Uplift, shortening, and steady-state topography in active mountain belts

We present a tectonic, surface process model used to investigate the role o f horizontal shortening in convergent orogens and the effects on steady-sta te topography. The tectonic model consists of a specified velocity field fo r the Earth's surface and includes a constant uplift rate and a constant ho rizontal strain rate which varies to reflect the relative importance of fro ntal accretion and underplating in an orogenic wedge. The surface process m odel includes incision of a network of rivers formed by collection of appli ed precipitation and diffusive hillslope mass transfer. Three non-dimension al parameters describe this model: a ratio of the maximum horizontal veloci ty to the vertical velocity, a Peclet number expressing the efficiency of t he hillslope diffusion relative to the uplift rate, and a fluvial "erosion number" reflecting the fluvial incision efficiency relative to the uplift r ate. A series of models are presented demonstrating the resultant steady-st ate landforms parameterized by these three numbers. A finite velocity ratio results in an asymmetric form to the model mountain range, although the ma gnitude of the asymmetry also depends on the Peclet number. Topographic ste ady 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 activ e mountain ranges in Taiwan, New Zealand, and the Olympic Mountains of Wash ington state. All these examples exhibit asymmetric topographic form with t he asymmetry consistent with the polarity of subduction, suggesting that ho rizontal tectonic motion is affecting the macro-geomorphic form of these ra nges.