Uplift, Shortening, and Steady State Topography in Active Mountain Belts

We present a tectonic, surface process model used to investigate the role of horizontal shortening in convergent orogens and the effects on steady-state topography. The tectonic model consists of a specified velocity field for the Earth’s surface and includes a constant uplift rate and a constant horizontal strain rate which varies to reflect the relative importance of frontal accretion and underplating in an orogenic wedge. The surface process model includes incision of a network of rivers formed by collection of applied precipitation and diffusive hillslope mass transfer. Three non-dimensional parameters describe this model: a ratio of the maximum horizontal velocity to the vertical velocity, a Peclet number expressing the efficiency of the hillslope diffusion relative to the uplift rate, and a fluvial “erosion number” reflecting the fluvial incision efficiency relative to the uplift rate. A series of models are presented demonstrating the resultant steady-state landforms parameterized by these three numbers. A finite velocity ratio results in an asymmetric form to the model mountain range, although the magnitude of the asymmetry also depends on the Peclet number. Topographic steady-state is achieved faster for models with no horizontal component to the velocity field. With finite horizontal velocity, topographic steady state is achieved only at the scale of the entire mountain range; even the first order drainage basins are unstable with time in the presence of horizontal shortening. We compare our model results to topographic profiles from active mountain ranges in Taiwan, New Zealand, and the Olympic Mountains of Washington state. All these examples exhibit asymmetric topographic form with the asymmetry consistent with the polarity of subduction, suggesting that horizontal tectonic motion is affecting the macro-geomorphic form of these ranges.