龙门山南段二叠系—下三叠统的古地磁研究

为了约束龙门山南段的构造运动特征，文章对龙门山南段大川镇附近的下三叠统飞仙关组淡紫灰色泥岩、粉砂岩和宝兴地区的二叠系灰岩开展了古地磁研究。古地磁样品取自10个采样点，其中3个采点为二叠系灰岩；7个采点为飞仙关组淡紫灰色泥岩、粉砂岩。对样品开展了逐步热退磁、岩石磁学（等温剩磁获得曲线和三轴等温剩磁热退磁）及扫描电镜实验。80个样品进行的逐步热退磁实验结果显示，二叠系灰岩样品未分离出稳定的特征剩磁；飞仙关组样品分离出了稳定的特征剩磁，并通过了广义褶皱检验，其特征剩磁的平均方向为：Ds=36.9°，Is=16.5°，α95=5.9°，K=33.8，N=18，对应的古地磁极投在了华南视极移曲线的早三叠世段附近。岩石磁学实验结果表明飞仙关组样品的载磁矿物为磁铁矿，扫描电镜观察展示其为碎屑状的铁氧化物，且无明显成岩后自生特征。结合退磁曲线特征，扫描电镜微观特征，特征剩磁的古地磁极位置和岩石磁学结果，飞仙关组样品的特征剩磁很可能为原生剩磁。该结果表明龙门山褶皱冲断带与四川盆地的没有明显地相对构造旋转运动，自晚三叠世以来，其与龙门山北段以及四川盆地在动力学上是统一的构造单元。

Paleomagnetism of Permian-Lower Triassic Strata in the Southern Longmenshan Fold-Thrust Belt

In order to study and constrain the tectonic movement characteristics of the southern Longmenshan fold-thrust belt, the paleomagnetic study was carried out on the light purple gray mudstone and siltstone of the lower Triassic Feixianguan formation near Dachuan town and the Permian limestone in Baoxing in the southern Longmenshan fold-thrust belt. Paleomagnetic samples were collected from 10 sampling sites, of which 3 are Permian limestone and 7 are light purple gray mudstone and siltstone of Feixianguan Formation. We have conducted the experiments of stepwise thermal demagnetization, rock magnetism (isothermal remanent magnetization acquisition curves and triaxial isothermal remanent thermal demagnetization) and scanning electron microscope. Based on the stepwise thermal demagnetization experiments of 80 samples, we revealed a stable characteristic remanence from the Feixianguan Formation samples but not from the Permian limestone samples. The high temperature components pass simple generalized fold test, the tilt-corrected mean direction of the characteristic remanence is：Ds=36.9°, Is=16.5°，α95=5.9°, K=33.8, N=18 and the corresponding paleomagnetic pole position plots close to the Early Triassic mean pole of the apparent polar wander path for the south China Block. The results of rock magnetic show that the major magnetic mineral of Feixianguan formation is magnetite, and the scanning electron microscope observation shows that it is clastic iron oxide, and there is no obvious post diagenetic authigenic characteristics. Combined with the characteristics of demagnetization curve, scanning electron microscope, paleomagnetic pole position of characteristic remanence and rock magnetic results, the characteristic remanence of Feixianguan formation is likely to be primary remanence. The results show that there is no obvious relative tectonic rotation between the Longmenshan fold-thrust belt and Sichuan Basin. Since the Late Triassic, the northern and southern Longmenshan fold-thrust belt and Sichuan Basin are unified tectonic units in dynamics.