Deviatoric Compression Research Articles

Force-balance modelling of the impact of glacial erosion, trench sedimentation, megathrust weakening and glacial loading on the stress state of the crust at active continental margins

Force-balance analyses showed that the stress state at active continental margins is largely controlled by the gravitational force and the megathrust shear force and remains unchanged as long as subduction proceeds undisturbed. Glacially induced topographic changes and mass redistribution by glacial erosion, sediment transport and deposition may affect the force balance but the impact on the stress state and the style of deformation in the upper plate has not been investigated so far. Here, we use numerical force-balance models to investigate the stress changes in the upper plate resulting from (i) a reduction in mountain height in the arc by glacial erosion, (ii) a steepening of the arc front, (iii) a decrease in the megathrust shear force due to increased sediment subduction and fault weakening, (iv) an increase in sediment thickness in the trench, and (v) the load by an ice cap. Our model results show that each process causes distinct stress changes that affect different parts of the upper plate. The largest stress changes result from a reduction in mountain height, which increases compression in the arc interior, and fault weakening by increased sediment subduction, which decreases the megathrust shear force and hence deviatoric compression in the forearc and backarc. Smaller stress changes occur for a steepening of the arc front, increased sediment deposition in the trench and the load of the ice cap. The different stress changes may promote or suppress faulting in different parts of the upper plate. Application of our model to the North Patagonian Andes indicates that glacial erosion during late Cenozoic cold periods may explain the localization of deformation in the arc interior, whereas the reduced activity of thrust faults in the forearc and backarc may reflect a decrease in deviatoric compression caused by a decrease in the megathrust shear force.

Lattice discrete particle modeling of concrete under cyclic tension–compression with multi-axial confinement

Accurate modeling of concrete mechanical behavior under cyclic loading with different levels of confinement is crucial in design and analysis of structures subjected to seismic events. One robust model for simulating concrete behavior is the Lattice Discrete Particle Model (LDPM), a discrete model formulated at the scale of aggregate and composed of polyhedral cells connected through a lattice of nonlinear fracturing struts. To improve the performance of LDPM for the prediction of mechanical behavior under different cyclic loading schemes and multi-axial confinement, a comprehensive cycling constitutive model, a modified volumetric–deviatoric compression law, and a modified frictional behavior under compression are established in this study. An effective strain and a corresponding effective stress at the mesoscale are formulated and used to determine the stress–strain relationship under monotonic loading. In addition, the constitutive relationships related to the tension and shear stresses are established separately to ensure a smooth stress transition from tension to compression state. Two material parameters are used to control the stiffness attenuation: the residual plastic strain, and the energy dissipation of concrete during loading and unloading, under both tension and compression. Further, a new constitutive equation for the frictional behavior under compression is proposed to simulate the post-peak behavior under confinement. The proposed constitutive model is used to simulate the mechanical behavior and failure mode of concrete under tension–compression cycling and hydrostatic pressure. Triaxial tests under different levels of confinement and unconfined compression tests are also simulated. Model validation is performed using multiple data sets available in the literature on concretes of various strengths. Simulation results show that the established cyclic constitutive model can effectively characterize the mechanical response of concrete under different cycling loadings and confining pressures.

Slow Slip Triggers the 2018 Mw 6.9 Zakynthos Earthquake Within the Weakly Locked Hellenic Subduction System, Greece

AbstractSlow slip events (SSEs) at subduction zones can precede large‐magnitude earthquakes and may serve as precursor indicators, but the triggering of earthquakes by slow slip remains insufficiently understood. Here, we combine geodetic, Coulomb wedge and Coulomb failure‐stress models with seismological data to explore the potential causal relationship between two SSEs and the 2018 Mw 6.9 Zakynthos Earthquake within the Hellenic Subduction System. We show that both SSEs released up to 10 mm of aseismic slip on the plate‐interface and were accompanied by an increase in upper‐plate seismicity rate. While the first SSE in late 2014 generated only mild Coulomb failure stress changes (≤3 kPa), that were nevertheless sufficient to destabilize faults of various kinematics in the overriding plate, the second SSE in 2018 caused stress changes up to 25 kPa prior to the mainshock. Collectively, these stress changes affected a highly overpressured and mechanically weak forearc, whose state of stress fluctuated between horizontal deviatoric compression and tension during the years preceding the Zakynthos Earthquake. We conclude that this configuration facilitated episodes of aseismic and seismic deformation that ultimately triggered the Zakynthos Earthquake.

