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• Analogue experiments confirm the effect of isostacy on crater floor fracture formation by upward flexing of crater floors. • Long-term relaxation of crust below craters and their periphery is caused by lower crustal flow toward crater centres. • The geometry of floor fractures caused by relaxation depends on crater diameters and crustal strength. Crater floor fractures are prominent post-cratering structural vestiges that are known from large impact craters on rocky celestial bodies. Two mechanisms have been proposed to explain the formation of crater floor fractures: emplacement of horizontal igneous sheets below crater floors and isostatic re-equilibration of crust underlying target rocks, i.e., crustal relaxation. Here, we use two-layer analogue experiments to model the deformation of lower and upper crust following crater formation, scaled to the physical conditions on Earth, to explore the structural and kinematic consequences of crustal relaxation. Specifically, the structural evolution of model upper crust was systematically analysed for various initial depths and diameters of crater floors, gleaned from previous numerical models for average continental crust. The analogue modelling results provide quantitative estimates of the duration, geometry and distribution of deformation zones in the upper crust and, for the first time, a quantitative relationship between the diameter, depth and fracture geometry of crater floors. The experiments also show that crater floor uplift is accomplished by long-wavelength subsidence of the crater periphery, which may operate on time scales of hundreds of thousands of years in nature. We conclude that patterns of natural crater floor fractures, including impact melt rock dikes known from the Sudbury and Vredefort impact structures, can be caused by long-term uplift of the crater floor, compensated by lateral crustal flow toward the crater centre. [Display omitted] [ABSTRACT FROM AUTHOR]

Lava domes result from effusive eruption of high viscosity lava. These viscous lava extrusions range in shape from flat-topped domes with small height-to-width aspect ratios, to spine-like columns exhibiting large height-to-width aspect ratios. A primary control on morphology during early dome growth is thought to be the variation in rheological characteristics of extruded material. In this work, we present new scaled analogue models of lava dome growth that consider extrusion of a frictional plastic upper-conduit plug followed by viscous magma. We simulate the brittle plug using a sand-plaster mixture, the cohesion of which is varied by plaster content. We model the magma using sugar syrup, the viscosity of which is controlled by the weight percent of added crystalline sugar. The models both qualitatively and quantitatively reproduce part of the spectrum of natural dome morphology not previously obtained in most past analogue modelling studies. Model aspect ratios of 0.02 to 0.9 capture approximately 90 % of the reported aspect ratio variation in nature. Increasing plug cohesion results in extrusions with higher aspect ratios and spinier morphologies. Low viscosity fluid typically erupts through the brittle dome, whilst high viscosity fluid tends to promote endogenous growth or emerge as exogenous lobes. Particle Image Velocimetry shows that fracture localisation at the dome surface is cohesion-dependent, and eruption of fluid follows shear fractures within the dome. Where fluid remains contained within the dome, we see lateral spread leading to a wider and flatter dome morphology. Evolution of lava dome morphology, deformation, and associated hazards is guided by the complex rheological properties of the extruded material; we suggest that during episodic dome growth, these properties are largely defined in the conduit prior to their eruption. • New analogue experiments of lava dome growth following repose. • Sand-plaster mix used as solid, and sugar-syrup suspension used as fluid. • Cohesion is key influence on first-order geometric parameters of modelled domes. • High visc. fluid promotes lateral spread; low visc. fluid exploits fractures. • Experiments produce morphologies qualitatively and quantitatively similar to nature. [ABSTRACT FROM AUTHOR]

Analogue modelling of wrench tectonics typically utilizes a rigid basement with a velocity discontinuity under a brittle or brittle-viscous cover, such as in Riedel experiments, which confines fault localization in the overlaying model. However, such a set-up is hardly compatible with modeling brittle-ductile systems such as the upper and lower crust or a brittle sedimentary cover overlying a viscous evaporitic layer. To achieve a more realistic experimental approach, Bruno Vendeville designed an alternative experimental set-up decoupling the basement from the brittle overburden with a viscous layer in which the basement is not involved. In this configuration, strike-slip movement is driven laterally rather than from the base up, facilitated by "weak zones" that preferentially localize the deformation during shortening and enable sliding between compartments. This original approach provides greater flexibility for modeling complex strike-slip settings, allowing for more freedom for strike-slip structures to form and evolve through time. Although the experiments described in this work were conducted in the late 1990s, the co-authors have chosen to revisit and adapt this earlier work for this Special Issue to underscore Bruno's influence on another aspect of salt tectonics and his pioneering foresight in the field of analogue modelling. • Wrench zones have traditionally been modeled using basal discontinuities. • An alternative brittle-viscous set-up utilizing weak zones is presented. • Wrenching is triggered by shortening without to involve the rigid basement. • This set-up provides greater freedom for the formation and evolution of strike-slip structures. • The style of the wrench zone varies according to the degree of lateral confinement. [ABSTRACT FROM AUTHOR]

Megaflaps comprise steeply dipping to overturned panels of the oldest suprasalt strata flanking steep diapirs, and represent the roofs of early inflated salt. These large-scale structures result from salt-sediment interaction at minibasin scales and entail multiple kilometres of folding and vertical relief. They are divided into two end-member types (halokinetic and contractional) and form by some combination of limb rotation and kink-band migration. They can be difficult to image and interpret adjacent to flaring diapirs and beneath allochthonous salt due to steep bedding dips and suboptimal illumination. Using physical models, we investigate halokinetic megaflaps driven by differential loading. Models with vertically-stacked vs. laterally-shifting loading above a prekinematic layer have been run to determine the main processes and mechanisms controlling the growth and kinematic evolution of megaflaps. Parameters such as the thickness of the prekinematic cover, the width of the proto-salt wall, the synkinematic sedimentation rate, and variations in the mechanical properties of the prekinematic cover have been tested to evaluate their role in megaflap generation. The experimental results demonstrate that in absence of tectonic forces, halokinetic megaflaps are generated by a combination of 1) an early increase of pressure-head gradient between two adjacent minibasins with different rates of sedimentation and subsidence, and 2) the disappearance of this gradient that occurs when welding occurs beneath the more quickly subsiding minibasin. The geometry, kinematic evolution, and degree of small-scale deformation of the megaflaps in our analogue models are consistent with both exposed (e.g., Paradox Basin) and seismically imaged halokinetic megaflaps (e.g., deepwater northern Gulf of Mexico). • We investigate halokinetic megaflaps driven by differential loading using analogue models. • Modelled megaflaps stretch and thin upward when they are made by sand, and have a constant thickness when are more cohesive. • They formed by rapid subsidence of a minibasin once the adjacent one was welded and pressure-head gradient disappears. • Halokinetic megaflap folding was dominantly by bed rotation early, but mostly by kinkband migration late in the history. • Megaflap's development ended when allochthonous salt broke out and overrode them. [ABSTRACT FROM AUTHOR]

We conducted scaled analogue modelling to show the influence of varying single layer initial orientation on the geometry of folds and boudins in a bulk constrictional strain field. The initial angle between the plane of shortening and the competent layer (θ Z(i)) was incrementally increased from 0° to 90° by multiples of 11.25°. While the amount of layer thickening decreased with increasing θ Z(i) , the deformation structures produced range from pure dome-and-basin folds to coeval folds and boudins. Based on the attitude of fold axes, there are extension-parallel (F EPR) and extension-perpendicular (F EPP) folds, with axes subparallel and subperpendicular to the principal stretching axis (X), respectively. Coeval growth of F EPR folds and boudins occurred when θ Z(i) > ca. 25°. The F EPP folds can be subdivided into a first type which affect the entire layer (if θ Z(i) ranges between 11.25 and 78.75°) and a second type, referred to as FB EPP folds, which are affecting pre-existing boudins if θ Z(i) > 45°. The interlimb angle of all types of folds increases with increasing θ Z(i). Folds and boudins similar to the ones produced in this study can be found in salt domes and in tectonites of subduction zones. • Constrictional structures range from dome-and-basin folds to coeval folds and boudins. • Under bulk constriction, the competent layer rotates slower than a passive plane. • Extension-parallel and –perpendicular folds grow simultaneously. • Extension-perpendicular folds affect previous boudins. [ABSTRACT FROM AUTHOR]

Tunisia, located in the Northern African margin, was subjected to Late Permian-Early Cretaceous N–S extension subsequent to Pangea breakup and leading to the Tethyan opening and widening. During the Mid- and Late Cretaceous, the NE–SW crustal extension that established the Pantelleria-Malta-Sirt grabens system underlining the eastern margin stretched also the Tunisian Atlassic domain, creating and/or reactivating many NW-SE extensional structures as much as the grabens in central Tunisia and leading to volcanism and halokinesis along several weak zones. Neogene compressional tectonics inverted the earlier extensional structures and impacted most of the halokinetic features established within the post-Triassic cover. The M'Rhila-Labaied-Trozza fault relay zone, located in the middle of central Tunisia, shows both extensional and compressional structures with Triassic extrusive features and seems to be a key feature for understanding the geodynamic development of the area. In this work, we combined field and geophysical data together with analogue models to decipher the structure and the kinematic evolution of the study area. The results showed that the studied structures are associated with reactive Mesozoic salt ridges established along an extensional fault relay zone that utilized NW-SE, E-W and NE-SW inherited fractures. These initially created extensional corridors allowing Triassic evaporite extrusion and accumulation, then influenced the deformation of the area in transpressional and local pure compressional regimes depending on the fractures and stress orientations during tectonic inversion. Analogue guided interpretation of the successive deformational stages of the study area from salt ridge growth to its tectonic inversion. • Triassic salts occupy an abnormal stratigraphy positions in central Tunisia. • Analogue modelling is a suitable method to explain halokinetic movements in Central Tunisia. • Halokinetic movements highly affect structural evolution of the study area. [ABSTRACT FROM AUTHOR]

