关于夷平面的科学问题——兼论青藏高原夷平面

On Problems of Planation Surface -A Discussion on the Planation Surface in Qinghai-Xizang Plateau

长期地貌演化研究表明 ,夷平面的形成有 4种基本方式 :准平原、山麓剥蚀平原、双层水平面和冻融剥夷平原。它们的形成都需要上千万年至数亿年的构造相对稳定时期。青藏高原上的层状地貌面可以划分为两级夷平面和一级剥蚀面。山顶面形成于渐新世至中新世早期 ;主夷平面是以双层水平面或山麓剥蚀平原形式发育的 ,大致形成于 2 0～ 3 .6MaB .P .期间 ,完成时的高度低于 10 0 0m ;剥蚀面形成于 3 .6～ 1.7MaB .P .期间。

Planation surface is a remnant of landform in old age and the erosion surface is of a mature landform, according to the Davisian cycle. Studies on the long-term landscape evolution show that there are four types of planation surface, peneplain, pediplain, double leveling surfaces and cryoplain. The peneplain is formed through lowering of relief in wet region. The pediplain is developed by slope retreating in dry region. The double leveling surfaces appeares in Savanna landscape. The cryoplain is formed in periglacial environment. Measurement of erosion rates and retreating rates of slope indicated that formation of planation surface needs the tectonic equability for severl million years or thousands million years. In the the Qinghai-Xizang Plateau, there are two planation surfaces and one erosion surface, Summit Surface, Main Surface and Erosion Surface. The Summit Surface have formed in the Oligocene and early Miocene, and the Main Surface between 20- 3.6Ma B.P. Tectonics on the Qinghai-Xizang Plateau were relatively stable between 3.6Ma B.P. and 1.7Ma B.P. and the Erosion Surfaces was formed. Because the Main Surface is a 'jumping-off point' of the Qinghai-Xizang Plateau, studies on its mode of formation and height have great significance in determining the altitude of the Qinghai-Xizang Plateau before uplift. Synthesizing the proofs from mammal fauna, regolith and geomorphology, we think the Main Surface was developed as a typical double level surface. There could be the peneplain in the south of it and the pediplain on north. Its altitude should not exceed 1000 m when it was formed. The Intense tectonic uplift, which resulted in the present Qinghai-Xizang Plateau with an altitude of 4500-5000m, happened in last 3.6Ma when the Main Surface was destroyed.