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We used power-law viscous plasticine as a rock analogue to simulate chocolate tablet boudinage of rocks undergoing dislocation creep. A competent plasticine layer, oriented perpendicular to the main shortening direction, Z, underwent two phases of plane strain in a weaker plasticine matrix, with the principal stretching axis, X , and the axis of no-change, Y , replacing each other from the first to the second phase. In each phase of plane strain, boudinage was controlled by an initial phase of viscous necking followed by extension fracture along the neck domain. Increase in the magnitude of finite strain ( e ) and decrease in layer thickness ( H i ) result in a decrease in the boudin width ( W a ) and an increase in the number of boudins ( N ). Given the viscosity ratio between layer and matrix ( m ) is higher than ca. 5, the number of boudins decreases and the boudin width increases with increasing values of m . An unexpected result of the present study is that in each experiment, the number of boudins was significantly higher during the second phase of plane strain. This difference should be related to additional drag of the matrix plasticine on the stiff layer in the neck domains formed during the first phase of boudinage. The aspect ratio of the second generation of boudins ( W d = W a / H i ) is compatible with aspect ratios of natural boudins and with aspect ratios calculated using analytical solutions. [ABSTRACT FROM AUTHOR]

Sills represent a major mechanism for constructing continental Earth's crust because these intrusions can amalgamate and form magma reservoirs and plutons. As a result, numerous field, laboratory and numerical studies have investigated the conditions that lead to sill emplacement. However, all previous studies have neglected the potential effect magma solidification could have on sill formation. The effects of solidification on the formation of sills are studied and quantified with scaled analogue laboratory experiments. The experiments presented here involved the injection of hot vegetable oil (a magma analogue) which solidified during its propagation as a dyke in a colder and layered solid of gelatine (a host rock analogue). The gelatine solid had two layers of different stiffness, to create a priori favourable conditions to form sills. Several behaviours were observed depending on the injection temperature and the injection rate: no intrusions (extreme solidification effects), dykes stopping at the interface (high solidification effects), sills (moderate solidification effects), and dykes passing through the interface (low solidification effects). All these results can be explained quantitatively as a function of a dimensionless temperature θ , which describes the experimental thermal conditions, and a dimensionless flux ϕ , which describes their dynamical conditions. The experiments reveal that sills can only form within a restricted domain of the ( θ , ϕ ) parameter space. These experiments demonstrate that contrary to isothermal experiments where cooling could not affect sill formation, the presence of an interface that would be a priori mechanically favourable is not a sufficient condition for sill formation; solidification effects restrict sill formation. The results are consistent with field observations and provide a means to explain why some dykes form sills when others do not under seemingly similar geological conditions. [ABSTRACT FROM AUTHOR]

New materials and related apparatuses are welcome to advance analogue modelling techniques. In this contribution, we report on a first attempt to use magnetorheological (MR) fluids as analogue materials for simulating the mechanical behavior of mobile décollement layers that change their mechanical properties during deformation. For this purpose, a specific sandbox was designed to include the possibility of quickly applying and removing a magnetic field below a MR fluid layer, in order to induce an instantaneous change from a frictional to a viscous behavior in the basal décollement material. The simulation of gravitational gliding and sediment progradation above a basal mobile shale layer provided results that compare well with analogue models produced with other experimental techniques, and with natural structures like those developed in the Niger delta region. This pilot study thus encourages further research for optimizing the applicability of MR fluids to the analogue simulation of geological processes. [ABSTRACT FROM AUTHOR]

Abstract: Paraffins have been widely used in analogue modelling of geological processes. Petrolatum and paraffin oil are commonly used to lubricate model boundaries and to simulate weak layers. In this paper, we present rheological tests of petrolatum, paraffin oil and several homogeneous mixtures of the two. The results show that petrolatum and all petrolatum–paraffin oil mixtures are strain, strain rate and temperature dependent under typical experimental strain rates (10−3–10−1 s−1). For the same conditions, pure paraffin oil is a slightly temperature-dependent, linear, Newtonian fluid. All mixtures have yield stress and flow stress (strain softening) values that decrease with decreasing shear rate, and with increasing relative amounts of paraffin oil. The degree of strain rate dependence (shear thinning) also decreases with increasing paraffin oil content. Because these materials have rheologies that can be characterized and controlled, they are suitable for use in a large number of analogue model settings, either as a lubricant or to simulate weak layers. When used as a lubricant, mixtures with higher paraffin oil content should perform better than pure petrolatum. [Copyright &y& Elsevier]

Abstract: A change in contractional polarity occurs when the direction of tectonic transport switches along strike. This switch is conditioned by lateral variations in rheology or inherited asymmetries, such as contrasts in structure or changes in the polarity of subduction zones. The parameters controlling contractional polarity changes are less understood in situations when the strain is transferred at large distances from indenters. Analysing this type of strain transfer is critical for understanding the mechanics of thrusting in fore- or back-arc settings of orogenic areas. Comparison of crustal-scale analogue modelling with the inversion of the Black Sea back-arc and the formation of the New Guinea–New Britain fore-arc suggest that far-field changes in contractional polarity are related to rheological contrasts across inherited normal faults. The initial extension creates rheological weak zones that localize the subsequent far-field contractional deformation along groups of thrusts with opposite vergence along the strike of the system. The largest amount of far-field contractional deformation is recorded in the transfer zone located between the two indenters moving in opposite directions and is particularly high when inverting oblique extensional systems. [Copyright &y& Elsevier]

The purpose of the paper is to show that analogue electronic modelling is an extremely fast technique compared to conventional digital computing, especially when solving large sets of coupled nonlinear differential equations. An array of thirty FitzHugh–Nagumo type electronic oscillators, modelling dynamics of the brain neurons, is considered. The decrease of time consumption by a factor of several thousands is demonstrated. The work delivers a perspective of how to implement convenient analogue models of complex dynamical networks.

