藏南古近纪前陆盆地演化过程及其沉积响应

藏南地区从三叠纪至古近纪经历了从洋盆(喜马拉雅特提斯)的形成、扩张、衰减、关闭,直至转换成前陆盆地的过程。被动大陆边缘阶段(T-K),在印度陆块北缘形成了从碎屑岩陆架到碳酸盐台地的沉积序列。从古近纪初开始,西藏特提斯关闭,形成周缘前陆盆地体系(由褶冲带、前渊带、前隆带和隆后盆地等单元构成)。随着褶冲带的上叠式逆冲,形成前渊盆地。当前陆推覆体进一步向印度克拉通推进时,前陆隆起亦随之逐渐向克拉通方向迁移。该带表现出一个海平面相对上升的过程,形成碳酸盐缓坡。随着前陆推覆体进一步逆冲,前陆隆起继续隆升并最终露出水面,导致其后的隆后盆地转变为半局限环境。始新世晚期,前陆盆地回返,海水从东向西逐渐退出西藏地区。生物相和沉积相是盆地沉积环境演化的物质表现,在藏南古近纪沉积中可识别出13种生物相和14类沉积相。藏南古近系的超层序,是在印度板块与亚洲板块碰撞背景下形成的,其沉积环境是一个构造活动极为强烈的前陆盆地。前陆盆地在剖面上具明显的不对称性,靠近褶皱山系一侧为陡坡地形,靠近地台一侧为缓坡。每个大型的三级层序都是非对称的,以发育具有独特的岩性和古生物特征的低水位体系域、海进体系域和高水位体系域为标志。藏南前陆盆地的演化符合通行.

The Paleogene Foreland Basin and Sedimentary Responses in the Southern Tibet:Analysis on Sequence Stratigraphy

Triassic to Paleogene,Tethyan Himalaya went through the forming, exaggerating, attenuating, closing and finally transforming into foreland basin of the oceanic basin. During the passive continental margin stage (T-K),a set of sedimentary sequence of clastic shelf to carbonate platform was developed in the northern Indian continent margin. At the earliest Paleogene,Tibet-Tethys was closed down,causing the forming of the southern Tibet peripheral foreland basin system. In late Eocene,the returning of the southern Tibet foreland basin caused the sea-water to retreat gradually from Tibet area from east to west. The Paleocene supersequence of the southern Tibet was formed under the background of the India/Asia collision,the sedimentary environment of which was a foreland basin with strong tectonic activity. Sedimentary facies is the substance exhibition of depositional environments. In this study, 13 biofacies and 14 sedimentary facies were recognized from the Paleogene sediments of the southern Tibet. The supersequence with 200-2000m thickness and 30Ma age is composed of nine large-scale third-class sequences which could be regionally correlated on the outcrops. Small-scale third-class parasequence sets, which resulted from the global sea-level change and unbalancedly flexual subsidence of basin basement caused by collision and only can be locally correlated, formed larger-scale sequences. Every large-scale third class sequence is asymmetrical and characterized by LST,TST and HST with special lithologic associations and palaeontological assemblages. The evolution of the southern Tibet foreland basin is accordant with the usual evolutional model of foreland basin and its sedimentary sequence shows that it had a multi-cycle sediment-tectonic evolutional history.