Large boulders with a diameter of up to several tens of meters are globally observed in mountainous bedrock channel environments. Recent theories suggest that high concentrations of boulders are associated with changes in channel morphology. However, data are scarce and ambiguous, and process-related studies are limited. Here we present data from the Liwu River, Taiwan, showing that channel width and slope increase with boulder concentration. We apply two mass balance principles of bedrock erosion and sediment transport and develop a theory to explain the steepening and widening trends. Five mechanisms are considered and compared to the field data. The cover effect by immobile boulders is found to have no influence on channel width. Channel width can partially be explained by boulder control on the tools effect and on the partitioning of the flow shear stress. However, none of the mechanisms we explored can adequately explain the scattered width data, potentially indicating a long-timescale adjustment of channel width to boulder input. Steepening can be best described by assuming a reduction of sediment transport efficiency with boulder concentration. We find that boulders represent a significant perturbation to the fluvial landscape. Channels tend to adjust to this perturbation leading to a new morphology that differs from boulder-free channels. The general approach presented here can be further expanded to explore the role of other boulder-related processes.

Deep underground water injections induce seismicity. When the seismic fractures coalesce into far-reaching pathways for fluid migration, the migrating fluid may reach pre-existing faults, and by decreasing fault strength, can trigger major seismic events. We assume that the potential for building such pathways depends on closeness of hypocenters, similarity of fracture planes orientations, and closeness of radii taking off from the injection point, on which events locate. We define this potential as the average distance between seismic events in the space of parameters quantifying the above conditions. We show that in the studied case from The Geysers geothermal field, this potential is highly correlated with injection rate. When the overall level of injection rate is high, the higher the injection rate, the more the potential for building far-reaching pathways for fluid migration is reduced.

Geologists have documented at least fourteen occurrences of “giant ooids”, a geologically rare type of carbonate allochem, in Neoproterozoic successions at low paleo-latitudes. Recent experiments and modeling demonstrated that ooid size reflects an equilibrium between precipitation and abrasion rates, such that ooid size could be used as a geological proxy for CaCO3 mineral saturation state (Ω). Here, the documented sizes of Neoproterozoic giant ooids were applied to estimate seawater , which provided a novel approach to constraining temperature, partial pressure of CO2, and alkalinity preceding Neoproterozoic glaciations. The results suggest that giant ooid formation was most plausible with seawater alkalinity elevated over its present value by at least a factor of two, and either much warmer (40C) or much colder (0C) climate than modern tropical carbonate platforms, which have important and divergent implications for climate states and ecosystem responses prior to the initiation of each Neoproterozoic glaciation.

Geothermal activity in the Chilean Southern Volcanic Zone is strongly controlled by the regional Liquiñe-Ofqui Fault System (LOFS) and the Andean Transverse Faults (ATF). We analyzed fifteen thermal springs in the Liquiñe area to assess the origin and the main physicochemical processes related to the LOFS and ATF. Major, minor and trace elements identify two defined clusters spatially related to the regional fault systems. In both clusters, ionic relationships suggest that the principal hydrogeochemical processes are mainly dominated by water-rock interactions. Factorial analysis provided two factors: i) F1 (65.1%), saturated by Cl, HCO, Na, SiO, Li, B and Cs, represents water-rock interaction processes driven by temperature in presence of CO; ii) F2 (28.5%) represented by SO and Mo, represents a minor water-rock interaction enhanced by the presence of HS. Samples associated to the LOFS have high scores of both factors, while those from the ATF have only high factor 1 scores. Ionic ratios compared with literature data, clearly identify the samples spatially associated to the LOFS from the ones associated to the ATF with a fuzzy pattern. Water stable isotopes values suggest a meteoric origin with small deviations from local meteoric isotopic line. CO exchange with slightly high and low temperature water rocks interaction is present in most of the samples. Our results indicate that groundwater circulation along faults is a complex process where different constraints influence the final hydeogeochemistry and reaction intensity. Finally, the established processes at Liquiñe area are not upscalable at the whole Southern Volcanic Zone.

