Two-sided orogen: Collision and erosion from the sandbox to the Southern Alps, New Zealand

Abstract

In continental collision zones, the indentor is of approximately the same vertical dimension as the material being deformed, thereby allowing material to move over the indentor and out of the deforming orogen. This simple constraint on the height of the indentor differs from that imposed by the bulldozer model of wedge deformation and results in the formation of an orogen with two opposite-dipping topographic slopes. These two wedges are mechanically coupled, and the mechanics are strongly influenced by the erosion conditions imposed on the wedge. Where the growing mountains perturb prevailing, moisture-laden winds, the erosion pattern is highly asymmetric, erosional work being concentrated at the toe of the steep wedge facing the indentor (the inboard wedge). Concentration of erosional energy results in concentration of mechanical energy in the same region and produces the distinctive geological and geophysical patterns associated with continental collision. These include rapid uplift and exposure of deep crustal assemblages adjacent to the indentor accompanied by high crustal heat flow and anomalously low seismicity. The outcrop pattern along the inboard wedge evolves from that of a crustal cross section currently exposed in the Southern Alps to one dominated by repetitive nappe structures consisting predominantly of lower crustal units, as in the western Alps of Europe. The other wedge (the outboard wedge) undergoes relatively little erosion, is broad with a more gentle surface slope, and maintains an outcrop pattern consisting of upper crustal material.