Sandbox Models Research Articles

Variation in geometries and displacement along thrust faults: a quantitative analysis from sandbox models

The present work combines theoretical analysis, laboratory experiments, and field examples together with quantitative analysis to understand displacement variation along fault surfaces with varying dip angle. Two sandbox experiments are described to quantitatively analyze the interactions between shortening and sedimentation during late stages of deformation. The two models consisted of sand layers with single and two microbeads layer/s within the sandpack, sieved over horizontal frictional detachment. The models demonstrate that the overlying sediment load bears a major role on kinematic of developing structures. Fault displacement varied along imbricate thrusts due to variation in fault dip angle in the listric geometry. Maximum displacement occurred in the up-dip direction (up-section or near fault tip) along the faults where fault dip angles were shallower and favourable for thrust displacement. In contrast, the displacement was limited in the down-dip direction (down-section or near basal detachment) where fault dip angles were steeper. The variation in fault displacement with fault dip along the fault surfaces made it possible to establish an inverse relationship between the fault dip and related displacement values, i.e. lower is the dip angle, greater the displacement and vice versa. Despite the inherent limitations and assumptions of laboratory experiments, application of results to natural examples sheds new light on the mechanics and geometry of thrust faults from the deep to shallow structural levels as well as structural patterns that could appear as anomalous at a first sight.

Middle Miocene evolution and structural style of the Diapir Fold Zone, Eastern Carpathian Bend Zone, Romania: insights from scaled analogue modelling

Abstract The Diapir Fold Zone of the Carpathians is the most prolific onshore hydrocarbon area in Romania. Structural complexity, mainly due to the presence of salt, combined with poor seismic quality near and below the salt lead to contrasting structural models in the area. To gain insights into the mid-Miocene tectonic evolution, structural geometries and the effects of penetrative strain, we ran dual décollement scaled sandbox models with layered brittle and ductile materials. Results of two analogue models (20 and 33% shortening) revealed that the onset of the deformation sequence was mainly characterized by layer-parallel shortening. As shortening continued, a foreland-verging sequence of supra-salt detachment folds and sub-salt duplexes evolved. The sub-salt duplexes are located directly below the crests of the detachment folds, as the development of these large wavelength anticlines was related to sub-salt deformation. Salt flow was another controlling factor of the deformation style, as salt accumulated in the anticlinal cores and increased the coupling in the supra-salt synclinal axis. Our results offer insights into the effects of salt on the kinematic evolution of this area, help to predict geometries in areas of poor seismic quality, and highlight the important contribution of penetrative strain on deformation and reservoir quality.

The effect of overburden thickness on deformation mechanisms in the Keping fold-thrust belt, southwestern Chinese Tian Shan Mountains: Insights from analogue modeling

The structural evolution of the Keping fold-thrust belt in the South Chinese Tian Shan, NW China is poorly understood. Here, we present the results of a series of scaled sandbox models based on the structural architecture of the Keping fold-thrust belt in our investigation of the effect of overburden thickness on deformation mechanisms in fold-thrust belts. These models simulated the evolution of the Keping fold-thrust belt, where the western segment was shortened with a thicker overburden relative to the eastern segment. Model results confirm that fewer thrust faults with wider spacing develop in thicker cover units, whereas more thrust faults with narrower spacing develop in thinner cover units. Transfer zones also form as a result of faster and farther propagation in a segment with thicker overburden, consistent with the prediction of the critical taper theory. Our modeling further shows that due to the episodic accretion of thrust wedges and non-simultaneous formation of thrust faults, the deformation front may locally propagate farther in the thinner overburden. Alternatively, there may not be so much difference between two parts of sand layers with different thicknesses, an observation that is inconsistent with the theoretical prediction. The similarities between the model and real geological observations suggest that the variations in the overburden thicknesses across the Piqiang Fault transfer zone strongly influenced the development of the structural architecture of the Keping fold-thrust belt. They further indicate that the growth of fold-thrust belts in nature is episodic, and that the propagation of deformation front cannot be simply predicted by the critical taper theory. The overburden thickness variations across the Piqiang Fault transfer zone were produced by high angle reverse faulting along the pre-existing Piqiang-Selibuya Fault during the late Pliocene and associated crustal uplift and erosion of the hanging wall strata.

