Late Cenozoic Tectonic Activity Of The Northern Part Of Puqian-qinglan Fault And Structural Subsidence Of Dongzhai Port In North Hainan

Since the Late Cenozoic in northern Hainan Island,the tectonic activity has been intense and seismic activity has been frequent.The occurrence of a 7.5 magnitude earthquake in Qiongzhou in 1605 caused a large area of the Dongzhai Port area to become a sea.The study of earthquake-related tectonic activities is of great significance in revealing the settlement process and settlement mechanism of Dongzhai Harbor,major engineering construction,and earthquake prevention and mitigation.In this paper,through remote sensing interpretation,field structure and landscape survey,high-density electrical method and earth-magnetism(EH4)detection,fracture joint profile analysis,combined with ESR and OSL chronology tests,we have established the Late Cenozoic stratigraphic sequences of different tectonic geomorphic units in the Dongzhai Port area.And the Late Cenozoic structural features of the northern section of the Puqian-Qinglan fault have been analyzed,and the settlement process of the Dongzhai Port has been revealed.The settlement mechanism of the Dongzhai Port has been preliminarily discussed.The Dongzhai Port area consists of three structural tectonic units: the Yanfeng volcanic rock platform,the Dongzhai Port lagoon plain and the Jinshan abraded tableland.The Cenozoic strata developed on the surface of each feature are mainly the Miocene Changliu Formation,the Pliocene Haikou Formation,the Lower Pleistocene Xiuying Formation,the Upper Pleistocene Basuo Formation,the Holocene Qiongshan Formation and the Yandun Formation.The north section of the Puqian-Qinglan fault is composed of the east coast faults of the Dongzhai Port and the west coast faults of the Dongzhai Port.Three east dipping normal faults on the west bank of Dongzhai Port and seven west dipping normal faults on the east bank of Dongzhai Port constitute the Dongzhai Port mantle structure.The Dongzhai Port mantle and fault experienced three tectonic activities: Prior to the Miocene,the fault activity caused the Dongzhai Port region to show a step-like landform with gradually decreasing elevation from east to west.At the end of the Middle Pliocene and the beginning of the Pliocene,the faults on the west coast of Dongzhai Harbor began to form.Together with the east coast faults,they controlled the embryonic form of the Dongzhai Harbor.After the Late Pleistocene,the Puqian-Qinglan fault was further active.The 1605 earthquake caused the Dongzhai Port mantle to sink quickly,resulting in a large area of land collapsed into the sea.The sediment thickness map by 86 drilling data at different stages since the Neogene has been made.It shows that the sedimentation center is transferred from the west bank to the east bank and the average deposition rates also have significant change.The average deposition rates of the Mesozoic-Pliocene,Early Pleistocene,Late Pleistocene and Holocene were 12mm/ka,10mm/ka,67mm/ka and 1922mm/ka respectively,which suggest that deposition rate was slow before the Middle Pleistocene,and the deposition rate began to increase suddenly,and it was possible to reach the maximum since the Holocene.That is,the Dongzhai Port has undergone a rapid settlement process since the Late Pleistocene.The settlement of Dongzhai Port in the Middle-Upper Pliocene was mainly controlled by the combination of the NW-trending fault and the EW-trending fault;the settlement since the Early Pleistocene was mainly controlled by the NW-trending fault.Since the Late Pleistocene,the fault activity in the NW-trending direction has increased,and the internal activation of the Dongzhai Port fault-depression zone has accelerated the settlement.The block-type differential lifting movement caused by normal faulting under the north-west direction of the principal stress of the Puqian-Qinglan fault controls the subsidence of the Dongzhai Harbor mantle.