Analogue Sandbox Experiments Research Articles

Insights into lava dome and spine extrusion using analogue sandbox experiments

Lava dome formation is a common process at stratovolcanoes involving the shallow intrusion or extrusion of viscous lava and may lead to the rise of spines. Spines are protrusions observed to extrude episodically during lava dome growth, yet the structural and mechanical factors controlling their formation are only partially understood. Here, we provide new, detailed insight into lava dome growth and the production of spines using a novel set of analogue experiments extruding sand-plaster mixtures from a fixed-diameter conduit under isothermal conditions. We trace displacement and strain with photogrammetric methods for precise and detailed monitoring of the extrusion process. Results show initial dome growth forming a steep-sided and flat-topped shape through extrusion of new material, leading to slumping of oversteepening slopes, forming a talus. Spines are found to protrude at a later stage through the dome surface along discrete circular faults that originate from the conduit walls, starting a cycle of spine growth and collapse. As our spines only appear after prolonged extrusion, we relate their appearance to the compaction and strengthening of material within the conduit. We find that spine diameter, height and volume are positively correlated with increasing cohesion and therefore material strength. The spine diameter was also observed to be smaller or equal to the diameter of the underlying conduit, as shear extrusion occurs along vertical to outward-dipping fault planes. For natural domes, our findings imply that spine growth may be the consequence of compaction and densification via porosity loss, shearing and/or outgassing of conduit magma during ascent. More efficient compaction will yield wider and taller spines as a result of increasing rock strength. Our study further highlights the relevance of analogue experiments in the study of lava domes and spines, which remain one of the most hazardous and unpredictable features at dome-forming volcanoes worldwide.

Influence of the orientation of cohesive blocks upon the structural grain of fold-and-thrust belts: An appraisal by means of analogue modeling

Analogue sand-box experiments have been set up to illustrate and analyze the control exerted by lithological heterogeneities on the evolution of a model fold-and-thrust belt (FTB). The structural architecture within many FTBs is highly sensitive to pretectonic obstacles (e.g., basement highs or intrusive bodies). To address FTB evolution with emphasis on the latter, we perform a series of experiments that include tabular cohesive blocks surrounded by a layered sandpack of negligible cohesion. Two degrees of cohesion are tested (low vs high cohesion) as well as different geometrical boundary conditions (longer axis parallel vs oblique relative to the main far-field stress). Modeling results show a tendency of sequential faults to avoid rigid obstacles, enhancing frontal accretion and propagation of the deformation front. Structural traces have arcuate patterns in map-view, strongly consistent with the geometry of the underlying block/obstacle. High-cohesion models show major deviations on the structural pattern with respect to models with low-cohesion blocks. We find that the rheological contrasts between rock types have an important effect on the structure of the upper crust, controlled by the degree of the contrast and the geometry (orientation) of the discordant block. An interesting aspect of our results is that orientation of the block plays a considerable role on whether new thrusts cut the block or stresses propagate towards the foreland through the cohesive body. Block obliquity also influences the generation of back-thrusts. These features are compared to natural cases as well as to other physical models of FTBs. We suggest that irregular structural patterns in FTBs may shed light on the presence of buried rock bodies (intrusive bodies, metamorphic complexes or crystalline basement highs) within the components of the deformed rock sequence.

Forearc Basin Stratigraphy Resulting From Syntectonic Sedimentation During Accretionary Wedge Growth: Insights From Sandbox Analog Experiments