We modify the Modified Cam-Clay (MCC) model for large stress ranges encountered in geological applications. The MCC model assumes that the friction angle (ϕ) and the slope of the compression curve (λ) of a mudrock are constant and thus predicts constant values for the lateral effective stress ratio under uniaxial, vertical strain (K0) and undrained strength ratio (S_u/(σ_v^’ )). However, experimental work shows that λ, ϕ, and S_u/(σ_v^’ ) decrease and K0 increases substantially with stress over large stress ranges (e.g., up to 100 MPa). We incorporate the stress dependency of λ and ϕ into the MCC model and use the new model to predict S_u/(σ_v^’ ) and K0 ratios. The modified model, with only one additional parameter, successfully predicts the stress dependency of these ratios. We encode the modified model and use it for finite-element analysis of a salt basin in the deepwater Gulf of Mexico. The stresses that the new model predicts around salt differ significantly from those predicted using the original model. We incorporate the stress dependency of the friction angle into the analytical models developed for critical tapers, wellbore drilling, and the stability of submarine channel levees. We show that the decrease of the friction angle with stress 1) results in a concave surface for critical wedges, 2) shifts the drilling window to higher mud weights and makes it narrower for a vertical wellbore, and 3) causes deep-seated failure of submarine channel levees at lower angles. Our study could improve in situ stress and pore pressure estimation, wellbore drilling, and quantitative understanding of geological processes.

Sediment progradation and spreading is a key process during gravity-driven, thin-skinned deformation in salt-bearing passive margins. However, to what degree the size and shape of a progradational sedimentary wedge control gravity-driven deformation is still not clear. We use analogue modelling to compare two endmember configurations constrained by critical wedge theory, in which the sediment wedge has different initial depositional slopes: a 5° critical (stable) slope and a 27° unstable slope. In both configurations, differential loading initiates spreading characterized by a basinward migrating system of linked proximal extension and distal contraction with a translational domain in between. With a critical frontal slope, the translational domain expands as the contractional domain migrates forward with viscous flow evenly distributed. With a steep frontal slope, both extensional and contractional domains migrate at similar rate due to more localized viscous flow under the wedge toe producing diagnostic structures of late extension overprinting early contraction. In both cases, salt flow is dominated by Poiseuille flow with only a subordinate contribution from Couette flow, contrasting to classical gravity gliding systems dominated by Couette flow. Comparison with previous studies reveal similar structural styles and viscous flow patterns. Our study highlights the geometric variations of sedimentary wedges result in variable responses in gravity spreading systems. With a steep frontal slope, the sediment wedge is more likely to collapse and develop spreading associated structures. However, such steep slope systems may not be very common in salt-bearing passive margins as they are less likely to occur in nature.

Corridors of size-selected crescent-shaped dunes, known as barchans, are commonly found in water, air, and other planetary environments. The growth of barchans results from the interplay between a fluid flow and a granular bed, but their size regulation involves intricate exchanges between different barchans within a field. One size-regulating mechanism is the binary interaction between nearby dunes, when two dunes exchange mass via the near flow field or by direct contact (collision). In a recent Letter (Assis & Franklin, GRL, 2020), we identified five different patterns arising from binary interactions of subaqueous barchans, and proposed classification maps. In this paper, we further inquire into binary exchanges by investigating the motion of individual grains while barchans interact with each other. The experiments were conducted in a water channel where the evolution of pairs of barchans in both aligned and off-centered configurations was recorded by conventional and high-speed cameras. Based on image processing, we obtained the morphology of dunes and motion of grains for all interaction patterns. We present the trajectories of individual grains, from which we show the origin and destination of moving grains, and their typical lengths and velocities. We also show that grains from the impacting dune spread with a diffusion-like component over the target barchan, and we propose a diffusion length. Our results provide new insights into the size-regulating mechanisms of barchans and barchanoid forms found on Earth and other planets.

