Using different grain-size granular mixtures (quartz and K-feldspar sand) in analogue extensional models

We have performed a series of sandbox models addressing the influence of sand mixtures (quartz and feldspar sand in different proportions) with different grain-size on the development of normal faults. The overall model evolution suggests that fault orientation, width of the deformed area, and average subsidence are not significantly influenced by the different sand mixtures. Conversely, the number of secondary structures, mean fault length, and average fault dip are strongly dependent on the material and grain-size. Modelling results suggest that a Log-normal distribution of fault length and segment length distribution betst describes the experimental fault populations, particularly when the amount of fine-grained (K-feldspar) sand is similar to 30%. The length distribution and the number of faults are related, specifically with increasing the amount of the K-feldspar sand (i.e., higher the amount of fine-grained material) the shorter the minimum detectable fault length. This behavior is described by a Log-normal distribution, with a few long faults and many intermediate to short faults. We thus suggest that (for length ratios of 10(-5) to 2 10(-6)) a sand-mixture composed of 70% quartz sand and 30% K-feldspar sand is the one (among those we have tested) that provides the best compromise in terms of frictional properties, density, structural detail, number and length distribution of the faults, easiness of use, and general behavior.