Force‐Balance Analysis of Stress Changes During the Subduction‐Collision Transition and Implications for the Rise of Mountain Belts

AbstractMountain height at convergent plate margins is limited by the megathrust shear force, but it remains unclear how this constraint affects the topographic evolution and mountain building at the transition from subduction to collision. Generally, mountain height increases during the subduction‐collision transition in response to crustal thickening or processes like mantle delamination and slab breakoff, but the main parameters controlling how much mountain height increases remain poorly understood. Here we show, based on analytical and finite‐element force‐balance models, that the increase in mountain height depends on the magnitude of the megathrust shear force and the reduction of submarine margin relief. During the subduction stage, the shear force is balanced by the gravitational effect of the margin relief and the deviatoric stresses in the upper plate are low. When the submarine margin relief is reduced during the closure of the ocean basin, the effect of the gravitational force decreases and the upper plate experiences enhanced deviatoric compression, which allows the mountain height to increase until a near‐neutral stress state beneath the high mountains is restored. If the increase in mountain height cannot keep pace with the submarine relief reduction, the compression of the upper plate increases by a few tens of MPa, which promotes tectonic shortening and mountain building. Our analysis implies that mountain height can increase by hundreds of meters to a few kilometers during collision, depending primarily on the trench depth during the subduction stage and possible syncollisional changes of the megathrust shear force.

Analysis of the Seismic Moment Tensor of Acoustic Emission: Granite Fracture Micromechanisms During Three-Point Bending

An experimental study of granite fracture micromechanisms during three-point bending based on reconstruction of the seismic moment tensor components for acoustic emission events has been carried out. An original three-stage algorithm for refining and rejecting solutions for the seismic moment tensor of acoustic emission events is proposed, based on a hybrid iterative algorithm and the jackknife resampling method. After the formation of a catalog of acoustic emission events with a stable solution, their mechanisms were analyzed. It is shown that in the case of three-point bending of granite, acoustic emission events are localized in the zone of formation of the future main crack. It has been found that granite bending is accompanied by the formation and development of normal opening cracks with an additional shear component of varying intensity. The kinematics of the found mechanisms of acoustic emission events corresponds to the configuration of loads applied to the sample. The shear component is caused by the microstructural inhomogeneity of the sample. Variations in the orientations of the axes of maximum deviatoric compression (P-axis) and tension (T-axis) are determined, which are associated with the variability of the local stress-strain state of the material near acoustic emission sources. It is shown that the instantaneous magnitudes of acoustic emission events obey the Gutenberg–Richter law.

Structural evolution and fusion behavior of gold supercrystals under stress: Insights from atomistic simulations

Stress-driven assembly and sintering of nanocrystal (NC) supercrystals is an effective mechanical method for fabricating ordered 1D nanostructure arrays. Here, we preform atomistic molecular dynamics simulations for alkylthiol-coated gold supercrystal to reveal its structural evolution and fusion behavior under high-pressure-induced stress. On initial hydrostatic compression, the supercrystal reduces lattice dimension nonlinearly with pressure and displays a reversible pressure-dependent change of interparticle distance, in good agreement with the experiment. Subsequently, the deviatoric compression results in a distorted and noncubic superstructure, where an unexpected structural hysteresis is observed during a compression–release cycle. These structural changes are explained in terms of the molecular conformation of passivating ligands as well as its variations caused by the change in the external stress. In particular, when the pressure exceeds a threshold, neighboring NCs start to contact one another and consolidate into numerous dimers and trimers, which further evolve into short nanorods and finally lead to an irreversible formation of stable nanowires. The structural and stress change in the gold NCs during the compression process are also analyzed. This work is expected to provide useful insights into the mechanical response of supercrystals subjected to an external stress.

