Contents of this volume

This special volume of the Journal of the Virtual Explorer is dedicated to analogue modelling of large-scale tectonic processes and comprises nine papers. The contributions cover a wide range of tectonic settings, including strike-slip deformation, fold and thrust deformation, transpression, back-arc extension, rifting, diapir emplacement and subduction. Two papers present results of analogue experiments performed in a centrifuge [Mulugeta; Dietl and Koyi], while the remainder of the papers describe experiments performed in the Earth's gravitational field. From these seven papers, two papers present results of thermomechanical analogue models [Boutelier et al.; Wosnitza], while the other five papers present results of mechanical analogue models build with stratified brittle-viscous rheologies [Le Calvez and Vendeville; Schellart et al.; Schreurs et al.; Schreurs and Colletta; Schreurs et al.]. From this last group of papers, three papers present results of analogue experiments recorded in a CT-scanner, with which three-dimensional images of the analogue models were obtained [Schreurs et al.; Schreurs and Colletta; Schreurs et al.].

The first paper by Calvez and Vendeville reviews results of previous analogue models simulating the along-strike interaction between laterally offset faults, such as the formation of pull-apart basins in between strike-slip faults and relay zones in between laterally offset normal faults. A new model design is presented in which faults can freely propagate along strike after they have nucleated at a predetermined model-controlled location. The new design provides much more freedom for fault interaction within the relay zone and for fault-block rotation than previous models, and therefore, results significantly differ from those of previous models.

Schellart et al. present the results of 3-dimensional analogue modelling of asymmetric back-arc extension of an overriding lithosphere with a varying initial rheology. The results show that with increasing lithospheric brittle to viscous strength (BS/VS), the fault density decreases in magnitude, while the asymmetry in deformation pattern in the back-arc region increases. The extent of deformation is mainly dependent on the ratio of brittle strength to buoyancy force (BS/BF), i.e. the larger the ratio, the smaller the area of deformation. The experimental results have been compared with several arc - back-arc systems to explain the asymmetric structures observed in these systems.

The third paper by Schreurs et al. presents results of analogue experiments to investigate the development and evolution of transfer zones in fold and thrust belts, which were recorded in a CT-scanner to generate 3-D volumetric images. Models were build to investigate the difference in evolution between thrust domains that were underlain or not underlain by a thin viscous layer. In the brittle domain, closely spaced and dominantly forward propagating thrusts formed a narrow and high fold-and-thrust belt. In the brittle-viscous domain, the thrust belt was wider and lower, and there was no consistent vergence of thrusting and folding. Transfer zones formed in the transition zone between the two domains. The authors conclude that the location and orientation of these transfer zones are directly related to the geometry of the boundary between basal viscous layer and adjacent brittle layers.

Mulugeta discusses results of centrifuge experiments to investigate the interactive development of ramp-flat thrust styles for various rheological stratifications. The models exhibit various fault-fold geometries (e.g. fault bend folds, footwall synclines, wedge faults) as structural expressions of ductile ramp-flat accommodation. Depending on the rheological stratification, the ramps change shape and length as slip accumulates along the fault surfaces in flowing surroundings.

In the following paper by Boutelier et al., results of thermo-mechanical physical modelling of continental subduction are presented using new temperature sensitive analogue materials to model the lithospheric layers. The authors found that during subduction, the model crustal layer subducted to considerable depth before it underwent large and complex deformation including upward ductile flow of the deeply subducted portions, localised failure of the model upper crust at shallow depths and possible delamination of the model crust and mantle layer.

In the sixth paper, Schreurs and Coletta present analogue modelling results of continental transpression, which were recorded in a CT-scanner to generate 3-D volumetric images. The experiments were performed to investigate the control of the applied ratio of shear strain rate and shortening strain rate on the initial fault evolution. This ratio proved to be critical for the geometry of the fault zone and fault types (strike-slip faults, oblique-slip faults, thrust faults) that developed in such a fault zone.

In the paper from Dietl and Koyi results of analogue centrifuge models are presented to study the kinematics and dynamics of concentrically expanded plutons. From the modelling results, the authors conclude that concentrically expanded plutons can result from combined initial diapirism and subsequent ”ductile dyking“ and that multiple diapirs can form only when the overburden units deform in a ductile fashion during the different stages of diapirism.

In the eighth paper, Schreurs et al. present results of analogue experiments to study the influence of brittle-viscous multilayers on faulting during rifting. It was found that the presence of an interbedded viscous layer in between two brittle layers can lead to the development of two independent, decoupled conjugate normal fault systems in the upper and lower brittle layer, where the initial width between conjugate faults depends on the thickness of the brittle layers. It was also found that location and orientation of extensional transfer zones can be directly linked to the geometry of the interbedded viscous layer.

In the ninth and final paper, Wosnitza discusses a new recording technique to reconstruct the stress field in thermomechanical analogue models. The technique makes use of an infrared camera to allow the thermal field of the temperature dependent materials used in the experiment to be analysed during deformation. The viscosity distribution is then obtained from the temperature field and from rheological material properties. Combining the viscosity field with the strain rate field, it is then possible to calculate the stress field in the model. The author discusses the application of the new recording technique in relation to experimental modelling of orogenic processes.