湘南大义山地区中—新生代变形序列及其动力学背景

Mesozoic and Cenozoic Deformation Sequencesand Their Dynamic Backgroundin the Danyishan Area,Southern Hunan

大义山地区处于南岭构造带与钦杭构造带结合部位,自早中生代以来发育多方向、不同类型的构造,这些变形的先后序次对理解湘南及邻区的构造演化过程具有基础性意义,然而目前其构造变形特征和应力场演替过程仍缺乏精细剖析。本文运用构造解析方法对大义山地区地表基岩中的剪切破裂和褶皱等构造要素进行分期和配套,利用赤平投影法恢复剪切破裂的古构造应力场,结合中—新生代华南大地构造演化历史,厘定了研究区5期构造变形事件及其动力学背景:第1期NEE—SWW向挤压(D_(1))发生于中三叠世晚期印支运动早幕,其动力学背景可能为多板块围限下的扬子克拉通与华夏地块间继发性陆内俯冲汇聚,而局部与区域挤压方向的差异性,可能与NW向邵阳—郴州断裂左行走滑派生的近E—W向挤压应力场及走滑所产生的逆时针牵引作用有关;第2期近S—N向挤压(D_(2))形成于晚三叠世—早侏罗世印支运动晚幕,可能与华北克拉通向南的挤压和思茅—印支地块向北的挤压有关;第3期NWW—SEE向挤压(D_(3))为中侏罗世晚期早燕山运动的产物,其动力来源于古太平洋板块(或伊泽奈崎板块)对华南大陆的西向俯冲;第4期与岩浆作用相关变形(D_(4))包括褶皱、挤压面理、肿缩构造和张剪性破裂等,可能受制于早燕山运动后造山阶段晚侏罗世花岗质岩浆沿不同方向基底断裂的幕式上侵和侧向流动;第5期N(N)E—S(S)W向挤压(D_(5))发生于古近纪中晚期,可能与先存NNE向断裂带右行走滑所派生的局部挤压应力场有关,其动力学机制可能为印度板块与欧亚板块碰撞而引发的青藏高原物质东向逃逸。进一步分析认为,NW向邵阳—郴州断裂最强烈的走滑活动可能发生在印支运动早幕,并由此奠定了断裂周缘中—新生代构造格架。

The Dayishan area is located at the junction of the Nanling tectonic belt and the Qinhang tectonic belt.It has developed multi-directional and different types of structural systems since the Early Mesozoic.Studying the deformation sequences is essential for better understanding of the tectonic evolution along and adja-cent to South Hunan.However,detailed analysis of the deformation characteristics and evolution of the stress field remains unsolved.This study employs structural analysis to correlate structural elements such as shear fractures and folds in the bedrock of the Dayishan area.The paleo-stress field of shear fractures is reconstructed using the lower hemisphere stereographic projection method.Combining the regional tectonic evolution history of South China in the Mesozoic and Cenozoic,this study defines five deformation events based on their dynamic backgrounds.The first phase(D_(1))of NEE-SWW compression,occurring during the early stages of the In-dosinian Movement(late Middle Triassic),resulted from the oblique intraplate under-thrusting of the Yangtze Craton beneath the Cathaysia Block in a multi-plate confining setting.The difference in local and regional shortening directions may be related to the E-W compressional stress field derived from the NW-trending Shao-yang-Chenzhou fault and the sinistral drag caused by strike-slip motion.The second phase(D_(2))of S-N com-pression formed during the late stage of the Indosinian Movement(Late Triassic to Early Jurassic).This phase may be related to the southward compression of the North China Craton and northward compression of the Simao-Indochina Block.The third phase(D_(3))of NWW-SEE compression occurred during the Early Yanshanian Movement(latest Middle Jurassic).This phase was driven by the westward subduction of the Palaeo-Pacific Plate,or the Izanazaki Plate,beneath the South China Block.The fourth phase(D_(4))of magmatism-related de-formation includes folding,tectonic foliations,pinch-and-swell structures,and tension-shear fractures.These structures are possibly related to episodic upward invasion and lateral flow of Late Jurassic granitic magmas along variously orientated basement faults during the post-orogenic stage of the Early Yanshanian Movement.The fifth phase(D_(5))of N(N)E-S(S)W compression occurred during the Middle and Late Paleogene.This phase may be related to the local compressive stress field derived from dextral strike-slip motion along previous NNE-tren-ding fault zones.Its dynamic mechanism may be the eastward escape of the Tibetan Plateau caused by the colli-sion between the Indian and Eurasian plates.Further analysis suggests that the strongest strike-slip motion of the Shaoyang-Chenzhou fault zone might have occurred in the early stage of the Indosinian Movement,thereby establishing the tectonic framework of the Mesozoic and Cenozoic in the surrounding area of this fault zone.