AbstractForearc basin stratigraphy is expected to record a detailed history of the deformation and growth pattern of an accretionary wedge. However, the relationship between syntectonic basin sedimentation and growth of a wedge remains poorly understood, including how deformation of the wedge modifies the basin stratigraphy and how syntectonic sedimentation influences deformation of the wedge. This study reproduced accretionary wedges with and without syntectonic sedimentation by analog sandbox experiments. The results show that basin stratigraphy varied with changes of the growth pattern of the accretionary wedge. In the case that wedge growth was dominated by frontal accretion phase, the depositional area migrated trenchward. In contrast, underthrusting phase caused the sediment layers to be tilted landward and the depocenter to migrate landward. A prolonged underthrusting can result in combining a retrowedge basin with a wedge top basin and yield a wide area of accommodation space. The occurrence of two types of basin stratigraphy (trenchward and landward migration of the depocenter) reflects the two different types of deformation (frontal accretion phase and underthrusting phase), and these differences possibly depend on a contrast in strength of the basal shear resistance between the inner and outer parts of the wedge due to temporal and spatial variations of sedimentation on the wedge. A change in the magnitude of normal stress acting on the wedge base likely influenced the mode of deformation of the wedge. These results suggest that forearc basin stratigraphy is influenced by the growth pattern of accretionary wedges, which is itself affected by syntectonic sedimentation.

Growing Faults in the Lab: Insights Into the Scale Dependence of the Fault Zone Evolution Process

Analog sandbox experiments are a widely used method to investigate tectonic processes that cannot be resolved from natural data alone, such as strain localization and the formation of fault zones. Despite this, it is still unclear to which extent the dynamics of strain localization and fault zone formation seen in sandbox experiments can be extrapolated to a natural prototype. Of paramount importance for dynamic similarity is the proper scaling of the work required to create the fault system, Wprop. Using analog sandbox experiments of strike‐slip deformation, we show Wprop to scale approximately with the square of the fault system length, l, which is consistent with the theory of fault growth in nature. Through quantitative measurements of both Wprop and strain distribution we are able to show that Wprop is mainly spent on diffuse deformation prior to localization, which we therefore regard as analogous to distributed deformation on small‐scale faults below seismic resolution in natural fault networks. Finally, we compare our data to estimates of the work consumed by natural fault zones to verify that analog sandbox experiments scale properly with respect to energy, that is, that they scale truly dynamically.

Sandbox rheometry: Co-evolution of stress and strain in Riedel– and Critical Wedge–experiments

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

Kinematic History of a Salient-recess Junction Explored through a Combined Approach of Field Data and Analog Sandbox Modeling.

Within fold-thrust belts, the junctions between salients and recesses may hold critical clues to the overall kinematic history. The deformation history within these junctions is best preserved in areas where thrust sheets extend from a salient through an adjacent recess. We examine one such junction within the Sevier fold-thrust belt (western United States) along the Leamington transverse zone, northern Utah. Deformation within this junction took place by faulting and cataclastic flow. Here, we describe a protocol that examines these fault patterns to better understand the kinematic history of the field area. Fault data is supplemented by analog sandbox experiments. This study suggests that, in detail, deformation within the overlying thrust sheet may not directly reflect the underlying basement structure. We demonstrate that this combined field-experimental approach is easy, accessible, and may provide more details to the deformation preserved in the crust than other more expensive methods, such as computer modeling. In addition, the sandbox model may help to explain why and how these details formed. This method can be applied throughout fold-thrust belts, where upper-crustal rocks are well preserved. In addition, it can be modified to study any part of the upper crust that has been deformed via elastico-frictional mechanisms. Finally, this combined approach may provide more details as to how fold-thrust belts maintain critical-taper and serve as potential targets for natural resource exploration.

Deformation along a salient-transverse zone junction: An example from the Leamington transverse zone, Utah, Sevier fold-thrust belt (USA)

Within fold-thrust belts, the junctions between salients and recesses may hold critical clues to the overall kinematic history of fold-thrust belts. The deformation history within these junctions is best preserved in areas where thrust sheets extend from a salient through an adjacent recess. We examine one such junction within the Sevier fold-thrust belt (western United States) along the Leamington transverse zone, northern Utah. The Canyon Range thrust sheet can be traced continuously from the Leamington transverse zone to its adjacent salient to the south, the Central Utah segment. Deformation within the Canyon Range thrust sheet took place by faulting and cataclastic flow. Analyses of these fault networks preserved throughout the Canyon Range thrust sheet are used to develop a kinematic history of the Leamington transverse zone. Field data is supplemented by analog sandbox experiments. This study suggests that, in detail, deformation within the overlying thrust sheet may not directly reflect the underlying basement structure. Moreover, these junctions may contain several types of accommodating structures that helped to maintain critical-taper and that serve as potential targets for natural resource exploration.

