Extensional Stress Research Articles

Fabrication of PLA/CB composites with excellent electrical conductivity and stiffness-ductility balance based on coupling extensional stress with thermal field

A newfound scalable strategy of “annealing stretching-thermal treatment” is proposed to tune electrical conductivity and stiffness-ductility balance of polylactide/carbon black (PLA/CB) conductive composites. The extensional stress-induced orientation and crystallization lead to a significant increase in tensile strength (from 69.3 MPa to 102.0 MPa) and elongation at break (from 9.8 % to 48.7 %). Further thermal treatment effectively reconstructs the conductive network destroyed by the strong extensional stress, causing a sharp increase in electrical conductivity to 5.75 S/m, which is surprisingly ∼ 109 higher than that of neat PLA (5.61 × 10-9 S/m). Besides, the ductility (44.3 %) and strength (97.8 MPa) are well preserved after thermal treatment, attributed to the preservation of oriented amorphous phase and the perfection of oriented crystals, respectively. Moreover, the heat resistance is also improved after thermal treatment. This work could provide significant guidance for facile and efficient fabrication of high-performance bio-based conductive polymer composites.

Unravelling the geological and geomorphological evolution of the Terra Cimmeria-Nepenthes Mensae transitional zone, Mars

The Terra Cimmeria-Nepenthes Mensae Transitional Zone comprises two highly contrasting geologic-geomorphic domains bounded by the dichotomy escarpment in the equatorial region of Mars. This research provides insight into the geological and geomorphological evolution of the poorly explored NW Terra Cimmeria and SE Nepenthes Mensae. A detailed study largely based on cartographic work and complemented by multiple analyses focused on crater densities, spectral data and radar profiles reveals that (1) the exposed geological record of this transitional zone formed over a long time-span from the Early Noachian to recent times, (2) the inferred compressional and extensional tectonic stresses shaped the landscape at least from the Late Noachian to Late Hesperian, and (3) extensional tectonics, which post-dated the compressional deformation, has controlled the development and evolution of the 2-km-high dichotomy escarpment, the system of NW-SE-oriented basins that used to host paleolakes from the Late Noachian to Late Hesperian, and the fissure volcanism in their deepest areas. These findings shed light into the fundamental morphogenetic role played by compressional and extensional tectonics, and fluvio-lacustrine activity on the configuration of the landscape in these regions. This work provides a reference geologic-geomorphologic framework for future studies in this area and other sectors along the highland-lowland transitional zone of Mars.

Development of pulsating extensional stress induced melt mixer and its application for polypropylene/carbon fibers composites

In this work, a pulsating extensional stress induced melt mixer was developed. The compositions, structure, working principle, and melt mixing process of the device were introduced in detail. The mixer generated a full-process pulsating extension deformation effect on materials by the eccentric vane units and arc-shaped flow channels, which greatly promoted the dispersion and mixing of the materials. Polypropylene/carbon fibers (PP/CFs) composites were prepared by the new mixer, and the effects of different rotational speeds and mixing time on the properties of the composites were studied. The results showed that proper rotational speed and mixing time effectively promoted the dispersion of CFs in PP matrix, while the fibers still had long remaining lengths. As a result, the overall properties, especially the mechanical properties of the PP/CFs composites improved. When the CFs content was 30 wt.%, at the process parameters of 20 r/min and 2 min, compared with pure PP, the tensile strength (58.46 MPa) and modulus (1962 MPa) of the PP/CFs composite were increased by 78.6% and 263.3%, respectively.

