THE IMPORTANCE OF NEAR-FAULT RELIEF ELEMENTS IN DEVELOPING A “CLASSIC” STRIKE-SLIP LANDSCAPE

Strike-slip faulting is known to create characteristic landscape features including offset stream channels and linear, fault-parallel valleys. Such features are often used to identify fault location, determine sense of slip, and, when possible, to calculate slip rate. Strike-slip faults, however, differ in their topographic expression, and studies are needed to address why some geomorphic features—such as long channel offsets—occur in one setting but not in another. To approach this problem, we use the Channel-Hillslope Integrated Landscape Development (CHILD) model to simulate a linear mountain ridge cut by a strike-slip fault, and compare its predictions with topographic metrics in the Marlborough Fault System, South Island, NZ. Our simulations test the effects of shutter ridge length, shutter ridge height, and ratio of horizontal to vertical fault motion on channel offset length and stream capture frequency – opposing processes that together determine the length of stream offset at any given time. Based on our models, we find that shutter ridge length primarily sets the scale of channel offsets. In cases with no or very small shutter ridges, channel offset is not preserved despite sustained and long-term lateral fault motion. We also find that shutter ridge height is important, as taller shutter ridges modulate channel offset lengths by forestalling stream capture. These results are supported by topographic metrics from the Marlborough Fault System, where shutter ridges with greater relief correlate with longer channel offsets. In models with variable uplift rates, we find that “classic” strike-slip topography develops in response to forcing by shutter ridges even when the ratio of strike-slip to uplift at the range front is very low. Together, these results suggest that near-fault relief elements are important, perhaps even required, in creating characteristic strike-slip landscapes with offset river channels and thus, the presence (or absence) of deflected rivers should not be considered a litmus test for the existence or activity level of a strike-slip fault.