Abstract A series of analogue models were run to investigate oblique inversion of pre-existing grabens when overprinted by later shortening and the effect of these grabens on development of contractional structures. Obliquity angle ( α ) defining the initial trend of pre-existing grabens relative to the shortening direction, was systematically changed from 0°, 10°, 20°, 30°, 40°, 50°, 65° and 90°. Different structural styles are shown in different models and also in sections cutting across different parts of the models. Model results show that existence of multi-grabens enhances lateral discontinuity of overprinted thrusts in map view. With increasing the obliquity angle, more and longer lateral ramps developed sub-parallel to the graben trends. The pre-existing grabens were apparently rotated from their initial trends during shortening. Some of the normal faults bounding the grabens were partially inverted and resulted in bulging of the syn- and post-rift graben fill sediments. Most normal faults were displaced and rotated by thrusting, and provided relatively weak zones for propagation of thrusts. By comparing with observations from Qingxi graben in western China and from the SW Taiwan fold-and-thrust belt, where oblique inversion occurred, model results can be used to interpret unclear relationships between thrusts and pre-existing extensional structures during superimposed deformation.

Abstract Salt diapirs preferably rise above basement faults in extensional basins. A series of analogue and numerical models were developed in order to assess the supply of salt from the footwall and hanging wall to a diapir and to study the influence of basin inversion on the diapir development. The modelling scenario was based on the Kłodawa Salt Structure evolution (central Poland). The experiments show that the ductile material derived from the footwall constitutes the dominant portion of the diapir developed due to model extension, and this material occurs both in the footwall and hanging wall parts of the diapir. Shortening of the analogue models resulted in thinning of the diapir and shifting its stem onto the footwall. Ductile material become redistributed inside the diapir, however footwall material still prevails in the diapir structure. Results from the numerical models show that the magnitude of the basement fault governs the amount of salt supply to a diapir across the fault and that there is a differential salt supply from the hanging wall and footwall with time.

Abstract Stratigraphical, sedimentological and structural data and a Bouguer gravity map of Medjez-El-Bab (MEB) in Northern Tunisia are used to illustrate a Cretaceous example of salt extrusion on a passive continental margin. Located just south of the Teboursouk thrust front (a preferential décollement surface used by the continuous Tertiary shortening in this area), the MEB structure is a simple N40°E box anticline. Removing the two Tertiary foldings (Eocene and Miocene) leads to the exposure of the original feature of a simple submarine ‘salt glacier’. The Triassic salt rocks appear as an Albian interstratified body between two Cretaceous series with stratigraphic normal polarity, suggesting a bedding parallel extrusion (at the sediment–water interface) of the Triassic salt in Cretaceous times. The formation of such salt extrusions are associated with extensional faulting (probably both in the cover and basement), the presence of a slope and basinwards salt flow. This scenario is similar to the allochthonous salt described in other salt provinces, characterizing passive margins.

Abstract We report a direct comparison of scaled analogue experiments to test the reproducibility of model results among ten different experimental modelling laboratories. We present results for two experiments: a brittle thrust wedge experiment and a brittle-viscous extension experiment. The experimental set-up, the model construction technique, the viscous material and the base and wall properties were prescribed. However, each laboratory used its own frictional analogue material and experimental apparatus. Comparison of results for the shortening experiment highlights large differences in model evolution that may have resulted from (1) differences in boundary conditions (indenter or basal-pull models), (2) differences in model widths, (3) location of observation (for example, sidewall versus centre of model), (4) material properties, (5) base and sidewall frictional properties, and (6) differences in set-up technique of individual experimenters. Six laboratories carried out the shortening experiment with a mobile wall. The overall evolution of their models is broadly similar, with the development of a thrust wedge characterized by forward thrust propagation and by back thrusting. However, significant variations are observed in spacing between thrusts, their dip angles, number of forward thrusts and back thrusts, and surface slopes. The structural evolution of the brittle-viscous extension experiments is similar to a high degree. Faulting initiates in the brittle layers above the viscous layer in close vicinity to the basal velocity discontinuity. Measurements of fault dip angles and fault spacing vary among laboratories. Comparison of experimental results indicates an encouraging overall agreement in model evolution, but also highlights important variations in the geometry and evolution of the resulting structures that may be induced by differences in modelling materials, model dimensions, experimental set-ups and observation location.

