CONTROLS ON THE TIMESCALE OF POST-OROGENIC DECAY OF TOPOGRAPHY

We explore the geomorphologic controls on the timescale for the post-orogenic decay of topography and address the question of whether there is a viable geomorphologic explanation for the persistence of topography in ancient orogenic belts, or whether alternative geodynamic explanations must be sought. Using both approximate analytical solutions and numerical simulations we show that the widely used detachment-limited bedrock channel incision model predicts relatively short post-orogenic decay times of 1-10 Ma. Additional layers of complexity are introduced to this model, each of which serve to lengthen the decay timescale, including: isostasy and flexural strength, a transition to transport-limited conditions during topographic decline, and the incorporation of the combined effects of a critical threshold for erosion and the stochastic variability of flood magnitudes. Pure Airy isostatic rebound of a thick crustal root results in decay times that are at most a factor of six longer than the detachment-limited analytical solution. The transition to transport-limited conditions increases decay times by a factor of two to three. Finally, including a critical shear stress results increases decay time by approximately a factor of twenty. More importantly, however, erosion rates in this case decline rapidly and soon become sufficiently low that weathering rates probably dictate the pace of topographic decline, even while a significant portion of the initial topography remains. Thus a model combining isostasy, a transition to transport-limited conditions, and a critical shear stress could account for the persistence of residual topography for >100 Ma.