港仔地裂缝——雷州半岛西部一条纵贯晚全新世砂堤地裂缝的发现

港仔地裂缝发生于新生代以来强烈活动的琼雷凹陷北部雷州半岛的西海岸,沿构造带展布,形成于凸起和凹陷交界地区。地裂缝局部纵贯沿海岸展布的晚全新世砂堤,切过中更新世火山岩及其风化土和全新世海成阶地。根据其展布区的构造、地形地貌和形成特征,并与近几十年雷州半岛大量发生的地裂缝比较,认为港仔地裂缝可能属构造蠕变地裂缝。该地裂缝揭示了被纵贯砂堤的沉积学特征和形成于巨砾海滩和高能海浪下的古环境。砂堤上部约海拔5m砂堤岩取样¹⁴C测年为1360±60aB.P.,结合雷州半岛西海岸、西南海岸其他抬升特征,推测该地裂缝是在晚全新世该地海岸抬升或抬升后形成的。

A GROUND CRACK LONGITUDINALLY PENETRATING THE HOLOCENE SAND DYKE IN GANGZI,LEIZHOU PENINSULA IS DISCOVERED

The ground crack at Gangzi is located at the south side of Haikang Port(20°40'42″N,109°45'43″E)of Leizhou City in the middle part of west shore of Leizhou Peninsula.In geological structure,it is situated at the junction between the Qishui Protrusion and the Wushi Sag in the northern half of the Cenozoic Qiongzhou-Leizhou Depression.The north-east fracture and the north-west fracture intersect there.As an area of intensive Pleistocene activity,relic of Pleistocene volcano eruption is only 8km away.The ground crack spreads along the fracture,transecting the Middle Pleistocene volcanic rock and the marine first terrace,and partially longitudinally penetrating the sand dyke that spreads along the shore.The sand dyke,already cemented into sand dyke rock,is about 2km long,more than 100m wide,and striking NEE-NNE.200m of its southwestern end is penetrated longitudinally by the crack,exposing the profile of the dyke.It is disclosed that the sand dyke was accumulated by high-energy sea waves hitting the basalt boulder beach.Samples taken at 5m and 4m altitude above sea level form the sand dyke are 1360±60aB.P.and 4040±100aB.P.respectively by ¹⁴C dating.According to previous studies,the eustatic sea level at 1360±60aB.P.is the same as that of today,and the eustatic sea level at 4040±100aB.P.is about 1m lower than that of today,it is thus calculated that since 1360±60aB.P.and 4040±100aB.P.the local shore was uplifted by 3.00m and 3.13m respectively.This implies that since 1360±60aB.P.the local shore experienced an uplifting of relatively large extent,with an uplifting rate up to 2.2mm/a.It also indicates that the ground crack was formed after 1360±60aB.P.during or after the uplifting process.In 1969,a M6.4 strong earthquake happened in Yangjiang,an area 150km away from Leizhou Peninsula.In that year and ever since,ground cracks in Leizhou Peninsula extended largely and developed frequently.The area near Gangzi is also a zone where ground cracks developed in the past decades.Upon surveying,it was found out that ground crack in that zone was related to strong earthquakes of higher than 5 magnitude and the ground cracks were developed in the area of Leizhou Peninsular where sea level was lowered and crust was uplifted in recent decades as reported by the tide observation station there.However,No ground crack is directly caused by any earthquake.They were the result of earth surface expansion fissures caused by crust uplifting and upheaval.Field survey data indicates that since Late Holocene Epoch,crust at the shore in western and southwestern parts of Leizhou Peninsula has been uplifted.Based on analyses of local structural activities,topography,geomorphy,etc.,it is deduced that the ground crack at Gangzi is of structural creeping type.