广东莲花山断裂带中、新生代多期复合变形变质带的基本特征及其形成机制的探讨

DEFORMATIONAL AND METAMORPHIC FEATURES OF LIANHUASHAN FAULT ZONE DURING MESO-CENOZOIC TIME AND MECHANISM OF THEIR FORMATION

莲花山构造带位于中国东南陆缘带丽水-海丰大断裂之西南段,是一个北东向华夏系、新华夏系复合构造带。印支运动以来经历了多次的挤压与拉张作用,拉张期主要形成地堑、地垒构造并控制沉积建造和岩浆建造的形成和展布;挤压期主要形成褶皱、推覆剪切或叠瓦式逆冲断层并控制变质带的形成与展布,可划分出四个主要变形变质期:1.印支期:形成北东向复式褶皱和断裂,并发生轻微的区域动力变质;2.燕山二幕(中侏罗世):带内地层再次褶皱和断裂,并形成三条近雁列的北东东向由绢云母一千枚岩带、石榴石一二云母片岩带、十字石—红柱石片岩带组成的递进变质带,属低压—低压过渡型变质相系。变质带严格受断裂带控制,其变质因素为沿断裂上升的热流和侧向应力作用;3.燕山三幕(晚侏罗世末),断裂再次发生强烈左行压扭。受北东向基底断裂控制,在断裂带内形成一系列与主断裂斜交,走向北东东,产状平缓,以强烈塑性递进形变为特征的韧性剪切带。每一韧性剪切带从边缘到中心,片理与剪切带走向夹角、矿物粒度、石英组构均呈有规律变化并广泛出现动力变质分异和产生新生矿物,形变温度400—500℃,形变深度5—10 km。差异应力值90—170MPa;4.燕山晚期(白垩纪)以来,发生过多次拉张与挤压,在挤压期,带内地层发生宽缓褶皱,沿断裂带发生碎裂变形为主的动力作用。燕山晚期以来不断的抬升,使原来处于不同构造位的变形体叠加在一起共同暴露于现今剥蚀面上,通过构造筛分,可划分出:1.深构造位;2.中深构造位;3.浅构造位的成分。通过构造岩带形成深度和产状的恢复及其形变特征,建立了本区梨状断裂的模式。本区地热场经历了递增期(印支—早燕山期)到衰减期(燕山晚期至早喜山期)的变化,递增期最终形成以热力因素为主的热动力变质,衰减期形成应力相对增强的动力变质。文中还讨论了多期复合变形和变质及其与区域构造演化的相互关系。

The Lianhuashan fault zone is located at the Southwest part of the major Lishui-Haifeng fault,where the NE-trending Cathaysian compounds with the NNE-trending Neocathaysian System.It has suffered alternately compressions and tensions since the Indosinian movement.Horsts and gra- bens,which controlled the formation and distribution of the sedimentary and igneous rocks,might be formed during times when tension prevailed. And folds,nappes and imbricated reverse faults,which controlled the forma- tion and distribution of metamorphic rocks,might be formed during periods. Four dominated by compression major deformation and metamorphism sta- ges can be recognized in the fault zone. 1.Indosinian Period..composite NE-trending folds and faults were formed and low grade regional dynamic metamorphism took place. 2.The Second Episode of Yenshanian Period (the end of middle Jura- ssic):Rocks were re-folded and re-faulted,forming three ENE en chilon arrayed progressive metamorphic belts consisting of serieite-phyllite belt, garnet-muscovite-biotite schist belt and stauralite-andalusite schist belt(low pressure and transition series).Metamorphic belts due to upwelling heat.flow along the fractures and to lateral stress were strictly controlled by the fault zone. 3.The Third Episode of Yenshanian Period (the end of Late Jurassic): Intensive sinistral compresso-sheared movement took place again in the fault zone.As a result of the oontrol by NE basement faults,a series of NE duotile shear zones with very gentle dip and strong plastic deformation, which are diagonal to the major fault,emerged.Their main parts are made up of granitic mylonites and schistose rocks.From the margin to the center of each ductile shear zone,the angle between schistosity and shear zone tends to decrease,the size of strained minerals becomes smaller and quartz C- axis fabricst tend to be a type of I crossed-girdle.Metamorphism with the appearance of new minerals and dynamic differentiation occurred extensively in the ductile shear zone.It is determined that the deformation took place at a temperature of 400—500℃,a depth of 5—10 km,and a differential stress 900—1700 b. 4.Late episode of Yenshanian period (Cretaceous):During this time, tension and compression occurred several times in the fault zone.During compreesion,the strata were gently folded and cataclastic metamorphism took place along the faults. Since the late Yenshanian,continued crustal uplifting would have expo- sed the deformed rocks at different structural levels to the earth's surface. There can be divided into three structural levels of deformation after sifting. 1.Deep structural Level:Ductile shear zones and flow-age folds.The deformation was brought about mainly by dislocation-creep,recrystallization and pressure solution. 2.Medium Structural Level ductile-brittle transition faults and shear folds.These faults have medium dip angles and clear fissure planes.The tecnonites consist mainly of cataclasites,mylonitcs,schistose rocks and phyllontes.The depth of duCtile-brittle deformation transition may be 5—6 km. 3.Shallow structural level brittle faults with big dip angle and gentle fiexural folds.The major tectonite is cataclasite. A model for listric fault formation of this region has been established by analysis of the depths and fault plane attitudes as well as deformation features. Geothermal field in this region might have experienced a change from increasing heat (Caledonian-early Yenshanian) to decreasing heat (late Yenshanian-early Himalayan).Dynamothermal metamorphism accompanied increasing heat,and dynamic metamorphism decreasing heat. The repeated compounding of deformation and metamorphism and their relation to regional tectonic evolution are also discussed in this paper.