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Growth rate of salt diapirs usually oscillates depending on several factors. The growth can be arrested by depletion of the source layer or diapir burial, conversely, diapir reactivation occurs through erosion of the overburden and/or introduction of tectonic forces. Examples of reactivated diapirs can be observed in the Zagros Mountains in Iran. There, tectonic shortening responsible for development of the Zagros Fold and Thrust Belt simultaneously squeezes the diapirs, which extrude salt onto the surface. The top section of the diapir is usually affected by meteoric water, which dissolves the salt and leaves behind insoluble material embedded within the source layer, forming the so called caprock. This caprock can be assumed to be already present before the reactivation of the diapirs during shortening, hence it may play a role in the development of the salt extrusions. Geometry, composition and mechanical properties of the caprock can vary widely depending on factors such as original composition of diapiric material, dissolution and growth rates, etc. Additionally, exact mechanical properties of any caprock are difficult to determine and are currently largely unknown. Hence, we present a series of 2D numerical simulations utilising finite element method to investigate how different geometries and rheologies of the caprock affect the shape of the subaerial extrusions. The analysis was performed with three variable parameters (caprock viscosity, cohesion, and thickness) for three scenarios of diapirism (1 - purely shortening-based, with depleted source layer; 2 - purely buoyancy-based, with preserved source layer and no tectonic forces, and; 3) a combined scenario). We analysed the general deformation patterns as well as quantifiers such as velocities, displacements, strains, strain rates and ratios between vertical and horizontal components of the quantifiers. We investigated variability of averaged values of the quantifiers in time as well as detailed spatial distribution for the finite state of simulation. The simulations revealed the strong contrast between less (low cohesion and viscosity) and more competent rheologies in term of deformation patterns. The former tends to result in caprock material being thinly spread over the surface of the salt extrusion, whereas in the latter case the caprock fractures into “rafts” floating on top of the extrusion. The exact geometry of the rafts (size, spacing, distribution) is highly dependent on the geometry and type of diapirism. We also compare the resultant patterns to the quantifiers, especially velocities and their ratios, establishing clear ties between the patterns and deformation dynamics.

Basin-scale salt flow and the evolution of salt structures in rift basins is mainly driven by sub- and supra-salt faulting and sedimentary loading. Crustal extension is often accompanied and followed by thermal subsidence leading to tilting of the graben flanks, which might induce an additional basinward-directed driver for salt tectonics. We designed a new experimental analogue apparatus capable of integrating the processes of sub-salt graben extension and tilting of the flanks, such that the overlapping effects on the deformation of a viscous substratum and the brittle overburden can be simulated. The presented experimental study was performed to demonstrate the main functionality of the experimental procedure and setup, demonstrating the main differences in structural evolution between conditions of pure extension, pure tilting, and extension combined with tilting. Digital image correlation of top-view stereoscopic images was applied to reveal the 3D displacement and strain patterns. The results of these experiments suggest that in salt basins affected by sub-salt extension and flank inclination, the salt flow and downward movement of overburden affects the entire flanks of the basin. Supra-salt extension occurring close to the graben centre is overprinted by the downward movement; i.e. the amount of extension is reduced or extensional faults zones are shortened. At the basin margins, thin-skinned extensional faults developed as a result of gravity gliding. A comparison with natural examples reveals that such fault zones can also be observed at the margins of many salt-bearing rift basins indicating that gravity gliding played a role in these basins.

As a primary driving force, margin tilting is crucial for gravity-driven thin-skinned salt tectonics. We investigated how instant versus progressive margin tilting mechanisms influence salt tectonics using an analogue modeling setup where tilting rate could be controlled. Instant tilting resulted in initially high deformation rates, triggering widely distributed upslope extension and downslope contraction. Later, both the extensional and contractional domains migrated upslope as early extensional structures were successively deactivated, while deformation rates decreased exponentially. In contrast, progressive tilting led to downslope migration of the extensional domain by sequentially formed, long-lived normal faults. Contraction localized on a few, long-lived thrusts before migrating upslope. We attribute the distinct structural evolution of thin-skinned deformation, especially in the extensional domain, in the two tilting scenarios mainly to mechanical coupling between the brittle overburden and underlying viscous material. The coupling effect in turn is largely controlled by the deformation rate. By demonstrating the spatiotemporal variations of structural style and kinematic evolution associated with instant versus progressive tilting, we suggest that such variation is identifiable in nature and therefore can provide a new way to analyze margin tilting histories.

