Abstract
ABSTRACT. 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.
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