Abstract: Three-dimensional thermo-mechanical analogue experiments are employed to test the hypothesis that oceanic subduction along a seaward-concave plate boundary can generate trench-parallel shortening in the forearc near the axis of curvature. The model deformation is analyzed with a Particle Imaging Velocimetry (PIV) system that allows for comparison of forearc deformation along the oblique limbs of the curved plate boundary and near the axis of curvature. Moreover, PIV allows for separation of the trench-parallel and trench-perpendicular components of strain, regardless of trench orientation. The resulting deformation maps show a remarkable symmetry and indicate drag of the forearc above the interplate coupling area towards the axis of curvature. Trench-perpendicular profiles show that along the oblique limbs of the plate boundary, the forearc is submitted to trench-normal shortening and trench-parallel shearing but not trench-parallel shortening or extension. This contrasts with the situation near the axis of symmetry where the forearc is submitted to trench-parallel and trench-perpendicular normal shortening, but is not sheared. The experimental results confirm that trench-normal thrusts observed in the fore-arc of the Central-Andes can be a mechanical consequence of subduction along a seaward-concave plate boundary if the degree of interplate coupling is large. [Copyright &y& Elsevier]

Abstract: Eruptions and volcano internal growth are mostly fed by dykes. The comprehension of the control factors on dyke paths is fundamental for the assessment of areas prone to vent formation and to the general understanding of how volcanoes work. We analyse an understudied magma path system; field data of nine volcanoes show they have a rectilinear rift zone in the central part passing into fan-arranged dykes at the two opposite volcano flanks. The geological, geomorphological and structural characteristics of these volcanoes and their substrate suggest that the formation of these “diverging rifts” is not specifically linked to substrate lithology and mechanical behaviour. The studied volcanoes have elongation <0.88 and V >10km3 (mostly >300km3). Eight volcanoes have the central rift that is normal to the regional tectonic least principal stress (σ3reg) and in one case it is sub-perpendicular. Field data have been combined with scaled analogue modelling, suggesting that if the σ3reg is oblique to the volcano elongation axis, dyke geometry in the edifice axial zone is controlled by elongation and thus by local gravity σ3, but dyke strike becomes perpendicular to σ3reg when dykes intrude the more external areas of the volcano. If a dyke is injected under the volcano flanks with slope inclination >50°, it attains a geometry parallel to the slope. At lower slope inclinations at the edifice terminations, magma paths diverge outwards and crosscut slopes at high angle. Our data are in agreement with the assumption that regional tectonic stresses can affect large volcanoes up to the summit area guiding the development of a rectilinear thoroughgoing rift, both in extensional and transtensional regimes. The diverging pattern takes place due to reorientation of the local stress field guided by topography only when dyke inception localizes laterally respect to the edifice axis. [Copyright &y& Elsevier]

Abstract: Analogue sandbox models have been set up to study the impact of synkinematic deposits on the geometry and evolution of single thrusts and folds according to different sedimentation modes (a slow or rapid sedimentation rate that is constant or changing in space and time) and rheological profiles (thin or thick sedimentary series, with or without a basal décollement level). A first series of experiments documents the influence of synkinematic deposits according to their sedimentation rate and the rheology of the prekinematic materials. A second series investigates the influence of changes in the sedimentation rate through time. A third one considers the influence of changes in the sedimentation rate in space. All these experiments suggest that the geometry and evolution of single compressive growth structures vary according to the sedimentation rate. The number and dip of their frontal thrust segments change with the ratio R between the sedimentation rate at the footwall of the faults and the uplift rate of their hanging wall. The latter is then more or less uplifted depending on the dip of the thrusts. As a result, the overall structure has either a fault-bend fold or a fault-propagation fold geometry. These rules are verified when the ratio R changes in space or through time. In addition, the rheological profile of the models also affects the geometry and evolution of compressive growth structures. Their structural style, as well as the synsedimentary splitting and steepening of the associated thrusts, varies according to the occurrence and strength of the brittle and ductile layers. According to this modelling study, the ratio R and its changes in space and time, along with the rheology of the deformed materials, are key parameters to better understand the geometrical and kinematical complexities of natural growth thrusts and folds and to improve their interpretation. [Copyright &y& Elsevier]

Abstract: We adopt analogue modelling to investigate continental crustal stretching by simulating a Late Miocene–Early Quaternary extensional event occurred in Tunisia and in the Sicily Channel. The analogue model of the continental crust is composed of a lower ductile crust and an upper brittle crust including a low-viscosity layer to simulate the presence of evaporites. In our models, the extension is driven by tectonic stresses and not solely by gravity. All models are run under the same boundary conditions with a moveable lateral edge. Different physical hypotheses are tested to interpret the origin and geometry of this rifting event that affected the SW sector of the Mediterranean basin. The modelling of the continental crustal stretching produces small tectonic troughs with regular spacing that simulates the pattern observed in the Atlasic Rift System (Tunisia) as well as a large rift zone involving bulk crust thinning simulating the features of the Sicily Channel Rift Zone. In accordance with geological and geophysical data, extension in the Atlasic Rift Zone is controlled at depth by the Triassic evaporite layer and by inherited diapiric structures that create rheological “soft points” in the upper crust. The best accordance between models and nature are thus obtained when combining crustal-scale stretching, decoupling level within the upper-crust and pre-structuration (inheritance) of the evaporitic layer. We propose that the NE-trending extensional event occurring since the Pliocene to the early Quaternary in the Tunisian–Pelagian Sea area was driven by a transient eastward migration of the Tunisia–Pelagian–Ionian area (acting as a micro-plate with an undefined southern boundary) due to a far-field pull of the Western-Hellenic/Dinaride subduction. [Copyright &y& Elsevier]

