The tool of phase-field modeling for the prediction of chemical as well as microstructural evolution during crystallization from a melt in a mineralogical system has been developed in this work. We provide a compact theoretical background and introduce new aspects such as the treatment of anisotropic surface energies that are essential for modeling mineralogical systems. These are then applied to two simple model systems - the binary olivine-melt and plagioclase-melt systems - to illustrate the application of the developed tools. In one case crystallization is modeled at a constant temperature and undercooling while in the other the process of crystallization is tracked for a constant cooling rate. These two examples serve to illustrate the capabilities of the modeling tool. The results are analyzed in terms of crystal size distributions (CSD) and with a view toward applications in diffusion chronometry; future possibilities are discussed. The modeling results demonstrate that growth at constant rates may be expected only for limited extents of crystallization, that breaks in slopes of CSD-plots should be common, and that the lifetime of a given crystal of a phase is different from the lifetime of a phase in a magmatic system. The last aspect imposes an inherent limit to timescales that may be accessed by diffusion chronometry. Most significantly, this tool provides a bridge between CSD analysis and diffusion chronometry - two common tools that are used to study timescales of magmatic processes.

The paleomagnetic record of meteorites provides invaluable information about planetary formation and evolution. Yet, the potential of these magnetic records in advancing the field of planetary science is severely hindered by a widely used identification technique: application of hand magnets. Here we showcase the destructive effects of touching meteorites with magnets as exemplified by the oldest known Martian meteorite, the Northwest Africa (NWA) 7034 pairing group. We recommend that magnets not be applied to meteorites during collection and curation. Instead, a low-field susceptibility meter is a far more sensitive and completely nondestructive tool for meteorite classification.

Miocene-Pliocene strata in the Pacific Northwest preserve a rich record of landscape evolution and coincident faunal shifts. Considerable research efforts in the past century have been aimed at understanding major drainage reorganization and its relation to tectonics, volcanism, climate change, and aquatic biota. Many studies have focused on fish fossils, which show that Miocene fish diversity, particularly salmonoids, displays great adaptive plasticity. However, the details and mechanisms for river reorganization are still debated. Here we present new and recent detrital zircon provenance results from modern and ancestral river sands collected throughout Oregon, Washington, and Idaho. We synthesize our new results and interpretations with existing paleontological evidence for basin isolation and drainage capture. Detrital zircons from the Columbia Basin (CB) consistently show populations derived from the Snake River Plain (SRP) throughout late Miocene-Pliocene time. However, comparisons of Miocene-Pliocene detrital zircons from the CB to modern major rivers and tributaries in the CB and SPR show that the upstream eastern SRP is a major contributor. CB strata do not require zircons sourced from the western SRP, where Pliocene Lake Idaho existed in a large, deep, and occasionally internally drained basin. Based on the age and provenance results, we suggest that the transiting Yellowstone Hotspot divided the modern SRP into two basins: the western basin was isolated and possibly closed, while the eastern basin drained northward into the modern Clark Fork and Columbia Rivers. This scenario is consistent with fish, mollusk, and rodent fossil evidence from the SRP and CB. In addition, the detrital zircon data indicate a Miocene confluence of the Columbia and Clearwater rivers south of the Saddle Mountains anticline, but north of the current Columbia-Snake River confluence. We also find that the Salmon River may have been captured by the Clearwater River sometime between 4.6 and 8.5 Ma. Prior to this time, the Salmon River likely drained into the SRP. Lastly, we find that faunal localities in southern Oregon suggested to contain evidence for fluvial connection between the western SRP and California are ~2.5 Ma, younger than the incision of Hells Canyon.

In the northern Hikurangi margin, aseismic slip events known as slow slip events (SSEs) occur approximately every 18-24 months with a long duration (weeks to months), and have been shown to cause 1 to 3 cm of horizontal surface displacement onshore and 1.5 to 5.4 cm of vertical displacement (uplift) offshore. The discovery of SSEs has expanded our scientific understanding of slip behavior at subduction zones to encompass behaviors beyond earthquake-producing (fast) slip events. Using converted phase seismic signals from local earthquakes in addition to direct P and S arrivals allows us to further analyze the properties of the plate interface along the subduction zone, including its composition, ­topography and heterogeneity. It has been hypothesized that these properties play a role in the process of slow slip, therefore our investigation of converted waves focuses on the area offshore the east coast of the North Island of New Zealand, where dozens of SSEs have been observed in the past twenty years. We examine an earthquake catalog comprised of local slab earthquakes occurring recorded on ocean-bottom seismometers (OBSs) from the one-year-long Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) experiment as well as New Zealand’s permanent GeoNet on-land seismic network. Preliminary findings indicate strong secondary arrivals on the vertical component of the land stations which are likely Sp conversions from the plate interface. We aim to identify conversions from the plate interface on the OBSs from the HOBITSS experiment, as these data will help us resolve a part of the plate interface that was not investigated in previous converted wave studies. We will conduct a systematic search for phase-converted arrivals from the plate interface, initially focusing primarily on Sp conversions by examining the vertical component seismograms.

