Slip Gradient Research Articles

Role of microstructure on the development of local orientation gradients in polycrystals

In this work, we study the relationship between slip localizations and local orientation gradients in a polycrystalline material. We employ a 3D spatially resolved crystal plasticity-based micromechanics technique that includes explicit intragranular slip band modeling, called SB-FFT (slip band fast Fourier transform). EBSD analysis reveals the development of slip localizations, with a rare few generating relatively large local orientation gradients in the neighboring grain. Our results find that several conditions need to be met simultaneously for large zone of intense orientation gradients to form within a grain and with this, offer an explanation for their scarcity. These conditions entail the localization of slip to a sufficient high intensity in the grain neighbor, the grain experiencing higher stress levels than the average stress of its nearest grain neighborhood, and a crystallographic orientation suitable for activating secondary slip under the stress field produced by the slip localization. Analysis of the 3D orientation distributions as they evolve in strain indicates that orientation gradient zones cause the distribution to become highly skewed, with the extreme tails extending further as strain increases. The numerical simulations with various grain neighborhoods show that this feature is, however, not a sufficient signature for such orientation gradients associated with slip localizations since triple junctions or quadruple points can also produce intragranular orientation gradients that have a similar impact on the grain orientation distribution. Last, comparison with several previously proposed slip transmission criteria indicates that this phenomenon is not relevant, contrary to current belief.

The use of Cauchy-type singular integrals over neighboring intervals to compute induced slip in displaced faults

We present expressions to compute the inverse of a Cauchy-type singular integral equation representing the relation between a double-peaked Coulomb stress in a fault or fracture and the resulting slip gradient in two distinct collinear slip patches. In particular we consider a situation where the patches are close enough to account for the influence of the slip gradient in one patch on the slip-induced shear stress in the other patch and vice versa. This situation can occur during depletion-induced or injection-induced fault slip in subsurface reservoirs for, e.g., natural gas production, hydrogen or CO2 storage, or geothermal operations. The theory for a single slip patch is well-developed but the situation is less clear for a configuration with two patches although the monographs of Muskhelishvili (1953) and Weertman (1996) provide earlier results. We show that the general inverse solution for the coupled two-patch problem requires six auxiliary conditions to ensure six physical requirements: boundedness of the slip gradient at the four end points of the slip patches and vanishing of the integrals of the slip gradient over the patches. Mathematically, the presence of two additional conditions, as compared to earlier formulations, corresponds to two undetermined coefficients in the general solution of the governing integral equation. Numerical simulation confirms that at least one of these is always non-zero in the coupled situation. For a coupled double-patch case with a symmetric pre-slip Coulomb stress pattern, the general inverse solution requires three auxiliary conditions. Moreover the conditions for the asymmetric case may be reduced to a set of four again, but these are different from the sets of four obtained earlier by Muskhelishvili (1953) and Weertman (1996). We illustrate the theory with a numerical example in which the evaluation of the Cauchy integrals is performed with a modified version of augmented Gauss–Chebyshev quadrature that relies on analytical inversion.

Restraining geometrically-necessary dislocations to the active slip systems in a crystal plasticity-based finite element framework

Strain gradients have been cast in the form of geometrically-necessary dislocations (GND) to relate the length-scale dependence of strength and to determine potential sites for failure initiation. The literature contains various different incompatibility measures, the main ones being: the total form (∇×Fp), the rate form for large displacements (∇×γ̇anaFp), and the slip gradient form (∇γ̇a). Here, these different approaches are compared rigorously for the first time. Obtaining GND densities when using the total form is a rank-deficit linear problem, solved by singular value decomposition (SVD) known as the Least Squares Minimization (L2 method). Alternative methods for finding GND densities such as Karush–Kuhn–Tucker (KKT) optimization are also investigated. Both L2 and KKT methods predict unrealistic GND densities on inactive slip systems leading to excessive strain hardening; even for a single crystal single slip case. Therefore, the restriction of GNDs to the active slip systems by using a threshold based on the total slip is proposed. This restriction reveals relatively consistent results for various single crystal single slip cases including: simple shear, uniaxial tension, and four-point bending. In addition, the small numerical differences in the slip leads to large discrepancies in the flow stress due to error accumulation, even for strain-gradient-free uniaxial tension, hence a threshold for the GND density increment (2×102 m−2) is used in all models to avoid formation of erroneous GND densities. Finally, the proposed method is applied to the evolution of the GND density for a grain inside a polycrystal aggregate that posses a complex stress state. The lowest incompatibility error is obtained by both of the total forms that use the curl of the plastic deformation gradient with the active slip system restriction suggesting them to be the most reliable GND measures.