Abstract Results of scaled sandbox models, containing three viscous layers located at different geographic and stratigraphic Levels simulating three evaporitic units in the South Pyrenean Triangle Zone, and interpreted field data are presented here to explain structural variation and kinematics in shortened areas containing multiple weak horizons acting as detachments. In the Southern Pyrenean Triangle Zone, the Beuda, Cardona and Barbastro thrust fronts have similar geometric features to those developed in the models, suggesting that they could have formed and evolved in a similar way. These deformation fronts are not always perpendicular to the regional shortening direction. Instead, their direction is governed by the initial pinch-out of the viscous horizons. Model results show that triangle zones form when: (1) deformation is transferred to weak horizons located at higher stratigraphic levels, and (2) the deformation front reaches the pinch-out of the weak horizons. Model results also show that the rheology of the detachment horizons controls the geometry of the deformation front. Weak detachments (Cardona Formation, and pure silicone in the models) promote folding and back-vergent structures, and thus formation of triangle zones at the deformation front, irrespective of the location of the thrust front relative to the pinch-out of the viscous detachment. However, over strong (more viscous) detachments (Barbastro and Beuda formations, and impure silicone in the models), folds that eventually evolve to thrusts are dominant. In such cases, backthrusts form only at the pinch-out of the detachment layer. In cases where no viscous detachment is present, no backthrusts form, and therefore the thrust front does not develop a triangle zone geometry. Instead, a foreland-vergent piggyback sequence of thrusts forms. Model results show that the stratigraphic level of a detachment governs size, geometry and spacing of the imbricates formed above it.

ABSTRACT Dynamic restoration is achieved when one accounts for the changes that occur in area or volume during deformation. In contractional areas, layer-parallel shortening (LPS) cannot always be easily estimated or measured, although it is a significant component of deformation, as is gravitational compaction. Five model analogs with known initial dimensions and boundary conditions were shortened from one end. Profiles of these models were used to (1) estimate the amount of layer-parallel compaction (LPC), the main modality of layer-parallel shortening in granular analog materials; (2) outline variation of LPC with depth, lateral location, and percentage shortening; and (3) estimate the effect of lithology on LPC. During progressive deformation, a modeled accretionary wedge, which formed during the shortening of the models, did not undergo homogeneous compaction; instead, loss of area varied in both space (with depth and laterally) and time. Balancing the area of sequential sections of one of the sand models, which was shortened above a high-friction basal děcollement, shows that the layers experienced tectonic compaction during deformation and lost as much as 17% of their cross-sectional area during 50% bulk shortening. Restoration of two model profiles shows that LPC is three times greater in the model with high basal friction than in the model with low basal friction. In models where a sand layer was embedded within a viscous layer (a Newtonian material simulating rock salt), the layer accommodated all the shortening by folding and underwent no significant LPC. Examples from the Spanish Pyrenees are used to illustrate the significance of LPS in restoring profiles of contractional areas. In the eastern Spanish Pyrenees, on the basis of deformed raindrop marks and burrows, from 16 to 23% of total shortening is estimated to be by LPS, whereas only 6 to 10% of the total shortening is accommodated by folding. Model results illustrate the lateral and temporal variations of penetrative strain within shortened layers. Outlining this heterogeneous distribution of penetrative strain and any associated volume loss is important in distinguishing areas of reduced porosity, which are significant for hydrocarbon exploration.

ABSTRACT The transpressional (or transtensional) model of Sanderson and Marchini considered a horizontal displacement to be oblique to the walls of a vertical tabular shear zone that has no extrusion along the strike direction of the shear zone. In a more general model that includes lateral extrusion, the nature of deformation becomes much more complex. This chapter analyzes such a tabular shear zone in terms of simultaneous simple shearing and coaxial straining. In such a deformation, one of the principal axes of the strain ellipsoid is always vertical and parallel to the vorticity vector and the other two lie on the horizontal plane. I present a new approach for analyzing and classifying transpressional (and transtensional) tectonism. The classification enables us to compare the degree of similarity of the deformation types and to visualize their relation with the kinematic vorticity number. It has been shown that a transpression does not always produce a flattening. Similarly, a transtension does not always produce a constriction. The history of progressive deformation may be complex, involving switching of the direction of maximum stretching, development of a cleavage and its folding in the same continuous deformation, and folding of early boudin lines in later stages of deformation.

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