The 3‐D Structural Model of an Out‐of‐Sequence Earthquake in China: Implication for the Reactivation of Positive Inversion Structures Along the Northern Tianshan Fold‐and‐Thrust Belt

AbstractThe intracontinental earthquake behavior is not well understood due to its complexity in the interior of the continent. The Tianshan Mountain is located in the interior of the Eurasian plate, which was rejuvenated in the Cenozoic due to the India‐Asia collision and sourced several large earthquakes. The 2016 Hutubi earthquake occurred within the northern Tianshan, China, during rupture of a high‐angle back‐thrust toward the hinterland, which broke the long‐time quiescence in the mountain front. The geometry, dynamics, and hazard implications of this earthquake on an out‐of‐sequence thrust fault remains unclear. The 3‐D structural model shows that the coseismic fault is connected to the large‐scale blind Southern Junggar thrust, which sourced the 1906 Mw ~8 Manas earthquake. The geometry of the coseismic fault and the occurrence of extensional tectonics in the Jurassic suggest that the Hutubi earthquake occurred on a reactivated rift‐related structure. To test this hypothesis and study the kinematics, we performed sandbox modeling, with results suggesting that coseismic fault can form during positive inversion of a preexisting Jurassic rift, and that the Southern Junggar thrust and related structures may be recently active. Coal beds facilitated the transfer of fault slip and strain from mountain to basin and from basal to roof detachment. We propose that the preexisting rift and related sedimentation controlled the subsequent structure and geomorphology of the mountain belt through thrust wedge formation. This study provides a typical example for the study of intracontinental inversion structure and its earthquake behavior.

Control of strike‐slip and pull‐apart processes to tectonic transition of the southern East China Sea Shelf Basin

The East China Sea Shelf Basin (ECSSB) lies among the Eurasian Plate, the Pacific Plate, and the Indian Plate. Its formation and evolution since the Mesozoic was controlled by the combined effect of westward subduction of the Pacific Plate and actions of the large‐scale ductile shear zone on the South China Plate—the Changle–Nan'ao Fault Zone. This paper selects the southern ECSSB with well‐developed Mesozoic strata as the study area. By analysing the basin architectures and structures of each structural unit, calculating the fault growth index, and referring to the sandbox modelling results, we consider that the Oujiang Sag belongs to a strike‐slip pull‐apart basin controlled by the Changle–Nan'ao Fault Zone, while the Minjiang Sag and the Jilong Sag are extensional basins without strike‐slip property. The Mesozoic evolutionary process of the ECSSB can be divided into three stages on the basis of the three main strike‐slip movements of the fault zone. In the Middle–Late Jurassic, the subduction of the Izanagi Plate induced the dextral shearing in a ENE direction of the Changle–Nan'ao Fault Zone, which then formed NW‐SE‐directed transpressional structures inside the ECSSB. In the Late Jurassic–Early Cretaceous, the motion sense of the Izanagi Plate gradually shifted to the northwestwards, inducing the sinistral‐thrusting of the fault zone. Following this, the initial NW‐SE‐directed compressional structures were stretched into the original sag. By the Late Cretaceous, the retreat of the subducting Paleo‐Pacific Plate led to the turning of the regional stress field from compression to transtension. Being influenced by the dextral strike‐slip‐related structures of the Changle–Nan'ao Fault Zone, the Oujiang Sag developed extensive strike slipping‐related structures and was assembled into a left‐stepping en echelon pattern. The Minjiang and Jilong sags, however, gradually grew into extensional depressions under successive extension due to the great distance from the strike‐slip fault zone.

Ramp initiation in fold-thrust belts: Insight from PIV analysis of sandbox models

Do forethrust ramps propagate up-section from the tip line of a detachment or do they initiate within the deforming strata, connecting later with the detachment? To address this question, we acquired cross-sectional digital images of a sand layer undergoing shortening, in a sandbox analog model. Analysis of a succession of images, using particle-image velocimetry (PIV), defines spatial and temporal variations in strain and vorticity in the sand layer during shortening. Results show that the foreland edge of layer-parallel shortening in the sand layer defines the location of the detachment's tip line, and that initiation of a new forethrust ramp begins within the layer above the detachment, well to the hinterland of the detachment tip. Once initiated, the ramp grows both up-dip and down-dip until it links to the detachment to define a new branch line. As the forethrust ramp grows, the position of the tip line of the active detachment migrates to the hinterland, so that previously active portions of the detachment stop slipping. Once the ramp intersects the detachment, displacement on the detachment to the foreland of the new branch line ceases temporarily, while slip on the ramp accommodates all shortening.

