Development of pull-apart basins and associated structures by the riedel shear mechanism: insigth from scaled clay analogue models / Разработка бассейнов пул-апарт и связанных с ними структур с помощью механизма сдвига Риделя: выводы из моделирования

The physical and mechanical properties of clay materials required to scale the tectonic analogue experiments are analysed based on the modern soil mechanics concepts. A practical procedure for the determination of these properties is presented. Scaled clay experiments under Riedel shear mechanism produced a series of typical pull-apart structures. At the initial stages of the development of the deformation zone within the clay slab, the synthetic shear fractures (Riedel shears) display dilatational behaviour. With increasing basal displacement these dilated synthetic shears rotate and open further; long, narrow and deep troughs are formed. The shear displacements and the low angle of orientation to the direction of principal basal displacement definitely distinguish them from the tension fractures. Synthetic segments (Y-shears), which are parallel to the basal principal displacement fault, develop in a later stage and accommodate the continuing strike-slip deformation. They bound the troughs at the two tips of the initial Riedel shears and pull the sides further apart. Many rhomb shaped structures which characterise the pull-apart basins are then recognised. This development mechanism of pull-apart basins may explain why some en echelon segments of strike-slip faults display an extensional component in addition to the main shear displacement (trastension). The left-stepping arrangement of the Riedel shear fractures promotes the formation of push-up ridges at the overstepping areas separating them, of contractional features such as folds, reverse and thrust faults develop. The shear strength and the thickness of the clay slab control among other things the width of the deformation zone, the number and spacing of the Riedel shears, the presence or absence of the conjugate Riedel shears and the amount of basal shear displacement required for the shear fractures to appear at the surface. Pull-apart basins that may have formed by a mechanism corresponding to that observed in our experiment are found at different length scales on Gozo Island (Strait of Sicily rift zone), where major strike-slip systems were active from the Miocene to the Present. The coeval formation of push-up ridges can be followed in the complex strike-slip basin of the Marmara Sea.  <...>