青藏高原东南缘构造地貌、活动构造和下地壳流动假说

Tectonic geomorphology,active tectonics and lower crustal channel flow hypothesis of the southeastern Tibetan Plateau

利用高精度的SRTM数字高程模型(DEM),定量勾画出青藏高原东南缘大尺度地形地貌的特征。分析表明,高原东南缘地貌特征为"负地形",即海拔高程与地形坡度,与地形起伏度之间均为负相关关系,与高原中部的"正地形"——海拔高程或地形坡度与地形起伏度之间呈正相关关系,形成鲜明对比。但是,在高原东南缘,在河谷之间保留有高海拔、低起伏的残留面。这些残留面与高原内部的平坦面具相似的渐变地貌特征,从腹地的正地形逐渐变为川西的高海拔平坦面与深切河谷相间的负地形。虽然随着河流下切深度往南逐渐增加,残留面虽越来越少,但仍然可以识别,最终终止在雅砻江逆冲断裂带附近,该断裂带以南地区没有明显负地形特征。北东向展布的雅砻江逆冲断裂带对应着50～200 km宽的地形相对陡变带。综合区域新构造和构造地貌研究的最新成果表明:1)雅砻江逆冲断裂带可能代表着现今正经受侵蚀改造和弱化的高原老边界,该边界以北和以南地区抬升历史不同;2)三江地区的峰值抬升期已过,目前以侵蚀为主。虽然不能排除与河流侵蚀对应的均衡反弹抬升作用,但具有真正意义的地壳增厚型的构造抬升较弱。国际上流行的高原东缘下地壳流动模式的依据之一是从高原内外流分界线到南中国海,存在一个区域上延伸数千公里的抬升前低海拔"类夷平面"的残留面。地貌特征,构造和地质综合分析都表明高原东缘不存在这样的类夷平面,不支持解释高原东缘地形演化和相应构造变形的下地壳流动模式。

We use high-resolution 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＇, i. e., 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. High-elevation and low-relief （HELR） surface, which is preserved between large rivers in SE plateau, is generically linked to the low-relief 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 NE-trending 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 ＇ peneplain＇-type pre-uplift, low-relief surface first formed at low elevation, extending continuously all the way to the South China Sea shore,before being warped upward in the late Miocene-Pliocene by lower crustal channel flow.