Origin of basin-scale syn-extensional synclines on the southern margin of the Northern Carnarvon Basin, Western Australia

Fault geometry exerts a dominant structural control on the deformation of hanging wall sequences during extension and contraction. Numerical, kinematic and sandbox modelling studies have demonstrated that characteristic anticline–syncline pairs are produced during the extension of ramp-flat faults. These features are commonly recognized in smaller fault-scale structures, but remain underappreciated in larger, basin-scale settings. The Lewis Trough, situated within the Northern Carnarvon Basin, Western Australia, is a basin-scale, largely unfaulted syncline with an associated anticline along its western flank, rather than the fault-related graben typical of the region. We present kinematic models demonstrating that a SE-dipping, ramp-flat fault geometry can produce relative highs and lows in Jurassic strata as well as honouring the asymmetrical onlap pattern within the Lewis Trough. This study indicates that the Lewis Trough formed during the Early Jurassic, a period typically associated with high rates of extension and not during the Late Triassic Fitzroy Compression Event. This study also highlights the importance of the Locker Shale in partitioning deformation of the Permian and Mesozoic fault systems and as a diffuse zone that variably partitions displacement between stacked Permian and Mesozoic fault systems.

Area-constant strain and dilation in sandbox models: Insights from whole-model area balance

Line-length restoration of sandbox models of contractional structures typically shows that bed lengths are too short, implying that the models are not area balanced. Two alternative hypotheses have been proposed to explain this discrepancy: horizontal (or tectonic) compaction, a porosity loss caused by horizontal shortening, and area-constant shortening with bed thickening. Whole-model area-balance shows that two very different sandbox models, for which line-length restoration implies compaction, are actually area balanced with small dilations. The line-length imbalance is due to area-constant thickening that accompanies the shortening. This thickening may not be obvious to the eye and may be impossible to recognize in the deformed state unless the initial thickness is known independently because when the entire model is shortened and thickened, no undeformed thickness may remain.

Differential strain transfer, Longmen Shan thrust belt, eastern Tibetan Plateau margin: Implications for seismic hazards

Southeastward strain transfer in the middle-southern Longmen Shan (LMS) thrust belt not only can promote the development of fault-related folds and earthquakes in the piedmont, but also affect the seismogenic capacity of the middle-southern LMS thrust belt. However, the detailed transfer process remains elusive. In this work, we combine structural profile re-analysis, sandbox modeling and seismic statistics to illustrate the strain transfer process. Our main results indicate that: (1) the thickness of the shallow detachment (e.g. Middle-Lower Triassic gypsum-salt strata) is probably the main factor for differences in southeastward strain transfer within the domain between the middle-southern LMS thrust belt and Weiyuan-Moxi anticline. This is also supported by our sandbox model; (2) the seismogenic capacity in the southern LMS thrust belt is probably proportional to the corresponding shortening of the Range Front Blind Thrust (RFBT). Thus, seismogenic capacity in the seismic gap zone is weaker than those of other segments of the southern LMS thrust belt, which is probably < Ms 7.0. (3) The weak seismogenic capacity (<Ms 7.0) and formation of the seismic gap is probably attributed to weak petrophysical properties and enhanced southeastward strain transfer. It is worth noting that the 1970 Dayi Ms 6.2 earthquake probably released most energy of the seismic gap. Further, this work also sheds light on the evaluation of seismic hazard in the LMS and other orogenic belts.