Abstract: So as to investigate the parameters influencing subduction and back-arc extension, we have done three series of laboratory experiments (32 in all) on physical models. Each model consisted of adjacent oceanic and continental plates, floating on an asthenosphere. In experiments of Series A, a wide rigid piston, moving horizontally, controlled the rate of convergence of the oceanic and continental plates, whereas, in Series B or C, a wide or narrow piston produced lateral compression, parallel to the continent–ocean boundary (COB) and perpendicular to the subduction direction. The parameters that we tested were (1) the velocity of plate convergence (Series A), (2) the width of the compressing piston (Series B and C), and (3) the density ratio between oceanic and continental plates (Series B and C). This density ratio was a key factor. For a ratio of 1.4, the amount of extension in the continental plate increased regularly throughout time; for a ratio of 1.3, the extension remained small, until the piston stopped moving laterally; and for a ratio of 1.1, there was little or no extension. The width of the compressing piston had a smaller effect, although a narrow piston provided more space, into which the continental plate could extend. One possible application of our models is to Anatolia. Despite similar geological settings, the areas north of the Hellenic and Cyprus subduction zones differ, in that extension is large in the former and much smaller in the latter. We suggest that one of the main driving forces for Aegean extension may have been a high density ratio between subducting oceanic lithosphere and a Hellenic–Balkanic upper plate. [Copyright &y& Elsevier]

This paper presents the different analogue scenarios of the tectonic evolution of the northern Outer Carpathians (i.e. the Western and northern Eastern Outer Carpathians) which formed as an accretionary wedge in front of the East Alpine–Carpathian–Pannonian (ALCAPA) block during Oligocene–Miocene times. Currently, this fold-and-thrust belt forms an arc which is asymmetrically convex to the north and wider in its eastern part. Palaeomagnetic investigations have suggested that the rocks of the arc underwent counter-clockwise rotation along almost the whole arc, which is difficult to explain as an effect of simple indentation of the triangular indenter. In this case two branches of the arc should be rotated in the opposite directions. The structural evolution of the Western Outer Carpathians is characterized by superposition of two successive tectonic shortening events directed N–S and NE–SW. The results of the presented analogue modelling suggest that two scenarios of the geodynamic evolution of the studied belt could explain the occurrence of such differently oriented shortening events: (1) two phases of differently directed indentation (first to the N, then to the NE) and (2) indenter movement to the NE with simultaneous counter-clockwise rotation. However, the experiment in which the moving indenter is simultaneously rotated produces the most suitable model. The counter-clockwise rotation of the material is only possible in front of both sides of the convex indenter in this model. The results of the analogue modelling also prove that rotation of the ALCAPA block started after formation of the Magura nappe (the innermost nappe of the Western Outer Carpathians). [ABSTRACT FROM PUBLISHER]

Abstract: The combination of scaled analogue experiments, material mechanics, X-ray computed tomography (XRCT) and Digital Volume Correlation techniques (DVC) is a powerful new tool not only to examine the 3 dimensional structure and kinematic evolution of complex deformation structures in scaled analogue experiments, but also to fully quantify their spatial strain distribution and complete strain history. Digital image correlation (DIC) is an important advance in quantitative physical modelling and helps to understand non-linear deformation processes. Optical non-intrusive (DIC) techniques enable the quantification of localised and distributed deformation in analogue experiments based either on images taken through transparent sidewalls (2D DIC) or on surface views (3D DIC). X-ray computed tomography (XRCT) analysis permits the non-destructive visualisation of the internal structure and kinematic evolution of scaled analogue experiments simulating tectonic evolution of complex geological structures. The combination of XRCT sectional image data of analogue experiments with 2D DIC only allows quantification of 2D displacement and strain components in section direction. This completely omits the potential of CT experiments for full 3D strain analysis of complex, non-cylindrical deformation structures. In this study, we apply digital volume correlation (DVC) techniques on XRCT scan data of “solid” analogue experiments to fully quantify the internal displacement and strain in 3 dimensions over time. Our first results indicate that the application of DVC techniques on XRCT volume data can successfully be used to quantify the 3D spatial and temporal strain patterns inside analogue experiments. We demonstrate the potential of combining DVC techniques and XRCT volume imaging for 3D strain analysis of a contractional experiment simulating the development of a non-cylindrical pop-up structure. Furthermore, we discuss various options for optimisation of granular materials, pattern generation, and data acquisition for increased resolution and accuracy of the strain results. Three-dimensional strain analysis of analogue models is of particular interest for geological and seismic interpretations of complex, non-cylindrical geological structures. The volume strain data enable the analysis of the large-scale and small-scale strain history of geological structures. [Copyright &y& Elsevier]

Abstract: The balance between extension and contraction in back-arc basins is very sensitive to a number of parameters related to on-going subduction and collision processes. This leads to complex back-arc geometries, where a lateral transition between crustal blocks with contrasting rheologies is often recorded. One good example is the back-arc region of the Balkanides–Pontides orogens, where lateral variations in rheologies are observed between the Balkanides–Moesian block and the Pontides–Western Black Sea Basin. The latter opened during Cretaceous–Eocene, and has been inverted together with the former starting during late Middle Eocene. The inversion generated contrasting geometries along the orogenic strike, with a narrow zone of high deformation in the Balkanides–Moesia region, wide areas of thrusting with low offsets in the Pontides–Western Black Sea Basin and a transitional zone characterized by highly curved geometries. This overall type of inversion is investigated here by the means of analogue modelling testing the role of inherited crustal geometries during inversion. Our modelling suggests that the contrasting architecture of inverted structures observed in the Balkanides–Pontides domain are the result of pre-existing crustal stretching geometries of various blocks inherited from the Cretaceous–Eocene extension. The stretched and weak back-arc basins can transfer contraction deformation at large distances, explaining structures derived by observational studies. The collisional deformation recorded in the Pontides was transmitted at large distances that are in the range of the contraction structures observed in the centre and northern part of the Western Black Sea. In the light of analogue modelling results we argue that the Western Black Sea was a rheologically weaker domain when compared with the adjacent western onshore at the beginning of the inversion, in contrast with previous results derived from numerical modelling studies that argued for a strong West Black Sea domain at the beginning of inversion. [Copyright &y& Elsevier]

