1.Key Laboratory of Continental Collision and Plateau Uplift|Institute of Tibetan Plateau Research|Chinese Academy of Sciences|Beijing |100085； |2.Laboratoire de Tectonique|Institut de Physique du Globe| 75005 |Paris|France； |3.Institute of Geology|Chinese Academy of Geological Sciences|Beijing |100037)
We use highresolution SRTM DEMs to quantify the large scale topography and geomorphology in southeastern margin of the Tibetan Plateau. The southeastern plateau region is characterized as ‘negative topography’，ie.，a topography where there is an inverse or negative correlation between elevation and relief and between elevation and mean slope gradient. This is in sharp contrast with the positive topography in plateau interior，where elevation is positively correlated with relief and mean slope. Highelevation and lowrelief(HELR)surface，which is preserved between large rivers in SE plateau，is generically linked to the lowrelief surface in plateau interior and shows a gradual transition to less preservation because of southeastwardly increased river incision. The HELR surface appears to stop near the NEtrending Yalong thrust belt，which coincides with a 50～200 km wide relatively steep drop in elevation across this plateau margin. Morphometric analysis，in conjunction with recent studies on regional tectonic and tectonic geomorphology leads us to propose that 1)The Yalong thrust belt defines an eroded old plateau margin，the region to the northwest may have different uplift history than the region to the southeast； 2)The Three Rivers region has passed its major phase of uplifting，and now has been undergone degradation by fluvial erosion，tectonic uplift is weak. Our analysis does not support the inference of a ‘peneplaintype preuplift，lowrelief surface first formed at low elevation，extending continuously all the way to the South China Sea shore，before being warped upward in the late MiocenePliocene by lower crustal channel flow.