Analogue Model Study Research Articles

Salt-pillow formation during inversion of evaporite-filled half graben – Insights from seismic data interpretation and integrated analogue-numerical modelling

The Mid-Polish Anticlinorium is a regional structure that formed during Late Cretaceous inversion of the Permian-Mesozoic Polish Basin. Within the anticlinorium, local salt pillows built of the Upper Permian (Zechstein) evaporites are often located above reverse faults that accommodated basement inversion. Seismic data from the Szubin area in central Poland were used to guide a combined analogue and numerical modelling study to test whether locally thicker evaporites deposited within a half-graben, could indeed give rise to a salt pillow formed above the half-graben's hanging wall during its inversion. The results of the two approaches are internally consistent and prove that such a genetic relationship is fully viable, with the most important variables being the size of the half-graben, the viscosity of the salt, and the presence of any erosion of the pillow structure. Thus, the existence of salt pillows along the major inversion zones of the Mid-Polish Anticlinorium could possibly be used as indicators for the location of syn-depositional half-grabens during deposition of the Zechstein evaporites. This in turn might suggest that even in the basin center, shallower areas might have existed above the footwalls of such half-graben during Zechstein deposition, characterized by smaller thicknesses and somewhat different facies arrangement. Similar concepts likely apply to other intracontinental salt basins that experienced rifting and then inversion.

The link between Somalian Plate rotation and the East African Rift System: an analogue modelling study

Abstract. The East African Rift System (EARS) represents a major tectonic feature that splits the African continent between the Nubian Plate situated to the west and the Somalian Plate to the east. The EARS comprises various rift segments and microplates and represents a key location for studying rift evolution. Researchers have proposed various scenarios for the evolution of the EARS, but the impact of continent-scale rotational rifting, linked to the rotation of the Somalian Plate, has received only limited attention. In this study we apply analogue models to explore the dynamic evolution of the EARS within its broader rotational-rifting framework. Our models show that rotational rifting leads to the lateral propagation of deformation towards the rotation axis, which reflects the general southward propagation of the EARS. However, we must distinguish between the propagation of distributed deformation, which can move very rapidly, and localized deformation, which can significantly lag behind the former. The various structural-weakness arrangements in our models (simulating the pre-existing lithospheric heterogeneities that localize rifting along the EARS) lead to a variety of structures. Laterally overlapping weaknesses are required for localizing parallel rift basins to create rift pass structures, leading to the rotation and segregation of microplates such as the Victoria Plate in the EARS, as well as to the simultaneous north- and southward propagation of the adjacent Western Rift. Additional model observations concern the development of early pairs of rift-bounding faults flanking the rift basins, followed by the localization of deformation along the axes of the most developed rift basins. Furthermore, the orientation of rift segments with respect to the regional (rotational) plate divergence affects deformation along these segments: oblique rift segments are less wide due to a strike-slip deformation component. Overall, our model results generally fit the large-scale present-day features of the EARS, with implications for general rift development and for the segregation and rotation of the Victoria Plate.

An Analog Model Study on Water–Sand Mixture Inrush Mechanisms During the Mining of Shallow Coal Seams

An Analog Model Study on Water–Sand Mixture Inrush Mechanisms During the Mining of Shallow Coal Seams

Sedimentological signatures of river-shelf processes in a wave-dominated delta front

A wave-dominated delta depositional model is characterized by the combined action of riverine-induced processes and sediment reworking by waves and currents. The combination of these natural-driven processes will lead to the formation of a delta-associated sedimentary record that can potentially register changes in prevailing oceanographic and hydrological conditions, including episodic or extreme events, and also anthropogenic-induced changes to the watershed and sediment load. The main objective of this study is to recognize the sedimentological signatures associated with distinct riverine-inner shelf processes in a wave-dominated delta setting. The study is based on high resolution grain size investigation, applying a grain size x standard deviation analysis. The results allowed to differentiate a typical riverine sediment deposit from the sedimentological signature of episodic or more responsive events related to higher flooding and storm-induced wave/current processes. The delta-associated inner shelf deposits vary according to their distance and orientation from the river mouth and water depth (south-north). This indicates that inner shelf sedimentary records show the spatial influence of riverine plume dispersion/deposition, and marine processes (waves resuspension, coastal currents and tides). These results can be considered for analogue model studies, but also to a first assessment related to medium to long term sedimentation pattern and riverine particulate contaminant dispersion and deposition.

