闽西南坳陷中部中‒晚三叠世构造变形特征

Characteristics of Middle-Late Triassic Structural Deformation in Central Southwestern Fujian

闽西南地区中‒晚三叠世变形构造是中生代华南陆缘由被动向主动转化的最早构造记录,但由于植被覆盖严重,长期以来对这一相对短暂的变形过程缺乏精细的研究。本文首先对闽西南坳陷中部玮埔花岗岩体及其东南侧的花岗“岩脉”开展了LA-ICP-MS锆石U-Pb同位素测年,证实这两个曾被认为印支期的岩体(岩脉)均形成于早古生代末,而它们与上覆的上古生界不同地层单元之间的关系由原先认为的侵入接触变更为正断接触。在此基础上,通过对玮埔花岗岩体东南侧上古生界中露头和显微尺度变形构造的观测和分析,我们建立了研究区的中生代构造框架,识别出中‒晚三叠世的三期变形构造,由老到新依次为:①NE向展布、常伴有轴面劈理的区域褶皱,②顶部朝向SSW的滑脱断层,③朝向E或SE运动的区域逆冲断层。早中生代区域构造体制由此经历了挤压‒伸展‒挤压两次转变,很可能归结于由早中生代火山岛弧与华南大陆碰撞产生的远场挤压和由深部异常高热流上涌诱发的近场伸展的共同作用。

The Middle-Late Triassic structural deformation in the southwestern Fujian province is the earliest record of the passive to active continental margin transformation of South China during the Early Mesozoic.However,this relatively short deformation process has not been studied in detail due to vegetation cover.We carried out LA-ICP-MS zircon U-Pb isotopic dating of the Weipu granite and its southeastern‘dikes’in the central part of Southwestern Fujian.The analytical results show that these granites,once considered to be formed in the Indosinian period,were formed at the end of the Early Paleozoic.Accordingly,the relationship between the granites and the overlying Upper Paleozoic different strata should be normal fault contact rather than the once believed intrusive contact.On this basis,we established the Mesozoic structural framework of the study area and identified three-stage deformation structures during the Middle and Late Triassic through field observations and structural analysis of the Upper Paleozoic strata on the southeast edge of the Weipu granite.From early to late,1)NE-trending regional folds commonly associated with axial cleavages,2)Top-to-SSW detachment faults,and 3)Top-to-E or-SE regional thrust.It reflects that the Early Mesozoic regional tectonic system experienced two transformations:from compression to extension to compression,which may be attributed to the combination of the far-field compression caused by arc-continent collision and the near-field extension induced by the upwelling of deep abnormal high heat flow.