In sparsely fractured crystalline rock, aperture variability exhibits significant control of the flow field through the fracture network. However, its inclusion in models is hampered due to a lack of field measurements and adequate numerical representation. A model for aperture generation is developed based on self-affine methods which includes two key parameters, the Hurst exponent and a scaling parameter, and which accounts for relative anisotropy and correlation between the adjacent surfaces forming the fracture. A methodology for analysing and extracting the necessary parameters from 3D surface scans of natural rock fractures is also developed. Analysis of the Hurst exponent and scaling parameter space shows that input combinations following a linear upper bound can be used to generate aperture fields which accurately reproduce measurements. It is also shown that the Hurst and scaling parameters are more sensitive than the correlation between the upper and lower fracture surfaces. The new model can produce an aperture ensemble that closely corresponds with the aperture obtained from the surface scans, and is an improvement on previous methods. The model is also successfully used to up-scale fracture apertures based on measurements restricted to a small sub-section of the sample. Thereby, the aperture fields generated using the model are representative of natural fracture apertures and can be implemented in larger scale fracture network models, allowing for numerical simulations to included representation of aperture internal heterogeneity.

A document by Sergio León-Ríos. Click on the document to view its contents.

Coral reefs protect coastlines from inundation and flooding, servicing over 200 million people globally. Wave transformation has previously been studied on coral reef flats with limited focus on forereef zones where wave transformation is greatest during high-energy conditions. This study investigates the role of forereef spur and groove (SaG) morphology on wave energy dissipation and overtopping on coral reefs. Using XBeach on LiDAR-derived bathymetry, we reproduced dissipation rates comparable to SaG field studies. Our results emphasize accurate bathymetries’ role in forereef wave energy dissipation models by including morphological features (e.g., groove sinuosity, irregular forereef slopes) that control the mode of wave energy dissipation (frictional and breaking). We then investigated changes to wave energy dissipation and wave overtopping based on IPCC AR5 low and high emission scenarios (RCP2.6 and RCP8.5) and a total disaster scenario (TD) for the year 2100 considering changes to SaG morphology, wave power and relative sea-level rise. For RCP2.6, an increase in wave heights of 0.8 m and an increase in water level of 0.3 m resulted in a two-fold increase in dissipation rates. For RCP8.5 and TD, with no increase in incident wave height, dissipation rates were 29% and 395% lower than RCP2.6. This resulted in increased overtopping at the reef crest by 1.8 m and 2.7 m for RCP8.5 and TD scenarios, respectively, when compared to RCP2.6. Decreased dissipation rates and increased wave overtopping in forecasted climate conditions suggest the need for strategies to promote coral growth to facilitate high dissipation rates in the future.

A significant shift in Earth’s climate characterizes the Neogene, transitioning from a single-ice-sheet planet to the current bipolar configuration. This climate evolution is closely linked to changing ocean currents, but globally-distributed continuous high-resolution sedimentary records are needed to fully capture this interaction. The Ocean Drilling Program (ODP) Site 752, located on Broken Ridge in the Indian Ocean, provides such a Miocene-to-recent archive. We use X-ray fluorescence (XRF) core scanning to build an eccentricity-tuned age-depth model and reconstruct paleoceanographic changes since 23 Ma. We find two intervals of enhanced productivity, during the early and middle Miocene (18.5 – 13.7 Ma) and late Pliocene/early Pleistocene (3 – 1 Ma). We also report a mixed eccentricity-obliquity imprint in the XRF-derived paleoproductivity proxy. In terms of grain size, three coarsening steps occur between 19.2 – 16 Ma, 10.8 – 8 Ma, and since 2.6 Ma. The steps respectively indicate stronger current winnowing in response to vigorous Antarctic Intermediate Water flow over Broken Ridge in the early Miocene, the first transient onset of Tasman Leakage in the Late Miocene, and the intensification of global oceanic circulation at the Plio-Pleistocene transition. High-resolution iron and manganese series provide a detailed Neogene dust record. This study utilized a single hole from an ODP legacy-site. Nevertheless, we managed to provide novel perspectives on past Indian Ocean responses to astronomical forcing. We conclude that Neogene sediments from Broken Ridge harbor the potential for even more comprehensive reconstructions. Realizing this potential necessitates re-drilling of these sedimentary archives utilizing modern drilling strategies.

