山东胶莱盆地五莲地区早白垩世红层古地磁研究及其构造意义

Paleomagnetic study of Early Cretaceous red beds from Wulian section,Jiaolai basin in Shandong Province and its tectonic implications

胶东半岛中生代以来的旋转变形对限定郯庐断裂的剪切走滑活动具有重要意义,然而目前对胶莱盆地中生代旋转变形与否及其幅度仍存在很大争议。本文选择胶莱盆地南部五莲地区早白垩世莱阳群龙旺庄组红层开展系统古地磁学研究,共采集11个采点82个定向古地磁岩芯样品。岩石磁学实验结果表明,样品的载磁矿物主要为赤铁矿,部分样品同时含有少量磁黄铁矿和磁铁矿;系统热退磁实验共分离出8个采点58个样品的稳定特征剩磁方向,并认为其可能代表岩石沉积时期的原生剩磁方向。8个采点的特征剩磁方向平均值为Ds=169.5°、Is=33.2°、ks=40.9、α_(95)=8.8。相对于稳定东亚极,五莲地区发生了显著的(22.8°±8.8°)逆时针旋转变形。结合前人在胶莱盆地白垩纪沉积岩中获得的古地磁结果,指示胶莱盆地晚白垩世以来可能经历了整体的逆时针(10°~20°)旋转变形,而沿五莲断裂带向南西方向的逆时针旋转变形幅度增大,表明受郯庐断裂带左行走滑的影响增强。此外,沿郯庐断裂带不同部位在不同时期发生了不同方式和幅度的旋转变形,其不仅受郯庐断裂带左行剪切活动影响,还与断裂带周缘不同地体的性质及其所处的大地构造背景有关。

Understanding the Mesozoic vertical-axis rotation of the Jiaodong Peninsula is crucial for constraining the sinistral shearing of the Tan-Lu fault.However,debates still exist regarding whether rotation occurred and,if so,the magnitude of rotation in the Jiaolai basin during the Mesozoic.In this study,we conducted a detailed paleomagnetic investigation of the Early Cretaceous red beds from the Longwangzhuang Formation at the Wulian locality in the southern Jiaolai basin.Detailed rock magnetic results indicate that hematite is the main magnetic carrier,while pyrrhotite and magnetite act as subordinate magnetic carriers in some samples.The Fisherian mean direction of the 8 sampling sites was determined as Ds=169.5°,Is=33.2°,ks=40.9,and α_(95)=8.8,which is proposed to be the primary remanence acquired during deposition.Our results indicate that the Wulian region underwent an obvious counterclockwise rotation of 22.8°±8.8°relative to stable East Asia.Together with previous paleomagnetic results from the Jiaolai basin,they probably indicate a holistic counterclockwise rotation(10°~20°)of the Jiaolai basin after the Late Cretaceous.Moreover,an increasing magnitude of counterclockwise rotation towards the southwest along the Wulian fault was observed and most likely attributed to the increasing influence of sinistral strike-slip faulting along the Tan-Lu fault to the west.Different rotation senses and magnitudes were observed in different periods along the Tan-Lu fault,which were not only influenced by the sinistral shearing of the Tan-Lu fault but also related to the different nature and tectonic backgrounds of nearby blocks.