Following Beaumont et al. [1992], I begin with a relatively narrow orogenic belt with a single deep thrust (Figure 1). In the absence of erosion, a high mountain range forms from uplifted material. Rocks originally overlying the fault plane (that is, the ``hanging wall of the fault'') flow out over the rocks originally at the surface on the downthrown side of the fault (that is, over the ``foreland''). Once forces associated with the uplift have approached the driving forces, further movement on the fault is limited by the ability of material to spread laterally over the foreland. If erosion is rapid, no high mountain range or extensive foreland thrust develops. With rapid erosion, tectonic exhumation of material can continue exposing deeper levels of the crust, permitting large horizontal convergence.
The final states of these end-member orogenic belts, once they erode to base level, are different. A gradual change in metamorphic grade occurs across an orogen that was not extensively eroded at the time of its formation because foreland rocks that were originally near the surface are deeply buried beneath the thrust. Conversely, a sharp metamorphic front is associated with rapidly eroded orogens because deeply exhumed, hanging-wall rocks are juxtaposed against unaffected foreland rocks. The Grenville front is a well studied example of a rapidly eroded orogen [ Beaumont et al., 1992; Hoffman and Grotzinger, 1993; Haggart et al., 1993].
A feedback exists between tectonics and climate because mountain ranges produce their own orographic precipitation and rain shadows. The evolution of thrust belts on the windward and leeward side of mountain ranges thus differs [ Beaumont et al., 1992; Hoffman and Grotzinger, 1993]. Leeward thrusts, including the Alleghany and Wopnay orogens, developed broad fold and thrust belts. Windward orogens, including the Alpine fault of New Zealand, the Grenville front, the Mauritanide belt, and the Thelon orogen, developed sharp metamorphic fronts. Interactions between orogens also occur. For example, during a late stage of its evolution, the Thelon orogen came within a rain shadow when uplift began on the Wopnay orogen [ Hoffman and Grotzinger, 1993].