Fault Coalescence, Slip Distribution, and Stress Drop of the February 2023 Southeast Türkiye Earthquakes from Joint Inversion of SAR, GNSS, and Burst Overlap Interferometry

Abstract Two devastating earthquakes struck southeastern Türkiye and northwestern Syria on 6 February 2023: an Mw 7.8 mainshock, followed 9 hr later by an Mw 7.6 aftershock. To recover and separate the subsurface geometry and slip distributions along the two earthquake faults, we jointly invert Interferometric Synthetic Aperture Radar, Synthetic Aperture Radar pixel offset tracking, burst overlap interferometry (BOI), Global Navigation Satellite System, and aftershock datasets. We introduce a new Kalman filter-based approach for merging spatially dense azimuth offset (AZO) data with the more precise yet spatially sparse BOI data. This procedure yields improved measurements of the displacements parallel to the near north-south satellite tracks, which are critical for resolving slip along most of the Mw 7.8 fault segments. We optimize the inversion using a new metric for assessing the degree of spatial correlation between the coseismic slip gradients and early aftershocks, resulting in a stable solution honoring the complementarity between the geodetic and aftershock datasets. The analysis suggests that the Mw 7.8 rupture consisted of three large segments and two short fault branches, covering about 300 km along the East Anatolian fault (EAF), whereas the Mw 7.6 rupture consisted of three segments extending for about 160 km along the nearby Sürgü fault (SF). On the basis of moment-to-stress-drop scaling relations, we show that the Mw 7.6 stress drop is four times larger than the Mw 7.8 stress drop, consistent with the larger recurrence intervals for Mw > 7 earthquakes on the SF than on the EAF. The moment released during the 2023 Mw 7.8 earthquake is 2–4 times larger than the sum of the moments released during individual historical Mw > 7 earthquakes along the three segments of the 2023 Mw 7.8 earthquake. Thus, when considering moment release for multisegment earthquakes, one should note that the final moment of fault coalescence is likely larger than the arithmetic sum of individual segment ruptures.

Spatial Relationships Between Coseismic Slip, Aseismic Afterslip, and On‐Fault Aftershock Density in Continental Earthquakes

AbstractDamaging aftershock sequences often exhibit considerable spatio‐temporal complexity. The stress changes associated with coseismic slip and aseismic afterslip are commonly proposed to drive aftershock sequences, but few systematic studies exist and do not always support strong, universal driving relationships. To investigate the roles that these two sources of stress changes may play in driving aftershocks, we assess the spatio‐temporal relationships between coseismic slip, afterslip, and on‐fault (within 5 km) aftershock density following seven Mw6.0–7.6 continental‐settings earthquakes, using available high‐quality slip models and regional seismic data. From previous empirical work and frictional considerations, near the mainshock we expect coseismic slip and afterslip to be anti‐correlated, and aftershocks to occur where coseismic slip is low/zero, near high slip gradients, and/or to migrate with afterslip. However, we find that spatial relationships between afterslip and coseismic slip, and between afterslip and aftershock density differ between earthquakes. Aftershock density correlates with coseismic slip following five of the earthquakes, and with total cumulative slip (coseismic slip + afterslip) following six: indicating that on‐fault aftershock distributions may be approximated by total slip (at current resolutions). Additionally, we find that the gradients of coseismic slip and afterslip (proxies for new stress concentrations) do not clearly correlate with aftershock distributions and that the choice of spatial domain over which relationships are tested can affect results significantly. A possible explanation of these results is that fault zones contain considerable fine‐scale structural and frictional heterogeneity. Nonetheless, the empirical evidence for frequently assumed relationships between coseismic slip, afterslip and on‐fault aftershocks is mixed.