Passive margin inversion controlled by stability of the mantle lithosphere

Contractional deformation of passive continental margins may resemble early stages of induced subduction initiation. Mechanical instabilities are required to permit underthrusting of the oceanic plate. Therefore, the success of developing a new subduction zone will largely depend on the rheology of the mantle lithosphere. In this physical analogue modelling study, a range of mantle viscosities subject to different convergence rates serve as a proxy to simulate contraction of differently aged passive margins with the purpose of describing and quantifying passive margin deformation and mantle stability using the buoyancy number. The experiments illustrate distinct differences in geometry and length-scale of deformation as a function of lithospheric mantle strengths. The results indicate that deformation occurs at passive margins for intermediate strength lithospheric mantle and high strain rate. In contrast, low and high strength mantle lithospheres lead to dominantly intra-oceanic deformation or long wavelength buckling of the entire model, respectively. Our experiments portray an evolution in which early-stage deformation commences at the ocean-continent transition and is controlled by the ductile lower crust of the continent. In the next stage, shear localization through the formation of a decollement within the ductile passive margin crust favors underthrusting of the oceanic lithosphere, leading to a reduction of the area affected by deformation. Prior to underthrusting, the primary response of the lithosphere to compression is by folding at scaled wavelengths of 100–300 km and 500–1000 km, controlled by the strength of the mantle lithosphere.

3D-thrust fault pattern control on negative inversion: An analogue modelling perspective on central Italy

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.

Strain localization mechanisms for subduction initiation at passive margins

It is widely accepted that subduction initiation at modern Earth passive margin systems critically depends on the buoyancy and strength of the oceanic lithosphere and requires failure of the load-bearing crustal and mantle layers. As such, subduction initiation upon orthogonal convergence is controlled by the age of the oceanic lithosphere and thus the strength contrast at the margin. However, it is still unclear where along the margin and how subduction initiates. In particular, rheologically-controlled mechanisms are poorly understood and require further investigation. Therefore, this combined analogue and numerical modelling study aims at exploring the effects of first order rheological and kinematic conditions on subduction initiation at passive margins. Our results highlight the sensitivity of early stages of subduction initiation to the initial rheological setup as well as evolving thermo-mechanical feedback mechanisms. Additionally, they provide more insights on the conditions responsible for the locus of subduction initiation. We infer that the locus of subduction is controlled by the rheology of both the crust and the mantle lithosphere at the margin, which is in strong correlation with the thermal age of the oceanic lithosphere. Suitable conditions for subduction initiation at passive margins correspond to an intermediate age (ca. 80 Myr) of the oceanic lithosphere. In all other cases deformation localizes within the oceanic lithosphere or affects the entire continent. We advocate the significance of crust-mantle decoupling at the passive margin for its inversion and possible evolution towards a subduction zone. Additionally, the development of a self-sustaining subduction zone will fail in the absence of weakening mechanisms taking place in the mantle lithosphere such as thermal softening (shear heating) and low temperature plasticity (Peierls mechanism). • Analogue and numerical models are used to infer favorable mechanical conditions for subduction initiation at passive margins. • Ductile continental crust is a key element for localizing deformation during the early stages of subduction initiation. • Subduction initiation is controlled by the strength of the mantle lithosphere at the margin. • Evolution towards sustainable subduction is controlled by both Peierls creep mantle rheology and shear heating.

Analogue modelling of marginal flexure in Afar, East Africa: Implications for passive margin formation

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.

