Our model sheds light on undercover mineral exploration and inspires future multi-disciplinary studies for a more comprehensive understanding of the mineral systems in Australia.

Analog Models of Lithospheric‐Scale Rifting Monitored in an X‐Ray CT Scanner

AbstractRifting and continental break‐up are fundamental tectonic processes, the understanding of which is of prime importance. However, the vast temporal and spatial scales involved pose major limitations to researchers. Analog tectonic modeling represents a great means to mitigate these limitations, but studying the complex internal deformation of lithospheric‐scale models remains a challenge. We therefore present a novel method for lithospheric‐scale rifting models that are uniquely monitored in an X‐ray CT scanner, which combined with digital image correlation (DIC) techniques, provides unparalleled insights into model deformation. Our first models illustrate how the degree of coupling between competent lithospheric layers, which are separated by a weak lower crustal layer, strongly impacts rift system development. Low coupling isolates the upper crust from the upper lithospheric mantle layer below, preventing an efficient transfer of deformation between both layers. By contrast, fast rifting increases coupling, so that deformation in the mantle is efficiently transferred to the upper crust, inducing either a symmetric or asymmetric (double) rift system. Furthermore, oblique divergence may lead to en echelon graben arrangements and delayed exhumation of the lower crustal layer. The successful application of our novel modeling approach, yielding these first‐order insights, provides a clear incentive to continue running lithospheric‐scale rifting models, and to apply advanced monitoring techniques to extract as much information from models as possible. There is indeed a broad range of opportunities for follow‐up studies within (and beyond) the field of rift tectonics.

Gravitational Instability in the Earth's Viscoelastic Crust

This paper studies instability of a heavy inclusion in the Earth's upper layers by the linear theory method for small perturbations. The existence of such inclusions with increased density is associated with chemical inhomogeneity or phase transitions. The viscoelasticity of the geomaterial is described by the Maxwell rheological model. Two layouts of the inclusion with increased density are considered. The heavy inclusion in the cold upper elastic layer of the crust does not change its location under small perturbations, i.e., it is stable according to the linear theory. The heavy inclusion which is located in the hot viscous crustal layer underlying the upper cold layer, is unstable (slowly sinking into the underlying viscous mantle layers).

Crustal Structure Constraints From the Detection of the SsPp Phase on Mars

AbstractThe shallowest intracrustal layer (extending to 8 ± 2 km depth) beneath the Mars InSight Lander site exhibits low seismic wave velocity, which is likely related to a combination of high porosity and other lithological factors. The SsPp phase, an SV‐ to P‐wave reflection on the receiver side, is naturally suited for constraining the seismic structure of this top crustal layer since its prominent signal makes it observable with a single station without the need for stacking. We have analyzed six broadband and low‐frequency seismic events recorded on Mars and made the first coherent detection of the SsPp phase on the red planet. The timing and amplitude of SsPp confirm the existence of the ∼8 km interface in the crust and the large wave speed (or impedance) contrast across it. With our new constraints from the SsPp phase, we determined that the average P‐wave speed in the top crustal layer is between 2.5 and 3.2 km/s, which is a more precise and robust estimate than the previous range of 2.0–3.5 km/s obtained by receiver function analysis. The low velocity of Layer 1 likely results from the presence of relatively low‐density lithified sedimentary rocks and/or aqueously altered igneous rocks that also have a significant amount of porosity, possibly as much as 22%–30% by volume (assuming an aspect ratio of 0.1 for the pore space). These porosities and average P‐wave speeds are compatible with our current understanding of the upper crustal stratigraphy beneath the InSight Lander site.

