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The study aims to understand geometry and evolution of accommodation zones along restraining and releasing bends in strike slip systems which is often complex and characterized by sharp change in polarity, resulting in a difficult characterization of traps location. A batch of sandbox modelling experiments was performed using a layered scaled sand material. The setup parameters of the wooden baseplates used in the first two experiments (SS1 and SS2) are basement offset of 6.5 cm/3 cm, 90°/90° stepovers in releasing and restraining bends of total 96 cm length and 25 cm width. The third experiment (SS3) was performed with basement offset of 6.5 cm/3 cm, 156°/126° stepovers in releasing and restraining bends of total 96 cm length and 25 cm width. The experiments were performed with special attention to the role of syn-kinematic sedimentation and the pre-existing structure of the basement. A sequence of pop-up and pull-apart structures was produced. Along the main structures, complex fault trend was compatible with a sinistral riedel distribution and a counterclockwise rotation of pre-existent elements. Change of fault's polarity at depth and dip direction flipping of high angle faults in accommodation zones were clearly observed. Even more, progressive propagation of the pop-up structures produces a spectacular inversion of adjacent extensional structures. Comparing results of the model with the Seagap fault zone, it is possible to put in evidence similar fault orientation and distribution. Internal structures developed were strongly influenced by syn-kinematic sedimentation as observed by characteristic helicoidal shape of growth faults. The analogue models clearly describe how boundary faults of the pull-apart basin along a paired bend, initially characterized by a normal kinematic can be subsequently inverted in the later stages, which can potentially have a strong impact on the petroleum system.

The role of pre-existing structures during the building of the Zagros fold-and-thrust belt is a matter of debate. The orientation of the pre-existing basement faults varies in different parts of the Zagros belt. Major directions of the basement faulting in the Arabian plate and Zagros are N–S, NE–SW, NW–SE and E–W. Reactivation of these inherited pre-existing faults occurred at the Cretaceous. In this study, analogue modelling was constructed to test the effect of various orientations of the pre-existing basement faults on the deformation style in the northern boundary of Dezful Embayment. The model consisted of sand layers that partly overlay a viscous layer of silicon above wooden plates. Model results explain the development of deflection in the deformation front and the incompatibility location of faults in the basement and surface. The presence of the basal decollement horizon between the basement and overlying sedimentary sequences separated faulting geometry in the basement from sedimentary cover. Model results suggest that the flexure of the Zagros Mountain Front Faults (ZMFF) in the north part of Dezful Embayment is resulted by reactivation of pre-existing Bala Rud fault (BRF) which is an oblique ramp. The close similarity between the major structures produced during the analogue experiments and those observed in nature, suggests that BRF as a pre-existing basement fault has a major role in the deformation of the structures in the study area.

Mountain building (orogeny) has a strong causal relationship with compression induced by plate drifting. It is generally accepted that orogenic belts are distributed perpendicular to the direction of plate movement. Extensive Late Mesozoic intra-continental deformations occurred around the North China Craton (NCC) in three different directions (i.e. EW-, WNW- and NNE-striking), which were previously attributed to three different geodynamic processes; the closure of the Mongolia-Okhotsk Ocean, convergence of the NCC and the South China block, and subduction of the paleo-Pacific plate. Remarkably, all these tectonic events were synchronised. Analogue modelling experiments were used to constrain far-field stress and basement shapes, to reconsider tectonic evolution around the NCC during this period. Results showed that all of these tectonic deformations could be plausibly interpreted as a unitary geodynamic regime governed by the westward indentation of the wedge-shaped NCC, driven by subduction of the paleo-Pacific plate. Mountain chains were formed along the edges of the wedge, indicating that the distribution of orogenic belts is not necessarily correlated with the direction of plate movement.

Pre-existing faults in the mechanical basement are believed to play an important role in controlling deformation of the thin-skinned Jura Mountains fold-and-thrust belt, which constitutes the northernmost extension of the European Alps. We use brittle-viscous analogue models to investigate the influence of frontal and oblique basement steps on the subsequent evolution of structures during thin-skinned shortening. Vertical offset between two rigid baseplates (simulating the mechanical basement) causes the formation of reverse faults and grabens in the overlying brittle layers that are not reactivated during subsequent thin-skinned shortening. However, baseplate steps localise deformation, causing a temporary frontward propagation of deformation in an early stage and inhibiting propagation afterwards. Downward baseplate steps induce very strong deformation localisation and foster the formation of fault-bend folds. Models featuring upward steps develop step-controlled pop-up structures with imbricated fronts and viscous ramps that shorten dynamically with progressive contraction. We find that deformation localisation increases both with higher step-throws and lower obliquity (α) of the strike of the step (e.g. frontal step α = 0°). With increasing step-throws, α = 30° and α = 45° oblique upward-steps lead to a characteristic imbrication of the brittle cover with laterally confined thrust-slices and step-parallel oblique-thrusts, which rotate up to 15° about a vertical axis over time. Step-controlled backthrusts preceding the formation of thrust-slices do not show notable rotation and hence constitute excellent indicators for the orientation of oblique upward-steps. The topographic patterns of oblique-step models resemble individual thin-skinned structures of the Internal Jura (i.e. Pontarlier and Vuache fault zones, the nappe system SE of Oyonnax and the Chasseral anticline), strongly suggesting that pre-existing NNE-SSW and NW-SE striking oblique upward-steps in the basement controlled deformation in the overlying cover. Our model results may be applied to other thin-skinned fold-and-thrust belts worldwide that formed above pre-existing basement structures.

Based on 3D seismic data, the evolution mechanism and characteristics of faults were investigated to reveal the structural origin and its control on differential hydrocarbon accumulation through comprehensive analyses, including structure style analysis, fault activity analysis, analogue modelling and comparison among the wells. The complex fault system with differently trending faults resulted from strike-slip and rifting in Paleogene was partly activated, developed successively and stretched obliquely by the near-NS extensional stress field in Neogene. In the area little affected by pre-existing faults, new faults nearly perpendicular to the extension direction developed. The structural development in the study area was not caused by transpressional strike slip. Under the oblique extension effect of pre-existing faults, if the angle between the strike of pre-existing fault and the extensional direction is different, the strike-slip and extensional stresses are different in ratio. The larger the angle between the two is, the stronger the extensional component, the poorer the sealing ability of the fault, and the stronger the oil and gas migration capacity will be. Conversely, the smaller the angle between the two is, the stronger the strike-slip component, the better the sealing ability of the fault, and the poorer the oil and gas migration capacity will be. The accumulation condition analysis results considering the fault trend are in good agreement with the oil and gas shows in wells drilled in this area. Key words: oblique extension, pre-existing faults, strike-slip component, extension component, Paleogene, Neogene, Bohai Sea