• Elongated calderas were often considered proxy for the regional extension. • We run circular caldera collapse analogue models afterward elongated by extension. • We prove that extension justifies only 13% of elongation for calderas in the MER. • Only specific sectors of caldera faults are capable of reactivation under extension. • Caldera structures influence the architecture of newly forming regional faults. Caldera collapses are paramount volcano-tectonic features because they form during hazardous explosive volcanic eruptions, they are ideal sites for geothermal development and mineral resources exploitation, and also because they preserve the evidence of the interaction between caldera magmatism and the regional tectonic processes. Despite this, many aspects of the caldera collapse process remain unclear, particularly concerning the interaction between caldera and tectonic related fault systems. We therefore used analogue models 1) to quantify the effect of regional strain on caldera elongation in extensional settings, such as the Main Ethiopian and the Kenya rifts, 2) to describe the effect of regional strain on caldera structures and, vice-versa 3) to document, for the first time, "the other side of the coin", that is how caldera structures affect the formation of newly forming regional extensional faults. Our models showed that tectonic extension only explains a small proportion (e.g. 13% for the Main Ethiopian Rift) of the elongation of most rift calderas. Furthermore, we showed how specific segments of caldera faults may accommodate regional extension by reactivating, therefore precluding caldera elongation. Finally, we showed how the presence of caldera structures may influence the geometry of newly forming regional normal faults, that display a marked curvature, "faking" caldera ring faults. We have suggested that these "fake" curved caldera ring faults may lead to incorrect estimations of caldera elongation in nature. In addition, such faults may also mislead geothermal fluid exploration, as they are likely disconnected from the caldera structures or the caldera plumbing system, and less likely the locus of hydrothermal fluid flow. [Display omitted] [ABSTRACT FROM AUTHOR]

Basement inherited structures represent a significant factor affecting thrust propagation dynamics during the growth of fold-and-thrust belts. In this study inspirited from seismic data analysis of Gaoquan anticline in the Northern Tianshan foreland basin, we devised an experimental approach to investigate the structural and kinematical evolution of deformation from preexisting basement restraining bend to subsequent contractional deformation. Tested parameters included reactivation of the basement restraining bend and erosion. Results indicated that when preexisting basement restraining bend was reactivated and folded an overlying décollement , subsequent thrust nucleated preferentially at the top of the folded décollement. Erosion helped localize deformation, thereby reducing the width of the deformation zone and promoting "out-of-sequence" thrusting during compression. Finally, as we employed silicone polymer to simulate overpressured mudstone layer in the major décollement , our experiments also provide insights into a better understanding of the relationship between shallow salt-detached thrusting and deep inherited basement structures, such as in the Jura Mountains structures. • Experimental models with wrench and compressional tectonics are presented. • asement restraining bends are reactivated or not during subsequent compression. • Restraining bend reactivation affects the mode of fold and thrust propagation. • Erosion during restraining bend reactivation affects sequence of thrusting. [ABSTRACT FROM AUTHOR]

Inspired by the structural evolution of the South-Central Pyrenean Thrust Salient we have designed an experimental program to investigate the role of the geometry of viscous detachments on the formation of thrust salients. The precontractional wedge geometry of the salt overburden facilitates propagation of the deformation front to the forward edge of the viscous detachment. As deformation progresses, interaction between structures formed above the viscous detachment and the adjacent frictional detachment mostly depends on the angles of the lateral edges of the weak layer with respect to the shortening direction. The orientation of the oblique structures defining thrust salients does not generally mimic the edges of the viscous detachment. They can even form at a high angle to the shortening direction if active structures in the frictional and viscous detachments connect along-strike. Thrust wedges above frictional and viscous detachments show different kinematic evolution. During the late stages of deformation frictional wedges propagate forward whereas viscous wedges deform internally by break-back thrust sequences. Both wedges show along-strike differences of the surface taper that have an impact on the evolution of the topography and related synorogenic sediment dispersal. Experimental results are consistent with the structural evolution and tectono-sedimentary relationships observed in the South-Central Pyrenees. [ABSTRACT FROM AUTHOR]

The European Alps result from the closure of a former Triassic to Cretaceous rifting system in which significative accumulation of Upper Triassic salt deposited. The thickness and spatial distribution of these salts had a major impact on the morphology and dynamic of the Alpine orogen all over the Western Alps. Following a bibliographical review, four regional cross-sections are revisited over the External Western Alps to emphasise along-strike variations of the orogenic wedge influenced by the presence or absence of Triassic salts. Each section is then compared with analogue models to examine the controlling factors explaining the structural styles variations. In the Northern Subalpine Chains, the absence of salt favoured a frictional decollement which promoted a narrow and thick orogenic wedge in the Bornes, Chartreuse, and Vercors massifs. Discrepancies in the spacing between the thrusts and the number of folds were however influenced by the thickness of incompetent layers in the sedimentary column. In the Jura and the SW Alps, the occurrence of Triassic salts promoted a viscous decollement and thus a wide and relatively thin orogenic wedge. However, in the SW Alps, thicker salt deposits allowed the development of pre-orogenic salt structures. These later constituted vertical heterogeneities having exerted a strong structural inheritance during the following compression, by accommodating a substantial part of the Alpine shortening, in squeezed diapirs especially. Triassic salt remobilization during the orogeny also locally promoted the development of syn-orogenic minibasins. This review highlights the significant role played by Triassic salt in shaping the Western Alps, and paves the way for possible new interpretations of the dynamics of the Alps regarding the exhumation of crystalline basement blocks, or new structural interpretations of Alpine domains, such as the Vocontian Domain. [ABSTRACT FROM AUTHOR]

This work includes, first, a synthesis of experimental analogue modelling programs assessing the influence of along-strike thickness variations in the development of oblique structures in fold-and-thrust belts. As a second part, we present a new experimental program testing, systematically, the influence of tapered décollement-cover thrust wedges. Previous experiments show that frictional, brittle models feature numerous, short-wavelength structures where the cover is thinner. Deformation fronts are totally or locally oblique to the backstop when the cover thinning is progressive or sharp, respectively. Low (<30°) or higher structural obliquities occur when thickness variations are progressive or sharp, respectively. The addition of a basal or intermediate décollement, commonly entails the transference of deformation towards its external pinch-out. Consequently salients occur and transfer zones and oblique structures form over décollement boundaries. Their location and orientation strongly depend on the pre-compressional shape of the décollements. Furthermore, as demonstrated by our modelling results, tapered brittle covers over viscous décollements result in asymmetric thrust wedges formed by structures that end or change their vergence laterally, resulting in obliquities generally <30°. Two natural case studies, showing strong (South Pyrenean Central Salient) or moderately oblique (Keping Shan fold-and-thrust belt) structures are revisited and compared to the described analogue models. • We study structural obliquities related to along-strike thickness changes. • We synthesized series of analogue models focused on the origin of oblique structures. • Previous models are completed with new ones involving tapered salt-detached systems. • New results and synthesized models are compared to three different natural examples. [ABSTRACT FROM AUTHOR]

In contractional (subduction/collisional) settings, convergence is accommodated by the formation of thin- and thick-skinned thrust and fold belts. The transmission of such deformation over larger distances into orogenic foreland areas is influenced by the inherited rheological characteristics of continental lithosphere. Lateral rheological variations parallel to the strike of continental foreland areas creates contrasting geometries and sequences of deformation that interact during orogenic build-up. We investigate the far-field transmission of strain within a continental lithosphere characterized by a laterally variable rheology through physical analogue modelling. Rheological weak crustal zones were introduced at distance from an advancing backstop to study the progressive along strike linkage and interference of structures during contraction. The results reveal that rheologically weak crustal zones localise far-field contractional deformation. When the size of weak zones, by means of their horizontal extend to depth ratio, is large, deformation localises at the boundaries of the weak zone where they lead to the formation of large-offset faults. Subsequently the faults migrate along-strike into areas that are rheologically stronger. When the size of the weak zone is reduced, a large-scale contractional step-over forms in orogenic forelands, where rheologically contrasting domains transmit out-of-sequence deformation by a gradual migration of thrust offsets and fold amplitudes along their strike. These results show that crustal scale orogenic step-overs do not always reflect variations in the geometry of the plate boundary (indenter) or along-strike gradients in shortening rates. Such features may also form in response to variations in rheology, as the ones created by inherited extensional basins situated at large distances from plate boundaries in the orogenic foreland. • Crustal weak zone(−s) localise strain at far distance from the indenter. • Contractional step over might reveal the presence of crustal weak zone. • Crustal deformation at weak zone will be associated with lithospheric folding. • The symmetry of lithosphere fold can reveal the lithosphere strength. [ABSTRACT FROM AUTHOR]

