EVOLUTION AND EFFICIENCY OF RESTRAINING BENDS IN STRIKE-SLIP FAULT SYSTEMS

Strike-slip fault systems, such as the San Andreas Fault system in southern California, can be effectively modeled by the use of analog clay box models in the lab. These models are constructed from wet kaolin, which is clay that has the same scale:strength ratio as rock material within the Earth’s crust. This allows for accurate simulation of fault system slip rates in a matter of hours instead of centuries. Recent advancements in the Physical Modeling Laboratory at UMass provide continuous documentation of the horizontal and vertical deformation of the wet kaolin experiments. This provides a wealth of information for analyzing the evolving efficiency of fault systems that cannot be collected from the Earth’s crust. A series of kaolin models have been tested to observe changes in mechanical efficiency within restraining bends in strike-slip faults, in regards to varying fault propagation angle and stepover distance. Specifically we have looked at restraining bends with stepover distances of 2, 5, and 10 cm and fault angles of 15° and 30°. We hypothesize that the fault with the smallest stepover distance (2 cm) and an angle of 15° will be the most efficient while the fault with the largest stepover distance (10 cm) and an angle of 30° will be the least efficient. This is expected since faults with lower restraining bend angles (15°) continue to slip along the original fault, while systems with greater angles (30˚) abandon the original fault necessitating the growth of new faults. Fault systems with smaller stepover distance begin to slip sooner in plate displacement than the larger stepover systems. Mechanical efficiency of each fault system increases with new fault propagation reaching a steady value once the new faults link across the restraining bend. Experiments show that with new fault growth, systems with similar stepover but different angle will evolve to similar steady state mechanical efficiency. This steady state efficiency of the mature fault system increases with decreasing stepover distance. This suggests that, within the crust, the stepover distance between fault segments in restraining bends limits the efficiency of the fault system.