A systematic comparison of experimental set-ups for modelling extensional tectonics

Analogue modellers investigating extensional tectonics often use different machines, set-ups and model materials, implying that direct comparisons of results from different studies can be challenging. Here we present a systematic comparison of crustal-scale analogue experiments using simple set-ups simulating extensional tectonics, involving either a foam base, a rubber base, rigid basal plates or a conveyor base system to deform overlying brittle-only or brittle-viscous models. We use X-ray computed tomography (CT) techniques for a detailed 3-D analysis of internal and external model evolution. We find that our brittle-only experiments are strongly affected by their specific set-up, as the materials are directly coupled to the model base. Experiments with a foam or rubber base undergo distributed faulting, whereas experiments with a rigid plate or conveyor base experience localized deformation and the development of discrete rift basins. Pervasive boundary effects may occur due to extension-perpendicular contraction of a rubber base. Brittle-viscous experiments are less affected by the experimental set-up than their brittle-only equivalents since the viscous layer acts as a buffer that decouples the brittle layer from the base. Under reference conditions, a structural weakness at the base of the brittle layer is required to localize deformation into a rift basin. Brittle-viscous plate and conveyor base experiments better localize deformation for high brittle-to-viscous thickness ratios since the thin viscous layers in these experiments allow deformation to transfer from the experimental base to the brittle cover. Brittle-viscous-base coupling is further influenced by changes in strain rate, which affects viscous strength. We find, however, that the brittle-to-viscous strength ratios alone do not suffice to predict the type of deformation in a rift system and that the localized or distributed character of the experimental set-up needs to be taken into account as well. Our set-ups are most appropriate for investigating crustal-scale extension in continental and selected oceanic settings. Specific combinations of set-up and model materials may be used for studying various tectonic settings or lithospheric conditions. Here, natural factors such as temperature variations, extension rate, water content and lithology should be carefully considered. We hope that our experimental overview may serve as a guide for future experimental studies of extensional tectonics

2D Thermo-Mechanical Simulations of Flat Subduction - Supporting Data for Research Manuscript

In \u3c 10% of global subduction zones, the downgoing oceanic plate “flattens” just beneath the overriding plate, attaining near-horizontality for hundreds of km before diving steeply into the mantle. Despite decades of inquiry, the rarity of flat subduction and its causes are not fully understood. Previous studies established a need for oceanward retreat of the trench caused by an advancing overriding plate but combined that with distinct causative factors for each flat slab occurrence, such as subducting slab buoyancy, a cratonic root in the overriding plate, large plate convergence rate, or mantle wedge suction. Here we propose a more universal mechanism for slab flattening based on the evolution of lithosphere and mantle rheology common to all long-lived subduction zones. Using thermomechanical models, we show that the necessary and sufficient conditions for slab flattening are long-lasting resistance to slab penetration into the lower mantle beneath an advancing continental plate, strong lithospheric cores, and viscous coupling restricted to the wedge corner. The advancing overriding plate advects cooler basal-lithospheric mantle into the mantle wedge. Resistance to subduction into the lower mantle hinders the equilibration of pressure between the top and bottom slab surfaces at the wedge corner. Together, these processes increase the slab-wedge viscous coupling within a compact

Complex fault interaction controls continental rifting

Continental rifting and break up processes are poorly constrained in the early stages. Here, the authors using high-resolution numerical simulations to show how early formed faults in continental extension can then control subsequent structure evolution of rifts

Benchmarking numerical models of brittle thrust wedges

We report quantitative results from three brittle thrust wedge experiments, comparing numerical results directly with each other and with corresponding analogue results. We first test whether the participating codes reproduce predictions from analytical critical taper theory. Eleven codes pass the stable wedge test, showing negligible internal deformation and maintaining the initial surface slope upon horizontal translation over a frictional interface. Eight codes participated in the unstable wedge test that examines the evolution of a wedge by thrust formation from a subcritical state to the critical taper geometry. The critical taper is recovered, but the models show two deformation modes characterised by either mainly forward dipping thrusts or a series of thrust pop-ups. We speculate that the two modes are caused by differences in effective basal boundary friction related to different algorithms for modelling boundary friction. The third experiment examines stacking of forward thrusts that are translated upward along a backward thrust. The results of the seven codes that run this experiment show variability in deformation style, number of thrusts, thrust dip angles and surface slope. Overall, our experiments show that numerical models run with different numerical techniques can successfully simulate laboratory brittle thrust wedge models at the cm-scale. In more detail, however, we find that it is challenging to reproduce sandbox-type setups numerically, because of frictional boundary conditions and velocity discontinuities. We recommend that future numerical-analogue comparisons use simple boundary conditions and that the numerical Earth Science community defines a plasticity test to resolve the variability in model shear zones