一个中生代的拉分盆地——宁芜盆地的形成及演化

宁芜盆地是中国东部郯庐断裂系中沿方山—南陵左行剪切断裂发育的中生代拉分盆地,形成于晚侏罗世,消亡于晚白垩世。根据宁芜盆地的构造变动、沉积建造、火山及岩浆活动和成矿作用等特点,文中提出了宁芜拉分盆地的生成及演化模式,并将宁芜拉分盆地的发展分为:(1)萌芽期——盆地核;(2)年轻期——“S”型盆地;(3)成熟期——菱形盆地和(4)消亡期——最终发展四个演化阶段。宁芜拉分盆地以具有强烈的火山活动为特征,特别是“S”形盆地末期,由于碱性的中基性次火山岩的侵入,形成了宁芜地区重要的玢岩铁矿。宁芜拉分盆地的构造应力场随着盆地的发展而不断变化,总的变化趋势是盆地内的拉张区逐渐扩大,在最终发展阶段这种拉张作用已遍及整个盆地。数值模拟的结果表明,NNE—NE向的拉张应力比NWW—NW向的挤压应力大数倍,宁芜拉分盆地正是在这种强大的拉张应力作用下得以形成及发展的。

FORMATION AND EVOLUTION OF THE NINGWU BASIN AS A MESOZOIC PULL-APART BASIN

The Ningwu basin is a mesozoic pull-apart basin that developed along the Fangshan-Nan-ling sinistral shear fault and the Binjiang sinistral shear fault in the Tancheng-Lujiang fault zone in eastern China, It formed in Late Jurassic and withered in Late Cretaceous.According to the characteristics of sedimentary formation, volcanicity, magma activity and mineralization of the Ningwu district, a model for formation and evolution of the Ningwu Pullapart basin has been put forward, by which the whole evolution process of the basin is divided into four stages: (1) germinal stage-basin nucleation, (2) young stage-"s-shaped" basin, (3) mature stage-roomboidal basin and, (4) withered stage-extreme development. The Ningwu pull-apart basin was characterized by intensive volcanic activity. Especially because of su-bvolcanic intrusion of intermediate-basic rocks with more alkili and less silica, the important porphyrite iron deposits of the Ningwu district were formed during late "s-shaped" basin stage.The tectonic stress field of the Ningwu pull-apart basin has been changed continuously with development of the basin. The general terned of the change is that the extension area of the basin expanded gradually and at the end, the extension was throughout the basin in the extreme development stage. The result of numerical simulation shows that the tension stress trending NNE-NE is several times large than the compression stress trending NWW-NW. It means that the Ningwu pull-apart basin has formed and developed under the action of this strong tension stress.