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Abstract:We report results of a study comparing numerical models of sandbox-type experiments. Two experimental designs were examined: (1) A brittle shortening experiment in which a thrust wedge is built in material of alternating frictional strength; and (2) an extension experiment in which a weak, basal viscous layer affects normal fault localization and propagation in overlying brittle materials. Eight different numerical codes, both commercial and academic, were tested against each other. Our results show that: (1) The overall evolution of all numerical codes is broadly similar. (2) Shortening is accommodated by in-sequence forward propagation of thrusts. The surface slope of the thrust wedge is within the stable field predicted by critical taper theory. (3) Details of thrust spacing, dip angle and number of thrusts vary between different codes for the shortening experiment. (4) Shear zones initiate at the velocity discontinuity in the extension experiment. The asymmetric evolution of the models is similar for all numerical codes. (5) Resolution affects strain localization and the number of shear zones that develop in strain-softening brittle material. (6) The variability between numerical codes is greater for the shortening than the extension experiment.Comparison to equivalent analogue experiments shows that the overall dynamic evolution of the numerical and analogue models is similar, in spite of the difficulty of achieving an exact representation of the analogue conditions with a numerical model. We find that the degree of variability between individual numerical results is about the same as between individual analogue models. Differences among and between numerical and analogue results are found in predictions of location, spacing and dip angle of shear zones. Our results show that numerical models using different solution techniques can to first order successfully reproduce structures observed in analogue sandbox experiments. The comparisons serve to highlight robust features in tectonic modelling of thrust wedges and brittle-viscous extension.Numerical and analogue modelling methods represent two different techniques with which the evolution of geological structures, such as fold-and-thrust belts and sedimentary basins, can be investigated. The underlying assumption with both methods is that their results approximate the development of structures in the real Earth in a reasonable manner. We may then expect that the results of analogue and numerical models look similar when applied to the same (2) to test the similarity of numerical and analogue models, in order to help establish robust features of tectonic models on the scale of the upper crust.The companion paper (Schreurs et al. 2006) presents the results of an analogue comparison study with ten participating modelling laboratories. Two experimental set-ups were tested: (1) a brittle convergent thrust wedge experiment and (2) a brittle-viscous extension experiment. The reproducibility of modelling results between the laboratories was found to be fai...

Continental rifts often develop from linkage of distinct rift segments under varying degrees of extension obliquity. These rift segments arise from rift initiation at non-aligned crustal heterogeneities and need to interact to develop a full-scale rift system. Here, we test the effects of 1) oblique extension and 2) initial heterogeneity (seed) offset on continental rift interaction with the use of an improved analogue model setup. X-Ray computer tomography (CT) techniques are used to analyse the 3D models through time and the results are compared with additional numerical models and natural examples. The experimental results reveal that increasing extension obliquity strongly changes rift segment structures from wide rifts in orthogonal settings to narrower rifts with oblique internal structures under oblique extension conditions to narrow strike-slip dominated systems towards the strike-slip domain. We also find that both decreasing seed offset and increasing extension obliquity promote hard linkage of rift segments through the formation of continuous rift boundary faults at the surface. (Initial) soft linkage through the formation of relay ramps is more likely when seed offset increases or extension is more orthogonal. Rather than linking at depth, the rift boundary faults curve around each other at depth and merge towards the surface to form a continuous trough. Orthogonal extension promotes the formation of intra-rift horsts, which may provide hydrocarbon traps in nature.

12 13We report the material properties of 26 granular analogue materials used in 14 ana-14 logue modelling laboratories. We determined physical characteristics such as bulk 15 density, grain size distribution, and grain shape, and performed ring shear tests to 16 determine friction angles and cohesion, and uniaxial compression tests to evaluate 17 the compaction behaviour. Mean grain size of the materials varied between c. 100 18 and 400 µm. Analysis of grain shape factors show that the four different classes of 19 granular materials (14 quartz sands, 5 dyed quartz sands, 4 heavy mineral sands 20 and 3 size fractions of glass beads) can be broadly divided into two groups consist-21 ing of 12 angular and 14 rounded materials. Grain shape has an influence on friction 22 angles, with most angular materials having higher internal friction angles (between 23 c. 35° and 40°) than rounded materials, whereas well-rounded glass beads have the 24 lowest internal friction angles (between c. 25° and 30°). We interpret this as an ef-25 2 fect of intergranular sliding versus rolling. Most angular materials have also higher 26 basal friction angles (tested for a specific foil) than more rounded materials, sug-27 gesting that angular grains scratch and wear the foil., Most materials have an inter-28 nal cohesion in the order of 20-100 Pa except for well-rounded glass beads, which 29show a trend towards a quasi-cohesionless (C <20 Pa) Coulomb-type material. The 30 uniaxial confined compression tests reveal that rounded grains generally show less 31 compaction than angular grains. We interpret this to be related to the initial packing 32 density after sifting, which is higher for rounded grains than for angular grains. Ring-33 shear test data show that angular grains undergo a longer strain-hardening phase 34 than more rounded materials. This might explain why analogue models consisting of 35 angular grains accommodate deformation in a more distributed manner prior to 36 strain localisation than models consisting of rounded grains.