During large strike-slip earthquakes, the displacement at the ground surface, only partially measured, is often under-estimated in comparison with the amount of slip inferred at depth. The resulting concept of shallow slip deficit is challenged by the precise measurements of surface deformation of on- and off-fault deformation by space imaging techniques, showing that a significant amount of deformation might be accommodated through distributed damage in a zone several hundred meters to kilometers wide around the fault. In this study, analogue modeling is used to quantify the distribution of on/off-fault surface deformation along strike-slip faults over the long term and to understand how it relates to the deep structure of the fault.To do so, we used a 1.5 m x 1.34 m PVC box, and studied the deformation of a homogeneous sand pack deposited above a straight basal fault, with sand thicknesses varying from 2 to 8 cm. During strike-slip fault experiments, the first structures to appear are the Riedel shears (R-shears) followed by the synthetic shears (S-shears). These structures eventually coalesce to form an anastomosed fault zone, made of a succession of segments separated by geometrical complexities of variable size. Optical imagery is used, at every stage of the strike-slip fault formation, to (1) describe the 3D surface displacement and (2) precisely quantify on/off-fault deformation. At the initiation of the fault before the formation of the Riedels, a zone of diffuse deformation is highlighted by a positive divergence of the displacement. This diffuse zone is also characterized by a vertical deformation that forms a bulge.When the displacement Ux parallel to the basal fault has a gradient dUx/dy >= 0.1, we consider that it is "on-fault" deformation, and it is "off-fault, when that gradient is between 0.02 and 0.1.At the Riedel shear stage, we find 40% of off-fault deformation over a unique Riedel fault and about 60% if deformation is distributed over two Riedels.Once the strike-slip fault is formed, the ratio drops between 0 to 5 % of off-fault deformation over a fault segment, but the ratio increases to 20% along geometrical complexities.Moreover, we also show that off-fault deformation around the early Riedel structures partly control the long-lived segmentation and morphology of the strike-slip fault.Experimental results are then compared to observations and measurements of near-field and far-field deformation obtained along the 2013 Mw 7.7 Balochistan earthquake by Vallage et al. (2015) and Gold et al. (2015). Azimuthal displacements measured in a relay zone (Vallage et al. 2015) are consistent with those observed along our experimental relay zones. Although our experiments were only run with sand, we found a similar distribution of the deformation at the surface. These observations suggest that the distribution of the surface deformation of strike-slip fault earthquakes is inherent to the fault structure, possibly inherited from the Riedel shear stage, and not induced by earthquakes dynamics.

Large-scale strike-slip faults are associated with significant strain partitioning in releasing/restraining bends and often display map-view curvatures ending in horse-tail geometries. Such faults are commonly associated with indentation tectonics, where shortening in front of indenters is transferred laterally to transpression, strike-slip and the formation of transtensional/extensional basins. We investigate how these structurally distinct domains are kinematically linked by the means of a crustal-scale analogue modelling approach where a deformable crust is moved against a stable and rigid indenter. The modelling demonstrates that the geometry of the indenter is the major controlling parameter driving strain partitioning and deformation transfer from thrusting and transpression to strike-slip and transtension, whereas the rotation of the mobile plate controls the opening of triangular shaped transtensional basins. Flow of the ductile crust leads to the distribution of deformation over a wider area, facilitating strike-slip splaying into transtension/extension behind the indenter. Our results show a very good correlation with the Moesian indentation in the Carpatho-Balkanides system of South-Eastern Europe, where strain is partitioned around the dextral Cerna and Timok strike-slip faults and transferred to thrusting in the Balkanides part of the Moesian indenter and to transtension/extension in the neighbouring South Carpathians.

Anisotropy of magnetic susceptibility can provide insights into strain distribution in models simulating fold-and-thrust belts. Models with layers of sand and magnetite mixture shortened above adjacent decollements with high and low friction, are used to study the effect of decollement friction on the magnetic fabric. Above high-friction decollement, an imbricate stack produced a ‘tectonic’ fabric with magnetic foliation parallel to thrusts. In contrast, above the low-friction decollement deformation propagated farther into the foreland, and deformation intensity is gradual from the foreland to the hinterland by defining a transition zone in between. In this zone, magnetic lineation rotated parallel to the deformation front, whereas in the hinterland the principal axes do not show a preferred orientation due to different deformation mechanisms between “thrust-affected” and “penetrative-strain affected” area. Above both decollement types, the principal axes of susceptibility developed tighter clustering with depth. Along the boundary between the two decollements, a deflection zone formed where rotation of surface markers and magnetic fabric reflect the transition between structures formed above the different decollements. Through quantifying magnetic fabric, this study reemphasises the clear link between decollement friction, strain distribution and magnitude in fold-and-thrust belts.

[Abstract Pull‐apart basins are structural features linked to the interactions between strike‐slip and extensional tectonics. Their morphology and structural evolution are determined by factors such as extension rate, the basin length/width ratio, and changes in extension direction. In this work, we investigate the effect of a change in the plate motion direction on a pull‐apart basin's structure, using analogue modelling experiments with a two‐layer ductile‐brittle configuration to simulate continental crust rheology. We initially impose orthogonal extension on an interconnected rift and strike‐slip system to drive pull‐apart development. Subsequently, we rotate the relative motion vector, imposing transtensional deformation and continuing with this new relative motion vector to the end of the experiment. To compare with natural examples, we analyse the model using seismic interpretation software, generating 3D fault structure and sedimentary thickness interpretations. Results show that the change in the direction of plate motion produces map‐view sigmoidal oblique slip faults that become normal‐slip when deformation adjusts to the new plate motion vector. Furthermore, sediment distribution is strongly influenced by the relative plate rotation, changing the locus of deposition inside the basin at each model stage. Finally, we compare our observations to seismic reflection images, sedimentary package thicknesses and fault interpretations from the Northern Gulf of California and find good agreement between model and nature. Similar fault arrays occur in the Bohai Basin in northern China, which suggests a rotational component in its evolution. More broadly, such similar structures could indicate a role for oblique extension and fault rotation in any pull‐apart basin., The evolution of a pull‐apart basin can be considerably influenced by a change of plate motion that introduces transtension in the basin. Identifying structures of this type of pull‐apart basin evolution are asymmetric triangular basins and wide principal displacement zones with oblique‐normal faults. We identify a number of these basins across the world and model their evolution through physical analogue models. ]

Plutons in crustal shear zones may exploit inherited structures, interfere with strain localizing or be deformed passively. To constrain the relative timing of such tectono-magmatic relationships in natural settings is not always straight-forward. We here present sandbox-type analogue model experiments simulating magma emplacement into simple and transtensional crustal shear zones to test the diagnostic potential of pluton shape with respect to timing and setting. Observations based on surface deformation and intrusion shape exemplify the interplay between evolving and inherited tectonic structures and magma uprising. We observe markedly asymmetric intrusions in association with dikes reflecting the regional stresses, fault pattern and finite strain field. At the same time, the presence of an intrusion modifies the tectonic evolution, but only transiently, resulting in short-lived faults, reactivation and inversion. Diagnostic attributes include the pluton's aspect ratio, its orientation and amplitude as well as dike association. Accordingly, syn-tectonic intrusions show the highest pluton amplitudes, but only intermediate elongation compared to other scenarios. They are oriented parallel to Riedel shears in simple shear, respectively to the compression direction in transtension. Post-tectonic intrusions are least elongated, have medium amplitudes and exploit Riedel shears. Pre-tectonic intrusions are characterized by lowest amplitudes but the highest aspect ratios and are parallel to the finite elongation direction. Intrusions in transtensional shear zones are generally of less elongate than those in simple shear zones. Experimental results are tested against observations from natural examples validating the diagnostic potential of pluton shape for the timing and the tectonic setting of the emplacement.

