华南华北地块中生代沉积岩的磁倾角浅化校正研究

Compaction-induced inclination shallowing in sedimentary rocks is a recent focus in paleomagnetism international community. Several key problems concerning this issue need to be solved urgently. These problems include whether or not all sedimentary rocks, especially continental sedimentary rocks, suffer from significant inclination shallowing phenomenon; whether or not all sedimentary rocks possess uniform or nearly uniform inclination shallowing correction factor; and how to identify and correct the inclination shallowing in each sedimentary unit formed in different sedimentary environments. In particular, since the Late Paleozoic to Jurassic paleomagnetic data for the North and South China blocks (NCB and SCB) are mostly isolated from continental clastics, it is of great significance to identify and correct the compaction-induced inclination shallowing in these available paleomagnetic data, which are basically main quantity constraints on reconstruction of paleogeography and tectonic evolution model in East Asian continent. Therefore, we prefer to carry out a subject focusing on the mechanism and amount of compaction-induced inclination shallowing in Mesozoic continental clastic rocks from the NCB and SCB. We intend to obtain inclination shallowing factor for different Mesozoic sedimentary units, and thus update the Mesozoic apparent polar wander paths for the NCB and SCB. Our final aim is to propose a reliable model of paleogeography and tectonic evolution of the NCB and SCB as well as their tectonic relationship with adjacent blocks during the most Mesozoic times.

沉积岩中存在由于压实而形成的磁倾角偏低现象近年来为国际学术界所关注。但是否所有沉积岩，尤其是陆相沉积岩中均存在显著的磁倾角偏低现象？沉积岩是否具有统一或接近统一的磁倾角浅化校正因子？如何识别和校正不同沉积环境下形成的各沉积单元中存在的磁倾角浅化异常？等问题亟待解决。特别地，由于华北和华南等东亚主要陆块的晚古生代-侏罗纪古地磁数据主要来自于陆相碎屑岩沉积，识别和校正这些古地磁数据中的磁倾角偏低，对正确认识基于这些古地磁数据所进行的板块古地理重建和不同块体间的碰撞配合模型具有重大意义。为此，本项研究拟通过对华南、华北陆块中生代陆相碎屑沉积岩磁倾角偏低机理及磁倾角偏低程度进行研究；厘定华南、华北中生代主要沉积单元的磁倾角浅化因子；修订其中生代古地磁视极移曲线；为正确认识华南、华北地块中生代的古地理位置及与周边块体的碰撞拼合过程提高可靠制约。

本项目针对华南、华北地块中生代碎屑岩磁倾角浅化效应及其影响因素/机制等这一构造磁学的基础性问题，以华南、华北地块早-中三叠世碎屑岩为主要研究对象，开展了一系列古地磁学等研究，取得了如下创新性成果：（1）在获得华北早三叠世刘家沟组和华南中三叠世巴东组的可靠古地磁记录前提下，精确厘定出其磁倾角浅化因子分别为f = 0.59和f = 0.63。经基于hf-AIR和单颗粒赤铁矿的剩磁各向异性（a值）确定的磁倾角浅化因子（f值）进行磁倾角浅化校正后的早-中三叠世古地磁极，仍支持华南、华北地块在中生代经历了剪刀式旋转碰撞拼合。（2）厘定出一套对以高矫顽力赤铁矿为主要载磁矿物的碎屑岩磁倾角浅化因子的实验方法与技术。提出hf-AIR方法是识别和校正红层磁倾角浅化效应的最有效方法；而E/I校正法，只有在没有显著迹象表明其采样剖面/地区之间存在相对构造运动的前提下，对红层磁倾角浅化的识别和校正才是行之有效的。（3）发现有且仅有剩磁组构才是判别碎屑沉积岩特征剩磁是否遭受了成岩/后期构造应力影响的充分和必要条件；揭示出构造应变（如平行于层面的水平挤压）可导致褶皱不同翼记录的特征剩磁（ChRM）磁倾角或浅化，或变陡，形成“同褶皱重磁化”假象。（4）发现对含细小磁性包裹体的硅酸岩矿物记录的沉积剩磁（DRM）而言，沉积剩磁倾角的浅化因子取决于硅酸岩矿物颗粒大小：相对于大颗粒硅酸岩矿物而言，小颗粒硅酸岩矿物更可能产生相对较大的磁倾角浅化误差（对应于相对较小的磁倾角浅化因子）。相关研究成果培养硕士研究生2名，部分资助培养博士研究生2名；资助已发表SCI论文3篇和北大核心期刊论文1篇。相关成果对已有华南、华北中生代碎屑岩古地磁学数据的正确使用和未来碎屑岩古地磁学研究等具有重要的指导意义。