In Situ Formation of Relic Landscapes By Fluvial Processes

Abstract:

Elevated low-relief landscapes are often interpreted as “relic” and characteristics such as low erosion rates or low relief are assumed to reflect pre-uplift tectonic conditions. We present here an alternative model whereby high elevation, low-relief landscapes develop in situ by purely fluvial processes. We argue that changes in tectonic regime result in change of uplift rate, but also deform the surface, thereby inducing changes in the river channel network through capture and divide migration. Loss of drainage area leads to lower erosion rate through lower river discharge and thus to higher surface uplift rate as erosion fails to keep up with tectonic uplift. Branches of a river network that lose area are thus raised to higher elevation where they have a predisposition towards further area loss, triggering a positive feedback, potentially including runaway conditions in which erosion rates and drainage area tend towards zero. Such conditions produce high-elevation, low relief, low erosion-rate branches of a river network that could be misconstrued as relic landscapes. We test this model by analyzing river profiles of several previously identified relic landscapes in the eastern Tibetan plateau region and in the Central Range of Taiwan. We apply two tests. First we check kinematic wave travel times to the relic and surrounding regions to test if there is a common uplift history with an increase in uplift rate that has not yet reached the relic. Second, we measure the channel steepness and channel profiles inside and outside the relic region to test for divide mobility and evidence of area capture. In every case examined, we find no common uplift history and widespread evidence that divides surrounding a relic landscape are moving inward, pirating drainage area and lowering erosion rates. This supports the model for in situ formation of these landscapes by progressive drainage area loss.