Suture Reactivation, Slip Partitioning, and a Protracted Strike‐Slip Rate Gradient in the Denali Fault System, Southern Alaska, USA

AbstractActive strike‐slip fault systems commonly display along‐strike Quaternary slip rate gradients associated with fault bends and splay faults, which generate surface uplift by dip‐slip faulting or distributed “off fault” deformation. By analogy, the documentation of long‐term (107 yr) slip gradients on some continental strike‐slip fault systems implies long‐term coevolution of strike‐slip and dip‐slip fault systems. Here we leverage the observed ≥33 Myr right‐lateral slip gradient on the Denali fault, Alaska, USA to investigate the role of splay thrust systems in accommodating the slip gradient. We focus on the Broxson Gulch thrust system, which splays southwestward from the Denali fault in the eastern Alaska Range. Apatite and zircon (U‐Th)/He and fission‐track cooling ages from metasedimentary and metaplutonic rocks intersected by the thrust system record an along‐strike decrease in cooling ages commensurate with an increase in late Oligocene‐Neogene bedrock exhumation and shortening with proximity to the Denali fault. The dominant structure in the Broxson Gulch thrust system is the Valdez Creek fault, which is an upper crustal reactivation of the Valdez Creek shear zone–the main Late Cretaceous suture between western North America and outboard accreted arc terranes. After reactivation of the Valdez Creek shear zone at ca. 30 Ma, the thrust system grew by south‐vergent imbrication of the upper crust along thrust and reverse faults until at least 6 Ma. Incorporating results from the Broxson Gulch thrust system into the regional structural evolution of the Denali fault system reveals significant spatiotemporal heterogeneity in shortening adjacent to the Denali fault. Moreover, nearly all of the late Oligocene‐Neogene shortening south of the Denali fault was focused along reactivated terrane boundaries inherited from Mesozoic assembly of the North American Cordillera, and the spatial distribution of the inherited structures appears to control slip partitioning behavior of the Denali fault system across time scales ranging from 101 (historic seismicity) to 107 yr. The slip partitioning behavior of the Denali fault system highlights the mechanical importance of inherited structures leading to protracted shortening on splay thrust systems, which siphon slip from the master strike‐slip fault. We contend that the weakness of nearby reactivated terrane boundaries should be considered among other mechanisms commonly evoked to explain the partitioning behavior of continental strike‐slip fault systems (e.g., stress field rotation, obliquity angle, and strength of master strike‐slip fault).

Stress and strain evolution in fault-related folds: insights from 2D geomechanical modelling

Fault-related folds are intriguing geological structures that develop in compressional and extensional regimes. These folds serve as structural traps for hydrocarbon resources, making their numerical models crucial for understanding the stress and strain evolution of hydrocarbon reservoirs. In our research, we utilize the two-dimensional finite element technique to simulate three representative categories of fault-related folds. Our investigation encompasses their geometric transformation over time, the distribution of stress and strain, variations in slip and uplift, and the effects that various mechanical properties have on these gradients. In our study, we uncovered essential findings about the behavior of fault-related folds. We ascertained that the fault slip gradient in the fault-bend fold model is less than in the fault-propagation fold model. Regarding the uplift gradient, the fault-propagation and fault-bend fold models displayed the greatest and the least degree of change, respectively. The trend of stress-strain evolution on the fold surface in all models was consistent, starting with an increase, transitioning to a constant phase, and ending with a decrease. This pattern proved to be more intricate and divergent than what was evident on the fault surface. Importantly, the internal friction angle, a crucial mechanical characteristic, had a significant influence on the development of these structures. This angle affected both the degree of uplift and stress; an increased angle resulted in enhanced uplift and stress, while a decrease resulted in a decline. Furthermore, the internal friction angle determined the compactness of the fold and the thickness of the forelimb, the part of the fold that inclines towards the advancing direction. These findings have enriched our knowledge of fault-related folds, highlighting the need to consider mechanical properties when studying their formation and evolution.

Absent drumlins beneath southern lobes of the Laurentide Ice Sheet: A new hypothesis based on Des Moines Lobe dynamics inferred from landforms