Tuning structure in 3D-printed scaffolds of polylactide by extensional stress and its influence on properties

In response to the increasing demand for biological scaffolds for bone defect repair in tissue engineering, fabricating biocompatible porous scaffolds with controllable mechanical properties has been a significant challenge. In this work, we successfully designed a unique Triply Periodic Minimal Surface (TPMS) structure along with delicate uniaxial stretching on it, which was integrated as a “three-dimensional (3D) printing-uniaxial stretching” strategy. The lightweight (0.505 g/cm3) and superior impact strength (25.63 ± 1.4 kJ/m2) of the 3D-printed polylactide (PLA) scaffold are attributed to the smooth and continuous porous structure, which is also beneficial for the uniformly deformed during the uniaxial stretching. With the increase of the draw ratio, the volume proportion of the inner hollow porous progressively expanded, allowing the density to decrease from 0.505 g/cm3 to 0.428 g/cm3 (a decrease of 15.2%). Crucially, the mechanical properties of the 3D-printed porous scaffold are significantly improved after uniaxial stretching. Impact strength and yield strength increased significantly to 400% and 355% of the original 3D-printed PLA scaffold, climbing to 101.16 ± 1.68 kJ/m2 and 13.88 ± 0.68 MPa, respectively. The relative modes of action are discussed in detail. This work provides a novel method to improve the application range of 3D-printed porous scaffolds in biomedical applications of tissue engineering and engineering materials.

Contemporaneous crust-derived I- and S-type granite magmatism and normal faulting on Tinos, Delos, and Naxos, Greece: Constraints on Aegean orogenic collapse

Granitoids of varying mineralogy are exposed on the Cycladic islands of Greece; they include both hornblende-bearing I-type granites and garnet ± muscovite−bearing S-type granites, suggesting heterogeneous magma sources. In this contribution, we present new field observations, major- and trace-element geochemistry, Sr-Nd isotopes, and U-Pb geochronology of granitoids from Tinos, Delos, and Naxos that provide insight into these magma sources, along with the timing of adjacent extensional structures. I-type (biotite and hornblende-biotite) granites have initial 87Sr/86Sr = 0.70956−0.71065 and εNd(t) = −6.3 to −9.3, and S-type (garnet ± tourmaline-muscovite) leucogranites have overlapping initial εNd(t) = −7.5 to −10.1, with initial 87Sr/86Sr values overlapping as well as extending to higher values (0.70621−0.73180). These isotope signatures are comparable to those of the Variscan-age Cycladic basement, but not the Hellenic arc. We suggest that both I- and S-type granites were derived via crustal anatexis of variable sources, dominantly metaigneous and metasedimentary, respectively, during the climax of Barrovian metamorphism between ca. 17 and 12 Ma, and critically, they are not related to the Hellenic subduction zone. I-type granitoids were likely derived from dehydration melting of igneous Variscan- or Cadomian-aged basement protoliths, whereas S-type leucogranites formed by muscovite dehydration melting of sedimentary protoliths. Top-to-the-(N)NE shear zones on Naxos and Tinos were active from ca. 20 to 15 Ma and are folded and cut by later low- and high-angle normal faults. S-type leucogranites at Livada Bay, Tinos, dated at ca. 14 Ma, are cut by domino-style normal faults, placing a maximum age on the timing of extension. This is similar to ca. 15−14 Ma dates from NNE-SSW horizontally boudinaged S-type granites on Naxos. We propose that the concurrent intrusion of both I- and S-type granitoids with the onset of normal faulting marked the transition from an overall compressional to an extensional stress field associated with orogenic collapse at ca. 15 Ma.

The Gibraltar slab dynamics and its influence on past and present-day Alboran domain deformation: Insights from thermo-mechanical numerical modelling