Previous studies have interpreted Last Interglacial (LIG; ~129-116 ka) sea-level estimates in multiple different ways to calibrate projections of future Antarctic ice-sheet (AIS) mass loss and associated sea-level rise. This study systematically explores the extent to which LIG constraints could inform future Antarctic contributions to sea-level rise. We develop a Gaussian process emulator of an ice-sheet model to produce continuous probabilistic projections of Antarctic sea-level contributions over the LIG and a future high-emissions scenario. We use a Bayesian approach conditioning emulator projections on a set of LIG constraints to find associated likelihoods of model parameterizations. LIG estimates inform both the probability of past and future ice-sheet instabilities and projections of future sea-level rise through 2150. Although best-available LIG estimates do not meaningfully constrain Antarctic mass loss projections or physical processes until 2060, they become increasingly informative over the next 130 years. Uncertainties of up to 50 cm remain in future projections even if LIG Antarctic mass loss is precisely known (+/-5 cm), indicating there is a limit to how informative the LIG could be for ice-sheet model future projections. The efficacy of LIG constraints on Antarctic mass loss also depends on assumptions about the Greenland ice sheet and LIG sea-level chronology. However, improved field measurements and understanding of LIG sea levels still have potential to improve future sea-level projections, highlighting the importance of continued observational efforts.

Meaningful analysis of uranium-series isotopic disequilibria in basaltic lavas relies on the use of complex forward numerical models like dynamic melting (McKenzie, 1985) and equilibrium porous flow (Spiegelman and Elliott, 1993). Historically, such models have either been solved analytically for simplified scenarios, such as constant melting rate or constant solid/melt trace element partitioning throughout the melting process, or have relied on incremental or numerical calculators with limited power to solve problems and/or restricted availability. The most public numerical solution to reactive porous flow, UserCalc (Spiegelman, 2000) was maintained on a private institutional server for nearly two decades, but that approach has been unsustainable in light of modern security concerns. Here we present a more long-lasting solution to the problems of availability, model sophistication and flexibility, and long-term access in the form of a cloud-hosted, publicly available Jupyter notebook. Similar to UserCalc, the new notebook calculates U-series disequilibria during time-dependent, equilibrium partial melting in a one-dimensional porous flow regime where mass is conserved. In addition, we also provide a new disequilibrium transport model which has the same melt transport model as UserCalc, but approximates rate-limited diffusive exchange of nuclides between solid and melt using linear kinetics. The degree of disequilibrium during transport is controlled by a Damköhler number, allowing the full spectrum of equilibration models from complete fractional melting (Da = 0) to equilibrium transport (Da = ∞).

Wind erosion is integral to the evolution of arid landscapes on Earth and Mars, but the nature of long-term wind erosion of bedrock is poorly understood. Here we describe the Salina del Fraile (SdF) depression in the Puna Plateau of the Central Andes, NW Argentina, as a landform excavated by wind over several million years. New structural cross-sections and a compilation of chronostratigraphic ages rule out the hypothesis that the depression was created by transtensional tectonics. Dated remnant lacustrine and alluvial deposits in the floor of the depression constrain the rate and timing of erosion. Late Oligocene–Miocene compressional folding uplifted friable strata that were preferentially eroded, resulting in the high-relief (900 m) depression. Up to 1.95 km and an average of 1.05 km of strata were eroded during the last 8.2 to 17 Ma, at rates of 0.06 to 0.23 mm/yr. These rates are similar to long-term average wind erosion rates reported in other regions. Coarse-grained eolian megaripples, yardangs, and elongated ridges indicate ongoing eolian abrasion and deflation, aided by salt weathering, of the floor of the depression. Megaripple migration across stony lag surfaces exposes fresh bedrock to continued erosion. The SdF also contains kilometerscale mesas and ridges that we interpret as erosional remnants. These landforms are similar to megayardangs and erosional topography identified on the lower flanks of Mount Sharp, Gale crater, Mars. In such hyperarid landscapes characterized by lithologic heterogeneities, high relief landforms can be generated and sustained by wind erosion, without significant fluvial or glacial incision.

