Abstract: The geomorphologic structure in the southeastern Tibetan Plateau is one of the important indexes for the expansion and deep dynamic process of Tibet. There are two different understandings for the geomorphologic structure in the southeastern Tibetan Plateau, i.e. gradual change and abrupt change. The gradient model suggests a gradual topographic reduction towards southeast which is an important evidence for the lower crust channel flow. The abrupt model considers that the southeast boundary of the plateau shows an abrupt change of topography in a zone of 50~200km wide which is controlled by the Yarlung-Yulong fault system. Here, we describe the morphotectonic feature in detail of the Sichuan-Yunnan block on the southeast edge of the plateau through the digital elevation model(DEM)analysis, further review the structural controls on the geomorphologic structure by combining the tectono-thermochronology analysis, and evaluate the southeastward spreading mode of the plateau.
The topographic arithmetic progression ranking by using the DEM of the Sichuan-Yunnan block reveals three geomorphologic steps gradually lowering from the northwest to southeast. The switching of hypsometric integral(HI)value and the anomaly of SL/K value(where SL is stream length-gradient index and K is altitude of the profile)of river systems all occur on the edge of terraces. The high terrace is located on the north of Muli-Yulong with average elevation~4 200m; the secondary level of terrace extends to the Yanyuan-Lijiang area with average elevation~3 000m; and the third level is the region between the Jinhe-Qinghe and Anninghe with average elevation~1 800m.
Structure investigation reveals that all the topographic boundaries between different terraces are consistent with regional major faults. The Muli thrust fault and Yulong thrust fault control the southeast edge of the high terrace, the Jinhe-Qinghe thrust fault separates the second and third level of terrace. The coincidence between topography boundaries and faults suggests that the formation of the stepped geomorphology on the southeast edge of the plateau were induced by the fault activities, reflecting the fault-controlled southeastward stepped-expanding mode of the plateau.
The fission-track(FT)dating of the granites at the hanging wall of the Yuling-Muli Fault reveals fast uplift during~27~22Ma BP, reflecting the major thrusting along the Yulong-Muli Fault, which is consistent with the early-stage activity (~30~25Ma BP) of the Longmenshan Fault. Therefore, the high terrace was formed during the Oligocene to early Miocene with the thrusting of the Yulong-Muli Fault. Tectono-thermochronology analysis also reveals the major thrusting of the Jinhe-Qinghe Fault occurred during~18~11Ma BP, indicating the middle terrace was formed in the middle Miocene, which also could correspond to the middle Miocene(~15~10Ma BP) activity of the Longmenshan Fault.
Therefore, the thrusting faults controlled stepped terrace geomorphologic structure and the stepwise expanding mode under combined movements of large-scale thrusts and strike-slip faults at the southeast edge of Tibetan Plateau during the late Cenozoic do not support the lower crust channel flow model.

Key words: southeastern Tibet, Chuandian block, stepped geomorphological structure, Muli-Yulong Fault, Jinhe-Jinghe thrust fault

摘要： 青藏高原东南部的地貌结构是高原隆升深部动力过程与高原扩展的重要指标之一，存在受下地壳流驱动的渐变模型和受宽约50~200km的雅砻-玉龙断裂系控制的陡变模型2种不同认识。文中基于30m分辨率的SRTM数据进行数字高程分析，利用高程和水系参数对研究区地貌加以提取和分析，结合野外地貌和构造调研的结果以及前人的相关研究，对高原东南缘川滇地块中部构造地貌细结构进行了详细的解析。研究认为，青藏高原东南边界具有明显的台阶式构造地貌结构，不同台阶梯度带受不同时期发育的NE-SW向断裂控制。其中一级边界位于木里-玉龙断裂，控制了平均海拔4 200m的高原面的东南边界，是渐新世-中新世早期构造抬升的结果；二级边界受中新世中期逆冲活动的金河-箐河断裂控制，其构成丽江-盐源一带海拔中等（约3 000m）、相对低起伏区域的东南边界。高原东南边界的台阶式构造地貌结构反映了高原向SE的前展式逆冲扩展。这种扩展模式并不支持下地壳管道流连续变形模型。

关键词: 青藏高原东南缘, 川滇地块, 台阶式构造地貌, 木里-玉龙断裂, 金河-箐河断裂