AbstractSpatial distributions of drumlin fields encode information about ice sheet dynamics. No drumlins formed beneath the most lobate parts of the Laurentide Ice Sheet's southern margin, in South Dakota, Iowa and Illinois, whereas ice lobes to the northeast generally produced drumlins. This pattern may have resulted from northerly ice overriding permafrost. Here we propose a new hypothesis for this pattern by constructing a LiDAR‐based landform map and applying a model of drumlin formation to account for the absence of drumlins beneath the largest of the ice sheet's southern lobes, the Des Moines Lobe. Broad belts of hummocky topography, ice‐walled lake plains, doughnuts and minor moraines, which together cover 90% of the lobe's upland area in Iowa, attest to widespread ice stagnation, as does the lobe's scarcity of eskers. Most stagnation topography is subtle, with insufficient relief to have obscured drumlins that might have formed before stagnation. Minor moraines are crevasse‐squeeze ridges diagnostic of surging, and their ubiquity indicates that during surging, the lobe's soft bed was weak nearly everywhere. End moraines are generally parallel to a minor moraine setup‐glacier, implying that surge‐driven advances were more numerous than indicated by the three major end moraines of the lobe. In the only physically based model of drumlin formation that includes surging, till deposition occurs during surges when effective pressure is uniformly low, whereas drumlins develop during quiescent flow between surges, when basal slip and low‐pressure R‐channels create the spatial gradients in effective pressure necessary to sculpt drumlins by differential erosion. Landforms of the lobe, however, indicate stagnation and down‐wasting during quiescence, without significant basal slip or hydraulic potential gradients necessary for R‐channels. We hypothesize that for the three southernmost lobes of the Laurentide Ice Sheet, surging followed by widespread down‐wasting of stagnant ice prevented drumlin formation, whereas beneath northern lobes it was permitted by climatically‐forced ice advance.

Unraveling size-affected plastic heterogeneity and asymmetry during micro-scaled deformation of CP-Ti by non-local crystal plasticity modeling

Titanium alloy is prominent in manufacturing of high-performance miniature devices, while size-affected plastic heterogeneity and asymmetry in micro-scaled deformation are not well understood due to its complex deformation mechanism. How the interplay among dislocation slip, deformation twinning, and strain gradient affects the size-dependent plastic heterogeneity and asymmetry is an intractable and non-eluded issue that needs to be unraveled. In this research, a non-local crystal plasticity model (NLCPM) considering discrete twins was established and implemented into a finite element framework. The proposed NLCPM successfully represented the size-dependent asymmetric mechanical response, primary and secondary twinning, and texture evolution. The size-dependent fracture behavior, plastic heterogeneity, and T-C asymmetry during micro-scaled deformation of commercial pure titanium (CP-Ti) were explored by coupling micro-tension/compression test and full-field simulation. Results showed that T-C asymmetry in flow stress was enhanced in the coarse-grained and small-diameter samples. For the sample with D≥0.6 mm, the ductility of CP-Ti was improved with the increase of grain size as there are more prevailing twins and the concentrating stress could be relieved by the abundant slip rather than cracking. Grain-level strain is more homogeneous in compression when d was increased from 60 to 110 μm since more non-prismatic slips were activated that accommodated strain along various directions. Meanwhile, plastic strain was allocated within discrete twin bands and thus alleviated strain concentration. Furthermore, the hardening effect of twin boundaries cancelled out the softening effect near the free surface, resulting in a narrower softening layer in the coarse-grained samples compared to its grain size. With the decrease of the sample size, the proportion of the softening layer increased, while the volume fraction of twinning decreased, leading to a reduction in macroscopic stress and fracture strain. This work enhances the in-depth understanding of size-affected plastic heterogeneity and asymmetry during micro-scaled deformation of CP-Ti and supports the forming of high-quality micro components under complex deformation path.

Influence of chemical reaction and thermal convective condition on the heat and mass transport in boundary layer flow over a magneto-radiated wedge with cross diffusion

The analysis of Boundary Layer Flows (BLFs) is of huge significance due to its large-scale applications most likely in aerodynamics, manufacturing of vehicles front faces and many other advanced research areas. Therefore, this study is organized by exercising significant important engineering parameters like surface convection, radiative flux, MHD and thermal and mass diffusion. The developed model embedded the concrete effects of velocity slip, diffusion convection, thermal radiations, magnetic field, chemical reaction and thermo-mass gradients. Later, the numerical treatment is made via RK scheme and the model efficaciously is tackled with desired accuracy. Then the results are decorated over the region (wedge and sheet) and deeply examined. It is observed that slippery surface of wedge and sheet [Formula: see text] enlarges the fluid movement rapidly. The directed magnetic field ([Formula: see text] controlled the fluid motion which can be beneficial from industrial view point. Further, temperature and concentration distributions upsurge by strengthening convective heat ([Formula: see text] and diffusion convection processes. Moreover, skin friction and Nusselt number improved against the parameters under consideration.