Changed hydrological regimes, sea-level rise, and accelerated subsidence are all putting river deltas at risk across the globe. Deltas may respond to these stressors through the mechanism of avulsion. Decades of delta avulsion studies have resulted in conflicting hypotheses that avulsion frequency and location are upstream (water and sediment discharge) or downstream (backwater and sea-level rise) controlled. In this study, we use Delft3D morphodynamic simulations to investigate the main controls over delta avulsion. Avulsion timing and location were recorded in six scenarios modelled over a 400-year period with varying alluvial slopes upstream of a delta slope break (1.13x10-4 to 3.04x10-3) within a range representative global deltas. We measure several independent morphometric variables including avulsion length, delta lobe width, channel width at avulsion, delta topset slope and sediment load. Correlating these variables with the avulsion timescales observed in our model shows that avulsion timescale is mostly controlled by sediment load, which in turn is controlled by the alluvial slope upstream of a delta slope break. With higher stream power index in steeper alluvial slopes, more sediment can be carried within a channel, resulting in more frequent avulsions. Our results are consistent with the avulsion timescale derived from an analytical solution, 19 natural deltas and downscaled physical laboratory deltas. These results help mitigate delta avulsion risk by focusing management efforts on variables that primarily control avulsion in a river delta, but also induce further debate over whether sea-level rise may, or may not, trigger more avulsions in river deltas.

Dataset: https://doi.org/10.7910/DVN/T2LESU

We examined data from the American Geophysical Union (AGU), the world’s largest earth and space science society, to characterize cohort demographics of multiple milestones in a biogeoscientists’ career. Geoscientists of color and White women make up a smaller proportion of those participating in activities critical to transitioning from student to professional (submitting manuscripts, getting published, and being asked to review) in comparison to White men. However, gender parity for biogeoscientists appears within reach at earlier career stages, with 37% AGU Biogeosciences members and 41% of Biogeosciences attendees at the Fall Meeting identifying as women in 2020. Unfortunately, data is lacking to make the same assessment for geoscientists of color. A large proportion of manuscripts are submitted by men (73%), many of which have no co-authors that identify as women or non-binary geoscientists, which likely points to inequitable resources and a greater service burden for scientists from historically excluded groups. Further, our communities’ bias of who we suggest as reviewers results in 85% of the reviewer invites going to White geoscientists and 63% going to men. Thus, while representation of diverse communities has improved in some areas, barriers to publishing results in journals not reflecting society: 25% and 22% of manuscripts were led by or included non-White geoscientists, respectively, and fewer than 5% and 7% were led by or included non-White, women geoscientists, respectively. Therefore, in sectors like academia where publishing remains critical for advancement, this process represents a significant obstacle for biogeoscientists not already part of the majority.

Volcanic ash provides information that can help understanding the evolution of volcanic activity during the early stages of a crisis, and possible transitions towards different eruptive styles. Ash consists of particles from a range of origins in the volcanic system and its analysis can be indicative of the processes driving activity. However, classifying ash particles into different types is not straightforward. Diagnostic observations for particle classification are not standardized and vary across samples. Here we explore the use of machine learning (ML) to improve the classification accuracy and reproducibility. We use a curated database of ash particles (VolcAshDB) to optimize and train two ML-based models: an Extreme Gradient Boosting (XGBoost) that uses the measured physical attributes of the particles, from which predictions are interpreted by the SHAP method, and a Vision Transformer (ViT) that classifies binocular, multi-focused, particle images. We find that the XGBoost has an overall classification accuracy of 0.77 (macro F1-score), and specific features of color (hue_mean) and texture (correlation) are the most discriminant between particle types. Classification using the particle images and the ViT is more accurate (macro F1-score of 0.93), with performances across eruptive styles from 0.85 in dome explosion, to 0.95 for phreatic and subplinian events. Notwithstanding the success of the classification algorithms, the used training dataset is limited in number of particles, ranges of eruptive styles, and volcanoes. Thus, the algorithms should be tested further with additional samples, and it is likely that classification for a given volcano is more accurate than between volcanoes.