During the last decades, central Italy has been struck by severe seismic sequences. Some authors have suggested that inherited thrust faults may have played an important role in controlling the recent-to active extensional deformation in the area, particularly during the 2016 Central Italy seismic sequence. To explore this hypothesis, we performed an analogue modelling study that addressed the structural evolution of a composite, heterogeneous multilayer affected by polyphase deformation. The overall three-dimensional geometry resulting from the modelling consists of a flat-ramp extensional fault system, with flats being localised along the weaker stratigraphic units, and ramps locally reactivating the pre-existing thrust ramps at depth. At shallower levels, deformation can localize at newly-formed short-cut faults depending on the geometry of deeper structures. The experimental results provide further support to the hypothesis of a link between Quaternary normal faults and inherited thrust faults during the 2016 seismic sequences. Specifically, analogue modelling results suggest that the Quaternary fault pattern may be controlled by the interaction between the active stress field and the pre-existing compressional structures. This scenario bears significant implications regarding the empirical relationship between the geometry of surface faulting and the magnitude of earthquakes, with considerable repercussions on seismic risk assessment.

Fault damage zones present a renewal of interest to better understand stress perturbations around faults, earthquake’s ground motions and fluid flow in the upper crust. Although numerous studies provide significant amounts of data from a broad variety of rocks, the processes controlling fault damage development are not clearly understood and scaling properties in carbonate rocks remain poorly studied. D-T (displacement - DZ thickness) data compilations show strong scattering and are acquired using different methods and at different places along the faults (including tip, wall, link, or inner and outer damages), therefore rendering difficult a proper definition of the scaling relationship. First, we analyse fault/fracture systems at the outcrop and map scale and define displacement - thickness (D-T) scaling of fault damage zones using scanlines, in carbonate rocks in France and Spain. We determine fault displacement and damage zone thickness perpendicular to fault planes and far from fault tips for 12 selected faults in four study sites. The data show a logarithmic decrease of fracture frequency from the fault cores. This decrease is characterized by local frequency peaks corresponding to variably-linked secondary fault segments and abandoned tips within the fault damage zone. D-T data comprised between 0 and 100 m of net fault displacement show a nearly linear scaling with very little scattering. Including two additional data for D > 100 m, the best fit corresponds better to a power law. The linear scaling is explained by well-known processes of fault growth such as stress perturbations around faults and fault segment linkage. The non-linear trend shown by the largest faults suggests that at this scale the faults become restricted at their lower tips by the base of the brittle crust. Secondly, we analyse fault damage growth using analogue modelling of normal faults in a sand box. The model is composed by a 5 cm thick layer of dry sand deposited above a 2 cm thick ductile “kinetic sand” (sand and silicone) layer. The experiment is analysed in cross-section using image correlation allowing to calculate the velocity field and strain tensor over the fault zones including their damage pattern. Fault damage thickness obtained using the strain field appears to grow linearly with respect to shear displacement when the fault is contained into the dry sand layer. When the fault lower tip reaches the kinetic sand, fault damage begins to growth non-linearly with shear displacement, revealing that the brittle layer thickness is the main parameter governing the non-linear scaling.

Using analogue techniques, we attempted to model the complex tectonic deformation pattern observed along the North-Anatolian Fault in the Sea of Marmara from morpho-bathymetry and seismic reflection images. In particular this paper focuses on the so-called Cinarcik segment of the fault connecting the eastern Izmit segment, which entirely ruptured during the Mw 7.4, 1999 earthquake, to the western segment of the Central High. The Cinarcik segment, potentially loaded after the Izmit earthquake, is expected to rupture during an earthquake occurring in the near future, possibly the next decades, with a high potential to affect the Istanbul metropolitan area. Our analysis suggests that the development of the observed structures accommodating strike-slip, transtensional and transpressional deformations, could be explained by changes in the geometry of fault segments within a right-lateral strike-slip tectonic regime. Tectonic deformations were reproduced in the analogue model by imposing a small (about 10°) and sharp difference in the relative orientations of the strike-slip segments at the edges of a major releasing bend. In the model slower strain accumulation occurs along the analogue of the Cinarcik segment than along the analogue of the Izmit segment of the fault. This would predict a delay for earthquakes triggered by stress transfer between the Izmit and Cinarcik segments. The model further predicts that most of the deformation in the Cinarcik basin is controlled by the sharp changes in the geometry of the fault itself.

The Afar region in East Africa is a key locality for studying continental break-up. Within Afar, passive margins are developing, of which the Southern Afar Margin (SAM) contains synthetic (basinward) faulting, whereas crustal flexure, antithetic faulting and marginal grabens occur along the Western Afar Margin (WAM). Numerous conflicting scenarios for the evolution of the WAM exist. In this analogue modelling study we test various factors that may affect the development of a WAM-style passive margin: brittle crustal thickness, (en echelon) rheological contrasts, sedimentation and oblique extension. Our experimental results illustrate how marginal flexure due to a weak lower crust below Afar can elegantly account for the structural features of the WAM. Brittle crustal thickness controls what structures occur: a thinner brittle crust accommodates flexure internally, whereas increasing brittle thicknesses lead to faulting. Large escarpment faults develop early on, followed by late-stage antithetic faulting and marginal grabens. A thicker brittle crust also causes enhanced subsidence, and increased strength contrasts between lower crustal domains leads to more localized deformation. Basin-wide sedimentation causes enhanced subsidence, as well as longer activity along large synthetic (escarpment) faults. Finally, oblique extension clearly prevents the development of marginal grabens, which only form in near-orthogonal extension. These results support a tectonic scenario involving initial oblique extension due to Arabian plate motion, creating echelon synthetic escarpment faults along the WAM. After the Danakil Block started its independent rotation, near-orthogonal extension conditions were established, allowing (enhanced) marginal flexure, antithetic faulting and marginal graben formation along the older en echelon escarpment. Differences in extension obliquity may also explain the differences in structural architectures between the WAM and SAM. The characteristics of the WAM are typical of magma-rich passive margins, and the margin has great potential for studying continental break-up and (magma-rich) passive margin formation.