Investigation of the Effect of Confining Pressure on the Mechanics-Permeability Behavior of Mudstone under Triaxial Compression

Injecting CO2 into a reservoir disturbs the geostress field, which leads to variations in the permeability of caprock and affects its sealing performance. In this paper, the evolution characteristics of the permeability of Yingcheng mudstone were experimentally studied during deviatoric compression under different confining pressures. As the confining pressure increased, the strength of the mudstone increased bilinearly, the angle between the fault and the maximum principle stress increased, and the fault became flatter. During compression, the permeability of mudstone first decreased and then increased and the turning point of the permeability was between the onset of dilatancy and the turning point of volumetric strain; when the fault formed, the permeability increased sharply and the fault-induced increment was reduced exponentially with increasing confining pressure. In addition, the mudstone transformed to the ductile failure mode when the effective confining pressure was greater than 35 MPa, which means that the permeability did not jump within a small strain. Finally, a practical strain-based model of permeability evolution that separately considers compaction and dilatancy was proposed, and the predicted permeability values were in good agreement with the experimental results. This study revealed the effect of confining pressure on permeability evolution during compression and can help evaluate the sealing ability of mudstone caprock.

Formation of the Iberian‐European Convergent Plate Boundary Fault and Its Effect on Intraplate Deformation in Central Europe

AbstractWith the Late Cretaceous onset of Africa‐Iberia‐Europe convergence Central Europe experienced a pulse of intraplate shortening lasting some 15–20 Myr. This deformation event documents area‐wide deviatoric compression of Europe and has been interpreted as a far‐field response to Africa‐Iberia‐Europe convergence. However, the factors that governed the compression of Europe and conditioned the transient character of the deformation event have remained unclear. Based on mechanical considerations, numerical simulations, and geological reconstructions, we examine how the dynamics of intraplate deformation were governed by the formation of a convergent plate boundary fault between Iberia and Europe. During the Late Cretaceous, plate convergence was accommodated by the inversion of a young hyperextended rift system separating Iberia from Europe. Our analysis shows that the strength of the lithosphere beneath this rift was initially sufficient to transmit large compressive stresses far into Europe, though the lithosphere beneath the rift was thinned and thermally weakened. Continued convergence forced the formation of the plate boundary fault between Iberia and Europe. The fault evolved progressively and constituted a lithospheric‐scale structure at the southern margin of Europe that weakened rheologically. This development caused a decrease in mechanical coupling between Iberia and Europe and a reduction of compressional far field stresses, which eventually terminated intraplate deformation in Central Europe. Taken together, our findings suggest that the Late Cretaceous intraplate deformation event records a high force transient that relates to the earliest strength evolution of a lithospheric‐scale plate boundary fault.

Influence of stress and high-temperature treatment on the permeability evolution behavior of sandstone

Permeability is an important property of rock in gas and oil exploration engineering; environmental temperature and geo-stress have great influence on it. This paper aims to analyze the influence of thermal treatment on the permeability of sandstone under triaxial compression. Based on the gas seepage tests on a sandstone specimen after different thermal treatment temperatures with different gas pressures, hydrostatic stresses and deviatoric stresses, the thermal effect on the physical properties of sandstone is firstly analyzed. The results show that the mass of the sandstone specimen decreases with the increase of temperature; some spalling damage and tensile cracks occur on the lateral surface of the specimen at 400 °C. According to the seepage test results with various gas pressures, an exponential relationship has been found between the permeability coefficient and temperature. The permeability coefficient is approximately 100 times as large as the initial value when the temperature increases from 20 °C to 800 °C. The permeability evolution of the heated sandstone under hydrostatic and deviatoric compression has also been analyzed. A simplified double-pore texture model is proposed which can describe well the permeability evolution of sandstone under compression with hydrostatic stress and deviatoric stress, and it can be helpful to estimate the permeability of thermally treated sandstone under elastic triaxial compression.