Damage zones are important to the rupture dynamics, evolution and fluid coupling of earthquakes. However, information about the damage zone at depth is limited. It is unclear if damage zones increase or decrease in intensity with depth. Here we use marine 3-D seismic surveys and modern fault detection methods to address the depth-dependent structure of damage zones. We use two overlapping legacy industry seismic volumes collected offshore of Los Angeles span approximately 20 km of the Palos Verdes strike-slip fault. The data here allows visibility of the damage zone in the sedimentary formations to 2,200 meters depth, which is comparable to the constraints provided by SAFOD and other studies. Using both interpreted mapped primary fault strands and seismic attributes to identify subsidiary faults, we map and quantify spatial variations in damage zone size and intensity. The damage zone consists of subsidiary faults, or linked discontinuities in the seismics selected within assigned ranges of geometries to the primary strands. Damage was identified using a variation of the seismic attribute semblance, or multi-trace similarity. This method allows interrogation of damage zone in response to changes sedimentary lithology and fault geometry. Subsidiary faults delineate the damage zone to approximately 1 km in width and fracture density decays with distance from the primary fault strands for all sedimentary lithologies in the study area. The damage zone narrows with depth, but fracture density increases because the intensity of fracturing more than compensates for the decreased width. In the thickest formation we find that fracture density increases as Z1.8, where Z is depth in meters. These results are then compared to resolution changes with depth. The damage intensity increase and localization potentially provides a strong constraint for efforts to determine an appropriate rheology for producing damage zones and studying their effects.

The North Qilian Ocean (NQO) was the northernmost branch of the Proto-Tethys separating the Central Qilian Terrane (CQT) from the North China Block (NCB) since the Neoproterozoic Rodinia breakup. An enhanced knowledge on its evolutionary history would greatly improve our understanding on the tectonics of the Proto-Tethys and the assembly of the East Asia. However, the timing of the NQO closure onset remains unsolved with assumptions ranging from the end-Ordovician to the Devonian. To address this issue, integrated studies of stratigraphy, petrology and geochronology were conducted on the Ordovician strata in the SWNCB and the eastern North Qilian Accretionary Belt (ENQAB). Stratigraphic and paleontologic syntheses demonstrate that the pre-Katian strata in the SWNCB are shallow-marine deposits containing abundant benthonic faunas, while the Katian successions atop an unconformity are dominated by deep-water calcareous debrites and siliciclastic turbidites with the dominance of planktonic graptolites. Provenance analysis reveals an evolving source from the NCB basement to the CQT orogen since the Katian. The pre-Katian quartz arenites in the SWNCB contain zircons of ca. 1600–2800 Ma significantly older than their depositional timing, in contrast, the Katian turbidites in the SWNCB and the ENQAB display similar age patterns dominated by ca. 450–900 Ma ages. These clues imply a noteworthy basin-filling shift from passive margin to underfilled peripheral foreland separated by a forebulge unconformity at the Sandbian/Katian boundary. The first arrival of CQT-originated detritus onto the SWNCB at ca. 453 Ma is the oldest stratigraphic constraint for the initial elimination of the northern Proto-Tethys.