Volcanoes display diverse morphologies as a result of the complex interplay of several constructive and destructive processes. Scaled analogue models offer a tool to isolate and characterize the different processes. Here, we investigate the interplay between volcano growth and deformation caused by an underlying strike-slip fault through simple analogue models. In particular, we analyze the morphometry of analogue volcanoes resulting from different growth-to-deformation ratios. Deformation elongates the volcano edifice at an angle of 10-45º from the fault trace along the extensional quadrants and generates a summit graben structure oriented perpendicular to the edifice elongation. The overall steepness of the edifice decreases, but steep slopes are preserved or increased on the lower compressional flanks, commonly related to small avalanches. Growth can partially to totally mask these features depending on the growth rate to strike-slip velocity ratio. The summit graben is easily masked even by low growth rates, whereas edifice elongation has the best preservation potential. Scaling of the experiments suggests that at volcanoes with growth rate (km3/yr) to strike-slip velocity (km/yr) ratios ≤ 3.8 km3/km, deformation features should be clearly preserved, whereas at volcanoes with ratios ≥ 15 km3/km, deformation features should be completely masked. The typical growth rates of volcanoes (0.01 to 1 km3/ka) and the typical velocities of strike-slip faults (1 to 20 mm/yr) suggest that in nature, growth rate to strike-slip velocity ratios can range over 3 orders of magnitude, spanning both types of end-members. Using examples of both active and inactive volcanoes located on strike-slip faults with variable elongation intensities and orientations, we highlight that the analogue models account for some of the morphometric variability observed at volcanoes in nature, although the role of vent distribution can be a key factor. Fil: Grosse, Pablo. Fundación Miguel Lillo. Dirección de Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; Argentina Fil: Poppe, Sam. Vrije Universiteit Brussel; Bélgica Fil: Delcamp, Audray. Vrije Universiteit Brussel; Bélgica Fil: van Wyk de Vries, Benjamin. Université Clermont Auvergne; Francia Fil: Kervyn, Matthieu. Vrije Universiteit Brussel; Bélgica

Most natural examples of transpression zones developed at oblique convergence regime are inherently 3D and have inclined boundaries. A 3D finite element model with an elasto-plastic rheology is used to investigate the structural and mechanical evolution of inclined transpression zones in a rock sequence above a frictional basal detachment. Inelastic constitutive relationships allow permanent strains to develop in response to the applied loads. FE-modelling results show that oblique convergence is accommodated by discrete deformation at the main pre-existing inclined faults (¼70 ) and by distributed brittle and ductile deformation at active blocks. Oblique contraction at the active blocks resulted mainly in layer-parallel shortening, orthogonal to the model outer boundaries, whereas thickening in the horizontal and vertical directions was accommodated via layer-parallel, fault strike-parallel extension and up-dip extrusion (i.e., inclined extrusion). Lateral extrusion should have compensated the rest and/or volume loss took place. Folding and thickening of the mobile backstop produced a non-cylindrical, asymmetric, bi-vergent anticline where permanent strains developed principally in the steep forelimb. Secondary, conjugate fault zones also accommodate oblique slip and contribute to uplift. Displacement vectors within the transpression zone are rotated counter-clockwise (ca. 20 –30 ) with respect to vectors in the fixed backstop. Areas with higher rotation values seem to correlate with those showing higher ellipticity values. The presence of pre-existing faults favored strain partitioning from the onset of deformation. FE-modelling results compared with analytical, natural example, and analogue modelling results show that our mechanical modelling can overall match inclined transpression zones geometry that present different modes of strain partitioning and localisation., Seyed Tohid Nabavi thanks Prof. Jordi Carreras and Dr. Elena Dru-guet for their help, and constructive discussions on progressive deformation, shear zones, and transpressive settings during scientific field trips in the Cap de Creus, the Roses and the Axial Zone of Pyrenees during his research visit period in Spain. Moreover, Seyed Tohid Nabavi thanks Dr. Jonas B. Ruh for teaching and discussions on the finite difference method programming during his research visit period in the Institute of Earth Sciences “Jaume Almera” (CSIC-ICTJA), Barcelona, Spain. Manuel Díaz-Azpiroz and Carlos Fernández gratefully acknowledge financial support from the Spanish Ministry of Science, Innovation and University (projects PGC2018-100914-B-I00 and PGC2018-096534- B-I00). This work forms part of the Ph.D. thesis of Seyed Tohid Nabavi at Shahid Beheshti University, Tehran, Iran. We thank reviewers Prof. Fernando O. Marques and Prof. Soumyajit Mukherjee for constructive reviews, comments and helpful suggestions which allowed us to improve the original manuscript. Editorial and handling efforts by Prof. Ian Alsop are also gratefully acknowledged.

Volcanic edifices are unstable accumulations of eruptive and intrusive products with complex internal structures. A documentation of these structures facilitates the assessing of the various hazards associated with volcanoes, e.g. flanks instabilities and eruptive events. However, structures are not easily assessed in the field, as volcanoes are regularly covered by eruptive products and are rapidly eroded. In the case of recent and active volcanoes, internal structures are generally extrapolated from the external morphology of the edifice, which can be documented using Digital Elevation Models (DEM) and geodetic data. Interpretation of such data greatly benefits from analogue models, which are idealised representations of reality used to test the impact that various processes may have on the shape of a volcanic edifice. This short review focuses on the contribution of analogue models to our comprehension of the internal structure of composite volcanic cones. The other volcanic morphologies (e.g. caldera) are excluded from this review, as they have distinct structure and evolution that have already been thoroughly reviewed. The reviewed models were used to investigate the impact that regional faults, magma injections and ductile substratum, among other processes, have on the internal structure and surface morphology of many volcanic edifices. These models were also used to locate unstable flanks and eruption sites.

The study of extensional structures such as normal faults, grabens and rift systems using analogue modelling techniques has been widely used to understand the evolution of sedimentary basins. These have been particularly helpful to understand the generation of oil reservoirs and structural traps. The present work was inspired by the discussion on the origins of two sub-parallel grabens located in the intracratonic Parecis Basin in the centre-west of Brazil. Parallel, oblique or intersecting grabens may form by the reactivation of inherited basement weaknesses that can act as velocity discontinuities, with their geometry and proximity controlling the evolution of the resulting grabens. In this work, we use sandbox analogue experiments to investigate the geometric and kinematic evolution of two nearby, sometimes intersecting, grabens. We do this by varying the distance and the geometry of two basal velocity discontinuities (VDs). In all experiments, an extension of 3 cm (scaling to 12 km) was prescribed to affect a 3 cm thick sand pack. The materials were deformed using a plexiglass box with two basal velocity discontinuities attached to two lateral movable walls. In a first set of experiments, we varied systematically the distance between the VDs. In the second set, we varied the angles between the VDs with different configurations, including situations in which the VDs intersect forming triple junctions. The results show that when the VDs are initially close ( 2 cm, the grabens develop independently and do not interfere. Furthermore, it is possible to conclude that whenever (not interfering) VDs are prescribed to the base of a sand pack it always leads to the formation of asymmetric grabens, except when the two VDs are initially close to each other. In the experiments with conjugate triple-junction VDs, the extension was shared between two independent asymmetric grabens where the VDs were at a certain distance from each other, and always led to the formation of a wide graben where the VDs were close or intersect each other.