The Predicament of Fintechs in the Environment of Traditional Banking Sector Regulation – An Analysis of Regulatory Sandboxes as a Possible Solution

Recently, “Financial Technology-companies” (FinTechs) are increasingly changing the financial services industry worldwide and impose considerable challenges for regulators tasked to solve the arising trade-off between sound regulation and innovation support. In this regard, regulatory sandboxes, which were recently introduced in several jurisdictions, provide a promising solution, as they imply a liberalization of regulatory requirements in order to enable FinTechs to test their innovative services. However, we observe that no comparable initiative exists in Germany, even though the German regulator identified a need for action on this subject in order to maintain its international competitiveness. Thus, based on a detailed analysis of various sandbox models worldwide, this paper develops a set of own recommendations as a basis for the implementation of a sandbox concept which might be applicable in the German regulatory environment. In doing so, we identify current theoretical as well as practical regulatory issues within the context of the rapid FinTech evolution. To the best of our knowledge, this paper represents the first study on key international sandboxes as a basis to design guidelines specifically for the German financial market. Thereby, we contribute to the literature as we evolve an effective regulation within the new setting of innovative financial technologies. Moreover, our findings contribute to the practical solution of current challenges faced by both regulators and affected companies. Even though our derived implications focus on the German financial sector, the results may potentially be applicable in further jurisdictions with similar regulatory requirements.

Formation and inversion of salt-detached ramp-syncline basins. Results from analog modeling and application to the Columbrets Basin (Western Mediterranean)

The widespread extensional deformation that took place during Jurassic to Cretaceous times in Western Europe and the North Atlantic margin resulted in the formation of several rift systems. Some of the resulting basins associated with these rifts show broad synclines detached on pre- or syn-kinematic Permian or Triassic salts, and are filled by thick sedimentary successions. The development of these salt-detached ramp-syncline basins has been associated to the extensional motion of ramp/flat extensional sub-salt faults. The shape and kinematics of such faults have usually been established using the architecture of syn-kinematic units and by assuming complete coupling of the hanging wall rocks. Therefore, there are fault interpretations that do not consider the role played by the deep salt layers, which clearly act as an effective detachment, decoupling sub- and supra-salt deformations. Moreover, the complexity of these scenarios further increases due to some of these basins, which, during latest Cretaceous and Cenozoic times, were partially inverted and were often incorporated into fold-and-thrust belts.Based on analog models and using the Mesozoic Columbrets Basin (Western Mediterranean) as a case study, the aim of this research is: to decipher the influence of the ramp/flat extensional fault during the syncline development and the interaction with a pre-kinematic salt; and to infer how salt-detached ramp-syncline basins are subsequently inverted.To achieve this goal we carried out an experimental program consisting of eight different sandbox models. Our results show that the main structure formed at the end of the extension is a salt-detached ramp-syncline and that its geometry not only depends on the dip and length of the fault panels, but also on the fault displacement and salt thickness. The inversion of these salt-detached ramp-synclines resulted in a major thick-skinned fault-bend anticline with minor thin-skinned contractional structures.

Bulk strain in orogenic wedges based on insights from magnetic fabrics in sandbox models

Anisotropy of magnetic susceptibility (AMS) analysis is used as a petrofabric indicator for a set of four identical-setup sandbox models that were shortened by different amounts and simulate contra ...

Growth of bending-moment faults due to progressive folding: Insights from sandbox models and paleoseismological implications

Bending moment normal faults (BMF) form during outer-arc extension of a folding layer. These faults, observed in contractional and extensional tectonic environments, can represent the only tectonic features potentially available for paleoseismological investigations (i.e., trenching) in order to date the movement of deeper structures. Yet, the reliability of BMF as a paleoseismological feature is debated in terms of its potential to continuously record coseismic deformation and their relationship with progressive flexure.We made use of analogue modeling in order to explore a simple model of progressive orthogonal flexure and the development of BMF. We collected data on mode of BMF inception and growth by analyzing time-series of cross section deformation, measuring dip-parallel BMF displacement profiles, and constructing a three-dimensional model of fault architecture by slicing of the experiment.Our results point to a model of BMF development typical of mechanically constrained faults, where limits on self-similar growth of faults is imposed once a maturity stage of displacement accumulation is attained. The BMF can accumulate displacement until the structure reaches a maturity stage when further slip is inhibited. This could result in an incomplete paleoseismological record of further fold movements. However, this fault stage can be recognized by the D-shaped displacement profile, typical of mature BMF. A simple two-dimensional constant-area balancing model is proposed, allowing us to derive incremental changes in flexuring, given the area of the collapsed sector with BMF and the thickness of the faulted layer. These results support the use of BMF as a paleoseismological proxy for deeper structures.