Crustal deformation in front of an indenter is often affected by the indenter's geometry, rheology, and motion path. In this context, the kinematics of the Jaufen- and Passeier faults have been studied by carrying out paleostress analysis in combination with crustal-scale analogue modelling to infer (1) their relationship during indentation of the Adriatic plate and (2) their sensitivity in terms of fault kinematics to the geometry and motion path of Adria. The field study reveals mylonites along the Jaufen fault, which formed under lower greenschist facies conditions and is associated with top-to-the-west/northwest shear with a northern block down component. In addition, a brittle reactivation of the Jaufen shear zone under NNW-SSE to NW-SE compressional and ENE-WSW tensional stress conditions was deduced from paleostress analysis. The inferred shortening direction is consistent with fission track ages portraying Neogene exhumation of the Meran-Mauls basement south of the fault. Along the Passeier fault, deformation was only brittle to semi-ductile and paleostress tensors record that the fault was subjected to E-W extension along its northern segment varying into NW-SE compression and sinistral transpression along its southern segment. In the performed analogue experiments, a rigid, triangular shaped indenter was pushed into a sand pile resulting in the formation of a Passeier-like fault sprouting from the indenter's tip. These kinds of north-trending tip faults formed in all experiments with shortening directions towards the NW, N, or NE. Consequently, we argue that the formation of the Passeier fault strongly corresponds to the outline of the Adriatic indenter and was only little affected by the indenter's motion path due to induced strain partitioning in front of the different indenter segments. The associated fault kinematics along the Passeier fault including both E-W extension and NNW to NW shortening, however, is most consistent with a northward advancing Adriatic indenter. [ABSTRACT FROM AUTHOR]

Abstract: Lithospheric-scale analogue models are used to analyse the parameters controlling the typical evolution of deformation during continental narrow rifting, characterized by early activation of large boundary faults and basin subsidence, followed by localization of tectonic activity in internal faults at the rift axis. Integration of current and previous experiments shows that the evolution of deformation, in particular the amount of extension needed for the abandonment of boundary faults and migration of deformation to in-rift faults, is dependent on at least five boundary conditions: (i) thickness of brittle layers (including syn-rift sediments); (ii) thickness of ductile layers; (iii) extension rate; (iv) width of the weak zone localizing extension; and (v) rift obliquity with respect to the extension direction. An increase in the amount of extension corresponding to the inward migration of faulting (i.e., a longer phase of slip on boundary faults) is observed for (a) an increase in the thickness of both brittle and ductile crustal layers and syn-rift sediment accumulation, (b) a decrease in extension rate and width of the weak zone, and (c) a decrease in rift obliquity. A unified account of these correlations is presented, based on the hypothesis that fault migration occurs when boundary faults can no longer accommodate the imposed bulk extension, leading to time–space variations of internal strain and strain rate (and consequently stress) in the ductile layers which overcome the total resistance of brittle layers to thoroughgoing faulting. [Copyright &y& Elsevier]

In physical modelling using sand, the construction of a sand pack is one of the main experimental difficulties. Existing solutions are generally based either on delicate manipulations (pouring and scraping), or on motorized sedimentation devices, and the qualities of packs thus produced are known to affect experimental outcomes. However, uniformity, planarity, reproducibility and efficiency can be achieved with a simple sedimentation device without motors. A rectangular reservoir pierced with holes rests on a support plate pierced with wider holes. Sand flows when the reservoir is displaced so that its holes match those of the support plate. Sand jets are diffused by planar horizontal sieves, below which the sand sets into the experimental box. Tests on a 250μm median grain size sand show that the density is at its maximum value, reproducible, and uniform. The spatial variations are only of ±0.4% of the mean density. Thickness of the sand layers shows spatial variations around ±2% of the mean thickness, and 6% near the side walls. Very fine grains (90μm median grain size) produce less uniform and less planar packs because of their greater sensitivity to air currents caused by the sedimentation. According to direct shear tests the sand pack has a well defined static friction coefficient decreasing to a lower dynamic value after about 3mm of slip with dilatancy. In contrast, poured sand packs, which are initially less dense, develop only the dynamic friction coefficient (no peak shear stress during slip), without dilatancy. Hole diameters, hole spacing, and the number and openings of the sieves are the parameters controlling the qualities of the sand pack for a given grain size distribution. [ABSTRACT FROM AUTHOR]

Abstract: We applied scaled physical analogue experiments to investigate the early development of salt diapirs induced by differential sedimentary loading in an intra-continental basin realm (e.g. the North German Basin). During the experiments, deformation in a salt-analogue viscous layer was initiated by variations in the thickness of an overlying brittle material and subsequent accumulation of the brittle material further sustained deformation. A 2D optical image correlation system was used to monitor the strain evolution in the salt analogue material. Our models indicate that the formation of salt pillow structures can be achieved by minimum variations in the overburden loading. The increase of differential loading by adding synkinematic layers in the subsided areas causes not only an active piercing of the viscous layer through the brittle overburden but also an additional uplift in the adjacent areas. These elevations, named “secondary structures”, act as origins for a successive generation of diapirs. Consequently, an initial perturbation of the salt–sediment-interface can lead to a lateral propagation temporally shifted diapirs. The linkage between primary and secondary structures is reflected in the synkinematic overburden layers such as overlapping peripheral sinks in the transition zone between two diapirs. These sinks, in turn, are a frequently observable phenomenon around salt structures of the North German basin indicating that “secondary diapirism” is an underestimated process – besides regional tectonic stresses – influencing the evolution of salt structures. [Copyright &y& Elsevier]