Cretaceous - Miocene sedimentary rocks of northern Borneo preserve records of subduction of the Paleo-Pacific and Proto-South China Sea, providing important evidence for reconstructing the tectonic evolution of Southeast Asia since the Mesozoic. However, the genesis and tectonic setting of these sediments remain controversial. In this study, new Sr isotope, combined with Nd isotope data were used to determine the provenance contribution of the Cretaceous – Late Eocene Lubok Antu mélange and the Rajang Group. Detrital zircon ages and sedimentary geochemistry data of the Cretaceous - Miocene strata are also used to better understand the tectonic evolution of Borneo. Results show that more than 60% of the sediments came from a magmatic belt during the Late Cretaceous to Early Paleocene, and more than 50% from the Malay Peninsula during the Paleocene to the Late Eocene. The proportion of different detrital zircon ages and sedimentary geochemical characteristics in Borneo changed from west to east during the Cretaceous to the Miocene, which may be related to drainage changes caused by the gradual closure of an ocean basin. Subduction ceased in central Borneo during the Early Paleocene, slightly later than Late Cretaceous cessation in western Borneo. The collapse of magmatic belt lead river drainages from the Malay Peninsula to flow into Borneo. Whereas subduction continued in Eastern Borneo until the Miocene. Opening of the South China Sea cut off the drainage from the Malay Peninsula, and the inner rocks in Borneo once again became the main source of sediments.

Barrier island overwash occurs when the elevation of wave runup exceeds the dune crest and induces landward transport of sediment across a barrier island and deposition of a washover deposit. Washover deposition is generally attributed to major storms, is important for the maintenance of barrier island resilience to sea-level rise and is used to extend hurricane records beyond historical accounts by reconstructing the frequency and extent of washover deposits preserved in the sedimentary record. Here, we present a high-fidelity three-year record of washover evolution and overwash at a transgressive barrier island site. During the first year after establishment, washover volume and area increased 1,595% and 197%, respectively, from at least monthly overwash. Most of the washover accretion resulted from the site morphology having a low resistance to overwash, as opposed to being directly impacted by major storms. Washover deposits can accrete over multi-year time scales; therefore, paleowashover deposits are more complex than simply event beds.

Located in northern Dominican Republic, the Early Cretaceous Rio Boba mafic-ultramafic plutonic sequence constitutes a lower crust section of the Caribbean island arc, made up by gabbroic rocks and subordinate pyroxenite. Modal compositions, mineral chemistry, whole-rock compositions and thermobarometric calculations indicate that pyroxenites and gabbronorites represent a cumulate sequence formed by fractionation of tholeiitic magmas with initially very low H2O content in the lower crust of the arc (0.6-0.8 GPa). Melts evolved along a simplified crystallization sequence of olivine ® pyroxenes ® plagioclase ® Fe-Ti oxides. The magmatic evolution of the Rio Boba sequence and associated supra-crustal Puerca Gorda metavolcanic rocks is multi-stage and involves the generation of magmas from melting of different sources in a supra-subduction zone setting. The first stage included the formation of a highly depleted substrate as result of decompressional melting of a refractory mantle source, represented by a cumulate sequence of LREE-depleted IAT and boninitic gabbronorites and pyroxenites. The second stage involved volumetrically subordinate cumulate troctolites and gabbros, which are not penetratively deformed. The mantle source was refractory and enriched by a LILE-rich hydrous fluid derived from a subducting slab and/or overlying sediments, and possibly by a LREE-rich melt. The third stage is recorded in the upper crust of the arc by the Puerca Gorda ‘normal’ IAT protoliths, which are derived from an N-MORB mantle source enriched with a strong subduction component. This magmatic evolution has implications for unravelling the processes responsible for subduction initiation and subsequent building of the Caribbean island arc.

Geologic carbon storage is required for achieving negative CO2 emissions to deal with the climate crisis. The classical concept of CO2 storage consists in injecting CO2 in geological formations at depths greater than 800 m, where CO2 becomes a dense fluid, minimizing storage volume. Yet, CO2 has a density lower than the resident brine and tends to float, hindering the widespread deployment of geologic carbon storage. Here, we propose for the first time to store CO2 in supercritical reservoirs to eliminate the CO2 leakage risk. Supercritical reservoirs are found at drilling-reachable depth in volcanic areas, where high pressure (p>21.8 MPa) and temperature (T>374 ºC) imply CO2 is denser than water. We estimate that 100 injection wells could eventually provide a CO2 storage capacity in the range of 50-500 Mt yr-1. Carbon storage in supercritical reservoirs is an appealing alternative to the traditional approach.