The origin and tectonic evolution of the Gibraltar Arc system is the result of a complex geodynamic evolution involving the convergence of the Eurasian and African plates and the dynamic impact of the Gibraltar slab. Although geologic and geophysical data collected in the last few years have increased our knowledge of the Gibraltar Arc region, it is still unclear which are the mechanical links between the Gibraltar slab and the past deformation of the overriding Alboran lithosphere, as well as to which degree this subduction system is presently active. In this study, we use 2D numerical modelling to investigate the impact of the Gibraltar slab dynamics on the deformation of the overriding Alboran lithosphere. Our model simulates a WE generic vertical section at an approximate latitude of 36°N and considers an initial setup at about Burdigalian times (∼20 Ma), when the subduction front position is relatively well constrained by recent tectonic reconstructions. Our modelling shows a switch in the overriding plate (OP) stress state from extensional stresses during the slab rollback to compressional stresses near the trench when the rollback velocity decreases, caused by the change in slab-induced mantle flow. We also find that much of the crustal and lithospheric deformation occur during fast slab rollback and OP extension in the first 10 Myr of evolution, while after that only moderate deformation associated with subduction is predicted. Finally, we find that despite the subduction rollback ceases, the ongoing motion of the deeper portion of the slab induces a mantle flow that causes some amount of west-directed basal drag of the Alboran lithosphere. This basal drag generates interplate compresional stresses compatible with the distribution of intermediate-depth earthquakes in western Alboran.

The effect of along-strike variable plate deflection on bending stress and seismicity at the southern Mariana Trench

Bending-induced faults are often recognized as sources of extensional or compressional earthquakes in the outer rise region, and have been extensively investigated in 2-D plate flexural models. However, such 2D models have difficulty in explaining observed earthquakes caused by reactivation of pre-exsiting fabrics inherited from mid-ridge, especially when the fabrics is oblique to the subduction. Here, we develop a 3-D flexural model to investigate the plate deflection at the southernmost Mariana subduction zone. The Particle Swarm Optimization (PSO) method was used to invert flexural parameters of the subducting plate. It shows that the boundary loading near the Challenger Deep is nearly twice that in the region further east. Under this deformation, the bending stresses within the outer rise vary significantly along the strike of the subduction zone. We find that the location of abrupt stress changes coincides with the outer rise earthquakes which were considered to be reactivation of remnant faults. We proposed a model to illustrate the influence of along-strike variation of plate deflection on bending stress and outer rise earthquakes: The variation of plate deflection along the trench may cause the change of the direction of the maximum extensional bending stress in the variable zone which provides greater possibilities for reactivation of preexisting weak faults, resulting in unevenly distributed outer rise earthquakes, reactivation of remnant fabric or earthquake.

Breakup of Pangea and the Cretaceous Revolution

AbstractA 250 Ma, Pangea had just reached an equatorial position of dynamic equilibrium, after a 60° northward migration due to True Polar Wandering. It then began oscillating about itself for the next 150 Myr. The resulting extensional stresses triggered three successive phases of breakup, controlled by the mechanical resistance of a crescent of thick lithosphere, surrounding the Tethyan realm, which had adjusted the supercontinent to its hemispheric shape. The fracturing of the crescent was produced in three successive generations, each new generation corresponding to Coulomb fractures, conjugates of the preceding set. Flood basalts were associated with these deep fractures within the thick lithosphere crescent. We consider unlikely that this highly ordered pattern of fracturing was determined by the locations of the impacts of successive plumes. Between 260 and 180 Ma, thermal isolation was maximal and the asthenosphere of Pangea was about 150°C warmer than below Panthalassa. From 180 to 100 Ma, the breakup elongated Pangea by about 3,000 km in a north‐northwest–south‐southeast direction, producing gaps in the subduction girdle. Lateral mixing began, leading to a continuous rise in global sea level and progressive return to a globally homogeneous upper mantle with sea‐level at its maximum 100 Ma. This Cretaceous Revolution marked the end of the Pangea tectonics, radically different from our present plate tectonics. Neither post‐Cretaceous plate kinematic inferences, nor mantle dynamic and associated planetary cooling inferences are extendable to Pangea times.