Retrogressive Thaw Slumps (RTSs), a highly dynamic form of mass wasting, are accelerating geomorphic change across ice-cored permafrost terrain, yet the main controls on their activity are poorly constrained. Questions over the spatial variability of environmentally sensitive buried massive ice (MI) bodies and a paucity of high-spatial and temporal resolution topographic data have limited our ability to project their development and wider impacts. This research addresses these key problems by investigating RTS processes on Peninsula Point — the type site for intra-sedimental MI in the Western Canadian Arctic. Utilizing high-resolution topographic data from drone surveys in 2016, 2017 and 2018 we (1) measure the temporal and spatial variations in headwall properties and retreat rates, (2) determine the spatial pattern of subsurface layering using passive seismic monitoring and (3) combine these to analyse and contextualise the factors controlling headwall retreat rates. We find that headwall properties, namely MI thickness and overburden thickness, are significant controls over rates of headwall retreat. Where persistent ice exposures are present and overburden thickness remains < 4 m, headwall retreat is typically more than double that of other headwalls. Furthermore, a 3D site model was created by combining photogrammetric and passive seismic data, highlighting the variability in internal layering, demonstrating the limitations of extrapolations based on headwall exposures, and improving predictions of headwall retreat rates compared to long term averages and extrapolations from the previous year. These results provide fresh insights into the controls on headwall retreat rates and new approaches to improve their predictability.

Anthropogenic subsurface urban heat islands (SUHIs) in groundwater under cities are known worldwide. SUHIs are potentially threats to springs because much spring fauna, like trout, amphipods, and rare plants, is cold stenothermal. The city of Minneapolis, Minnesota, USA, has a SUHI documented by the temperature of an underground spring, dubbed “Little Minnehaha Falls,” inside Schieks Cave, which is located 23 m below the central core of the city. In 2000 the temperature of that spring was elevated 11°C above regional background groundwater temperatures (8°C) at this latitude (45°N). A thermometric survey of the cave and nearby tunnel seepages in 2007 found that an abandoned drill-hole through the bedrock ceiling of the cave was discharging groundwater with a temperature of 17.9°C. By comparison, groundwater in the deep water-table below the cave was closer to natural background temperatures for the region. The unusually warm groundwater was thereby localized to the strata above the cave. This is the strongest signal of anthropogenic groundwater warming in the state of Minnesota and is attributed to vertical heat conduction from basements and pavements. Minneapolis is unique among SUHIs in that a cave forms a natural collection gallery deep below the city surface, whereas the literature is almost exclusively based on data from observation wells.

A portion of pore water is typically in a state of unfrozen condition in frozen soils due to the complex soil-water interactions. The variation of the amount of unfrozen water and ice has a significant influence on the physical and mechanical behaviors of the frozen soils. Several empirical, semi-empirical, physical and theoretical models are available in the literature to estimate the unfrozen water content (UWC) in frozen soils. However, these models have limitations due to the complex interactions of various influencing factors that are not well understood or fully established. For this reason, in the present study, an artificial neural network (ANN) modeling framework is proposed and the PyTorch package is used for predicting the UWC in soils. For achieving this objective, extensive UWC data of various types of soils tested under various conditions were collected through an extensive search of the literature. The developed ANN model showed good performance for the test dataset. In addition, the model performance was compared with two traditional statistical models for UWC prediction on four additional types of soils and found to outperform these traditional models. Detailed discussions on the developed ANN model, and its strengths and limitations in comparison to different other models are provided. The study demonstrates that the proposed ANN model is simple yet reliable for estimating the UWC of various soils. In addition, the summarized UWC data and the proposed machine learning modeling framework are valuable for future studies related to frozen soils.

Openly accessible space-borne lidar, ICESat-2 datasets along with Pleiades stereo datasets provide a unique opportunity for estimation and monitoring of fragile sites in relatively inaccessible complex terrain for their changes in elevation and state. Two sets of over 100 lidar points (footprints) from ICESat-2 Track ID: 1354, dated 27 March 2019 were chosen in the flood-impacted Rishiganga and Dhauliganga valleys, i.e. at the place of the rock slide and a confluence Junction on the downstream side towards the severely affected Raini Village. These two locations depict the large-scale changes that occurred due to the flash flood initiated by the rockfall on 7th February 2021. The pre-and post- datasets from Google Earth optical images depict the large variations that occurred due to the event. Digital elevation model generated from Pleiades stereo datasets acquired on 10th February 2021 (post-event), is used for analysis with ICESat-2 datasets (pre-event). Before the event at the analyzed junction location, there was a width of about 30m channel with boulders and had tree-covered surrounding slopes. After the event lot of mud and debris have accumulated in a width of about 300 m without tree cover. The trees got unearthed in the processes involved in the event. The analysis shows a clear sign of erosion on the banks and the accumulation of debris along the river channel as well. The analysis depicted an accumulation of debris that raised the elevation from 0.1m to 44.86m at places with an average of 11.34m. Whereas the erosion varies from 0.15m - 15.76m with an average of 4.85m, mainly on the eroded river banks.