The geometric evolution of brittle fault systems in inverted oblique and offset rift systems has been simulated using scaled sandbox analogue models. Dry fine-grained quartz sand was used to represent the brittle upper crust. Extensional faults geometries in the models were governed by the geometry and orientation of a stretching zone at the base of the models. Oblique rift models were characterized by segmented en-echelon border fault systems trending parallel to the rift axis and the underlying zone of basement stretching. Offset rift models promoted highly-segmented border faults as well as offset sub-basins within the rift. In both types of models, intra-rift fault arrays were oriented sub-perpendicular to the extension direction. Inversion of the oblique and offset extensional models was achieved by horizontal shortening. This resulted in partial inversion of the border and intra-rift faults as well as the formation of new reverse faults. The geometries, distribution, orientations and number of these new reverse faults were strongly controlled by the earlier-formed fault extensional architectures. At the margins of the rift zone, shortening was mainly accommodated by partial inversion of the border faults together with the formation of hanging-wall bypass faults and footwall shortcut thrusts. Inversion of the offset rift models produced reactivation of the extensional accommodation zones as soft-linked transfer zones between new thrust faults. The analogue model results have been compared with natural inversion structures in the Atlas Mountains of Morocco and the Ukrainian Donbas fold belt. The analogue modelling results suggest that the High Atlas formed as the result of oblique inversion of an oblique rift system, and the contractional structures in the Ukranian Donbas belt were generated by partial inversion of the earlier-formed Donbas extensional graben via two major newly developed short-cuts that uplifted and exhumed the basin.

Abstract: The surface expression of a lateral polarity change of continental mantle lithosphere subduction has been studied by using lithosphere-scale physical models. Key parameters investigated were: the degree of lateral coupling between adjacent domains of opposing subduction polarity, the width of the zone separating the domains, and the lithosphere geometry and rheology. The model results illustrate an asymmetric lithospheric structure induced by deformation of the downgoing plates, which have been separated by a narrow transition zone. A wide and symmetric orogenic wedge overlying a region of thickened mantle lithosphere and hampered subduction characterizes this transition zone. In addition, interaction between the neighboring subduction domains caused downbending of the upper plates and resulted in the lateral termination of crustal structures and lowering of surface topography. The lateral extent of interaction between the domains strongly depends on the degree of coupling between the domains, the rheology of the mantle lithosphere and the amount of bulk shortening. The modelling results have major implications on the interpretation of seismic and tomographic data from the European Alps in terms of the crust and lithosphere geometries. It appears that an observed lateral change of subduction polarity at mantle depth can explain the variations of wedge build-up between the Western/Central and Eastern Alps. [Copyright &y& Elsevier]

Abstract: The magnetite fabrics measured by anisotropy of magnetic susceptibility (AMS) and by shape preferred orientation (SPO) optical methods are classically used as flow kinematics indicators in lava flows. The development of magnetite fabrics during simple shear strains γ≤20 was performed using a suspension of 1% volume fraction of multidomain magnetite randomly contained in a mixture of silicone and wax. We measured AMS fabric and SPO ellipsoids by calculating a quadratic shape tensor from oriented thin-sections. For γ <8, fabrics obey to the theoretical model of rotation of Jeffery (1922). Fabrics are usable to determine the flow kinematics, including the amount of applied finite strain. For γ >8, fabric elements, foliation and lineation, are stabilised closely parallel to the flow plane and the shear direction, respectively. Two- and three-dimensional numerical simulations using measured aspect ratios of magnetite point out that the large scattering of aspect ratios and the initial orientation distribution of particles are together responsible for a wide-ranging loss of periodicity. The stable AMS and SPO fabrics observed at large strains in experiments are the result of these primary fabric properties combined to collisions between particles and, possibly, their complex three-dimensional shapes. In addition, the constant angular relationship observed at large strains between fabrics and flow components is related to the transient collisions. Consequently, the determination of the lava flow kinematics by using fabric properties measured either by AMS or by SPO analyses should be indubitably associated to a detailed study of the three-dimensional shape of the solid carriers. Regularly shaped populations of low elongated particles will be capable to produce cyclic to oscillating fabrics, while the fabric of elongated particles will be more sensitive to the shape parameters and collisions, ultimately favouring stable fabrics at large strains. [Copyright &y& Elsevier]

Abstract: Intraplate strike-slip zones commonly display intricate 3-D geometries, with rapid changes in structural style along strike and with depth. Strike-slip deformation typically results in complex vertical and horizontal sections that can be difficult to interpret coherently. Physical modelling of strike-slip fault systems is a powerful and graphic tool to help in providing a unified picture of the evolution of strike-slip zones with considerable spatial and temporal detail. A large number of experimental studies have investigated different aspects of strike-slip tectonics using materials such as dry sand, wet clay, or silicone. The choice of analogue material and experimental design exerts a strong control on the structures that form in the model. Here we present a review of different experimental setups used to investigate intraplate strike-slip tectonics, from the classical Riedel experiment to more sophisticated setups using brittle and viscous analogue materials. We review our current understanding from models of distributed shear, transtension, transpression, pull-apart basins formed in releasing stepovers, and popups formed in restraining stepovers. In addition, we present the results of two new experimental series that investigated (1) the effect of crustal weak zones on strike-slip fault-zone segmentation and (2) strike-slip and transpressional reactivation of extensional basins. [Copyright &y& Elsevier]

