青藏高原的剥蚀与构造抬升*

利用宇宙成因核素¹⁰ Be和²⁶ Al对西昆仑和可可西里北部地表基岩的剥蚀速率进行了测定,得到的结果是:西昆仑的平均剥蚀速率为12m/Ma, 可可西里北部的平均剥蚀速率为15m/Ma。裂变径迹和宇宙成因核素这两种手段所得到的平均剥蚀速率的时间尺度是不同的,前者得到的是几个至数十百万年的平均剥蚀速率,而后者得到的是十几至几十万年的平均剥蚀速率。比较通过这两种手段得到的青藏高原北部和中部的平均剥蚀速率可以发现其平均剥蚀速率从20Ma以来的100m/Ma以上减少到了最近几十万年以来的10m/Ma,我们认为这一剥蚀速率下降的趋势反映了青藏高原从中新世到第四纪晚期构造活动的减弱,据此推断青藏高原北部和中部的隆升应该主要发生在第四纪晚期之前,而不是在最近的几十万年。

EROSION AND UPLIFT OF TIBETAN PLATEAU

We have measured average erosion rates, using cosmogenic nuclide ^10Be and ^26Al dating technique, in northwest and the northern part of Tibet. The average erosion rates are 12m/Ma and 15m/Ma, respectively. Cosmogenic nuclide dating and fission-track thermochronology are techniques which can confine landform denudation rate in different time scales. The former confines erosion rates in a time interval of hundreds thousands years and the latter confines erosion rates in a time scale of some million years. Comparison of erosion rates obtained via the two methods we could find that the average erosion rate in northern and interior Tibetan Plateau decreased from over 100m/Ma in Miocene to 10m/Ma in the last hundreds thousand years. We attribute the decrease of erosion rates to reduced tectonic activities in the last hundreds thousands years in the Tibetan Plateau. Therefore the major uplift of Tibetan Plateau should have occurred before Late Quaternary.