Fractures formed when igneous rocks cool below the surface of the Earth are of considerable current interest in studies of hydrology, mineral systems and radioactive waste disposal. However, little is known about the geometry and kinematics of such fractures. Conceptual models suggest that early fractures result from emplacement forces (the Cloos model) while two-dimensional numerical modelling suggests that fractures may result from the contraction of cooling bodies. We use analogue modelling to investigate the geometry of fractures formed in a cylindrical static contracting body. Our experiments show that dehydration of buried starch flour is a workable analogue. Fractures in the analogue material result from drying and contraction of a cylindrical volume which differs from previous experiments in which drying was from a single planar surface and resulted in the formation of columnar joints. In buried analogue models two key fracture sets form internally, producing sub-vertical radial fractures, and concentric fractures that curve downwards with depth. Fracture density decreases towards the centre and bottom of the samples. Samples that were buried more deeply have fewer and less curved concentric fractures than those with shallow burials. Radial fractures have similar orientations to those predicted from numerical models, and concentric fractures are comparable to marginal fissure orientations of the Cloos model. The analogue models suggest that both radial and concentric fractures may result from the contraction of plutonic bodies. • Analogue models to understand fracturing of plutons during cooling. • Radial and concentric, steepening downwards, fractures are formed in contracting cylindrical volumes. • Results depend on rate of contraction and burial depth. [ABSTRACT FROM AUTHOR]

Crustal shearing of pre-tectonic weak bodies (e.g. salt pillows, igneous pockets) associated with basement strike-slip fault systems is common in nature. Yet the structural pattern arising from the interference between a brittle shear zone and such weak rheological heterogeneities is still not fully explained. In the present work, different sand-box analogue modelling experiments were carried out to better understand, not only the shear zone perturbation effect caused by a strictly rheologic (viscous-weak) anomaly, but also the one arising from a combined morphological perturbation (i.e. traverse topographic crest). Obtained results show that the development of different shear zone deformation patterns is in each case essentially determined by the interplay between: (i) ductile wrenching strain accommodation in the (viscous) weak body; and (ii) coincidence vs. mismatch between main brittle-viscous rheological boundaries and (crest-related) topographic steps. Comparison of the experimentally obtained results with complying natural examples is further discussed, and essentially focused on the NE Atlantic tectono-magmatic interference between the Gloria transform fault and the Tore-Madeira submarine ridge. • Analogue modelling of strike-slip brittle shear propagation across a pre-tectonic weak (viscous) anomaly. • Combined (morpho-rheologic) effect exerted by the viscous anomaly and by a (90° or 120°) trasverse topographic crest. • Coincidence vs. mismatch between rheological and morphological boundaries and its influence on the resulting structural pattern. • Control on the distribution of different structures around opposite quarters of the viscous anomaly. • The natural example of the transform Gloria fault - Tore Madeira rise tectono-magmatic interference in NE Atlantic. [ABSTRACT FROM AUTHOR]

The PIV (particle image velocimetry) method became a standard tool for the calculation of displacement fields in physical geodynamic models. For understanding the deformation dynamics of geodynamic models, in our study, we implemented several post-processing algorithms on the derived displacement field and calculated the velocity and strain(-rate) components, such as the divergence of the velocity field, vorticity and shear strain-rate. In the model of oroclinal buckling, we focused on strain analysis of the upper crust and correlated the shear strain-rate, vorticity and divergence anomalies with visual deformation patterns in the upper crust. The divergence of velocity fields in these models correspond to the pop-up and pop-down belts oriented along the axial trace of the oroclinal bends. High shear strain-rate domains correlate with horizontal, isovolumic shear zones alongside these belts, while vorticity shows rotational trend of fold axial traces of the pop-up and pop-down belts, around orocline inflection. In another series of models, we simulated the development of melt-cored crustal scale detachment folds and employed the same set of parameters to investigate the ductile deformation visible in side-view of the model domain. We developed a method that allows tracing the divergence in subcells locked on target subdomains. We tracked and quantified melt flow between the melt source area at the bottom of the model and progressively developing folds. This mass transfer analysis revealed polyphase fold evolution, where initial fold perturbations quickly amplify as the melt accumulates in the triangular hinge sector below and between the rotating fold limbs. While the early amplification leads to decompression driving the melt into the hinge zone area between the limbs, the fold lock-up stage and continued attenuation of the vertical limbs is associated with melt expulsion from the fold interlimb domain back into the source layer, where it can be transferred laterally to the foreland. • Models of oroclinal buckling and detachment folding were investigated by the PIV method. • Several model parameters related to the velocity and strain were used to analyze the deformation features in both models. • The divergence of the velocity field reflects distortion of the model surface. • Analysis of the divergence reflects mass transfer budget in detachment folds. • The fold growth rate is driven by the degree of melt flux into the axial zone. [ABSTRACT FROM AUTHOR]

Three-dimensional analogue models are employed to investigate whether oroclines may develop by horizontal buckling. A first series of experiments demonstrates that a crustal ribbon carried by a subducting plate cannot buckle and detach from its mantle root because it weakens and deforms when entering the subduction zone, such that little compressive stress is transferred through the ribbon. A second series of experiments shows that the aspect ratio of the ribbon impacts the wavelength of buckling and that the experimental tank employed is too small (maximum equivalent length is < 1500 km) to generate multiple buckles. Finally, a third series of experiments shows that if the plate boundaries surrounding the ribbon resist its horizontal lateral motion, thrusts or strike-slip fault systems may be generated in the ribbon thereby preventing buckling. We conclude that oroclinal buckling is favoured when a crustal ribbon is pulled by subduction, causing backarc extension. Hence, buckling and bending models for orocline formation are not mutually exclusive but reinforce each other. • Oroclines do not form by buckling of crustal ribbon. • Lithospheric-scale orocline have large (>1500 km) wavelength. • Buckling occurs when assisted by subduction of negatively buoyant lithosphere. [ABSTRACT FROM AUTHOR]

The Pyrenees are a collisional orogen striking WNW-ESE. However, in the Ainsa oblique zone in the south-central part of the Pyrenees, faults and folds rotate to ∼N–S over a westward thinning evaporitic layer. The influence of lateral change in basal friction due to declining evaporitic layer thickness in a fold-and-thrust belt is studied by analogue modelling simulating the Ainsa oblique zone. In the experiments the transition zone is simulated by a viscous silicone mixture sheet that laterally changes in thickness adjacent to frictional-plastic quartz-sand. The models confirm that a transition between weak and strong basal friction causes oblique structures to develop. Weak detachment zones are prone to fault propagation whereas strong detachment zones are prone to imbrication. At the transition zone those structural styles are combined, resulting in a complex domain. The differences in deformation style cause dextral shear and clockwise rotation in the transition zone and larger horizontal deformation propagation in the weak basal friction zone. Structures produced in the analogue models are compared to structures in the Ainsa oblique zone and found to be representative. Therefore, this study shows that transition between weak and strong detachment layers can explain the regional structural deformation style in the Ainsa oblique zone. • The Ainsa oblique zone has oblique fold and fault structures likely due to an unevenly distributed basal viscous salt layer. • A new analogue set-up simulates lateral progressive thinning of a viscous silicone sheet representing the Ainsa oblique zone. • Experimentally obtained oblique fold and fault angles over a thinning viscous sheet match the Ainsa zone structural angles. • Transition between high and low basal friction combines two deformation styles causing dextral shear and clockwise rotation. [ABSTRACT FROM AUTHOR]

Better understanding of stress perturbations, strain propagation and fluid flow in the upper crust require characterisation of fault damage zone evolution. Outcrop studies help understand the different processes operating during fault movement; but capture little about the evolution through time. In this study, we investigate damage zone evolution using high-resolution analogue modelling; simulating the growth of a normal fault population. The incremental strain reveals that early deformation stages occur by strain localisation into corridors of distributed deformation. Active deformation within these corridors becomes narrower as segment linkage occurs, leading to the formation of master faults, encouraging further localisation of incremental strain. This results in wide zones of cumulative strain around the fault, accommodated by different types of fault damage geometry, which formed at very different stages of the fault system growth. We also highlight the new concept of "fault system damage", that shows similarities with observations in nature. These first descriptions of the fault damage evolution allow an understanding of the composite content of damage zones. This gives support to assess zones of permeability enhancement in naturally fractured reservoirs, and proposes a view of damage distribution for targeting fluid flow, geothermal resources, leaks, micro/macro seismicity and mechanical properties of fault zones. • Fault damage zones evolution is described and grow by fault segment linkage. • Damage zone types are described in out-of-plane dimension. • Damage at fault system scale occurs by the interaction of several master faults. • A new concept of fault system damages is described and compared to nature. [ABSTRACT FROM AUTHOR]

Abstract Dome-in-dome structures are frequent in salt provinces and in high-grade rocks of orogens. Their origin is only poorly understood. In the present study we have modelled dome-in-dome structures, which result from constrictional (radial) in-plane shortening of a competent layer embedded in a rising less competent matrix. Both the layer and the matrix consist of non-linear viscous plasticine, which display a viscosity contrast of 5. The deforming competent layer is characterized by (1) a striking downward drag along the margins, (2) a sample-scale first-order dome, and (3) numerous second-order domes and basins straddling the first-order dome. With increasing layer thickness and strain, the amplitude, A , arc-length, L , and wavelength, λ , of the second-order domes and basins increase. However, their growth rate is significantly smaller than that of non-rising domes and basins, which result from a bidirectional flow. This restricted growth of rising domes and basins is attributed to the simultaneous growth of the first-order dome, and explains, why hair-pin type folds – typical for non-rising domes and basins - are lacking, even at higher finite strain. Similar dome-in-dome structures like those produced in the present study have been described from deeper levels of salt and gneiss domes or from foliation triple points of interfering diapirs. Highlights • Dome-in-dome structures develop during in-plane constriction. • Dome-in-dome structures occur in salt and gneiss domes. • Rising dome-and-basin structures are growing on top of a single superior dome. • Rising dome-and-basin structures do not develop a dominant wavelength. • Rising dome-and-basin structures are characterized by a reduced growth rate. [ABSTRACT FROM AUTHOR]