An alternative way to describe thermodynamically-consistent higher-order dissipation within strain gradient plasticity

In the context of strain gradient plasticity (SGP), description of higher-order dissipation is the subject of extensive on-going discussions. In most existing SGP theories including thermodynamically-consistent higher-order dissipation, higher-order dissipative processes are described based on the decomposition of the higher-order stresses into recoverable and unrecoverable parts. This higher-order stress decomposition represents the basis of the so-called non-incremental (Gurtin-type) SGP theories, which are the most commonly used in the literature. As formulated, these theories satisfy the thermodynamic requirement of non-negative dissipation. However, they generally lead to unusual effects for some boundary value problems, such as the occurrence of elastic gaps under non-proportional loading conditions. The present work proposes an alternative way to describe higher-order dissipative effects, with an illustration within strain gradient crystal plasticity (SGCP) framework. Inspired by rheological models in series like Maxwell model, the higher-order stress decomposition is replaced by a decomposition of the plastic slip gradients into recoverable and unrecoverable parts. Effects of this decomposition technique are studied and compared with those obtained using higher-order stress decomposition. Capabilities of such a technique to deal with elastic gaps are also investigated.

Mechanism of LaFeO3 perovskite and gradient strain nanocrystalline improving carbon atom diffusion and tribological properties of carburizing steel

A composite layer of gradient nanocrystals and LaFeO3 perovskite oxide was prepared. The influence of the composite layer on the diffusion of carbon atoms and the change of wear mechanism was studied. The experimental data and related mechanisms show that the formation mechanism of gradient nanocrystalline layer mainly involves dislocation slip and strain gradient evolution at grain boundaries. The oxygen vacancies provided by nano oxides improve the surface activity of the samples and promote the diffusion of carbon to deep layers. The existence of composite layer changes the wear mode of different depths. The oxide film formed on the surface promoted by rare earth provides the effect of anti-wear and lubrication. Oxygen vacancies improves the anti-spalling ability of oxide film.

Study of the interplay between lower-order and higher-order energetic strain-gradient effects in polycrystal plasticity

Strain-gradient (SG) plasticity refers to a class of non-local theories in which gradients of plastic slip determine the storage of geometrically necessary dislocations, introducing a length-scale dependence in the mechanical behavior of crystalline materials, which is otherwise lacking in local theories. In this work, we incorporate lower-order (LO) and higher-order energetic (HOE) strain-gradient effects into a crystal plasticity fast Fourier transform (FFT)-based formulation to investigate the interplay of the length scale that each strain-gradient term introduces at the microscale, and the mechanical properties that result at the macroscale. For an applicable range of length scales, we consider two systems: a 1-D two-phase face centered cubic (FCC) laminate and a 3-D FCC polycrystal, and two uniaxial deformation modes: monotonic tension and cyclic tension–compression. We show that increases in the individual LO and HOE length scales increase the hardening rate and strength of the material, respectively. When combined, the strong LO hardening is less pronounced than the effect alone due to the lowering of the gradients due to the HOE microstress. We demonstrate that the LO and HOE hardening manifest as “isotropic” (yield surface expansion) and “kinematic” (yield surface shift) effects, respectively, consistent with their theoretical origins. We show that in cyclic loading, the Bauschinger effect emerges in both local and non-local calculations and link its origins and severity to the behavior in the strain field, slip-system rates, and the HOE microforce.

Thermal bowing of reinforced concrete sandwich panels using time-domain coupled-field finite element analysis

Thermal bowing in concrete sandwich panels (CSPs), which is an undesirable deflection due to temperature differentials between inner and outer surfaces, has not been studied sufficiently and its effects are still not well understood. In this study, a transient thermal-mechanical finite element technique in ANSYS software, featuring two separate but interrelated time-domain simulations, a heat transfer and a non-linear structural, has been developed to study the bowing phenomenon in CSPs. The model predictions were validated by comparing with experimental data from full-scale panels in the literature subjected to heating on one wythe that produced an increasing temperature differential up to 30 °C. Validation included thermal bowing, slip and temperatures gradient. The structural component of the model was also validated by comparing with panels tested under flexural loads. The coupled model was then used to study the effects of several key variables that were not examined in the experiments, including cooling load, degree of composite action (kp), and wind loading combined with thermal loading. Results showed that very cold temperatures can result in bowing that exceeds serviceability limits and can cause compression failure in connectors. Bowing increased by 129% when kp increased from zero to 85%. Up to 50% increase in bowing was observed when adding wind effects to thermally determined bowing for the thermal range investigated in the parametric study.

