The effect of humidity on ductile behavior of wet plaster, in physical modelling, the case study of strike-slip faults system

The purpose of physical modeling in structural geology is to reconstruct the formation, development and evolution of structures on a laboratory scale with materials similar to crustal rocks. The physical-mechanical properties of the materials used in these modeling are of particular importance; So that the characteristics of these materials should be similar to the physical and mechanical characteristics of crustal rocks, and there should be a proportionality between the model and nature in terms of rheology, viscosity, and frictional resistance. Sand, mud-clay, gypsum and silicon are among the materials used in physical modeling. Gypsum powder, plaster, or hemi-hydrate in dry and wet form (killed gypsum) has also been used in modeling. In this study, the effect of humidity on viscous behavior of wet plaster in physical modelling of strike- slip fault system has been investigated. In these modellings wet plaster with 320%, 285% and 175% of humidity as a viscous material has been used. In determination of physical and mechanical (Atterberg limits) properties of wet plaster, the liquid limit of 178% and plastic limit of 96% was obtained. This study also provides data on behavior of wet plaster with changes in its humidity.

Atterberg measurements were first used to measure the water content of wet (or dead) plaster. Based on this criterion, three limits are defined for the amount of wetness in the material, which includes the shrinkage limit, the plastic limit, and the liquid limit. In this study, due to the non-use of wet plaster in the humidity range of the shrinkage limit in physical modeling, the shrinkage limit test was not performed. In the plastic limit test, the minimum moisture required to turn the soil into mud is measured. In this regard, some balls of mud prepared from wet plaster are placed on a glass and by rotating the hand on the plaster, a rod with a diameter of 3mm is prepared. If cracks on the mud is developed, its humidity is at the level of mud. To determine the liquid limit of wet plaster, the penetrator cone device was used. Then the physical modeling of strike-slip faulting has been done using wet plaster with different wetness percentages. Three tests of physical modeling of strike-slip fault system in the basement and investigation of the effect of these movements on the sedimentary cover have been carried out using wet plaster with different humidity levels of 320%, 285% and 175%.

The results of Physical modelling on strike- slip fault system shows that by decrease in humidity of wet plaster from 320% to 175%, the Riedel shear fractures distance is increased from 0.3cm to 1.9 cm and their lengths have increased from 2.5cm to 4.5cm. These results obtained from the distribution and frequency of shear fractures in the form of a decrease in the number of fractures, an increase in the distance and length of fractures, and the uneven development of fractures qualitatively indicate a decrease in the elastic limit of wet plaster because of the decrease on its humidity. Reduction on the humidity of the wet plaster resulted in development fault related folding in the modeling. The variations on the numbers, distance, type and lengths of shear fractures in wet plaster with different percent of humidity demonstrates that wet plaster acts as viscous behavior when its humidity is between plastic limit (96%) and liquid limit (178%). These experiments also confirm the results of previous studies on the brittle behavior of plaster at lower wettnes levels.

The results obtained from this study in the investigation of Riddle shear fractures and folds related to the fault in the physical modeling of the strike-slip fault system showed that the water content in wet plaster to the extent of the plastic limit causes its plasticity behavior. The increase of water content more than this amount due to the relative decrease of adhesion and viscosity is associated with the decrease in the plasticity behavior of wet plaster. These changes are associated with the increase in the number of Riddle shear fractures and the decrease in the development of folds related to the thrust fault has been accompanied in the experiments. This conclusion is in agreement with the former studies that documented the brittle behavior of plaster with lower humidity than the humidity of the plastic limit.