Fluid-overpressure Driven Sediment Mobilisation and Its Risk for the Integrity for CO2 Storage Sites – An Analogue Modelling Approach

During the process of sediment mobilisation unconsolidated sediments are deformed due to fluid overpressure. Structures formed by sediment mobilisation can serve as high permeable pathways for vertical fluid migration through a low-permeable seal layer and, therefore, have to be considered in the risk assessment of CO2 storage sites. In our study, we designed and conducted physical sandbox models to investigate structural characteristics and the dynamic evolution of sediment mobilisation resulting from pore fluid overpressure. Preliminary results show that basically two different modes of cover deformation occur: uplift in cohesive sediment and pipe formation in low-cohesive sediments.

Evolution of structure and permeability of normal faults with clay smear: Insights from water‐saturated sandbox models and numerical simulations

AbstractClay smear is difficult to predict for subsurface flow applications and would benefit from an improved understanding of the processes controlling clay smear. We present water‐saturated sandbox experiments with large clay smear surfaces (~500 cm2) that couple cross‐fault fluid flow measurements with structural analysis of excavated clay smears. We compare measured flow data to numerical flow simulations to develop a tool to evaluate the evolving fault structure. Results show diagnostic relationships between fault structures and cross‐fault flow. In experiments with one or two clay layers and a cumulative thickness of 10 mm at 100 mm displacement, normally consolidated clay in a structural domain of graben faulting initially yields hybrid brittle/ductile failure with early breaching of the clay layer and increased cross‐fault flux. This is followed by fault backstepping, formation of clay smears, and reworking of clay fragments within the fault. Early formed holes remain open during the evolution of the faults. Fault zones are segmented by fault lenses, breached relays, and clay smears in which sand and clay mix by deformation. Experiments with two clay layers show that holes rarely form at the same position on the fault plane, producing a layered sand‐clay fault rock with greater flow path tortuosity and lower permeability than in one‐layer experiments. We compare our results with observations of faults in nature and discuss progress toward models with sufficient detail and understanding to allow prediction of flow across evolving faults, first in laboratory models and then in the subsurface.

Comment on “Sandbox modeling of evolving thrust wedges with different preexisting topographic relief: Implications for the longmen Shan thrust belt, eastern Tibet” by C. Sun et al.

AbstractSun et al.'s (2016) sandbox modeling takes preexisting topographic relief into account for faulting activity at the compressional orogenic belt, which promotes better understanding of Longmenshan thrust belt's (LTB) orogenic process and faulting behavior. However, topographic relief in sandbox modeling is contradictory to central LTB's actual topographic relief, and as a result, both the comparison between sandbox modeling and actual tectonics and topography and the conclusion that Xiaoyudong Fault is a tear fault should be reconsidered. Actually, based on Sun et al.'s (2016) sandbox modeling, the Xiaoyudong Fault cannot be treated as a tear fault.

Reply to comment by Tan et al. on “Sandbox modeling of evolving thrust wedges with different preexisting topographic relief: Implications for the Longmen Shan thrust belt, eastern Tibet”

AbstractTan et al. comment that the preexisting topographic relief in our sandbox is opposed to its prototype in the central Longmen Shan. Therefore, the comparison between our sandbox modeling and the natural topography is questionable and does not agree with our conclusion that the Xiaoyudong fault is a tear fault. First, we are grateful to the authors for their approval of our sandbox modeling and its contribution to understanding fault behavior within thrust wedges. However, after reading the comment carefully, we found that they misunderstood the meaning of topographic relief we conveyed. In response, we would like to address the differences between the topography in their comment and the orogen‐scale topography we investigated in our modeling to defend our conclusion.

