ALBERT

Description: 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 brittleviscous 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-viscious 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.

Description: 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.

Description: 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 Klodawa 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.

Description: 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 horizonts. 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 horizonts 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.

Description: Protrusions and lenses of serpentinite-matrix melanges occur at several places along the thrust faults of the Zagros Suture Zone. They separate the lower allochthonous thrust sheet, the Lower Allochthon' (i.e. Walash-Naopurdan nappe), of Paleocene-Eocene age from sediments of the Arabian platform and the upper thrust sheet of Mesozoic, ophiolite-bearing terranes termed the Upper Allochthon' (i.e. Gemo-Qandil nappe). The serpentinite-matrix melanges occur mostly as stretched bodies (slices) on both sides of the Lower Allochthon (Hero, Halsho and Pushtashan (HHP) and Galalah, Qalander and Rayat (GQR)). Their overall chondrite-normalized rare earth element (REE) patterns form two main groups. Group One exhibits enrichment in the total REEs (〉 1 x chondrite) whereas the Group Two pattern shows depletion (i.e. 〈 1 x chondrite). Bulk-rock MORB-normalized profiles of Group Two are almost flat in the MREE-HREE region with flattening profiles in the Gd-Lu range (〉 3 times the MORB composition). In comparison with Group One, Group Two has extremely high REE content and displays variable depletions in the moderately incompatible high-field-strength elements (HFSEs) (Zr, Hf, Y) relative to their adjacent REEs. The REEs in the GQR serpentinite-matrix melanges have a noticeably high LREE content, and a positive Eu anomaly, and their HREE content never reaches more than 1 x chondrite (i.e. 〈 0.01 to 1 x chondrite). The latter indicates that the hemipelagic sedimentary, melt-like components (i.e. high LREE, U/La, La/Sm and low Ba/Th) control the geochemical peculiarities of this type of serpentinite. The HHP serpentinite-matrix melanges, however, are either equally divided between the two REE pattern groups (e.g. Hero, Halsho) or inclined towards Group One (e.g. Pushtashan). Contrary to GQR serpentinites, the variation in HHP serpentinite-matrix melanges spans a compositional spectrum from U/La-rich to more Ba/Th-rich. Such ratio variations reflect the large variation in these two subducted sedimentary components (i.e. carbonate and hemipelagic sediment mix). The obvious differences in the trace element signatures of the GQR and HHP serpentinite-matrix melanges might be related to plate tectonic parameters such as convergence rate, subduction age and thickness and type of subducted slab. It is more likely that the influx of subducted components to the mantle wedge relied heavily on the composition of the sedimentary inputs. These vary considerably with time from the relatively deepwater hemipelagic sediments (Qulqula Radiolarite Formation) to platform carbonate sediments (Balambo limestone). The trace element signatures of the GQR and HHP serpentinite-matrix melanges might suggest multi-staging of the allochthonous sheet emplacement on the Arabian platform sediments.

Description: Accessory chrome spinels are scattered throughout the serpentinite masses in two allochthonous thrust sheets belonging to the Penjween-Walash sub-zone of the northwestern Zagros Suture Zone in Kurdistan. Based on field evidence, the serpentinites are divided into two groups: (1) highly sheared serpentinites (110-80 Ma), which occupy the lower contact of the ophiolitic massifs of the Upper Allochthonous sheet (Albian-Cenomanian age), and (2) ophiolitic melange serpentinites of mixed ages (150 and 200 Ma) occurring along thrust faults on the base of the volcano-sedimentary segment (42-32 Ma) of the Lower Allochthonous sheet. The Cr-spinels of both groups show a wide range of YCr (Cr/(Cr + Al) atomic ratio) from 0.37 to 1.0, while the XMg (Mg/(Mg + Fe2+) atomic ratio) ranges from 0.0 to 0.75. Based on the Cr-spinel compositions of the entire dataset and in conjunction with back-scattered electron imaging, from core to rim, three spinel stages have been recognized: the residual mantle stage, a Cr-rich stage and a third stage showing a very narrow magnetite rim. These three stages are represented by primary Cr-spinel, pre-serpentinization metamorphosed spinel and syn- or post-serpentinization spinel, respectively. The chemical characteristics of primary (first-stage) Cr-spinels of both serpentinite groups indicate a tectonic affinity within a fore-arc setting of peridotite protoliths. The second stage indicates that Cr-spinels have undergone subsolidus re-equilibration as a result of solid-solid reaction during pre-serpentinization cooling of the host rock. Here the primary Cr-spinel compositions have been partly or completely obscured by metamorphism. During the third stage, the Cr-spinels have undergone solid-fluid re-equilibration during syn- or post-serpentinization processes. Both the second and third stages point to diachronous metamorphic paths resulting from continuous tectonic evolution influenced by either slow or fast uplift of mantle protoliths. In the fast metamorphic paths, the primary chrome spinels are flanked by a very narrow magnetite rim. The presence of two groups of distally separated serpentinites with different emplacement ages and fore-arc tectonic affinity could indicate that the closure of the Tethys Ocean culminated in two fortuitous subduction processes.