Clastic rock geochemistry of the Sirohi Basin, Aravalli Craton, NW India: Implications for paleoweathering, provenance and geodynamics

• Petrographic and geochemical data of clastic rock formations in the Sirohi Basin. • Two wacke horizons with distinct textural and chemical characteristics. • Geological evolution of the Sirohi Basin and relation with geodynamics of the Indian Shield. Sirohi Basin of Trans-Aravalli Region, Rajasthan preserves a thick clastic sequence composed of phyllite and metasandstone. Based on their petrochemical properties, metasandstone is classified as K-feldspar rich wacke (Wacke I) and plagioclase rich wacke (Wacke II). Wacke I is characterised by higher K 2 O, Fe 2 O 3 and lower CaO, Na 2 O, and Na 2 O/K 2 O ratios than Wacke II. Chemical Index of Alteration (CIA) and the A-CN-K systematic prescribe least chemical weathering for the sediments of Wacke II, moderate for Wacke I and moderately intense for phyllite of the Sirohi Basin. Chemical traits perceived from UCC spidergrams, discrimination function diagrams, etc. in conjunction with immobile element ratios (e.g., Al 2 O 3 /TiO 2 , Th/Sc, La/Sc, La/Co, Th/Co, Eu/Eu*, La/Sm N and Gd/Yb N ) establish dual-source terrains for Sirohi clastics, one with preponderance of felsic componets and other mafic plus felsic ingrediatns. The petrochemical signatures of Sirohi clastics, though suggest procurement of detritus form diffrent sources but consanguineous evolution in the Sirohi Basin, in response to extensional stresses. :

Structures, deformation history and dynamic background of the Qianyingzi Coal Mine in the Huaibei Coalfield, eastern China

The Qianyingzi Coal Mine is located in the west of the Suxian Mining District of the Huaibei Coalfield, eastern China. The study on structural development patterns and genetic mechanisms in this mine lays an important foundation for safe and efficiently underground mining, and is also the key to understanding the regional tectonic evolution. In this study, based on the analysis of three-dimensional seismic, drilling and underground measured data and regional tectonic correlation, the structures, evolution history and dynamic background of the Qianyingzi Coal Mine are discussed. The Carboniferous-Permian coal measure strata in the mine are generally a gentle syncline with a NNE-trending axis, and cut by a series of faults. The faults developed in this mine are mainly medium- and small-sized with a throw of less than 20 m, and the number of reverse faults is significantly greater than that of normal faults. The strikes of reverse and normal faults are both mainly NE, followed by NNE and nearly N‒S. According to the characteristics of structural geometry, tectonic association, fault property and cross-cutting relation, the structural deformation of coal measure strata in the Qianyingzi Coal Mine can be divided into five stages, and the corresponding tectonic stress fields are NWW‒SEE compressive stress, nearly E‒W compressive stress, NW‒SE compressive stress, nearly E‒W and NW‒SE extensional stresses, respectively. It developed the Fengjia Syncline with a NNE-trending axis in the first stage and nearly N‒S-striking reverse faults in the second stage, which were the results of foreland deformation and subsequent continent-continent collision during the convergence of the North China Craton and South China Plate in the Indosinian period. The NNE-striking reverse sinistral faults and NE-striking reverse faults developed in the third stage is related to the rapid oblique subduction of the Izanagi Plate toward the East Asian continental margin at the beginning of the Early Cretaceous in the western Pacific region. Later, the fourth and fifth stages of the nearly N‒S- and NE-SW-striking normal faults were developed under the backarc extensional background in eastern China during the Early Cretaceous. These new results can be used to guide the rational arrangement for underground mining and also provide a new understanding for regional tectonic evolution of the Huaibei Coalfield.

