DEVELOPMENT OF SLIP PARTITIONING WITHIN WET KAOLIN AND DRY SAND OBLIQUE-CONVERGENCE EXPERIMENTS

Slip partitioned systems can accommodate oblique convergence with different slip rake on two or more faults and are well documented; however, the evolution of slip partitioned crustal systems is unconstrained. Carefully scaled physical experiments in crustal analogs inform our understanding of fault evolution because we can control the loading and directly observe the ensuing deformation. Using experiments in both dry sand and wet kaolin clay we explore how slip partitioning evolves under different convergence angles. Previous cohesionless dry sand experiments documented that convergence angle and fault strength control slip partitioning. In contrast to dry sand, the non-zero cohesion of wet kaolin produces long-lived fault structures that easily reactivate. Scaled experiments using both dry sand and wet kaolin with identical boundary conditions provide insights on the role of material properties in slip partitioning. We use motorized movement of rigid blocks with 30° dipping contacts overlaid by a 2.5 cm layer of dry sand or wet kaolin clay to approximate crustal deformation due to oblique subduction zone convergence. Digital image correlation combined with stereovision techniques provides evolution of horizontal strain and uplift that constrain fault geometry and slip vectors along the faults. Additionally, force gauges record the evolution of fault-normal forces throughout slip partitioning, such as stress drops associated with fault growth. Within the dry sand, an oblique-slip forethrust-backthrust pair forms first followed by a late stage through-going strike-slip fault. In contrast, at shallow convergence angles within the clay, the strike-slip fault forms first and a backthrust never develops. As convergence accumulates in the clay, a forethrust forms dipping toward the underlying discontinuity. The lack of cohesion in dry sand may prevent the concentration of mode III stresses that leads to the early vertical strike-slip fault growth observed in the clay. Since a forethrust/backthrust pair requires greater fault surface area than a vertical strike-slip fault, the cohesion of the clay inhibits backthrust formation. The cohesion of the clay, which is similar to crustal rock strength, may facilitate the maintenance of slip partitioned fault systems.