Thermal history models are interpretive tools that incorporate data from chronometers and implement the published kinetics in the context of independent constraints on a sample’s known geologic history, in order to explore specific gaps in geologic knowledge. Despite their central role in the interpretation of thermochronologic datasets, our community has no standards for what characterizes a “robust” thermal history model result, how a model result’s rigor can and should be demonstrated, and how to communicate the key layers of interpretation produce a preferred thermal (and geologic) history. As a result, and through no fault of any one study or modeling program, published models are a patchwork of modeling philosophies, assumptions, and auxiliary hypotheses that are rarely sufficiently explored—to the frustration of authors, reviewers, and readers. This patchwork can give rise to conflicting conclusions and generate apparent controversies that distract from the geologic questions at hand. Therefore, our community needs to both embrace a diversity of modeling approaches and collectively discuss and set broad expectations for what constitutes thermal history modeling best practices. Here, we argue that the fundamental characteristic of any robust thermal history model result is that it is accompanied by a clear articulation of the “why”—e.g., the reason(s) that a model produces a distinctive history, be it the power of a geologic constraint, a grain’s age, a spatial relationship between samples, the choice of kinetic model, etc. We demonstrate this approach using (U-Th)/He data from basement rocks in the Front Range, CO, which when modeled require a distinctive Neoproterozoic thermal history: heating to 235-280°C after ca. 650 Ma and then cooling to <60°C during Paleozoic time. We demonstrate “why” through a suite of models that add, modify, and remove geologic constraints and data from the preferred model. We find that a heating event is required to produce the observed zircon He age-[eU] trend because (1) there is no more than ~600 My of radiation damage accumulated in the zircon crystals, (2) the geologic record places the samples at the surface prior to 650 Ma, and (3) published Ar ages require that these rocks were colder than ~250˚C for most of the last 1.5 Gy. By identifying these key factors, our sensitivity test facilitates comparisons to other studies and directs further discussion to how confident we are in the parts of our data and model set-up that produce this distinctive result. More broadly, this exercise demonstrates one of the challenges of deep-time thermochronology: the potential to accumulate multiple auxiliary assumptions that control the model result in ways that are not obvious, even to the experienced model user, without deliberate exploration of alternative solutions—further underscoring the need for more open discussion of this topic.

The interaction between form and process within a river produces the variety of morphodynamics we observe in channels. This poster presents a method using a simple index of channel behaviour that quantitatively represents the style of deformation a river reach undergoes. We term this index the throughput ratio (ζ), and it is calculated by comparing the volume of morphologic change recorded during an event to the volume of sediment transported during the event. The ratio of these two volumes represents a change in behaviour from exchange-based deformation of the channel (ζ < 1) to a more resilient throughput channel state where material is sourced from upstream, does not interact with the reach in question and is transmitted through (ζ > 1). A pair of experiments that developed different morphodynamics whilst sharing the same initial width, slope, discharge and grain size were used to demonstrate this methodology and interpretation of the results. The difference in morphodynamics between the channels was due to the presence of inerodible banks in one experiment, and a freedom to widen in the other. The inclusion of fixed banks prevented the system from being able to adjust its channel cross-section as freely, and maintained a high but variable sediment throughput over the experiment. In the system with mobile banks, the channel widened and exhibited a greater capacity to store sediment inside and outside of the active channel, causing the sediment transport rate to decline to zero during the experiment. In both, the rate of morphologic change tended to zero despite their marked differences in sediment transport over time. As a result, the throughput ratios depict two contrasting evolutions of channel behaviour. The differences in trajectory are due to the processes available to each system and their feedback with channel form. This approach provides a new method of representing channel character that may act to supplement existing analyses of river behaviour.