Abstract During the early evolution of rift systems, individual rift segments often develop along pre-existing crustal weaknesses that are frequently non-continuous and laterally offset. As extension progresses, these initial rift segments establish linkage in order to develop a continuous rift system that might eventually lead to continental break-up. Previous analogue and numerical modelling efforts have demonstrated that rift interaction structures are influenced by structural inheritances, detachment layers, magma bodies, rate and direction of extension, as well as the distance between rift segments. Yet to date, the effects of syn-tectonic sediments have been largely ignored or only modelled in 2D. In this study we therefore assess the influence of sedimentation on rift segment and rift interaction zone evolution in orthogonal and oblique extensional settings, by means of 3D brittle-ductile analogue models, analysed with 4D X-ray computed tomography (XRCT or CT) methods and digital volume correlation (DVC) techniques. Our models show that syn-rift sedimentation does not significantly influence the initial large-scale evolution of rift segments and rift interaction zones. Nevertheless, syn-rift sedimentation can strongly affect rift-internal structures: sedimentary loading reinforces the rift wedge, decreasing rift wedge faulting and increases subsidence within the rift basin. These effects are strongest in areas where most accommodation space is available, that is, along the main rift segments. In contrast, rift segments that undergo high degrees of oblique extension develop less accommodation space and are therefore less influenced by sedimentary loading. Rift interaction structures are least affected by sediment influx, as they experience relatively low amounts of subsidence so that little accommodation space is available. Our conclusions are valid for the early stages of rift development, when a high sediment influx could delay continental break-up, as other processes are likely to become dominant during later stages of continental extension. Finally, state-of-the-art DVC analysis of CT data proves to be a powerful tool to extract and fully quantify 3D internal model deformation in great detail and could be useful for comparing and calibrating analogue and numerical models. Highlights • Syn-rift sedimentation does not significantly influence large-scale rift structures. • Yet sedimentary loading effects can strongly influence internal rift structures. • Extension direction and inherited structures largely control how rifts connect. • DVC can now quantify full 3D deformation in brittle-viscous analogue models. • Material may also move out of plane under orthogonal extension conditions. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we validate three-dimensional numerical models of subduction against a laboratory experiment. The design of our numerical models follows that of the laboratory, and employs a single subducting plate fixed at its trailing edge without an overriding plate. Rheology and buoyancy are not thermally dependent, but importantly the model has a free surface boundary. All the models include a sticky air layer, which is an approximation to a free surface boundary condition, as opposed to a free-slip condition that unrealistically binds the plate to the upper surface. We use a parallel, Python, particle-in-cell, finite-element scheme, and present 8 different models that differ in their viscosity at the interfaces of the subducting plate. We consider harmonic and arithmetic averaging of the viscosity together with a plate composite rheology associated with arithmetic averaging, which similarly to the harmonic averaging results in two lubricated sub-layers at the plate lower and upper interfaces. Despite showing a similar subduction-induced poloidal and toroidal flow in the mantle, the agreement of the models with the experiment is overall ordinary. The rates at which the slab tip sinks and the plate hinge retreats are approaching the laboratory counterparts to within 7–18%, at best, when using the composite plate models. Errors in the characterization of the laboratory parameters can account for 5% of such differences. As the composite plate sub-layers are made stiffer, the plate hinge also begins to depart from the steady retreat observed in the laboratory experiment. All numerical models exhibit a secondary Rayleigh-Taylor instability along the plate edge that is not observed in the laboratory experiment. The development of this instability is further delayed when the numerical spatial resolution is increased, or when the composite plate sub-layers are made stiffer. This benchmark exercise shows that 1) the glucose surface tension in the laboratory, not taken into account in the numerical modelling, is sufficient to prevent the flank instability during the retreat, and 2) numerical models are usually free from it due to their use of stiffer plate sub-layers and/or their display of a limited amount of retreat compared to this study. [ABSTRACT FROM AUTHOR]

The METYSS project ( M essinian E vent in the T yrrhenian from S eismic S tudy) is based on high-resolution seismic data acquired along the Eastern Sardinian margin, Western Tyrrhenian Sea. The main aim is to study the Messinian Salinity Crisis (MSC) in the Western Tyrrhenian Basin, but we also investigated the thinning processes of the continental crust and the timing of crustal vertical movements across this backarc domain. Our first results shown that rifting ended before the MSC, but that crustal activity persisted long after the end of the rifting. This has been particularly observed on the proximal margin, the East-Sardinia Basin, where the Mobile Unit (MU, mobile Messinian salt) is thin or absent. In this study, we examined the distal margin, the Cornaglia Terrace, where the MU accumulated during the MSC and acted as a décollement , thus potentially decoupling the basement from the sedimentary cover. Our observations provide evidence for lateral flow and gravity gliding of the salt and its brittle sedimentary overburden along local basement slopes generated by the post-MSC tilting of some basement blocks formerly generated during the rifting. We also investigated an intriguing wedge-shaped body of MU located in a narrow N-S half graben bounded to the west by a major, east-dipping, crustal normal fault. Classically, one could think that this salt wedge is related to the syn-tectonics deposition of the MU, but we propose an original scenario, in which the post-rift vertical motion of the major fault has been cushioned by lateral flow of an initially tabular salt layer, leaving the supra-salt series apparently unaffected by the crustal motions of the basement. We tested this scenario by comparing natural data and physical (analogue) modelling data. Our results reveal that salt tectonics provides a powerful tool to understand the deep crustal tectonics of the margin and to constrain the timing of vertical motions in the Western Tyrrhenian Basin, results that can be applied to rifted salt-bearing margins worldwide. [ABSTRACT FROM AUTHOR]

It is well known that ductile layers play a major role in the style and location of deformation. However, at the scale of a single normal fault, the impact of rheological layering is poorly constrained and badly understood, and there is a lack of information regarding the influence of several décollement levels within a sedimentary cover on the single fault geometry under purely extensive deformation. We present small-scale experiments that were built with interbedded layers of brittle and ductile materials and with minimum initial constraints (only a velocity discontinuity at the base of the experiment) on the normal fault geometry in order to investigate the influence of controlled parameters such as extension velocity, rate of extension, ductile thickness and varying stratigraphy on the 3D fault geometry. These experiments showed a broad-spectrum of tectonic features such as grabens, ramp-flat-ramp normal faults and reverse faults. Forced folds are associated with fault flats that develop in the décollement levels (refraction of the fault angle). One of the key points is that the normal fault geometry displays large variations in both direction and dip, despite the imposed homogeneous extension. This result is exclusively related to the presence of décollement levels, and is not associated with any global/regional variation in extension direction and/or inversion. [ABSTRACT FROM AUTHOR]

The Eastern Sichuan–Xuefeng fold-thrust belt (CXFTB) located in South China has received wide attention due to its distinctive deformation styles and close relationships with natural gas preservation, but its deformation evolution still remains controversial. In order to study further this issue, we designed three sets of analogue models. Based on the results of the models, we suggest that: 1) the deformation in the CXFTB may simultaneously initiate along two zones nearby the Dayong and Qiyueshan faults at ∼190 Ma, and then progressively propagate into the interiors of the Western Hunan-Hubei and Eastern Sichuan domains at ∼140–150 Ma, and finally reach the front of the Huayingshan fault at ∼120 Ma; 2) the difference in décollement depth is the main factor determining the patterns of folds in different domains of the CXFTB; and 3) the Eastern Sichuan domain may have a basement significantly different from those of the Western Sichuan and Western Hunan-Hubei domains. [ABSTRACT FROM AUTHOR]

Gravity-driven salt-related deformation on passive margins is commonly interpreted as kinematically-linked domains of updip extension and downdip contraction with an intermediate, undeformed zone of translation. Our study uses analogue models to show how salt flow is affected by the subsalt geometry resulting in complex deformation and the alternation of extensional and contractional domains in space and time in translational salt provinces. In our work, the analogue models incorporate subsalt relief associated with half-graben and graben that are realistic geometries along passive margins and combine the effects of gravity spreading in which deformation is driven by differential sedimentary loading, and an early-stage of gravity gliding induced by basin tilt. This approach allows for analysis of how salt flow and overburden deformation are affected by: (1) smoothly dipping subsalt relief; (2) subsalt relief characterized by half-graben with a gently landward dipping extensional fault; (3) subsalt half-graben connected laterally with a distal symmetric graben. For each of these models, we evaluate: 1) the interplay between slipping deformation corresponding to gravity gliding, gravity spreading mechanisms, and subsalt geometry; 2) the spatial and temporal distribution of salt-related structural domains; and 3) the contrasting styles of salt tectonics for different subsalt geometries. In addition, some of the experiments have been repeated considering thicker salt and pre-kinematic overburden and different sedimentation rates. We show that the structures of the subsalt relief variably resist salt flow producing flow mismatch. This variable resistance controls the orientation of the extension and contraction supra-salt structures. Salt surplus and contraction occur over landward dipping base-salt ramps, and a zone of salt depletion and subsidence limited by extensional and contractional hinges occurs over the basinward dipping base-salt ramp. Variations in width and the dip of these base-salt ramps result in stronger flow mismatches and deformation complexity. We also discuss variations in salt flow profile in response to the gravitational forces that drive deformation, allowing a good first-order approximation of viscous salt flow at a regional scale that can be directly compared to examples from various salt-bearing continental margins, particularly the relatively less understood but extremely economically important distal domains of Brazil's Campos and Santos basins. • Analogical simulations of subsalt relief on gravity-driven salt tectonics. • Structural distribution related to salt flux variations over subsalt relief. • Variably-dipping subsalt relief result in flow mismatches and complex deformation. [ABSTRACT FROM AUTHOR]