An experimental investigation on deformation and failure behavior of carbonaceous Garau shale in Lurestan Basin, west Iran: Application in shale gas development

The safety and efficiency of commercial shale gas development strategies are tightly related to accurate evaluation of rock deformation and brittle failure processes. In contrast to extensively developed siliciclastic shale gas resources in the world, the mechanical behaviors of the carbonaceous shales and their causal mechanisms are not thoroughly understood. In this study, a geomechanical characterization program including unconfined and confined deviatoric compression experiments together with complementary compositional and microstructural analyses was therefore carried out to explain the stress-strain behaviors of the carbonaceous organic-rich Garau shales in Iran. Deformation and progressive fracturing process of the collected rock samples at varying confinement states were evaluated and discussed in terms of well-established mechanistic concepts and theoretical models. In addition, the applicability of some of widely used expressions for assessing rock brittleness was also investigated. According to the obtained results, in spite of substantially varying deformation and failure characteristics, all collected samples over the range of applied confinement states up to 40 MPa follow the typical brittle failure process. However, the degree of brittle failure in terms of brittleness is considerably variable and dependent to the established definition. Furthermore, the diagnosed range of stress-strain characteristics are dominantly correlated with variations of carbonate depositional textures. Accordingly, as the stiff fine-grained micritic fabric of a carbonaceous shale gradually transforms to relatively large sparitic texture, rock generally tends to fail at relatively lower strength but higher degree of brittleness. This relation can be advantageous to an accurate determination of the relative brittleness successions in carbonaceous shale gas layers for practical field operations.

Neotectonics and tectonic stresses of the Sakhalin Island

The paper describes the neotectonics of the Sakhalin Island and analyzes the latest and recent tectonic stresses in the study area in order to establish their differences in the Amur and Okhotsk microplates, which boundary is confined to the Tym-Poronaisk fault, the largest NS-striking fault in the Central Sakhalin (Fig. 1). Our map of the structural geomorphological features of the study area (Fig. 2) shows three longitudinal zones: the western and eastern uplifts, and the Central Sakhalin basin between the uplifts. In the Southern Sakhalin, neotectonic stresses were studied by a combination of tectonophysical methods and the method of structural geology (Figures 3 to 6, and Table). Our study shows that the regional axes of maximum and minimum compressive principal normal stresses are primarily of the subhorizontal orientations (Fig. 5, Д ) . In the Northern and Central Sakhalin, neotectonic stresses were reconstructed by the structural geomorphology method. The compression axes are oriented sublatitudinally, with the NE-trending strike in the Northern Sakhalin (Fig. 7, A ), and the extension axes are oriented submeridionally; in the Northern Sakhalin, respectively, they are oriented in the NW direction. The results of our study of neotectonic stresses were used to construct a map of recent geodynamics of Sakhalin (Fig. 7, Б ), which shows zones differing in the geodynamic settings of the most recent faulting. According to the analysis of the recent tectonic stress with respect to the earthquake focal mechanisms in the period from 1978 to 2015 (Fig. 8), recent stresses dominating in Sakhalin have mainly the sublatitudinal low-angle orientations of the deviatoric compression axis. The submeridional low-angle orientations of the deviatoric extension axes are observed in the Northern Sakhalin and partly in the north of the Southern Sakhalin (see Fig. 8). The high-angle axes of deviatoric extension are typical of the western and central parts of the Southern Sakhalin, and such extension leads to horizontal compression and reverse faulting. In some areas of the recent stress field, the deviatoric axes of compression and extension have unstable orientations. The latitudinal boundaries of such areas are nearly coincident with the boundaries of the zones that differ in the geodynamic settings of the most recent faulting, which means that these areas and zones are reliably identified. The relative inhomogeneity of the neotectonic and recent stress fields in the Southern Sakhalin does not give grounds to distinguish differences in the state of crustal stresses in the areas located on the sides of the Southern Sakhalin fault. As a consequence, a boundary between the Amur and the Okhotsk Plate in the South Sakhalin cannot be drawn along this fault. It is most likely that this boundary coincides with the Western-Sakhalin fault in the southern areas of the study region. Our data on the Central and Northern Sakhalin does not contradict with the conclusion in [ Savostin et al., 1982 ] concerning this boundary.