Sediment progradation and spreading is a key process during gravity-driven, thin-skinned deformation in salt-bearing passive margins. However, to what degree the size and shape of a progradational ...

Sediment spreading is a key process during gravity-driven deformation in salt-bearing passive margins. Whether and how progradational sedimentary wedges control gravity-spreading is still under deb...

The Zagros fold-and-thrust belt in Southern Iran is famous for its spectacular outcrops of salt diapirs. Most of these diapirs already existed prior to the onset of the Zagros orogeny, but tectonic shortening caused their reactivation and extrusion of the salt. Thus, the diapir exposures often provide access to intense internal deformation of the Hormuz salt series and its entrained interlayers. However, highly soluble evaporites (mainly halite) were already dissolved in many of the exposures leaving behind degraded ‘caprock’, which is built of a multi-compositional residuum of less soluble minerals and rocks. Based on geological field studies on two iconic salt diapirs in Southern Iran, the Karmostaj (Gach) and the Siah Taq diapir, we ascertained that the caprock is also intensively deformed. The accessible part of the caprock is roughly 200 m thick and consists of a fine-grained, laminated gypsum containing fragments of brecciated carbonates and siliciclastics. Especially in the down- and mid-slope regions of the salt exposure, this mixture is sheared and folded, but also dissected by thrust faults. Since such deformation processes in the caprock were not described before, there is a lack in explanations for the timing, the depth of formation and the structural evolution of these structures. For instance, it is unclear if the ductile shearing of the relatively competent gypsum matrix and the brecciation of the clasts took place near the surface or in larger depths (a few hundreds of meters), where confining pressure is higher.In this study, we want to classify the observed structures in the caprock, characterize deformation mechanisms and differentiate typical deformation domains. Based on that, we speculate about the timing and structural evolution of the caprock deformation and suggest that three scenarios can be imagined: (1) Pre-extrusion deformation: The caprock exposed today was buried by a thicker caprock package and, therefore, is compacted and mechanically strong. With the onset of the Zagros orogeny, tectonic shortening of the buried diapir caused lateral deformation before the salt extrusion. (2) Syn-extrusion deformation: The caprock is relatively young and was mechanically weak after its formation. Thus, it was deformed during diapir extrusion and, then, solidified during degradation of the salt. (3) Post-extrusion deformation: The caprock was mainly formed after salt extrusion, but it remained relatively immobile. The caprock matrix is occasionally weakened by the infiltration of meteoric water, and continued to be deformed due to gravitational gliding even after the dissolution of the rock salt. In order to test these hypotheses, we intend to carry out analogue experiments in which we try to model a squeezed diapir. In a parameter study, the thickness and the material of the covering layer simulating the caprock will be varied to assess possible differences in the deformation patterns.