Comparison of Earthquake Clusters in a Stable Continental Region: A Case Study from Nordland, Northern Norway

Abstract Nordland, Northern Norway, is a seismically active stable continental region. Along its coast, clusters of small earthquakes controlled by local extensional stresses are observed. In this study, we present a comparison between two adjacent seismically active areas along the Nordland coast: Jektvik and Rana, which have distinct spatiotemporal patterns. The seismicity in Jektvik, which presents a swarm-like behavior, shows outward activity progression from its center hinting at triggering between earthquake clusters. In contrast, the seismicity in Rana, where swarms are also observed, does not exhibit such pattern. Earthquakes in the Rana cluster occur within isolated spots and show repeating earthquake behavior. Singular spectrum analysis shows that seismicity in Jektvik has a dominant annual periodicity and is modulated by hydrological load, which is also observable on Global Navigation Satellite Systems stations. Although hydrological load changes also affect Rana, its seismicity does not exhibit an annual periodicity. We hypothesize that the Jektvik seismicity occurs within a fluid-rich fracture system that is affected by hydrological modulation, whereas the Rana seismicity occurs within fault irregularities, which accumulate stress and rupture repeatedly. This study presents a case where adjacent areas within an intraplate setting can have significantly different seismogenic behaviors.

Bayesian regional moment tensor from ocean bottom seismograms recorded in the Lesser Antilles: implications for regional stress field

SUMMARY Seismic activity in the Lesser Antilles (LA) is characterized by strong regional variability along the arc reflecting the complex subduction setting and history. Although routine seismicity monitoring can rely on an increasing number of island stations, the island-arc setting means that high-resolution monitoring and detailed studies of fault structures require a network of ocean bottom seismometers (OBS). As part of the 2016–2017 Volatile recycling at the Lesser Antilles arc (VoiLA) project, we deployed 34 OBS stations in the forearc and backarc. During the deployment time, 381 events were recorded within the subduction zone. In this paper, we perform full-waveform regional moment tensor (RMT) inversions, to gain insight into the stress distribution along the arc and at depth. We developed a novel inversion approach, AmΦB—‘Amphibious Bayesian’, taking into account uncertainties associated with OBS deployments. Particularly, the orientation of horizontal components (alignment uncertainty) and the high noise level on them due to ocean microseisms are accounted for using AmΦB. The inversion is conducted using a direct, uniform importance sampling of the fault parameters within a multidimensional tree structure: the uniXtree-sampling algorithm. We show that the alignment of the horizontal OBS components, particularly in high noise level marine environments, influences the obtained source mechanism when using standard least-squares (L2) RMT inversion schemes, resulting in systematic errors in the recovered focal mechanisms including high artificial compensated linear vector dipole (CLVD) contributions. Our Bayesian formulation in AmΦB reduces these CLVD components by nearly 60 per cent and the aberration of the focal geometry as measured by the Kagan angle by around 40 per cent relative to a standard L2 inversion. Subsequently, we use AmΦB-RMT to obtain 45 (Mw > 3.8) regional MT solutions, out of which 39 are new to any existing database. Combining our new results with existing solutions, we subsequently analyse a total of 151 solutions in a focal mechanism classification (FMC) diagram and map them to the regional tectonic setting. We also use our newly compiled RMT database to perform stress tensor inversions along the LA subduction zone. On the plate interface, we observe the typical compressional stress regime of a subduction zone and find evidence for upper-plate strike slip and normal fault behaviour in the north that becomes a near arc-perpendicular extensional stress regime towards the south. A dominant slab perpendicular extensional stress regime is found in the slab at 100–200 km beneath the central part of the arc. We interpret this stress condition to be a result of slab pull varying along the arc due to partial slab detachment along previously hypothesized lateral slab tear near Grenada, at the southern end of the LA arc, leading to reactivation of pre-existing structures around the subducted Proto-Caribbean ridge.