Description: Fingerlike bodies of evaporite rocks have been observed in many regions affected by multiple tectonic phases, such as the Atlas, the Pyrenees, or the Zagros Mountains, where they have been interpreted as fault-plane diapiric injections or collapse-related salt welds. In this article, we suggest a new interpretation of these structures as squeezed diapirs based on detailed structural and sedimentological field data of the Bicorb–Quesa diapir (eastern Prebetics) and offer five analog models, which simulate the polyphase deformation of the eastern Prebetics. In this area, diapirs formed from the Oligocene to Langhian during an extensional phase related to the opening of the Valencia Trough. These diapirs were later affected by a Serravallian contractional phase, which inverted the preexisting grabens and created new folds and thrusts. The preexisting diapirs were necked and/or squeezed, forming secondary welds with a fingerlike geometry that isolated the diapir bulbs from their source layer. Extrusion of diapiric material was also accelerated during this phase, but no new diapirs formed. Finally, the area was again affected by an extensional phase during the Tortonian, which reactivated the normal faults and created a new set of diapirs. The new diapirs formed where the overburden was thinner, that is, at the toe of the major reactivated faults. Commonly, these faults coincide with the bounding faults of the major grabens formed during the first extensional phase, and therefore, the new diapirs grow close to the location of the squeezed diapirs. The models also show that the faults created during the initial extension prevailed as the main focus for deformation during the polyphase history. Deformation in the overburden and the viscous layer was mainly accommodated along the major grabens formed during the first extensional stage. During shortening, the initial major grabens deformed as complex anticlines, and during the subsequent extensional phase, most deformation occurred by the collapse of these anticlines along preexisting faults, fault welds, and the flank of the squeezed diapirs. The source layer is compartmentalized, accumulating and withdrawing material in the same locations (the initial grabens and horst). As a result, the source layer is easily depleted beneath the initial horst, forming primary welds. Eduard Roca is a professor of structural geology at the Universitat de Barcelona. He received his Ph.D. in 1992 from the Universitat de Barcelona, and then he worked for the Institut Français du Pétrole in Paris for one year until he joined the Universitat de Barcelona. His research interests include the structure of thrusts and fold belts, salt tectonics, tectonosedimentary relationships, and the tectonics of the western Mediterranean.Maura Sans received her M.Sc. degree in 1992 and her Ph.D. in 2000 from the Universitat de Barcelona. She worked on salt tectonics from 1990 to 2001 in extensional settings in the western Mediterranean and in contractional settings in the Pyrenees in collaboration with the potash mining companies in the Pyrenean foreland. Since 2001, she has been working as an independent consultant. Hemin Koyi is a professor in tectonics and geodynamics at the Department of Earth Sciences in Uppsala. He is a native of Kurdistan (Iraq), where he received his B.Sc. degree. He received his M.Sc. degree and his Ph.D. in tectonics and geodynamics from Uppsala University (Sweden). He was a research fellow at the Bureau of Economic Geology (Austin, Texas) between 1991 and 1993. His main research interest is the modeling of geologic processes with special emphasis on salt and thrust tectonics. He is a member of the editorial board of the Journal of Petroleum Geology and the consulting editor's board of the Geological Quarterly .