Finite-Frequency Kernels for Pg Wavetrains

ABSTRACT Pg waves, which propagate at high frequencies through the crust, are important for tomography and explosion monitoring, and can be among the most prominent P waves observed from small seismic events. Much of what we know about Pg propagation, however, comes from ray approximations, 1D Earth models, and other simplified treatments. For an improved understanding, we use full wave-equation modeling and the adjoint-state method to calculate 0.5 Hz Pg sensitivity kernels corresponding to a range of Earth models, source depths, and epicentral distances. The resulting Pg sensitivity kernels are in many cases dominated by diffraction effects, waveguide mode effects, and other nonray behavior. For a source at the surface observed at less than 4° epicentral distance, first-arriving Pg energy shows little sensitivity to the lower crust. Deeper events channel Pg sensitivity into whatever crustal layer the event originates. In all cases, the simple picture of Pg as a wave reverberating throughout the whole crust with similar sensitivity across upper, middle, and lower crustal layers is found to be inadequate. Other details of the sensitivity kernels can be understood in terms of reflections and conversions at the free surface, which have a greater overall effect on the Pg wavetrain than reflections or conversions at the Moho. Because P–P reflection coefficients at the surface are affected by changes in shear velocity, Pg travel times depend not only on compressional wave velocity, but also on shear-wave velocity. By comparing sensitivity kernels from 1D and 3D velocity models, we show that this strong dependence on near surface velocities, in turn, imparts strong importance to shallow 3D velocity variations. These results represent, to our knowledge, the first application of the adjoint-state method to regional Pg wavetrains.

Mars Seismology

For the first time, from early 2019 to the end of 2022, Mars’ shallow and deep interiors have been explored by seismology with the InSight mission. Thanks to the performances of its seismometers and the quality of their robotic installation on the ground, 1,319 seismic events have been detected, including about 90 marsquakes at teleseismic distances, with Mw from 2.5 to 4.7 and at least 6 impacts, the largest ones with craters larger than 130 m. A large fraction of these marsquakes occur in Cerberus Fossae, demonstrating active regional tectonics. Records of pressure-induced seismic noise and signals from the penetration of a heat flow probe have provided subsurface models below the lander. Deeper direct and secondary body wave phase travel time, receiver function, and surface wave analysis have provided the first interior models of Mars, including crustal thickness and crustal layering, mantle structure, thermal lithospheric thickness, and core radius and state. ▪ With InSight's SEIS (Seismic Experiment for Interior Structure of Mars) experiment and for the first time in planetary exploration, Mars’ internal structure and seismicity are constrained. ▪ More than 1,300 seismic events and seismic noise records enable the first comparative seismology studies together with Earth and lunar seismic data. ▪ Inversion of seismic travel times and waveforms provided the first interior model of another terrestrial planet, down to the core. ▪ Several impacts were also seismically recorded with their craters imaged from orbit, providing the first data on impact dynamic on Mars.

Nature of slab-mantle interactions recorded by coupled δ13C–δ15N–δ18O signatures and elemental compositions of Koidu diamonds and their inclusions

Eclogitic diamond formation can be associated with interactions between subducted slabs and ambient mantle. To gain a better understanding of the extent of chemical exchange between slabs and ambient mantle and the mechanisms of coeval diamond formation, we examined 16 diamonds with eclogitic garnet inclusions from the Koidu kimberlite complex in Sierra Leone (West African Craton). We analyzed the 16 garnet inclusions for major element, trace element and oxygen isotope (δ18O) compositions and their host diamonds for carbon (δ13C) and nitrogen (δ15N) isotope compositions.The garnet inclusions have δ18O values ranging from +5.4 to +12.1‰ (median = +11.3‰) and all but two have δ18O values ≥+9.9‰. Such high δ18O values indicate a link to protoliths that had undergone extensive low-temperature alteration by seawater, which occurs in the uppermost basaltic layer of oceanic crust, prior to subduction. Diamonds hosting the high δ18O garnets have a crustal δ13C signature (−29.6 to −19.4‰) paired with δ15N values (−5.6 to +1.3‰, with one outlier at +9.9‰) implying typically only minor 15N enrichment. This apparent decoupling of δ18O and δ13C from δ15N indicates diamond formation in an eclogitic substrate where garnet oxygen isotope compositions are inherited from altered low-pressure protoliths, diamond carbon is principally derived from biogenic carbonate and organic matter, and diamond nitrogen is variably added by an external, mantle-derived fluid that prompted diamond formation. Of the two remaining garnet inclusions, one has mantle-like δ18O (+5.4‰) and a positive Eu anomaly, suggesting derivation from gabbroic protoliths originally located deep in the oceanic crust where significant alteration by seawater did not occur. Based on mantle-like δ13C (−4.7‰) and δ15N (−6.9‰), the associated diamond formed from mantle-derived fluids/melts. Relative to the other Koidu garnet inclusions, the final garnet has a small majorite component (formation pressure ∼8 GPa), high Mg# (79.0), elevated Cr# (0.90), and low δ18O (+6.3‰), suggesting encapsulation during infiltration of slab-derived melts into surrounding asthenospheric peridotite, associated with a high degree of chemical exchange between slab- and mantle-derived components.

