构造、气候与砾岩——以积石山和临夏盆地为例

TECTONIC EVENTS,CLIMATE AND CONGLOMERATE: EXAMPLE FROM JISHISHAN MOUNTAIN AND LINXIA BASIN

探讨构造、气候与砾岩的关系对于研究青藏高原隆升的时间和方式具有重要意义。裂变径迹热年代学表明积石山地区于8MaB．P．开始构造变形，与碎屑颗粒裂变径迹结果和生长地层结果一致，而明显早于积石砾岩出现的时间（3．6MaB．P．）。通过区分岩体隆升与地面隆升之间的差别，文章提出一种新的模型，以解释隆升、气候和砾岩之间的关系。积石山岩体于8MaB．P．开始隆升，隆升初期，尽管岩体隆升1500—2000m，但是由于积石山上覆的新生代地层易于剥蚀，在花岗岩基底被剥露到地表之前，地表只有少量隆升或没有隆升。随着沉积地层被剥蚀殆尽，基岩暴露于地表，地面隆升速率加快。约3．6MaB．P．，积石山隆升约200—900m，造成了地形雨和发源于积石山的横向河流的出现，这些横向河流把积石山的花岗岩搬运到盆地中沉积下来，形成积石砾岩。

The timing and mechanisms of the uplift of Tibetan Plateau are among key issues of continental tectonics. They are still poorly understood because of the lack of a widespread and precise constraint on timing determination of the deformation of the Plateau as well as the complicated relationship between rock uplift and surface uplift of the Plateau, the latter of which is caused by the competition between rock uplift rate and erosional exhumation rate which are controlled by tectonic and climatic factors as well as by variation in rock properties. Jishishan Mountain is located at the Qilian Mountain and the northern edge southeastern termination of the Lajishan of Qinling Mountains is at its south. Mountain, which is a branch of They together comprise of the northeastern margin of Tibetan Plateau. Jishishan Mountain separates the Linxia Basin and Xunhua Basin. Although Jishishan area has been well studied, the onset time and style of deformation are still debating issues. In this paper, we present the fission track data to constrain the onset time of deformation in Jishishan area. Modeling result of the apatite fission track data sampled from Jishishan Mountain suggests that a rapid cooling occurred at about 8MaB. P. This is consistent with the onset time of deformation suggested by both the detrital apatite fission track data and the stratigraphic data obtained from the Linxia Basin, but earlier than the onset time of the Jishishan conglomerate deposition （3.6MaB. P. ）. In addition, the distribution of Jishishan conglomerate is well correlated to the traverse river originated from the Jishishan Mountain. This suggests that Jishishan conglomerate was resulted from the traverse river. The surface uplift is determined by the result of competition between rock uplift and erosion. Noticing the difference and relationship between surface uplift and rock uplift, we suggest that the deformation in Jishishan area （when rock uplift started） occurred at about 8MaB. P. and sustained until present. Presuming that both the climate and tectonic deformation are stable （with a constant rock uplift rate）, the erosion rate change is resulted from the change of lithology. The development of Jishishan Mountain in Late Cenozoic is thus inferred as follows. In early period （8 - 5MaB. P. ） of the deformation, the overlain Cenozoic sediment which was subject to erosion was quickly removed. Therefore, the surface was hardly uplifted although the rock uplift was about 1500 - 2000m. The surface uplift rate was not enhanced until the resistant bed rock was exhumed to surface at about 5MaB. P. At about 3.6MaB. P. , Jishishan uplifted to about 200 -900m, high enough to result in the relief precipitation and traverse river origination. The conglomerate deposition in Linxia Basin was consequently formed by the traverse river transportation, thus the Jishishan conglomerate cannot be used as a proxy of onset deformation and uplift of Jishishan Mountain.