<p>The mechanism governing late Cenozoic tectonic deformation in the Southeastern Tibetan Plateau (SETP) remains debated. Two mainstream theoretical models dominate the discussion: the “lateral block extrusion model” and the “continuous deformation model.” The central debate focuses on whether strong deformation is concentrated at block boundaries or dispersed within blocks. In this context, the Litang fault system (LTFS), as a large strike-slip fault within the Sichuan–Yunnan rhombiform block, has remained activity since the Quaternary. Given that rivers are sensitive geomorphic units that respond to tectonic activities, a systematic analysis of their geomorphic indices provides a valuable approach for exploring the dynamic mechanisms of regional tectonic deformation. Thus, studying the index of relative active tectonics (IAT) along the LTFS offers an opportunity to verify the deformation mechanism within the SETP. We analyzed geomorphic indices from 78 basins along the fault and found that 72% of them fall into IAT class 1, indicating strong tectonic activity. The continuous deformation model may be a key mechanism for late Cenozoic deformation in the SETP. Furthermore, the proportion of class 1 IAT basins in the LTFS is significantly higher than that of the Xiaojiang fault on the block boundary (24.7%), despite the latter exhibiting higher slip rates in GPS observations. We propose that this discrepancy arises because the mid-south segment of the LTFS lies on the deforming edge of the SETP, and that preexisting high relief amplifies the geomorphic signature of tectonic activity.</p>