Reducing the Erodibility of Sandy Soils Engineered by Cyanobacteria Inoculation: A Laboratory Investigation

Windblown and water-induced erosion cause substantial soil losses worldwide, especially for drylands. Any sustainable management program that increases soil organic matter and improves the stability of the crustal layer could considerably enhance soil productivity and the preservation of erosion-prone land. This paper presents a laboratory investigation of cyanobacteria-inoculated medium sand and fine sand soils studied for severe runoff conditions that were simulated using an erosion function apparatus (EFA). Loosely deposited sand specimens prepared by air-pluviation were inoculated with a single native filamentous-cyanobacterium strain (investigating both Nostoc sp. and Calothrix sp.) and then incubated under high exposure to white light for 32- or 48-day periods. Well-developed bio-crusts were produced on the specimens’ top surface that achieved substantial improvements in erosion resistance, as was demonstrated for a wide range of hydraulic shear stress investigated using EFA experiments. Relative improvements in hydraulic erosion resistance were explained in terms of the nature of the cyanobacteria-developed microstructures (cyanobacteria filament infiltration of pore-void spaces and exopolysaccharide excretion), as were observed by scanning electron microscope examinations. The developed microstructure depended on the cyanobacterium strain employed and the nominal pore-void sizes that are related to the sand gradation and density state. The encouraging findings of this experimental investigation suggest a tailored approach (i.e., employing a suitable native cyanobacterium strain chosen for its compatibility with the soil’s physical properties) could lay the basis for developing a novel technology for soil protection.

Geochemical features and seismic imaging of the tectonic zone between the Tibetan Plateau and Ordos Block, central northern China

The Tibetan Plateau is growing by both vertical uplift and horizontal extension. It is a continuing debate how the Tibetan Plateau interacts with its surrounding plates and blocks. Due to intense tectonic activity, which produced catastrophic earthquakes, the tectonic zone between the northeast margin of the horizontal extending Tibetan Plateau and the stable Ordos Block has garnered considerable interest. This study investigated the spatial distribution of gas geochemical anomalies (e.g., high flux of CO2 in correspondence of the main faults) at regional scale together with the seismic tomography in correspondence of this tectonic zone with the aim to figure out the domain of convergent boundary between the Ordos block and Tibetan plateau, and trace the tectonic discontinuities which are able to transfer fluids through the crustal layers between the two main geological units. From northwest to southeast, obvious difference of spatial distributions of geochemical and geophysical features in the tectonic zone between the northeast margin of the Tibetan Plateau and the Ordos Block is inferred. The northeast area (Zone A) is dominated by thrust and strike-slip faults with clear velocity boundary underneath, where low crack density (ε), saturation rate (ξ) and Poisson’ ratio (σ) in the middle-lower crust coincided with the low values of heat flow and CO2 emissions, tectonic compression and regional locked-fault can be inducements. The southeast area (Zone C) is dominated by extensional tectonics with roughly E-W fast-velocity direction (FVD) of P-wave azimuthal anisotropy, where high permeability and porosity can be deduced from crustal high ε, ξ and relatively high σ anomalies, resulting in high heat flow, CO2 concentrations and fluxes at the surface, and predominantly crustal-derived gases. The intermediate area (Zone B) also dominated by thrust and strike-slip faults is an extraordinary zone, where intensely locked-fault were clearly revealed, while the predominant anisotropic FVDs in the middle crust changed obviously, more contribution of shallow gas component was detected, and CO2 flux, heat flow, and regional ε, ξ, and σ in the upper crust were higher, compared with those in Zone A, which indicated the regional crushing fragmentation underneath Zone B. The adopted multidisciplinary approach demonstrated that Zone B is the convergent boundary between the Tibetan Plateau and the Ordos Block.