Integration of Cretaceous Morro do Chaves rock properties (NE Brazil) with the Holocene Hamelin Coquina architecture (Shark Bay, Western Australia) to model effective permeability

Coquinas are significant producing facies in the pre-salt reservoirs of Brazil and Angola. This paper describes an analogue model study to help understand the reservoir characterization of coquina reservoirs. Porosity and permeability measured in 39 core plugs from a single coquina bed in a Lower Cretaceous outcrop in NE Brazil have been combined in a three-dimensional geological model for use in reservoir simulation. A training image derived from the geometry of a Holocene coquina analogue in Western Australia controlled the distribution of the bed-scale petrophysical properties from the outcrop. A synthetic well test showed that the effective permeability of the system lies between the geometrical and harmonic averages of the plug data in the layer. There is also moderate flow anisotropy, with preferential fluid flow aligned along the beach ridges. This paper demonstrates how the combination of outcrop data and an appropriate modern environment might be used to improve our understanding of the behaviour of coquina reservoirs and to guide future reservoir studies.

Structure of the actively deforming fold-thrust belt of the St. Elias orogen with implications for glacial exhumation and three-dimensional tectonic processes

Previous studies in the Yakataga fold-thrust belt of the St. Elias orogen in southern Alaska have demonstrated high exhumation rates associated with alpine glaciation; however, these studies were conducted with only a rudimentary treatment of the actual structures responsible for the deformation that produced long-term uplift. We present results of detailed geologic mapping in two corridors across the onshore fold-thrust system: the Duktoth River transect just west of Cape Yakataga and the Icy Bay transect in the Mount St. Elias region. In the Duktoth transect, we recognize older, approximately east-west–trending structures that are overprinted by open, northwest-trending fold systems, which we correlate to a system of northeast-trending, out-of-sequence, probably active thrusts. These younger structures overprint a fold-thrust stack that is characterized by variable structural complexity related to detachment folding along coal-bearing horizons and duplexing within Eocene strata. In the Icy Bay transect, we recognize a similar structural style, but a different kinematic history that is constrained by an angular unconformity at the base of the syntectonic Yakataga Formation. At high structural levels, near the suture, structures show a consistent northwest trend, but fold-thrust systems rotate to east-west to northeast trends in successively younger structures within the Yakataga Formation. We present balanced cross sections for each of these transects where we project the top of basement from offshore seismic data and assume a subsurface structure with duplex systems similar to, but simplified from, structures observed in the onshore transects. These sections can account for 150–200 km of shortening within the fold-thrust system, which is Our section restorations also provide a simple explanation for the observed elongate bullseye pattern of low-temperature cooling ages in the thrust belt as a consequence of exhumation above the growing duplex and/or antiformal stack. Comparison with analog model studies suggests that structural feedbacks between erosion and development of decollement horizons in coal-bearing strata led to this structural style. Although previous studies based on thermochronology suggested an active backthrust at the northern edge of the thrust belt, section restorations indicate that a backthrust is allowable but not required by available data. The Yakataga fold-thrust belt has been treated as a dominantly 2D system, yet our work indicates that 3D processes are prominent. In the Duktoth transect, we interpret a group of northeast-trending thrusts as younger, out-of-sequence structures formed in response to the rapid destruction of the orogenic wedge by glacial erosion and deposition immediately offshore. We infer that these northeast-trending thrusts transfer slip downdip into a duplex system that forms the antiformal stack modeled in cross-section restorations, and we infer that these structures represent thrusting stepping back from the active thrust front attempting to rebuild an orogenic wedge that is being destroyed as rapidly as, or more rapidly than, it is being rebuilt. In the Icy Bay transect, we use the relative chronology provided by an angular unconformity beneath the syntectonic Yakataga Formation to infer that early, northwest-trending fold-thrust systems were formed along the Fairweather transform as transpressional structures. Continued strike slip carried these structures into the tectonic corner between the Fairweather and Yakataga segments of the orogen, producing a counterclockwise rotation of the shortening axis until the rocks reached their present position.

The effects of basement ramps on deformation of the Prebetics (Spain): A combined field and analogue modelling study

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.