Structural evolution and mechanical behaviour of Pt nanoparticle superlattices at high pressure.

High pressure is an effective means for tuning the interparticle distances of nanoparticle (NP) superlattices and thus for modifying their physical properties and functionalities. In this work, we determined the evolution of inter-NP distances of a Pt NP superlattice with increasing pressure using an in situ synchrotron small-angle X-ray scattering (SAXS) technique in a diamond-anvil cell (DAC). Transmission electron microscopy (TEM) was used to characterize the microstructures of pre- and post-compression samples. Our results demonstrate that the evolution of Pt NP assemblies with increasing pressure consists of four stages: (1) ligand elastic response, (2) uniform compression, (3) ligand detachment from NP surfaces, and (4) deviatoric compression of ligands between neighboring NPs. By controlling the magnitudes of applied pressure and deviatoric stress, one can sinter NPs into novel architectures such as nanowires and nanoceramics.

Accurate focal depth determination of oceanic earthquakes using water-column reverberation and some implications for the shrinking plate hypothesis

Investigation of oceanic earthquakes is useful for constraining the lateral and depth variations of the stress and strain-rate fields in oceanic lithosphere, and the thickness of the seismogenic layer as a function of lithosphere age, thereby providing us with critical insight into thermal and dynamic processes associated with the cooling and evolution of oceanic lithosphere. With the goal of estimating hypocentral depths more accurately, we observe clear water reverberations after the direct P wave on teleseismic records of oceanic earthquakes and develop a technique to estimate earthquake depths by using these reverberations. The Z–H grid search method allows the simultaneous determination of the sea floor depth (H) and earthquake depth (Z) with an uncertainty less than 1 km, which compares favorably with alternative approaches. We apply this method to two closely located earthquakes beneath the eastern Pacific. These earthquakes occurred in ∼25 Ma-old lithosphere and were previously estimated to have similar depths of ∼10–12 km. We find that the two events actually occurred at dissimilar depths of 2.5 km and 16.8 km beneath the seafloor, respectively, within the oceanic crust and lithospheric mantle. The shallow and deep events are determined to be a thrust and normal earthquake, respectively, indicating that the stress field within the oceanic lithosphere changes from horizontal deviatoric compression to horizontal deviatoric tension as depth increases, which is consistent with the prediction of lithospheric cooling models. Furthermore, we show that the P-axis of the newly investigated thrust-faulting earthquake is perpendicular to that of the previously studied thrust event, consistent with the predictions of the shrinking-plate hypothesis.

Global correlations between maximum magnitudes of subduction zone interface thrust earthquakes and physical parameters of subduction zones

