Benchmarking analogue models of brittle thrust wedges
Résumé
We performed a quantitative comparison of brittle thrust wedge experiments to evaluate the variability
among analogue models and to appraise the reproducibility and limits of model interpretation. Fifteen
analogue modeling laboratories participated in this benchmark initiative. Each laboratory received a
shipment of the same type of quartz and corundum sand and all laboratories adhered to a stringent
model building protocol and used the same type of foil to cover base and sidewalls of the sandbox. Sieve
structure, sifting height, filling rate, and details on off-scraping of excess sand followed prescribed
procedures.
Our analogue benchmark shows that even for simple plane-strain experiments with prescribed
stringent model construction techniques, quantitative model results show variability, most notably for
surface slope, thrust spacing and number of forward and backthrusts. One of the sources of the variability
in model results is related to slight variations in how sand is deposited in the sandbox. Small changes in
sifting height, sifting rate, and scraping will result in slightly heterogeneous material bulk densities,
which will affect the mechanical properties of the sand, and will result in lateral and vertical differences
in peak and boundary friction angles, as well as cohesion values once the model is constructed. Initial
variations in basal friction are inferred to play the most important role in causing model variability.
Our comparison shows that the human factor plays a decisive role, and even when one modeler repeats
the same experiment, quantitative model results still show variability. Our observations highlight
the limits of up-scaling quantitative analogue model results to nature or for making comparisons with
numerical models. The frictional behavior of sand is highly sensitive to small variations in material state
or experimental set-up, and hence, it will remain difficult to scale quantitative results such as number of
thrusts, thrust spacing, and pop-up width from model to nature.
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