西秦岭晚白垩世原型盆地——新生代青藏高原隆起的背景

Late Cretaceous proto-type basin in the western Qinling:Background of Cenozoic uplifting of Tibet Plateau

青藏高原是新生代隆升的构造地貌。本文试图通过对青藏高原东北缘的西秦岭上白垩统的研究,揭示新生代青藏高原隆升之前的晚白垩世原型盆地和构造地貌背景,这对探索青藏高原隆起过程的起始至关重要。西秦岭腹地岷县地区分布着一套角度不整合于下伏不同时代地层之上且沉积序列相近的上白垩统红层地层。该套红层现今呈离散分布,故多被认为是西秦岭陆内造山阶段不同山间盆地或走滑拉分盆地的沉积物。对不同高程和露头上的该套红层与下伏地层之间角度不整合面地质特征对比分析,特别是对不整合面之上含砾砂岩和砂岩的粒度组成和颗粒的显微结构研究表明,该套红层底部的胶结砂砾岩和其上的红色砂岩皆具有沙漠沉积的特征,也就是说西秦岭晚白垩世曾出现过干旱沙漠环境。沙漠环境的出现不仅需要干旱炎热气候条件,而且需要相对平坦的地形地貌空间条件。据此,本文提出了西秦岭在晚白垩世可能处于相对平缓的古地貌状态。现今这套红层不连续分布在相对平坦的山顶面,其下部以洪积砾岩、河床砾岩和砂岩、沙漠相砂岩互层,上部则以河一湖相红色泥岩、粉砂岩和细砂岩等细碎屑沉积为主。经研究分析认为西秦岭在上白垩统红层开始沉积之时,总体已处于地形起伏不大的洪积平原和宽谷型河流地貌,而晚期则演变为平坦湖盆地貌,其原始盆地为统一宽缓的湖相盆地。现今上白垩统红层地层和角度不整合的弥散性分布是在印度板块-欧亚板块碰撞汇聚的动力学背景下,青藏高原崛起和逆冲-走滑作用以及地壳不均匀抬升-侵蚀的结果。西秦岭晚白垩世相对平坦的古地貌状态确定可为研究西秦岭中新生代陆内构造过程和高原隆升与板内变形在东北缘的扩展提供重要线索和参考标志。

The Qinghai-Tibet Plateau is a large and high elevation tectonic relief having uplifted since Cenozoic. Its temporal and spatial uplifting processes is one of many key issues in research on the Qinghai-Tibet Plateau. By studying on the Upper Cretaceous red bed strata in the western Qinling, this article attempts to restore Late Cretaceous prototype basin and its reflecting relief features of western Qinling, the northern margin of the Qinghai-Tibet Plateau. This study will provide constrain on essentially starting background before the Cenozoic uplift of the Qinghai-Tibet Plateau. There are a set of Late Cretaceous red bed strata in Minxian-Wushan area, West Qinling Mountains. These red bed strata overlaid on different-aged strata separated by angular unconformities, having a similar sedimentary sequence. These strata are presently dispersed, being considered as different intermontane basins or strike-slipping pull-part basins during intracontinental tectonic processes. The observations and comparisons on the unconformities between these red bed strata and their underlying strata from different outcrops and various altitudes and the study on grain size distributions and microscopic features of clastic particles, show that the cemented sands of conglomerates and red sandstones over the unconformity has similar characteristics to a desert dune deposition, which may mean ever existence of an arid desert environment in the West Qinling Mountains. It is well known that appearance of desert environment requires not only the hot and arid climate conditions, but also need relatively flat topography in spatial conditions. Therefore, it is suggested that the present- day West Qinling Mountains was ancient relatively-flatted topographic landform at the beginning of the Late Cretaceous. In addition, Upper Cretaceous red bed strata in West Qinling Mountains, at present, distributed discontinuously on the relatively-flatted top surface. The bottom is characterized by conglomerates and sandstones interbedded with desert sediments and the upper is mainly composed of red mudstones, sihstones and fine sandstones in river- lacustrine facies fine clastic sediments. Based upon above data, this paper speculates that the West Qinling Mountains was broad-range river topographic relief at the Early Late Cretaceous and evolving into the flat-like geomorphology of lacustrine basin at the end of Late Cretaceous. The proto-type basin should be wholly one wide lacustrine basin. The present dispersed distribution of Upper Cretaceous disconformities and red beds resulted from regional differentiated uplift of the crust and erosion during Cenozoic Tibetan Plateau rising, thrusting, strike-slipping due to India-Eurasia plate collision. Relatively-flatted topographic relief of the West Qinling Mountains in Late Cretaceous provides important clues and reference for understanding the spreading of Tibetan Plateau uplifting and intraplate deformation in its northeast margin.