循化-贵德地区黄河水系河流纵剖面形态特征及其构造意义

青藏高原东北缘循化-贵德地区晚新生代构造活动强烈,同时晚新生代黄河的发育和演化也提供了研究高原构造隆升扩展以及黄河侵蚀下切过程的理想场所。黄河水系通过调整河流纵剖面的形态在垂向上响应不同的构造隆升以及河床岩性的变化。利用数字高程模型数据,提取黄河水系河流纵剖面形态,利用基岩河道河流水力侵蚀模型（stream-power incision model）为依托,尝试揭示晚新生代以来强烈构造活动在黄河水系河流纵剖面形态特征上的表现。分析发现,在循化-贵德地区岩性分布均匀的新生代盆地内部,黄河水系纵剖面在河流水力侵蚀模型坡度-面积双对数图解中大部分表现为上凸特征,即基岩河道的河床随时间的变化为正值,指示了基岩的隆升速率大于河流的下切侵蚀速率。在河流流经盆地内部不同岩性地区,特别是前新生界地层区,河流纵剖面也表现为一定的上凸特征,表明了新生界与前新生界地层岩性差异对河流纵剖面的控制作用。综合来看,青藏高原东北缘循化-贵德地区黄河水系河流纵剖面形态特征是晚新生代强烈构造活动和岩性差异共同控制的结果,揭示了循化-贵德地区造山系统地形发育仍然处于前均衡或接近均衡状态,同时也从侧面表明青藏高原东北缘地区是整个高原隆升扩展的最新和最前缘部位的时空格局特征。

CHARACTERISTICS OF THE HUANGHE RIVER LONGITUDINAL PROFILES AROUND XUNHUA-GUIDE AREA(NE TIBET)AND THEIR TECTONIC SIGNIFICANCE

Xunhua-Guide area is a part of Late Cenozoic tectonic orogenic systerm along the northeastern Tibetan Plateau.The section of the Huanghe River in this area provides an ideal site to unravel the process of the plateau uplift and incision of the Huanghe River.Although some terrace studies within this region have shown a coupling pattern between the plateau uplift and Huanghe River incision,more analysises are still needed to test the impact of the ongoing uplift on river evolution.It is known that longitudinal profile can be well adjusted to respond the vertical differential uplift and lithological change along a river thus in this paper we have carried out a study on longitudinal profile of the Huanghe River and its 38 tributaries.By utilizing the digital elevation model database,longitudinal profiles along the Huanghe River around Xunhua-Guide area were extracted and analyzed.Our study indicates that 65.8%(25/38)of the longitudinal profiles are convex up,while 34.2%(13/38)show concave features.Determination of the knickpoint location along the profile indicates that 64.0%(16/25)of the 25 convex profiles have their knickpoints near the basin boundary or the contact between basin and range systerm.On the basis of the stream-power incision model,the tectonic expression on the profile is determined.Our logrithmic slope-area plots for the Huanghe River drainage declare that 68.4%(26/38)are upward-convex and the left are almost near the linear state.Consequent analysis reveals that the upward-convex features of the profiles and slope-area plots can be explained by the elevation of the bedrock profile getting higher and higher and the uplift being beyond the incision around the target area.Additionally,we know that in the areas where the rivers run across the pre-Cenozoic high resistant bedrocks,the longitudinal profiles also show a similar upward-convex feature,which indicates that the differential lithology distribution is also another control of the profile feature.In a general view,the longitudinal profiles of the Huanghe River tributaries are well controlled by the Late Cenozoic tectonics and lithological difference,which indicates that the landscape around Xunhua-Guide area is in the pre-steady state or near to the steady state.Meanwhile,our analysis also agrees,to some degree,that the northeastern Tibetan Plateau is one of the newly uplifted parts in the whole plateau frame.