河源伸展剥离断层(博罗-龙川段)及其第四纪活动特征

Discussions on the Heyuan Extensional Detachment Fault From Boluo to Longchuan and Its Quaternary Activities

在实地调查测量和资料分析的基础上,对河源断裂带主体(博罗长宁-龙川段)伸展剥离构造及其第四纪活动特征进行讨论。该断裂带自燕山晚期以来主要表现为缓倾角的伸展剥离正断层,下盘形成了最大厚度约200m的糜棱岩和糜棱岩化带,上盘下滑并控制着K2-E裂陷盆地的形成和发展。博罗-龙川一带的河源伸展剥离断层,南段和北段滑距较大,上盘脆性域K2-E红层以剥离断层为界与下盘前寒武纪变质核杂岩或燕山期花岗岩相接触;中段滑距较小,K2-E红层多与由J1构成的中间韧性层相接触,之间为脆性剥离断层,下剥离盘为糜棱岩化花岗岩,但变质核杂岩未出露。河源断裂带下盘顶部的糜棱岩在后期活动中曾发生强烈的硅化,形成宽度4～30m的硅化岩带。K2-E红层形成之后,断裂带在新生代晚期再次发生错动,其结果是在硅化岩带的顶部或在K2-E红层与中间韧性层J1浅变质粉砂岩之间产生新的脆性断层面,并往往形成20～80cm厚的断层泥。地质、地貌、形变测量、地震以及断层物质测年等资料表明,博罗-龙川段河源断裂带在第四纪直至现今仍有活动,但自晚更新世以来的近地表活动减弱;其第四纪活动在空间上还表现出分段特征:(1)东源黄田以北段活动性较弱;(2)博罗石坝至东源黄田段活动性较强;(3)博罗石坝以南现代活动性也较弱。

Based on the field surveying and data analyzing,this paper discussed on the extensional detachment structure of the Heyuan fault striking northeast from Boluo County to Longchuan County and its Quaternary activities.The Heyuan fault mainly exhibited as an extensional low-angle normal faulting since the late Yanshan stage in the Mesozoic.At the top of the footwall,a layer of mylonite and mylonitized rocks is developed,where the maximum thickness is 200 m.Due to the downward movement of the hanging wall of the fault,the late Cretaceous -Paleogene graben basins were formed and developed.The displacements in the southern and the northern sectors were larger than that in the middle part of the Heyuan extensional detachment fault in the Boluo-Longchuan area.In the southern and the northern parts of the detachment fault,the Cretaceous and Paleogene red beds in the hanging wall as a brittle tectonic facies contacted with the Sinian-Cambrian metamorphic core complex or the Yanshan granites in the footwall.In the middle part,the complex metamorphic core was not exhumed.The Cretaceous-Paleogene red beds contacted with the median ductile layer composing of the lower Jurassic by a brittle fault.While obvious solidification and cataclasis occurred in the upper part of the mylonite,cataclastic silicified rocks would be formed with 4～30m thickness.After the late Cretaceous-Paleogene red beds is formed,brittle fault surfaces would be formed on the top of the silicified belt or between the late Cretaceous -Paleogene red beds and the Jurassic low -rank metamorphic rocks.Within the brittle fracture belt,the thickness of fault gouge ranges from 20 cm to 80 cm.The fault is still active in the Quaternary even at present but its activities becomes weaker gradually since the late Pleistocene.In addition,the fault activities in the Quaternary showed a piecewise feature:(1) the weak activity in norther part of Huangtian,Dongyuan County;(2) the strong activity in the region from Huangtian,Dongyuan County to Shiba,Boluo County;and(3) the weak activity in southern part of Shiba,Boluo County.