Analogue sandbox experiments have been used for a long time to understand tectonic processes, because they facilitate detailed measurements of deformation at a spatio-temporal resolution unachievable from natural data. Despite this long history, force measurements to further characterise the mechanical evolution in analogue sandbox experiments have only emerged recently. Combined continuous measurements of forces and deformation in such experiments, an approach here referred to as “sandbox rheometry”, are a new tool that may help to better understand work budgets and force balances for tectonic systems and to derive constitutive laws for regional scale deformation. In this article we present an experimental device that facilitates precise measurements of boundary forces and surface deformation at high temporal and spatial resolution. We demonstrate its capabilities in two classical experiments: one of strike-slip deformation (the Riedel set-up) and one of compressional accretionary deformation (the Critical Wedge set-up). In these we are able to directly observe a correlation between strain weakening and strain localisation that had previously only been inferred, namely the coincidence of the maximum localisation rate with the onset of weakening. Additionally, we observe in the compressional experiment a hysteresis of localisation with respect to the mechanical evolution that reflects the internal structural complexity of an accretionary wedge. [ABSTRACT FROM AUTHOR]

Various studies have demonstrated the intrinsic interrelationship between tectonics and sedimentation in salt-related rift basins during extension as well as during their inversion by compression. Here, we present seven brittle–ductile analogue models to show that the longitudinal or transverse progradation of sediment filling an elongate extensional basin has a substantial impact on the growth of diapirs and their lateral geometrical variations. We use five extensional models to reveal how these prograding systems triggered diapir growth variations, from proximal to distal areas, relative to the sedimentary source. In the models, continuous passive diapir walls developed, after a short period of reactive–active diapiric activity, during syn-extensional homogeneous deposition. In contrast, non-rectilinear diapir walls grew during longitudinal prograding sedimentation. Both longitudinal and transverse post-extensional progradation triggered well-developed passive diapirs in the proximal domains, whereas incipient reactive–active diapirs, incipient roller-like diapirs, or poorly developed diapirs were generated in the distal domains, depending on the modelled sedimentary pattern. Two models included final phases of 6% and 10% shortening associated with basin inversion by compression, respectively, to discriminate compressional from purely extensional geometries. With the applied shortening, the outward flanks of existing diapir walls steepened their dips from 8°–17° to 30°–50°. Likewise, 6% of shortening narrowed the diapir walls by 32%–72%, with their fully closing (salt welds) with 10% of shortening. We compare our results with the distribution of salt walls and minibasins of the Central High Atlas diapiric basin in Morocco, which was infilled with a longitudinally prograding mixed siliciclastic and carbonatic depositional sequence during the Early–Middle Jurassic with a minimum thicknesses of 2.5–4.0 km. [ABSTRACT FROM AUTHOR]

A series of analogue experiments simulating intra-continental subduction contemporaneous with lateral extrusion of the upper plate are performed to study the interference between these two processes at crustal levels and in the lithospheric mantle. The models demonstrate that intra-continental subduction and coeval lateral extrusion of the upper plate are compatible processes leading to similar deformation structures within the extruding region as compared to the classical setup, lithosphere-scale indentation. Strong coupling across the subduction boundary allows for the transfer of stresses to the upper plate, where strain regimes are characterized by crustal thickening near a confined margin and dominated by lateral displacement of material near a weak lateral confinement. The strain regimes propagate laterally during ongoing convergence creating an area of overlap characterized by transpression. When subduction is oblique to the convergence direction, the upper plate is less deformed and as a consequence the amount of lateral extrusion decreases. In addition, strain is partitioned along the oblique plate boundary resulting in less subduction in expense of right lateral displacement close to the weak lateral confinement. Both oblique and orthogonal subduction models have a strong resemblance to lateral extrusion tectonics of the Eastern Alps (Europe), where subduction of the adjacent Adriatic plate beneath the Eastern Alps is debated. Our results imply that subduction of Adria is a valid mechanisms to induce extrusion-type deformation within the Eastern Alps lithosphere. Furthermore, our findings suggest that the Oligocene to Late Miocene structural evolution of the Eastern Alps reflects a phase of oblique subduction followed by a later stage of orthogonal subduction conform a Miocene shift in the plate motion of Adria. Oblique subduction also provides a viable mechanism to explain the rapid decrease in slab length of the Adriatic plate beneath the Eastern Alps towards the Pannonian Basin. [ABSTRACT FROM AUTHOR]

We present new analogue modelling results of crustal thrust-systems in which a deformable (brittle) hanging wall is assumed to endure passive internal deformation during thrusting, i.e. exclusively as a consequence of having to adapt its shape to the variable geometry of a rigid footwall. Building on previous experimental contributions, we specifically investigate the role of two so far overlooked critical variables: a) concave-convex (CC) vs. flat-ramp-flat (FRF) thrust ramp geometry; and b) presence vs. absence of a basal velocity discontinuity (VD). Regarding the first variable, we compare new results for considered (CC) smoother ramp types against classical experiments in which (FRF) sharp ramp geometries are always prescribed. Our results show that the considered sharp vs. smooth variation in the thrust-ramp geometry produces important differences in the distribution of the local stress field in the deformable hanging wall above both (lower and upper) fault bends, with corresponding styles of strain accommodation being expressed by marked differences in measured morpho-structural parameters.
Regarding the second variable, we for the first time report analogue modelling results of this type of experiments in which basal VDs are experimentally prescribed to be absent. Our results critically show that true passive hanging wall deformation is only possible to simulate in the absence of any basal VD, since active shortening accommodation always necessarily occurs in the hanging wall above such a discontinuity (i.e. above the lower fault bend). In addition, we show that the morpho-structural configuration of model thrust-wedges formed for prescribed VD absence conditions complies well with natural examples of major overthrusts, wherein conditions must occur that approximate a frictionless state along the main basal thrust-plane. [ABSTRACT FROM AUTHOR]

The East China Sea Shelf Basin lies between the Pacific Subduction and Indian-Eurasian Collision tectonic domains and records Cenozoic tectonic inversion, especially in the Xihu Sag. To improve the understanding of the evolution and mechanism of tectonic inversion, this paper employs analogue modelling to reproduce the evolutionary process. Combined with the structural analysis of seismic profile, this paper determines the pattern of basement-involved faults. Simulation results show that under the transtension, two subsidence centers developed and a number of normal faults assembled in two flower structures. When the stress field turned into transpression, the geometry and deformation of inversion basin inherited the previous transtensional basin and pre-existing faults, respectively. The geometry and fault patterns in models are well consistent with those observed in the Xihu Sag, which indicates the plausibility of similar deformation controls. The formation of the tectonic inversion is related to the variation in stress field caused by the changes in the rates and directions of the subduction of the Pacific Plate and the collision of the Indian Plate with Eurasian Plate. [ABSTRACT FROM AUTHOR]

During the process of sediment mobilisation unconsolidated sediments are deformed due to fluid overpressure. Structures formed by sediment mobilisation can serve as high permeable pathways for vertical fluid migration through a low-permeable seal layer and, therefore, have to be considered in the risk assessment of CO 2 storage sites. In our study, we designed and conducted physical sandbox models to investigate structural characteristics and the dynamic evolution of sediment mobilisation resulting from pore fluid overpressure. Preliminary results show that basically two different modes of cover deformation occur: uplift in cohesive sediment and pipe formation in low-cohesive sediments. [ABSTRACT FROM AUTHOR]

This work simulates the inversion and incorporation of sub-thrust segmented half-graben basins into tectonic wedges by means of sandbox analogue models. Segmented half-graben basins - striking at 90°, 45° and 15° to the extension direction - were created first, and then shortened using different angles for the basal detachment and topographic slope. A shallow viscous polymer layer located above the half-graben basin was included in one of the models. The experiments were analysed using time-lapse photography, topography laser scans and image-based 3D voxels. The results indicate a deformation sequence characterised by layer-parallel compaction, fault reactivation, thrust propagation and related folding. Fault reactivation was associated with the layer-parallel compaction accomplished by slip along the basal detachment, prior to and in between pulses of thrusting. Results reveal the fundamental control imposed by the vertical load of the tectonic wedge and its integrated strength profile in the inversion of sub-thrust basins. Results are compared to the Alpine-Carpathian fold-and-thrust belt. [ABSTRACT FROM AUTHOR]