The dark-PMT: A novel directional light dark matter detector based on vertically-aligned carbon nanotubes

The ANDROMeDa project, recently funded by the italian ministry of research with a 1M€grant, has the objective of developing, over the course of the next three years, a novel dark matter (DM) detector based on carbon nanotubes: the ‘dark-PMT’. Such a detector would be sensitive to DM-electron recoils in the eV energy range, and could have world-leading sensitivity for DM masses below 30 MeV with an exposure of only 1g⋅year. Significant R&D is needed to produce carbon nanotubes with ideal properties for a DM target. In particular two by-products of synthesis (non-aligned crust layer and the sub-μm ‘waviness’) are expected to reduce electron transmission probability, and therefore need to be minimized. This will be done via a precise tuning of the evaporation and growth parameters, and of post-growth plasma etching.

Uncovering the chemistry of color change in rock art in Leang Tedongnge (Pangkep Regency, South Sulawesi, Indonesia)

The color change process is among the various threats endangering the preservation of the Maros-Pangkep rock art. It is necessary to be concerned about this issue because it can cause rock art to lose its original hue, fade, obscure, or darken. Among the rock art sites in the Maros-Pangkep Region experiencing the color change of rock art pigments is Leang Tedongnge. The color change process is indicated by several hand stencils with bright red and yellow colors, while others show a dark, dull red hue or a greyish tone. In this work, we aim to assess the mechanism of the color change process by studying the physicochemical properties of the pigment. Here, we employed a combined multi-analytical characterization, including optical microscopy observation, elemental analyzes of X-ray fluorescence (XRF), scanning electron microscopy coupled with energy dispersive electron spectroscopy (SEM-EDS), and Raman spectroscopy. The result indicates that the color-change process of the rock art of Leang Tedongnge was due to the formation of a greyish crust layer made up predominantly of gypsum minerals. The layer caused the pigment to become dull, fade or darken and obscured the original color of the rock art. The layer is formed by the reaction between calcium ion dissolved in the seeping of karst water with sulphate ions from the minerals deposited at the rock's surface. Moreover, the reaction might also involve the sulfate ions derived from anthropogenic sulfur emissions produced by post-harvesting straw burning near the rock art site. To the best of our knowledge, this work is the first report to present a detailed-physicochemical assessment of pigment material regarding the color change for Indonesian rock art.

Paleoproterozoic thick-skinned tectonics in the Central Indian Tectonic Zone: implications on the tectonic reconstructions of cratonic nuclei

ABSTRACT The Satpura Mountain Belt, or the Central Indian Tectonic Zone (CITZ), is a Paleoproterozoic mountain belt of Central India, with Mesoproterozoic and Neoproterozoic rejuvenation history. Structural studies carried out in the southern margin of this ancient orogenic belt have identified a two-phase evolution. The documentation of mapped fold interference of Type 2 ‘boomerang-shaped’ pattern and the spread of early lineations over the hinge area of second-generation folds provide an estimate of ~40-43° angle between the regional axial directions of the folds of the two orogenic episodes. The map patterns indicate the presence of basement-involved recumbent fold nappes placed above the sedimentary cover sequence. The identified décollement structure on the surface provides information regarding the deformation of the upper crust in ductile manner. The patterns of the deep crustal reflectors mapped through previous deep seismic reflection study interpreted during the present investigation brought out the thick-skinned deformation in the region as well as identification of crustal low-velocity zones of lower strength and a domal geometry of the lower crust. The regional thrusts are found to deform deeper crustal layers, one thrust affecting the crust-mantle boundary. The behaviours of the upper and lower crust in terms of rheological layering have been used to interpret the regional structural style of the lithosphere. The structural features of the region do not support the argument regarding the status of the Central Indian Shear as a suture, but indicate it to be the outermost emergent thrust towards the foreland side. The patterns of dip of the thrusts and thrust transport directions indicate a northward dip of the South Indian Block during the subduction stage and the position of the collisional suture of the Satpura orogen to the north of the Sausar belt. A tectonic model has been proposed here to explain the formation of the Satpura Mountain Belt or CITZ involving subduction-collision at ~2250 Ma, followed by erosion, stretching and basin development between 2100 and 1500 Ma. A collisional event in the eastern and western craton margins of India, at ~1500 Ma, caused the second uplift of the Satpura Mountain Belt and its northern margin, developing a temporal unconformity of ~300 Ma gap between the Semri and Kaimur Groups (Vindhyan Supergroup). A third event in the region at ~950 Ma was similar to the second event. No continental amalgamation took place along the CITZ during the tectonothermal events at ~1500 Ma and ~950 Ma; these involved reactivation of old thrusts and basal accretion which were the effect of far-field stresses emanating from the orogenic events at the eastern margin of the Bastar Craton and western margin of the Aravalli Craton.