Salt flow in rift basins is mainly driven by sub- and supra-salt extension imposing shear stresses and differential loading on the salt layer. In many rift basins, the graben flanks are tilted as a result of thermal subsidence and sediment load. Such tilt induces additional basin-ward directed stresses potentially causing downward directed salt flow and gravity gliding of the supra-salt overburden. However, sediment loading in extensional basins is usually largest in the basin centre, which would lead to an upward directed salt expulsion and might act as an effective buttress resisting downward gliding.Our aim is to investigate the opposing influence of sub-salt extension, sedimentary loading and tilting on deformation patterns in the viscous salt and the brittle overburden. We try to assess under which geological configurations (e.g. minimum basin slope or topographic gradient) upward directed salt flow and downward directed gravity gliding are the dominating deformation processes in extensional basins. Therefore, we developed a new analogue modelling apparatus enabling to simulate the processes of tectonic extension of a graben structure and the gradual tilting of the graben flanks, acting either simultaneously or separately. Using digital image correlation technique, temporal and spatial changes of the displacement and strain patterns can be analysed. Cross sections through the final experiments enable to identify structures characteristic for specific driving processes.Here, we present results of a preliminary experimental study in which the basic influence of flank tilting and syn-kinematic sedimentation on salt tectonics in rift basins is examined. In case that the graben flanks remain flat during extension, widespread extensional fault zones develop on the footwall sides near the graben faults. In case that the flanks are tilted simultaneously with basal extension, additional extensional fault zones evolve at the upslope basin margins resulting from downward gliding of the overburden. In the downslope basin centre, this peripheral extension is balanced by reduced amounts of extension near the graben and later by shortening above the graben bounding faults and the hanging wall graben centre. If syn-kinematic sedimentation is introduced, downslope gravity gliding is significantly reduced and extensional fault zones are rather localized. Peripheral extensional structures observed in the experiments resemble typical thin-skinned extensional structures occurring at the flanks of many salt-bearing rift basins, e.g. the Polish Basin and Norwegian-Danish Basin. Thus, such structures might serve as diagnostic indicators for the occurrence of gravity gliding in rift basins.

Basin-scale salt flow and the evolution of salt structures in rift basin is mainly driven by sub- and supra-salt faulting and sedimentary loading. Crustal extension is often accompanied and followed by thermal subsidence leading to tilting of the graben flanks, which might induce an additional basinward directed driver for salt tectonics. We designed a new experimental analog apparatus capable of integrating the processes of sub-salt graben extension and tilting of the flanks, such that the overlapping effects on the deformation of a viscous substratum and the brittle overburden can be simulated. The presented experimental study was performed to demonstrate the main functionality of the experimental procedure and setup demonstrating the main differences in structural evolution between conditions of pure extension, pure tilting and extension combined with tilting. Digital image correlation of top view stereoscopic images was applied to reveal the 3D displacement and strain patterns. Results of these experiments suggest that in salt basins affected by sub-salt extension and flank inclination, the salt flow and downward movement of overburden affects the entire flanks of the basin. Supra-salt extension occurring close to the graben centre is overprinted by the downward movement, i.e. amount of extension is reduced or extensional faults zones are shortened. At the basin margins, thin-skinned extensional fault developed, which resemble fault zones observed on basin flanks offset from the central graben zone.

In this paper, a literature‐based compilation of the timing and history of salt tectonics in the Southern Permian Basin (Central Europe) is presented. The tectono‐stratigraphic evolution of the Southern Permian Basin is influenced by salt movement and the structural development of various types of salt structures. The compilation presented here was used to characterize the following syndepositional growth stages of the salt structures: (a) “phase of initiation”; (b) phase of fastest growth (“main activity”); and (c) phase of burial’. We have also mapped the spatial pattern of potential mechanisms that triggered the initiation of salt structures over the area studied and summarized them for distinct regions (sub‐basins, platforms, etc.). The data base compiled and the set of maps produced from it provide a detailed overview of the spatial and temporal distribution of salt tectonic activity enabling the correlation of tectonic phases between specific regions of the entire Southern Permian Basin. Accordingly, salt movements were initiated in deeply subsided graben structures and fault zones during the Early and Middle Triassic. In these areas, salt structures reached their phase of main activity already during the Late Triassic or the Jurassic and were mostly buried during the Early Cretaceous. Salt structures in less subsided sub‐basins and platform regions of the Southern Permian Basin mostly started to grow during the Late Triassic. The subsequent phase of main activity of these salt structures took place from the Late Cretaceous to the Cenozoic. The analysis of the trigger mechanisms revealed that most salt structures were initiated by large‐offset normal faults in the sub‐salt basement in the large graben structures and minor normal faulting associated with thin‐skinned extension in the less subsided basin parts.

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.

