藏东南帕隆藏布现今河流地貌特征及其晚第四纪演化

Fluvial geomorphological characteristics and its evolution of the Parlung Zangbo in Southeast Tibet

河流沉积与地貌对构造与气候的变化极为敏感,可记录区域构造活动、气候变化和环境演变等多方面的丰富信息.由于独特的构造背景与气候条件,帕隆藏布不仅成为雅鲁藏布水系水量最大的支流,而且其流域在藏东南地区占有重要的地位.帕隆藏布流域内地表过程活跃且河流地貌演化过程快速,是揭示青藏高原东南部构造地貌演化的重要载体.通过对该河流地貌的形态学和沉积学分析发现,帕隆藏布河流形态具有明显的线状特征,其干流近似直线展布,而主要支流呈羽状分布,两者多呈直角交汇,表明河流形态明显受到嘉黎断裂带的构造形迹控制.进一步利用光释光和14C定年方法,对帕隆藏布的晚第四纪河流地貌演化,尤其是干流和东久河支流的晚第四纪河流阶地进行研究后发现,末次冰期以来的气候变化导致帕隆藏布的晚第四纪河流地貌呈现出典型的分段式特征,根据海拔高度主要可划分为3段:1)海拔2600 m以下的河谷地貌呈V形峡谷,河谷比降大,阶地沉积年龄均在9.0~2.0 kaBP间,沉积属性以河流相和坡积相为主,表明是全新世以来气候变暖条件下形成的;2)海拔2600~3300 m的中游段河谷呈冰蚀围谷盆地、U形槽谷等,河谷比降小,河岸谷坡坡度小,主谷两岸冰碛垄发育,存留了古冰缘地貌遗迹,阶地沉积属性以古湖相、冰水相及河流相为主,测年结果在29.8~10.9 kaBP和50.9~39.8 kaBP间,显示其曾经为末次冰期和冰消期冰缘湖泊体系,后被现今的帕隆藏布所贯通;3)海拔3300 m以上河流地貌为典型的冰川U形槽谷,谷底平坦,发育现代冰湖,仅发育Ⅰ级阶地并上面覆有冰碛物堆积体,有末次冰期的冰缘地貌遗迹,但主要受周围海洋性冰川作用,呈现现代冰缘地貌特征.整体上看,帕隆藏布的现今河流地貌上、下游两端年轻,主要形成于全新世期间;中游的河流地貌出现较早,残留了末次冰期和冰消期的冰缘地貌特征,并保留了广泛的古冰湖相沉积物.因此,帕隆藏布现今的河流形态主要出现在末次冰期以来.

The river system shapes the landscape of the mainland and controls the transport of erosive material to the ocean. Fluvial geomorphology is extremely sensitive to tectonic movement and climate change, and records rich information on regional and global tectonic activities, climate change and environmental evolution events by topography, sedimentology and geochronology. As the largest tributary of the Yarlung Zangbo river system, Parlung Zangbo occupies an important position in the southeastern Tibet. Due to the unique tectonic setting and climatic conditions, the surface processes in the Parlung Zangbo Basin are active and the fluvial geomorphology is rapidly evolving, but it has not received enough attention of scholars. In this paper, by field investigation and sampling, morphological and sedimentological analysis of fluvial geomorphology;using OSL and 14C dating, combined with relevant research results, the fluvial geomorphology evolution of the Parlung Zangbo Basin, especially the terraces formation and evolution of the main stream and the Dongjiu River tributary research has been carried out systematically. We found, firstly, the shape of the Parlung Zangbo is relatively straight and linear. The main stream is approximately linear, the tributaries are pinnately distributed, and the intersection of the main stream and the tributary is at right angles, indicating that the river form is affected by the Jiali fault zone. Secondly, the climate change since the last glacial period has divided the fluvial geomorphology of the Parlung Zangbo into three segments. 1)The valleys below 2 600 m above sea level are “V” shaped canyons. The deposit ages of the terraces are between 9.0 kaBP and 2.0 kaBP. The sedimentary properties are dominated by river facies and slopes, which proves to be the climate warming conditions since the Holocene. 2)The middle reaches of the valley with an altitude of 2 600~3 300 m are ice-eroded valley basins and U-shaped troughs. The ice ridges on both sides of the main valley are developed, and the remains of ancient ice margins are preserved. The deposits are mainly composed of paleo-lake phase, ice-water phase and river facies. The dating results are between 29.8~10.9 kaBP and 50.9~39.8 kaBP, indicating that it was once the ice-edge lake system, and it dredged by the current Parlung Zangbo. 3)The elevation of the river above 3 300 m is a typical glacier U-shaped trough, with a flat bottom and a modern icy lake. It only develops the I terrace and is covered with glacial deposits. It has the remains of the last glacial ice margin;but it mainly shaped by the warm glaciers. Showing the characteristics of modern ice margins. On the whole, the upper and lower reaches of the Parlung Zangbo River formation are late, which is the landform formed since the Holocene;the middle reaches of the river are relatively old, retaining the ice margin geomorphology of the last glacial periods, and retaining a wide range of paleolake deposits. The lacustrine deposits show that the current fluvial topography of Parlung Zangbo has been formed since the last glacial period, especially since the Holocene.