Experimental study of fracture slip due to stress perturbation in fractured geo-resources

Anthropogenic activities such as underground fluid injection/extraction, well stimulation, and dam impoundment will perturb the state-of-stress in the subsurface, which could activate critically stressed pre-existing fractures and faults. Fault and fracture activation can induce seismicity, change flow behavior, and damage asperities on the fracture surface to alter reservoir permeability. In this study, triaxial shear tests were conducted on saw cut and rough fracture Barre granite specimens to better characterize how a decrease in normal stress and increase in shear stress will influence crystalline rock fracture hydromechanical properties. In a triaxial set up, confining pressure (CP) and differential stress (DS) were used to induce normal and shear stress on a 45° oriented fracture. Fracture slip and the corresponding stress relaxation was observed when the confining stress decreased. Fracture permeability increased when the fracture slipped. Moreover, new micro-cracks observed by x-ray images indicated the creation of new flow paths for fluid movement. The experimental outcomes of this study indicate that decrease in normal stress due to stress perturbations along the faults/fractures in the extensional stress regime can induce the slip along the fracture. This could have a significant influence on slope stability in quarries, stimulation and injection strategy in deep geo-resources, and potentially minimize the risk of induced seismicity. The observed stress dependency of hydraulic aperture in rough fracture during pre- and post-slip stages could provide key inputs for field-scale simulation of geo-resources.

Numerical simulations on the settling dynamics of an ellipsoidal particle in a viscoelastic fluid

The settling dynamics of a rigid ellipsoid in an unbounded viscoelastic fluid under inertialess conditions is studied through direct numerical simulations. The governing equations are solved by the finite element method with an Arbitrary Lagrangian–Eulerian formulation to handle the particle motion. The viscoelastic fluid is modeled through the Giesekus constitutive equation. Simulations are carried out up to a value of Deborah number of 5. The settling of prolate and oblate spheroidal particles is first addressed. The sedimentation, lift, and angular velocities are computed as a function of the orientation angle and for aspect ratios from 1/8 to 8. Regardless of the particle shape, initial orientation, and Deborah number, the particle rotates to align its longest axis along the force direction. A high extensional stress region behind the particle is observed at high aspect ratios due to the large curvature of the tip, leading to a fast decay of the axial fluid velocity downstream and to the appearance of a negative wake. Similarly, a triaxial ellipsoid reaches a final orientation with major axis parallel to the falling direction. The particle shape affects the orientational dynamics, both in terms of the orbits followed by the orientation vectors and the time needed to reach the equilibrium orientation, and the steady-state settling velocity. The fastest sedimentation rate is observed for a prolate spheroid with aspect ratio of about 2 whereas the slowest one for a high aspect ratio oblate spheroid. Triaxial ellipsoids settle with rates in between these two limiting behaviors.

Hot or Fertile Origin for Continental Break-Up Flood Basalts: Insights from Olivine Systematics

Abstract The break-up of supercontinents is often temporally and spatially associated with large outpourings of basaltic magmas in the form of large igneous provinces (LIPs) and seaward dipping reflectors (SDRs). A widespread view is that the upwelling of hot mantle plumes drives both continental break-up and generation of associated LIPs. This is supported by petrologic estimates of the temperature from olivine-melt thermometers applied to basaltic magmas. These thermometers must be applied to a primary mantle-derived magma, requiring the selection of an appropriate primitive magma and an assumption of how much olivine is to be back-added to correct for fractional crystallization. We evaluated the effects of these assumptions on formation temperatures by compiling and analyzing a database of North Atlantic igneous province (NAIP) and Central Atlantic magmatic province (CAMP) lavas and olivines. Ni and FeOT systematics suggest that many picrite magmas have undergone olivine addition and are not true liquids, requiring careful selection of primitive magmas. The maximum amount of back-added olivine was determined by constraining mantle peridotite melt fractions for a range of possible mantle potential temperatures and continental lithosphere thicknesses. Using an empirical relationship between melting degree and forsterite (Fo) content, we show that the possible maximum olivine forsterite content in equilibrium with NAIP magmas is 90.9, which is lower than the maximum olivine forsterite content observed in the NAIP olivine population. We infer primary magmas that lead to mantle potential temperatures of 1420°C for the NAIP and 1330°C for CAMP. Using a similar approach for consistency, we estimate a mantle potential temperature of 1350°C for mid-ocean ridge basalts (MORB). Our results suggest that LIPs associated with continental break-up are not significantly hotter than MORB, which suggests that continental break-up may not be driven by deep-seated thermal plumes. Instead, we suggest that such voluminous magmatism might be related to preferential melting of fertile components within the lithosphere triggered by far-field extensional stresses.