Observations noise and stability of geodetic variable slip fault models

SUMMARY Variable slip models of seismic faults represent ill-posed (or underdetermined) problems with infinity of solutions; a single solution is typically obtained using constraints imposed by the L-curve theory, through an experimentally derived coefficient which optimizes the trade-off between fault roughness (smoothness, mean slip gradient) and mean misfit of observations to the fault model. However, in some cases, diverse variable slip fault models (VSFM) have been presented, even using the same data sets. In this paper, we investigate the problem of stability (sensitivity) of VSFM to input geodetic (GNSS/GPS, INSAR) ground slip measurements, based on statistics of synthetic data: statistically similar sets of synthetic ground displacements were the only degree of freedom in an analysis leading to comparable VSFM, regarded as images of slip amplitude and of rake. Differences of synthetic models from the reference fault model for different levels of noise, both for filtered and for PCA-filtered were examined, and results were summarized in images depicting the differences and the variability (standard deviation) of each fault slip patch, both for slip and rake. From this ‘truth’-based approach, it was found that an increase in observations noise is reflected in increase of variability (instability) of fault models, especially away from the fault centre, higher for slip than in rake. Analysis with a different virtual observations system indicates that stations above the fault tend to show spurious stress concentration areas on the fault surface, in agreement with previous studies. Analysis with synthetic data appears as a promising strategy to validate a VSFM based on geodetic data.

Coseismic slip gradient at the western terminus of the 1920 Haiyuan Mw 7.9 earthquake

Stepovers in strike-slip fault systems play important roles in controlling the propagation of earthquake ruptures, depending partially on the competition between the stepover width and the coseismic slip gradient approaching the step. The 1920 Mw 7.9 Haiyuan earthquake is the most recent major earthquake that has occurred along the Haiyuan fault. The earthquake rupture broke through multiple stepovers along the fault and finally ended at the 4 km-wide Jingtai pull-apart basin (releasing stepover) at the western end. To understand the process of this termination of the Haiyuan earthquake rupture, we conducted detailed mapping of the surface rupture geometry, coseismic slip measurement and slip gradient calculation in the vicinity of the endpoint based on the 0.2 m-resolution SfM-derived DEM along the 20 km section east of the Jingtai pull-apart basin. Combining coseismic slip measurements from this study and published slip data from fieldwork in the 1980s, we calculated slip gradients of 84–160 cm/km as the rupture approached the Jingtai releasing stepover. These values are high for the 4 km-wide Jingtai releasing stepover compared to those from a dataset of worldwide historical earthquakes compiled by Elliott et al. (2009). The high values imply that the rupture could have a relatively high likelihood of breaching through the Jingtai releasing stepover. Alternatively, the slip gradient may be overestimated. Detailed mapping and field investigation, however, show that the rupture may extend ~1.86 km farther west of the end location indicated in previous studies. We acquired the new slip gradient with lower value of 50–82 cm/km, which is still considerably high compared to dataset of Elliott et al. (2009). We speculate that the slip gradient could be larger variation along the stepover boundary to stop fault slip in different geologic setting. Another factor control slip termination is related to fault properties. Creep along the southern boundary fault of the Jingtai stepover, a velocity-strengthening region, may also have played a role in stopping the rupture at this location. Our observations further indicate that the increasing LOS velocity of InSAR data within the Jingtai stepover is probably related to fault behavior rather than a nontectonic signal of subsidence.

Geomechanical modelling of fault-propagation folds: Estimating the influence of the internal friction angle and friction coefficient

The interplay between faulting and folding mechanisms has been recognised in extensional and contractional settings. Numerous investigations have documented that the analysis of structural aspects of fault-related folds is important for the exploration and exploitation of hydrocarbon resources and in seismic data interpretation. In this study, we used four series of 2-D elastic-plastic finite element models (A, B, C, and D) to investigate the influence of internal friction angle, friction coefficient, and an additional detachment on the fault slip and uplift gradients, stress-strain patterns, and structural evolution of fault-propagation folds (FPFs). We found that: 1) the stress-strain patterns are similar in different sections of a FPF; 2) the plastic strain is concentrated at the fault tip, fault surface, and forelimb of the FPF; 3) among the internal friction angle, friction coefficient, and additional detachment parameters, only additional detachment affects the stress-strain patterns at the crest and backlimb of the FPF; 4) to create a FPF, not only must the fault friction be less than the layers, but the layer friction also has to be less than a threshold; 5) increase in the internal friction angle, decrease in the friction coefficient, and generation of an additional detachment lead to the formation of tight folds whereas, decrease in the internal friction angle and increase in the friction coefficient create wide folds; and 6) the internal friction angle and friction coefficient are two key factors of the fault-related fold style.