The maximum earthquake magnitude recorded for subduction zone plate boundaries varies considerably on Earth, with some subduction zone segments producing giant subduction zone thrust earthquakes (e.g. Chile, Alaska, Sumatra–Andaman, Japan) and others producing relatively small earthquakes (e.g. Mariana, Scotia). Here we show how such variability might depend on various subduction zone parameters. We present 24 physical parameters that characterize these subduction zones in terms of their geometry, kinematics, geology and dynamics. We have investigated correlations between these parameters and the maximum recorded moment magnitude (MW) for subduction zone segments in the period 1900–June 2012. The investigations were done for one dataset using a geological subduction zone segmentation (44 segments) and for two datasets (rupture zone dataset and epicenter dataset) using a 200km segmentation (241 segments). All linear correlations for the rupture zone dataset and the epicenter dataset (|R|=0.00–0.30) and for the geological dataset (|R|=0.02–0.51) are negligible-low, indicating that even for the highest correlation the best-fit regression line can only explain 26% of the variance. A comparative investigation of the observed ranges of the physical parameters for subduction segments with MW>8.5 and the observed ranges for all subduction segments gives more useful insight into the spatial distribution of giant subduction thrust earthquakes. For segments with MW>8.5 distinct (narrow) ranges are observed for several parameters, most notably the trench-normal overriding plate deformation rate (vOPD⊥, i.e. the relative velocity between forearc and stable far-field backarc), trench-normal absolute trench rollback velocity (vT⊥), subduction partitioning ratio (vSP⊥/vS⊥, the fraction of the subduction velocity that is accommodated by subducting plate motion), subduction thrust dip angle (δST), subduction thrust curvature (CST), and trench curvature angle (αT). The results indicate that MW>8.5 subduction earthquakes occur for rapidly shortening to slowly extending overriding plates (−3.0⩽vOPD⊥⩽2.3cm/yr), slow trench velocities (−2.9⩽vT⊥⩽2.8cm/yr), moderate to high subduction partitioning ratios (vSP⊥/vS⊥⩽0.3–1.4), low subduction thrust dip angles (δST⩽30°), low subduction thrust curvature (CST⩽2.0×10−13m−2) and low trench curvature angles (−6.3°⩽αT⩽9.8°). Epicenters of giant earthquakes with MW>8.5 only occur at trench segments bordering overriding plates that experience shortening or are neutral (vOPD⊥⩽0), suggesting that such earthquakes initiate at mechanically highly coupled segments of the subduction zone interface that have a relatively high normal stress (deviatoric compression) on the interface (i.e. a normal stress asperity). Notably, for the three largest recorded earthquakes (Chile 1960, Alaska 1964, Sumatra–Andaman 2004) the earthquake rupture propagated from a zone of compressive deviatoric normal stress on the subduction zone interface to a region of lower normal stress (neutral or deviatoric tension). Stress asperities should be seen separately from frictional asperities that result from a variation in friction coefficient along the subduction zone interface. We have developed a global map in which individual subduction zone segments have been ranked in terms of their predicted capability of generating a giant subduction zone earthquake (MW>8.5) using the six most indicative subduction zone parameters (vOPD⊥, vT⊥, vSP⊥/vS⊥, δST, CST and αT). We identify a number of subduction zones and segments that rank highly, which implies a capability to generate MW>8.5 earthquakes. These include Sunda, North Sulawesi, Hikurangi, Nankai-northern Ryukyu, Kamchatka-Kuril-Japan, Aleutians-Alaska, Cascadia, Mexico-Central America, South America, Lesser Antilles, western Hellenic and Makran. Several subduction segments have a low score, most notably Scotia, New Hebrides and Mariana.

A new driving mechanism for backarc extension and backarc shortening through slab sinking induced toroidal and poloidal mantle flow: Results from dynamic subduction models with an overriding plate

AbstractWe present numerical subduction models to investigate overriding plate deformation at subduction zones. All models show forearc shortening, resulting predominantly from shear stresses at the subduction zone interface and opposite‐sense mantle shear stresses at the base of the forearc lithosphere. Models dominated by backarc extension show that it results from trench‐normal positive velocity gradients in the mantle below the overriding plate. Such gradients result from toroidal mantle flow induced by slab rollback, with velocities below the leading part of the backarc faster than the overriding plate velocity. The velocity gradients induce basal shear stresses that increase trenchward and cause trenchward overriding plate motion at a velocity (vOP⊥) whose spatial average is below the trench retreat velocity (vT⊥). The combination of basal shear stresses and average vOP⊥ < vT⊥ causes trench‐normal deviatoric tension in the backarc and backarc extension. Models dominated by backarc shortening show that it results from a relatively immobile subduction hinge and trenchward overriding plate motion driven by poloidal mantle flow. The poloidal mantle flow is induced by downdip slab sinking and causes the average vOP⊥ > vT⊥. This results in trench‐normal deviatoric compression and shortening in the leading part of the overriding plate as it collides with the subduction hinge. Ultimately, the geodynamic models demonstrate that backarc extension is favored for narrow slabs and near lateral slab edges and is driven by rollback induced toroidal mantle flow, while backarc shortening is favored for the center of wide slabs and is driven by poloidal mantle flow resulting from downdip slab motion.

