拉萨地区晚第四纪地壳的抬升与拉萨河的向南迁移

根据数字高程模型、遥感资料分析及实地调查,拉萨地区在近南北向挤压的作用下,与NWW向的主构造格架断裂走向相一致,近东西向的分水岭和沟谷沿南北方向相间分布。自分水岭附近的山顶剥蚀面至盆地,坡度由平缓、较陡、平缓逐渐过渡,反映了山顶剥蚀区、强剥蚀区、堆积区的依次变化,表现了一个完整的构造、气候控制的地貌剥蚀、堆积过程。在同样的气候条件下,拉萨地区晚第四纪以来的新构造运动整体表现为自北向南的掀斜,造成北侧剥蚀较强,下游河流得以携带更多的泥沙物质,北侧冲、洪积扇汇入拉萨河的物质较南侧多,加之地壳自北向南的掀斜,使得拉萨河拉萨段向南迁移,堆龙曲河口冲积扇也向南迁移。

LATE QUATERNARY CRUSTAL UPLIFT AND SOUTHWARD SHIFT OF THE LHASA RIVER IN THE LHASA AREA, TIBET

On the basis of the field investigation,digital elevation modeling and remote sense data analysis,the authors think that the nearly E-W-trending watersheds and valleys in the Lhasa area are distributed alternately from north to south under the nearly N-S compression,parallelly to the WNW-trending main faults.From the erosional surfaces of the summits near the watersheds to catchment basins,the hill slope changes from being gentle to being relatively steep and again to being gentle,indicating a successive change from the hilltop erosion zone through intense erosion zone to sedimentation zone and a complete process of erosion and sedimentation under the tectonic-climatic control.Under the same climatic conditions,the Lhasa block tilted to the south during the late Quaternary neotectonic movement,and as a result,the northern Lhasa block has been eroded more intensely than the southern block,and therefore the downstream reaches carry more sand and mud.As more alluvial-diluvial fan deposits from the northern block flow to the Lhasa River than those from the southern block and in addition the crust tilts from north to south,the Lhasa reach of the Lhasa River has shifted toward the south and so has the alluvial fan of the Doilung Qu river mouth.