Physical analog modeling of pull-apart basin evolution

Abstract

Pull-apart basins are structural depressions formed by localized extension along strike-slip fault systems, typically at releasing bends or steps in the fundamental strike-slip system. Analog modeling is used to evaluate the sequence of structural evolution of pull-apart basins and factors that control their degree of structural asymmetry and geometry. Basin evolution, internal structure and overall symmetry are investigated for oblique releasing step angles and with varying rates of displacement between brittle and ductile crust on opposing sides of the strike-slip system, while maintaining relative rates between brittle crust on opposing sides of the basin. Pull-apart basin evolution is separated into three stages: incipient, early, and mature. Incipient pull-apart basins are characterized by formation of a normal-fault bounded graben or half-graben parallel to the oblique step between main strike-slip zones. In the early stage of formation, additional normal faults form toward the basin interior from the original bounding faults, and cross-basin strike-slip faults cut diagonally across the basin interior; basin-bounding normal fault systems are characterized by lateral variations of fault throw and localized relay ramps. In the mature stage of evolution, strike-slip and normal faults join to completely bound the pull-apart basin. Analog model results indicate that displacement associated with cross-basin faults causes development of a through-going strike-slip fault that links the two main strike-slip displacement zones, ultimately resulting in a decline in normal fault activity. Asymmetric, symmetric, and hybrid pull-apart basins all follow the same overall deformation sequence just described. The asymmetry of a pull-apart basin is controlled by the degree of decoupling between brittle and ductile crust beneath the two crustal blocks in relative motion. This is modeled by maintaining a constant relative rate of motion between opposing fault blocks in all models, but varying the rate between the blocks and the fixed (model) basement (‘absolute’ rate). Models in which one side of the detachment is fixed with respect to the basement form asymmetric pull-apart basins defined by a half-graben with the master fault on the mobile side. In models where opposing sides of the fault system are equally decoupled from the basement, symmetrical pull-apart basins form, defined by horst and graben structures and master fault dominance switching sides along the length of the basin. Fault segmentation associated with relatively immature pull-apart basins may be capable of arresting earthquake rupture. Late linkage of main strike-slip zones by a cross-basin fault could extend potential rupture area, dramatically increasing the possibility for large-magnitude earthquakes.