Strong, ductile and durable Poly(glycolic acid)-based films by constructing crystalline orientation, entanglement network and rigid amorphous fraction

Poly(glycolic acid) as a promising substitution for traditional petroleum-based polymers suffers from its inherent brittleness and rapid hydrolysis. The improvement of toughness and hydrolysis resistance is usually accompanied with a sacrifice of strength and stiffness. In the present work, we propose a simple and effective strategy to address this obstacle by introducing poly (butylene adipate-co-terephthalate) (PBAT) and combining extensional stress and temperature field. The robust, ductile and durable PGA/PBAT films are fabricated via “high temperature melting-quenching-stretching-annealing” (QSA) process. Highly oriented crystallites constructed by stretching and annealing lead to the improvement of strength and stiffness. Stress-induced strengthened entanglement network contributes to stabilization of craze growth and slipping, thus leading to the ductile behavior. As the result, the QSA films exhibit 89 MPa and 131% in tensile strength and the elongation at break, which are increased by 195%, 126% respectively compared to the unstretched film. More impressively, stretching resulted in abundant rigid amorphous chains, which worked with oriented crystallites to impart enhanced heat and hydrolysis resistance for the films. This work paves an avenue toward a wider range of applications for PGA materials and might be guidance for other semi-crystalline polymer blends.

Dyke-induced graben formation in a heterogeneous succession on Mt. Etna: Insights from field observations and FEM numerical models

The most common way of magma transfer towards the surface is through dyking. Dykes can generate stresses at their tips and the surrounding host rock, initiating surficial deformation, seismic activity, and graben formation. Although scientists can study active deformation and seismicity via volcano monitoring, the conditions under which dykes induce grabens during their emplacement in the shallow crust are still enigmatic. Here, we explore through FEM numerical modelling the conditions that could have been associated with dyke-induced graben formation during the 1928 fissure eruption on Mt. Etna (Italy). We use stratigraphic data of the shallow host rock successions along the western and eastern sections of the fissure that became the basis for several suites of numerical models and sensitivity tests. The layers had dissimilar mechanical properties, which allowed us to investigate the studied processes more realistically. We investigated the boundary conditions using a dyke overpressure range of 1–10 MPa and a local extensional stress field of 0.5–2 MPa. We studied the effect of field-related geometrical parameters by employing a layer thickness range of 0.1–55 m and a variable layer sequence at the existing stratigraphy. We also tested how more compliant pyroclastics, such as scoria, (if present) could have affected the accumulation of stresses around the dyke. Also, we explored how inclined sheets and vertical dykes can generate grabens at the surface. We propose that the mechanical heterogeneity of the flank succession and the local extensional stress field can largely control both the dyke path and dyke-induced graben formation regardless of the tested dyke overpressure values. Similarly, soft materials in the stratigraphy can greatly suppress the shear stresses in the vicinity of a propagating dyke, encouraging narrow grabens at the surface if only the fracturing condition is satisfied, while inclined sheets tend to form semigrabens, respectively. Finally, we provide some insights related to the structural evolution of the 1928 lateral dyking event. All the latter can be theoretically applied in similar case studies worldwide.