High fracture density in fault damage zones not only reduces the elastic stiffness of rocks but may also promote time-dependent bulk deformation through the sliding of fracture surfaces and thus impact the stress evolution in fault zones. Comparing the damage zones of the three faults in the Chelungpu fault system encountered in the Taiwan Chelungpu fault Drilling Project (TCDP), the youngest damage zone showed pronounced sonic velocity reduction even though fracture density is the same for all three fault zones, consistent with the shorter healing time of the youngest fault. Caliper log data showed a time-dependent enlargement of the borehole wall at the damage zone. These damage zones record lower differential stress than the surrounding host rock, which cannot be explained by the reduced elastic stiffness in the damage zone. Stress relaxation caused by time-dependent bulk deformation in the damage zone may be responsible for the observed low differential stress.

Quantifying interseismic deformation of fault networks which are predominantly deforming in a north-south direction is challenging, because GNSS networks are usually not dense enough to resolve deformation at the level of individual faults. The alternative, synthetic aperture radar interferometry (InSAR), provides high spatial resolution but is limited by a low sensitivity to N-S motion. We study the active normal fault network of Western Anatolia, which is undergoing rapid N-S extension, using InSAR. In the first part of this study, we develop a workflow to assess the potential of decomposing InSAR line-of-sight (LOS) velocities to determine the N-S component. We use synthetic tests to quantify the impact of noise and other velocity components and outline the requirements to detect N-S deformation in future studies. In its current state, the N-S deformation field is too noisy to allow robust interpretations, hence in the second part we complement the study by including vertical deformation. Since most faults in the study region are normal faults, the high-resolution vertical velocity field provides new insights into regional active faulting. We show that tectonic deformation in the large graben systems is not restricted to the main faults, and seemingly less active or inactive faults could be accommodating strain. We also observe a potential correlation between recent seismicity and active surface deformation. Furthermore, we find that active fault splays causing significant surface deformation can form several kilometres away from the mapped fault trace, and provide an estimate of current activity for many faults in the region.

Paleomagnetic records of middle Neoproterozoic (820-780 Ma) rocks display high amplitude directional variations that lead to large discrepancies in paleogeographic reconstructions. Hypotheses to explain these data include rapid true polar wander, a geomagnetic field geometry that deviates from a predominantly axial dipole field, a hyper-reversing field (> 10 reversals/Ma), and/or undiagnosed remagnetization. To test these hypotheses, we collected 1057 oriented cores over a 85 m stratigraphic succession in the Laoshanya Formation (Yangjiaping, Hunan, China). High precision U-Pb dating of two intercalated tuff layers constrain the age of the sediments between 809 and 804 Ma. Thermal demagnetization isolates three magnetization components residing in hematite which are not time-progressive but conflated throughout the section. All samples possess a north and downward directed component (in geographic coordinates) at temperatures up to 660°C that is ascribed to a Cretaceous overprint. Two components isolated above 660°C reveal distinct directional clusters: one is interpreted as a depositional remanence, while the other appears to be the result of a mid-Paleozoic (460-420 Ma) remagnetization, which is likely widespread throughout South China. The high-temperature directions are subtly dependent on lithology; microscopic and rock magnetic analyses identify multiple generations of hematite that vary in concentration and distinguish the magnetization components. A comparison with other middle Neoproterozoic paleomagnetic studies in the region indicates that the sudden changes in paleomagnetic directions, used elsewhere to support the rapid true polar wander hypothesis (ca. 805 Ma), are better explained by mixtures of primary and remagnetized components, and/or vertical axis rotations.