Deformation patterns caused by the subduction of a single seamount or aseismic ridge have been well studied in analogue and numerical models. However, the effects of sequential multi-seamount subduction on accretionary wedge deformation have rarely been investigated in details. We performed a series of analogue modelling experiments of sequential subduction involving two seamounts of variable shape, spacing and rheological properties of strata to better understand the deformation mechanisms of an accretionary wedge with multi-seamount subduction. The results demonstrate that a seamount significantly hinders the seaward propagation of the accretionary wedge and facilitates lateral propagation. Two structural quiet zones form at the leading and trailing edges of the subducted seamount in the early stage of collision. As the seamount deeply penetrates into the wedge, the structural quiet zone in the leading edge is remoulded by a duplex structure, which may help the upward transport of deep subducted sediments back into the shallow area. Comparatively, the structural quiet zone in the trailing edge remains undeformed because it is situated in the stress shadow of the seamount. Deformation of the strata between two subducting seamounts may occur by thrusts laterally propagating into the seamount gap, which is facilitated by the decollement layer. The modelling results provide insights into the complex deformation mechanisms related to seamounts collision observed in the accretionary wedge offshore from the Costa Rica margin.

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.

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.

International audience; Structures developing above long‐term growing shallow‐level magma reservoirs, such as resurgent domes, may contain information on the reservoir itself. To understand the formation of such tectonic features, we have investigated the deformation process around a shallow pressurized magma reservoir embedded in a damaging elastic volcanic edifice. Our model allows evidencing the effect of the progressive damage in producing the fault pattern associated to tectonic surface deformation. Damage is first isotropic around the cavity and constitutes a damaged zone. Then the free‐surface effect appears and an anisotropic shear strain develops from the boundary of the damaged zone; it localizes on reverse faults that propagate upward to the surface. When the surface deformation is sufficient, normal faulting appears. Finally, the complete structure shows an undeformed wedge above the damaged zone. This structure is similar to those found by analogue modelling and from field geologic observations. From this model, we found a relation to estimate the reservoir radius and depth from the graben and dome widths. From limit analysis, we deduced an analytical expression of the magma reservoir pressure which provides a better understanding of the magma pressure build‐up during doming. The dip of reverse faults limiting the dome can be inferred from the minimal pressure required to rupture the crust around the reservoir. Finally, the magma reservoir overpressure, the dip of the faults, the reservoir depth and the damaged zone radius is inferred from three parameters: the ratio ρR computed from the dome and graben widths, the cohesion and the friction angle.

We have performed a series of sandbox models addressing the influence of sand mixtures (quartz and feldspar sand in different proportions) with different grain-size on the development of normal faults. The overall model evolution suggests that fault orientation, width of the deformed area, and average subsidence are not significantly influenced by the different sand mixtures. Conversely, the number of secondary structures, mean fault length, and average fault dip are strongly dependent on the material and grain-size. Modelling results suggest that a Log-normal distribution of fault length and segment length distribution betst describes the experimental fault populations, particularly when the amount of fine-grained (K-feldspar) sand is similar to 30%. The length distribution and the number of faults are related, specifically with increasing the amount of the K-feldspar sand (i.e., higher the amount of fine-grained material) the shorter the minimum detectable fault length. This behavior is described by a Log-normal distribution, with a few long faults and many intermediate to short faults. We thus suggest that (for length ratios of 10(-5) to 2 10(-6)) a sand-mixture composed of 70% quartz sand and 30% K-feldspar sand is the one (among those we have tested) that provides the best compromise in terms of frictional properties, density, structural detail, number and length distribution of the faults, easiness of use, and general behavior.

Cenozoic sedimentary basins typically contain young, unconsolidated sediments, potentially suitable for CO2 storage, e.g. the Utsira sand in the Norwegian North Sea Basin. During burial and compaction, formation fluids are squeezed out of the sedimentary deposits. These fluids can mobilize and transport sediments to overlying strata, forming seismic-scale clastic intrusions and shallow gas accumulations, to the Earth’s surface, or to the sea floor, where they are expelled through pockmarks or mud volcanoes, respectively. These deformation processes are described as subsurface sediment remobilization (SSR). Little attention has been paid to the formation mechanisms of SSR in the search for CO2 storage formations, though they may be relevant for storage safety, either occurring naturally, triggered by CO2 injection, or being fossil features providing pathways for fluid migration in the subsurface. Structural characteristics of SSR features are well known, but formation processes and dynamics are poorly understood. We have developed a scaled analogue experimental approach and performed systematic laboratory experiments in order to comprehend and quantify the conditions controlling and triggering SSR, the critical rheological properties of sediments, and storage structures (layer thickness, anticlines, faults). Experimental results are presented and potential implications for the selection of storage sites and storage safety are discussed in this article.

The role of inherited weaknesses in metamorphic basements with respect to the geometry and tectonic evolution of subsequent orogens formation forms a cornerstone of structural reconstructions, especially in non-inverted orogens with old Variscan basements. In this study, we analyse the geometry, kinematics, and tectonic evolution of the complex western termination of the Alpine Pyrenean–Cantabrian orogen using an integrated approach based on analogue modelling, constrained by structural mapping and geochronology, to provide new insights into the role of pre-existing structures and the shift in the orientation of the stress field. Our results demonstrate that pre-existing structures are necessary to explain the complex western termination, which does not follow the same tectonic architecture as the rest of the orogen. The reason for this is the different orientation of the pre-existing Variscan structures in this area, which do not follow the general E–W trend. These western structures (including NNE–SSW and NW–SE strike-slip fault systems and a greater number of E–W thrusts) accommodate the shortening, but have a less favourable orientation. The reactivation of the NNE–SSW strike-slip faults and the southern E–W thrusts in the recent stages indicates a late intraplate tectonic regime, mainly following the shift of the Shmax (from N–S to NNW–SSE) which occurred when the convergence between the Iberian, Eurasian, and African plates changed.

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 decollement, 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 decollement (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 decollement 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 decollement layer on absorbing the slip along the PFT. Our results provide a new mechanical interpretation of the deformation of Wupoer FTB, NE Pamir.

We present here the results of the first lithospheric-scale centrifuge models of pull-apart basins. The experiments simulate relative displacement of two lithospheric blocks along two offset master faults, with the presence of a weak zone in the offset area localising deformation during strike–slip displacement. Reproducing the entire lithosphere–asthenosphere system provides boundary conditions that are more realistic than the horizontal detachment in traditional 1 g experiments and thus provide a better approximation of the dynamic evolution of natural pull-apart basins. Model results show that local extension in the pull-apart basins is accommodated through development of oblique–slip faulting at the basin margins and cross-basin faults obliquely cutting the rift depression. As observed in previous modelling studies, our centrifuge experiments suggest that the angle of offset between the master fault segments is one of the most important parameters controlling the architecture of pull-apart basins: the basins are lozenge shaped in the case of underlapping master faults, lazy-Z shaped in case of neutral offset and rhomboidal shaped for overlapping master faults. Model cross sections show significant along-strike variations in basin morphology, with transition from narrow V- and U-shaped grabens to a more symmetric, boxlike geometry passing from the basin terminations to the basin centre; a flip in the dominance of the sidewall faults from one end of the basin to the other is observed in all models. These geometries are also typical of 1 g models and characterise several pull-apart basins worldwide. Our models show that the complex faulting in the upper brittle layer corresponds at depth to strong thinning of the ductile layer in the weak zone; a rise of the base of the lithosphere occurs beneath the basin, and maximum lithospheric thinning roughly corresponds to the areas of maximum surface subsidence (i.e., the basin depocentre).