Analogue modelling of a reactivated, basement controlled strike-slip zone, Sierra de Albarracín, Spain: application of sandbox modelling to polyphase deformation

Abstract This paper presents the results of an analogue modelling study on the reactivation of Riedel shears generated by basement-induced sinistral strike-slip faulting. It is based on a natural example in the Sierra de Albarracín, Iberian Range (Spain). The area has a polyphase deformation history, defined by the Variscan and Alpine orogenies. Late Variscan deformation was concentrated in a wide NW-SE shear zone with accompanying kilometre-scale E-W Riedel shears, which divided the Palaeozoic basement into large fault blocks. Alpine reactivation resulted in differential movements on the Riedel shears, as evidenced by a NW-SE chain of Palaeozoic inliers surrounded by a Mesozoic cover that generally shows minor deformations except near the E-W Riedel shears, where strata locally appear in near-vertical to overturned position. Sandbox analogue modelling was applied to improve insight into the structural history. It focused on the kinematics of spontaneously developed en echelon Riedel shears, reactivated in a rotated stress field. Sand with a controlled added strength was used to form Riedel shears in a first deformation phase to act as weak zones for a second phase. The modelling showed that in the first deformation phase large pop-up structures developed between the Riedel shears in a basement-induced sinistral strike-slip zone. Later reactivation in the N060°E and N135°E shortening directions was taken up respectively by sinistral-reverse and dextral-reverse shear along the pre-existing Riedel shears, but only if the sand on one side of the fault zone was allowed to move freely along the other. Scissor faulting along the Riedel shears with their complex 3D-geometry increased the height of the up-squeezed blocks. For experiments with fixed boundaries and no oil-water emulsion layer between the base plate and sand pack, thrusting at the backstop occurred rather than reactivation of the Riedel shears. This approach provided robust insights on the 4D development of the Sierra de Albarracín area.

Ice-flow dynamics and meteorite collection in Antarctica

Typical targets for meteorite searches in Antarctica are blue ice fields in front of major bedrock obstacles, where the meteorite-bearing ice slows down, is uplifted and exhumed by ablation. However, in some areas collection occurs in the lee of topographic barriers, as documented in the downstream side of the Frontier Mountain, a major emerged bedrock barrier. In this study we present an analogue modelling study of ice flow at Frontier Mountain, which has been chosen as example study area because an exceptionally complete set of glaciological data is available to investigate the causes leading to meteorite concentration. Results of modelling show how the boundary conditions (mainly the bedrock topography) control ice flow and how this flow, coupled to high ablation, may give rise to a stable meteorite trap downstream of major topographic obstacles. Modelling results support the contention that, as erosion is not a mountain builder, ablation does not represent a driving force in ice-flow dynamics, although it plays a significant role in shaving stranding surfaces and exhuming meteorites.

Viscous anisotropy of two-phase composites, and applications to rocks and structures

Anisotropy of viscosity in rocks has a significant effect on the development of geological structures, as demonstrated in several theoretical and analogue model studies. This paper develops the results of a self-consistent mechanics approach to modelling the bulk viscous properties of two-phase composites, to reveal the strength of the viscous anisotropy. Anisotropy factor, δ, is defined as the ratio of normal to shear viscosity, μ N/ μ S. The value of δ is determined for different types of composites of two viscous phases with varying viscosity ratio ( m=5, 10, 100), ranging from bilaminates to those with elliptical shape fabrics given by axial ratio, R. Bilaminates with equal layer thickness are the most highly anisotropic systems, where δ simply depends on m. These results are applied to rocks, modelled as two-phase composites of competent and incompetent lithologies, with the aim of quantifying the viscous anisotropy that is associated with natural rock deformation and the formation of geological structures. It is deduced that m=5 or 10 is an appropriate maximum viscosity contrast for many metasedimentary rocks (e.g. psammite/pelite composites), but that m=100 might be effectively reached in limestone/marl pairs, or in rocks containing surfaces of slip or zones of recrystallisation or grain-boundary sliding. These m values give rise to maximum δ values of 3, 5.5 and 25.5, respectively. An anisotropy factor of δ≫25 is effectively a composite of a rigid and a viscous phase, and may only be achieved in solid–melt mixtures. Anisotropic rocks, whether layered or containing a strong shape fabric, will develop folds more easily than isotropic rocks. Characteristic structures are similar folds, chevron folds, kink bands and shear bands. Geometric features of folds, especially fold angularity, may provide methods of quantifying the viscous anisotropy of rocks and composites.