Modeling the interaction between presalt seamounts and gravitational failure in salt-bearing passive margins: The Messinian case in the northwestern Mediterranean Basin

The northwest Mediterranean Basin includes a thick Messinian salt sequence composed of three evaporitic units. From these, the intermediate unit, which is dominantly composed of halite, acted as a gravitational detachment favoring the downslope failure of the overlying sediments in a thin-skinned deformation regime. As a result, the structure of the margin is characterized by an upper extensional domain with basinward-dipping listric normal faults and a lower contractional domain that accommodates upslope extension by folding, salt inflation, or diapir squeezing. Lower to middle Miocene volcanic seamounts (presalt reliefs) located at the upper extensional domain locally disrupted the evaporitic units and produced salt flow perturbations. They acted as passive buttresses during the gravitational failure modifying the structural zonation of the margin. Using an experimental approach (sandbox models), we analyze the role played by seamounts during the kinematic evolution of passive margins and how they alter salt flow and suprasalt deformation during gravitational gliding. The experiments found that the seamounts locally interrupt the structural zonation of the margin because they hindered downdip salt flow during early deformation. Seamounts initially compartmentalize the margin architecture, resulting in the development of two gravitational subsystems with two extensional/contractional pairs that are subsequently reconnected when the accumulation of salt analog upslope of the relief is enough to overthrust it. From this point onward, the cover is passively translated downslope as a regional system. The changes in the viscous layer flow velocity related to the dip differences between the flanks and edges of the seamount determine the kinematic evolution of this system. Our experiments also provide geometric constraints to consider during interpretation of these structures, which are commonly poorly imaged in seismic data.

Interpretation of salt-cored frontal structures in the Southern Pyrenees guided by analog modeling, surface and subsurface data

Southern Pyrenean frontal thrusts are usually blind or partially covered by syn- and post-tectonic sediments of the Ebro Foreland hampering their interpretation. We have investigated the geometry and evolution of the Súria Anticline, a frontal structure of the Southern Pyrenees, which is characterized by the presence of multiple buried and blind thrusts displaying a complex triangle zone and several fishtail structures at depth. To characterize the geometry and structural evolution, a combination of surface geology, subsurface data interpretation, and analog modeling have been integrated into a unique 3D geologic model. The surface geology of this area depicts a rather simple structure, characterized by a north-directed backthrust and a gentle anticline, both striking west–southwest/east–northeast. In contrast, geophysical data indicate that the subsurface structure is more complex, exhibiting several northward- and southward-directed thrusts and folds detaching on the Pyrenean foreland Eocene evaporites. Integration of surface (geologic maps and dip measurements) and subsurface data (seismic and wells) allowed us to reconstruct the 3D structure of this area. To produce a more robust kinematic model, we used an experimental approach to investigate the evolution of frontal structures in folds and thrust belts on evaporitic detachments. Different viscous layer configurations were tested in a series of sandbox models comparing them with the evolution of the Súria Anticline. Taking into account the deformation and that seismic quality is in general poor, the experimental results allow us to better characterize the structures developed in this area. Our structural evolution is characterized by an early stage of salt inflation and fold nucleation. With increasing shortening, the structure evolved to a detachment anticline eventually developing a northward-directed thrust that broke through the northern anticline limb. The present-day geometry developed in a later stage of fold reactivation, when a new regional fold formed, and tightened the preexisting structure.

Intermediate decollement activation in response to the basal friction variation and its effect on folding style in the Zagros fold-thrust belt, an analogue modeling approach

Although the role of various basal and intermediate decollement levels on structural style is well documented individually in many folded terrains, the interaction between basal and intermediate decollements is poorly constrained. This study uses results of two scaled sand-box models shortened from one end to study the variation in structural development in response to varying basal friction and its consequent interaction with intermediate decollement horizons. Two models with similar incompetent intermediate decollement, but with different basal friction (with and without a thick basal decollement), were prepared analogous for the eastern and the western parts of the Razak basement fault in the Fars Region of the eastern part of the Zagros fold thrust belt (ZFTB). Combined results of scaled models with geological observations are used to argue that the basal decollement friction characteristics govern propagation of deformation front. In addition, model results, analogues to north-south direction, show that deformation complexity and disharmonic folding exist in the section where the intermediate decollement has been activated in response to the shortening without the basal decollement (throughout the western part of the Razak basement fault where less thickness of the Hormuz series as the basal decollement has been documented compared to its eastern part). In other words, the complexity in deformation is less portrayed along sections where basal friction beneath the model decreases (e.g. the eastern part of the Razak basement fault). We argue here that, in addition to other parameters (not presented in this study) interaction of intermediate decollement levels with basal decollement friction characteristics could explain decoupling between structures within the sedimentary column of the Fars Region of the eastern part of the Zagros fold thrust belt.