Scale-model experiments, involving pure shear shortening of a single incompetent layer embedded in a competent matrix, were carried out to study the influence of the bulk strain geometry and an inclined layer on the growth rate and shape of mullions. The viscosity ratio between non-linear viscous layer and matrix varied from 1/4–1/35. If layer parallel shortening (e Z) is the same, the growth rate of mullions is highest under bulk constriction, moderate under bulk plane strain, and lowest under bulk flattening. Because of their low growth rate in a flattening strain field, formation of mullions is hardly possible. Initial layer inclination under bulk constriction led to antisymmetric mullions. Increasing the viscosity ratio between layer and matrix results in (1) faster rotation of the layer towards the main stretching axis, X , (2) earlier attainment of a stable mullion wavelength, and (3) earlier formation of flames. The style of deformation (coeval folding and boudinage vs. shearing) of a thin competent sheet, inserted into the incompetent layer, is controlled not only by the viscosity ratio, but also by the bonding between competent and incompetent material. The new results shed light on the deformation behavior of Sederholm-type dikes and sills at deep crustal levels. • Mullions grow fast under bulk constriction. • Mullions grow very slow under bulk flattening. • Flames are supported by high viscosity ratio. • Mullions grow earlier at high viscosity ratio. • Oblique layers result in antisymmetric mullions. [ABSTRACT FROM AUTHOR]

The analogue materials play a critical role in dynamically scaled experiments, defining the processes that can be simulated and the structures observable in the model. The dynamic scaling enables the direct comparison between the model and its natural counterpart. To obtain such a model the physical and mechanical properties of the analogue material must be scaled with respect to the rock prototype. A large variety of materials have been applied in analogue modelling studies to address the physical and mechanical requirements for the simulation of (i) upper crust, (ii) middle crust and (iii) lower crust and mantle processes. Nevertheless, the development of new model materials represents a continuous improvement of the analogue modelling techniques. We investigated the mechanical and physical properties of a new Granular Rock-Analogue Material (GRAM) (introduced in Massaro et al. (2022)) and its component materials. GRAM is an ultra-weak artificial sandstone composed of quartz sand cemented with gypsum, capable to deform by tensile and shear failure under variable stress conditions. GRAM aggregates in different mixing ratios (from 1% to 4% in weight of hemihydrate powder) were systematically tested with ring-shear tests and uniaxial compression tests. The relationships between the hemihydrate content and the mechanical properties of GRAM were examined. Additionally, the sample preparation procedure of GRAM was investigated, evaluating the impact of the residual water content on the mechanical properties of GRAM aggregates and defining a standard preparation procedure. Finally, GRAM was compared to natural rocks and to other granular materials applied in analogue modelling studies, in terms of physical and mechanical properties, application to physical modelling and dynamic scaling. It was underlined how the application of GRAM aggregates in dynamically scaled experiments can enhance the comprehension of the fault and fracture processes occurring at the scale of the damage zone. • The physical and mechanical properties of a new model material were investigated. • Preparation workflow and moisture content impact the mechanical properties. • Compressive and shear strength, elastic properties and strain softening analysed. • Dynamic scaling and model-nature length ratio modified by varying the mixing ratios. • Simulation of brittle deformation processes in fault zones at a novel model resolution. [ABSTRACT FROM AUTHOR]

Understanding magma behaviour during emplacement within the crust is vital for understanding the dynamic processes occurring in volcanic systems. However, linking the static record of magma flow to its dynamic origin is challenging, particularly as macroscopic indicators of magma flow are often not observed, absent and/or have been modified after emplacement. Anisotropy of magnetic susceptibility (AMS) has been used as an important tool in field studies to infer magma flow direction, using the magnetic fabric as a record of the magma intrusion dynamics and to identify magma source regions. Here, we describe a new method to explore magnetic fabric development in magma intrusions and lava flows using scaled analogue laboratory experiments. Coloured mixtures of Plaster of Paris (the magma analogue) seeded with magnetite particles were loaded concentrically into a piston and injected through a central port in the base of a box filled with compacted fine-grained wheat flour (the crust analogue). This created a series of interconnected sheet and tube-like 'magma' intrusions which eventually breached the surface to feed a model 'lava flow'. Once solidified, the intrusions were excavated and sampled for AMS, with the results showing that magnetic fabrics were preserved. A new dynamic scaling analysis shows the plaster mixture represents the intrusion of dacite magma into the shallow crust. These models provide proof-of-concept that this new methodology and scaling analysis can be used to explore AMS development in viscous (dacite) magma intrusions in nature, with the potential for direct comparison with field-based indicators of magma flow dynamics. • New analogue experiments show how magnetic fabrics develop during magma intrusion. • Compacted wheat flour represents the crust and liquid Plaster represents the magma. • Scaling analysis shows liquid Plaster is a good analogue for dacite magma. • Dyke-like intrusions were created for the first time, and the intrusion morphologies agree well with nature. • Anisotropy of magnetic susceptibility fabrics were formed by magnetite crystals and record flow. [ABSTRACT FROM AUTHOR]

Morphometric parameters have been used to describe and classify the shape of volcanic edifices, but the processes that control these parameters are not fully understood. Here we investigate the influence of the changes in eruptive volume and magma viscosity on the edifice morphology of a simple polygenetic volcano with a fixed stable vent using analogue experiments in the laboratory. We ran the experiments by ejecting vegetable oil repeatedly at a static location in a temperature-controlled room with an adequate interval between ejections to allow this material to solidify. The experiments comprised four different scenarios: constant ejection volume and viscosity, linearly decreasing ejection volume with constant viscosity, exponentially decreasing ejection volume with constant viscosity, and increasing viscosity with constant ejection volume. We built digital elevation models from images captured by digital cameras after each ejection using photogrammetry method. We describe the shapes of the edifices using morphometric parameters, such as height, basal width, H/W ratio, volume, slope, circularity, and regularity. The experiments with decreasing ejection volume produced taller edifices with steeper slopes, especially near the summit, compared to the edifice produced with constant ejection volume. A similar finding was also observed for the edifice resulting from the experiment with increasing viscosity. The circularity and regularity indexes were insignificantly influenced by ejection volume and viscosity changes. Instead, these parameters vary with the height fraction of the edifice where the lower part is more circular, but irregular compared to the upper part. Based on the changes of morphometric variables throughout the experiments, we propose three development stages of volcanic edifice growth: basal foundation, flank construction, and vertical build-up. The development stage in which the edifice currently grows can be inferred from the relative changes in the eruptive volume and the magma viscosity. Therefore, the quantitative characterization of a natural volcanic edifice's morphology can be interpreted to explain the processes that influenced it and its stage of growth. [ABSTRACT FROM AUTHOR]

The detachment or breakoff of subducted lithosphere is investigated using scaled three-dimensional thermo-mechanical analogue experiments in which forces are measured and deformation is monitored using high-speed particle imaging velocimetry (PIV). The experiments demonstrate that the convergence rate in a subduction zone determine if and when slab detachment occurs. Slow subduction experiments (with scaled convergence rates ∼1 cm yr −1 ) have lower Peclet numbers and are characterized by lower tensile strength subducted lithosphere, causing detachment to occur when the downward pull force exerted by a relatively short subducted slab is relatively low. Therefore when continental collision is preceded by slow oceanic subduction, the hot and weak subducted lithosphere need not be very long or extremely negatively buoyant to cause detachment. Under such conditions, detachment may occur sooner after the onset of continental subduction than previously predicted. In contrast, if collision is preceded by rapid subduction (∼10 cm yr −1 ), breakoff will be delayed and occur only when the convergence rate has slowed sufficiently to thermally weaken the slab and cause its eventual failure. The analogue experiments further confirm that slab detachment occurs diachronously as it propagates along the plate boundary. Stereoscopic PIV reveals a characteristic strain pattern that accompanies the detachment. Horizontal contraction and subsidence (with scaled values up to 1200 m) in the trench and forearc area precedes the passage of the detachment, and is followed by horizontal extension and uplift (up to 900 m). High-frequency monitoring captures rapid propagation of the detachment along the plate boundary at scaled rates of up to 100 cm yr −1 . However this rate is not constant and interaction between the slab and lower mantle or opening of a backarc basin in the upper plate can reduce or stop slab breakoff propagation altogether. [ABSTRACT FROM AUTHOR]

Continental rifts often develop from linkage of distinct rift segments under varying degrees of extension obliquity. These rift segments arise from rift initiation at non-aligned crustal heterogeneities and need to interact to develop a full-scale rift system. Here, we test the effects of 1) oblique extension and 2) initial heterogeneity (seed) offset on continental rift interaction with the use of an improved analogue model set-up. X-ray computer tomography (CT) techniques are used to analyse the 3D models through time and the results are compared with additional numerical models and natural examples. The experimental results reveal that increasing extension obliquity strongly changes rift segment structures from wide rifts in orthogonal settings to narrower rifts with oblique internal structures under oblique extension conditions to narrow strike-slip dominated systems towards the strike-slip domain. We also find that both decreasing seed offset and increasing extension obliquity promote hard linkage of rift segments through the formation of continuous rift boundary faults at the surface. (Initial) soft linkage through the formation of relay ramps is more likely when seed offset increases or extension is more orthogonal. Rather than linking at depth, the rift boundary faults curve around each other at depth and merge towards the surface to form a continuous trough. Orthogonal extension promotes the formation of intra-rift horsts, which may provide hydrocarbon traps in nature. [ABSTRACT FROM AUTHOR]