Most thermochronological studies aimed at constraining exhumation rates rely on bedrock datasets. Often, they involve the analysis of samples collected along an elevation profile in terrains with high relief. However, there are several limitations to this approach, most importantly access to an appropriately steep traverse and sufficient relief to overcome uncertainties, and to have a broad enough range in closure ages as a function of elevation. Detrital thermochronology offers an alternative approach which can mitigate these challenges through coordinated dating of modern river sediments using multiple thermochronologic methods. Modern detrital sediments from active catchments provide an excellent source of material, typically rich in rock-forming and accessory minerals. Detrital thermochronologic data for material in sedimentary basins has been used widely to infer exhumation histories of sedimentary source terrains, reconstruct paleorelief, and evaluate spatial and temporal variations in erosion rates; however, there have been comparably fewer studies that apply this technique to evaluate regional exhumation patterns using detrital samples from active catchments. Based on the approach presented in Gallagher and Parra, (2020), we are exploring the capability of detrital thermochronologic data to infer regional exhumation patterns in the southeastern Sierra Nevada, CA. Here the uplift history remains debated and the potential mechanism of uplift has yet to be thoroughly constrained. Many catchments along the eastern side of the Sierra Nevada exhibit advantageous characteristics for detrital thermochronologic studies, including steep topography and high relief (that make it more difficult to sample bedrock), limited lithologic variability (which minimizes point-source biasing), relatively simple geologic structure, and relatively easy access to detrital sampling localities. Additionally, the dominant source of the southeastern Sierra Nevada catchments, the igneous units of the Sierra Nevada batholith, include abundant rock-forming minerals for 40Ar/39Ar thermochronology (hornblende, biotite, and sometimes muscovite) as well as abundant accessory minerals for (U-Th)/Pb geochronology(zircon), (U-Th)/Pb thermochronology (apatite), and (U-Th)/He thermochronology (zircon and apatite). Collectively, detrital thermochronological data from these minerals can elucidate much of the post-crystallization thermal history of the eastern flank of the Sierra Nevada. Preliminary results of this technique demonstrate the potential of this cost- and labor-efficient approach for exhumation history studies.

The weathering of continental surfaces and the transport of sediments via rivers into the oceans is an integral part of the dynamic processes that shape the Earth’s surface. To understand how tectonic and climatic forcings control regional rates of weathering, we must be able to identify their effects on sedimentary archives over geologic timescales. Cosmogenic nuclides are a valuable tool to study rates of surface processes and have long been applied in fluvial systems to quantify basin-wide erosion rates. However, in large rivers, continual processes of erosion and deposition during sediment transport make it difficult to constrain how long sediments spend within the fluvial system. In this study, we examine the role of rivers in transmitting and buffering perturbations to the continental erosional signal by constraining the timescales of fluvial transport in large rivers across the world. We apply a stochastic numerical model based on measurements of cosmogenic nuclides concentrations and calculate sediment residence times of 10^4-10^5 years in large rivers. These timescales are equal to or longer than climatic cycles, entailing that changes to rates of weathering brought on by climatic variations are buffered during transport in large rivers and are not manifested in the sedimentary record.

The Ultraviolet Laser Ablation Microprobe (UVLAMP) method of releasing helium from samples is an excellent, but under-utilized, tool in the diverse toolkit of gas extraction approaches available to researchers working with the (U-Th-Sm)/He thermochronology method. So far, most applications have involved some form of Laser Ablation (U-Th-Sm)/He dating (LAHe) or combined LAHe and Laser Ablation U-Th/Pb double dating (LADD) (e.g. 1, 2, 3, 4, 5, 6, 7). Other applications using UVLAMP have focused on 2D-mapping of helium distributions within zircon crystals (8) and stepwise Laser Ablation Depth Profiling (LADP) of induced helium diffusional loss profiles in apatite and zircon (9, 10). Based on the latter examples the stepwise helium LADP method would appear to be an excellent method to study the intricacies associated with a variety of aspects of the (U-Th-Sm)/He dating method and the interpretation and modeling of its results. Given that it creates high resolution helium profiles from the crystal margin to its core without the need to heat the sample to release the gas. Thus, it avoids issues of within-experiment radiation damage annealing, diffusional flattening of helium zonation, and/or the sudden release of helium from fluid and/or melt inclusions that can be associated with approaches using step heating of samples to acquire similar information about the helium distribution within a sample. In this contribution we focus on the results of high spatial resolution helium LADP experiments in a variety of accessory minerals (apatite, zircon, monazite, and titanite). The experiments are intended to a) empirically determine the alpha ejection distance and how those results compare to the distance for each mineral derived from SRIM calculations (11) and b) image natural helium distribution profiles from rim to core in zircons to produce data that are equivalent to those produced by 4He/3He thermochronology (12) experiments, but without the need to proton irradiate the sample. Initial LADP results on Durango apatite yielded an alpha ejection distance that is within error of the theoretical value, while results from several larger (>5 mm) zircon crystals did not yield profiles consistent with the presence of a straightforward alpha ejection zone. The helium depth profile results from the zircons were suggestive of either natural diffusional loss profiles, showing evidence of U-Th zoning, or a combination thereof. 1 Boyce et al. GCA 70, 2006; 2 Vermeesch et al. GCA 79, 2012; 3 Tripathy-Lang et al. JGR-ES 118, 2013; 4 Evans et al. JAAS 30, 2015; 5 Horne et al. GCA 178, 2016; 6 Horne et al. CG 506, 2019; 7 Pickering et al. CG 548, 2020; 8 Danisik et al. Sci Adv 3, 2017; 9 Van Soest et al. GCA 75, 2011; 10 Anderson et al. GCA 274, 2020; 11 Ziegler and Biersack, 1985; 12 Shuster and Farley EPSL 217, 2004.