Soil Microorganisms: Characteristics, Importance and their Functional Role

Soil is the upper layer of the Earth's crust that has been altered by weathering, physical/chemical, and biological processes. It is made up of mineral particles, organic matter, water, air, and living organisms that are organised in genetic soil horizons. Different soils represent the effects of the various underlying factors in their formation, and as their physicochemical characteristics shift along different axes (surface and subsurface horizons), variability exists from site to site and within a given site in the range of micro niches. Such strange characteristics convert/transform soils into a composite of very diverse ecosystems, making its study difficult because very diverse communities can coexist in a very small scale of the same sample. Soil organisms involve macro/megafauna, mesofauna, and microfauna/flora, and despite accounting for less than 1% of the total mass of a soil, they play critical functional roles in maintaining the soil ecosystem. This study describes various approaches to structural and functional characterising key soil microorganisms such as bacteria, archaea, plant growth promoting bacteria, arbuscular mycorrhizas, and nematodes.

Electrical resistivity structure across the Late Cenozoic Abaga-Dalinor volcanic field, eastern China, from 3-D magnetotelluric imaging and its tectonic implications

The Abaga-Dalinor volcanic field (ADVF) in Inner Mongolia, China, located to the west of the North-South Gravity Lineament (NSGL), is the largest and less known area of Late Cenozoic intraplate volcanism in Northeast China. Knowledge of the subsurface structure beneath the ADVF will improve our understanding of its evolution process and formation mechanisms. New broadband magnetotelluric (MT) data were collected along a profile of about 300 km that crosses the major basaltic areas exposed at the surface in the ADVF and a series of east-north-east (ENE) trending faults. The electrical resistivity structure of the crust and uppermost mantle across the ADVF was generated by 3-D inversion of full impedance tensor and tipper data. This model reveals a thick high-resistivity layer (>1000 Ωm) in the upper crust that may represent the Precambrian basement and Late Paleozoic-Mesozoic granitic plutons. In contrast, high conductivity anomalies dominate the middle-lower crust. The northward-dipping high-conductivity feature at depths of 20–40 km below sea level under the Chagan Obo fault is suggested to be the involvement of the Early Paleozoic subducted oceanic crust. A remarkably high-conductivity zone (1–20 Ωm) exists in the middle-lower crust beneath the Abaga and Dalinor volcanic areas. It is explained to be a saline fluid zone of ∼0.45%–0.48% that exsolved from the partial melts as magma ascent rapidly. The impermeable upper crust traps these fluids in the middle-lower crust. A moderately high-conductivity anomaly in the uppermost mantle below near the Solonker Suture Zone (SSZ) is attributed to a minimum partial melt of ∼3–4% associated with the localized asthenospheric upwelling, which feeds the Abaga and Dalinor volcanos as a common magma source. Integrating our resistivity model with previous geophysical and geochemical studies, we suggest that intraplate volcanism in the ADVF is mainly controlled by the pre-existing tectonic weak zones. The lithosphere within the SSZ is susceptible to thermal perturbation from the upwelling mantle, providing the preconditions for melt generation. A set of ENE trending faults and an inferred NW-SE trending fault jointly dominated the vertical transport of magma and lateral extension along the faults, which may have influenced the spatial distribution of basalts and volcanic cones at the surface in the ADVF. In addition, we tend to suggest that Late Cenozoic intraplate volcanism in the ADVF could be the result of decompressing melting within the localized small-scale asthenospheric upwelling induced by the edge-driven convection near the NSGL, which is indirectly related to the westward subduction and retreat of the (Paleo-)Pacific plate during the Late Mesozoic to Cenozoic.

