Transient landscapes at fault tips

Fault growth produces patterns of displacement and slip rate that are highly variable in both space and time. This transience is most pronounced near fault tips, where along‐strike displacement gradients vary in time as the fault array lengthens. We use a set of statistical and field observations to quantify the response of catchments and their associated fans in three large normal fault arrays to transient patterns of displacement and slip rate. Catchments near the fault tips show distinct scaling of channel slope with drainage area compared with catchments near the strike center. This scaling becomes uniform beyond about ∼10 km from the fault tips and is therefore like footwall relief, largely decoupled from the fault displacement profile. The estimated catchment response times to a change in slip rate also vary between fault tips and strike center. The response times for tip catchments are much longer than the inferred time since fault activity began, indicating that they are unlikely to be in equilibrium with the current fault displacement field. This disequilibrium, combined with the decoupling of slope‐area scaling from displacement, indicates that landscapes are most sensitive to fault activity near fault tips. Active faults characterized by along‐strike variation in slip rate thus provide excellent opportunities to explore the transient response of landscapes to tectonic forcing

Landscape evolution at extensional relay zones

It is commonly argued that the extensional relay zones between adjacent crustal-scale normal fault segments are associated with large catchment-fan systems that deliver significant amounts of sediment to hanging wall basins. This conceptual model of extensional basin development, while useful, overlooks some of the physical constraints on catchment evolution and sediment supply in relay zones. We argue that a key factor in the geomorphic evolution of relay zones is the interplay between two different timescales, the time over which the fault array develops, and the time over which the footwall catchment-fan systems are established. Results of numerical experiments using a landscape evolution model suggest that, in isolated fault blocks, footwall catchment evolution is highly dependent on the pattern and rate of fault array growth. A rapidly linked en echelon fault geometry gives rise to capture of relay zone drainage by aggressive catchment incision in the relay zone and to consequent increases in the rate of sediment supply to the hanging wall. Capture events do not occur when the fault segments are allowed to propagate slowly toward an en echelon geometry. In neither case, however, are large relay zone catchment-fan systems developed. We propose several physical reasons for this, including geometric constraints and limits on catchment incision and sediment transport rates in relay zones. Future research efforts should focus on the timescales over which fault array development occurs, and on the quantitative variations in catchment-fan system morphology at relay zones

What sets topographic relief in extensional footwalls?

We use three large normal fault arrays in the northeastern Basin and Range Province, western United States, to document catchment development and relief production during fault growth. Fault slip and slip rates increase systematically along strike from zero at the fault tips. Catchment relief and across-strike range width both increase as slip accumulates but reach maximum values at a distance of 15 km from the fault tips and remain uniform along strike over much of the footwalls. Catchment outlet spacing also increases away from the fault tips but does not reach a uniform value and may vary by a factor of 5–6 along strike. We infer that catchments first elongate in the across-strike direction as slip accumulates and the range half-width increases. Once the half-width reaches its maximum value, continued catchment growth is possible only by along-strike capture, which increases outlet spacing but not relief. The close correspondence between catchment relief and range half-width suggests that geomorphically limited hillslope and channel gradients are achieved within the 15 km tip zone. Thus, the limiting factor in footwall development is the width of the range, which is controlled by two external agents: the geometry and spacing of the major faults, and the elevations of base level on both flanks

Footwall topographic development during continental extension

We examine the progressive development of footwall topography associated with a set of active normal faults in the northeastern Basin and Range Province of the western United States. Fault length and displacement increase monotonically from northeast to southwest in the study area, allowing us to track both variations in footwall morphology with increasing displacement and along-strike changes in morphology on a single fault. We show that patterns of catchment area, footwall relief, and catchment outlet spacing vary predictably and are related to the growth of the range-bounding normal fault array. In this semiarid region, full parsing of footwall drainage area and removal of antecedent topography do not occur until fault arrays grow beyond two crustal-scale segments. Multiple-segment faults with lengths of up to 150 km have footwall relief that is limited to ∼1000 m in the center of the footwall and that decays to zero at the fault tips over a length scale of ∼15 km. We hypothesize that this relatively uniform footwall relief is erosionally limited and reflects the efficacy of surface processes in removing footwall material in the center of the footwall. If the fault array grows by relatively steady propagation of the tips, we suggest that the 15 km length scale required to reach uniform relief is related to a timescale of relief generation by the fault tip propagation rate. While such propagation rates are poorly known, an average rate of 10 mm yr−1 would imply footwall relief generation over a timescale of ∼1 Myr