In spite of many specific studies focussing on the Sea of Marmara segment of the North Anatolian Fault (NAF), its deformation and stress accumulation pattern remain difficult to understand. In part this is due to the complexity of the transform fault system which here combines a releasing and restraining bend. In this study, we use analogue modelling to reproduce and monitor the strain patterns across a releasing and restraining bend pair. We also compare the strain evolution with the evolution of topographic changes. The experiments reveal how the master right-lateral strike-slip fault system and newly formed fault zones change their geometry as displacement accumulates across a releasing and restraining bend pair. We find that the master shear zone develops from a single to a multi-branch fault system, with different branches active and dominant at different times. Comparison with the tectonic setting of the Sea of Marmara suggests that the western portion of the basin may be characterized by a fault shortcut associated with both a compressional regime and uplift of the Ganos Mountain.

Canary Islands constitute an active volcanic archipelago. From the time immediately before the Castilian conquest of the islands, 17 volcanic eruptions have occurred: 2 prehispanic and 15 historical, some of them with multiple eruptive vents, affecting the islands of Lanzarote, Tenerife, La Palma and El Hierro. In order to carry out the inventory of geosites for the Canary Islands at a regional scale, it has been applied a methodology consisting of two sequential phases: the first one address the selection of geosites that will be part of the inventory, and the second one deals with the characterization and assessment of the selected geosites. In this methodology, geosites are selected within geological frameworks previously established given their regional significance for Canary Islands. With this aim, 12 geological frameworks representative of the geodiversity of Canary Islands have been identified, which include the essential elements, processes and morphologies of the Canarian geology, covering all stages of construction of the islands as well their geological evolution, including processes, morphologies, fossils and deposits associated both to the volcanism and the external geological agents. The selection of geosites is then constrained and facilitated by its representativeness within each geological framework. One of these 12 geological frameworks corresponds to “Historical and prehistorical volcanism”. The scarce number of volcanic eruptions, their low frequency and their general characteristics –fissure mafic eruptions, low VEI and strombolian eruptive styles- determine an apparent geological homogeneity. Nevertheless, the high variety of processes, morphologies and deposits associated to this framework, their good conservation status, as well as the information from the historical chronicles, have permitted to identify 19 geosites of high scientific value. They highlight the existence of eruptive styles ranging from hawaian to vulcanian, with short phases of water-magma contact, quiet emissions of water or in geysers, phreatomagmatic explosions, etc. These geosites constitute a unique and representative record of the volcanology, geomorphology, tectonics and petrology characterizing the most recent mafic volcanism of the Canarian archipelago.

We conducted sandbox analogue experiments for subduction of trench-fill sediments beneath accretionary wedge and backstop in order to explain how protoliths of high-pressure/low-temperature (HP-LT) metamorphic rocks are transported to high pressure environment. At accretionary-type subduction zones, it is commonly difficult that coarse-grained sandy trench-fill deposits subduct deeper than high pressure environment (>10 km in depth), because they are accreted at the shallower part of the wedge (<5 km) in association with stepping down of decollement due to progressive dewatering under the accretionary wedge. However, ancient exhumed accretionary complexes sometimes accompany with low-grade accretionary rocks from trench-fill turbidites and HP-LT metamorphic rocks including psammitic and even conglomeratic schists, whose provenance and depositional ages are similar to each other. Therefore, we need a model to explain growth of accretionary wedge and subduction of coarse-grained trench-fill sediments beneath the wedge at the same time. In this study, we attempt to identify an importance of seafloor roughness for transportation of trench-fill sediments to deep during subduction. For this purpose, we performed sandbox analogue experiments by using an unfixed rigid backstop on a subduction channel with the cases of smooth surface (Exp. A) and rough surface representing a seamount or ridge on subducting lower plate (Exp. B). The results of Exp. A showed progressive thickening of the accretionary wedge pushed the backstop down, meaning stepping down of the decollement and narrowing the subduction channel. On the other hand, Exp. B showed a subducting seamount lifted up the backstop, stepped up the decollement, and then widened the subduction channel. Subduction of a rigid material like seamounts is a possible mechanism to open subduction channels for transportation of terrigenous sediments from the trench to high-pressure condition. Significant sediment supply to the trench and rough surface of subducting oceanic plate are required to enable subduction of protolith of HP-LT metamorphic rocks and accretion of trench-fill sediments at the shallow part.