The Red Sea represents an incipient ocean basin that is in transition from continental rifting to ocean initiation. In this paper we assess how pre-existing mantle and crustal heterogeneities within the African and Arabian lithospheres influenced rift development and propagation using three-dimensional (3D) brittle-ductile analogue laboratory experiments with a rotational boundary condition. Here we investigate the effect of three different configurations: (i) pre-existing rheological heterogeneities in the crust, (ii) in the lithospheric mantle, and (iii) in both. The experimental outcomes show that a linear weak zone in the lithospheric mantle layer exerts a first-order control on the overall mechanical behaviour of extending lithosphere, rift architecture and style of rifting. Crustal heterogeneities control second-order features, such as the trends of the main boundary escarpments along the rift axis. Our experiments reproduce the geological history of the Red Sea and provide key insights on its deformation history in space and time. We provide, for the first time, 3D dynamic constraints on how pre-existing weak zones in the lithosphere have controlled the temporal evolution of the Red Sea and determined its distinctive present-day morphology. The results find that certain morphological elements of the Red Sea can only be explained by the combined effects of crustal and mantle lithosphere heterogeneities. When applied to other natural examples, our findings provide significant new insights on the role of crustal and lithospheric heterogeneities during continental rifting.

The Nepal Himalaya fold-and-thrust belt is a classical two-decollement thrust–fold system. The belt has been extensively studied by analogue and numerical modeling, but the formation mechanism of the mid-crustal duplex structure beneath the northern part of the Lesser Himalaya remains debated. The formation of this thrust duplex has been attributed either to tectonic underplating along new ramps in response to the subducting Indian Plate, or to out-of-sequence thrusting along preexisting ramps. We conducted a series of experiments using analogue models, employing a model setup that was faithful to the geological setting of the Nepal Himalaya. We used industrial computed tomography (CT) and particle image velocimetry (PIV) to generate detailed deformation maps. The analogue models successfully reproduce the main structure of the Nepal Himalaya fold-and-thrust belt, showing that the two competing processes (i.e., underplating along new ramps and out-of-sequence thrusting along preexisting ramps) operate alternately to form the duplex structure. The models also show that underplating is associated with extensive foreland sedimentation, analogous to the foreland molassic sedimentation in the Sub-Himalaya, which leads to a systematic recession of the deformation front and a smaller horse. Finally, the recognition that out-of-sequence thrusting occurs in the interval between major underplating events provides important insights into the origin of the 2015 M W 7.8 Gorkha earthquake.

A new type of fault relay zone in extensional contexts, dominated by distributed along-strike or longitudinal fractures, is defined. It contrasts with the classical models reported in the literature, in which transverse connecting faults controlled by the own relay kinematics prevail. The new model is based on structural features of the Teruel graben system, as well as on analogue modelling. Relay zones between the NW-SE to NNW-SSE striking faults that delimit the eastern boundary of the Jiloca Graben (Calamocha, Sierra Palomera and Concud faults), together with the Teruel Fault, have been studied. All of these relay faults show recent (Neogene-Quaternary) ruptures at different scales, mostly parallel to the macrostructural trend and to the maximum horizontal stress (SHmax) trajectories (i.e., orthogonal to the ENE-WSW regional extension direction that characterises the nearly biaxial or radial stress regime active during Upper Pliocene-Quaternary times). Transverse ruptures are almost absent, with the exception of the northern relay zone (Calamocha-Sierra Palomera), where an incipient NE-SW striking connecting fault does exist. Analogue models have been run under a biaxial extension regime similar to the regional one. They allowed analysing the main factors controlling fracture propagation, depending on the ratio of extension velocities and the orientation of the master faults relative to extension directions. Laboratory fracture patterns, as in the natural studied examples, are mostly controlled by the inherited anisotropies and, in a greater extent, by the imposed extension trajectories, which results in a clear prevalence of longitudinal fractures. Such external controls, usually disregarded in numerical and analogue modelling, tend to induce fault coalescence through along-strike (parallel or at very-low-angle) propagation resulting in a final braided fault pattern.

Catastrophic volcanic eruptions triggered by landslide collapses can jet upwards or blast sideways. Magma intrusion is related to both landslide-triggered eruptive scenarios (lateral or vertical), but it is not clear how such different responses are produced, nor if any precursor can be used for forecasting them. We approach this problem with physical analogue modelling enhanced with X-ray Multiple Detector Computed Tomography scanning, used to track evolution of internal intrusion, and its related faulting and surface deformation. We find that intrusions produce three different volcano deformation patterns, one of them involving asymmetric intrusion and deformation, with the early development of a listric slump fault producing pronounced slippage of one sector. This previously undescribed early deep potential slip surface provides a unified explanation for the two different eruptive scenarios (lateral vs. vertical). Lateral blast only occurs in flank collapse when the intrusion has risen into the sliding block. Otherwise, vertical rather than lateral expansion of magma is promoted by summit dilatation and flank buttressing. The distinctive surface deformation evolution detected opens the possibility to forecast the possible eruptive scenarios: laterally directed blast should only be expected when surface deformation begins to develop oblique to the first major fault.

Geological field evidence and laboratory modelling indicate that volcanoes constructed on slopes slide downhill. If this happens on an active volcano, then the movement will distort deformation data and thus potentially compromise interpretation. Our recent GPS measurements demonstrate that the entire edifice of Mt. Etna is sliding to the ESE, the overall direction of slope of its complex, rough sedimentary basement. We report methods of discriminating the sliding vector from other deformation processes and of measuring its velocity, which averaged 14 mm year−1 during four intervals between 2001 and 2012. Though sliding of one sector of a volcano due to flank instability is widespread and well-known, this is the first time basement sliding of an entire active volcano has been directly observed. This is important because the geological record shows that such sliding volcanoes are prone to devastating sector collapse on the downslope side, and whole volcano migration should be taken into account when assessing future collapse hazard. It is also important in eruption forecasting, as the sliding vector needs to be allowed for when interpreting deformation events that take place above the sliding basement within the superstructure of the active volcano, as might occur with dyke intrusion or inflation/deflation episodes. Electronic supplementary material The online version of this article (10.1007/s00445-018-1209-1) contains supplementary material, which is available to authorized users.

In this work we use analogue modelling to analyse the effect of sudden ice shelf breakup on the flow of ice draining an ice sheet grounded below sea level. Experimental results confirm that the removal of the buttressing effect exerted by the ice shelf results in significant acceleration of inland glaciers: the models show indeed a pronounced increase in ice velocity close to the grounding line. However, this effect does not significantly propagate upstream towards the internal portions of the ice sheet and rapidly decays with time. Therefore, the ice sheet is almost unaffected by flow perturbations induced by the disintegration of the ice shelf.

