琼北新生代火山构造的航磁异常特征及其地质意义

琼北地区发育大量新生代火山岩,是了解东南亚新生代构造演化的重要窗口之一。本文利用琼北地区最新航磁资料,采用航磁ΔT原始数据进行化极、垂向一阶导数等频率域转换处理等方法,并结合琼北新生代火山岩研究的其它资料,对琼北新生代火山岩区域航磁特征以及典型地区航磁异常特征进行了分析。研究结果表明:琼北东部与西部区域磁场特征存在一定的差异,即其东部具有平缓变化的负磁场特征,而中—西部具有强烈升高正磁场特征,这可能与琼北平缓变化的负背景场上叠加的新生代火山岩磁性强弱以及在空间上分布的不均匀有关。琼北新生代火山构造航磁异常具有两种不同的类型,第一类其内环航磁异常表现为不规则椭圆状负磁异常,负异常周围被环形升高的串珠状正磁异常所包围,该类航磁异常可能由塌陷式盾状火山锥群引起;第二类其外环由多个圈闭不规则的串珠状正磁异常组成,环形构造内部总体为负异常,但中心分布两个强度较大的正异常,该类航磁异常可能与早期塌陷式盾状火山锥群并叠加晚期磁性玄武质岩浆活动有关。

Aeromagnetic Anomalies of Cenozoic Volcanic Structure in Northern Hainan Island and Its Geological Significance

There are massive Cenozoic volcanic rocks in the northern Hainan Island, and it is thought that the Cenozoic volcanic rock is one of the important windows to recognize the Cenozoic tectonic evolution of the Southeast Asian continent. By using the new aeromagnetic data, applying the reduction to the pole, vertical first order derivation frequency domain conversion processing, and combining with the volcanic rocks other data, we analyze the regional aeromagnetic anomaly features of the Cenozoic volcanic rocks and the local aeromagnetic anomaly features of the typical area from the northern Hainan Island. Our results indicated that there are some obvious differences of the regional aeromagnetic anomaly between the east and west of the northern Hainan Island. In other word, the regional magnetic feature of the eastern region of the northern Hainan Island is gentle negative field, and the regional magnetic feature of the west and middle region is strong positive field. It is speculated that it is responsible to both the strength of magnetic and the inhomogeneity of the space distribution of the Cenozoic volcanic rocks which overlap with the gentle negative regional background field of the northern Hainan Island. Two types of the aeromagnetic anomalies of the typical Cenozoic volcanic structures are identified in the northern Hainan Island. The first kind of the aeromagnetic anomalies is characterized by the irregular elliptical negative anomaly of the inner ring, which was surrounded by the beaded rising positive anomaly, what the type of the aeromagnetic anomaly above mentioned are possibly caused by the collapse and shield like volcanic cones swarm. The second kind of the aeromagnetic anomalies is characterized by the multiple irregular beaded positive anomalies of the outer rings and the overall negative anomalies of the inside of the rings, which are overprinted by the two positive anomalies. It is suggested that the reason of the second kind of the aeromagnetic anomaly of the volcanic structures is possibly caused by the collapse and shield like volcanic cones swarm and the overprinting of the later strong magnetic basaltic magmatism.