How the South China Sea rifted has long been a puzzling question that is still debated, particularly with reference to the Southwest Sub-basin (SWSB). Analogue modelling remains one of the most useful tools for testing rift models and processes. Here, we present and discuss a series of analogue modelling experiments designed to investigate the rifting process of the SWSB. Convincing geophysical results were compiled to provide realistic constraints to test the experimental results and interpretations. A heterogeneous lithosphere model with a varied lithospheric structure showed tectono-morphological features similar to the natural case of the SWSB, indicating that the initial thermal condition and rheological stratification of the lithosphere should have a dominant effect on the rifting process of the SWSB. Rigid tectonic blocks existed in the continental margin, such as the Macclesfield Bank and the Reed Bank, and they played important roles in both the shaping of the continent–ocean boundary and the coupling between the crust and mantle. The initial thermal condition and rheological stratification of the lithosphere under the South China Sea controlled the propagated rifting process of the SWSB. Extension was centred on the deep troughs between the rigid blocks, and the break-up occurred in these areas between them. The westward rifting propagation is best explained with a heterogeneous lithosphere model characterized by varied lithospheric structure, and it was responsible for producing the V-shaped configuration of the SWSB. [ABSTRACT FROM AUTHOR]

Palaeo-uplifts often exist in fold-and-thrust belts. However, their effects on the deformational process have not yet been well understood. To evaluate such effects, six analogue models were systematically run based on geological features of the Wupoer fold-and-thrust belt (FTB), NE Pamir, where the Wulagen palaeo-uplift with overlying gypsum bed has been clearly identified. Our analogue results demonstrated that the palaeo-uplift (its location and inhomogenous distribution), accompanied with overlying gypsum bed that serves as ductile décollement, plays a critical role in localizing the front thrust fault and shaping it into arc form. The results indicate that the front thrust fault slides along the ductile décollement (gypsum bed), and breaks through onto the surface at the region where the palaeo-uplift develops, forming a piggy-back basin in the hanging wall. It suggests that the palaeo-uplift with consequent topographic variation of the overlying ductile décollement localizes the breakthrough point of the front thrust fault. In addition, the results indicate that the front thrust fault (Pamir Front Thrust, PFT) initially broke through in the location where it develops the Wulagen palaeo-uplift and propagated aside. This resulted in the distance between the PFT and the basement-involved fault (Main Pamir Thrust, MPT) to decrease from the region with palaeo-uplift to the areas aside without palaeo-uplift, thereby forming the arc-shaped PFT in map view. The results of this study also provide a revised geological model, which emphasizes the effect of décollement layer on absorbing the slip along the PFT. Our results provide a new mechanical interpretation of the deformation of Wupoer FTB, NE Pamir. [ABSTRACT FROM AUTHOR]

Orogenic wedges locally present chaotic tectonostratigraphic units that contain exotic blocks of various size, origin, age and lithology, embedded in a sedimentary matrix. The occurrence of ophiolitic blocks, sometimes huge, in such “mélanges” raises questions on (i) the mechanisms responsible for the incorporation of oceanic basement rocks into an accretionary wedge and (ii) the mechanisms allowing exhumation and redeposition of these exotic elements in “mélanges” during wedge growth. To address these questions, we present the results of a series of analogue experiments performed to characterize the processes and parameters responsible for accretion, exhumation and tectonosedimentary reworking of oceanic basement lithospheric fragments in an accretionary wedge. The experimental setup is designed to simulate the interaction between tectonics, erosion and sedimentation. Different configurations are applied to study the impact of various parameters, such as irregular oceanic floor due to structural inheritance, or the presence of layers with contrasted rheology that can affect deformation partitioning in the wedge (frontal accretion vs basal accretion) influencing its growth. Image correlation technique allows extracting instantaneous velocity field, and tracking of passive particles. By retrieving the particle paths determined from models, the pressure-temperature path of mélange units or elementary blocks can be discussed. The experimental results are then compared with observations from ophiolite-bearing mélanges in Taiwan (Lichi and Kenting mélanges) and Raman spectroscopy of carbonaceous material (RSCM) Thermometry data on rocks from the northern Apennines (Casanova mélange). A geological scenario is proposed following basic observations. The tectonic evolution of the retroside of doubly vergent accretionary wedges is mainly controlled by backthrusting and backfolding. The retro wedge is characterized by steep slopes that are prone to gravitational instabilities. It triggers submarine landslides inducing huge mass transfers. This erosion combined with backthrusting could favour exhumation of the ophiolitic fragments formerly accreted at the base of the wedge along the rough seafloor-sediments interface. Such an exhumed material can be reworked and deposited as debris- flows in proximal basins located at the foot of the retrowedge slope forming a tectono-sedimentary mélange. These syntectonic basins are continuously deformed and involved in prograding backthrusting-induced deformation. [ABSTRACT FROM AUTHOR]

We present a review of the analogue modelling method, which has been used for 200 years, and continues to be used, to investigate geological phenomena and geodynamic processes. We particularly focus on the following four components: (1) the different fundamental modelling approaches that exist in analogue modelling; (2) the scaling theory and scaling of topography; (3) the different materials and rheologies that are used to simulate the complex behaviour of rocks; and (4) a range of recording techniques that are used for qualitative and quantitative analyses and interpretations of analogue models. Furthermore, we apply these four components to laboratory-based subduction models and describe some of the issues at hand with modelling such systems. Over the last 200 years, a wide variety of analogue materials have been used with different rheologies, including viscous materials (e.g. syrups, silicones, water), brittle materials (e.g. granular materials such as sand, microspheres and sugar), plastic materials (e.g. plasticine), visco-plastic materials (e.g. paraffin, waxes, petrolatum) and visco-elasto-plastic materials (e.g. hydrocarbon compounds and gelatins). These materials have been used in many different set-ups to study processes from the microscale, such as porphyroclast rotation, to the mantle scale, such as subduction and mantle convection. Despite the wide variety of modelling materials and great diversity in model set-ups and processes investigated, all laboratory experiments can be classified into one of three different categories based on three fundamental modelling approaches that have been used in analogue modelling: (1) The external approach, (2) the combined (external + internal) approach, and (3) the internal approach. In the external approach and combined approach, energy is added to the experimental system through the external application of a velocity, temperature gradient or a material influx (or a combination thereof), and so the system is open. In the external approach, all deformation in the system is driven by the externally imposed condition, while in the combined approach, part of the deformation is driven by buoyancy forces internal to the system. In the internal approach, all deformation is driven by buoyancy forces internal to the system and so the system is closed and no energy is added during an experimental run. In the combined approach, the externally imposed force or added energy is generally not quantified nor compared to the internal buoyancy force or potential energy of the system, and so it is not known if these experiments are properly scaled with respect to nature. The scaling theory requires that analogue models are geometrically, kinematically and dynamically similar to the natural prototype. Direct scaling of topography in laboratory models indicates that it is often significantly exaggerated. This can be ascribed to (1) The lack of isostatic compensation, which causes topography to be too high. (2) The lack of erosion, which causes topography to be too high. (3) The incorrect scaling of topography when density contrasts are scaled (rather than densities); In isostatically supported models, scaling of density contrasts requires an adjustment of the scaled topography by applying a topographic correction factor. (4) The incorrect scaling of externally imposed boundary conditions in isostatically supported experiments using the combined approach; When externally imposed forces are too high, this creates topography that is too high. Other processes that also affect surface topography in laboratory models but not in nature (or only in a negligible way) include surface tension (for models using fluids) and shear zone dilatation (for models using granular material), but these will generally only affect the model surface topography on relatively short horizontal length scales of the order of several mm across material boundaries and shear zones, respectively. [ABSTRACT FROM AUTHOR]

Dome and basin folds are structures with circular or slightly elongate outcrop patterns, which can form during single- and polyphase deformation in various tectonic settings. We used power-law viscous rock analogues to simulate single-phase dome-and-basin folding of rocks undergoing dislocation creep. The viscosity ratio between a single competent layer and incompetent matrix was 5, and the stress exponent of both materials was 7. The samples underwent layer-parallel shortening under bulk pure constriction. Increasing initial layer thickness resulted in a decrease in the number of domes and basins and an increase in amplitude, A , arc-length, L , wavelength, λ , and layer thickness, H f . Samples deformed incrementally show progressive development of domes and basins until a strain of e Y=Z = −30% is attained. During the dome-and-basin formation the layer thickened permanently, while A , L , and λ increased. A dominant wavelength was not attained. The normalized amplitude ( A / λ ) increased almost linearly reaching a maximum of 0.12 at e Y=Z = −30%. During the last increment of shortening ( e Y=Z = −30 to −40%) the domes and basins did not further grow, but were overprinted by a second generation of non-cylindrical folds. Most of the geometrical parameters of the previously formed domes and basins behaved stable or decreased during this phase. The normalized arc-length ( L / H f ) of domes and basins is significantly higher than that of 2D cylindrical folds. For this reason, the normalized arc length can probably be used to identify domes and basins in the field, even if these structures are not fully exposed in 3D. [ABSTRACT FROM AUTHOR]