Tectonism of Late Noachian Mars: Surface Signatures from the Southern Highlands

Upwelling mantle plumes often instigate extensional stress within the continental crust of Earth. When stress exceeds crustal strength, extensional structures develop, reducing the effective stress and trigger magmatic processes at the crust–mantle boundary. However, such processes and their relationship to the formation of many surface structures remain poorly characterized on Mars. We identified a series of extensional structures in the southern highlands of Mars which collectively resemble continental rift zones on Earth. We further characterized these extensional structures and their surrounding region (area of ~1.8 M km2) by determining the surface mineralogy and bulk regional geochemistry of the terrain. In turn, this constrains their formation and yields a framework for their comparison with extensional structures on Earth. These terrains are notable for olivine and high-Ca pyroxene with a high abundance of potassium and calcium akin to alkali basalts. In the case of Mars, this Earth-like proto-plate tectonic scenario may be related to the plume-induced crustal stretching and considering their distribution and temporal relationship with the Hellas basin, we conclude that the plume is impact-induced. Overall, the findings of this work support the presence of mantle plume activity in the Noachian, as suggested by thermal evolution models of Mars.

The effect of hydrology and crevasse wall contact on calving

Abstract. Calving is one of the main controls on the dynamics of marine ice sheets. We solve a quasi-static linear elastic fracture dynamics problem, forced by a viscous pre-stress describing the stress state in the ice prior to the introduction of a crack, to determine conditions under which an ice shelf can calve for a variety of different surface hydrologies. Extending previous work, we develop a boundary-element-based method for solving the problem, which enables us to ensure that the faces of crevasses are not spuriously allowed to penetrate into each other in the model. We find that a fixed water table below the ice surface can lead to two distinct styles of calving, one of which involves the abrupt unstable growth of a crack across a finite thickness of unbroken ice that is potentially history-dependent, while the other involves the continuous growth of the crack until the full ice thickness has been penetrated, which occurs at a critical combination of extensional stress, water level and ice thickness. We give a relatively simple analytical calving law for the latter case. For a fixed water volume injected into a surface crack, we find that complete crack propagation almost invariably happens at realistic extensional stresses if the initial crack length exceeds a shallow threshold, but we also argue that this process is more likely to correspond to the formation of a localized, moulin-like slot that permits drainage, rather than a calving event. We also revisit the formation of basal cracks and find that, in the model, they invariably propagate across the full ice shelf at stresses that are readily generated near an ice shelf front. This indicates that a more sophisticated coupling of the present model (which has been used in a very similar form by several previous authors) needs modification to incorporate the effect of torques generated by buoyantly modulated shelf flexure in the far field.

An Alzheimer’s Disease Mechanism Based on Early Pathology, Anatomy, Vascular-Induced Flow, and Migration of Maximum Flow Stress Energy Location with Increasing Vascular Disease

This paper suggests a chemical mechanism for the earliest stages of Alzheimer's disease (AD). Cerebrospinal fluid (CSF) flow stresses provide the energy needed to induce molecular conformation changes leading to AD by initiating amyloid-β (Aβ) and tau aggregation. Shear and extensional flow stresses initiate aggregation in the laboratory and in natural biophysical processes. Energy-rich CSF flow regions are mainly found in lower brain regions. MRI studies reveal flow stress "hot spots" in basal cisterns and brain ventricles that have chaotic flow properties that can distort molecules such as Aβ and tau trapped in these regions into unusual conformations. Such fluid disturbance is surrounded by tissue deformation. There is strong mapping overlap between the locations of these hot spots and of early-stage AD pathology. Our mechanism creates pure and mixed protein dimers, followed by tissue surface adsorption, and long-term tissue agitation ultimately inducing chemical reactions forming more stable, toxic oligomer seeds that initiate AD. It is proposed that different flow stress energies and flow types in different basal brain regions produce different neurotoxic aggregates. Proliferating artery hardening is responsible for enhanced heart systolic pulses that drive energetic CSF pulses, whose critical maximum systolic pulse energy location migrates further from the heart with increasing vascular disease. Two glymphatic systems, carotid and basilar, are suggested to contain the earliest Aβ and tau AD disease pathologies. A key to the proposed AD mechanism is a comparison of early chronic traumatic encephalopathy and AD pathologies. Experiments that test the proposed mechanism are needed.