The Southern Central Graben is a prominent evaporite-influenced rift structure in the Central European North Sea region. The evolution of the graben probably started in the Permian followed by several minor rifting phases during the Mid to Late Triassic. After a phase of regional uplift from the latest Early to Mid Jurassic, a major rifting phase occurred during the Late Jurassic and lasted until Early Cretaceous. In the Late Cretaceous, the graben was partly inverted due to tectonic compression. The southern part of the graben, located in the German and Dutch North Sea sector, is characterized by a half-graben structure bounded by a complex normal fault system at the eastern flank. The internal structure of the graben is differentiated into several sub-basins. More importantly, the Post-Permian graben fill was affected by mobilization of the Upper Permian Zechstein evaporites. Numerous salt diapirs and adjacent depocentres developed due to salt redistribution in particular during the Jurassic and Cretaceous.Even if the overall tectonic and sedimentary evolution of the graben is well known, a detailed survey on the extensional phases and the influence of salt movement on the subsidence patterns has not yet been carried out. In this study, we intend to investigate these aspects based on interpretation of 2D and 3D seismic data and cross section restoration. With special focus on the Late Jurassic to Early Cretaceous main rift event, we aim to distinguish the effects of extensional tectonics and salt movement on the local subsidence patterns. In the study area (northern Dutch, German and southern Danish North Sea sectors), we mapped 15 stratigraphic Post-Permian horizons in total of which 9 belong to the Jurassic and Early Cretaceous. Based on the depth-converted seismic interpretations, depth and thickness maps were constructed. Subsidence analysis and 2D backstripping of the depocentres enabled us to constrain timing and spacing of the tectonic activity and the salt migration.The outcomes of this study show that depocentres at the graben edges are mainly controlled by fault movement at the base of the salt. In contrast, structural patterns of post-Permian layers in the graben interior are often decoupled from base-salt features, i.e. faults in the base do not penetrate into supra-salt layer. Instead the post-Permian strata is gently folded, affected by thin-skinned extension, and penetrated by salt diapirs. The subsidence patterns of the Mesozoic strata were controlled by differential movement of sub-salt faults and overprinted by movement of the underlying Zechstein evaporites. We suggest that salt migration and its influence on subsidence patterns were strongest during the Late Jurassic probably accompanied by the initiation and acceleration of salt diapir growth and the lateral migration of depocentres.

Current gravitational tectonics models illustrating the structural style of passive margin salt basins typically have domains of upslope extension and corresponding downslope contraction, separated by a domain of rather undeformed mid-slope translation. However, such a translational domain is rarely observed in natural systems where extensional and contractional structures may interfere in the mid-slope area. In this study, we use sandbox analogue modelling analyzed by 4D digital image correlation (DIC) to investigate how the pre-kinematic layer thickness, differential sediment loading and sedimentation rate control the structural evolution of translational domains. As in nature, experimental deformation is driven by slowly increasing gravitational forces associated with continuous basal tilting. The results show that a translational domain persists throughout the basin evolution when the pre-kinematic layer is evenly distributed, although a thin (1 mm in the experiment, 100 m in nature) pre-kinematic layer can render the translational domain relatively narrow when comparing to settings with a thicker (5 mm) pre-kinematic layer. In contrast, early differential sedimentary loading in the mid-slope area creates minibasins intervened by salt diapirs overprinting the translational domain. Similarly, very low sedimentation rate (1 mm per day in the experiment, equates to

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 formation of salt structures exerted a major influence on the evolution of subsidence and sedimentation patterns in the Gluckstadt Graben, which is part of the Central European Basin System and comprises a post-Permian sediment thickness of up to 11 km. Driven by regional tectonics and differential loading, large salt diapirs, salt walls and salt pillows developed. The resulting salt flow significantly influenced sediment distribution in the peripheral sinks adjacent to the salt structures and overprinted the regional subsidence patterns. In this study, we investigate the geometric and temporal evolution of salt structures and subsidence patterns in the central Gluckstadt Graben. Along a key geological cross section, the post-Permian strata were sequentially decompacted and restored in order to reconstruct the subsidence history of minibasins between the salt structures. The structural restoration reveals that subsidence of peripheral sinks and salt structure growth were initiated in Early to Middle Triassic time. From the Late Triassic to the Middle Jurassic, salt movement and salt structure growth never ceased, but were faster during periods of crustal extension. Following a phase from Late Jurassic to the end of the early Late Cretaceous, in which minor salt flow occurred, salt movement was renewed, particularly in the marginal parts of the Gluckstadt Graben. Subsidence rates and tectonic subsidence derived from backstripping of 1D profiles reveal that especially the Early Triassic and Middle Keuper times were periods of regional extension. Three specific types of salt structures and adjacent peripheral sinks could be identified: (1) Graben centre salt walls possessing deep secondary peripheral sinks on the sides facing away from the basin centre, (2) platform salt walls, whose main peripheral sinks switched multiple times from one side of the salt wall to the other, and (3) Graben edge pillows, which show only one peripheral sink facing the basin centre.

