闽西南安砂水库南岸上古生界的多期褶皱特征及其意义

Characteristics and Implication of Polyphase Folds in Upper Paleozoic Strata,South of Ansha Reservoir,Southwestern Fujian Province

闽西南安砂水库库区省道307的新路段清晰出露了上、下古生界,为研究闽西南地区中生代变形构造提供了良好条件。野外构造观测和构造复原表明上古生界发育3期褶皱:第一期呈NE向展布,两翼相对紧闭,发育特征的轴面片理;第二期呈NW向展布,相对宽缓,发育NW走向的透入性皱纹面理;第三期为近平行第一期褶皱的宽缓褶皱,尺度最大,伴有局部发育的NE向皱纹面理。它们依次形成在中三叠世、晚三叠世和中侏罗世末‒晚侏罗世初,分别对应着NW-SE、NE-SW和NW-SE向区域挤压作用,反映在近场洋壳俯冲和远场陆块碰撞的共同作用下,华南陆缘在中生代经历了复杂的构造变形过程。目前主流的两期共轴挤压认识是对该构造变形过程的一级近似。

The upper and lower Paleozoic strata are well exposed along the cuts of the provincial route 307 on the southern bank of the Ansha reservoir in the southwestern Fujian,which provides excellent conditions to study the temporal and spatial variation of deformation during the Mesozoic.Field observations and structural restoration show that there are three folding phases in the upper Paleozoic strata.The first-phase folds are NE-trending with close interlimb angles and characterized by axial foliation(S1).The second-phase folds are NW-trending,relatively open,associated with NW-trending and widespread crenulation cleavage(S2).The third-phase folds are basically coaxial with the first-phase folds but have open interlimb angles and larger sizes,accompanied by the NE-trending and local distributive crenulation cleavage(S3).These polyphase folds were formed successively in the Middle Triassic,the Late Triassic,and the late stage of Middle Jurassic–early stage of Late Jurassic,corresponding to the regional compression of NW-SE,NE-SW and NW-SE,respectively.It reflects the complexity of structural deformation process on the South China continental margin during Mesozoic,which occurred corresponding to the subduction of the western Pacific oceanic basin(s)and the collision between the Indochina and the South China continents.In this light,the widely accepted viewpoint about two-phases coaxial compression is the first-order approximation of the rather complicated deformation process.