Abstract: Seismic sections across the NW part of the Polish Basin show that thrust faults developed in the sedimentary units above the Zechstein evaporite layer during basin inversion. These cover thrust faults have formed above the basement footwall. Based on the evolution of the basin, a series of scaled analogue models was carried out to study interaction between a basement fault and cover sediments during basin extension and inversion. During model extension, a set of normal faults originated in the sand cover above the basement fault area. The distribution and geometry of these faults were dependent on the thickness of a ductile layer and pre-extension sand layer, synkinematic deposition, the amount of model extension, as well as on the presence of a ductile layer between the cover and basement. Footwall cover was faulted away from the basement only in cases where a large amount of model extension and hanging-wall subsidence were not balanced by synkinematic deposition. Model inversion reactivated major cover faults located above the basement fault tip as reverse faults, whereas other extensional faults were either rotated or activated only in their upper segments, evolving into sub-horizontal thrusts. New normal or reverse faults originated in the footwall cover in models which contained a very thin pre-extension sand layer above the ductile layer. This was also the case in the highly extended and shortened model in which synkinematic hanging-wall subsidence was not balanced by sand deposition during model extension. Model results show that inversion along the basement fault results in shortening of the cover units and formation of thrust faults. This scenario happens only when the cover units are decoupled from the basement by a ductile layer. Given this, we argue that the thrusts in the sedimentary infill of the Polish Basin, which are decoupled from the basement tectonics by Zechstein evaporites, developed due to the inversion of the basement faults during the Late Cretaceous-Early Tertiary. [Copyright &y& Elsevier]

Abstract: Fluid overpressure at the base of low-permeability strata reduces effective stress, allowing for gravitational sliding of the overlying cover. The force driving sliding is the slope-parallel component of the weight of the cover, whereas the resisting forces are the friction at the base of the cover and the buttressing resistance to shortening, which can be critically reduced by incision at the base of the slope. We developed an analytical model and undertook a series of analogue experiments to better understand the evolution of a sedimentary cover sliding above a low-permeability layer subjected to fluid overpressure. Where a downslope buttress was present, the sliding sheet length decreased with increasing pore-fluid pressure. In the absence of such buttress, the slide''s length increased exponentially with increasing pore-fluid pressure. Another important difference dealt with geometry and kinematics. Buttressed slides consisted of one large slope-parallel mass rigidly translated and bounded by downslope thrusts and upslope normal faults. With increasing pore-fluid pressure, the contractional structures propagated upslope. By contrast, non-buttressed slides showed intense strain: deformation started with normal faults forming near the incision, then propagating upslope throughout the slide''s evolution. [Copyright &y& Elsevier]

Abstract: The cause of upper-crustal segmentation into rhomb-shaped, shear zone-bound domains associated with contractional sedimentary basins in hot, wide orogens is not well understood. Here we use scaled multilayered analogue experiments to investigate the role of an orogen-parallel crustal-strength gradient on the formation of such structures. We show that the aspect ratio and size of domains, the sinuous character and abundance of transpressional shear zones vary with the integrated mechanical strength of crust. Upper-crustal deformation patterns and the degree of strain localization in the experiments are controlled by the ratio between the brittle and ductile strength in the model crust as well as gradients in tectonic and buoyancy forces. The experimental results match the first-order kinematic and structural characteristics of the southern Central Andes and provide insight on the dynamics of underlying deformation patterns in hot, wide orogens. [Copyright &y& Elsevier]

Abstract: On the one hand, it is not yet understood why transform faults commonly form orthogonal to rifts, because this cannot be explained by simple theories of fracture mechanics. On the other hand, rock experimental data have shown that a very strong layer (elastic core) should exist between brittle and ductile layers of the lithosphere. Therefore, we investigated the influence of an elastic core on the generation of a transform fault/rift orthogonal configuration. We used fully gravitational scaled physical experiments of brittle-viscous models, and the results indicate that the elastic core and localized heat flow can be responsible for the observed common pattern of transforms orthogonal to rifts. [Copyright &y& Elsevier]

Abstract: Deformation of rocks produces structures at all scales that are in many cases periodic (folding or boudinage), with variable amplitude and wavelength. Here we focus on boudinage, a process of primordial importance for tectonics. In the present study, we carried out measurements of natural boudins and experimentally tested the effects of two variables on boudinage: layer thickness and compression rate. The models were made of a competent layer (mostly brittle, as in nature) of either elastic (soft paper) or viscoelastoplastic (clay) material embedded in a ductile matrix of linear viscous silicone putty. The competent layer lied with its greatest surface normal to the principal shortening axis and greatest length parallel to the principal stretching axis. The model was then subjected to pure shear at constant piston velocity and variable competent layer thickness (Model 1), or at different piston velocity and constant layer thickness (Model 2). The results of Model 1 show an exponential dependence of boudin width on competent layer thickness, in disagreement with data from the studied natural occurrence. This indicates that variables other than competent layer thickness are hidden in the linear relationship obtained for the natural boudinage. The results of Model 2 show that the higher the velocity the smaller the boudin width, following a power-law with exponent very similar to that of analytical predictions. The studied natural boudinage occasionally occurs in two orthogonal directions. This chocolate tablet boudinage can be the result of two successive stages of deformation: buckling followed by stretching of competent sandstone layers, or buckling followed by rotation of reverse limbs into the extensional field of simple shear. [Copyright &y& Elsevier]

Abstract: Compressional inversion of former extensional sedimentary basins is a particularly common phenomenon, given that passive margins share the same destiny of being subsequently incorporated into fold-and-thrust belts. During basin inversion pre-existing faults may concentrate stress and localise future thrust ramps, or may be reactivated recording reversal of movement from extension to reverse. This process has attracted attention largely because of its economic implications related to petroleum and ore deposit prospects, and seismic hazard assessment. Since more than two decades, analogue modelling has been used to indentify and/or test a number of parameters relevant for the inversion process. This paper aims therefore to offer a reasoned review of the analogue modelling work done on the subject, coupling such previous results with those of a new experimental series inspired to the Northern Apennines stratigraphy and evolution. Past models have explored the role of main factors governing fault reactivation susceptibility, such as fluid pressure, fault weakness, fault steepness, angle of shortening, and sediment loading. The new models have addressed the role played by geometry and strength of a basal ductile layer during inversion. Two sets of sand–silicone models were first extended orthogonally at different velocities to produce dissimilar pre-inversion internal geometries, and then shortened coaxially at different velocities to vary the brittle–ductile coupling. The experimental results confirm the selectiveness of fault inversion, and reveal strong similarities with structural styles of tectonic inversion reported from the Northern Apennines and various areas worldwide. The preferred geological model inferred from this modelling involves contribution of (1) syn-inversion rotation of pre-existing faults to shallower dip, and (2) rotation of principal stress axes, which could explain the invariable reactivation of only one of the two oppositely-dipping graben bounding faults. These factors would thus interact with the other well-established parameters controlling the fault reactivation process. [Copyright &y& Elsevier]