Understanding the stress distribution around shallow magma chambers is vital for predicting eruption sites and magma propagation directions. To achieve accurate predictions, comprehensive insight into the stress field surrounding magma chambers and near the surface is essential. Existing stress models for magma chamber inflation often assume a homogenous elastic half-space or a heterogeneous crust with varying mechanical properties in horizontal layers. However, as many volcanoes have complex, non-horizontal, and heterogeneous layers, we enhance these assumptions by considering mechanically diverse layers with varying dips. We employed the Finite Element Method (FEM) to create numerical models simulating two chamber shapes: a circular form and a sill-like ellipse. The primary condition was a 10 MPa excess pressure within the magma chamber, generating the stress field. Layers dips by 20-degree increments, with differing elastic moduli, represented by stiffness ratios (EU/EL) ranging from 0.01 to 100. Our findings validate prior research on heterogeneous crustal modeling, showing that high stiffness ratios disrupt stress within layers and induce local stress rotations at mismatched interfaces. Layer inclination further influences stress fields, shifting the location of maximum stress concentration over varying distances. This study underscores the significance of accurately understanding mechanical properties, layer dip in volcanoes, and magma chamber geometry. Improving predictions of future eruption vents in active volcanoes, particularly in the Andes with its deformed, folded, and non-horizontal stratified crust, hinges on this knowledge. By expanding stress models to incorporate complex geological structures, we enhance our ability to forecast eruption sites and the paths of magma propagation accurately.

The Carrascoy and Palomares faults are two major active faults of the Eastern Betic Shear Zone (SE Iberia), both controlling conspicuous mountain fronts. However, the area in between both faults, corresponding to the Mazarron Graben (MG), is a nearly flat plain bounded by a relief of smooth hills whose tectonic origin and evolution remains uncertain. By means of a morphotectonic analysis, geophysical survey and paleoseismological trenching we point out that this is area of distributed deformation controlled by folds of variable amplitude nucleated in high angle reverse faults with sinistral component without a well-defined deformation front. The MG developed a marine basin during the Upper Miocene evolving into an alluvial environment with calcrete pedogenic development through the Pleistocene, which formed a tableland landscape that favors the identification of tectonic structures. In this study we demonstrate how some of the ancient normal faults controlling the graben were reactivated as reverse during the late Middle Pleistocene within a regional frame of positive tectonic inversion. Such inversion is evidenced by several emblematic structures: (i) presence of harpoon folding, and (ii) newly formed high angle reverse faults, which dips increase and ruptures become younger backwards on the hanging wall. Based on the timing of the observed deformation, we also suggest that the onset of the regional tectonic inversion might be related to the tectonic evolution of the neighboring Carrascoy and Palomares faults, producing a local stress tensor varying dramatically from extension to compression within the neotectonic period in a regional convergence tectonic frame.

Stratovolcanoes are common globally, with high altitude summit regions that are often glacier-clad and intersect the seasonal and perennial snow line. Explosive eruptions from stratovolcanoes can generate pyroclastic density currents (PDCs). When PDCs are emplaced onto and propagate over glacierised substrates, melt and steam are generated and incorporated into the flow, which can cause a transformation from hot, dry granular flow, to a water-saturated, sediment-laden flow, termed a lahar. Both PDCs and ice-melt lahars are highly hazardous due to their high energy during flow and long runout distances. Knowledge of the physics that underpin these interactions and the transformation to ice-melt lahar is extremely limited, preventing accurate descriptions within hazard models. To physically constrain the thermal interactions we conduct static melting experiments, where a hot granular layer was emplaced onto an ice substrate. The rate of heat transfer through the particle layer, melt and steam generation were quantified. Experiments revealed systematic increases in melt and steam with increasing particle layer thicknesses and temperatures. We also present a one-dimensional numerical model for heat transfer, calibrated against experiment data, capable of accurately predicting temperature and associated melting. Furthermore, we present similarity solutions for early-time melting which are used to benchmark our numerical scheme, and to provide rapid estimates for meltwater flux hydrographs. These data are vital for predicting melt volume and incorporation into PDCs required to facilitate the transformation to and evolution of ice-melt lahars.