Dyke propagation and sill formation in a compressive tectonic environment

Sills could potentially form as a result of dykes modifying their trajectory in response to remote tectonic compression. Here, we use analogue experiments to investigate how a buoyant vertical dyke adjusts its trajectory to a compressive remote stress to form a sill, and over which vertical distance this sill formation does occur. Our investigation is restricted to an intrusion propagating through a homogeneous solid, which enables us to identify the characteristic length‐scale over which a dyke responds to remote stress compression, independently of the presence of crustal layers. The experiments involve the injection of air in a gelatine solid that experiences lateral deviatoric compression. The response of the buoyant air crack to the compressive stress in not instantaneous but operates over some distance. An important observation is that some cracks reach the surface despite the compressive environment. Dyke‐to‐sill rotation occurs only for large compressive stress or small effective buoyancy. Dimensional analysis shows that the length‐scale over which this rotation takes place increases exponentially with the ratio of crack effective buoyancy to horizontal compressive stress. Up‐scaled to geological conditions, our analysis indicates that a dyke‐to‐sill transition in response to tectonic compression in homogeneous rocks cannot occur over less than two hundred meters and would need several kilometers in most cases. This is typically greater than the average thickness of lithological units, which supports the idea that crustal heterogeneities play an important role in determining the fate of dykes and in controlling where sills could form.

Experimental study and constitutive modelling of elasto‐plastic damage in heat‐treated mortar

AbstractThis study investigates the effect of a heat‐treatment upon the thermo‐mechanical behaviour of a model cement‐based material, i.e. a normalized mortar, with a (w/c) ratio of 0.5. First, a whole set of varied experimental results is provided, in order to either identify or validate a thermo‐mechanical constitutive model, presented in the second paper part. Experimental responses of both hydraulic and mechanical behaviour are given after different heating/cooling cycling levels (105, 200, 300, 400∘C). The reference state, used for comparison purposes, is taken after mass stabilization at 60∘C. Typical uniaxial compression tests are provided, and original triaxial deviatoric compressive test responses are also given. Hydraulic behaviour is identified simultaneously to triaxial deviatoric compressive loading through gas permeability Kgas assessment. Kgas is well correlated with volumetric strain evolution: gas permeability increases hugely when εv testifies of a dilatant material behaviour, instead of contractile from the test start. Finally, the thermo‐mechanical model, based on a thermodynamics approach, is identified using the experimental results on uniaxial and triaxial deviatoric compression. It is also positively validated at residual state for triaxial deviatoric compression, but also by using a different stress path in lateral extension, which is at the origin of noticeable plasticity. Copyright © 2009 John Wiley & Sons, Ltd.

Active stress from earthquake focal mechanisms along the Padan–Adriatic side of the Northern Apennines (Italy), with considerations on stress magnitudes and pore-fluid pressures