Abstract: The principal concern of this paper is with the accuracy and validity of experimental sand box models of geological structures – and of thrust faults in particular – as influenced by side wall friction. Side wall friction can produce undesirable artifacts, in particular in experiments aimed at reproducing plane deformations. The importance of such artifacts is revealed in the present study by the results of experiments that are affected in opposite ways by side wall friction. Two distinct experimental configurations have been chosen for this purpose for a set-up that comprises a body of sand placed between vertical side walls on a basal plate against a back wall and a front wall, all made of glass. In configuration M the bottom plate and attached front wall move relative to the side walls towards a fixed back wall, while in configuration F the back wall moves relative to the side walls and attached bottom plate towards a fixed front wall (‘fixed’ = ‘fixed relative to the side walls’). In both configurations, thrust traces on the top surface of the sand become curved near the side walls because of wall friction. The outcome of altogether 31 experiments in the two configurations shows that the magnitude of the experimental bias depends on the ratio SL /SB of the area of contact of the sand body with the side walls (SL ) to its area of contact with the basal surface (SB ). The bias is the difference in thrust locations in the two configurations. It is measured on photographs taken through the side walls during the experiments. Away from the side walls, the bias is estimated by the thrust curvatures observed in top views. For SL /SB  ≤ 0.1, i.e. very little contact with side walls, there is no identifiable bias. Furthermore, a 2D theoretical solution reproduces well the results. For SL /SB increasing to 0.25, curvatures increase, yielding rather different cross-sections at the side walls, but still similar cross-sections in the central part of the sand body. For SL /SB  ≥ 0.3, thrust planes occupy different regions of the box for each configuration, although their curvature is decreasing. At SL /SB  = 0.9, curvatures have almost disappeared but the location of the thrusts and the length of the décollement depend strongly on the experimental configuration. Departures from the 2D theoretical solution are clearly stronger with configuration M than F. Another important conclusion is that the absence of curvature of the thrust traces is not a proof of the 2D character of the experiment. Mechanical equilibrium of the total forces recorded at both front and back walls during shortening shows that the thrust locations are controlled by the difference between basal and side wall shear forces. [Copyright &y& Elsevier]

Abstract: Fundamental aspects of mountain building such as the dominant deformation mechanism, strain localisation, and topography development in response to the presence of decoupling zones, either along boundaries of weak and strong lithospheres or within weak domains of continental lithosphere, are studied by analogue lithospheric models. Scenarios are investigated where a weak part of the lithosphere is bordered on either side by stronger lithospheres. Geometries of the weak zone vary from box- to wedge- to graben shaped representing lateral strength variations as a consequence of earlier deformation phases. The rheologic and geometric configuration at the onset of shortening is therefore considered as representative for continental collision and intra-plate settings. The modelling results show that the geometric and topographic evolution of mountain belts through time is sensitive to the presence of decoupling horizons along plate or tectonic boundaries. This is because the decoupled boundaries localise strain, control the dominant deformation mechanism and hence the locus of uplift and subsidence. Accordingly decoupled boundaries favour deformation by thrusting and particularly, in the case of an initially wedge-shaped weak zone, the development of mountain belts with overall triangular shape. In contrast, lithospheres with coupled boundaries dominantly deform by buckling irrespective of the initial geometry of the weak zone. As this deformation mechanism affects all model domains it produces a distinguishable pattern of uplift and subsidence controlled by buckling. Strong decoupling at mid-crustal levels within the weak zone favours vertical strain partitioning and the formation of plateau shaped mountains. The modelling results suggest that natural collisional systems should respond to an increase in plate coupling through time by widespread uplift of the mountain belt including its foreland basins, which were formerly created under decoupled conditions. Due to the absence of decoupling zones, like plate contacts, weak domains in intra-plate settings are predicted to deform dominantly by buckling. [Copyright &y& Elsevier]

Abstract: In petroleum industry, the difference between pore pressure (Pp ) and minimum horizontal stress Sh (termed the seal or retention capacity) is of major consideration because it is often assumed to represent how close a system is to hydraulic failure and thus the maximum hydrocarbon column height that can be maintained. While Sh and Pp are often considered to be independent parameters, several studies in the last decade have demonstrated that Sh and Pp are in fact coupled. However, the nature of this coupling relationship remains poorly understood. In this paper, we explore the influences of the spatial pore pressure distribution on Sh /Pp coupling and then on failure pressure predictions and trap integrity evaluation. With analytical models, we predict the fluid pressure sustainable within a reservoir before failure of its overpressured shale cover. We verify our analytical predictions with experiments involving analogue materials and fluids. We show that hydraulic fracturing and seal breach occur for fluid pressure greater than it would be expected from conventional retention capacity. This can be explained by the impact of the fluid overpressure field in the overburden and the pressure diffusion around the reservoir on the principal stresses. We calculate that supralithostatic pressure could locally be reached in overpressured covers. We also define the retention capacity of a cover (RC) surrounding a fluid source or reservoir as the difference between the failure pressure and the fluid overpressure prevailing in shale at the same depth. In response to a localized fluid pressure rise, we show that the retention capacity does not only depend on the pore fluid overpressure of the overburden but also on the tensile strength of the cover, its Poisson’s ratio, and the depth and width of the fluid source. [Copyright &y& Elsevier]