Salt flow in sedimentary basins is mainly driven by differential loading and can be described by the concept of hydraulic head. A hydraulic head in the salt layer can be imposed by vertically displacing the salt layer (elevation head) or the weight of overburden sediments (pressure head). Basement faulting in salt-bearing extensional basins is widely acknowledged as a potential trigger for hydraulic heads and the growth of salt structures. In this study, scaled analogue experiments were designed to examine the kinematics of salt flow during the early evolution of a salt structure triggered by basement extension. In order to distinguish flow patterns driven by elevation head or by pressure head, we applied a short pulse of basement extension, which was followed by a long-lasting phase of sedimentation. During the experiments viscous silicone putty simulated ductile rock salt, and a PVC-beads/quartz-sand mixture was used to simulate a brittle supra-salt layer. In order to derive 2-D incremental displacement and strain patterns, the analogue experiments were monitored using an optical image correlation system (particle imaging velocimetry). By varying layer thicknesses and extension rates, the influence of these parameters on the kinematics of salt flow were tested. Model results reveal that significant flow can be triggered in the viscous layer by small-offset basement faulting. During basement extension downward flow occurs in the viscous layer above the basement fault tip. In contrast, upward flow takes place during post-extensional sediment accumulation. Flow patterns in the viscous material are characterized by channelized Poiseuille-type flow, which is associated with subsidence in regions of "salt" expulsion and surface uplift in regions of inflation of the viscous material. Inflation of the viscous material eventually leads to the formation of pillow structures adjacent to the basement faults (primary pillows). The subsidence of peripheral sinks adjacent to the primary pillow causes the formation of additional pillow structures at large distance from the basement fault (secondary pillows). The experimentally obtained structures resemble those of some natural extensional basins, e.g. the North German Basin or the Mid-Polish Trough, and can aid understanding of the kinematics and structural evolution of sedimentary basins characterized by the presence of salt structures.

Basement faulting is widely acknowledged as a potential trigger for salt flow and the growth of salt structures in salt-bearing extensional basins. In this study, dynamically scaled analogue experiments were designed to examine the evolution of salt pillows and the kinematics of salt flow due to a short pulse of basement faulting and a long-lasting phase of successive sedimentation. Experiments performed in the framework of this study consist of viscous silicone putty to simulate ductile rock salt, and a PVC-beads-quartz sand mixture representing a brittle supra-salt layer. In order to derive 2-D incremental displacement and strain patterns, the analogue experiments were monitored by an optical image correlation system (Particle Imaging Velocimetry). By varying layer thicknesses and extension rates, the influence of these parameters on the kinematics of salt flow were tested. Model results reveal that significant strain is triggered in the viscous layer by minor basement faulting. During basement extension downward flow occurs in the viscous layer above the basement fault tip. In contrast, upward flow takes place during post-extensional sedimentation. Lateral redistribution of the viscous material during post-extensional sedimentation is associated with subsidence above the footwall block and uplift adjacent to the basement faults leading to the formation of pillow structures (primary pillows). Decoupled cover faulting and the subsidence of peripheral sinks adjacent to the primary pillow causes the formation of additional pillow structures at large distance from the basement fault (secondary pillows). Experimental results demonstrate that the development of salt pillows can be triggered by basement extension, but requires a phase of tectonic quiescence. The potential for pillow growth and the displacement rate in the viscous layer increase with increasing thickness of the viscous layer and increasing extension rate, but decrease with increasing thickness of the overburden. The experimentally obtained structures resemble those of some natural extensional basins, e.g. the North German Basin or the Mid-Polish Trough, and can help to understand the kinematics during the structural evolution.

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