Understanding the mountain–basin coupling relationship is fundamental to placing constraints on the tectonic evolution of the Ailao Shan–Red River mylonite shear zone, the key feature accommodating relative movement between the Tibetan Plateau and SE Asia, because a contemporary basin bounds its middle segment on the northeast, along which the shear zone is bent from northwest–southeast to roughly east–west. The basin comprises two units: the Mubang Breccia and the Lengdun Conglomerate of Early Oligocene and Late Oligocene–Early Miocene age, respectively. This study reveals evidence indicating that the Wubang Breccia marks a high-strain zone, resulting from top-to-north shear (range-front detachment (RFD)), along which the mylonite on the footwall experienced northward bending. Moreover, the Lengdun Conglomerate on the hanging wall was deposited as growth strata, overlying a thrust belt to the north. The latter marks the southern rim of the Yangtze block, composed of landslide blocks, whose northward displacement along the toe of the RFD was synchronized with the north–south extension across the Red River basin. The spatial and temporal relationships between the Red River basin and the Ailao Shan–Red River shear zone indicate that basin formation was controlled by the change in geometry of the shear zone. The Red River basin can be viewed as an extensional step-over in the left-lateral strike-slip field, in which all sedimentary and deformation processes are the manifestation of the gravitational collapse, accommodated by the RFD. This indicates that the sedimentary detritus, including both landslide blocks and the Langdun Conglomerate, were all shed from the top of the Ailao Shan mylonite belt. The cause of bending of the shear zone is attributed to the northward movement between India and South China.

Individual lava flows in flood basalt provinces are composed of sheet pāhoehoe lobes and the 10-100 m thick lobes are thought to form by inflation. Quantifying the emplacement history of these lobes can help infer the magnitude and temporal dynamics of these prehistoric eruptions. Here we use a phase-field model to describe solidification and re-melting of sequentially-emplaced lava flows to explore additional processes that may lead to thick flows. We calibrate model parameters using field measurements at Makaopuhi lava lake. We vary the thickness of individual flows and the time interval between eruptions to study the interplay between thermal evolution, flow thickness and emplacement frequency. Our theoretical analysis shows that, if the time between emplacement is sufficiently short, reheating and re-melting may merge sequentially emplaced flows — making flows appear thicker than they actually were. Our results suggest that fused flows could be another mechanism that creates apparently thick lava flows.

Slickenlines are lineations thought to record slip motion and mechanical wear within shear fractures. Their formation mechanisms and effect on friction and fault rheology are poorly understood. We investigate natural slickenlines from strike-slip, normal, and low-angle detachment faults formed in volcanic, quarzitic, and mylonitized sedimentary lithologies, respectively. Slickenline surfaces exhibit non-Gaussian height distributions and anisotropic self-affine roughnesses with corresponding mean Hurst exponents in directions parallel– 0.53±0.07– and perpendicular –0.6±0.1– to slip. However, there is a significant variability in the fractal roughness descriptors obtained from multiple hand samples per fault surface. Microstructural analyses reveal that the principal slip surface is formed by a thin (≤100 µm) nanoparticulate- and phyllosilicate-rich layer, followed by a ~10 μm thick layer of increased cohesion, wherein several smaller grains coalesce into bigger aggregates. These microstructures are present in most analyzed samples suggesting that they commonly form during fault slip regardless of lithology or tectonic setting. Our results 1) suggest that deformation immediately adjacent to fault surfaces is energetic enough to comminute the rocks into nanometric grains and 2) highlight the intricacies of fault systems not fully captured by current models, which are likely to impact stress distributions and frictional responses along faults.