Abstract: The far-field strain rate is a crucial parameter that controls the transition between brittle and ductile deformation. We have used analogue experiments to study the strain rate influence on the development of brittle fractures in a ductile composite material. Plasticine multilayer models were deformed under coaxial boundary conditions at three different strain rates to analyse the transition from non-localised deformation to the development of a brittle fracture network that accommodates part of the deformation. The results show that tension cracks and voids are the first macroscopic structures that nucleate after an early stage of ductile deformation. Coalescence and collapse of these structures lead to the development of brittle shear fractures. The evolution of fracture orientations, lengths and displacements was systematically analysed. The ratio of the accumulated fracture displacement vs. fracture length (d max /L) depends not only on the total deformation, but also on the strain rate at which the system is deformed. The accumulated displacement with respect to fracture length increases with strain rate. [Copyright &y& Elsevier]

Abstract: In this study we test whether pre-existing basement faults control deformation in the Prebetic zones of the Betic Cordillera. We focus on an area in the northern part of the Prebetics which is dominated by a transpressional fault system consisting of a NNE–SSW striking fault zone and E–W striking small-scale thrusts. Field observations show that both sets of faults formed in a regional NW–SE to NNW–SSE directed compressional stress field during the Miocene. We propose that the faults formed on top of a basement containing basement ramps overlain by a décollement of Late Triassic evaporites and clays. These basement ramps have either an E–W or NNE–SSW orientation and result from extensional faulting prior to the compressional phase. The style of faulting in the overlying sediments is controlled by the variation in orientation of the pre-existing basement ramps in a uniform stress field. Where the ramps have an E–W orientation small-scale thrusting and folding occurs, where the ramps are oriented NNE–SSW the fault zone acts as an oblique-slip fault. The model was tested by analogue tectonic experiments incorporating a ductile layer, representing the décollement, overlain by a brittle layer representing the overlying sediments. The pre-existing basement ramps were represented by rigid material. The close similarity between the structures produced during the analogue experiments and those observed in the field suggests that pre-existing basement faults exert a major role on deformation of the Prebetics. [Copyright &y& Elsevier]

Abstract: Onshore slides are driven by gravitational forces that are either related to a basal or a surface slope. Resisting these driving forces are the friction at the base of the slide, and the strength to compressional failure at the downslope edge of the slide. Two distinct processes can reduce these resisting forces and thereby promote slides. On one hand, fluid overpressure at the base of low-permeability layers decreases the effective stress, shifting the Mohr circle closer to the failure envelope. On the other hand, river incision removes the downslope buttresses. We undertook analogue experiments to investigate the combined influence of both processes on promoting landsliding. We applied air pressure at the base of horizontal or tilted models made of high and low-permeability layers to induce basal overpressure, combined with local incision similar to river incision in nature. We also tested the differences in deformation as a function of whether the incision was continuous throughout the models’ evolution. No deformation occurred in the regions not subjected to overpressure. In models subjected to continuous incision, normal faults formed first along the valley flanks, then propagated upslope retrogressively. Where incision was not continuous through time, a downslope buttress progressively formed with the sliding mass comprised of an extensional domain upslope, a long, translated but non-deformed slab at mid-slope, and a shortened domain downslope. In our models, the size of the deformed area increased with incision depth and/or increasing basal slope. These results show that river incision, combined with fluid overpressure is a potential landslide-triggering factor, as suggested by field data gathered at the Waitawhiti landslide complex, North Island, New Zealand. [Copyright &y& Elsevier]

Abstract: The monitoring of materials elastic parameters during geological analogue experiments would highly improve the interpretation of involved processes. It would more particularly help the implementation of joint analytical or numerical analyses. We propose laser-Doppler vibrometry as a probing tool for the systematic characterisation of analogue models. We therefore develop an experimental set-up to record small-scale seismic lines at the surface of an unconsolidated granular medium in laboratory conditions. Pressure-wave first arrival times and surface-wave dispersion are then inverted for a one-dimensional Pressure- and Shear-wave velocity structure. Inferred profiles appear to match previously thoroughly estimated properties of the probed medium, thus validating our approach. [ABSTRACT FROM AUTHOR]

Fold and thrust belts developed in deepwater environments have generally been subjected to greater net sedimentation and different sedimentation patterns compared to subaerial or shallow marine environments. In this study, the effects of syn-kinematic sedimentation on fold and thrust belt evolution have been evaluated by varying the rates and patterns of syn-contractional sedimentation in 2D analogue models of simple thrust wedges. All models produced critically-tapered Coulomb wedges with topographic slopes of 7-10°. Increased rates of syn-kinematic sedimentation caused greater thrust spacings, increased the total wedge height and wedge length, and generally decreased the number of major thrust structures. Progradational sedimentation caused conspicuous out-of-sequence movement in thrusts at the rear of the model thrust wedge and frontal thrusts became buried and inactive. Aggradational sedimentation caused the rapid and sequential burial of thrusts, rendering them inactive and forcing the deformation to the front of the wedge. The geometries of the front of the Nankai accretionary complex, offshore Japan, and the offshore Niger Delta, Gulf of Guinea, compare well with models in this study and demonstrate that the addition of sedimentation during shortening of fold and thrust belts may dramatically affect the style of deformation as well as the timings and amounts of fault displacements in such systems. [ABSTRACT FROM AUTHOR]

The integration of surface, subsurface and paleomagnetic data allows the 3D reconstruction of the geometry of the northern border of the Cameros basin for its extensional, pre-inverted stage. In the model proposed, the Cameros basin represents a lens-shaped, large-scale growth syncline over a major basement fault with maximum thickness of deposits of about 8000 m in its depocenter, gradually diminishing toward the north and south. A series of analogue models validate this geometry, pointing to the role of interlayered detachment levels on the development of syncline-basin geometries. [ABSTRACT FROM AUTHOR]