Intrinsic versus extrinsic variability of analogue sand-box experiments – Insights from statistical analysis of repeated accretionary sand wedge experiments

Analogue models are not perfectly reproducible even under controlled boundary conditions which make their interpretation and application not always straight forward. As any scientific experiment they include some random component which can be influenced both by intrinsic (inherent processes) and extrinsic (boundary conditions, material properties) sources. In order to help in the assessment of analogue model results, we discriminate and quantify the intrinsic versus extrinsic variability of results from “sandbox” models of accretionary wedges that were repeated in a controlled environment. The extrinsic source of variability, i.e. the parameter varied is the nature of the décollement (material, friction and thickness). Experiment observables include geometric properties of the faults (lifetime, spacing, dip) as well as wedge geometry (height, slope, length).For each variable we calculated the coefficient of variance (CV) and quantified the variability as a symmetric distribution (Normal, Laplacian) or asymmetric distribution (Gamma) using a Chi squared test (χ2). Observables like fault dip/back thrust dip (CV = 0.6–0.7/0.2–0.6) are less variable and decrease in magnitude with decreasing basal friction. Variables that are time dependent like fault lifetime (CV = 0.19–0.56) and fault spacing (CV = 0.12 – 0.36) have a higher CV consequently affecting the variability of wedge slope (CV = 0.12–0.33). These observables also increase in magnitude with increasing basal friction. As the mechanical complexity of the evolving wedge increases over time so does the CV and asymmetry of the distribution. In addition, we confirm the repeatability of experiments using an ANOVA test. Through the statistical analysis of results from repeated experiments we present a tool to quantify variability and an alternative method to gaining better insights into the dynamic mechanics of deformation in analogue sand wedges.

Relay ramps as pathways for turbidity currents: a study combining analogue sandbox experiments and numerical flow simulations

Unlike for subaerial settings, the impact of subaqueous relay ramps on sediment dispersal is still poorly understood. A combination of analogue laboratory experiments in a sandbox with numerical flow calculations is used to simulate relay ramp topographies on rifting continental margins and to analyse the resulting turbidity current pathways and their deposits. Various scenarios are investigated, including inflow perpendicular and oblique to the relay ramp axis as well as flow constrained by an incised channel on the ramp and by a landward-directed tilt of the ramp. Without channelling, most sedimentation takes place on the basin floor because the bulk of the flow follows the steepest gradient down the fault and into the rift basin. With a channel along the relay ramp, significant flow occurs initially down the ramp axis, but channel spillover and basinward ramp tilting combine to redirect much of the sediment down the fault slope into the basin. When the relay ramp has a landward-oriented tilt, most of the current flows down the ramp and deposits its sediment load there and at the foot of the ramp. However, also here a considerable amount of the flow is shed over the hanging wall fault and into the basin, forming a secondary depocentre, while ponding redistributes thin deposits over a wider area of the basin. The quantitative dependence of these results on the specific ramp geometries remains to be investigated further but may bear great importance for refined sedimentary models in subaqueous rifted settings as well as for hydrocarbon exploration therein.

Digital sandbox modelling of Indian collision to Eurasia

Summary Discrete Element Method (DEM) is a powerful tool as a digital sandbox simulator to numerically reproduce fault related structures and to analyse the deformation quantitatively. Similar to analogue sandbox experiments, DEM approximates the geologic body as an assembly of particles that can properly simulate the brittle behaviour of the upper crust. We examined the collision process of the Indian sub-continent to the Eurasian Plate by using the DEM. The simulations reproduced the deformation geometry similar to published analogue experiments. Velocity and stresses of each particle extracted from the simulation were quite unstable showing a characteristic feature of the brittle behaviour of the upper crust. A comparison of these results with GPS and in-situ stress data of the eastern Asia suggests that the ductile deformation of the lower crust and mantle may have major roles to control the real deformation. The results also suggest that Tapponnier’s tectonic model may have a strong boundary effect, particularly to the stress field within the model.

