Episodic Mesozoic constructional events of central South China: constraints from lines of evidence of superimposed folds, fault kinematic analysis, and magma geochronology

The South China block (SCB), located in the convergence zones of the western Pacific subduction tectonic domain and Tethyan tectonic domain, has experienced complex tectonic processes, including the continent–continent collision caused by the closure of the Palaeo-Tethys Ocean, and the subduction of the Palaeo-Pacific plate. However, due to complex intracontinental deformation and abundant magmatism, there are serious divergences in the corresponding records of the tectonic transformation from Palaeo-Tethys to Palaeo-Pacific. We have analysed the map-scale superimposed fold system developed in central SCB and also inverted the palaeo-stress field based on the fault–slip vectors. On this basis, the deformation styles and superposition mechanism of two-stage folds were recovered to establish the tectonic stress field in early Mesozoic and tectono-magmatic events by combining the chronology of the accompanying syntectonic magma. The early E–W/WNW-trending folds and Triassic magmatic system were identified; these were controlled by the NE–SW-trending compressive stress field, to coordinate with the collisions between the SCB and the Indochina block in the southwest, and the North China block in the north. The late NE/NNE-trending folds superimposed on the early folds in an orthogonal way to form a large-scale dome-basin superimposed fold system, which were controlled by the WNW–ESE-trending compressive stress field. According to the strata relations involved in deformation and the chronology data of magmatic rocks, it could be determined that NE/ENE-trending folds would be formed during the Mid- to Late Jurassic, corresponding to the westward subduction events of the Palaeo-Pacific plate. The establishment of the large-scale superimposed fold styles and the identification of fold deformation in the Triassic and Jurassic are important for understanding the early Mesozoic tectonics of South China, and even for all East Asia continent. In particular, it can provide important temporal and spatial constraints to explain the complex deformation process and geodynamic settings of South China in the early Mesozoic.

华南地块（South China block）位于西太平洋俯冲构造域（western Pacific subduction tectonic domain）与特提斯构造域（Tethyan tectonic domain）的交汇地带，经历了复杂的构造演化过程，其中包括古特提斯洋（Palaeo-Tethys Ocean）闭合引发的陆陆碰撞，以及古太平洋板块（Palaeo-Pacific plate）的俯冲作用。然而，由于陆内变形复杂且岩浆作用广泛发育，关于古特提斯构造域向古太平洋构造域转换的相关记录存在严重分歧。本研究针对华南地块中部发育的图幅级叠加褶皱系统（map-scale superimposed fold system）开展了分析，并基于断层滑动矢量（fault–slip vectors）反演了古应力场（palaeo-stress field）。在此基础上，结合伴生同构造岩浆（syntectonic magma）的年代学（chronology）数据，恢复了两期褶皱的变形样式与叠加机制，进而重建了早中生代构造应力场与构造-岩浆事件序列。研究识别出早阶段东西向/北西西向褶皱与三叠纪岩浆系统，其受控于北东-南西向挤压应力场，这与华南地块西南侧与印支地块（Indochina block）、北侧与华北地块（North China block）的碰撞作用相协调。晚阶段北东/北北东向褶皱以正交叠加的方式覆于早期褶皱之上，形成大型穹盆叠加褶皱系统，其受控于北西西-南东东向挤压应力场。根据变形涉及的地层接触关系以及岩浆岩年代学数据，可确定北东/北东东向褶皱形成于中侏罗世至晚侏罗世（Mid- to Late Jurassic），对应古太平洋板块的西向俯冲事件。本研究建立的大型叠加褶皱样式，以及三叠纪与侏罗纪褶皱变形的识别，对于理解华南乃至整个东亚大陆的早中生代构造演化均具有重要意义，尤其可为解释华南早中生代复杂的变形过程与地球动力学背景提供关键的时空约束。