磁异常三维解析信号所揭示的中国东部及邻近海域岩浆岩特征

MAGNETIC ACTIVITIES IN EASTERN CHINA AND ADJACENT SEAS REVEALED BY 3D ANALYTIC SIGNALS OF MAGNETIC DATA

根据中国东部及邻近海域总磁异常计算的三维解析信号振幅值比总磁异常,能精确地刻画磁源体的位置、深度和边界。结果显示在中国东部地区不同构造块体内的岩浆岩深度差异很大,华北块体的磁异常解析信号振幅值较高、尺度大,磁源体埋深达10 km左右或更深,反映了这个古老克拉通块体的中—下地壳内部较为强烈的岩浆活动过程与活化。受构造抬升和岩浆上涌影响,苏鲁—大别造山带的岩浆岩体埋深则相对较浅,多小于5 km。中、下扬子块体的岩浆岩体的埋深也普遍较大,但是岩浆活动微弱,解析信号振幅值很低。受晚中生代古太平洋俯冲的影响,华南块体及其东北海域分布比较孤立的岩浆岩体,其深度大都小于5 km。在东海,受新生代张裂作用的影响,岩浆岩体的深度与基底隆起或沉降呈明显对应关系。琉球群岛对应平静的磁异常并显示较深的岩浆岩体分布,表明琉球群岛不是菲律宾海板块俯冲形成的火山弧而可能是由俯冲增生楔物质和部分从欧亚大陆分离出来的块体构成,其西侧的岩浆岩带才是真正的火山弧。

Analytical signals estimated from total field magnetic anomalies in eastern China and adjacent seas are found to be more accurate in detecting locations,depths,and boundaries of magmatic bodies than original magnetic anomalies.The depth distributions of magmatic sources differ significantly between different tectonic blocks.The North China Craton shows high analytical signal amplitudes and large lateral extensions,but the detected magmatic sources are mostly deeply buried at depths up to 10 km or larger.This indicates strong magmatic activities in the lower to middle crust of the craton from tectonic reactivation.Due to tectonic uplifting and magmatic intrusions,the Sulu-Dabie Orogen has mainly shallowly buried magmatic bodies with depths smaller than 5 km.The Yangtze Block shows predominantly deeply buried magmatic sources,but unlike North China Craton,the magmatic activities in the Yangtze Block appear to be very weak and the calculated analytical signal amplitudes there are very small.The Cathaysia Block and its offshore northeastward extension is dominated by relatively isolated and shallow magmatic bodies（mostly 5 km） that are mainly caused by the subduction of the Paleo-Pacific Plate in the late Mesozoic.Depths of magmatic sources in the East China Sea correlate well with basement uplifts and depressions caused by Cenozoic continental margin rifting.The Ryukyu Archipelago corresponds with a very quiet magnetic zone and shows deeply buried magmatic sources,and these observations support the reasoning that the Ryukyu Archipelago itself is not the volcanic arc from the subduction of the Philippine Sea Plate but is composed of accretionary materials and/or continental blocks separated from the Eurasian Plate by opening of the Okinawa Trough.The true volcanic arc is represented by a volcanic belt slightly to the west of the Ryukyu Archipelago.