4-D evolution of rift systems: Insights from scaled physical models

The four dimensional (4-D) evolution of brittle fault systems in orthogonal, oblique, and offset rift systems has been simulated by scaled sandbox models using dry, cohesionless, fine-grained quartz sand. Extensional deformation in the models was controlled by the orientation and geometry of a zone of stretching at the base of the model. The results of these analog model studies are compared with natural examples of rift fault systems. Rift basins produced by orthogonal and oblique rifting are defined by segmented border fault systems parallel to the rift axes and by intrarift fault systems that are subperpendicular to the extension direction. Segmentation of the rift margin increases with increase in obliquity of the rift axis, resulting in a consequent increase in displacement on intrarift fault systems. Offset rift models are characterized by highly segmented border faults and offset subbasins in the rift zone. Along-strike displacement transfer in the model rifts occurred as a result of formation of two types of accommodation zones. High-relief, extension-parallel accommodation zones typically are found in 60 degrees rifts and above left steps in offset rift systems. Changes in fault polarities in these accommodation zones were achieved by interlocking arrays of conjugate extensional faults. The second type of accommodation zone was generally oblique to the extension direction and consisted of conjugate fault arrays having rotated tips that bounded a low-relief oblique-slip zone or grabens. These typically are found in highly oblique rift systems (<45 degrees) and above right steps in offset rift models.

Analogue model study of EM induction in elongated conductors—2D and 3D induction arrow responses

Magnetic induction arrow responses of elongated conductors embedded in a uniformly resistive host earth and underlain by a conductive substratum are studied with the aid of laboratory analogue model measurements. The dependence on period, the overburden depth, and the underlying conductive basement depth, are examined for three different conductor lengths that were chosen so as to include the transition in the responses from two dimensional (2D) to those of three dimensional (3D) structures. The characteristic period T c (the period at which the in-phase anomalous response of the elongated conductor model is maximum and the quadrature response is minimum) is found to depend on the conductor length, the overburden depth, and the conductive basement depth. A decrease in conductor length or conductive basement depth, each have the effect of decreasing both the response amplitude and the characteristic period, while increasing the overburden depth has the effect of increasing the characteristic period and decreasing the response amplitude. At short periods, say below the characteristic period, the responses appear to be roughly 2D, and to become progressively more 3D-like with increasing period above the characteristic period.

EM responses of an elongated conductor near an ocean — analogue model studies

Magnetic induction responses of an elongated conductor at sites near an ocean are studied with the aid of laboratory analogue model measurements. The dependence on the period, the distance from the ocean, and the depth extent of the conductor is examined for the case of the elongated conductor parallel to the ocean coastline. The Parkinson induction arrows at sites near the conductor reverse directions from pointing towards the conductor at short periods to pointing towards the ocean at longer periods, and the period at which the reversals occur differs for the in-phase and the quadrature arrows. It is also shown that except for sites right at the coastline and at the conductor, the responses of the conductor alone and the ocean alone can be added to yield the responses of the model that includes both the ocean and the conductor. These results have application to the subtraction of ocean induction effects from geomagnetic measurements in coastal regions to delineate the responses of any nearby elongated anomalous conductors.

Analogue model studies of induction effects at auroral latitudes

Abstract. In addition to field observations and numerical models, geomagnetic induction effects can be studied by scaled analogue model experiments. We present here results of analogue model studies of the auroral electrojet with an Earth model simulating the Arctic Ocean and inland conductivity structures in northern Fennoscandia. The main elements of the analogue model used were salt water simulating the host rock, an aluminium plate corresponding to the ocean and graphite pieces producing the inland highly conducting anomalies. The electrojet was a time-harmonic line current flowing at a (simulated) height of 100 km above northern Fennoscandia. The period simulated was 9 min. The analogue model results confirmed the well-known rapid increase of the vertical field when the coast is approached from the continent. The increase of the horizontal field due to induced ocean currents was demonstrated above the ocean, as well as the essentially negligible effect of these currents on the horizontal field on the continent. The behaviour of the magnetic field is explained with a simple two-dimensional thin-sheet model. The range, or the adjustment distance, of the ocean effect inland was found to be some hundreds of kilometers, which also agrees with earlier results of the Siebert-Kertz separation of IMAGE magnetometer data. The modelled inland anomalies evidently had too large conductivities, but on the other hand, their influence decayed on scales of only some tens of kilometers. Analogue model results, thin-sheet calculations, and field observations show that the induction effect on the horizontal magnetic field Bx near the electrojet is negligible. On the other hand, the vertical component Bz is clearly affected by induced currents in the ocean. Evidence of this is the shift of the zero point of Bz 0-1° southwards from the maximum of Bx. The importance of these results are discussed, emphasizing the determination of ionospheric currents.