International audience; The Main Ethiopian Rift, East Africa, is characterized by the presence of major, enigmatic structures which strike approximately orthogonal to the trend of the rift valley. These structures are marked by important deformation and magmatic activity in an off-axis position in the plateaus surrounding the rift. In this study, we present new structural data based on a remote and field analysis, complemented with analogue modelling experiments, and new geochemical analysis of volcanic rocks sampled in different portions of one of these transversal structures: the Goba-Bonga volcano-tectonic lineament (GBVL). This integrated analysis shows that the GBVL is associated with roughly E–W-trending prominent volcano-tectonic activity affecting the western plateau. Within the rift floor, the approximately E–W alignment of Awasa and Corbetti calderas likely represent expressions of the GBVL. Conversely, no tectonic or volcanic features of similar (E–W) orientation have been recognized on the eastern plateau. Analogue modelling suggests that the volcano-tectonic features of the GBVL have probably been controlled by the presence of a roughly E–W striking pre-existing discontinuity beneath the western plateau, which did not extend beneath the eastern plateau. Geochemical analysis supports this interpretation and indicates that, although magmas have the same sub-lithospheric mantle source, limited differences in magma evolution displayed by products found along the GBVL may be ascribed to the different tectonic framework to the west, to the east, and in the axial zone of the rift. These results support the importance of the heterogeneous nature of the lithosphere and the spatial variations of its structure in controlling the architecture of continental rifts and the distribution of the related volcano-tectonic activity.

Netherlands journal of geosciences : NJG 96(05), s87-s104 (2018). doi:10.1017/njg.2017.19 special issue: "Induced Seismicity in the Groningen Gas Field, the Netherlands", Published by Cambridge Univ. Press, Cambridge

Magma emplacing at shallow crustal levels may cause significant deformation in the overlying country rock (i.e., forced folding, fracturing and faulting), both at a local and/or regional scale. To get insights into these processes, we investigated in the laboratory the development of forced folds and associated fracture/fault networks. An analogue magma, simulated by polyglycerols, was intruded into a sand pack representing the brittle crust. The scaled analogue models reproduced different 3D deformation structures depending on the model parameters (e.g., magma viscosity, injection rate, volumetric flux, and the rheology and thickness of the host and cover rocks). However, all models support the observation that the emplacement of shallow magmatic bodies may result in the growth of dome-shaped forced folds, and associated development of tensional and compressional deformation in the host-rock. Although the models involve simplifications, these results provide useful hints for geothermal research, as fractures and faults associated with magma emplacement can significantly influence the distribution and migration of superhot geothermal fluids. These structures can therefore be considered potential targets for geothermal and/or ore deposit exploration. In this perspective, the results of analogue models may provide useful geometric constraints for field work, numerical modelling, and particularly seismic interpretation, allowing production and a better understanding of integrated conceptual models concerning the circulation of supercritical fluids.

Heterogeneous deformation is intrinsic in natural deformation, but often underestimated in the analysis and interpretation of mesoscopic brittle shear faults. Based on the analysis of 11,222 faults from two distinct tectonic settings, the Central Andes in Argentina and the Sudbury area in Canada, interpolation of principal strain directions and scaled analogue modelling, we revisit controversial issues of fault-slip inversions, collectively adhering to heterogeneous deformation. These issues include the significance of inversion solutions in terms of (1) strain or paleo-stress; (2) displacement, notably plate convergence; (3) local versus far-field deformation; (4) strain perturbations and (5) spacing between stations of fault-slip data acquisition. Furthermore, we highlight the value of inversions for identifying the kinematics of master fault zones in the absence of displaced geological markers. A key result of our assessment is that fault-slip inversions relate to local strain, not paleo-stress, and thus can aid in inferring, the kinematics of master faults. Moreover, strain perturbations caused by mechanical anomalies of the deforming upper crust significantly influence local principal strain directions. Thus, differently oriented principal strain axes inferred from fault-slip inversions in a given region may not point to regional deformation caused by successive and distinct deformation regimes. This outcome calls into question the common practice of separating heterogeneous fault-slip data sets into apparently homogeneous subsets. Finally, the fact that displacement vectors and principal strains are rarely co-linear defies the use of brittle fault data as proxy for estimating directions of plate-scale motions.

Many volcano summits host craters that are partially overlapping. The formation of such nested craters has been commonly interpreted as vent migration. Here, we present an additional mechanism that may explain the geometry of nested craters at volcanoes. Lascar Volcano, the most active volcano of the Central Volcanic Zone in the Chilean Andes, hosts ENE-WSW trending summit craters that are partially overlapping (nested). Details on the evolution and interaction between the different craters remain unclear. To create a robust dataset, Terrestrial Laser Scanner (TLS) data were collected at the summit of Lascar in 2013. The resulting topographic data set, consisting of more than 15 million data points with centimetre sampling, allows visualising almost the complete eastern edifice of the volcano's summit. From the TLS data, a Digital Elevation Model (DEM) and a slope map were generated allowing us to create a lineament map and quantify the observed morphological and structural features. To further improve our understanding of the processes responsible for the formation of the craters and geomorphology, we designed sandbox analogue models. Results suggest that one of the craters is a ‘parasite’ crater, formed as a consequence of ongoing activity in the adjacent crater. Our data suggest that the nested craters have all been modified since the last major eruption in 1993, by near surface effects associated with cooling, compaction and gravitational sliding of the crater floor infill. As the active crater deepens, the adjacent inactive crater extends and partially slumps towards the active one. Understanding the structural development of these nested craters is relevant for assessing potential future eruption sites, thus making Lascar a dynamic target for a detailed morphology study. These findings may similarly be applied to other volcanoes, where nested craters have developed.

Of the features that characterize large shield volcanoes on Mars, flank terraces remain the most enigmatic. Several competing mechanisms have been proposed for these laterally expansive, topographically subtle landforms. Here we test the hypothesis that horizontal contraction of a volcano in response to the down-flexing of its underlying basement leads to flank terracing. We performed a series of analogue models consisting of a conical sand–plaster load emplaced on a basement comprising a layer of brittle sand–plaster atop a reservoir of viscoelastic silicone. Our experiments consistently produced a suite of structures that included a zone of concentric extension distal to the conical load, a flexural trough adjacent to the load base and convexities (terraces) on the cone’s flanks. The effects of variations in the thickness of the brittle basal layer, as well as in the volume, slope and planform eccentricity of the cone, were also investigated. For a given cone geometry, we find that terrace formation is enhanced as the brittle basement thickness decreases, but that a sufficiently thick brittle layer can enhance the basement’s resistance to loading such that terracing of the cone is reduced or even inhibited altogether. For a given brittle basement thickness, terracing is reduced with decreasing cone slope and/or volume. Our experimental results compare well morphologically to observations of terraced edifices on Mars, and so provide a framework with which to understand the developmental history of large shield volcanoes on the Red Planet.

