Evidence for and against landscape transience in the Northern Qinling Mountains, China

Abstract

The Northern Qinling Mountains, China, have experienced large earthquakes over the last millennium, including the 1556 Shaanxi earthquake, thought to be the deadliest earthquake in recorded history. Past studies suggest rapid Holocene fault throw rates, but exhumation and basin wide erosion rates are an order of magnitude slower. We explored channel profiles and normalized channel steepness index (ksn) values to evaluate evidence for landscape transience and potential acceleration in channel incision triggered by changing tectonic activity. We find that precipitation and lithological differences cannot explain the differences in channel steepness in the area. We integrate drainage area along channel profiles to plot a transformed coordinate, χ, against elevation, a technique that can identify signatures of changing channel incision rates. If there is acceleration in fault throw rates in a fault crossing the outlet of multiple basins, one would expect channel convexities located at similar χ locations as well as clustering of knickpoints at similar elevations. Instead, we find channel profiles are remarkably straight in χ-elevation space in basins that drain across the Qinling Northern Piedmont Fault (QNF) and Huashan Piedmont Fault (HPF). The landscape does contain a number of knickpoints but these do not cluster in elevation. Some basins in the area contain convexities but these convexities to not reappear in adjacent basins. These lines of evidence suggest that if fault throw rates have accelerated regionally along the QNF and HPF, they have not had time to substantially perturb channel profiles. Simple calculation of adjustment timescales for steady state channels suggests that the erosion rates, at least in channels not containing convexities, have been stable for timescales on the order of a million years.