The first-order shape of a mountain range over a blind thrust fault is controlled by the competition between the geometry of the underlying thrust fault and local base levels that flank the emerging range. We use an elastic half-space model to show that the surface displacements caused by slip across a blind thrust are sensitive to the geometry and depth of the fault, as well as to its growth history. Accumulation of this displacement field over time creates a tectonic topography; in the absence of erosion, this is the same as the topography of the resulting range. Erosion modifies this tectonic topography by creating local relief on both flanks of the range and forcing divergence between the tectonic and topographic divides. Importantly, erosion is modulated by the relative base levels on either side of the range. We show that in many cases the position of the topographic divide is predicted by relative base level through a simple geometric ratio, such that the average slope of both range flanks is the same. Importantly, attributes of the topography on one flank of an emerging range depend on what happens to base level on the opposite flank. The role of relative base levels in determining the first-order range shape illustrates the importance of regional-scale events (e.g., sediment supply to piggyback basins or the evolution of a regional fluvial network) in the development of mountainous topography in a complex fold-and-thrust belt system.