Have the southernmost Andes been curved since Late Cretaceous time? An analog test for the Patagonian Orocline

The kinematic evolution of the enigmatic arc-shaped southernmost Andes of Patagonia and Tierra del Fuego has been a subject of debate for most of the past century. We compared the results from analog sandbox experiments with the tectonic evolution and actual configuration of the mountain chain in order to elucidate whether oroclinal bending took place during the Tertiary, or if the southernmost Andes have been a curved orogen since at least Late Cretaceous time. Experiments simulating oroclinal rotation produced strong along-strike variations in shortening and failed to account for structural data compiled from the Fuegian Andes. Results from experiments simulating an L-shaped, concave-to-foreland indenter were in agreement with the known Tertiary structural evolution of the southernmost Andes. The diachronicity of principal shortening events previously recognized in Patagonia and Tierra del Fuego could only be reproduced by moving the indenter in two successive orthogonal directions: first approximately northward to form the Fuegian fold-and-thrust belt, and then approximately eastward to propagate thrusting in the Patagonian Andes. This two-phase evolution is consistent with a recorded change in the convergence direction of the Farallon-Nazca plate that occurred at ca. 27 Ma.

Polygonal fault systems and channel boudinage: 3D analysis of multidirectional extension in analogue sandbox experiments

A systematic set of analogue models has been carried out to study the structural evolution of a sedimentary pile consisting of superposed brittle and ductile lithologies in a regional context of passive margins. The models are composed of silicone representing a ductile basal layer of evaporites or overpressured shales, and sand progressively sedimented on top to simulate brittle overburden sediments. Channels with various shapes are embedded in the silicone. Deformation is triggered by gravitational instability of the growing sedimentary pile and basal slope angles ranging from 0 to 5°. The main results follow. • A polygonal fault pattern developed, due to the 3D boudinage of the overburden sand above the silicone layer during multidirectional extension. • If a sandy channel was embedded in the ductile base, it was immediately cut by faults, which were perpendicular to the channel boundaries, no matter what the angle between channel and slope direction. • The boundaries of the channel preferentially localised faults, which propagated through the sedimentary pile above. Ongoing deformation along these faults therefore strongly influenced further accumulation of sediments.

The numerical sandbox: comparison of model results for a shortening and an extension experiment

Abstract We report results of a study comparing numerical models of sandbox-type experiments. Two experimental designs were examined: (1) A brittle shortening experiment in which a thrust wedge is built in material of alternating frictional strength; and (2) an extension experiment in which a weak, basal viscous layer affects normal fault localization and propagation in overlying brittle materials. Eight different numerical codes, both commercial and academic, were tested against each other. Our results show that: (1) The overall evolution of all numerical codes is broadly similar. (2) Shortening is accommodated by in-sequence forward propagation of thrusts. The surface slope of the thrust wedge is within the stable field predicted by critical taper theory. (3) Details of thrust spacing, dip angle and number of thrusts vary between different codes for the shortening experiment. (4) Shear zones initiate at the velocity discontinuity in the extension experiment. The asymmetric evolution of the models is similar for all numerical codes. (5) Resolution affects strain localization and the number of shear zones that develop in strain-softening brittle material. (6) The variability between numerical codes is greater for the shortening than the extension experiment. Comparison to equivalent analogue experiments shows that the overall dynamic evolution of the numerical and analogue models is similar, in spite of the difficulty of achieving an exact representation of the analogue conditions with a numerical model. We find that the degree of variability between individual numerical results is about the same as between individual analogue models. Differences among and between numerical and analogue results are found in predictions of location, spacing and dip angle of shear zones. Our results show that numerical models using different solution techniques can to first order successfully reproduce structures observed in analogue sandbox experiments. The comparisons serve to highlight robust features in tectonic modelling of thrust wedges and brittle-viscous extension.

