Abstract:

The NE-trending Xinyi-Lianjiang fault zone is a tectonic belt, located in the interior of the Yunkai uplift in the west of Guangdong Province, clamping the Lianjiang synclinorium and consisting of the eastern branch and the western branch. The southwestern segment of the eastern branch of Xinyi-Lianjiang fault zone, about 34km long, extends from the north of Guanqiao, through Lianjiang, to the north of Hengshan. However, it is still unclear about whether the segment extends to Jiuzhoujiang alluvial plain or not, which is in the southwest of Hengshan. If it does, what is about its fault activity? According to ‘Catalogue of the Modern Earthquakes of China’, two moderately strong earthquakes with magnitude 6.0 and 6.5 struck the Lianjiang region in 1605 AD. So it is necessary to acquire the knowledge about the activity of the segment fault, which is probably the corresponding seismogenic structure of the two destructive earthquakes. And the study on the fault activity of the segment can boost the research on seismotectonics of moderately strong earthquakes in Southeast China. In order to obtain the understanding of the existence of the buried fault of the southwestern segment, shallow seismic exploration profiles and composite borehole sections have been conducted. The results indicate its existence. Two shallow seismic exploration profiles show that buried depth of the upper breakpoints and vertical throw of the buried fault are 60m and 4~7m(L5-1 and L5-2 segment, the Hengshan section), 85m and 5~8m(L5-3 segment), 73m and 3~5m(Tiantouzai section), respectively and all of them suggest the buried fault has offset the base of the Quaternary strata. Two composite borehole sections reveal that the depth of the upper breakpoints and vertical throws of the buried segment are about 66m and 7.5m(Hengshan section) and 75m and 5m(Tiantouzai section), respectively. The drilling geological section in Hengshan reveals that the width of the fault could be up to 27m. Chronology data of Quaternary strata in the two drilling sections, obtained by means of electron spin resonance(ESR), suggest that the latest activity age of the buried fault of the southwestern segment is from late of early Pleistocene(Tiantouzai section) to early stage of middle Pleistocene(Hengshan section). Slip rates, obtained by Hengshan section and Tiantouzai section, are 0.1mm/a and 0.013mm/a, respectively. As shown by the fault profile located in a bedrock exposed region in Shajing, there are at least two stages of fault gouge and near-horizontal striation on the fault surface, indicating that the latest activity of the southwestern segment is characterized by strike-slip movement. Chronology data suggest that the age of the gouge formed in the later stage is(348±49) ka.

Key words: the southwestern segment of the eastern branch of Xinyi-Lianjiang fault zone, buried fault, fault activity

摘要：

通过浅层地震勘探和钻孔联合剖面探测，揭示了信宜-廉江断裂带东支西南段隐伏断裂的存在。2条浅层地震勘探剖面资料显示，信宜-廉江断裂带东支西南段隐伏断裂的上断点埋深和垂直断距分别为60m和4～7m（L5-1和L5-2测段，横山镇剖面）、85m和5～8m（L5-3测段）、73m和3～5m（L6测线，田头仔村剖面），均断错了第四系底部。2条钻孔联合剖面揭示了上断点埋深和垂直断距分别为66m和7.5m（横山镇剖面）、75m和5m（田头仔村剖面），其中横山镇钻孔剖面显示断裂带宽约27m。采用电子自旋共振方法对钻孔联合剖面中的第四系年代进行测定，显示信宜-廉江断裂带东支西南段隐伏断裂最新活动时代为早更新世晚期（田头仔村剖面）到中更新世早期（横山镇剖面）。由横山镇钻孔联合剖面和田头仔村钻孔联合剖面获得的滑动速率分别为0.1mm/a和0.013mm/a。基岩出露区沙井断层剖面的断面上发育有2期断层泥，由电子自旋共振方法测得晚期的断层泥年龄为（348±49）ka，沿断面还发育有近水平擦痕，反映断裂的最新活动方式以右旋走滑运动为主。

关键词: 信宜-廉江断裂带东支西南段, 隐伏断裂, 活动性