浙东南地球物理场和基底构造

GEOPHYSICAL FIELD AND BASE STRUCTURE OF SOUTHEASTERN ZHEJIANG, CHINA

利用地球物理参数块体填图并结合地表资料,推断浙东南中生代火山岩覆盖层下的中深部构造特征:变质基底呈东西向展布,为早元古代华夏板块的一部分。其后,发展为浙西钱塘裂陷槽、华夏古陆和沿海裂隙槽,进而形成巨形花岗岩环、中生代酸性火山岩以及沿NW向深断裂活动的双峰式火山岩。现尚处于脉动式裂解,蠕散的初级阶段。因此,构造活动非常活跃。

In southeastern Zhejiang covered by extensive Mesozoic volcanics, according to geophysical field features, the author used parameter massive mosaic mapping method, compiled an extrapolating map, and found out characeristics of the basement structures as follows 1. Nearly east-west extending characteristics of metamorphic basement: parameter massive bodies featured by weak magnetism and high density extend in east-west direction. According to physical parameter, combined with metamorphic window found in surface, the massive bodies are infered to be basement metamorphic rocks. Furthermore, we studied metamorphic rocks in Longquan and found the first intensive folding of deep metamorphic rocks of 2400 Ma also developed in east-west direction. Therefore, it is believed that almost east-west metamorphic rock series infered by geophysical field is early Proterozoic metamorphic basement, which belongs to a part of Cathysian ancient plate. 2.East-west spade-like faults and basement faults: It is found that gravity gradient belt in Jiangshan-Xiangshan and strong magnetic anomaly belt in Xianxialing-Tiantaishan are the dividing boundary of gravity and magnetic field. Through quantantive computation of gravity anomaly, the author suggested the belt represent Jiangshan-Xiangshan Spade-like fault to the south of the fault there is southern Jiangxi-Fujian-Zhejiang grade Ⅱ fault block, while to the north there is Jiangnan-Zhoushan grade Ⅱ fault block. 3. Northeastern extending characteristics of covering strata: In Cathysian ancient plate, northeastern belt-like, Qiangtan rift-depression trench, coastal rift-depression trench and rhomb Cathysian developed first. The upper crust having high-speed layer in the rift-depression trenches probably is caused by welling-up of mantle or lower crust. On the surface, we found late Proterozoic volcanic series and early palaeozoic flysch formation, not cut off untill Caledonian movement. In rift-depression trenches, northeastern Indo-China folds developed further. However, in rigid cathysia, extensive granites, which thicken the upper crust, were developed, and basement metamorphic rocks like residual block are on granit layer. Granites show circular shape. In early Yanshanian, the crust began to become extending, so Mesozoic acid volcanic rocks were formed. 4. Deep faults and bimodal volcanics: The deep faults characterized by strong magnetic anomaly include coastal Nanao-Dongshan fault (or 50 meter isobaths fault), continental northwestern deep faults and so on. These faults are believed to be arc back tension faults. Because of welling-up of mantle along seaside, thinning of lower crust and then sinking of upper crust, basement faults reacted and deeply cut, and magmas derived from deep source cmplaced and effused. It is believed that in late Yanshanian abundant mineral resourcaes are closely associated with these magmatism. In summary, basement in this area is not an orogenic zone but an ancient plate. It is suggested that island arc of western Pacific coast also belong to fragment of the ancient plate, which is propelling toward Pacific Ocean.