Strike-slip deformation in the Confidence Hills, southern Death Valley fault zone, eastern California, USA

The Confidence Hills form a well exposed, composite, positive flower structure developed in Pliocene to Recent lacustrine and alluvial fan sediments. The structure has developed along the current trace of the southern Death Valley dextral strike-slip fault zone. California, USA. NW–SE-striking fault zones bound the Confidence Hills. In 3D, these fault segments are inferred to link at depth to a common basal fault system. The flower structure is formed by doubly plunging anticlines that roughly parallel the bounding fault segments. Fold development was aided by: (a) the presence of a basal salt deposit and numerous detachment horizons within the sedimentary pile and (b) buttressing and uplift along and against the bounding oblique-slip reverse faults. Structural and palaeomagnetic evidence indicates that the anticlines developed with axial surfaces parallel and sub-parallel to the trace of both fault zones. Folding appears to have initially developed adjacent to and above the southeastern fault segment and then propagated outwards and to the northwest. The latest displacements along the current southern Death Valley fault zone are probably only in the orders of hundreds of metres and as young as 0.9 Ma to Recent. This young fault zone is highly segmented along the length of the floor of southern Death Valley: the high proportion of oblique-slip faulting consistent with the strike-slip zone being immature. The Confidence Hills structure displays close geometric similarities with other natural examples of positive flower structures and to the features found in scaled. analogue sandbox experiments of strike-slip faulting.

Geomechanics of the South Furious field. An integrated approach towards solving complex structural geological problems, including analogue and finite-element modelling

The South Furious oil field, offshore Sabah (Malaisia), is an asymmetric anticline bounded on the steep southern flank by a series of steep E-W-striking reverse faults and affected by intense E-W crestal normal faulting immediately next to and contemporaneous with the reverse faults. In spite of the very good sand development and a high sand percentage, the reservoir performance is very poor. A study was undertaken to describe and understand the development and the mechanics of this type of structure and to explain their presence in almost undisturbed sediments around them. The regional tectonic development was studied to provide a basis for a geological model of the structure. A structural geological model was necessary to be able to predict the type of faults and fractures and their location in the structure. The proposed structural development was modelled with analogue sandbox experiments and numerically simulated with a finite-element program (GEOFEP). The results indicate that basement faulting related to the closing of the South China Sea, induced differential sediment loading, triggering asymmetric clay diapirism in the core of the anticline. The asymmetry resulted in contemporaneous formation of steep reverse faults at the leading edge of the migrating anticline and normal faults in a crestal collapse system. The reverse faults and associated shear fractures (as seen in cores) have a sealing cataclastic fabric. This explains the severe compartmentalisation of the reservoir with consequently poor production behaviour, in particular in the reverse faulted part of the field.

Computerized X-ray tomography analysis of sandbox models: Examples of thin-skinned thrust systems

Computerized X-ray tomography applied to analog sandbox experiments performed in a normal gravity field makes possible the analysis of the kinematic evolution, as well as the three-dimensional geometry, of models that simulate tectonic deformations. Most of the plastic or viscous analog materials generally used in a normal gravity field for such models have X-ray attenuations compatible with medical scanner images. Models using soft, cohesionless granular materials were studied. Faults in granular materials corresponding to dilatant zones with lower (by 30%-50%) X-ray attenuation are thus clearly visible in the tomography images. Kinematic analysis was done on imbricate thrust propagation. In materials with an internal friction angle of 30°, thrust propagation is controlled mainly by a triangular pop-up structure resulting from the nucleation of two conjugate thrusts at the basal decollement. The pop-up structure is created immediately in front of the previous thrust. Three-dimensional geometry analysis was tested on lateral ramps parallel to the transport direction. Lateral ramps were induced by changing the frictional boundary conditions at the bottom of the model. Though the lateral ramps strike almost parallel to the transport direction, their dip angle is comparable to the dip angle of frontal ramps.