Between 81º30’ E and 83ºE the Himalayan range’s “perfect” arcuate shape is interrupted by an embayment. We hypothesize that thrust geometry and duplexing along the megathrust at mid-lower crustal depths plays a leading role in growth of the embayment as well the southern margin of the Tibetan plateau. To test this hypothesis, we conducted thermokinematic modeling of published thermochronologic data from the topographic and structural embayment in the western Nepal Himalaya to investigate the three-dimensional geometry and kinematics of the megathrust at mid-lower crustal depths. Models that can best reproduce observed cooling ages suggest that the megathrust in the western Nepal Himalaya is best described as two ramps connected by a long flat that extends further north than in segments to the east and west. These models suggest that the high-slope zone along the embayment lies above the foreland limb of an antiformal crustal accretion zone on the megathrust with lateral and oblique ramps at mid-lower crustal depths. The lateral and oblique ramps may have initiated by ca. 10 Ma. This process may have controlled along-strike variation in Himalayan-plateau growth and therefore development of the topographic embayment. Finally, we analyze geological and morphologic features and propose an evolution model in which landscape and drainage systems across the central-western Himalaya evolve in response to crustal accretion at depth and the three-dimensional geometry of the megathrust. Our work highlights the importance of crustal accretion at different depths in orogenic-wedge growth and that the mid-lower crustal accretion determines the location of plateau edge.

Rock texture has a critical influence on the way rocks weather. The most important textural factors affecting weathering are grain size and the presence of cracks and stylolites. These discontinuities operate as planes of mechanical weakness at which chemical weathering is enhanced. However, it is unclear how different rock textures impact weathering rates and the size of weathered grains. Here, we use a numerical model to simulate weathering of rocks possessing grain boundaries, cracks, and stylolites. We ran simulations with either synthetic or natural patterns of discontinuities. We found that for all patterns, weathering rates increase with discontinuity density. When the density was <~25%, the weathering rate of synthetic patterns followed the order: grid >honeycomb >Voronoi >brick-wall. For higher values, all weathering rates were similar. We also found that weathering rates decreased as the tortuosity of the pattern increased. Moreover, we show that textural patterns strongly impact the size distributions of detached grains. Rocks with an initial monomodal grain size distribution produce weathered fragments that are normally distributed. In contrast, rocks with an initial log-normal size distribution produce weathered grains that are log-normally distributed. For the natural patterns, weathering produced lower modality distributions.

The Indus Fan, located in the Arabian Sea, contains the bulk of the sediment eroded from the Western Himalaya and Karakoram. Scientific drilling in the Laxmi Basin by the International Ocean Discovery Program (IODP) provides an erosional record from the Indus River drainage dating back to 10.8 Ma, and with a single sample from 15.5 Ma. We dated detrital zircon grains by U-Pb geochronology to reconstruct how erosion patterns changed through time. Long-term increases in detrital zircon U-Pb components of 750–1200 Ma and 1500–2300 Ma show increasing preferential erosion of the Himalaya relative to the Karakoram at 7.99–7.78 Ma and more consistently starting by 5.87 Ma. An increase in the contribution of 1500–2300 Ma zircons starting by 1.56 Ma indicates significant unroofing of the Inner Lesser Himalaya (ILH) by that time. The trend in zircon U-Pb age populations is consistent with bulk sediment Nd isotope data implies greater zircon fertility in Himalayan bedrock compared to the Karakoram and Transhimalaya. The initial change in spatial erosion patterns at 7.0–5.87 Ma occurred during a time of drying climate in the Indus foreland. The increase in ILH erosion postdates the onset of dry-wet glacial-interglacial cycles suggesting some role for climate control. However, erosion driven by rising topography in response to formation of the Lesser Himalayan thrust duplex, especially during the Pliocene may also be important. The influence of the Nanga Parbat Massif to the bulk sediment flux is modest, in contrast to the situation in the eastern Himalaya syntaxis.