Development of surface ruptures by hanging-wall extension over a thrust ramp along the Ragged Mountain fault, Katalla, Alaska, USA: Applications of high-resolution three-dimensional terrain models

AbstractHigh-resolution three-dimensional terrain models are used to evaluate the Ragged Mountain fault kinematics (Katalla, Alaska, USA). Previous studies have produced contradictory interpretations of the fault’s kinematics because surface ruptures along the fault are primarily steeply dipping, uphill-facing normal fault scarps. In this paper, we evaluate the hypothesis that these uphill-facing scarps represent extension above a buried thrust ramp. Detailed geomorphic mapping along the fault, using 20-cm-resolution aerial imagery draped onto a 1-m-resolution lidar (light detection and ranging) elevation model, was used to produce multiple topographic profiles. These profiles illustrate scarp geometries and prominent convex-upward topographic surfaces, indicating significant disturbance by active tectonics. A theoretical model is developed for fault-parallel flow over a thrust ramp that shows the geometric relationships between thrust displacement, upper-plate extension, and ramp dip. An important prediction of the model for this study is that the magnitude of upper-plate extension is comparable to, or greater than, the thrust displacement for ramps with dips greater than ∼45°. This model is used to analyze profile shapes and surface displacements in Move software (Midland Valley Ltd.). Analyses of scarp heights allow estimates of hanging-wall extension, which we then use to estimate slip on the underlying thrust via the model. Assuming a low-angle (30°) uniformly dipping thrust and simple longitudinal extension via normal faulting, variations in extension along the fault would require a slip gradient from ∼8 m in the north to ∼22 m in the south. However, the same north-south variation in extension with a constant slip of 8–10 m may infer an increase in fault dip from ∼30° in the north to ∼60° in the south. This model prediction has broader implications for active-fault studies. Because the model quantifies relationships between hanging-wall extension, fault slip, and fault dip, it is possible to invert for fault slip in blind thrust ramps where hanging-wall extension is the primary surface manifestation. This study, together with results from the St. Elias Erosion and Tectonics Project (STEEP), clarifies the role of the Ragged Mountain fault as a contractional structure within a broadly sinistral shear system in the western syntaxis of the St. Elias orogeny.

Late Quaternary slip rate of the Aksay segment and its rapidly decreasing gradient along the Altyn Tagh fault

AbstractConstraining the fault slip rate on a fault can reveal the strain accumulation and partitioning pattern. The Aksay segment, the eastern segment of the Altyn Tagh fault, as the starting area where the slip rate of the Altyn Tagh fault decreases, is a strain partitioning zone. The spatial and temporal distribution of its fault slip rate is of great significance to clarify the strain-partitioning pattern of the eastern Altyn Tagh fault. In this study, we determined the slip rates at four sites along the Aksay segment. The results demonstrated that the slip rate decreases dramatically, with an overwhelmingly high slip gradient of ∼9.8 mm/yr/100 km (a 9.8 mm/yr reduction of slip rate occurs over a distance of 100 km) within a distance of ∼50 km. The slip rate gradient along strike at the Aksay segment is four times that of the Subei segment to the eastward termination of the Altyn Tagh fault. Our results indicate that the slip rate gradient along the Altyn Tagh fault is not uniform and decreases eastward with variable slip rate gradients on different segments, resulting in the uplift of the mountains oblique to the Altyn Tagh fault.

A new mechanism of strain transfer in polycrystals

At the grain boundaries of plastically deforming polycrystals, strain transfer mechanisms can accommodate the shear strain carried by slip bands and mechanical twins to prevent stress build-ups and damage. So far, only the accommodation obtained through slip (and twinning) alone has been considered in the mechanism known as slip (and twin) transfer. Here, a strain transfer mechanism that also requires the rotation of the crystal lattice is demonstrated. A region of accumulated slip develops perpendicular to the active slip plane in the impinged grain. The slip gradients enable a localized lattice rotation that accommodates the shear strain in the incoming band, preventing the build-up of interfacial stresses. The mechanism operates preferentially at the boundaries between highly misoriented grains. Facilitating strain transfer at these interfaces opens up new possibilities to improve the mechanical properties of polycrystals, as discussed.