Erosional control of active compressional orogens

Abstract

Denudation has long been acknowledged as a process that contributes to the unroofing of compressional orogens. It has, however, mainly been considered as a passive process, not one that can dictate or control the tectonic evolution. This view prevails despite the knowledge that the style of deformation is controlled by the interplay of gravitational and tectonic stresses: an interplay that is sensitive to the mass removed by denudation. This paper presents a model designed to investigate the syn-tectonic style of a compressional orogen that is subject to large-scale denudation. The model couples tectonics and erosion in the following manner. The rate of lithospheric deformation, including surface uplift, is calculated by a plane-strain rigid plastic and/or viscous model of the lithosphere's response to tectonic compression. A digital erosion model acts on the current planform surface ofthe orogen to redistribute mass by a combination of short-range (hillslope) diffusion and long-range (fluvial) transport. The fluvial network is recharged by a model of orographic rainfall in which the distribution is related to the current model topography. Tectonic uplift is added to the planform surface at the beginning of each erosional timestep. The current planform topography after erosion is averaged along the direction normal to compression and is then used to define the surface for the next step of the tectonic model. Preliminary results compare favourably with the orogen-scale precipitation, morphology and tectonics of the Southern Alps of South Island, New Zealand. The potential importance of at-mospheric coupling to orogenic evolution is illustrated by results from contrasting models ofthe Southern Alps in which: (I) there is no erosion; (2) rainfall is derived from a westerly source, and; (3) rainfall is derived from an easterly source. If the model is correct, the structural and metamor-phic style of this orogen is controlled by fluvial denudation of its western flank charged by the orographic precipitation. Can erosion control the evolution of compressional orogens at the scale of the orogen? This paper investigates the way in which orographic rainfall and consequent erosion of a mountain belt may be linked to the tectonics of the underlying deforming orogen. If this analysis is correct, palaeoclimatic information may be required before the evolution of ancient orogens can be fully understood.