Sills are an integral part of the formation and development of larger plutons and magma reservoirs. Thus sills are essential for both the transport and the storage of magma in the Earth's crust. However, although cooling and solidification are central to magmatism, their effects on sills have been so far poorly studied. Here, the effects of solidification on sill propagation dynamics and morphology are studied by means of analogue laboratory experiments. Hot fluid vegetable oil (magma analogue), that solidifies during its propagation, is injected as a sill in a colder layered gelatine solid (elastic host rock analogue). The injection flux and temperature are maintained constant during an experiment and systematically varied between each experiment, in order to vary and quantify the amount of solidification between each experiments. The oil is injected directly at the interface between the two gelatine layers. When solidification effects are small (high injection temperatures and fluxes), the propagation is continuous and the sill has a regular and smooth surface. Inversely, when solidification effects are important (low injection temperatures and fluxes), sill propagation is discontinuous and occurs by steps of surface-area creation interspersed with periods of momentary arrest. The morphology of these sills displays folds, ropy structures on their surface, and lobes with imprints of the leading fronts that correspond to each step of area creation. These experiments show that for a given, constant injected volume, as solidification effects increase, the area of the sills decreases, their thickness increases, and the number of propagation steps increases. These results have various geological and geophysical implications. The morphology of sills, such as lobate structures (interpretation of 3D seismic studies in sedimentary basin) and ropy flow structures (field observations) can be related to solidification during emplacement. Moreover, a non-continuous morphology as observed in the field does not necessarily involve multiple injections, but could instead reflect a continuous, yet complex morphology induced by solidification effects during emplacement. Also, a discontinuous sill propagation induced by solidification effects should be associated with bursts of seismic activity. Finally, our study shows that once a sill has initiated, the dimensionless flux influences the sill thermal state, and in turn its propagation, and final extent and thickness. In restricting the lateral extent of sills, magma cooling and solidification are likely to impact directly the size of plutons constructed by amalgamated sills. [ABSTRACT FROM AUTHOR]

Analogue models often require that materials with specific physical properties be engineered to satisfy scaling conditions. To achieve this goal we investigate the rheology of aqueous solutions of Natrosol 250 HH, a rheology modifier employed in various industries to thicken viscous solutions. We report the rheological properties as functions of the concentration and temperature and discuss the advantages and limitations of these materials in view of their use in analogue modelling experiments. The solutions are linear visco-elastic for low stresses (or strain-rates), becoming shear-thinning for larger stresses. For the typically slow analogue experiments of tectonics, the solutions can be considered linear visco-elastic with a Maxwell relaxation time much smaller than the characteristic observation time. This simplification is even more appropriate when the solutions are employed at temperatures higher than 20 °C, since the solutions then display a behaviour that is more viscous, less elastic at the same shear-rate, while the Newtonian viscosity reduces and the shear-rate limit between Newtonian and shear-thinning behaviours increases. The Newtonian viscosity is shown to increase non-linearly with concentration and decrease non-linearly with temperature. With concentrations between 0 and 3% and temperature between 20 and 40 °C, the viscosity varied between 10 −1 and 4000 Pa s, while the density remained close to the density of water. Natrosol 250 HH thus offers the possibility to control the viscosity of a solution without significantly affecting the density, thereby facilitating the design and setup of analogue experiments. [ABSTRACT FROM AUTHOR]

TecPIV is a MATLAB-based, open source application aiming to facilitate and accelerate the Particle Image Velocimetry (PIV) analysis of analogue tectonic models. Since tectonic modelling experiments are usually slow, low sensitivity but high resolution digital cameras can be employed to monitor displacements and calculate strain from PIV analysis. The presented work-flow package provides an integrated environment to import raw files, calibrate, enhance, undistort and rectify the views of the model, calculate the incremental displacements between successive images, the spatial derivatives, and the Eulerian cumulative displacements. The calibration procedure employs a combination of linear and polynomial functions allowing correction of both lens and parallax distortions. The cross-correlation uses a parallelized FFT-method to accelerate the processing of large images. The performance is tested using synthetic images designed to evaluate the precision of the produced velocity field for various settings as well as the ability to capture sharp features characteristic of experimental tectonics. [ABSTRACT FROM AUTHOR]

By physical modelling we investigated the buckling and post-buckling behaviour of an elastic layer hosted in a viscous medium, as analogue of the elastic response of crustal rocks. The experiments were performed by embedding thin elastic layers of finite length in a linear viscous medium, and in two successive stages: a first stage of layer-parallel shortening in pure shear, followed by a second stage of post-buckling unfolding with zero velocity at the bounding walls. The experimental results show that the fold wavelength varies inversely with applied piston velocity, following an exponential function for which the higher the piston velocity the higher the number of waves. With cessation of layer-parallel shortening, the buckled layers underwent post-buckling relaxation in response to the elastic strain accumulated during the buckling stage. Such relaxation reduces the number of waves by successive elimination of the lower amplitude folds. The relaxation process gives rise to larger folds of higher amplitude in the centre of the elastic layer, concomitantly with the production of a train of gentle folds in the outer domains by outward motion of the ends of the elastic layer. This means that relaxation is more efficient away from the central domain. Comparison of our experimental results with available analytical solutions reveals significant discrepancies. Finally, we suggest that a similar process of relaxation of the elastic strain, although of lower amplitude, may explain the late stage open folding and the formation of overlying extensional basins observed in past orogens like the Caledonides in western Norway. [ABSTRACT FROM AUTHOR]

• The fault structure is acquired very early in its history. • The fault structure is controlled by the Riedels and the width of diffuse zone. • Off-fault deformation is localized in the relay zones and along the old shear bands. • Fault zone width and OFD ratio decrease with displacement, to reach a stable value. • Explains why uplift is a good marker of off-fault deformation zones. Surface deformation associated with strike-slip faults can be distributed in space, with deformation located either along the primary fault strand or around it and referred to as off-fault deformation (OFD). Fault displacement hazard evaluation require to identify and estimate surface slip rates along active fault strands. We calculate the horizontal and vertical displacement of the analogue models surfaces with optical image correlation and photogrammetry, to investigate the OFD's development with increasing cumulative deformation. The criterion uses the gradient of the horizontal displacement norm perpendicular to the basal fault. Below 0.005 (noise level), there is no deformation, up to 0.03, it is off-fault-deformation, above 0.03, it is on-fault. We confirm previous observations made on analogue models that the surface deformation starts with a broad diffuse deformation, then produces fault strands alternating with relay zones that may be abandoned and reactivated. OFD is located first between Riedels, then between synthetic shears, and finally takes place in the relay zones. We also show that the OFD initially accommodates 100% of the applied slip (no faults), then decreases abruptly during the Riedels stage down to 20 to 30% to finally remain stable for the rest of the experiment. The abandonment and reactivation of the relay zones has the consequence of maintaining the OFD ratio on a stable value. Our experiments show that, like the OFD ratio, the width of the fault zone decreases with cumulative displacement to reach a stable value. Consequently, the OFD is correlated with this fault zone width and its geometric complexities. The ratios of OFD observed in this study are also consistent with measurements of OFD made on seven natural faults that exhibit different cumulative displacements. Hence our models suggest that strike-slip faults will never reach a continuous, linear geometry, and will always maintain a minimum amount of OFD. [ABSTRACT FROM AUTHOR]

The salt layer thickness variability and the timing of salt deposition in relation to rifting (prerift salt, synrift salt, and late synrift/postrift salt) influence salt flow and the resulting growth, geometry, and kinematics of salt bodies under different extensional conditions. We apply analogue modelling to analyze the influence and interplay of each of these parameters through three series of experiments focusing on extensional salt tectonics. The first (Series I) simulates thick-skinned extension of a tabular salt layer in the absence of basement structures and explore the effects of variable salt thicknesses and extensional structural styles. Series II tests the degree of mechanical coupling of sub- and supra-salt faults according to contrasting salt thicknesses over a rotating rift block. Series III explores thin-skinned, gravity-driven deformation implementing models of i) pure-gliding (S 1), ii) pure-spreading (S 2) and iii) a combination of both (S 3). Series I shows that the degree of symmetry and the kinematics of the structures vary due to the influence of the thickness of the salt layer. Thick salt layers tend to nucleate symmetric rifts and associated diapirs, while thin salt layers form rifts that are strongly asymmetric and rotated towards the direction of extension. Series II demonstrates that the number and dimensions of peripheral post-salt grabens, rollers, and reactive diapirs depends on the degree of coupling between sub- and supra-salt, which in turn is controlled by how the salt layer is distributed over active rift structures. Series III shows that deformation is compartmentalized into proximal, transitional, and distal domains which are characterized by extension, translation and contraction, respectively. The style and lateral extent of these domains however vary. In models where deformation is driven exclusively by tilting of the salt layer (S 1) and open-toe salt advance, there is development of linear grabens cored by reactive-passive diapirs throughout most of the model length. This produces a more abrupt transition between the extensional and contractional domains and limited downdip salt inflation. In the pure-spreading model (S 2), where deformation is driven solely by differential loading there is formation of a wide zone of salt inflation at the toe-of-slope and an overall greater number of contractional structures at the downdip end of the model, beyond the slope. In S 3 there is development of a more variable suite of salt and overburden structures as typically observed along salt-bearing passive margins. In this model, deformation is characterized by the development of rollover and normal growth faults in the proximal domain that transition downdip onto upright diapirs associated with bowl-shaped minibasins, halokinetic sequences rotated and megaflaps. Our study improves our understanding of the extensional salt tectonic systems and the controls on different salt and post-salt geometries and kinematics. • Analogical simulations of salt tectonics under extensional conditions. • Influence of salt thickness and the spreading and gliding mechanisms. • Effects of modelling parameters in the resulting salt structural styles. [ABSTRACT FROM AUTHOR]