The active tectonic regime along the outer Northern Apennines (Padan–Adriatic area) is a matter of debate. We analyse the active tectonic regime by systematically inverting earthquake focal mechanisms in terms of their driving stress field, comparing two different stress inversion methods. Earthquakes within the area often deviate from Andersonian conditions, being characterized by reverse or transpressional slip on high-angle faults even if the regime is almost purely thrust faulting (e.g. Reggio Emilia 1996 and Faenza 2000 earthquakes). We analyse the stress conditions at faulting for the Reggio Emilia and Faenza earthquakes in order to infer the stress magnitudes and the possible role of fluid pressures. The stress analysis defines a consistent pattern of sub-horizontal active deviatoric compression arranged nearly perpendicular to the eastern front of the Padan–Adriatic fold-and-thrust system, independent of the stress inversion method used. The results are consistent with active compression operating within the Padan–Adriatic belt. The stress field is thrust faulting (sub-vertical σ3), except for the Cesena–Forlì and Ancona areas, where a strike–slip regime (sub-vertical or steeply-plunging σ2) operates. The strike–slip regimes are interpreted as being caused by the superposition of local tensional stresses due to oroclinal bending (i.e. rotations of the belt about vertical axes) on the regional compressional stress field. Kinematic complexities characterize the 1996 Reggio Emilia seismic sequence. The distribution of these complexities is not random, suggesting that they are due to local variations of the regional stress field within the unfaulted rocks surrounding the coseismic rupture. The stress conditions at faulting for the Reggio Emilia 1996 and Faenza 2000 earthquakes, coupled with the observation that seismicity in the Padan–Adriatic area often occurs in swarms, suggest that high pore-fluid pressures (Pf ≥ 70% of the lithostatic load) operate within the compressed crust. The estimated stress difference ( σ1– σ3) is ≤ 460–560 MPa at 15–20 km depth.

A Comparative Study on the Effect of Down‐Pulling Lithospheric Root Beneath the West and East Mountain Ranges in the Qinghai‐Tibet Plateau

AbstractIn order to understand the tectonic stress mechanism caused by crust equilibrium and lithospheric root pull in the West and East Qinghai‐Tibet plateau, two‐dimensional viscoelastic models are established and computed on the basis of geophysical data. The results show that the horizontal deviatoric compression mainly comes from down‐pulling lithospheric root and the northward convergence of Indian plate is secondary in the West Section, forming the topography “Mountain rising and Basin sinking”. While the horizontal deviatoric compression in the rigid upper crust induced by the down‐pulling lithospheric root is relatively weak in the East Section. By comparing the numerical modeling results, the difference of deep material and tectonic stress system may imply different evolution stages of mountain uplift in the West and East Qinghai‐Tibet plateau.

Initiation of Subduction Zones as a Consequence of Lateral Compositional Buoyancy Contrast within the Lithosphere: a Petrological Perspective

Tonga and Mariana fore-arc peridotites, inferred to represent their respective sub-arc mantle lithospheres, are compositionally highly depleted (low Fe/Mg) and thus physically buoyant relative to abyssal peridotites representing normal oceanic lithosphere (high Fe/Mg) formed at ocean ridges. The observation that the depletion of these fore-arc lithospheres is unrelated to, and pre-dates, the inception of present-day western Pacific subduction zones demonstrates the pre-existence of compositional buoyancy contrast at the sites of these subduction zones. These observations allow us to suggest that lateral compositional buoyancy contrast within the oceanic lithosphere creates the favoured and necessary condition for subduction initiation. Edges of buoyant oceanic plateaux, for example, mark a compositional buoyancy contrast within the oceanic lithosphere. These edges under deviatoric compression (e.g. ridge push) could develop reverse faults with combined forces in excess of the oceanic lithosphere strength, allowing the dense normal oceanic lithosphere to sink into the asthenosphere beneath the buoyant overriding oceanic plateaux, i.e. the initiation of subduction zones. We term this concept the ‘oceanic plateau model’. This model explains many other observations and offers testable hypotheses on important geodynamic problems on a global scale. These include (1) the origin of the 43Ma bend along the Hawaii‐ Emperor Seamount Chain in the Pacific, (2) mechanisms of ophiolite emplacement, (3) continental accretion, etc. Subduction initiation is not unique to oceanic plateaux, but the plateau model well illustrates the importance of the compositional buoyancy contrast within the lithosphere for subduction initiation. Most portions of passive continental margins, such as in the Atlantic where large compositional buoyancy contrast exists, are the loci of future subduction zones.