A large strike-slip earthquake occurred in the Caribbean Sea on 28 January 2020. We inverted teleseismic P-waveforms from the earthquake to construct a finite-fault model by a new method of inversion that simultaneously resolves the spatiotemporal evolution of fault geometry and slip. The model showed almost unilateral rupture propagation westward from the epicenter along a 300 km section of the Oriente transform fault with two episodes of rupture at speeds exceeding the local shear-wave velocity. Our modeling indicated that the 2020 Caribbean earthquake rupture encountered a bend in the fault system associated with a bathymetric feature near the source region. The geometric complexity of the fault system triggered multiple rupture episodes and a complex rupture evolution. Our analysis of the earthquake revealed complexity of rupture process and fault geometry previously unrecognized for an oceanic transform fault that was thought to be part of a simple linear transform fault system.

High water cut has been an issue in the Delaware basin for many years now. Volume of produced water continue to increase, resulting in adverse environmental impacts and higher reservoir-management costs. To address these problems, a data-driven workflow has been developed to pre-emptively identify the high water-cut wells. The workflow uses unsupervised pseudo-rock typing followed by supervised classification trained on well logs from 17 wells in the Delaware basin. The workflow requires a suite of 5 well logs from a 500-ft depth interval surrounding the kick-off points of these wells, which includes 200 ft above and 300 ft below the KOP. First, the well logs are clustered into 5 pseudo-rock types using multi-level clustering. Using statistical features extracted from these 5 pseudo-rock types, 3 supervised classifiers, namely K-nearest neighbor, support vector machine, and logistic regression, are trained and tested to detect the high water-cut wells. Over 100 cross validations, the 3 classifiers perform at a median Matthew’s Correlation Coefficient (MCC) of 0.90. The kurtosis of the neutron porosity log response of the pseudo-rock type A0, interpreted as a shale lithology, is the most The submitted paper is currently under review. Dr. Sid Misra is the lead investigator on this topic. informative/relevant signature associated with high water cut. Next, the presence of pseudo-rock type A1, interpreted as high-permeability lithology, is an informative signature of low water-cut wells. The kurtosis of the density porosity log response of the pseudo-rock type B0, interpreted as carbonate lithology, and the presence of pseudo-rock type B1, interpreted as a tight sandstone lithology, serve as informative signatures for differentiating high water cut wells from low water cut wells.

The shape of soil-mantled hillslopes is typically attributed to erosion rate and the transport efficiency of the various processes that contribute to soil creep. While climate is generally hypothesized to have an important influence on soil creep rates, a lack of uniformity in the measurement of transport efficiency has been an obstacle to evaluating the controls on this important landscape parameter. We addressed this problem by compiling a data set in which the transport efficiency has been calculated using a single method, the analysis of hilltop curvatures using 1-m LiDAR data, and the erosion rates have also been determined via a single method, in-situ ¬cosmogenic 10Be concentrations. Moreover, to control for lithology, we chose sites that are only underlain by resistant bedrock. The sites span a range of erosion rates (6 – 922 mm/kyr), mean annual precipitation (39 – 320 cm/yr), and aridity index (0.08 – 1.38). Surprisingly, we find that hilltop curvature varies with the square root of erosion rate, whereas previous studies predict a linear relationship. In addition, we find that the inferred transport coefficient also varies with the square root of erosion rate but is insensitive to climate. We explore various mechanisms that might link the transport coefficient to the erosion rate and conclude that present theory regarding soil-mantled hillslopes is unable to explain our results and is, therefore, incomplete. Finally, we tentatively suggest that processes occurding in the bedrock (e.g., fracture generation) may play a role in the shape of hillslope profiles at our sites.

Here we report for the first-time shock-induced incongruent melting of olivine in an ordinary chondrite. Several olivine grains (Fo74), entrained in the shock-melt vein of the Kamargaon L6 chondrite were dissociated into magnesiowüstite (XFe = 0.71) and orthoenstatite (XFe = 0.22). We propose that the breakdown of olivine took place as a result of incongruent melting to produce magnesiowüstite and Mg-rich liquid. We suggest that bridgmanite may have crystallized as the second phase from the olivine melt which back-transformed to a low-pressure phase of orthoenstatite from subsequent high-temperature and low-pressure events. In this case, olivine grains may have experienced pressure and temperature of ~25 GPa and ~2500 °C, respectively. Our results suggest that the incongruent melting of olivine may possibly operate as one of the alternative mechanisms of dissociation reaction driving the phase transformation of olivine in the natural systems.

