Field work and topography analysis show that remnant, local areas of a low-relief landscape or erosion surfaces are geographically continuous across the southeastern Tibetan Plateau margin. We correlate these remnant surfaces as a paleolandscape that formed at low elevation. Remnants of this paleolandscape are preserved because incision of the fluvial system has been largely limited to major rivers and principal tributaries and has not yet progressed throughout the entire fluvial network. The incomplete adjustment of the fluvial system signals initiation of rapid bedrock incision into a developing plateau margin, and erosional denudation is concentrated in the major river channels. This interpretation contradicts earlier notions that low-gradient, regional topography is the product of regional elevation reduction by intense landscape dissection due to the presence of several large southeast flowing rivers. The modern altitude of the reconstructed paleolandscape (or "relict landscape") constrains the vertical displacement of the plateau surface in response to crustal thickening and subsequent erosion during the lateral growth of the Tibetan Plateau. Regional preservation of the relict landscape is consistent with minor surface disruption by late Cenozoic thrust faults and folds and supports a model of distributed lower crustal thickening. Because significant erosion is limited to narrow river gorges, an increase in the plateau elevation due to isostatic rebound is minor. Therefore we propose that the modern elevation of the relict landscape reflects isostatically compensated thickening of the lower crust.