Fault kinematic analysis
Fault-segment boundaries initiate, evolve and die as a result of the propagation, interaction and linkage of normal faults during crustal extension. However, little is known about the distribution, evolution and controls on the development of relay ramps, which are the key structures developed at synthetic segment boundaries. In this study, we use a series of scaled physical models (wet clay) to investigate the distribution and evolution of fault-segment boundaries within an evolving normal-fault population during orthogonal extension. From the models, we can establish a simple geometrical classification for segment boundaries, analyse their spatial and temporal evolution, and identify key factors that influence their variability.
Development of overlapping fault tips is a prerequisite for fault growth via segment linkage. Synthetic segment boundaries are the most common segment boundary type developed in the models. The proportion of synthetic segment boundaries in the total fault population increases with increasing strain, whereas conjugate (antithetic) segment boundaries are very rare. Hanging-wall-breached relay ramps are the most common type (>70%) of breached-segment boundary, followed by footwall-breached relay ramps (<25%). Transfer faults are uncommon in our models. The type of breached segment boundary that develops cannot be predicted based on fault overlap to fault spacing aspect ratio alone. Instead, we show that fault linkage occurs in a range of styles across a wide range of fault overlap to fault spacing ratios (1:1–7:1). Furthermore, we show that fault spacing is constrained by stress-reduction shadows at the time of fault nucleation, whereas fault overlap changes during fault growth and interaction. Our study thus shows that scaled physical models are a powerful tool to assess the style, distribution and controls on the evolution of synthetic segment boundaries developing in rifts. Predictions from these models must now be assessed with data from natural examples exposed in the field or imaged in the subsurface.