Measuring surface deformation is critical in analogue modelling of Earth science phenomena. Here, we present a novel application of the Microsoft Kinect sensor to measure vertical deformation in a scaled analogue model of Nisyros volcano (Greece), simulating two magmatic sources and related surface deformation. The Kinect permits capture of real time, 640 × 480 pixel, true-colour images (RGB) and a grid of distances to the modelled surface with a horizontal and vertical resolution of ±1 mm. Using recorded distances permits quantification of vertical deformation in terms of maximum and average down- and uplift during deflation and inflation phases respectively, which is of crucial importance for defining the kinematics of faults formed during the modelling, determined from interpretation of the RGB images. Although other techniques have demonstrated similar or higher accuracy, our study demonstrates the cost-effectiveness of the Kinect in analogue modelling of volcanoes. © 2014 John Wiley & Sons Ltd.

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).

International audience; 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.

International audience; The Gulf of Mexico and the offshore Campos and Santos basins of Brazil provide good examples of sub-salt detachments, by shearing in the presence of overpressure. We have developed new physical models of such situations, including layers of ductile silicone putty, which simulated an evaporite, and fluid overpressure in porous brittle layers. We studied two configurations, in which the maximal overpressure was either beneath a single ductile layer of silicone (simulating salt), or between two such layers of silicone (simulating an evaporite sequence). The results of our analogue modelling showed that detachment faults appeared and persisted at the bases of the ductile layers, even though some ductile deformation occurred simultaneously within these layers. For models where maximal overpressure was between two ductile layers, detachment occurred mainly at the base of the upper silicone layer. On comparing our models with structures in the Gulf of Mexico or offshore Brazil, we find strong similarities, which lead us to suspect that detachment is indeed possible at the base of an evaporite sequence in nature. This conclusion has strong implications, not only for the understanding of petroleum systems, but also for better management of petroleum productionin sedimentary basins.

The combination of analytical and analogue models gives new opportunities to better understand the kinematic parameters controlling the evolution of transpression zones. In this work, we carried out a set of analogue models using the kinematic parameters of transpressional deformation obtained by applying a general triclinic transpression analytical model to a tabular-shaped shear zone in the external Betic Chain (Torcal de Antequera massif). According to the results of the analytical model, we used two oblique convergence angles to reproduce the main structural and kinematic features of structural domains observed within the Torcal de Antequera massif (α = 15° for the outer domains and α = 30° for the inner domain). Two parallel inclined backstops (one fixed and the other mobile) reproduce the geometry of the shear zone walls of the natural case. Additionally, we applied digital particle image velocimetry (PIV) method to calculate the velocity field of the incremental deformation. Our results suggest that the spatial distribution of the main structures observed in the Torcal de Antequera massif reflects different modes of strain partitioning and strain localization between two domain types, which are related to the variation in the oblique convergence angle and the presence of steep planar velocity – and rheological – discontinuities (the shear zone walls in the natural case). In the 15° model, strain partitioning is simple and strain localization is high: a single narrow shear zone is developed close and parallel to the fixed backstop, bounded by strike-slip faults and internally deformed by R and P shears. In the 30° model, strain partitioning is strong, generating regularly spaced oblique-to-the backstops thrusts and strike-slip faults. At final stages of the 30° experiment, deformation affects the entire model box. Our results show that the application of analytical modelling to natural transpressive zones related to upper crustal deformation facilitates to constrain the geometrical parameters of analogue models.

The morphodynamic response of large tidal inlet/basin systems to future relative sea level rise (RSLR), incorporating both Eustatic sea level rise and local land subsidence effects, is qualitatively investigated using the state-of-the-art Delft3D numerical model and the Realistic analogue modelling philosophy. The modelling approach is implemented on a highly schematised morphology representing a typical large inlet/basin system located on the Dutch Wadden Sea (Ameland Inlet) over a 110-year study period. Three different RSLR Scenarios are considered: (a) No RSLR, (b) IPCC lower sea level rise (SLR) projection (0.2 m SLR by 2100 compared to 1990) and land subsidence, and (c) IPCC higher SLR projection (0.7 m SLR by 2100 compared to 1990) and land subsidence. Model results indicate that, for the 110-year study duration, the existing flood dominance of the system will increase with increasing rates of RSLR causing the ebb-tidal delta to erode and the basin to accrete. The rates of erosion/accretion are positively correlated with the rate of RSLR. Under the No RSLR condition, the tidal flats continue to develop while under the high RSLR scenario tidal flats eventually drown, implying that under this condition the system may degenerate into a tidal lagoon within the next 110 years. The tidal flats are stable under the low RSLR scenario implying that, at least for the next 100 years, this may be the critical RSLR condition for the maintenance of the system. Essentially the results of this study indicate that, as the Eustatic SLR is likely to be greater than the apparently critical rise of 0.2 m (by 2100 compared to 1990), the tidal flats in these systems will at least diminish. In the worst, but not unlikely, scenario that the Eustatic SLR is as high as the IPCC higher projections (0.7 m by 2100), the tidal flats may completely disappear. In either case, the associated environmental and socio-economic impacts will be massive. Therefore, more research focusing on the quantification of the physical and socio-economic impacts of RSLR on these systems is urgently needed to enable the development of effective and timely adaptation strategies.

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.

Magmatic sheet intrusions contribute significantly to the upper crustal magma transport network. The emplacement mechanism of the magmatic sheets controls the final geometry of the intrusions and the characteristics of host rock deformation. Previous observations have highlighted the preponderance of brittle structures, associated with shallow-level sheet intrusions. However, recent studies have suggested that non-brittle host rock behaviour also occurs, particularly related to the formation of magma fingers during shallow-level sill intrusion. Here, we examine both brittle and non-brittle intrusion mechanisms and expand upon them with field observations from a series of widespread and variable magmatic systems. Non-brittle emplacement appears primarily associated with viscous flow of the host rock during intrusion and is therefore intimately linked to the contemporaneous host rock rheology as well as magma dynamics. Purely brittle and non-brittle emplacement processes are found to be end members with many intrusions containing evidence of both behaviours. Deriving the host rock characteristics is therefore important for discerning potential diagnostic intrusion indicators and intrusion geometries both within the field and in modelling. Incorporation of variable host material behaviours in numerical and analogue modelling, tuned using direct field observations, may consequently further our understanding of the controls on shallow-level intrusion.

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.

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. © 2010 Elsevier B.V.

Terra Nova, 22, 390–395, 2010 Abstract We present the results of coupled analogue and numerical models that provide new insights into the relationships between volcanoes and thrusts. The effects of both upper-crustal magma chambers and the load of volcanoes on the geometry of thrust systems were investigated. Analogue modelling points to a strong influence exerted by a magma chamber on thrust geometry, which, as suggested by the numerical models used to rationalize these results, is related to the stress redistribution around the weak heterogeneity. The low-viscosity body below a volcanic edifice localizes compressional deformation and causes a curvature of the thrusts towards the magma chamber, opposite to the direction of tectonic transport. In these conditions, the volcanic load has a negligible effect on the structural geometry. These results are in contrast with those of previous studies, where intrusions or the load of major volcanoes generated a curvature of the thrusts away from volcanic edifices in the direction of tectonic transport.