Water release in subduction zones is not only an important part of the deep Earth’s water cycle, but also plays an essential role in the physical and chemical properties of rocks constituting the deep Earth. To understand water release processes, it is important to know properties of dehydration in hydrous phases of the downgoing slab. Although it is widely accepted that phengite can be stable to greater depth in subduction environment, behavior of hydroxyl and lattice of it at high temperature and high pressure are less investigated in contrast to other hydrous phases. Here, using IR and Raman spectroscopy, we characterize hydroxyl and lattice of ammonium-bearing and ammonium-free phengite at high temperature and high pressure. No proton transferring and structural phase transition in phengite were observed over the measured temperature and pressure range. Both pressure and temperature induce hydroxyl band shifting to lower frequencies, and pressure has a greater impact. The band width of hydroxyl increases with temperature and pressure. Hydroxyl bond weakening and hydrogen disordering at high temperature and high pressure should be responsible for the spectra variations. On the other hand, the lattice modes soften with increasing temperature whereas stiffen under compression, and ammonium plays an important role in the Grüneisen parameters of the lattice modes, especially the K-O mode. These features of hydroxyl and lattice at high temperature and high pressure could benefit for further understanding dehydration, thermodynamic properties and stability of phengite in subduction zones.

Barrovian-grade pelites in the Greater Himalayan Crystallines and Lesser Himalayan Formations exposed in the Himalayan core are separated by the Main Central Thrust (MCT). This fault system accommodated a significant amount of India-Asia convergence and is the focus of several models that explore ideas about the development of the range and collisional belts in general. Units separated by the MCT provide critical information regarding the mechanisms of heat transfer within collisional belts. Garnets collected across the MCT record their growth history through changes in chemistry. These chemical changes can be extracted and modeled using a variety of thermodynamic approaches. Here we describe and apply particular thermobarometric techniques to decipher the metamorphic history of several garnet-bearing rocks collected across the MCT in central Nepal, the Sikkim region, and NW India. Comparisons are made between the results of previously-reported conventional rim P-T conditions and P-T paths extracted using the Gibb’s method to isopleth thermobarometry and high-resolution P-T path modeling using the same data and assemblages. Regardless of calibrations used, the P-T conditions and paths, along with previously-reported timing constraints, are consistent with an imbrication model that suggest the MCT shear zone developed as rock packages within the LHF were progressively transferred. In this model, samples within the LHF travel along the MCT at a 5 km/Ma speed rate from 25 to 18 Ma. The hanging wall speed rate is 10 km/Ma, and topography progressively accumulates until a maximum height of 3.5 km. Once the topography is achieved at 18 Ma, a period of cessation is applied to the MCT between 18 and 15 Ma, and topography is reduced at a rate of 1.5 km/Ma. The model returns to activity within the MCT shear zone with the activation of the MCT footwall slivers from 8 to 2 Ma. P‐T changes recorded by the footwall garnets result from thermal advection combined with alterations in topography. For most MCT footwall samples, the P-T paths match the model predictions remarkably well. The P-T paths for some samples in central Nepal are also consistent high exhumation rates (>12mm/year) within the MCT shear zone since the Pliocene, a scenario predicted by this imbrication model.

The shergottite family of meteorites shows a remarkable petrographic and geochemical variety, revealing information about mantle processes and basalt formation on Mars. Northwest Africa (NWA) 11115, found in Morocco in 2016, is one of the newest meteorites in this family. We report bulk-rock major and trace element abundances of NWA 11115, bulk oxygen isotope systematics, and the petrography and mineralogy of a thick section, and compare the geochemistry of this recent find to other martian rocks. NWA 11115 is an olivine-phyric shergottite with an enriched rare earth element pattern, and shares similarities with NWA 1068, another enriched olivine-phyric shergottite. The large (< 2.5 mm) olivine phenocrysts are likely to be cumulates, similar to NWA 1068. However, the abundant maskelynite (~30 vol. %) in NWA 11115 places the bulk chemistry somewhat closer to the basaltic shergottites. We suggest that NWA 11115 is genetically linked to NWA 1068, perhaps crystallizing slightly above in the same cumulate pile. NWA 11115 contains one of the lowest K/Th ratios among the martian meteorites (K/Th = 2987 ± 810), and far lower than the surface of Mars (K/Th = 5300). Finally, while NWA 11115 contains abundant (~0.4 vol. %) fracture-filling calcite (presumably from hot desert alteration during its terrestrial residence), diagnostic bulk element mass ratios were not indicative of the presence of terrestrial alteration (Th/U ≈ 4.09, Sr/Nd ≈ 12.26, K/La ≈ 526.94, Ce/Ce* ≈ 1.01).