The Teisseyre-Tornquist Zone (TTZ) is the longest pre-Alpine tectonic lineament in Europe. Its nature and structural evolution are controversially debated. In this study, we show its structural evolution beneath the southern Baltic Sea both on crustal and basin scale by using three seismic reflection profiles combined with 2-D potential field data. The results demonstrate that the southern Baltic Sea is underlain by a thick crust of the East European Craton (EEC) with a Moho depth in the range of 38-42 km. The overall crustal architecture is shaped by three phases of localized crustal stretching in early Paleozoic, Devonian-Carboniferous, and Permian-Mesozoic. The most spectacular feature of the southern Baltic Sea are zones of thick-skinned compressional deformation produced by Alpine inversion along the TTZ and Sorgenfrei-Tornquist Zone (STZ). Both zones include a system of thrusts and back thrusts penetrating the entire crust in an 80-90 km wide inversion zone superimposed on the EEC crust and its sedimentary cover. ENE-vergent thrusts are traced from the top of the Cretaceous down to the Moho and they are accompanied by back thrusts of opposite vergence, also reaching the Moho. Inversion tectonics resulted in the uplift of a block of cratonic crust as a pop-up structure, bounded by thrusts and back thrusts, and the displacement of the Moho within the STZ and TTZ. The similar mechanism of intra-cratonic inversion was recognized for the Dnieper-Donbas Basin in Ukraine, and it may be characteristic of rigid cratons, where deformation is localized in a few preexisting zones of weakness.

Seismic tomography of Earth’s mantle images abundant slab remnants, often located in close proximity to active subduction systems. The impact of such remnants on the dynamics of subduction remains under explored. Here, we use simulations of multi-material free subduction in a 3-D spherical shell geometry to examine the interaction between visco-plastic slabs and remnants that are positioned above, within and below the mantle transition zone. Depending on their size, negatively buoyant remnants can set up mantle flow of similar strength and length scales as that due to active subduction. As such, we find that remnants located within a few hundred km from a slab tip can locally enhance sinking by up to a factor 2. Remnant location influences trench motion: the trench advances towards a remnant positioned in the mantle wedge region, whereas remnants in the sub-slab region enhance trench retreat. These motions aid in rotating the subducting slab and remnant towards each other, reducing the distance between them, and further enhancing the positive interaction of their mantle flow fields. In this process, the trench develops along-strike variations in shape that are dependent on the remnant’s location. Slab-remnant interactions may explain the poor correlation between subducting plate velocities and subducting plate age found in recent plate tectonic reconstructions. Our results imply that slab-remnant interactions affect the evolution of subducting slabs and trench geometry. Remnant-induced downwelling may also anchor and sustain subduction systems, facilitate subduction initiation, and contribute to plate reorganisation events.

The southeastern margin of the Tibetan Plateau has experienced complex deformation since the Cenozoic, resulting in a high level of seismicity and seismic hazard. Knowledge about the seismic anisotropy provides important insight into the deformation mechanism and the regional seismotectonics beneath this tectonically active region. In this study, we conduct a fullwave multi-scale tomography to investigate the seismic anisotropy in the southeastern margin of the Tibetan Plateau. Broadband records from 470 teleseismic events at 111 permanent stations in the region are used to obtain 5,216 high-quality SKS splitting intensity measurements, which are then inverted in conjunction with 3D sensitivity kernels to obtain the anisotropic model for the region with a multi-scale resolution. Resolution tests show that our dataset recovers anisotropy anomalies reasonably well on the scale of 1º x 1º horizontally and ~100 km vertically. Our result suggests that in the southeastern margin of the Tibetan Plateau the deformations in the lithosphere and asthenosphere are decoupled. The anisotropy in the lithosphere varies both laterally and vertically as a result of the dynamic interactions of neighboring blocks as well as lithospheric reactivation. The anisotropy in the asthenosphere largely follows the direction of regional absolute plate motion, i.e. southeastward under the Songpan-Ganzi Terrane and the Yangtze Craton and nearly east-west south of 26ºN latitude. The SKS splitting observed at the surface can be interpreted as the vertical integration of the contributions from lithosphere and asthenosphere.

