Particle-Based Modeling of Pull-Apart Basin Development

A scale-independent modeling approach based on the discrete element method has been established to investigate the development of pull-apart basins. The main findings can be summarized as follows. Thirty degree underlapping models produce pull-apart basins that evolve from spindle-shaped through lazy-Z-shaped to rhomboidal and stretched rhomboidal basin. Ninety degree nonoverlapping and 150 degrees overlapping models generate rhomboidal pull-apart basins without going through spindle-shaped and lazy-Z-shaped stages. The shape of a pull-apart basin is the consequence of both the initial strike-slip fault geometry and its various evolution stages. Rhomboidal basins, which have larger basin length than the amount of motion, form in overlapping systems and do not progress through the spindle-shaped and lazy-Z-shaped stages such as the Dead Sea basin. Rhomboidal basins with cross-basin faults tend to form in underlapping systems. All the numerical models show nearly the same trend for maximum principal stress versus relative extension epsilon(x)* (horizontal opening divided by fault separation). Peak stress and onset of crack propagation are observed for epsilon(x)* of similar to 0.035. The relative extension to form the first depression is similar to 0.155. Therefore, for a pull-apart to form in nature, the displacement and time needed to form the first cracks and depression area can be estimated from the corresponding epsilon(x)* and the slip rate of the strike-slip faults. The time needed to form the first depression can be considered as the minimum age of initiation for the pull-apart basin. This method to deduce the starting age of pull-apart basin development can be used for basins which are still active.