羌塘盆地中央隆起带的重磁场证据及其构造意义

Characteristics of the central uplift zone in Qiangtang Basin and its tectonic implications:Evidences from airborne gravity and magnetic data

对于羌塘盆地是否存在横贯东西的中央隆起带,目前学术界仍有分歧.本文提供的最新高精度航空重、磁资料证实存在呈东西向贯通羌塘盆地的中央隆起带,并对该带的构造特征进行了精细刻画.隆起带受南北两侧深大断裂控制,其空间跨度(宽度)由西向东逐渐收敛,并被一组近南北向的隐伏断裂系切割、左滑错动.重、磁场资料还显示中央隆起带在双湖东、西两侧存在明显差异:西段基底大规模隆起,基岩深度一般在3~5 km以内,明显浅于南北羌塘坳陷7~15 km的基底埋深;东段基底隆起幅度明显降低,主要表现为潜伏的低隆起,其中双湖—雅曲段基底埋深5~7 km,雅曲—岗尼段基底埋深7~9 km;即中央隆起带基底自西向东“台阶状”降低,隆起的幅度和分布范围受到近南北向断裂控制.构造分层表明,与南羌塘地块相比,北羌塘地块的基底隆起幅度小、稳定性更好.南北羌塘基底地球物理属性的显著差异说明羌塘盆地并不存在统一的前寒武系变质基底,中央隆起带的形成应该与古特提斯洋关闭时形成的混杂岩带有关.

Academia still has different opinions on whether there is a consecutive central uplift belt in the Qiangtang Basin.The latest high-precision airborne gravity and aeromagnetic data provided in this article verify the existence of the central uplift belt,which is controlled by two faults,traversing the Qiangtang Basin from east to west.A set of near-north-south left-lateral slip fault systems divide this uplift belt into several parts.The uplift belt,gradually converged from west to east,has obviously different characteristics between the east and west sides of Shuanghu County.The west section shows large-scale uplift of the basement,and the bedrock depth is generally within 3~5 km,which is much shallower than those in the North and South Qiangtang depression with the basement depth of 7~15 km.However,the uplift amplitude of the eastern segment decreases significantly,mainly manifested as a low base uplift.In addition,the central uplift belt shows obvious“multi-segments”and“step-like”decreasing trend from west to east,controlled by near-north-south faults.The basement depth of the Shuanghu-Yaqu section is between 5 and 7 km,and that of the Yaqu-Gonney section is between 7 and 9km.According to the structural stratification comparison,it is reveraled that the North Qiangtang Block has a smaller uplift amplitude and better stability than the South Qiangtang Block.The differences in geophysical properties of the basement between the North and South Qiangtang depression indicate that there is not an unified Precambrian metamorphic basement in the Qiangtang Basin,and the formation of the central uplift zone may be related to the melange zone formed during the closure of the Paleo-Tethys Ocean.