Shear wave velocity structure at the Fukushima forearc region based on H/V analysis of ambient noise recordings by ocean bottom seismometers

SUMMARYThis study presents the shear wave velocity (VS) structures of sedimentary sequences and a section of the upper crustal layer in the Fukushima forearc region of the Japan Trench subduction zone, which were obtained by analysing the horizontal-to-vertical (H/V) spectral ratios of ambient vibration records. The H/V curves were derived using 31 d of continuous seismic data from 3 broad-band and 16 short-period ocean bottom seismometer (OBS) stations. Using the broad-band data, H/V ratios from 0.01 to 10 Hz were derived, but the ratios below 0.1 Hz frequencies were unusually large and temporally unstable. Characterization of seismic noise energy from ∼1 yr of seismic data of three broad-band OBSs revealed variable and elevated energy conditions below 0.1 Hz due to typical long-period oceanic noise; we link these observations with the unstable H/V ratios below this frequency. Therefore, H/V analysis was performed in the frequency range of 0.1–10 Hz for both broad-band and short-period OBSs to obtain subsurface VS profiles. For the forward calculation of the H/V ratios in the inversion process, we used the recently developed ‘hvgeneralized’ method, which is based on the diffuse field assumption, and accounts for the water layer on top of stratified media. Moreover, available prior geological and geophysical information was utilized during the inversion of the H/V curves. We found that subsurface VS ranged from approximately 30 m s−1 at the seabed to approximately 4900 m s−1 at 7000 m below the sea floor (mbsf). Starting with the best model candidate at each OBS location, the effect of the water layer on the H/V curve in the deep ocean was investigated by comparing synthetic H/V curves with and without the water layer. The synthetic H/V analysis revealed that the water layer had a significant effect on H/V amplitudes at higher frequencies (&amp;gt;1 Hz), whereas comparatively little effect was observed at lower frequencies (&amp;lt;1 Hz). This study provides an empirical basis for H/V analysis using OBS data to determine VS down to several kilometres of sedimentary sequences to the upper crust with high-resolution.

Factors influencing the development of microtextures on cold‐climate aeolian quartz grains revealed by experimental frost action

AbstractAeolian‐originated quartz grains of coarse‐sand size (0.5–1 mm) were subjected to experimental frost weathering. A total of 1,000 freeze–thaw cycles with temperature ranges from −5 to +10°C were simulated under full water availability conditions. Scanning electron microscope microtextural analysis of grain surfaces conducted after 0, 50, 100, 300, 700, and 1,000 freeze–thaw cycles resulted in different‐sized conchoidal fractures and breakage blocks as frost‐induced microtextures. The vast majority of these microtextures were encountered on the most convex parts of aeolian grains and their number increased with ongoing freeze–thaw cycles. However, the number of recorded frost‐originated microtextures remained relatively small up to 700 freeze–thaw cycles and increased after 1,000 freeze–thaw cycles. Transmission electron microscope microstructural analysis of grains after 0, 100, and 1,000 freeze–thaw cycles showed both primary (e.g., inclusions, grain boundaries) and secondary (e.g., cracks) defects in quartz crystals. The frequency of the latter remained unexpectedly low. The susceptibility of aeolian‐originated sand‐sized quartz grains to frost‐induced modifications is interpreted here to depend mainly on their internal characteristics. These include aeolian‐driven development of a subsurface impact zone that determines the depth to which frost‐originated microtextures develop. The outer impact zone consists of a thin layer of surficial crust and a series of more or less parallel ridges arranged into mechanically upturned plates. The inner impact zone consists of intact or cracked quartz crystals. The susceptibility of aeolian‐originated quartz grains to frost‐induced modifications depends therefore on a combination of internal (i.e., original crystallography of quartz grains) and external (i.e., aeolian and frost processes acting upon the grains) factors.

Undrained capacity of circular shallow foundations on two-layer clays under combined VHMT loading

Wind turbines are typically designed based on fatigue and serviceability limit states, but still require an accurate assessment of bearing capacity. Overconsolidated clay deposits in Canada often have a thin layer of crust with a relatively high undrained shear strength. However, existing bearing capacity design methods do not consider surficial crusts. This paper studies the undrained VHMT (vertical, horizontal, moment, and torsional) failure envelope of circular foundations founded on a surficial crust underlain by a uniform soil using finite element analysis. Crust correction factors have been introduced to account for the effects of the stiff layer on the vertical and moment capacities. The same forms of equation that are used for uniform soils, but with different parameters provide satisfactory fits for the failure envelopes for a soil with a crustal layer. An analytical expression for the 4-D VHMT failure envelope is derived, and an application of this method is provided.