Astronomical cycles recorded in stratigraphic sequences offer a powerful data source to estimate Earth’s axial precession frequency k, as well as the frequency of rotation of the planetary perihelia (gi) and of the ascending nodes of their orbital planes (si). Together, these frequencies control the insolation cycles (eccentricity, obliquity and climatic precession) that affect climate and sedimentation, providing a geologic record of ancient Solar system behavior spanning billions of years. Here we introduce two Bayesian methods that harness stratigraphic data to quantitatively estimate ancient astronomical frequencies and their uncertainties. The first method (TimeOptB) calculates the posterior probability density function (PDF) of the axial precession frequency k and of the sedimentation rate u for a given cyclostratigraphic data set, while setting the Solar system frequencies gi and si to fixed values. The second method (TimeOptBMCMC) applies an adaptive Markov chain Monte Carlo algorithm to efficiently sample the posterior PDF of all the parameters that affect astronomical cycles recorded in stratigraphy: five gi, five si, k, and u. We also include an approach to assess the significance of detecting astronomical cycles in cyclostratigraphic records. The methods provide an extension of current approaches that is computationally efficient and well suited to recover the history of astronomical cycles, Earth-Moon history, and the evolution of the Solar system from geological records. As case studies, data from the Xiamaling Formation (N. China, 1.4 Ga) and ODP Site 1262 (S. Atlantic, 55 Ma) are evaluated, providing updated estimates of astronomical frequencies, Earth-Moon history, and secular resonance terms.

Multispectral mapping data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) provide a unique opportunity to characterize south polar ice deposits at higher spectral sampling, spatial resolution, or spatiotemporal coverage than previous work. This new perspective can help to constrain the nature and distribution of different mixtures of CO2 ice, H2O ice, and dust that influence the formation, evolution, and preservation of Mars climate records. We processed 1103 CRISM observations spanning southern summer of six Mars Years (MY) through a combination of k-means clustering and random forest classification. Using a set of 12 spectral endmembers directly tied to previous work with high-resolution CRISM targeted data, we made a series of temporally restricted mosaics showing surface spectral variation over time. The mosaics show the effects of the MY 28 dust storm on the removal of the seasonal CO2 ice cap that year and new details of how this process differed from the years that followed. A mosaic showing residual ice surfaces displays broad agreement with previous compositional maps while resolving new details in the distribution of H2O ice-rich material around the periphery of the bright CO2 ice cap. By showing how surface composition varies across a broad swath of the south polar region though time, the endmember set and classified mosaics produced in this work can provide critical context for future studies of the dynamic processes that shape south polar ice deposits.

Curved mountain belts are spectacular natural features, which contain crucial 3D information about the tectonic evolution of orogenic systems. The Mesozoic units exposed at the Cordilleran Mexican Fold and Thrust belt in NE Mexico show a striking curvature that has not been explained nor included in the existent tectonic models of the region. We have investigated with paleomagnetism and rock magnetism the kinematic history of that curvature, which is observed in the rocks of the Jurassic Nazas igneous province and its overlying red beds. Our results show a complex history of remagnetizations that occurred during the Late Jurassic and Cretaceous, as well as clockwise and counterclockwise vertical axis rotations of up to 50˚ respectively in each limb of the curvature. Although our data cannot provide precise timing for such rotations yet, our results confirm that the Mexican Fold and Thrust Belt underwent post-Late Jurassic orocline bending or bucking in NE Mexico.

