珠江口区域滨海断裂带晚第四纪活动特征及年代学约束

Active structure mainly dominates the strong intracontinental earthquakes, and Littoral Fault Zone (LFZ) is a main active fault in the north of South China Sea, which influenced the historical earthquakes along the off-shore area of Guangdong, Hongkong and Macao region. The results of a wide-angle onshore–offshore seismic experiments carried out in northern South China Sea, using large volume airgun sources at sea and seismic stations on land, reveal that the LFZ cross the whole crust with a width more than 10 km. However, the latest stratigraphy of LFZ cutting through remains unclear since the late Pleistocene. We select the cored holes distributed along the main fault to analysis the deposition system and construct the stratigraphy sequence based on the results of the high-resolution multichannel seismic profiles and sub-bottom profiles. Constrained by the ages of AMS14C and photoluminescence (OSL), we retrieve the paleomagnetic polarity events or excursions as well as reconstructing the relative intensity and secular variations of the earth magnetic field to correlate and date the sediments in millennial time scale. So, the high-resolution chronology of the stratigraphy since the middle to late Pleistocene can be estimated. As the key tied points using the geomagnetic field variations, the high-frequency cycle sequences can be used to refine the links of the stratigraphy when the environmental parameters and the fraction of hematite and goethite constitute the proxies combined the detrital grain size distribution. After stratigraphy correlation, the active properties of LFZ, such as the displacement of fault and the latest period of fault cutting across the stratigraphy can be achieved.

滨海断裂带是南海北部最大的活动断层之一，粤港澳大湾区等沿海地区历史期间的强震几乎都与该断裂相关。深反射地震剖面揭示滨海断裂带穿透整个岩石圈，但与强震活动紧密相关的晚第四纪以来断裂带的活动性质和切穿地层未有清晰的结论，影响对断裂带未来致震风险的认识。本申请在前期高分辨率多道地震和浅地层剖面调查的基础上，选择已获取的、分布在断裂带两侧的沉积钻孔，开展多方法的沉积学特征及沉积体系分析，建立较为完整的断裂带区域晚第四纪以来的沉积序列。基于AMS14C和光释光年龄，获取沉积物记录的地磁极性事件或漂移，重建地球磁场的长期变化，构建钻孔地层千年尺度的时间标尺。最后以地球磁场的变化为锚点，以表示陆源碎屑物质粒度、浓度等变化的环境磁学等参数为指标，以旋回地层学为基本原理，进行断层两侧地层的精细对比，定量确定主断层晚第四纪以来的活动性质，为断层强震危险性判定和强震孕育发生机理的研究提供科学依据。

分别选择位于滨海断裂带南、北两侧的两个沉积钻孔NC3(114°32.3′ E，22°03.1′ N)和NC2(114°16.46′ E, 21°4.05′ N)开展年代学和沉积学方面的相应研究，确定了全新世以来断层两侧高分辨率的对比方案，初步分析滨海断裂带全新世期间的活动规律。其中NC3钻孔水深42.4 m，钻孔岩心长84.27 m，古地磁学和AMS14C方法相结合，确定钻孔底部沉积年龄约约为340 ka，41-10.1 ka之间缺少了约三万年的沉积。整个钻孔可划分为7个沉积旋回，中晚更新世期间，以潮坪-河流相沉积为主体的六个沉积旋回，全新世期间的一个沉积旋回由潮坪-浅海相构成。NC2钻孔水深53.3 m，岩心长约40.4 m，AMS14C和光释光年龄相结合，确定钻孔记录了约16 ka以来的沉积。钻孔沉积由河流相-潮坪相-浅海相一个沉积旋回构成，与断裂带北侧NC3钻孔的全新世沉积旋回为同一时期沉积。以两个钻孔沉积物磁化率的旋回变化为标志，将其全新世的地层可划分为四个对比层位。四个对比层位沉积速率的显著差异，反映滨海断裂带在晚更新世末期-全新世早期、早全新世和全新世中期不同的活动特征。