Apatite fission track (AFT) analyses for granitoid and metamorphic bedrock samples from the Western Superior Province (Ontario), the Churchill-Rae Province (Melville Peninsula and Southampton Island, Nunavut), and the Slave Province (Northwest Territories) show a broad range of single grain effective uranium concentrations (eU) (<1 to ~300 ppm) and some of the oldest reported AFT ages in North America. Although most of our samples are characterized by near-endmember fluorapatite composition with implied low track retentivity (<0.1 apfu Cl, rmr0 ~0.85-0.82), single-grain AFT ages are statistically overdispersed and ages decrease with increasing eU content. This eU-age relationship is resonant of the Hendriks and Redfield (2005) Earth Planet. Sci. Lett. 236 (443-458) argument for α-radiation enhanced fission track annealing (REA) and is analogous to the negative age-eU correlations observed in published zircon and titanite (U-Th)/He data from slowly-cooled cratonic rocks. In all cases, the samples fail the canonical χ2 test (<5%), generally considered to indicate that the ages are unlikely to be drawn from a single Poissonian distribution with a discrete mean value and may represent multiple populations. The high intra-sample age variability for low-Cl bedrock apatites with protracted histories (>200-500 m.y.) at <100°C since the Precambrian suggests strong REA control on AFT ages. Conversely, some low Cl AFT samples with a narrower eU range show less age dispersion and a weak apparent age-eU correlation. A complex trade-off between radiation damage, chemical composition (e.g. low Cl and REE enrichment), and thermal history is implied when eU and rmr0 are positively correlated. Previous assessments of the influence of REA on AFT age were based on evaluating central age and mean track length, which potentially mask high single-grain age scatter and REA effects due to the modal nature of central age determination. REA is also supported by and compatible with materials science and nuclear waste studies of radiation damage in different apatite groups, therefore it is crucial that bedrock samples exhibiting high age scatter are evaluated in terms of intra-sample compositional heterogeneity. AFT samples with relatively low Cl concentrations are especially prone to greater REA control of cooling ages and this underscores the need for routine acquisition of compositional data for AFT datasets. Our broad range in single-grain AFT ages (with no other clear, strong compositional controls) supports the notion that radiation damage affects both the AFT and (U-Th)/He thermochronometers in slowly-cooled settings and must be accounted for during thermal history modeling and interpretation.

Barchans are dunes of crescentic shape found on Earth, Mars and other celestial bodies, growing usually on polydisperse granular beds. In this Letter, we investigate experimentally the growth of subaqueous barchans consisting of bidisperse grains. We found that the grain distribution within the dune changes with the employed pair, and that a transient stripe appears on the dune surface. We propose that observed patterns result from the competition between fluid entrainment and easiness of rolling for each grain type, and that grains segregate with a diffusion-like mechanism. Our results provide new insights into barchan structures found in other environments.

Theoretical studies predict that during earthquake rupture faults slide at non-constant slip velocity, however it is not clear which source time functions are compatible with the high velocity rheology of earthquake faults. Here we present results from high velocity friction experiments with non-constant velocity history, employing a well-known seismic source solution compatible with earthquake source kinematics. The evolution of friction in experiments shows a strong dependence on the applied slip history, and parameters relevant to the energetics of faulting scale with the impulsiveness of the applied slip function. When comparing constitutive models of strength against our experimental results we demonstrate that the evolution of fault strength is directly controlled by the temperature evolution on and off the fault. Flash heating predicts weakening behaviour at short timescales, but at larger timescales strength is better predicted by a viscous creep rheology. We use a steady-state slip pulse to test the compatibility of our strength measurements at imposed slip rate history with the stress predicted from elastodynamic equilibrium. Whilst some compatibility is observed, the strength evolution indicates that slip acceleration and deceleration might be more rapid than that imposed in our experiments.