祁连盆地第三纪沉积物磁性地层和岩石磁组构初步研究  

Magnetostratigraphy and magnetic anisotropy of the Neogene sediments in the Qilian Basin

作　　者：刘彩彩[1] 王伟涛[1] 张培震[1] 庞建章[1] 俞晶星[1] 

机构地区：[1]中国地震局地质研究所地震动力学国家重点实验室,北京100029

出　　处：《地球物理学报》2016年第8期2965-2978,共14页Chinese Journal of Geophysics

基　　金：国家自然科学基金(41474053);中国科学院先导性专项课题(XDB03020200);地震动力学国家重点实验室自主课题(LED2014A03);中国地震局地质研究所中央级公益性科研院所基本科研业务专项(IGCEA1218)资助

摘　　要：祁连山山间盆地内的新生代沉积物是研究新生代以来祁连山构造演化的重要材料.本文以位于祁连山中部祁连盆地内的新生代沉积物为研究对象,利用磁性地层学方法结合碎屑颗粒裂变径迹定年方法获取其沉积时代框架,在此基础上,结合岩性变化与沉积环境变迁分析祁连山构造演化历史.野外实测剖面显示该盆地内的第三系可划分为下部砾岩组和上部砂岩组两大岩性单元.古地磁结果显示砾岩组的沉积时代约为10—14.3Ma.砾岩组沉积大约在14.3 Ma开始形成,指示祁连山14.3 Ma以来构造活动变强烈.磁组构结果显示砾石组顶部沉积形成时的受力方向与现今祁连盆地周缘断层分布所指示的应力方向一致,表明这些断层大约在10 Ma附近开始活动.我们的结果揭示祁连山中部山脉14.3 Ma以来尤其在10 Ma附近构造活动较强烈.这与过去低温热年代学所获得的祁连山山体的快速冷却年龄及祁连山两端大型盆地内的第三系所记录的构造事件发生的时间基本吻合.而砂岩组的古地磁结果并未通过褶皱检验,其古地磁记录发生了后期重磁化,无法获得地层的准确沉积年龄.Cenozoic sediments in intermountain basins of the Qilian Mountains are good recorders for Cenozoic tectonic events in this area. We have studied the Neogene sediments in the Qilian Basin, one of the intermountain basins of the Qilian Mountains. This basin located in the middle part of this mountain range. Detailed magnetostratigraphic and detrital fission track dating analyses combined with the study of sedimentary facies and lithology have been carried out on these sediments to study the tectonic evolution history of the Qilian Mountains. The Neogene sediments in this basin can be divided into two parts：the lower conglomerate and the upper sand layers. Magnetostratigraphic results suggest that the conglomerate layer formed during 10-14.3 Ma. The conglomerate started to form at about 14.3 Ma, suggesting an increasing tectonic activity of the Qilian Mountains since that time. Magnetic fabrics of the upper conglomerate section suggest that the tectonic stress acted on these sediments was consistent with that on the faults around the Qilian Basin. This may indicate that shear on these faults took place near 10 Ma. Our results suggest that the tectonic activity of the Middle Qilian Mountain might have largely increased since about 14.3 Ma, especially intensified at about 10 Ma. This is consistent with the ages of tectonic events in the Qilian Mountains revealed by low-temperature thermal-chronological data and sedimentary recording in great basins to the north and south. The characteristic magnetic remanence directions of samples from the sand layer fail the folding test. This suggests that the original depositional remanence may be remagnetized by post-folding magnetization. Therefore, we can not obtain the depositional age of the sand layer by magnetostratigraphy.

关 键 词：祁连盆地 第三纪沉积 磁性地层学 重磁化 

分 类 号：P318[天文地球—固体地球物理学]