Evaluation of the approximate magnitude of the gross discrepancy between the volume of sediment produced on the hinterland and the volume deposited in the basin, over long time and length scales, is required to make source-to-sink sediment mass-balance calculations more accurate so that multiple sources for a single widespread stratigraphic unit, or bypass of the unit, might be more easily detected. This paper outlines a method to characterize the sources of sediments, or provenance lithotypes, according to their relative ability to produce dissolved ions, clay minerals, and unaltered residue at different levels of weathering. Estimating the relative proportion of the hinterland that is dissolved supports mass-balance analysis comparing hinterland denudation with basinal deposition, whereas estimating the relative proportion of clay (both original clay, eroded from mudstone, for example, as well as newly created clay produced by weathering of feldspar) supports potential identification of multiple sediment sources. The method is illustrated with a practical example from the Bohemian Massif and documented with an Excel workbook. This is a mineralogical approach based on mineral inventories of weathering profiles. Even if the prediction is necessarily uncertain because the mineralogical representation of the PLs are gross abstractions, the modelled transformation processes are crude cartoons, and the extent of transformation under different environmental conditions is wild speculation based on sparse examples, quantitative provenance analysis will be more accurate and more precise than it would be if dissolution and alteration were not explicitly accounted. There is ample opportunity for the community to improve the procedure!

Plate Motion Change at ca. 6 MaJacob L. Rosenthal1, Paul G. Fitzgerald1, Jeffrey A. Benowitz2, James R. Metcalf2, Paul M. Betka31Department of Earth and Environmental Sciences, Syracuse University , Syracuse NY 13244, USA2Department of Geological Sciences, University of Colorado Boulder , Boulder Co 80309, USA3Atmospheric, Oceanic, and Earth Sciences Department , George Mason University, Fairfax VA 22030, USACorresponding author: Jacob Rosenthal (jlrosent@syr.edu)

Mid-lithosphere discontinuities are seismic interfaces likely located within the lithospheric mantle of stable cratons, which typically represent velocities decreasing with depth. The origins of these interfaces are poorly understood due to the difficulties in both characterizing them seismically and reconciling the observations with thermal-chemical models of cratons. Metasomatism of the cratonic lithosphere has been reported by numerous geochemical and petrological studies worldwide, yet its seismic signature remains elusive. Here, we identify two distinct mid-lithosphere discontinuities at ~89 and ~115 km depth beneath the eastern Wyoming craton and the southwestern Superior craton by analyzing seismic data recorded by two longstanding stations. Our waveform modeling shows that the shallow and deep interfaces represent isotropic velocity drops of 2–9% and 3–10%, respectively, depending on the contributions from changes in radial anisotropy and density. By building a thermal-chemical model including the regional xenolith thermobarometry constraints and the experimental phase-equilibrium data of mantle metasomatism, we show that the shallow interface probably represents the metasomatic front, below which hydrous minerals such as amphibole and phlogopite are present, whereas the deep interface may be caused by the onset of carbonated partial melting. The hydrous minerals and melts are products of mantle metasomatism, with CO2-H2O-rich siliceous melt as a probable metasomatic reagent. Our results suggest that mantle metasomatism is probably an important cause of mid-lithosphere discontinuities worldwide, especially near craton boundaries, where the mantle lithosphere may be intensely metasomatized by fluids and melts released by subducting slabs.

Along the strike of subduction zones, tectonic tremor activity is segmented on a geologic scale, indicating local variations of the tremor-generating process. Here, we study how strong temporal clustering and long-term recurrence of activity can emerge from the synchronization of elementary tremor sources, as they interact through fluid pressure transients. We model tremor sources as rapid openings of low-permeability valves in the permeable fault zone channeling the upward flow of deep metamorphic fluids. Valve openings trigger fast pressure transients that generate seismic waves. In such a system, tremor activity is thus shaped by unsteady fluid circulation. Using numerical simulations of fluid flow for a large number of different valve populations, we show that the synchronized, collective activity of sources generates episodic activity, and that along-strike variations of fluid flux and fluid transport properties can lead to the segmentation of tremor activity. Strong tremor bursts that coherently activate wide parts of the fault and recur with a long period are associated with patches densely populated with valves and characterized by below-average permeability. Long-term tremor episodicity emerges from the synchronous activity of valves in such patches and is responsible for fluid-pressure cycling at the subduction scale. In the tremor zone of the Shikoku, Japan, subduction interface, the most temporally clustered segment coincides with a downgoing seamount chain, suggesting that the segmentation of the fault zone permeability, and hence of tremor activity, could be inherited from the topography of the subducting oceanic plate.