Testing the Concept of Diapiric Rise and Fall Using 3D Seismic Data from the U.K. Southern Gas Basin: ABSTRACT

The U.K. Southern Gas Basin experienced a long and complicated developmental history-involving extension, salt tectonics, inversion, and subsidence-that began in the early Triassic and continued into the middle Tertiary. In our study areas, faulted Paleozoic rocks (Rotliegendes Fm. and older) are separated from a faulted overburden sequence (Bacton Grp. to Chalk) by the salt-dominated Zechstein Grp. Overburden structure is dominated by a series of broadly NW-SE trending graben and associated salt diapirs and walls. New insights into salt tectonics derived from scaled analog and numerical model studies, reveal that in areas with originally thick salt, regional extension of the overburden is important in triggering and controlling diapirism and graben growth. Diapiric piercement proceeds in a predictable series of modes-reactive, active, and passive. As extension continues, if for some reason the salt supply becomes restricted, the diapir can no longer grow and begins to fall. The diapir can then no longer support the graben floor and the graben collapses into the falling diapir. The change from diapiric rise to diapiric collapse is often marked by fundamental changes in sediment distribution patterns. We interpret graben and diapiric growth to be initiated by gravity-driven thin-skinned brittle extension of the overburden. The supply ofmore » salt to the developing diapirs eventually became restricted and continued extension lead to diapiric collapse. Collapse is marked by fundamental changes in sediment distribution patterns. While all graben continued to act as depocentres, many graben flanks, areas that were originally high, became syn-collapse depocentres; many integraben troughs, areas that had acted as depocentres during diapiric rise, became starved and received little sedimentation during diapiric collapse.« less

The geometries and kinematics of inverted fault systems: a review of analogue model studies

Abstract The geometries and kinematics of 2D inverted extensional fault systems are reviewed using the results of scaled physical models together with case histories from inverted basin fault systems. 2D analogue models of detached terranes, listric, planar, ramp/flat listric and domino arrays of planar faults were constructed from homogeneous sandpacks and from anisotropic sand/mica layers. The models were first extended and then subjected to horizontal compression in order to reactivate the extensional fault systems. Upon extension simple listric faults produce a characteristic roll-over anticline and crestal collapse graben. Inversion of this system produces reactivation of the main detachment with the development of a fault-bounded wedge of syn-extensional strata elevated above regional together with tightening of the crestal collapse graben. New thrust faults initiate from the tips of the crestal collapse extensional faults. Characteristic ‘harpoon’ structures develop associated with the reactivation of the main detachment. Similar inversion architectures were also produced for the planar fault systems. Extension of ramp-flat listric fault systems produces an upper roll-over and crestal collapse graben together with a ramp syncline and lower roll-over and crestal collapse graben. Inversion of this system only reactivates a part of the main detachment producing a hangingwall shortcut fault that bypasses the upper roll-over system. Inversion of domino fault arrays produced characteristic harpoon geometries of the syn-rift wedge together with shortcut faults in the footwalls of the main domino faults. Analysis of the progressive deformation during both extension and inversion has been carried out using marker points embedded in the models. In the analogue models of detached terranes hangingwall collapse during extension occurs along planar to curved shear surfaces, whereas upon inversion, different, lower angle shear trajectories are utilized. In the domino fault arrays more complicated shear paths are observed as a result of the footwall shortcut faults developed during inversion. These results have important implications for the analysis of extensional and inverted terranes and for fault reconstruction and section balancing techniques. The architecture of inverted fault systems developed in the analogue models show striking similarities to the inverted basin geometries found in natural fault systems such as those in the North Sea and South East Asia. Examples from these terranes are compared and contrasted with the results of the analogue models. Conceptual models for inversion kinematics and fault system architectures are presented.