International audienc

[1] 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 À1 would imply footwall relief generation over a timescale of $1 Myr.

As hypervelocity ejecta from the Chicxulub (Yucatán, Mexico) impact fell back to Earth, the surface may have received a deadly dose of thermal radiation suffi cient to ignite global wildfi res. Using a two-phase fl uid fl ow code, which includes ejecta and air opacities in a radiative transfer calculation, we modeled the atmospheric reentry of spherules arriving at distal sites. The models predict a pulse of thermal radiation at the surface peaking at 5-15 kW/m 2 , analogous to an oven set on "broil" (~260° C). Previous calculations, which ignored spherule opacity, yielded >10 kW/m 2 sustained over >20 min and such an extended pulse is thought to be required for wood ignition. However, the new modeling suggests that fl uxes only exceed the solar norm for ~30 min and are only >5 kW/m 2 for a few minutes. Previous models failed to consider the self-shielding effect of settling spherules, which block an increasing proportion of downward thermal radiation emitted by the later-arriving spherules. Such self-shielding may have prevented widespread wildfi re ignition, although the thermal pulse may have been suffi cient to ignite localized fi res and kill fauna lacking temporary shelter. An opaque cap of submicron dust in the upper atmosphere could, however, override the self-shielding effect.on June 8, 2015 geology.gsapubs.org Downloaded from

[1] The discovery of large amounts of soot in clays deposited at the Cretaceous-Paleogene (K-Pg) boundary and linked to the~65 Ma Chicxulub impact crater led to the hypothesis that major wildfires were a consequence of the asteroid impact. Subsequently, several lines of evidence, including the lack of charcoal in North American sites, were used to argue against global wildfires. Close to the impact site fires are likely to be directly ignited by the impact fireball, whereas globally they could be ignited by radiation from the reentry of hypervelocity ejecta. To-date, models of the latter have yet to take into account that ejection -and thus the emission of thermal radiation-is asymmetric and dependent on impact angle. Here, we model: (1) the impact and ejection of material, (2) the ballistic continuation of ejecta around a spherical Earth, and (3) the thermal pulse delivered to the Earth's surface when ejecta reenters the atmosphere. We find that thermal pulses in the downrange direction are sufficient to ignite flora several thousand kilometers from Chicxulub, whereas pulses at most sites in the uprange direction are too low to ignite even the most susceptible plant matter. Our analyses and models suggest some fires were ignited by the impact fireball and ejecta reentry, but that the nonuniform distribution of thermal radiation across the surface of the Earth is inconsistent with the ignition of fires globally as a direct and immediate result of the Chicxulub impact. Instead, we propose that the desiccation of flora by ejecta reentry, as well as the effects of postimpact global cooling/darkness, left much of the terrestrial flora prone to fires, and that the volume of soot in the global K-Pg layer is explained by a combination of syn-and postimpact wildfires.