柴达木盆地北缘中新生代地层的磁组构特征及其沉积-构造学意义

柴达木盆地沉积地层记载着青藏高原东北部的构造演化信息.对该盆地路乐河地区上中生界-新生界地层系统采样，获得千余块定向岩心样品.岩石磁学研究表明样品中的磁性矿物主要为赤铁矿和磁铁矿；磁组构研究表明为初始沉积磁组构特征.磁组构特征指示了自中侏罗统大煤沟组（J₂d）至早中新统下油砂山组（N₂¹y）7个地层单位沉积时期，古水流方向共经历了4次阶段性的变化，表明柴达木块体相应地发生了4次旋转.在中-晚侏罗世块体逆时针旋转约22°；至早白垩世，块体又顺时针旋转约65°；在65.5~32 Ma期间块体旋转方向再次改变，逆时针旋转约63°；到32~13 Ma阶段块体又发生约50°的顺时针旋转.柴达木块体的旋转及其方向的转换，可能与其南的羌塘块体、拉萨块体和印度板块阶段性北向碰撞挤压紧密相关.拉张环境与挤压环境的多次转换可能与中特提斯的关闭、新特提斯的张开和闭合、高原快速隆升后其边部松弛相联系.

Anisotropy of magnetic susceptibility of Mesozoic and Cenozoic sediments in the northern margin of Qaidam Basin and its sedimentary-tectonic significance

The sedimentary strata of the Qaidam Basin recorded the tectonic uplift information of the orogenic belt in the northeastern margin of the Tibetan Plateau. More than 1000 oriented core samples were collected systematically from the late Mesozoic strata to the Cenozoic strata at the Lulehe section. Rock magnetism analysis shows that hematite and magnetite were dominant magnetic carrier in the late Mesozoic and Cenozoic sediments. The magnetic fabrics reveal that the paleocurrent direction has changed 4 times during the Dameigou Formation (J₂d) of the Middle Jurassic and the Lower Youshashan Formation (N₂¹y) of the Early Miocene, indicating 4 periods of rotations of the Qaidam block:(i)~22° counterclockwise rotations from the Middle to Late Jurassic; (ii) successive clockwise rotations of~65° from Late Jurassic to the Early Cretaceous; (iii)~62° counterclockwise rotations during 65.5~32 Ma; (iv) subsequent~50° clockwise rotations during 32~13 Ma. The rotations of Qaidam block and the changes of its direction are possibly related to the periodic northward collision and compression of its southern Qiangtang block, Lhasa block and Indian plate. The multiple transformations of tensional and compressive environment may have connection with the closure of the Middle-Tethys, the opening and closure of the Neo-Tethys, and the relaxation of the edge of the plateau after rapid uplift.